Laura Bennati
email: lbennati@purdue.edu

Anthony Finizola

James A Walker

Dina L Lopez

I Camilo Higuera-Diaz

Claudia Schütze

Francisco Barahona

Rafael Cartagena

Vladimir Conde

Renan Funes

Cristobal Rios

Fluid circulation in a complex volcano-tectonic setting, inferred from self-potential and soil CO2 flux surveys: The Santa María-Cerro Quemado-Zunil volcanoes and Xela caldera (Northwestern Guatemala)

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Presence of water is one of the main parameters controlling terrestrial volcanic activity. Inside active volcanoes, water is the mobile phase responsible for transferring energy and mass, from depth toward the surface, generating hydrothermal systems. It can also be confined into aquifers. Usually hydrothermal systems within volcanoes form compartments that are just above a percolation threshold because of the interplay between flow, dissolution/ precipitation and fracturing related to stress and strain variations. Visible manifestations of hydrothermal fluid flow are fumaroles and thermal waters/springs. Sometimes, quiet hydrothermal fluid flow is disrupted by interaction with rising magma, triggering explosive phreatomagmatic activity. Between eruptive periods, hydrothermal activity creates zones of weakness inside volcanoes by hydrothermal alteration of volcanic rocks. These areas increase the risk of potential failure on edifice flanks [START_REF] Lopez | Catastrophic volcanic collapse: relation to hydrothermal processes[END_REF][START_REF] Frank | Surficial extent and conceptual model of hydrothermal system at Mount Rainier, Washington[END_REF][START_REF] Van Wyk De Vries | Sector collapse forming at Casita volcano, Nicaragua[END_REF][START_REF] Reid | Volcano collapse promoted by hydrothermal alteration and edifice shape[END_REF][START_REF] Reid | Massive collapse of volcano edifices triggered by hydrothermal pressurization[END_REF][START_REF] Aizawa | Classification of self-potential anomalies on volcanoes and possible interpretations for their subsurface structure[END_REF]. Therefore, studying fluid flow inside active volcanoes is important for complete volcanic hazard assessments. Moreover, the study of hydrothermal systems and their components is also the first step in discovering new sites for potential geothermal energy exploitation, or improving the knowledge and development plans of sites under exploitation [START_REF] Nieva | Geochemical exploration of the Chipilapa Geothermal Field[END_REF][START_REF] Mahar | A structural model guide for geothermal exploration in Ancestral Mount Bao, Leyte, Philippines[END_REF][START_REF] Negraru | Heat flow and geothermal potential in the South-Central United States[END_REF].

This study aims to identify the main axis of fluid flow and the hydrothermal systems extension across an active volcanic area. Our region of interest is located close to Quetzaltenango, the second largest city of Guatemala (~300,000 inhabitants), in Central America (Fig. 1A andB). This region contains six volcanoes: Santa María, Santiaguito, Cerro Quemado, Zunil, Santo Tomàs, and Siete Orejas, in a 300 km 2 area (Fig. 1C). Our fieldwork has been concentrated on the Santa María, Cerro Quemado, and Zunil volcanoes. We selected these Fig. 1. A: General plate tectonic setting around the studied area (modified from [START_REF] Muller | Age, spreading rates and spreading symmetry of the world's ocean crust[END_REF]. Colors in oceans are related to the age of the oceanic crust with red representing the youngest crust and blue, the oldest. Yellow lines represent plate boundaries. Yellow arrows represent the relative displacement along the transform system between the North American and Caribbean plates. "p." stands for "plate". B: Structural features of the northernmost portion of the Central American subduction zone, mapped on a 90 × 90 m grid digital elevation model (extracted from SRTM). White arrows indicate the directions of plate motion. "F." stands for "Fault system". Yellow lines show the locations of the Middle America trench and the important strike-slip faults in northern Central America. Yellow arrows represent the relative displacement along the strike-slip faults. Blue lines represent graben boundaries. Green lines represent country boundaries. The red square indicates the region of interest (modified from [START_REF] Foley | Geology and geophysics of the Zunil geothermal system, Guatemala[END_REF][START_REF] Caicedo Anchissi | Present status of exploration and development of the geothermal resources of Guatemala[END_REF][START_REF] Cáceres | Crustal deformation in northern Central America[END_REF][START_REF] Weber | Provenance ages of late Paleozoic sandstones (Santa Rosa Formation) from the Maya block, SE Mexico. Implications on the tectonic evolution on western Pangea[END_REF][START_REF] Cox | Investigation of late Quaternary fault uplift along the Motagua/Swan Islands fault system: implications for seismic/tsunami hazard for the Bay of Honduras[END_REF][START_REF] Ortega-Obregón | Middle-Late Ordovician magmatism and Late Cretaceous collision in the southern Maya block, Rabinal-Salamá area, central Guatemala: implications for North America-Caribbean plate tectonics[END_REF]. C: Large view from a digital elevation model (10 × 10 m) of the studied area. Red lines outline the main volcanic features: collapse areas, bases of volcanic edifices, dome extensions (modified from [START_REF] Foley | Geology and geophysics of the Zunil geothermal system, Guatemala[END_REF][START_REF] Asturias | Reservoir assessment of Zunil I and II geothermal fields, Guatemala[END_REF]. Blue dashed lines represent the Zunil Fault zone limits. Blue arrows represent the relative displacement along the Zunil Fault zone boundaries. "1, 2, 3, and 4" represent the 4 stages of domes construction for the Cerro Quemado dome complex (modified from [START_REF] Conway | Cerro Quemado, Guatemala: the volcanic history and hazards of an exogenous volcanic dome complex[END_REF]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) three edifices because (1) of the Plinian eruption of Santa María in 1902 of VEI 6 (Fig. 2A), (2) the recent lava dome extrusion of Cerro Quemado in 1818 (Fig. 2B), and (3) the present-day strong hydrothermal manifestations concentrated between Cerro Quemado and Zunil volcanoes, in two regions named Zunil-I and Zunil-II (Fig. 2C).

Fluid flow is channelized along zones of higher permeability; therefore identifying zones of fluid flow is valuable in highlighting the main weakness planes crossing a volcanic edifice [START_REF] Lénat | Structure et dynamique interne d'un volcan basaltique intraplaque océanique: Le Piton de la Fournaise (île de la Réunion)[END_REF][START_REF] Lénat | Etude de la zone sommitale du volcan Karthala (Grande Comore) par polarisation spontanée[END_REF][START_REF] Finizola | Fluid circulation at Stromboli volcano (Aeolian Islands, Italy) from self-potential and CO 2 surveys[END_REF][START_REF] Finizola | The summit hydrothermal system of Stromboli. New insights from self-potential, temperature, CO 2 and fumarolic fluid measurements, with structural and monitoring implications[END_REF][START_REF] Lewicki | Self-potential, soil CO2 flux, and temperature on Masaya volcano, Nicaragua[END_REF][START_REF] Revil | Geophysical investigations at Stromboli volcano, Italy: implications for ground water flow and paroxysmal activity[END_REF][START_REF] Finizola | Fluid circulation and structural discontinuities inside Misti volcano (Peru) inferred from self-potential measurements[END_REF][START_REF] Finizola | Importance of structural history in the summit area of Stromboli during the 2002-2003 eruptive crisis inferred from temperature, soil CO 2 , self-potential, and electrical resistivity tomography[END_REF]. Sometimes, these fault zones can be sealed (Revil and Cathles, 2002a,b;[START_REF] Ball | Constraining fault-zone hydrogeology through integrated hydrological and geoelectrical analysis[END_REF]. When these faults are more permeable, the fluid flow information is of first importance in volcanic hazard assessments as vent initiation and flank failure are often centered on areas of preexisting weakness. These weak regions can be old volcanic structural boundaries or tectonic faults. In order to study fluid flow, the following geophysical and geochemical methods have been coupled: (1) self-potential technique, sensitive to water flow associated with hydrogeological and hydrothermal systems and (2) soil diffuse degassing technique, sensitive to CO 2 ascent, generally , 1990;[START_REF] Foley | Geology and geophysics of the Zunil geothermal system, Guatemala[END_REF][START_REF] Conway | Cerro Quemado, Guatemala: the volcanic history and hazards of an exogenous volcanic dome complex[END_REF][START_REF] Asturias | Reservoir assessment of Zunil I and II geothermal fields, Guatemala[END_REF][START_REF] Walker | The chemistry of spring waters and fumarolic gases encircling Santa María volcano, Guatemala: insights into regional hydrothermal activity and implications for volcano monitoring[END_REF], and of the self-potential/ soil CO 2 traverse. Self-potential peaks (Q1, Q7, and Q8) are discussed in the text.

of a deep magmatic origin. Coupling these two techniques furnishes information about possible links between the shallower hydrothermal system and the deeper magmatic system.

Tectonic setting and historical volcanic activity

Tectonic setting

Guatemala, located in northern Central America, sits in a complex tectonic setting where crustal deformation results from interactions between the North American, the Cocos, and the northwestern corner of the Caribbean plates (Fig. 1A andB). Movements between the North American and Caribbean plates take place along several hundreds of kilometers of curvilinear left-lateral strike slip faults called the Swan, Motagua, and Polochic transform faults (Fig. 1A andB) [START_REF] Cáceres | Crustal deformation in northern Central America[END_REF][START_REF] Cox | Investigation of late Quaternary fault uplift along the Motagua/Swan Islands fault system: implications for seismic/tsunami hazard for the Bay of Honduras[END_REF][START_REF] Ortega-Obregón | Middle-Late Ordovician magmatism and Late Cretaceous collision in the southern Maya block, Rabinal-Salamá area, central Guatemala: implications for North America-Caribbean plate tectonics[END_REF]. The Swan transform fault (Fig. 1A) connects the southern end of the Cayman trough with the Guatemalan Motagua fault (Fig. 1B) [START_REF] Rosencrantz | SeaMARC II mapping of transform faults in the Cayman Trough, Caribbean Sea[END_REF]. The North American-Caribbean (NAC) plate boundary is seismically well defined. Historical earthquakes of note include: the series of earthquakes in 1816 on the Polochic fault (M7.5 to 7.75) [START_REF] White | The Guatemala earthquake of 1816 on the Chixoy-Polochic fault[END_REF], the 1856 earthquake on the Swan fault (M7 to 8), and the 1976 Motagua fault earthquake (M7.5), which produced about 2 m of slip motion [START_REF] Kanamori | Seismological aspects of the Guatemala earthquake of February 4, 1976[END_REF]. Slip rate along this plate boundary ranges between 11 and 25 mm/year [START_REF] Demelts | GPS geodetic constraints on Caribbean-North America plate motion[END_REF]. Along the Pacific coast, the Cocos plate is subducting beneath the Caribbean plate in a northeast direction. Its slip rate through Central America ranges from 60 to 85 mm/ year, with higher values toward the south [START_REF] Demelts | A new estimate for present-day Cocos-Caribbean plate motion: implications for slip along the Central American volcanic arc[END_REF]. Numerous earthquakes have also been recorded along the subduction zone with magnitudes reaching up to 8 [START_REF] White | Destructive upper-crustal earthquakes in Central America since 1990[END_REF]. Cocos subduction has generated the Central American volcanic front, which runs less than 100 km from the coast (Fig. 1B). Several segments have been identified in this volcanic front, from Guatemala to Costa Rica, defined by varying geological, tectonical or volcanological parameters, such as the strike of volcanic lineaments [START_REF] Stoiber | Quaternary volcanic and tectonic segmentation of Central America[END_REF][START_REF] Carr | Central America[END_REF][START_REF] Carr | Symmetrical and segmented variation of physical and geochemical characteristics of the Central American volcanic front[END_REF]. Despite the significance of the Cocos-Caribbean plate boundary for present-day volcanism, recent tectonic surveys suggest that a large fraction of the intraplate deformation in northern Central America is due to interaction along the NAC plate boundary [START_REF] Cáceres | Crustal deformation in northern Central America[END_REF]. Late Cretaceous transpressional tectonism has been recorded along the Motagua fault as high pressure-low temperature metamorphism [START_REF] Brueckner | Metamorphic reworking of a high pressurelow temperature mélange along the Motagua fault, Guatemala: a record of neocomian and Maastrichtian transpressional tectonics[END_REF]. The NAC plate boundary consists of a complex system of large-scale faults that separate blocks with compressional geologic features [START_REF] Donnelly | Northern Central America: the Maya and Chortís blocks[END_REF][START_REF] Giunta | The peri-Caribbean ophiolites: structures, tectono-magmatic significance and geodynamic implications[END_REF][START_REF] Rogers | Jurassic-Recent tectonic and stratigraphic history of the Chortis block of Honduras and Nicaragua (northern Central America)[END_REF][START_REF] Ortega-Gutiérrez | The Maya-Chortís boundary: a tectonostratigraphic approach[END_REF][START_REF] Phipps Morgan | Intra-arc extension in Central America: links between plate motions, tectonics, volcanism, and geochemistry[END_REF]. The Maya block (Fig. 1A andB) of southeastern Mexico, Belize, and Guatemala is the southernmost continental assemblage of the North American plate, whereas the Chortís block in Guatemala, Honduras, and the Nicaragua rise is the northernmost part of the Caribbean plate [START_REF] Mcbirney | Geology of a part of the central Guatemalan Cordillera[END_REF][START_REF] Weyl | Geology of Central America[END_REF][START_REF] Donnelly | Northern Central America: the Maya and Chortís blocks[END_REF][START_REF] Ortega-Gutiérrez | Oaxaquia, a Proterozoic microcontinent accreted to North America during the late Paleozoic[END_REF][START_REF] Keppie | Terranes of Mexico revisited: a 1.3 billion year odyssey[END_REF]. The collision between the Chortís and Maya blocks, responsible for ophiolite obduction, occurred between 74 and 65 Ma [START_REF] Ortega-Obregón | Middle-Late Ordovician magmatism and Late Cretaceous collision in the southern Maya block, Rabinal-Salamá area, central Guatemala: implications for North America-Caribbean plate tectonics[END_REF]. Estimates of Cenozoic displacements along the Motagua fault system are of about 170 km [START_REF] Donnelly | Northern Central America: the Maya and Chortís blocks[END_REF]. The present-day area located between the Polochic and Motagua faults constitutes an important shear zone [START_REF] Ortega-Obregón | Middle-Late Ordovician magmatism and Late Cretaceous collision in the southern Maya block, Rabinal-Salamá area, central Guatemala: implications for North America-Caribbean plate tectonics[END_REF]. Near to the western end of the Motagua fault, detailed geological studies have defined a major regional fault zone, with an approximate NE-SW direction, more or less perpendicular to the Motagua fault axis, named the Zunil fault zone (Fig. 1B andC) [START_REF] Newhall | Geology of the Lake Atitlán Region, Western Guatemala[END_REF][START_REF] Foley | Geology and geophysics of the Zunil geothermal system, Guatemala[END_REF][START_REF] Asturias | Reservoir assessment of Zunil I and II geothermal fields, Guatemala[END_REF]. Faulting in this area has been linked to the segmentation of the volcanic front proposed by [START_REF] Stoiber | Quaternary volcanic and tectonic segmentation of Central America[END_REF]. Although the relationship between the Zunil fault zone and the end of Motagua fault is not well established, faulting seems to be compatible with strain accommodation between the Maya and Chortís blocks motion [START_REF] Phipps Morgan | Intra-arc extension in Central America: links between plate motions, tectonics, volcanism, and geochemistry[END_REF]. The Zunil fault zone is also located on the eastern border of a huge ~15× 20 km caldera; the Xela (or Quetzaltenango) caldera (Fig. 1B andC) [START_REF] Foley | Geology and geophysics of the Zunil geothermal system, Guatemala[END_REF][START_REF] Duffield | Oblique synoptic images, produced from digital data, display strong evidence of a "new" caldera in south-western Guatemala[END_REF][START_REF] Asturias | Reservoir assessment of Zunil I and II geothermal fields, Guatemala[END_REF]. The age of the caldera is still a subject of investigation and is currently comprised between 84 ka and 12 Ma [START_REF] Duffield | Oblique synoptic images, produced from digital data, display strong evidence of a "new" caldera in south-western Guatemala[END_REF]. The linear shape of the eastern border of Xela caldera clearly suggests tectonic control by the Zunil fault zone.

Our study area is located inside this complex tectonic setting. It includes the Zunil fault zone, and the southern border of the Xela caldera, and contains seven volcanoes, three of which being active during historical times (Santiaguito, Santa María, and Cerro Quemado).

Volcanic and hydrothermal activity

Our area of interest (Figs. 1C and2) lies on a surface of Quaternary and Tertiary volcanic rocks underlain by older igneous and metamorphic rocks [START_REF] Williams | Volcanic history of the Guatemalan Highlands[END_REF]Rose, 1987a,b;[START_REF] Adams | Geochemistry and hydrology of the Zunil geothermal system, Guatemala[END_REF][START_REF] Foley | Geology and geophysics of the Zunil geothermal system, Guatemala[END_REF]. Our study area contains three historically active volcanoes: Santiaguito, Santa María, and Cerro Quemado, three inactive volcanoes, and one well-documented hydrothermal area.

Santa María stratovolcano (Figs. 1C, 2A, and C) has had three discrete eruptive stages: (1) stratovolcano construction, (2) Plinian eruption, and (3) dome growth (Rose et al., 1977a;[START_REF] Conway | Age and magma flux of Santa María volcano, Guatemala: correlation of paleomagnetic waveforms with the 28,000 to 25,000 yr B.P. Mono Lake excursion[END_REF][START_REF] Escobar-Wolf | 40 Ar/ 39 Ar and paleomagnetic constraints on the evolution of Volcán de Santa María, Guatemala[END_REF]. Using paleomagnetic and 40 Ar/ 39 Ar data, [START_REF] Escobar-Wolf | 40 Ar/ 39 Ar and paleomagnetic constraints on the evolution of Volcán de Santa María, Guatemala[END_REF] show that the first stage lasted from 103 ka to 35 ka, producing a modest cone (~8 km³). After a long period without any activity, about 5-10 km³ (DRE) of dacitic magma dramatically erupted in October 1902 (Rose, 1972a;[START_REF] Williams | The October 1902 Plinian eruption of Santa María volcano, Guatemala[END_REF]. This eruption was one of the largest of the 20th century, killing about 5000 people (Rose, 1972a;[START_REF] Conway | Age and magma flux of Santa María volcano, Guatemala: correlation of paleomagnetic waveforms with the 28,000 to 25,000 yr B.P. Mono Lake excursion[END_REF] and creating a huge crater/amphitheatre on the southwestern flank of the Santa María edifice. In 1922, after 20 years of quiescence, dacitic lava started to extrude in the 1902 crater [START_REF] Stoiber | Recent volcanic and fumarolic activity at Santiaguito volcano, Guatemala[END_REF][START_REF] Rose | Pattern and mechanism of volcanic activity at the Santiaguito volcanic dome, Guatemala[END_REF]Rose, , 1987a;;[START_REF] Harris | The thermal stealth flows of Santiaguito dome, Guatemala: implications for the cooling and emplacement of dacitic block-lava flows[END_REF][START_REF] Harris | The evolution of an active silicic lava flow field: an ETM perspective[END_REF], forming the large (~1 km 3 ), intricate Santiaguito dome, which remains active today [START_REF] Stoiber | Recent volcanic and fumarolic activity at Santiaguito volcano, Guatemala[END_REF]Rose, 1972b[START_REF] Rose | Pattern and mechanism of volcanic activity at the Santiaguito volcanic dome, Guatemala[END_REF]Rose, , 1987a;;[START_REF] Anderson | Mount St. Helens and Santiaguito lava domes: the effect of short-term eruption rate on surface texture and degassing processes[END_REF][START_REF] Barmin | Periodic behavior in lava dome eruptions[END_REF][START_REF] Harris | The thermal stealth flows of Santiaguito dome, Guatemala: implications for the cooling and emplacement of dacitic block-lava flows[END_REF][START_REF] Harris | Temporal trends in lava dome extrusion at Santiaguito 1922-2000[END_REF][START_REF] Harris | The evolution of an active silicic lava flow field: an ETM perspective[END_REF]. Dome construction has been characterized by alternating periods of rapid enlargement during intense extrusion periods lasting 3 to 6 years, and longer periods of reduced growth rates (Rose, 1987a;[START_REF] Harris | Temporal trends in lava dome extrusion at Santiaguito 1922-2000[END_REF]. Dome growth at Santiaguito has been occasionally accompanied by other styles of eruptive activity, including pyroclastics flows, vertical pyroclastics eruptions, and lahars [START_REF] Rose | Pattern and mechanism of volcanic activity at the Santiaguito volcanic dome, Guatemala[END_REF][START_REF] Rose | Nuée ardente from Santiaguito volcano, April, 1973[END_REF]Rose et al., 1977b;Rose, 1987a). Minor vertical ash explosions are a daily occurrence and are thought to be phreatomagmatic, resulting from infiltrated meteoric water, which is vaporized by the heat of the hot erupted rocks from a deep magma body [START_REF] Sanchez Bennett | Santa María, Guatemala: a decade volcano[END_REF]. Important hydrothermal manifestations have been observed on Santa María, most notably the numerous fumaroles seen on and by the Santiaguito dome since its birth in 1922 [START_REF] Stoiber | Recent volcanic and fumarolic activity at Santiaguito volcano, Guatemala[END_REF][START_REF] Stoiber | The geochemistry of Central American volcanic gas condensates[END_REF][START_REF] Rose | Nuée ardente from Santiaguito volcano, April, 1973[END_REF]. In addition, [START_REF] Walker | The chemistry of spring waters and fumarolic gases encircling Santa María volcano, Guatemala: insights into regional hydrothermal activity and implications for volcano monitoring[END_REF] highlighted the presence of thermal springs on the south-southwestern flank of Santa María.

Cerro Quemado, an exogenous dome complex, is located 10 km northeast of Santa María volcano (Figs. 1C, 2B, andC). It is the largest and most recent of a series of domes bordering the southern part of Quetzaltenango city [START_REF] Williams | Volcanic history of the Guatemalan Highlands[END_REF][START_REF] Conway | Cerro Quemado, Guatemala: the volcanic history and hazards of an exogenous volcanic dome complex[END_REF]. The volcanic activity at Cerro Quemado has been subdivided by [START_REF] Conway | Cerro Quemado, Guatemala: the volcanic history and hazards of an exogenous volcanic dome complex[END_REF] into four stages. The southern and northern parts of the volcano were formed by andesitic and dacitic lava flows, and domes (phases 1 and 2 on Fig. 1C). The third phase started in 1020 yr B.P. and was characterized by a large explosion with debris avalanches, a laterally directed pyroclastic blast, and pyroclastics flows (phase 3 on Fig. 1C). The explosion left a horseshoe-shaped scar on the western flank of the volcano (Fig. 2C). [START_REF] Conway | Cerro Quemado, Guatemala: the volcanic history and hazards of an exogenous volcanic dome complex[END_REF] showed that the blocky lava flows erupted in 1818 define the fourth and last stage of Cerro Quemado construction (phase 4 on Fig. 1C). The present activity of Cerro Quemado consists solely of hydrothermal manifestations, including fumaroles at about 500 m elevation, to the south of the horseshoe-shaped scar (Fig. 2C).

Zunil volcano is a deeply dissected edifice, located east of Santa María, and lying close to Santo Tomás volcano (Fig. 1C) [START_REF] Williams | Volcanic history of the Guatemalan Highlands[END_REF]. Neither volcano has shown any historical activity (Rose, 1987b).

Siete Orejas, the last volcano located in the studied area, is a stratovolcano, which experienced a large Plinian eruption (N2 km 3 of magma ejected) ca. 120-150 ka [START_REF] Williams | Volcanic history of the Guatemalan Highlands[END_REF][START_REF] Rose | An improved age framework for late Quaternary silicic eruptions in northern Central America[END_REF]. The volcano is now deeply dissected and has a wide south-opening caldera [START_REF] Williams | Volcanic history of the Guatemalan Highlands[END_REF][START_REF] Duffield | Oblique synoptic images, produced from digital data, display strong evidence of a "new" caldera in south-western Guatemala[END_REF].

Snaking between Santa María, Cerro Quemado, and Zunil volcanoes is the Samalá River, which is surrounded on both sides by numerous regional faults that form the Zunil fault zone. This later lies parallel and close to the eastern border of Xela caldera [START_REF] Foley | Geology and geophysics of the Zunil geothermal system, Guatemala[END_REF][START_REF] Duffield | Oblique synoptic images, produced from digital data, display strong evidence of a "new" caldera in south-western Guatemala[END_REF]. The principal direction of this fault system is NE-SW. A secondary fault system is primarily oriented NNW-SSE (Fig. 2C). Two regions of intense hydrothermal activity (characterized by fumaroles and thermal springs) are associated with these two faults systems, covering an area of about 4 km in diameter. The first zone, called Zunil-I, is located on the western side of the Samalá River and a few kilometers SE of Cerro Quemado [START_REF] Templeton | Hydrothermal systems at Volcan Santa María[END_REF]. Within Zunil-I, a geothermal power plant is producing 24 MW, supplying electricity for Quetzaltenango city. The second hydrothermal area is called Zunil-II, or Sulfur Mountain. It is located on the eastern side of the Samalá River and on the western flank of Zunil volcano, and it is surrounded by a semicircular caldera, 3 km in diameter (Figs. 1C and2C) [START_REF] Adams | Geochemistry and hydrology of the Zunil geothermal system, Guatemala[END_REF][START_REF] Foley | Geology and geophysics of the Zunil geothermal system, Guatemala[END_REF][START_REF] Giggenbach | Isotopic and chemical composition of water and gas discharges from the Zunil geothermal system, Guatemala[END_REF][START_REF] Duffield | Oblique synoptic images, produced from digital data, display strong evidence of a "new" caldera in south-western Guatemala[END_REF][START_REF] Lima Lobato | The Zunil-II geothermal field, Guatemala, Central America[END_REF][START_REF] Asturias | Reservoir assessment of Zunil I and II geothermal fields, Guatemala[END_REF]. A big debate is still going on about the exact location of the thermal/magmatic body responsible for these two intense geothermal zones. It was first suggested to be beneath Zunil volcano [START_REF] Giggenbach | Isotopic and chemical composition of water and gas discharges from the Zunil geothermal system, Guatemala[END_REF]. [START_REF] Barmin | Periodic behavior in lava dome eruptions[END_REF], on the other hand, suggested that it could be an extensive hydrothermal system, centered right below the 1902 Santa María crater and Santiaguito dome. Finally, using spring water isotopic geochemistry, [START_REF] Walker | The chemistry of spring waters and fumarolic gases encircling Santa María volcano, Guatemala: insights into regional hydrothermal activity and implications for volcano monitoring[END_REF] stated that the source responsible for Zunil-I and Zunil-II hydrothermal activity could be linked to the Cerro Quemado edifice, the nearest historically active volcano. Based on the new results obtained from our survey, we will discuss these three hypotheses in the following sections.

Methods

As described in the Introduction, self-potential and soil CO 2 diffuse degassing techniques have been used in this survey. The aim of coupling these techniques is to identify the higher permeability structures for fluid movement within this very complex volcanic region.

Self-potential

Self-potential measurements were carried out with a pair of nonpolarisable Cu/CuSO 4 electrodes. This method consists in measuring, directly at the ground surface, the natural difference of electrical potential between two points. This potential difference is measured between the reference electrode and a moving electrode with a high impedance voltmeter (sensitivity of 0.1 mV, internal impedance of 100 MOhm), and an isolated copper cable, 300 m-long. At each station, a ~15 cm deep hole was dug to improve the electrical contact between the electrode and the ground. The offset error between the two electrodes was always less than 1 mV. To avoid increasing this offset along the entire profile, we switched moving and reference electrodes for the next set of measurements.

On active volcanoes, the main source of a self-potential signal is electrokinetic coupling [START_REF] Corwin | The Self-Potential method in geothermal exploration[END_REF][START_REF] Zlotnicki | Review on morphological insights of self-potential anomalies on volcanoes[END_REF][START_REF] Ishido | Electrokinetic mechanism for the "W"-shaped self-potential profile on volcanoes[END_REF]. In short, the flow of ground water creates a macroscopic current density and an electrical field, called the streaming potential, measurable at the ground surface (Lorne et al., 1999a,b;Revil et al., 1999a,b;[START_REF] Revil | Hydroelectric coupling in a clayey material[END_REF]. Positive selfpotential anomalies associated with thermal anomalies are well explained by the electrokinetic theory. The zeta potential, a key parameter describing electrokinetic effects, is generally negative. This means that the interface between the rock (or mineral) and the fluid is negatively charged. This charge is counterbalanced by positive mobile charges in more external layers. Fluid flowing in the pore space (e.g. hydrothermal convecting water) carries these mobile charges downstream, thus making the downstream region positively charged [START_REF] Overbeek | Electrochemistry of the double layer[END_REF][START_REF] Macinnes | The Principles of Electrochemistry[END_REF]Revil, 2002). As a consequence, the electrokinetic effect associated with the uprising of steam/liquid water in volcanic hydrothermal systems will generally result in positive anomalies (positive charges moving in the flow direction), and the downward movement of fluids in negative anomalies [START_REF] Pribnow | Fluid flow in the resurgent dome of Long Valley Caldera: implications from thermal data and deep electrical sounding[END_REF][START_REF] Zlotnicki | Review on morphological insights of self-potential anomalies on volcanoes[END_REF][START_REF] Finizola | Fluid circulation and structural discontinuities inside Misti volcano (Peru) inferred from self-potential measurements[END_REF][START_REF] Revil | Geophysical investigations at Stromboli volcano, Italy: implications for ground water flow and paroxysmal activity[END_REF][START_REF] Revil | Inner structure of La Fossa di Vulcano (Vulcano Island, southern Tyrrhenian Sea, Italy) revealed by highresolution electric resistivity tomography coupled with self-potential, temperature, and CO2 diffuse degassing measurements[END_REF]Barde-Cabusson et al., 2009a,b;[START_REF] Finizola | Importance of structural history in the summit area of Stromboli during the 2002-2003 eruptive crisis inferred from temperature, soil CO 2 , self-potential, and electrical resistivity tomography[END_REF]. Self-potential surveys on active volcanoes have found anomalies of a few hundreds to thousands of mV in amplitude, corresponding to active hydrothermal systems and preferential rainwater infiltration. Positive anomalies are generally centered on summit crater areas [START_REF] Lénat | Etude de la zone sommitale du volcan Karthala (Grande Comore) par polarisation spontanée[END_REF][START_REF] Finizola | The summit hydrothermal system of Stromboli. New insights from self-potential, temperature, CO 2 and fumarolic fluid measurements, with structural and monitoring implications[END_REF][START_REF] Finizola | Fluid circulation and structural discontinuities inside Misti volcano (Peru) inferred from self-potential measurements[END_REF][START_REF] Finizola | Hydrogeological insights at Stromboli volcano (Italy) from geoelectrical, temperature, and CO 2 soil degassing investigations[END_REF][START_REF] Aizawa | Zeta potential estimation of volcanic rocks on 11 island arc-type volcanoes in Japan: implication for the generation of local self-potential anomalies[END_REF], on volcano flanks, or above lateral vents, eruptive fissures, and magmatic intrusions [START_REF] Zablocki | Mapping thermal anomalies on an active volcano by the self-potential method, Kilauea, Hawaii[END_REF][START_REF] Aubert | Evolution d'une intrusion magmatique dans le flanc Sud de l'Etna entre juin 1982 et juin 1983. Résultats de potentiel spontané (PS) et essai d'interprétation de l'éruption de[END_REF][START_REF] Jackson | Regional self-potential anomalies at Kilauea volcano: "Volcanism in Hawaii[END_REF][START_REF] Lénat | Structure et dynamique interne d'un volcan basaltique intraplaque océanique: Le Piton de la Fournaise (île de la Réunion)[END_REF][START_REF] Malengreau | Cartographie et surveillance temporelle des anomalies de Polarisation Spontanée (PS) sur le Piton de la Fournaise[END_REF][START_REF] Lewicki | Self-potential, soil CO2 flux, and temperature on Masaya volcano, Nicaragua[END_REF]. The location of the centered self-potential anomaly at summit crater areas is consistent with convective circulation cells driven by the heat released by magmatic chambers, as suggested by computer modeling results [START_REF] Todesco | Origin of fumarolic fluids at Vulcano (Italy). Insights from isotope data and numerical modeling of hydrothermal circulation[END_REF], physicochemical models of volcanic systems [START_REF] Henley | Geothermal systems ancient and modern: a geochemical review[END_REF][START_REF] Giggenbach | The chemistry of fumarolic vapor and thermal-spring discharges from the Nevado del Ruiz volcanic-magmatic-hydrothermal system, Colombia[END_REF], and three dimensional resistivity distribution [START_REF] Onizawa | Self-potential distribution on active volcano controlled by three-dimensional resistivity structure in Izu-Oshima, Japan[END_REF].

Soil CO 2 flux

CO 2 fluxes were acquired using the "accumulation chamber method", a passive geochemical technique for measuring CO 2 diffuse degassing from soils [START_REF] Chiodini | Soil CO 2 flux measurements in volcanic and geothermal areas[END_REF]. CO 2 soil diffuse degassing measurements were carried out by means of a portable diffuse flux meter developed by WestSystems corporation and equipped with an IR spectrometer (LICOR 820 CO 2 analyzer), an accumulation chamber Type A (internal volume: 0.003 m 3 ), and a Pocket PC to visualize the data. The gas-air mixture within the accumulation chamber is continuously analyzed by the IR spectrometer while concentration/ time curves are plotted on the PC. The accumulation chamber is placed on top of the soil and gases emitted by the soil accumulate within the chamber. The method is based on the measurement of the CO 2 concentration increasing inside the accumulation chamber; the slope of the curve (CO 2 concentration versus time) is directly proportional to the flux. In addition to natural organic sources, the CO 2 emitted by soils in volcanic areas can have volcanic origin. High CO 2 anomalies mainly have their origin in magma degassing inside volcanic systems. Magmatic gases follow the same high permeability pathways as hydrothermal fluids, providing information about zones where there is both vertical permeability and the presence of a source gas release at depth. CO 2 flux surveys performed on active volcanoes and/or tectonic active areas have highlighted huge anomalies associated with active vents, old crater boundaries, and tectonic faults [START_REF] Allard | Eruptive and diffuse emissions of CO 2 from Mount Etna[END_REF][START_REF] Chiodini | Diffuse emission of CO2 from the Fossa crater, Vulcano Island (Italy)[END_REF][START_REF] Etiope | Structural pattern and CO 2 -CH 4 degassing of Ustica island, Southern Tyrrhenian basin[END_REF][START_REF] Lewicki | Self-potential, soil CO2 flux, and temperature on Masaya volcano, Nicaragua[END_REF][START_REF] Revil | Inner structure of La Fossa di Vulcano (Vulcano Island, southern Tyrrhenian Sea, Italy) revealed by highresolution electric resistivity tomography coupled with self-potential, temperature, and CO2 diffuse degassing measurements[END_REF].

Data acquisition and results

Self-potential and CO 2 flux measurements were performed in January 2007 along a unique profile crossing several active volcanoes: Santa María, Cerro Quemado, and Zunil edifices, and also hypothesized structural discontinuities (Figs. 1C and2C). The direction of the profile is west-east, and its length is 16.880 km. 848 collocated data points were measured along the profile, every 20 m, for both self-potential and CO 2 flux. Data positions were recorded on a handheld GPS receiver and corresponding elevations have been extracted from a precise (10 × 10 m) digital elevation model.

Raw self-potential and CO 2 flux signals

The west-east profile is shown in Fig. 3, where topography (brown), self-potential (blue), and soil CO 2 flux (red) have been plotted versus distance. The self-potential signal displays large variations (close to 1000 mV). Based on the topography and selfpotential signal variations, we have subdivided the profile into four major segments: (1) Santa María (M), (2) Cerro Quemado (Q), (3) Samalá River (S), and (4) Zunil (Z). For each segment, based on the self-potential/elevation relationships, further subdivisions have been made.

The first major segment, M, is located on the eastern flank of Santa María volcano. In this segment, the self-potential signal displays a general "V" shape (M1 through M6 subsections). This "V" shape is slightly disrupted by the M4 sector. The self-potential signal remains nearly flat on subsection M7.

The second segment, Q, corresponds to the Cerro Quemado dome. It displays a fluctuating self-potential signal (from Q2 to Q6) bordered by sharp self-potential negative peaks on both sides. The negative peak on the western side of the dome, Q1, has an amplitude of about 450 mV, while the two negative anomalies, Q7 and Q8, on the eastern side of the dome, have lower amplitudes (about 200 mV for Q7 and 120 mV for Q8). The third (S) and fourth (Z) segments, corresponding respectively to the Samalá River and Zunil volcano, are characterized by a selfpotential signal, which exhibits an excellent negative correlation with the surface topography. The boundary between the third and fourth segments has been defined by a sharp break in the slope of the selfpotential signal.

CO 2 flux data display a weak oscillation along the entire profile (Fig. 3). No significant peak can be distinguished. The average degassing value along the profile is of about 21 g/m 2 d, and CO 2 flux data range from not measurable to 125 g/m 2 d. Such a value of soil CO 2 flux is low in comparison to average values measured on active degassing volcanoes such as Miyakejima: 235 g/m 2 d (Hernández Dark blue zones with strong negative self-potential peaks (Q1 and Q7-8) have been interpreted as dome emplacement faults bordering the Cerro Quemado dome complex allowing meteoric water infiltrations. Blue areas with negative self-potential/elevation gradients (M3, M5-7, Q2, Q6, S1-2, and Z1-2) are considered as hydrologic zones and yellow areas with positive self-potential/elevation gradients as active hydrothermal systems (M1-2, M4, and Q4). Areas with no obvious self-potential/ elevation gradients (n.d. = not determined) are in green (Q3 and Q5) and located on both side of Cerro Quemado hydrothermal system. CO 2 flux data (red) are smoothed by a FFTlow pass filter: blue color represents the five lowest CO 2 flux values (below 10 g/m² d) and orange color represents CO 2 flux values above the average (21 g/m² d). These areas are discussed in the text. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) et al., 2001); Vulcano: between 129 and 2750 g/m 2 d [START_REF] Granieri | Correlated increase in CO 2 fumarolic content and diffuse emission from la Fossa crater (Vulcano, Italy): evidence of volcanic unrest or increasing gas release from a stationary deep magma body?[END_REF]; summit of Teide: 717 g/m 2 d [START_REF] Hernández | Diffuse emission of carbon dioxide, methane, and helium-3 from Teide volcano, Tenerife, Canary Islands[END_REF]; Mammoth Mountain: between 897 and 2414 g/m 2 d [START_REF] Gerlach | Three-year decline of magmatic CO 2 emission from soils of a Mammoth Mountain tree kill: Horseshoe Lake, CA, 1995-1997[END_REF]; Solfatara of Pozzuoli: 1478 g/m 2 d [START_REF] Chiodini | Soil CO 2 flux measurements in volcanic and geothermal areas[END_REF]. Note that in the cited references, volcanic areas that present very high values of CO 2 degassing correspond to actively convective zones (surficial manifestations such as fumaroles, etc.) rather than zones where soil gases are discharged quietly in a diffuse way. Similar low range of soil CO 2 degassing values has been measured on Salvadorian volcanoes (e.g. San Miguel Volcano, [START_REF] Cartagena | Diffuse soil degassing of carbon dioxide, radon, and mercury at San Miguel volcano, El Salvador[END_REF]San Salvador Volcano, Pérez et al., 2004;Ilopango Caldera, Lopez et al., 2004), and Popocatepetl Volcano in Mexico [START_REF] Varley | The absence of diffuse degassing at Popocatépetl volcano, Mexico[END_REF]. Nevertheless, the average flux is above the normal soil degassing rates [START_REF] Scheffer | Lehrbuch der Bodenkunde[END_REF]. A FFT-low pass filter minimizes the influence of high-frequency noise and was applied to smooth the raw data (see black curve in Fig. 3). On this smoothed curve, a degassing minimum seems to coincide with the minimum in surface topography of the Samalá River valley. However, there is no evident correlation between self-potential and CO 2 flux signals.

Self-potential/elevation gradient analysis

As described in the "3. Methods" chapter, the general trend of a self-potential signal crossing a volcanic edifice is enough to discriminate between hydrogeological and hydrothermal zones [START_REF] Zlotnicki | Review on morphological insights of self-potential anomalies on volcanoes[END_REF][START_REF] Finizola | Fluid circulation and structural discontinuities inside Misti volcano (Peru) inferred from self-potential measurements[END_REF][START_REF] Ishido | Electrokinetic mechanism for the "W"-shaped self-potential profile on volcanoes[END_REF].

A more detailed analysis of a self-potential signal versus elevation often identifies linear segments separated by sharp breaks in slope. As shown by previous studies performed on Misti volcano [START_REF] Finizola | Fluid circulation and structural discontinuities inside Misti volcano (Peru) inferred from self-potential measurements[END_REF][START_REF] Tort | The buried caldera of Misti volcano, Peru, revealed by combining a self-potential survey with elliptic Fourier function analysis of topography[END_REF], these breaks in slope help distinguish the hydrogeologic transition between the ground surface and the water-saturated zone, and also the hydrothermal transition between the water evaporation level and the ground surface. A similar approach, studying self-potential/elevation gradients, has been applied on the entire west-east profile. For clarity, we have separated the self-potential/elevation plots into 3 parts: Santa María, Cerro Quemado and Samalá-Zunil (Fig. 4).

The Santa María area (Fig. 4A) can clearly been subdivided into seven self-potential/elevation gradient sectors. Among these, three have positive values: M1, M2, and M4, and one, M7, is nearly flat.

The Cerro Quemado area (Fig. 4B) displays a more complex relationship between self-potential and elevation. A symmetric behavior can be seen on both side of the dome. In the center of the dome, a positive self-potential/elevation gradient (Q4) is surrounded by two non-determinable gradient areas (Q3 and Q5), then by negative self-potential/elevation gradients (Q2 and Q6), and finally bordered by straight self-potential negative peaks (Q1 and Q7-Q8).

The Samalá River and Zunil volcano area (Fig. 4C) is characterized by four sectors with only negative gradients.

Discussion

Based on the results presented above, four main points will be discussed in this chapter: (1) the main present-day hydrothermal systems active in the study area, (2) the structural discontinuities encountered along the profile, (3) the relative low amplitude fluctuation in the soil CO 2 degassing, and (4) the possible location of the thermal/magmatic body responsible for the two intense geothermal zones named Zunil-I and Zunil-II.

Main hydrothermal systems

Our self-potential study identifies two main positive self-potential/ elevation gradients. The first one centered on top of Santa María volcano (M1/M2), and the second one within the Cerro Quemado dome (Q4). As mentioned above, these positive gradients likely delineate zones of rising hydrothermal fluids.

On the northeastern flank of Santa María volcano, the M1-M2 positive gradients suggest a hydrothermal system up to about 800 m away from the summit (Figs. 5 and6). The overall self-potential signal has a typical "V" shape similar to the signal associated with hydrothermal systems identified on other active volcanoes [START_REF] Finizola | Fluid circulation and structural discontinuities inside Misti volcano (Peru) inferred from self-potential measurements[END_REF][START_REF] Ishido | Electrokinetic mechanism for the "W"-shaped self-potential profile on volcanoes[END_REF][START_REF] Aizawa | Zeta potential estimation of volcanic rocks on 11 island arc-type volcanoes in Japan: implication for the generation of local self-potential anomalies[END_REF]. With only one selfpotential profile, it is hard to say if this hydrothermal system is fed by the thermal energy released by the residual magmatic body from the last eruption of Santa María in 1902 or by the magmatic body feeding the present-day Santiaguito dome activity, located only 2.5 km away from the summit of Santa María (Fig. 1C). However, the location of this hydrothermal system (see Figs. 5 and7), located in the upper third of the edifice of Santa María, strongly suggests that the source generating the hydrothermal activity here is a residual magmatic body related to the last Santa María activity in 1902 rather than the present-day Santiaguito activity. But the main point here is that Santa María volcano is not associated with a hydrothermal system larger than about 1.5 km in diameter.

On Cerro Quemado dome, only one positive gradient in sector Q4 has been identified (Fig. 4). This area is associated, at the surface, with a no vegetation zone corresponding to the lava dome extrusion of 1818 (Figs. 2 and6). These elements support the hypothesis of an underlying hydrothermal system. A 1 km large "transition" zone surrounds Q4, without determinable self-potential/elevation gradients (sectors Q3 and Q5), before it encounters a better-defined negative self-potential/elevation gradient suggesting downward motion of meteoric water (Fig. 6). The comparable values of the self-potential/ elevation gradients in these zones (Q2 = -0.9 mV/m and Q6 = -1.1 mV/m) and the fact that Q2 and Q6 cut the same geomorphological area bordering the Cerro Quemado dome (see the continuity of the break in slope in Figs. 1C and2C), suggest similar lithologies in both sectors.

In the Q4 sector, the positive SP anomaly occurs within the borders of the 1818 lava dome (Fig. 6). Moreover, the positive SP/elevation gradient disappears immediately going away from the 1818 lava dome. As a consequence, we can assume that the hydrothermal system of Cerro Quemado dome is mainly delimited by the boundary of the 1818 lava dome. We therefore hypothesize that the vaporization of the present-day hydrothermal system and the rising of these hydrothermal fluids toward the surface are induced by the thermal energy released by the cooling magma of the last 1818 eruption. It is interesting to note that the extension of the Cerro Quemado hydrothermal system is no more than 1.5 km in diameter, similar to that beneath Santa María volcano (Figs. 5 and6).

Main structural discontinuities

Previous studies comparing changes in surface topography and transitions in self-potential/elevation gradients in other volcanoes have distinguished major structural discontinuities in volcanic complexes [START_REF] Tort | The buried caldera of Misti volcano, Peru, revealed by combining a self-potential survey with elliptic Fourier function analysis of topography[END_REF].

On Santa María volcano, the topography displays a sharp break in slope at the boundary between sectors M4 and M5 (Fig. 3). From a self-potential/elevation gradient point of view, M4 appears as an isolated sector with a positive value, extending only on a few hundred meters (Fig. 4A). Such marked changes in topography and selfpotential/elevation gradient suggest the presence of an important structural discontinuity. Comparing this result and previous geological surveys coupling aerial photographs and field structural geology [START_REF] Foley | Geology and geophysics of the Zunil geothermal system, Guatemala[END_REF][START_REF] Asturias | Reservoir assessment of Zunil I and II geothermal fields, Guatemala[END_REF] indicates that this boundary corresponds to the border of Xela caldera (Fig. 6). In order to explain the positive self-potential/elevation gradient at this location, we hypothesize that Xela caldera is a structural boundary of higher permeability, allowing the uprising of hydrothermal fluids. It is also important to specify that the Zunil fault zone cannot explain the M4 SP anomaly. As shown in Fig. 2C, the SP profile does not cross the Zunil fault zone. The M4 SP anomaly is located about 2 km away from the Zunil fault zone, and areas located closer to the Zunil fault zone (as Q2-Q3 in Figs. 2C and3) have no positive hydrothermal SP anomaly. These geographic considerations also support our hypothesis that the M4 anomaly is linked to the Xela caldera.

The Cerro Quemado dome complex is characterized by a smoothed chaotic topography and well-defined external boundaries (Q1 and Q7 in Fig. 5). This change in morphology bordering the Cerro Quemado dome corresponds with sharp self-potential minima, but it is not associated with faults crossing the Zunil fault zone (Fig. 2). On the western side, Q1 peak could be related to a lithological transition within the dome or with bordering alluvium, but this is not likely the case on the eastern side for the Q7 and Q8 peaks. Similar short wavelength self-potential minima have been identified on Stromboli volcano and attributed to old crater boundaries through which meteoritic water preferentially flows [START_REF] Finizola | Fluid circulation at Stromboli volcano (Aeolian Islands, Italy) from self-potential and CO 2 surveys[END_REF]. In the case of Cerro Quemado, we are dealing with a dome complex, and dome growth is often accompanied by concentric faulting [START_REF] Buisson | Experiments on internal strain in lava dome cross sections[END_REF]. Therefore, the self-potential minima (Q1, Q7, and Q8) could also be interpreted as preferential rainwater infiltration along the faults induced by past stages of magma intrusion and dome growth. These dome related faults would have to be frequently reactivated for their permeability to stay high; and not be plugged by mineral precipitation. In the case of Cerro Quemado dome and due to its proximity with Quetzaltenango, it is also important to consider that such higher permeability areas bordering the dome represent weakness planes that could be used as preferential pathways for future magmatic injection in the dome complex.

Slight fluctuations in soil CO 2 degassing

On other volcanoes, good correlations have been shown between CO 2 peaks and self-potential anomalies. These have been correlated to (1) upward flow of water vapor close to heat sources and inside hydrothermal systems [START_REF] Finizola | The summit hydrothermal system of Stromboli. New insights from self-potential, temperature, CO 2 and fumarolic fluid measurements, with structural and monitoring implications[END_REF][START_REF] Lewicki | Self-potential, soil CO2 flux, and temperature on Masaya volcano, Nicaragua[END_REF], or with (2) downward flow of liquid water far away from heat sources and outside of hydrothermal systems [START_REF] Finizola | Fluid circulation at Stromboli volcano (Aeolian Islands, Italy) from self-potential and CO 2 surveys[END_REF]. In this study, no high CO 2 peak is associated with the high permeability zones evidenced by the self-potential signal (e.g., Q1 and Q7-Q8 in Fig. 5). Nevertheless, the flux of soil CO 2 degassing is not negligible and remains weak to medium along the profile (Fig. 3). Analyzing the CO 2 signal through a FFT-low pass filter, we considered an arbitrary threshold of 10 g/m² d, and then examined the CO 2 minima below this value (Fig. 5). Five sectors appear to have lower CO 2 flux values: S1, Q4, Q1, Z2, and M3. Three of them, S1, Z2, and M3, are characterized by a negative self-potential/elevation gradient and, hence, likely represent major downwellings of meteoric water. As just discussed, Q1, with its large self-potential minima, also is a likely site of downward meteoric flow. Such sites in this region are characterized by low diffuse degassing of CO 2 . Surface emissions of CO 2 are inhibited by water-saturated soil and by the dissolution of CO 2 into downward moving water [START_REF] Farrar | Magmatic carbon emissions at Mammoth Mountain, California[END_REF]. The low CO 2 emissions in the Q4 segment, which we have suggested is characterized by hydrothermal upwelling, are more puzzling. [START_REF] Finizola | Fluid circulation at Stromboli volcano (Aeolian Islands, Italy) from self-potential and CO 2 surveys[END_REF] saw a similar correlation between low CO 2 emanations and hydrothermal upwelling within Stromboli volcano, which they attributed to self-sealing of the hydrothermal system, which inhibits the release of CO 2 . This would also explain the very low level of fumarolic activity at Cerro Quemado. To conclude, our study area has a medium level of CO 2 diffuse degassing except where there is preferential water infiltration, as at the boundaries of the Xela caldera, and above the hydrothermal system at Cerro Quemado. These ideas need to be tested by additional surveys, for example by coupling resistivity tomography with SP and soil gas measurements, along new profiles crossing the entire region, like Barde-Cabusson et al. (2009b) did in Italy, on Vulcano edifice.

Thermal/magmatic body related to Zunil-I and Zunil-II

The results of the present study make possible to better constrain the ongoing debate about the thermal/magmatic source responsible for the Zunil-I and Zunil-II geothermal activities.

Although our self-potential profile did not reach the top of Zunil volcano, no hydrothermal system signature has been shown on the northern flank of Zunil, between 2050 and 2800 masl (Fig. 4C). It is therefore difficult to attribute the Zunil-I and Zunil-II fumarolic manifestations to a magma body beneath Zunil volcano as suggested by [START_REF] Giggenbach | Isotopic and chemical composition of water and gas discharges from the Zunil geothermal system, Guatemala[END_REF]. On the other hand, the Santa María and Cerro Quemado dome hydrothermal systems have diameters of less than 1.5 km, centered on the edifices. This fact is not in accordance with the hypothesis of [START_REF] Barmin | Periodic behavior in lava dome eruptions[END_REF] that posits an extensive hydrothermal system linked to Santa María-Santiaguito to explain both the Zunil-I and Zunil-II geothermal areas. Carbon isotopic data presented by [START_REF] Walker | The chemistry of spring waters and fumarolic gases encircling Santa María volcano, Guatemala: insights into regional hydrothermal activity and implications for volcano monitoring[END_REF] are also inconsistent with these hypotheses, and suggest a preferential link between Cerro Quemado and the Zunil-I and Zunil-II geothermal areas. Our survey also puts constraints on this suggestion.

As shown on a larger scale (Fig. 7), the Zunil-I and Zunil-II geothermal areas are located inside a highly complex tectonic area, called the Zunil fault zone, parallel and close to the eastern border of Xela caldera. This caldera boundary channels hydrothermal fluids within the Santa María edifice (sector M4 in Figs. 5 and6). Our data suggest that the historical volcanic activity of Santa María and Cerro Quemado only produces small hydrothermal systems, with restricted lateral extensions. Indeed, it is difficult to imagine a deep heat source for Cerro Quemado and Santa María, able to produce hydrothermal systems with a relatively small lateral extension (only 1.5 km in diameter). On the other hand, the positioning of the Zunil-I and Zunil-II, aligned along deep tectonic structures (Zunil fault zone), may suggest a deeper heat source. We therefore hypothesize the existence of a deeper and broader heat source to explain the intense geothermal manifestations observed at the Zunil-I and Zunil-II geothermal sites, and shallower heat sources, related to recent historical eruptive activity, to explain the hydrothermal systems of Cerro Quemado and Santa María (Fig. 7). The heat source of Zunil-I and Zunil-II could be a remnant of the Xela caldera system with fluid flow focused into the highly fractured Zunil area. Hot fluids rising along the caldera rim could be distributed by the highly fracturated ring fault system, as occurs in other caldera-related geothermal systems of Central America, such as the Concepción de Ataco caldera, the Ahuachapan Geothermal Systems [START_REF] González Partida | Plio-pleistocene volcanic history of the Ahuachapán geothermal system, El Salvador: the Concepción de Ataco Caldera[END_REF], and the Berlin Caldera and Berlin Geothermal System (D'Amore and Tenorio Mejia, 1999), both in El Salvador. This Xela heat source could also be responsible for the hypothesized hydrothermal fluid flow on the northeastern flank of Santa María (segment M4) and even that at Cerro Quemado, which would be in accordance with the carbon isotopic resemblance between water samples at Cerro Quemado and Zunil (Fig. 7) [START_REF] Walker | The chemistry of spring waters and fumarolic gases encircling Santa María volcano, Guatemala: insights into regional hydrothermal activity and implications for volcano monitoring[END_REF].

Conclusions

Contrary to other self-potential surveys carried out on Andean volcanoes that identified huge hydrothermal systems more than 6 km in diameter [START_REF] Finizola | Fluid circulation and structural discontinuities inside Misti volcano (Peru) inferred from self-potential measurements[END_REF], our survey crossing Santa María, Cerro Quemado, and Zunil volcanic complexes only identified 2-3 small, active hydrothermal systems no more than 1.5 km in diameter: one located below the top of Santa María; another on the northwestern flank of Santa María; and one located in the upper part of the Cerro Quemado dome complex. The restricted lateral extensions for these hydrothermal systems do not support existing hypotheses of the subsurface linkages between the volcanic complexes and the intense Zunil-I and Zunil-II geothermal manifestations. Therefore, we have to consider a deeper and more extensive heat source/magmatic body to explain the Zunil-I and Zunil-II geothermal sites located inside Xela caldera. Such a heat source would also explain the hydrothermal fluid rising along the Xela caldera boundary as suggested by the new self-potential survey. Due to the similar carbon isotopic ratios in water between Cerro Quemado and the Zunil-I and Zunil-II areas [START_REF] Walker | The chemistry of spring waters and fumarolic gases encircling Santa María volcano, Guatemala: insights into regional hydrothermal activity and implications for volcano monitoring[END_REF], we also hypothesize that the hydrothermal/volcanic activity at Cerro Quemado corresponds to a lateral extension of the heat source/magmatic body feeding the Zunil fault zone. Considering a large active magmatic system inside the Xela caldera, we can also better understand the near homogeneous CO 2 degassing inside the caldera, and the decrease of the CO 2 flux values outside the caldera boundaries.

Further multidisciplinary surveys such as those carried out on Vulcano Island by Barde-Cabusson et al. (2009a,b) where they coupled resistivity tomography, SP and soil gas measurements would be useful to better image the complex relationship between the volcano-tectonic structures and heat sources responsible for the Zunil-I and Zunil-II geothermal areas, because those three methods are all highly sensitive to fluid flow, highlighting preferential structures used for fluid circulation within volcanic edifices. From a hazard point of view, it is possible that an important seismo-tectonic event could disrupt the hydro-magmatic equilibrium in the Zunil fault zone. On the Cerro Quemado dome complex, new magma injections would likely occur along the highest permeability areas bordering the dome and near the heavily populated city of Quetzaltenango.

Fig. 2 .

 2 Fig. 2. A: Picture of Santa María and ash plume rising from Santiaguito, taken from the flanks of Zunil volcano. B: Picture of Cerro Quemado with the city of Quetzaltenango to its west, taken from the flanks of Santa María. C: Landsat image overlapped on a 10 × 10 m digital elevation model of the Santa María-Cerro Quemado-Zunil region, showing the locations of hydrothermal manifestations (modified from Caicedo Anchissi and Palma, 1990;[START_REF] Foley | Geology and geophysics of the Zunil geothermal system, Guatemala[END_REF][START_REF] Conway | Cerro Quemado, Guatemala: the volcanic history and hazards of an exogenous volcanic dome complex[END_REF][START_REF] Asturias | Reservoir assessment of Zunil I and II geothermal fields, Guatemala[END_REF][START_REF] Walker | The chemistry of spring waters and fumarolic gases encircling Santa María volcano, Guatemala: insights into regional hydrothermal activity and implications for volcano monitoring[END_REF], and of the self-potential/ soil CO 2 traverse. Self-potential peaks (Q1, Q7, and Q8) are discussed in the text.

Fig. 3 .

 3 Fig. 3. Elevation (brown), self-potential (blue), and soil CO 2 flux (red) versus distance along the studied profile. The black curve represents soil CO 2 flux data smoothed by a FFT-low pass filter. The CO 2 signal does not show any significant anomaly, while four distinct subdivisions can be distinguished via self-potential: M1-7, Q1-8, S1-2, and Z1-2, termed Santa María, Cerro Quemado, Samalá River, and Zunil, respectively. See details in the text. "Ref" refers to the self-potential reference measured on the border of Samalá River. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4 .

 4 Fig. 4. Self-potential signal versus elevation for: Santa María volcano area (A), Cerro Quemado dome complex (B), and Samalá River and Zunil volcano area (C). Letters represent subdivisions as defined in Fig. 3. Color-coded numbers represent values of self-potential/elevation gradients for select subdivisions (unit: mV/m). Four positive self-potential/ elevation gradients (M1, M2, M4, and Q4) and three short wavelength minima (Q1, Q7, and Q8) have been highlighted. Their significance is discussed in the text. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6 .

 6 Fig. 6. Landsat image overlapped on a 3D (10 × 10 m grid) digital elevation model of the Santa María-Cerro Quemado-Zunil area. Interpretation of fluid flows has been reported with different colors and arrows along the 16.880 km-long profile, highlighting the main structural discontinuities constraining fluid flows. Symbols are the same as in Fig. 5. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Acknowledgements

This survey was supported by a National Oceanic and Atmospheric Administration (NOAA) grant operated through the Analytical Center for Climate and Environmental Change (ACCEC) in the Department of Geology and Environmental Geosciences, Northern Illinois University, and in collaboration with the Universidad de El Salvador. We particularly appreciated the assistance of Barry Cameron and Kurt Roggensack in the field. The authors thank the local civil defense institution from Cantel, which ensured security during the fieldwork. We are very thankful to Bill Rose for the digital elevation model exchanges we made. We would like to thank the two anonymous reviewers for their greatly appreciated constructive comments. This is IPGP contribution number 2571.


V Sivakumar

D Moema

A Sharma

N Mbatha

C Bollig

S Malinga

G Mengistu

H Bencherif

P Keckhut

LIDAR FOR ATMOSPHERE RESEARCH OVER AFRICA -A TRILATERAL RESEARCH PROGRAMME (LARA -TRIP)

This paper describes the importance of LIDAR for atmosphere research over Africa and the initiatives being undertaken to foster such research. A mobile LIDAR system is to be employed for field campaign measurements over different locations around South Africa and Africa.

INTRODUCTION

In recent years, the importance of systematic monitoring of the atmospheric structure and dynamics has been highlighted by several atmospheric research campaigns over the globe. Laser radar, more popularly known as LIDAR (LIght Detection And Ranging), has become an excellent tool for monitoring the atmosphere in a relatively short period of time (a few seconds to minutes). Furthermore, interpretation of the results can accurately reflect the state of atmosphere at the time of the measurements. The discovery of different laser sources and improvements in detector technology, data collection and analysis techniques, have made the LIDAR a consistent remote sensing tool for atmospheric science. Both continuous wave and pulsed laser systems have been extensively used. These systems are currently operational and are used for the study of the atmospheric structure and dynamics, trace constituents, aerosols, clouds as well as the boundary layer and other meteorological applications. LIDARs are used notably in different parts of the globe to study aerosols/clouds (Mie Scattering), atmospheric density and temperature (Rayleigh Scattering), metallic ion species (Resonance Scattering), minor constituents and trace gases (Differential absorption), composition (Raman Scattering) and winds (Doppler LIDAR). For further details, one can refer to the recent chapter on 'LIDAR for ground and air borne trace gas detection' in a text book 'Lasers in chemistry' [START_REF]LIDAR for ground and air borne trace gas detection, Chapter in a text book[END_REF].

The first establishment and construction of a LIDAR system was made in 1961, by two groups at Hughes Aircraft Malibu and Culver City, respectively. However, this system was exploited for a range finding application only. Later, LIDAR studies were carried out using a Ruby-laser as source [START_REF] Ligda | Proceedings of the First conference on Laser Technology[END_REF][3] [START_REF] Fiocco | Observations of the aerosol layer at 20 km by optical radar[END_REF] but recent studies have taken advantage of high pulse repetition frequencies available with Neodymium (Nd)-Yttrium Aluminum Garnet (YAG) lasers, the frequency-doubled output at 532 nm being employed for atmospheric studies [START_REF] Chanin | LIDAR observation of Gravity and Tidal waves in the stratosphere and mesosphere[END_REF]. Advantages offered by the improved performance of Nd:YAG lasers, the use of shorter wavelengths, and the availability of improved blocking for rejecting the elastically backscattered return, have led to renewed interest in atmospheric density measurements. The Nd:YAG laser is also advantageous since it can be easily tuned to make atmospheric measurements on aerosols, clouds, water vapour, ozone and metallic ions.

LIDAR survey

A survey of ground based LIDARs around the world was conducted via a web search. It is noted here that the survey is not by any means comprehensive and conclusive. Further, the survey has been made with regards to LIDAR applications for atmosphere studies including pollutant monitoring. The following salient features are noted from the survey that:

around 80 % of the LIDARs are in the northern hemisphere region, only a few, approximately 20 %, exist in the southern hemisphere region with a high concentration in South America, -LIDARs in the African continent are limited to two system which includes the mobile LIDAR recently developed at Pretoria, even though there are many ground based standard LIDARs, there are not many mobile LIDAR systems for systematic atmosphere measurements over different locations.

Only two LIDAR systems are available in South Africa. One is located in Durban (South Africa) and operated under a South Africa -France association by University of Kwa-Zulu Natal (UKZN) [START_REF] Bencherif | Observation and first validation of stratospheric temperature profiles obtained by a Rayleigh-Mie LIDAR over Durban, South Africa[END_REF], [START_REF] Moorgawa | The Durban atmospheric LiDAR[END_REF] and the other one is the new mobile LIDAR system designed and developed at the National Laser Centre (NLC), Council for Scientific and Industrial Research (CSIR), Pretoria. Our aim is therefore to develop a mobile LIDAR system to serve as a research platform for monitoring the atmosphere over Africa.

OBJECTIVES AND DESCRIPTION

The research objective is to obtain a sustainable LIDAR for probing the atmosphere. At present, the research programme is focussed to obtain a better understanding of global climate change (GCC) and global warming (GW). The two important factors influencing GCC and GW are water vapour and ozone. Regular monitoring of these constituents is important for a better understanding of GCC and GW. To achieve this, we need a mobile instrument which would facilitate measurements at different regions. To accomplish the above requirements, the LIDAR will be installed in a mobile vehicle equipped with the necessary optics to make remote measurements. Our goal is to build the mobile LIDAR system and to measure the following in three years period of time; -Mie scattering of particulate size (µm) matter in the atmosphere ( Year -1 ) -Aerosol measurements and Cloud characteristics ( Year -1 ) -Water vapour measurements in the lower troposphere region up to 8 km ( Year -2 ) -Ozone measurements in the troposphere regions up to 18 km ( Year -3)

The study on particulate matter will address knowledge on particle size and concentrations in the atmosphere.

Making observations over different regions/places provide an idea of mass transport between the regions and through trajectory analysis to identifies the location of the source, e-g., Bio-mass burning. Aerosols/clouds play their roles in the earth-radiation budget and LIDAR has the unique capability of measuring the particle concentration at a particular place in an effective manner by illuminating different harmonics of the laser (Nd:YAG). At present, the radiative effects of aerosols create the largest uncertainty in global climate predictions to quantify climate forcing caused by manmade changes to the composition of the atmosphere. LIDAR retrieved measurements will provide reasonably accurate information. It also has the potential to locate the height and distribution of various clouds which are present. The relative thermodynamic phase distribution of the cloud, water/super-cooled water, ice and ice crystal, can be examined to some extent, based on the depolarization characteristics of the LIDAR back scattering and linear depolarization ratio. Once, we are capable of applying the LIDAR for aerosol/cloud measurements, the LIDAR system will be upgraded for measuring water-vapour in the lower troposphere. This would be performed with initial site measurements and then campaigns over other locations around South Africa/Africa. The water-vapour measurements over different regions will address the level of water vapour present in the atmosphere. It addresses the dry and wet regions, and thereby focuses on the ecological effects (such as, agriculture and water scarcity). Since watervapour is a primary green-house gas, the study further addresses issues such as GCC and GW. The aim is to upgrade the LIDAR system to be capable of making troposphere ozone measurements. It elucidates the amount of ozone in the troposphere height regions and one would be able to infer ozone depletion and increases in temperature (in other words, global warming due to increase in ground ozone).

Besides regular monitoring, measurements at different places would provide a better knowledge of concentrations and its localized variation with respect to place. The mobile LIDAR system will enable us to fulfil our objective of making measurement in different places, especially in South Africa and African countries (Ethiopia and Malawi).

This research programme is also aimed to improve research related relationships between the countries and our understanding of aerosol and cloud characteristics in the sub-tropical southern hemisphere. A kind of triangular LIDAR research network is established between the LIDAR located in NLC-CSIR Mobile LIDAR at Pretoria, Laboratoire de l'Atmosphère et des Cyclones de La Réunion, CNRS 8105 at Reunion and University of Kwazulu Natal (UKZN), Durban.

LIDAR SPECIFICATION

Based on the literature survey, we have proposed a typical specification of the planned mobile LIDAR. The details are given in the following table.

PRESENT SCENARIO

The mobile LIDAR system has been initially designed and aimed for first field campaign measurements in a few months and the system is to be employed for the backscatter measurements (Aerosols/clouds) and for the height region from ground to 35 km. Figures 1 and2 represent the external view of the mobile LIDAR van and the LIDAR system set-up respectively. A more detailed system description is explained in the companion paper [START_REF] Sivakumar | CSIR-NLC South Africa Mobile LIDAR system description[END_REF].

Figure 1 .

 1 Figure 1. The external view of mobile LIDAR van.

Figure 2 .

 2 Figure 2. The present LIDAR set-up inside the van.

Table 1 .

 1 Specifications of the planned mobile LIDAR

	Transmitter
	Laser Source	: Nd: YAG, Continuum
	Operating Wavelength	: 1064, 532, 355, 266 nm
	Average energy per pulse : ~200 mJ (at 532)
	Beam Expander	: 3 x
	Pulse width	: 7 nsec (Typical)
	Pulse repetition rate	: 10 Hz
	Beam Divergence	: 0.6 mRad
	Receiver
	Telescope type	: Newtonian
	Diameter	: 404 mm
	Field of View	: less than 1 mRad
	Filter band width	: 0.7 nm
	Raman cell
	Length	: 150 cm
	Diameter (inner/outer)	: 20/55 mm
	D 2 pressure / purity	: 10 bars / 99.7 %
	He pressure / purity	: 24 bars / 99.999 %
	Signal and Data Processing

ACKNOWLEDGEMENTS

The authors are thankful to the different South Africa funding agencies, Council for Scientific and Industrial Research-National Laser Centre (CSIR-NLC), Department of Science and Technology (DST), National Research Foundation (NRF) (Grant no: 65086) and African Laser Centre (ALC) for partial financial support to execute this project 'LIDAR for Atmosphere Research over Africa -a TRIlateral research Project (LARA -TRIP)'. Authors also acknowledge to Centre National de la Recherché Scientifique (CNRS) (France) for funding (under negotiation) in-terms of South Africa and France Researcher/student exchange programme.


Venkataraman Sivakumar
email: svenkataraman@csir.co.za

D Moema

C Bollig

A Sharma

Nkanyiso Mbatha

Sandile B Malinga

G Mengistu

Hassan Bencherif

Philippe Keckhut

LiDAR for Atmosphere Research over Africa (LARA)

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The importance of systematic monitoring of the atmospheric structure and dynamics has been the interest of several atmospheric research campaigns across the globe. Laser radar, more popularly known as LIDAR (LIght Detection and Ranging), has become an outstanding instrument for short-term sensing of the atmosphere (seconds to minutes). Lasers offer great advantages over conventional light sources in terms of peak power and narrow spectral/beam width. The discovery of different laser sources and improvements in detection technology, data collection and analysis techniques have made the LIDAR a consistent remote sensing tool for atmospheric studies. In general, LIDAR systems use both continuous wave and pulsed lasers. LIDAR systems are currently operational in different countries and are used for the study of atmospheric structure and dynamics, trace constituents. (Differential absorption), aerosols/clouds (Mie Scattering), atmospheric density and temperature (Rayleigh Scattering), metallic ion species (Resonance Scattering), minor constituents (Raman Scattering) and winds (Doppler LIDAR).

The first development of a LIDAR system took place in 1961, by two groups at Hughes Aircraft Malibu and Culver City, respectively. However, this system was exploited for target range finding. Later, LIDAR studies were carried out using a Ruby laser as source [START_REF] Ligda | Proceedings of the First conference on Laser Technology[END_REF][START_REF] Fiocco | Lidar studies of aerosol studies: An outline, MAP Handbook[END_REF][START_REF] Fiocco | Detection of Scattering Layers in the Upper Atmosphere (60Â-140 km) by Optical Radar[END_REF][START_REF] Fiocco | Observations of the aerosol layer at 20 km by optical radar[END_REF] but recent atmospheric studies have taken advantage of the high pulse repetition capability of Neodymium-Yttrium Aluminum Garnet (Nd:YAG) lasers which are usually frequencydoubled to provide a 532 nm output [START_REF] Chanin | LIDAR observation of Gravity and Tidal waves in the stratosphere and mesosphere[END_REF]. The improved performance of Nd:YAG lasers, the use of shorter wavelengths, and the availability of improved blocking, for rejecting the elastically backscattered return, have led to enhanced interest in atmospheric measurements. The Nd:YAG laser is also advantageous since it can be easily tuned to different wavelengths by means of harmonic generators which allow for different atmospheric measurements.

A web-based survey of ground-based LIDARs around the world was conducted. The location of lidar systems employed for different atmosphere studies is presented in Figure 1. It is noted here that the survey is not complete and needs further attention to provide any substantiate statement. Further, the survey has been made with regards to LIDAR applications for atmosphere studies including pollutant monitoring. It is evident from the figure that:

• around 80 % of the LIDARs are highly concentrated in the northern hemisphere region, • only a few, approximately 20 %, are located in the southern hemisphere region, with a high concentration in South America, • LIDARs on the African continent are limited to two, including the mobile LIDAR system developed at Pretoria (South Africa), • While there are many ground-based fixed LIDARs, there are not many mobile LIDAR systems for systematic atmosphere measurements.

Hence, LIDAR is yet to be considered as state-ofthe-art for South Africa and African countries. We have identified only two LIDAR systems in South Africa. One is located in Durban (South Africa) and operated under a South Africa -France association by the University of KwaZulu-Natal (UKZN) and the other one is the new mobile LIDAR system at the National Laser Centre (NLC), Council for Scientific and Industrial Research (CSIR), Pretoria [START_REF] Sivakumar | Lidar research in South Africa, The international society for optical Engineering (SPIE)[END_REF]. Our aim is therefore to develop a mobile LIDAR system which will serve as a research platform for monitoring the atmosphere over Africa (LARA).

At present, the NLC mobile LIDAR system is primarily designed for atmospheric studies, including monitoring of various pollutants that contribute to global climate change and global warming. This paper describes the present status of the LIDAR system and the research objectives, and discusses the initial research results obtained.

LARA objectives

The research objective is to develop a mobile LIDAR platform for remote sensing the atmosphere. The present research programme is focused towards a better understanding of global climate change (GCC) and global warming (GW). In future, it is planned to use the LIDAR system to measure water vapour and ozone in the atmosphere. Regular monitoring of these constituents is important for a better understanding of GCC and GW. To achieve this, we need a portable instrument which will facilitate measurements at different regions. To achieve the above requirements, the LIDAR has been installed in a mobile vehicle equipped with the necessary optics to make remote measurements possible. Our goal is to utilise the mobile LIDAR system to measure the following;

• Mie scattering of particulate size (µm) matter in the atmosphere • Aerosol measurements and cloud characteristics • Smoke/Plume detection • Lower troposphere water vapour measurements (from ground to 8 km).

• Troposphere ozone measurements (from ground to 18 km)

The study on particulate matter addresses knowledge on particle size and concentrations in the atmosphere. Making observations over different regions/places provides an idea of mass transport between the regions and is able to back-track through trajectory analysis to identify the location of the source, e-g. bio-mass burning.

The study on aerosols/clouds plays an important role in the earth-radiation budget (~30 %) and the LIDAR has the unique capability of measuring the particle concentration at a particular place in an effective manner by illuminating different harmonic of laser (Nd:YAG).

The radiative effect of aerosols contributes the largest uncertainty in global climate predictions to quantify climate forcing caused by man-made changes to the composition of the atmosphere [START_REF] Fiocco | Lidar studies of aerosol studies: An outline, MAP Handbook[END_REF]. The LIDAR measurements of aerosols provide reasonably accurate information of its concentrations. It also has potential to locate the height and vertical distribution of various clouds which are present. The relative thermodynamic phase distribution of the cloud, water/super-cooled water, ice and ice crystal, can be examined in some extent, based on the depolarization characteristics of the LIDAR back scattering, normally referred to as linear depolarization ratio [START_REF] Killinger | Laser remote sensing of the atmosphere[END_REF]Menyuk, 1987, Sassen et al., 1995). The successive plan is to upgrade the LIDAR system for measuring water vapour in the lower troposphere. The water-vapour measurements over different regions will address the level of water vapour present in the atmosphere. It addresses the dry and wet regions, and thereby focuses on the ecological effects (such as agriculture and water scarcity). Since water vapour is a primary green-house gas, the study aims to better understand global climate change and global warming. Besides the water vapour measurements, the aim is to further upgrade the LIDAR system to be capable of making troposphere ozone measurements. This would elucidate the amount of ozone in the different height regions (maximum ~18 km) of the atmosphere and one would be able to infer ozone depletion and increases in temperature (in other words, global warming due to increase in ground ozone).

Besides regular monitoring, measurements at different places will provide a better knowledge of concentrations and its localized variation with respect to place. The mobile LIDAR system will enable us to enhance our objective of making measurements in different places, especially in South Africa and African countries.

We are also focused on strengthening research relationships between South Africa and other countries to improve our understanding of the aerosol and cloud characteristics in the subtropical southern hemisphere. A triangular LIDAR research network is established between the LIDARs located at Pretoria (NLC-CSIR Mobile LIDAR), Reunion (Laboratoire de l'Atmosphère et des Cyclones de La Réunion, CNRS 8105) and Durban (University of KwaZulu-Natal (UKZN)).

(see Figure 1 with blue colored triangle).

Present Status

Using advanced techniques and instrumentation, a mobile Light Detection and Ranging (LIDAR) system is being designed and developed at the CSIR-National Laser Centre (NLC), Pretoria (25°45 ′ S; 28°17 ′ E) (see Figure 2).

At present, the system is capable of providing aerosol/cloud backscatter measurements for the height region from ground to 20 km with a 10 m vertical height resolution. The major advantage of the LIDAR is its

System description

The LIDAR system comprises a laser transmitter, optical receiver and a data acquisition system. The complete LIDAR system is custom-fitted into a van, using a shock absorber frame. Hydraulic stabilizer feet have been added to the vehicle suspension to ensure stability during measurements (see Figure 3). A Nd:YAG laser is used for transmission which is presently employed at the second harmonic (532 nm) at a repetition rate of 10 Hz.

The receiver system employs a Newtonian telescope configuration with a 16 inch primary mirror. The backscattered signal is subjected to fall on the primary mirror of the telescope and is then focused toward a plane mirror kept at an angle of 45 degrees. It is detected by the Photo-Multiplier Tube (PMT) and the PMT output signal is transmitted to the transient digitizer and PC for analysis and archival. A multimode optical fiber is used to couple the received backscatter optical signal from the telescope to the PMT. To accomplish accurate The data acquisition is performed by a transient recorder which communicates with a host computer for storage and offline processing of data. A Licel® transient digitizer was procured for this purpose. This unit is a data acquisition system optimized for fast, repetitive photomultiplier LIDAR signals in the analog voltage range of 0 -500 mV. The system is favored due to its capability of simultaneous analog and photon counting detection, which makes it highly suited to LIDAR applications by providing high dynamic range.

The schematic LIDAR setup inside the van is illustrated in Figure 4, and the more important specifications of the mobile LIDAR system are given in Table 1. More details about the system are cited in [START_REF] Sivakumar | CSIR-NLC South Africa Mobile LIDAR system description[END_REF].

Initial Results

After calibration and initial testing, the LIDAR was operated for the first time on 23 February 2008. The laser was directed vertically upward into the sky and the corresponding night was a cloudy sky (see Figure 5). There was a passage of cumulous clouds which is normally found at lower height region from 3 km to 5 km. Since these clouds are generally optically dense, light is prevented from passing through. For each single data set, the data range is first identified where both the analog and photon count signals are varying linearly. Then the scaled and dead time corrected photon counts are appropriately merged with some threshold values (obtained from the linear progression). The glued photon count data are displayed in Figure 6 (bottom left panel).

EUMETSAT©2008 EUMETSAT©2008

Figure 6 clearly distinguishes the cloud observation from normal scattering from background particulate matter. It shows the sharp enhancement during the presence of cloud around 4.2 km and slowly has moved down to 3.5 km. This figure demonstrates the capability of LIDAR to observe the cloud thickness (less than around 300 m). The measured high resolution data is also important when studying cloud morphology. Otherwise, the lower height regions indicate high intensity signal returns which is due to the presence of dense fog and the boundary layer evolutions. The planetary boundary layer evolution can be detected using the range corrected signal (Signal multiplied by the square of range) and found that the boundary layer is located around 2 km.

It is important to note here that the experiment has been carried out using two neutral density (ND) filters which attenuates 99% of the signal entering the PMT. This means that the backscattered signal represented in the colourmap and in the height profiles corresponds to only 1% of the signal. The ND filters are employed to avoid signal saturation due to the presence of dense cloud at lower height region which significantly interrupts the laser from passing through. In future, we shall experiment with different filter options to allow a greater percentage of the backscattered signal to observe and investigate for higher altitude regions.

Concluding remarks and future perspectives

The initial results show that the system is capable of providing aerosol/cloud backscatter measurements for the height region from ground to 40 km with a 10 m vertical height resolution. The measurements will elucidate the aerosol concentration, optical depth, cloud position, thickness and other general properties of the cloud which are important for a better understanding of the earth-radiation budget, global climate change and turbulence.

The LIDAR measurements will be calibrated with other in-situ (e-g., Sun Photometer, Radio-meter, Balloon borne), space-borne (Stratospheric Aerosol and Gas Experiment -SAGE, Cloud-Aerosol LIDAR and Infrared Pathfinder Satellite Observation -CALIPSO, Moderate Resolution Imaging Spectro-radiometer -MODIS) and different network data sets (AErosol RObotic NETwork -AERONET, Network for the Detection of Atmospheric Composition Change -NDACC).

Future plans include qualitative industrial pollutant measurements, 3-D measurements using an XY scanner, a two-channel LIDAR system, water-vapour measurements, the implementation of DIfferential Absorption LIDAR (DIAL) and ozone measurements.

Figure 1 :

 1 Figure 1: The LIDAR systems are deployed around the world. The lidar stations are represented by a star mark on the map.

Figure 2 .

 2 Figure 2. The external view of mobile LIDAR van. capability to provide the vertical cross-section of clouds including the thickness which is important for better understanding the cloud dynamics and the earth-radiation budget [2]. The cloud information is also useful for predicting convective systems and rain. The LIDAR measurements will also elucidate the aerosol concentration, optical depth, cloud position, thickness and other general properties of the cloud which are important for a better understanding of the earth-radiation budget, global climate change and turbulence. The initial results on cloud detection are presented in this paper.

Figure 3 :

 3 Figure 3: CSIR-NLC-Mobile LIDAR system alignment of the fiber tip, a motorized 3-D translation stage is used. The optical fiber is connected to an optical baffle which is positioned by the stage. The PMT is installed in an optical tube which converts the optical backscatter signal to an electronic signal and then transferred to PC for storage and analysis.

Figure 4 .

 4 Figure 4. The present LIDAR set-up inside the van.

Figure 5 :

 5 Figure 5: EUMETSAT picture illustrating the cloud over Pretoria (LIDAR site) The present LIDAR data acquisition is capable of simultaneously acquiring data with a high range resolution (10 m) via both the analog and photon count channels. The present observations were carried out for approximately two hours. The different forms of height-time-backscattered signal (analog, photon counts, Dead-time corrected and glued) returns are presented in Figure 6. The detected analog and photon count signals are displayed in the top panel of Figure 6. The signal has been post processed for the required/corrected information, dead time corrected (3.6 ns for our present case) (see Figure 6, bottom left panel). The simultaneous analog and photon count data are merged appropriately to create a single return signal. A linear correlation analysis was performed on the analog and photon count signals to retrieve the scaled photon counts, i.e, Scaled photon count = a * analog signal + b

Figure 6 :

 6 Figure 6: Height-time-colour map of LIDAR signal returns for 23 February 2008.

Acknowledgements

The author is grateful to the different South Africa funding agencies, the Council for Scientific and Industrial Research-National Laser Centre (CSIR-NLC), Department of Science and Technology (DST), National Research Foundation (NRF) (Grant no: 65086) and African Laser Centre (ALC). The authors also acknowledge the Centre National de la Recherché Scientifique (CNRS) (France) for funding (under negotiation) in terms of the South Africa/France researcher/student exchange programme. The authors also wish to thank the South African Weather Service for providing the cloud picture.


Verònica Martínez Ferreras

Fabrice Bigot

Fabrice Bigot New

New archaeometric data on Roman wine amphorae from Gallia Narbonensis. The pottery workshop at Lattara (Lattes, south France)