Megan Zecevic
email: meganzecevic@gmail.com

Guillaume Daniel
email: guillaume.daniel@magnitude-geo.com

Dana Jurick
email: dana.m.jurick@conocophillips.com

On the nature of LPLD seismic events detected during hydraulic fracturing

Long-period long-duration (LPLD) seismic events are low-amplitude tremor-like seismic signals that have been observed in some microseismic monitoring data sets acquired during hydraulic fracturing operations. LPLD events have been interpreted to be associated with slow-slip along preexisting fractures which are presumed to have either high clay content or be misaligned with respect to the current day principal stress directions.

However, a recent study shows that regional earthquakes, when recorded on vertical downhole monitoring arrays, have similar signal characteristics to LPLD events and that care must be taken when analyzing and interpreting such signals. Using data from a hydraulic fracturing microseismic data set where LPLD events have previously been identified and well documented, together with data from the EarthScope Transportable USArray, we investigated the hypothesis that the documented LPLD events are regional earthquakes. We determined that the LPLD events correspond with signals recorded on the USArray at distances of up to 350 km away from the injection well, although they were not listed in any regional earthquake catalogue. The spatial coverage of the USArray allows the sources of many of the LPLD events to be relocated outside of the treatment well area and thus suggests that they are regional earthquakes of magnitude smaller than M2.5 rather than locally sourced events related to the hydraulic fracturing stimulation process.

INTRODUCTION

Hydraulic fracturing is an engineering completions technology and method used to enhance and develop fracture networks in low permeability reservoirs. During fluid injection, the in situ pore pressure increases and the stress field of the injection interval changes which can result in the creation of new fractures and reactivation of preexisting fractures in critically stressed zones [START_REF] Fehler | Stress control of seismicity patterns observed during hydraulic fracturing experiments at the Fenton Hill hot dry rock geothermal energy site[END_REF][START_REF] Zoback | Injection-induced earthquakes and crustal stress at 9 km depth at the KTB deep drilling site[END_REF]. Failure along these fractures results in the generation of low magnitude seismic emissions usually termed microseismic events (MSEs). These events can be detected using seismic sensor arrays and their mapping can be used to characterize the fracture evolution in space and time. In recent years, a type of seismic event referred to as a long-period long-duration (LPLD) event was observed in various studies [START_REF] Das | Long-period, long-duration seismic events during hydraulic fracture stimulation of a shale gas reservoir[END_REF], 2013a;[START_REF] Mitchell | Detecting long-period longduration microseismic events during hydraulic fracturing in the Cline Shale Formation, West Texas: A Case Study[END_REF][START_REF] Kwietniak | Detection of the Long Period Long Duration (LPLD) Events in Time-and Frequency-Domain[END_REF] using geophone arrays in downhole monitoring wells acquired during hydraulic fracturing completion operations in a set of horizontal wells. The seismic signals associated with these LPLD events were characterized by emergent, low-amplitude tremor-like signals lasting from tens of seconds to minutes with recorded dominant frequencies between 10 and 80 Hz. Their source mechanisms are currently not fully understood, however Das and Zoback (2013b) hypothesized that these LPLD type events represented slowslip on preexisting fractures and thus may be an another mechanism of accommodating deformation within the hydraulically stimulated reservoir zone. The preexisting fractures on which these slow-slip events are thought to originate are presumed to have either high clay content or be misaligned with respect to the present day principal stress orientations in the region of the stimulation interval. If the source mechanisms of the LPLD events are truly related to slow-slip deformation of the reservoir interval as a consequence of hydraulic fracturing stimulation, then understanding the nature and cause of these LPLD events is important because it could impact reservoir permeability changes and thus influence approaches to reservoir management.

Another study of interest to LPLD-type event investigation [START_REF] Caffagni | Regional seismicity: A potential pitfall for identification of long-period long-duration events[END_REF] found that regional earthquakes recorded using typical microseismic downhole sensor arrays can have similar characteristics to LPLD events. Similarities are noted with respect to their time duration, frequency content and apparent velocity. Additionally, [START_REF] Caffagni | Regional seismicity: A potential pitfall for identification of long-period long-duration events[END_REF] addressed the waveform similarities between LPLD events and regional earthquakes. For example, regional earthquakes also have an emergent onset due to the P-and S-wave coda being strongly scattered within the crust along the path from the source to the receiver [START_REF] Aki | Analysis of the seismic coda of local earthquakes as scattered waves[END_REF][START_REF] Aki | Origin of the coda waves: Source attenuation and scattering effects[END_REF]. Furthermore, [START_REF] Caffagni | Regional seismicity: A potential pitfall for identification of long-period long-duration events[END_REF] also explain that crustal scattering and relatively short epicentral distances may result in the P-and Swave coda partially overlapping, thus producing what appears to be a continuous long-duration signal. These similarities may act as "a potential pitfall" for identifying and characterizing LPLD events as being related to engineering completion related signals. Such a misinterpretation could not only impact reservoir management but also affect decisions related to hydraulic fracturing operations, e.g. for operations within a traffic-light monitoring system.

In this study, we aim to test the hypothesis that the LPLD events documented in the "Barnett Data 2" data set by Das & Zoback (2013a, 2013b) are regional earthquakes, but are perhaps too small in magnitude to be registered in the U.S. Geological Survey's National Earthquake Information Center (NEIC) earthquake catalog. Publicly available data recorded by temporary seismic stations deployed under the EarthScope Transportable USArray program [START_REF] Usarray | Transportable Array[END_REF] are used in this study to provide additional observations. For the sake of clarity and continuity with previous work we shall continue to refer to these investigated events as "LPLD events".

DATA

Hydraulic fracturing microseismic data set

In late November 2009, two horizontal wells were multi-stage hydraulically fractured and monitored for microseismicity using a two well multi-level vertical monitoring array geometry in the Barnett Shale, Fort Worth Basin, Texas. The horizontal treatment wells were fractured in 18 sequential stages using a "zipper-frac" sequencing technique in which two adjacent horizontal wells are stimulated concurrently, with "treatment stages" alternating between adjacent isolated measured depth intervals on each of the horizontal wells as the injection progresses from toe (deepest measured depth) to heel (shallowest measured depth). The hydraulic fracturing operation was monitored by two vertical downhole arrays, each with forty 3component 15 Hz natural frequency OYO Geospace DS-150 geophones spaced 15 meters apart in each vertical array, using a sampling rate of 0.5 milliseconds. This configuration is expected to have high sensitivity to very weak signals. These arrays detected and recorded several thousand MSEs that were subsequently processed through location analysis. This data set also contained numerous LPLD events as documented by Das & Zoback (2013a, 2013b) who referenced the underlying data set as "Barnett Data 2".

Over fifty LPLD events were identified in this data set by Das & Zoback (2013a). However due to their emergent onset, low frequency content and the vertical array geometry it proved difficult to apply traditional event location methods to locate all the LPLD events (Das & Zoback, 2013b). Das & Zoback (2013b) located a subset of 10 large LPLD events identified in this data set using a source-scanning (i.e. beamforming) method. Using this method, the most probable volume for event locations consisted of two distinct cones expanding from the area between the two monitoring wells. In their work, Das & Zoback (2013b) assumed that the most likely source of these events existed within the cone that traversed the area of active stimulation at the approximate depth of the hydraulic stimulation interval (2400 m). The locations of each of the LPLD events were not very well constrained; therefore a source region was defined rather than a point-source location. Unlike the MSEs which are typically located in a region surrounding the active treatment stage, the LPLD events were located in a cluster within a limited region of the reservoir, regardless of the stage that was being injected (Figure 1).

We were able to extract from the raw microseismic data recordings eight of the same LPLD events (see Table 1 for event times and corresponding stage numbers) that were located by Das and Zoback (2013b). Due to corrupted data files in our copy of the data set, we were unable to extract the two other LPLD events. MSE and LPLD events are color-coded according to the active fracturing stage. The LPLD events are labelled according to the treatment well (first number) and the fracturing stage during which the event was recorded (second number). Reproduced from Das & Zoback (2013b). An approximate distance scale bar is included for reference purposes.

EarthScope Transportable USArray

The EarthScope Transportable USArray is a network of 400 3-

ANALYSIS & RESULTS

NEIC earthquake catalog

During the timeframe of the reservoir stimulation operation (hydraulic fracturing), the NEIC earthquake catalog recorded two regional earthquakes within a 500 km radius of the injection well (located in Oklahoma as a magnitude 3.0 event and in North Texas as a magnitude 2.9 event). The corresponding signals, when observed by the downhole vertical monitoring arrays, are remarkably similar to the LPLD events observed by Das & Zoback (2013a) in both waveform and frequency content (Figure 3). Note that the time difference between event onset (P-phase) and the later, more energetic phase (S-phase) of the regional earthquakes is attributed to different epicentral distances, however similar to the observations of [START_REF] Caffagni | Regional seismicity: A potential pitfall for identification of long-period long-duration events[END_REF] the waveforms appear as a continuous signal due to the overlapping P-and S-wave coda. The signals associated with the catalogued regional earthquakes have amplitudes that are up to an order of magnitude larger than the LPLD events detected within this particular microseismic data set.

However the catalogued regional earthquakes do not correspond in time with the 8 extracted LPLD events examined in this paper. We do not have times for the other LPLD events (more than 50 in total) identified by Das & Zoback (2013a, 2013b) in this microseismic data set and hence cannot reject the hypothesis that LPLD events are regional earthquakes by using the NEIC catalogue.

Transportable USArray Data

Upon examining the two-minute long USArray data records surrounding the LPLD arrival times (as determined on DHA2), it was apparent that coherent signals similar to the LPLD events could be observed on many of the USArray stations, even at distances of up to 350 km away from the injection well site. The seismic events observed on the USArray stations have first arrival times that are both earlier and later than the LPLD onset observed on the DHA2 recordings. Moveout analysis suggests that the seismic events propagating through the USArray network coincide in time with the occurrence of the LPLD events that are recorded in DHA2 (Figures 4567). Although we cannot determine if the USArray data and the downhole array data are "perfectly" synchronized in time, we can determine that the time delay between events observed on the borehole data and USArray stations (especially those closest to the injection pad) are within a reasonable limit to be expected due to both the vertical (2km) and lateral difference in locations of the different recording instruments, as illustrated in Figures 4567(right panels). It therefore seems unlikely that these LPLD events have a source location that originates within the stimulated reservoir interval as a consequence of the hydraulic fracturing completion operation.

Locating Events

The signals recorded on the USArray stations have both impulsive and emergent onsets, and overlapping P-and S-wave coda. The complex waveforms make it difficult to ensure that the same coherent S-phase is

DISCUSSION

Locations

Using the USArray data it is evident that the LPLD events observed by Das & Zoback (2013a, 2013b) for one of their datasets can be interpreted as regional earthquakes rather than locally sourced events directly related to the hydraulic fracture stimulation process (Figure 2 and Figures 4567). In the last 6 years, since approximately 2009, it has been documented that North Texas and Oklahoma have seen an increase in the occurrence and rate of regional seismicity (Ellsworth, 2013;[START_REF] Ellsworth | Increasing seismicity in the U. S. midcontinent: Implications for earthquake hazard[END_REF][START_REF] Frohlich | The Dallas-Fort Worth Earthquake Sequence[END_REF]Frohlich, 2012b;[START_REF] Hornbach | Causal factors for seismicity near Azle[END_REF][START_REF] Keranen | Sharp increase in central Oklahoma seismicity since 2008 induced by massive wastewater injection[END_REF]McNamara et al., 2015a;National Research Council, 2012). The presence of the USArray together with temporary local seismic networks within the North Texas-Oklahoma region has proved useful in improving seismic monitoring in this region (Frohlich, 2012a(Frohlich, , 2012b;;McNamara et al., 2015aMcNamara et al., , 2015b)) (Frohlich, 2012b) is shown in Figure 2, together with the relocated LPLD events. The relocated LPLD event epicenter locations are spatially consistent with the observed seismicity in the region, thus further supporting the evidence that LPLD events in this instance may be regional earthquakes of relatively low magnitude.

Das & Zoback (2013b) located the same LPLD events analyzed in this study within the stimulated reservoir area where very few MSEs were located (Figure 1). [START_REF] Roy | Integrated characterization of hydraulic fracture treatments in the Barnett Shale: the Stocker geophysical experiment: Interpretation[END_REF] integrated a variety of geological, geophysical and engineering data sets in order to obtain a better image of the subsurface for this particular hydraulic fracturing completions operation. Their interpretation shows a transition in lithological facies type approximately midway between the toe and heel of the completion wells, with a carbonate submarine fan to the West and primary reservoir rock (siliceous mudstones)

to the East. This transition can be further corroborated by the interpretation of microseismic data characteristics (Busetti et al., 2014;Busetti & Reches 2014). Based on proprietary moment tensor inversion work applied to these MSEs, the MSEs from this particular data set are categorized as hybrid in nature, with mixed tensile and shear components. By decomposing the moment tensors of the MSEs, the deviation angle of the slip vector from the fault plane can be used as a proxy for the principal mode of failure of the MSEs [START_REF] Vavryčuk | Inversion for parameters of tensile earthquakes[END_REF]. From the moment tensor decomposition results of Busetti et al. (2014), it is therefore apparent that the mode of failure of the MSEs varies spatially across the reservoir, consistent with the change in facies. Events located towards the toe (West) of the wells are predominantly tensile in nature, whereas MSEs located towards the heel of the wells have predominantly shear mechanisms. The LPLD source region interpreted by Das & Zoback (2013b) for one of their datasets appears to mark the transition zone between these two different failure zones. We hypothesize that the relatively rapid spatial lithological facies transition within the reservoir could act as a strong acoustic energy scatterer. Local scatterers can generate a weak "secondary" wavefield that will be present in the recorded signal. As Das & Zoback (2013b) assumed a local scale target when using the beamforming/source-scanning technique, it is possible that the location they obtained is a secondary wavefield source directly related to this facies transition. It can also be a more general artifact of the beam-forming location approach used, given that these authors fixed the depth of LPLD locations.

Magnitude

Two regional earthquakes located within a 500 km radius of the injection well were recorded on the NEIC catalog (located in Oklahoma, magnitude 3.0 and North Texas, magnitude 2.9) during the timeframe of the reservoir stimulation operation. These signals, when observed by the downhole monitoring arrays, have amplitudes that are up to an order of magnitude larger than the LPLD events detected within the data set (Figure 3). As the LPLD earthquakes are not recorded on the NEIC earthquake catalog, it is likely that they have magnitudes that fall beneath the approximate M 2.5 detection threshold of the NEIC earthquake catalog (McNamara et al., 2015b).

To test this theory, we re-calculated the magnitude of a relocated LPLD event. The seismic moment (M 0 ) of an event can be determined from the low frequency plateau of the displacement spectrum (𝛺 0 ) [START_REF] Brune | Tectonic stress and the spectra of seismic shear waves from earthquakes[END_REF] of the P-and S-phases of a signal using

𝑀 0 = 4𝜋𝜌𝑣 𝑝,𝑠 3 𝑟𝛺 0 /𝑅 𝑝,𝑠 , (1)
which in turn can be used to derive the moment magnitude (𝑀 𝑤 ) given by 𝑀 𝑤 = 2 3 log 10 (𝑀 0 ) -6.07 ;

(2)

where 𝜌 is the average density of the rock, 𝑣 𝑝,𝑠 is the P-or S-wave velocity around the source, 𝑟 is the source-to-receiver distance and 𝑅 𝑝,𝑠 is a coefficient correcting the observed amplitudes for the influence of the P-or Swave radiation pattern of the seismic source [START_REF] Hanks | A moment magnitude scale[END_REF].

Using an LPLD event with distinguishable P-and S-phases recorded on a USArray station with epicentral distance (𝑟) of ~80 km and with an observed 𝛺 0 of 2.03 x 10 -9 m.s and 1.12 x 10 -8 m.s for the P-and S-phases respectively, and assuming a crustal 𝑣 𝑝 of 6500 m/s and 𝑣 𝑠 of 3750 m/s (USGS, 2014), 𝜌 of 2700 kg/m 3 , and 𝑅 𝑝 of 0.52 and 𝑅 𝑠 of 0.63 [START_REF] Boore | Average body-wave radiation coefficients[END_REF], the moment magnitude of the LPLD event was calculated to be M w 2.2.

In their study, Das & Zoback (2013a) estimated the magnitude of one of these LPLD events using a similar method to the empirical Green's function method [START_REF] Frankel | Determination of rupture duration and stress drop for earthquakes in southern California[END_REF][START_REF] Mori | Source parameters for small events associated with the 1986 North Palm Springs, California, earthquake determined using empirical Green functions[END_REF], assuming that the LPLD event originated within a similar region as a MSE of known magnitude. The moment magnitude that they obtained was approximately M w 0.3. This calculation is likely to be an underestimation of the magnitude of this event due to the error in source location. If the error in the magnitude calculation is solely based on the source-to-receiver distance (𝑟), then the corrected moment magnitude (𝑀 𝑤𝑐 ) can be derived from the initial magnitude (𝑀 𝑤𝑖 ) estimate by rearranging equations 1 and 2 such that

𝑀 𝑤𝑐 = 2 3 log 10 ( 𝑟 𝑐 𝑟 𝑖 ) + 𝑀 𝑤𝑖 . (3)
Assuming that the initial assumed source-to-receiver distance (𝑟 𝑖 ) is 0.5 km and the corrected source-to-receiver distance (𝑟 𝑐 ) is 200 km (for an event located in Oklahoma), then equation 3 yields a corrected moment magnitude of approximately M w 2, which is comparable to our calculations above.

CONCLUSION

Recent work, such as that of [START_REF] Das | Long-period, long-duration seismic events during hydraulic fracture stimulation of a shale gas reservoir[END_REF], 2013a, 2013b), [START_REF] Mitchell | Detecting long-period longduration microseismic events during hydraulic fracturing in the Cline Shale Formation, West Texas: A Case Study[END_REF], and [START_REF] Kwietniak | Detection of the Long Period Long Duration (LPLD) Events in Time-and Frequency-Domain[END_REF], have identified and characterized a type of seismic event, known as an LPLD event, which have been detected

in microseismic data sets acquired during hydraulic fracturing operations.

These events have been interpreted to be associated with slow-slip along preexisting fractures which are presumed to either have high clay content or be misaligned with respect to the current day principal stress directions (Das & Zoback, 2013a, 2013b). A study by [START_REF] Caffagni | Regional seismicity: A potential pitfall for identification of long-period long-duration events[END_REF] suggested that regional earthquakes could be misinterpreted as LPLD events in vertical downhole seismic monitoring array data sets and that careful analysis must be undertaken when analyzing and interpreting such events. The LPLD events observed by Das & Zoback (2013a) in "Barnett Data 2" do not correlate with any recorded earthquakes within the NEIC earthquake catalog.

However, catalogued regional earthquakes (magnitude > 2.9) that were recorded during the timeframe of the hydraulic fracturing operation are remarkably similar in many important seismic waveform characteristics to the observed LPLD events. The signals of these regional earthquakes, however, have amplitudes which are up to an order of magnitude greater than the LPLD events. The similarities in the signal characteristics suggest that LPLD events could be regional earthquakes that are too small in magnitude to be recorded and published in the NEIC earthquake catalog.

The EarthScope Transportable USArray was deployed within the vicinity of the injection well during the timeframe of the hydraulic fracturing operation recorded by two vertical monitoring arrays. Signals associated in time with the LPLD events could be observed on many USArray stations, even at distances up to 350 km from the injection well. The spatial coverage of the USArray enabled all of the LPLD events to be relocated in the North Texas-Oklahoma region, outside of the stimulated reservoir volume. Their locations are consistent with other seismicity in the region, and thus suggest to us that they are regional earthquakes of relatively small magnitude (< M 2.5). We conclude that these LPLD events are not directly related to the hydraulic fracture stimulation process or the induced reservoir deformation process.

This study highlights the potential that other hydraulic fracturing microseismic data sets could have in identifying, although not necessarily locating or characterizing, low magnitude regional earthquakes in this area. Moreover, regional earthquake signals can contain some weak secondary wavefields which can reveal the location of local scatterers and thus potentially improve our knowledge of the reservoir. Furthermore, this study also emphasizes the potential value of utilizing a surface seismic network together with downhole seismic arrays for microseismic monitoring and interpretation during hydraulic fracturing operations.

According to Das & Zoback (2013a, 2013b) the LPLD events observed in "Barnett Data 2" are similar in character to other LPLD events observed in "Barnett Data 1". We therefore suggest that those LPLD events observed in the "Barnett Data 1" data set could be further investigated within the context of available USArray data. It is important to reiterate the recommendation of [START_REF] Caffagni | Regional seismicity: A potential pitfall for identification of long-period long-duration events[END_REF] that detected and identified LPLD events should be checked against a regional earthquake catalog or, as in this specific case, against independent seismic data acquired from nearby surface seismic monitoring networks, if they exist, in order to avoid misinterpretations when analysing local data.

Figure 1 .

 1 Figure 1. Map view of the treatment and monitor well geometry and MSE and

  component broadband seismic stations (Steckeisen STS-2, Streckeisen STS-2.5, Nanometrics Trillium 120, Nanometrics Trillium 240, Kinemetrics Episensor and Guralp CMG-3T sensors with Kinemetrics Quanterra Q330 digitizers) deployed in temporary sites across the United States of America in a several-hundred-kilometer-wide band extending from Canada to Mexico, with inter-station spacing of approximately 70 km (USArray, 2015). Each year, half of the stations are moved eastward. In mid-November 2009 the USArray stations were deployed in eastern Texas (Frohlich, 2012a), thus providing seismic monitoring coverage using vaultemplaced surface seismometers during the hydraulic fracturing operation discussed above. However at the time of the hydraulic fracturing operation, the injection wells were located on the eastern edge of the array deployment, hence the spatial coverage is good to the North, South and West of the injection well location, but poor towards the East (Figure 2). Two-minute long data records, spanning the onset time of each LPLD event recorded on the deepest sensor (2140 m depth) of the downhole array in Monitoring Well 2 (DHA2), are extracted from each of the USArray station archived data sets shown in Figure 2. The data is sampled every 25 milliseconds, resulting in a maximum useable (Nyquist) frequency of 20 Hz. Some of the USArray stations have signals that are dominated by low frequency noise and therefore all signals were band-pass filtered between 5 and 18 Hz to enhance their signal-to-noise ratio.

Figure 2 .

 2 Figure 2. Map view of USArray stations deployed at the time of the hydraulic

Figure 3 .

 3 Figure 3. North Texas (top left) and Oklahoma (top right) regional

  Figure7. This LPLD event is only observed at some stations along the

Figure 4 .

 4 Figure 4. Location and traces of LPLD event recorded during Stage 1 of Well

Figure 5 .

 5 Figure 5. Location and traces of LPLD event recorded during Stage 7 of Well

Figure 6 .

 6 Figure 6. Location and traces of LPLD event recorded during Stage 6 of Well

Figure 7 .

 7 Figure 7. Poorly constrained location and traces of LPLD event recorded

Figure 1 .

 1 Figure 1. Map view of the treatment and monitor well geometry and

Figure 2 .

 2 Figure 2. Map view of USArray stations deployed at the time of the

Figure 3 .

 3 Figure 3. North Texas (top left) and Oklahoma (top right) regional

Figure 4 .

 4 Figure 4. Location and traces of LPLD event recorded during Stage 1

Figure 5 .

 5 Figure 5. Location and traces of LPLD event recorded during Stage 7

Figure 6 .

 6 Figure 6. Location and traces of LPLD event recorded during Stage 6

Figure 7 .

 7 Figure 7. Poorly constrained location and traces of LPLD event

Table 1 .

 1 LPLD event time (UTC) and corresponding stage numbers

	Well	Stage	Event Time (UTC)
	1	1	11/28/2009 13:12:13
	2	1	11/29/2009 14:14:29

ACKNOWLEDGMENTS

Data from the Transportable USArray (TA) network are made freely available as part of the EarthScope USArray facility, operated by Incorporated Research Institutions for Seismology (IRIS) and supported by the National Science Foundation, under Cooperative Agreements EAR-0323309, EAR-0323311, EAR-0733069, and EAR-1261681. This work was elaborated in the framework of the FLOWTRANS Initial Training Network, a Marie Sklodowska-Curie action. We gratefully acknowledge support from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no 316889. We are also grateful to ConocoPhillips for permission to publish the data and our findings. REFERENCES