Master Event Research Articles

Localization for surface microseismic monitoring based on arrival time correction and VFOM

Abstract Unconventional resources have emerged as the primary source to meet the escalating demand for energy consumption, with hydraulic fracturing standing out as an effective means of boosting production. The utilization of microseismic monitoring is crucial for acquiring real-time or semi-real-time extension information of the fracture network to guide the fracturing process. The precise positioning of microseismic events is a fundamental aspect of microseismic monitoring. Traditional methods relying on (relative) arrival time significantly impact positioning accuracy due to picking errors. While waveform-based methods offer high accuracy, they require precise velocity models and are time-consuming. To overcome challenges associated with arrival time pickup and velocity accuracy, we introduce a virtual field optimization method (VFOM) based on arrival time correction. Initially, an equivalent velocity model is established, and the arrival time difference resulting from the model transformation of the master event is calculated to correct the observed arrival time of the target event. Subsequently, we match detector pairs, establish hyperboloids based on the corrected arrival time difference, and employ the intersection point of all hyperboloids as the positioning result. After that, we use the location results of the master event to enhance the accuracy of the target event. Finally, we apply the proposed method to both synthetic test and field datasets, demonstrating a significant improvement in the positioning accuracy and stability provided by the novel method. The robustness against arrival time error renders it a suitable choice for surface monitoring applications where signal quality is compromised. Furthermore, the simplified velocity model significantly diminishes the computational requirements in the positioning process, enhancing its efficiency, and consequently holds vast potential for application in real-time monitoring.

Locating clustered seismicity using Distance Geometry Solvers: applications for sparse and single-borehole DAS networks

Summary The determination of seismic event locations with sparse networks or single-borehole systems remains a significant challenge in observational seismology. Leveraging the advantages of the location approach HADES (eartHquake locAtion via Distance gEometry Solvers), which was initially developed for locating clustered seismicity recorded at two stations, through the solution of a Distance Geometry Problem, we present here an improved version of the methodology: HADES-R (HADES-Relative). Where HADES previously needed a minimum of 4 absolutely located master events, HADES-R solves a least-squares problem to find the relative inter-event distances in the cluster, and uses only a single master event to find the locations of all events, and subsequently applies rotational optimizer to find the cluster orientation. It can leverage iterative station combinations if multiple receivers are available, to describe the cluster shape and orientation uncertainty with a bootstrap approach. The improved method requires P- and S-phase arrival picks, a homogeneous velocity model, a single master event with a known location, and an estimate of the cluster width. The approach is benchmarked on the 2019 Ridgecrest sequence recorded at two stations, and applied to two seismic clusters at the FORGE geothermal test site in Utah, USA, with a microseismic monitoring scenario with a DAS in a vertical borehole. Traditional procedures struggle in these settings due to the ill-posed network configuration. The azimuthal ambiguity in such a scenario is partially overcome by the assumption that all events belong to the same cluster around the master event and a cluster width estimate. We are able to find the cluster shape in both cases, although the orientation remains uncertain. HADES-R contributes to an efficient way to locate multiple events simultaneously with minimal prior information. The method’s ability to constrain the cluster shape and location with only one well-located event offers promising implications, especially for environments where limited or specialised instrumentation is in use.

Double-Difference Method for Relocating the Hypocenter of Aftershock Earthquakes in Seririt Singaraja

A method that does not require a main earthquake (master event) that can be used simultaneously to relocate a very large number of earthquakes with wide hypocenter separation is called the double-difference method. A method used to relocate the aftershocks in Seririt Singaraja on November 14 2019 with coordinate positions 113.478 – 115.181 East Longitude and 8.357 – 7.894 South Latitude. The earthquake data used in this research was accumulated from 85 BMKG seismic stations. Data analysis uses cross-correlation time differences which can increase the accuracy of travel time between the receiving station and the earthquake, thereby reducing errors in calculations. The double difference method used to relocate the earthquake in the Seririt Singaraja area showed that there was a shift in the location of the earthquake hypocenter before and after it was relocated. Horizontally and vertically, the distribution of earthquake hypocenters before and after being relocated occurs when there is a collection of location shifts. The results of this research were able to relocate 152 aftershocks properly. The main earthquake after being relocated was at a depth of 17 km, while the distribution of aftershocks was at a depth of around 6-25 km, so that it can more accurately describe the position of the earthquake source and is able to show clearer and easier to interpret structural patterns.

Evolution of shallow volcanic seismicity in the hydrothermal system of La Soufrière de Guadeloupe following the April 2018 Mlv 4.1 earthquake

La Soufrière de Guadeloupe volcano is characterized by seismo-volcanic activity dominantly linked to an active hydrothermal system. This microseismicity is shallow, mainly triggered in swarms and characterized by repeating earthquakes. Four recurrent families of Volcano-Tectonic (VT) repeaters have been identified and the main repeater accounts for 80% of detections. Stacking repeating seismic waveforms produces MASTER events with a high Signal-to-Noise Ratio (SNR) and thanks to the deployment of a temporary seismic nodal array, their absolute locations can be better constrained. Because we observe systematic positive residuals of P-wave arrival times at almost all stations, we investigate potential bias in the velocity model in the shallow part of the dome. By increasing the P-wave velocity and decreasing the VP/VS ratio from 1.8 to 1.69, we reduce these residuals and improve the local velocity model. We use this new velocity model to locate each VT event relatively to is own MASTER hypocenter. This procedure offers a new image of the hydrothermal seismic activity that we locate under the acid lake of the summit Tarissan crater, along a sub-vertical conduit. We also define a linear relationship between the logarithm of the peak amplitude of seismic events and their duration to obtain a pseudo local magnitude. The approach enables to accurately and automatically estimate the magnitude during the event detection. We show that after the occurrence in April 2018 of the largest earthquake (Mlv 4.1) since the last phreatic eruption in 1976–1977, the swarm frequency and amplitude increased significantly, and a new family of VT repeater emerges, accounting for up to 14% of detections. We show a quiescence in the main repeaters activity in the month following the April 2018 earthquake, suggesting a significant stress release within the volcanic system, which is likely related to dynamic stress changes rather than static stress variations caused by the earthquake. Moreover, we detect the emergence of a new family of repeaters 3 months after the event, located ∼100 m above the main repeater family, that could be related to a fluid pressure increase induced by the regional hydrological cycle, superimposed to the internal forcing driven by the hydrothermal system. Finally, using a statistical approach, we highlight seasonal periodicities in the number of events and the released seismic moment at La Soufrière de Guadeloupe, with a dominant peak of seismic activity in October–November and a second, less significant, peak of in April.

Multi-master event waveform stacking microseismic location method based on time-frequency transformation

Waveform stacking is frequently employed in the analysis of microseismic data due to its adaptability, effectiveness, and noise immunity. However, the low signal-to-noise ratio and inaccurate velocity model have a considerable negative impact on the waveform stacking location method's performance. A time-frequency transform-based characteristic function and the addition of a multi-master event method to waveform stacking location are presented as solutions to the aforementioned issues. In this paper, non-smooth statistical features of characteristic function are extracted and enhanced using time-frequency transform. Based on the two-pair double-difference method, multi-master events are introduced and a reference criterion for selecting the master events is proposed. Experiments with simulated and actual microseismic data show that the conversion of time-domain signals into time-frequency signals by wavelet transform can enhance the peak capacity of the signal and reduce the background noise, enhancing the imaging resolution. The introduction of multi-master events not only reduces the sensitivity of the waveform stacking method to velocity error but also improves the clustering effect of the master event method.

Multi-station volcano tectonic earthquake monitoring based on transfer learning

Introduction: Developing reliable seismic catalogs for volcanoes is essential for investigating underlying volcanic structures. However, owing to the complexity and heterogeneity of volcanic environments, seismic signals are strongly affected by seismic attenuation, which modifies the seismic waveforms and their spectral content observed at different seismic stations. As a consequence, the ability to properly discriminate incoming information is compromised. To address this issue, multi-station operational frameworks that allow unequivocal real-time management of large volumes of volcano seismic data are needed.Methods: In this study, we developed a multi-station volcano tectonic earthquake monitoring approach based on transfer learning techniques. We applied two machine learning systems—a recurrent neural network based on long short-term memory cells (RNN–LSTM) and a temporal convolutional network (TCN)—both trained with a master dataset and catalogue belonging to Deception Island volcano (Antarctica), as blind-recognizers to a new volcanic environment (Mount Bezymianny, Kamchatka; 6 months of data collected from June to December 2017, including periods of quiescence and eruption).Results and discussion: When the systems were re-trained under a multi correlation-based approach (i.e., only seismic traces detected at the same time at different seismic stations were selected), the performances of the systems improved substantially. We found that the RNN-based system offered the most reliable recognition by excluding low confidence detections for seismic traces (i.e., those that were only partially similar to those of the baseline). In contrast, the TCN-based network was capable of detecting a greater number of events; however, many of those events were only partially similar to the master events of the baseline. Together, these two approaches offer complementary tools for volcano monitoring. Moreover, we found that our approach had a number of advantages over the classical short time average over long time-average (STA/LTA) algorithm. In particular, the systems automatically detect VTs in a seismic trace without searching for optimal parameter settings, which makes it a portable, scalable, and economical tool with relatively low computational cost. Moreover, besides obtaining a preliminary seismic catalog, it offers information on the confidence of the detected events. Finally, our approach provides a useful tentative label for subsequent analysis carried out by a human operator. Ultimately, this study contributes a new framework for rapid and easy volcano monitoring based on temporal changes in monitored seismic signals.

Master event based backazimuth estimation and its application to downhole microseismic monitoring

Microseismic monitoring provides a valuable tool for evaluating the effectiveness of hydraulic fracturing operations. However, robust detection and accurate location of microseismic events are challenging due to the low signal to noise ratio (SNR) of their signals on seismograms. In a downhole monitoring setting, P-wave polarization direction measured from 3-component records is usually considered as the backazimuth of the microseismic event, i.e., the direction of the event. The direction and arrival time difference between the P and S waves is used to locate the seismic event. When SNR is low, an accurate estimate of event backazimuth becomes very challenging with the traditional covariance matrix method. Here we propose to employ a master event and use a grid search method to find the backazimuth of a target event that maximizes the dot product of the two backazimuthal vectors of the master and target events. We compared the backazimuths measured with the proposed grid-search and the conventional covariance-matrix methods using a large synthetic dataset. We found that the grid-search method yields more accurate backazimuth estimates from low SNR records when measurements are made at single geophone level. When array data are combined, the proposed method also has some advantage over the covariance-matrix method, especially when the number of geophones is low. We also applied the method to a microseismic dataset acquired by a hydraulic fracturing project at a shale play site in southwestern China and found that the relocated microseismic events tend to align along existing faults more tightly than those in the original catalog.

Velocity calibration by incremental pseudomaster events for single-well microseismic monitoring

The velocity model plays a critical role in accurately determining microseismic event locations during hydraulic fracturing. Conventional velocity optimization methods, such as updating the velocity model using perforation shots, have the drawback of weak ray coverage during single-well monitoring. The microseismic events distributed in different areas can broaden the ray coverage and help improve the accuracy of the velocity model, whereas only a few perforation shots are available for a given stage. We have developed an incremental pseudomaster (IPM) method to reduce the velocity errors. The master event location technique is used to determine the positions of representative events in each iteration of the IPM method. These events then act as new pseudomasters to further optimize the velocity model using Bayesian inference. With progressive iterations, the number of pseudomaster events increases and their distribution range expands. The accuracy of the velocity model continuously improves, which means that the final IPM-based velocity model, generated based on the pseudomasters distributed throughout the study area, more accurately locates all microseismic events than the perforation shot alone method. One synthetic example and one field test have been conducted, and the results demonstrate that the IPM method could overcome the drawbacks of weak ray coverage and improve the accuracy of the inverted velocity model and event locations.

Tempo-spatial characteristics of repeating seismic events in the middle of Tianshan orogenic belt

Based on the seismic waveform data recorded in the stations of the Xinjiang seismic network from 2009 to 2017, the repeating earthquakes in the middle of Tianshan orogenic belt and its periphery in Xinjiang were determined and relocated by using the waveform correlation technique and the master event approach. The results show that 11 618 of the 30 181 events are repeating events, which consist of 2395 groups of doublets and clusters, accounting for 38.5% of the total events. According to the statistical results of the distance between doublets before and after repeating events relocation, it is estimated that the system location error in the research area is about 5—10 km. In addition, combined with the latest source classification results in this area, the results show that repeating earthquakes of different source types have different spatial and temporal distribution characteristics. Repeating quarry blasts appear mostly as clusters, 93.6% of them occur during the daytime, and they also exhibit a seasonal pattern with more events in summers and fewer ones in winters. Tectonic earthquakes occurred in various thrust faults in the Tianshan orogenic belt, and occurred randomly at any time of the day, and the monthly tectonic event frequency is relatively stable during the studied period. Repeating induced earthquake locations indicate that most of them are located near large gas/oil fields and water reservoirs, but some also geographically overlay tectonic earthquakes in some regions. The occurring time characteristics of induced earthquakes are similar to those of tectonic earthquakes, that is random distribution within 24 hours.

Earthquake location in tectonic structures of the Alpine Chain: the case of the Constance Lake (Central Europe) seismic sequence

A set of four magnitude Ml ≥ 3.0 earthquakes including the magnitude Ml = 3.7 mainshock of the seismic sequence hitting the Lake Constance, Southern Germany, area in July–August 2019 was studied by means of bulletin and waveform data collected from 86 seismic stations of the Central Europe-Alpine region. The first single-event locations obtained using a uniform 1-D velocity model, and both fixed and free depths, showed residuals of the order of up ± 2.0 s, systematically affecting stations located in different areas of the study region. Namely, German stations to the northeast of the epicenters and French stations to the west exhibit negative residuals, while Italian stations located to the southeast are characterized by similarly large positive residuals. As a consequence, the epicentral coordinates were affected by a significant bias of the order of 4–5 km to the NNE. The locations were repeated applying a method that uses different velocity models for three groups of stations situated in different geological environments, obtaining more accurate locations. Moreover, the application of two methods of relative locations and joint hypocentral determination, without improving the absolute location of the master event, has shown that the sources of the four considered events are separated by distances of the order of one km both in horizontal coordinates and in depths. A particular attention has been paid to the geographical positions of the seismic stations used in the locations and their relationship with the known crustal features, such as the Moho depth and velocity anomalies in the studied region. Significant correlations between the observed travel time residuals and the crustal structure were obtained.

Estimation of relative source locations from seismic amplitude: application to earthquakes and tremors at Meakandake volcano, eastern Hokkaido, Japan

Although seismic amplitudes can be used to estimate event locations for volcanic tremors and other seismic events with unclear phase arrival times, the precision of such estimates is strongly affected by site amplification factors. Therefore, reduction of the influence of site amplification will allow more precise estimation of event locations by this method. Here, we propose a new method to estimate relative event locations using seismic amplitudes. We use the amplitude ratio between two seismic events at a given station to cancel out the effect of the site amplification factor at that station. By assuming that the difference between the hypocentral distances of these events is much smaller than their hypocentral distances themselves, we derive a system of linear equations for the differences in relative event locations. This formulation is similar to that of a master event location method that uses differences in phase arrival times. We applied our new method to earthquakes and tremors at Meakandake volcano, eastern Hokkaido, Japan. Comparison of the hypocentral distributions of volcano-tectonic earthquakes obtained thereby with those obtained from phase arrival times confirmed the validity of our new method. Moreover, our method clearly identified source migration among three source regions in the tremor on 16 November 2008, consistent with previous interpretations of other geophysical observations in our study area. Our method will thus be useful for detailed analyses of seismic events whose onset times are ambiguous.

Fault imaging at Mt Pollino (Italy) from relative location of microearthquakes

SUMMARYRelative location of microearthquakes that occurred at Mt Pollino (Italy) from 2011 to 2013 have been analyzed with the aim of a detailed imaging of the geometry of active faults. We identified 27 clusters composed of a number of earthquakes from 9 to 33, with local magnitude in the range 0.6–2.7. The relative location shows that the distribution of hypocentres in each cluster is characterized by extension from few tens of meters to at most 350 m. For each cluster the hypocentre distribution was fitted by a plane to infer the fault orientation, and results were compared with the fault plane solutions corresponding to the focal mechanism of earthquakes of the same cluster. The comparison shows a good agreement in most of the cases. The relative location analysis, generally applied to earthquakes with similar waveform, has been improved to permit also the relative location of earthquakes characterized by not similar signals. To achieve this purpose a modified procedure that overcome the condition of very similar waveforms has been applied to estimate the time delay between first pulses of the master events. The relative location of master events of all clusters shows a precise imaging of the relative position of all analysed sources and allows also to follow with high accuracy the evolution in time of the seismic swarm within the selected periods. The hypocentre position of master events and the nearly parallel fitting planes of any clusters suggest that most of the analyzed earthquakes were produced by different patches of the same fault. The final results depict a main fault plane characterized by NW–SE strike, dip of about 35–45° and depth between 4.5 and 6.5 km b.s.l. Focal mechanisms, used also to evaluate the local stress field, are mostly of normal type with few strike slip solutions for the shallowest events. This result is in good agreement with the local tectonic stress regime that is characterized by predominant NE–SW transtension, as inferred from structural, seismological and geophysical data.

Low-SNR Microseismic Detection Using DirectP-Wave Arrival Polarization

ABSTRACTDetection and analysis of small magnitude events is valuable for better characterization and understanding of reservoirs in addition to developing strategies for mitigating induced seismicity. Three-component (3C) receivers, which are now widely used, are commonly deployed in boreholes to provide continuous seismic data amenable to novel and powerful analysis. Using multicomponent continuous records of ground motion, we utilize two principal features of the direct P-wave arrival: (1) linearity and (2) polarization in the direction along the ray path to the source region to detect small magnitude events undetectable by conventional methods. We evaluate the linearity of polarization and direction of arrival in the time and frequency domains by introducing the spectral matrix analysis method and combine them into a scalar characteristic function that is thresholded for event detection purposes. We boost the signal-to-noise ratio by stacking characteristic functions obtained at different 3C receivers along an empirical moveout of a master event known to have occurred in an area of interest. This allows us to detect smaller events and spatially tie them to a relatively small area around the large event. We apply our method to field data recorded at the Groningen gas field in the Netherlands. Our method detects all catalog events as well as several previously undetected events.

Characterising microseismicity in a low seismicity region: applications of short-term broadband seismic arrays in Dunedin, New Zealand

ABSTRACT We analyse seismic data collected from a short-term array deployment in the low seismicity area of Dunedin city, New Zealand. The addition of this temporary network to the permanent GeoNet national seismic network allows detection of earthquakes not captured by the GeoNet network. Using one of the newly detected earthquakes as a master event, we employ a template match-filter analysis to identify similar events in the area. Specifically, we record a cluster of micro-events at the northern extent of the Akatore Fault (<15 km from the city centre). A recent paleoseismic study has shown the Akatore Fault to have had multiple earthquake ruptures in the late Holocene after over 110 ka of quiescence, so the observed microseismicity is consistent with the fault being in a state of elevated activity. In contrast, other known faults in the area are quiescent with respect to both microseismicity and Holocene ruptures. The establishment of a permanent seismic array in and around Dunedin for the monitoring of microseismicity on the Akatore Fault and other local faults is warranted.

Illuminating the Rupturing of Microseismic Sources in an Injection‐Induced Earthquake Experiment

AbstractWe analyze source parameters of Mw −3.9 to −3.1 induced earthquakes during an in situ fluid injection experiment in France using the spectral ratio method based on empirical Green's function. We choose 10 event pairs with highly similar waveforms and resolve their spectral ratios using multiple S wave windows. We find that master events ruptured meter‐scale source patches with &lt;1‐μm slip in a preexisting fracture network oriented differently from the injected plane. The temporal correlation between master earthquake occurrence and injection pressure peak and the relatively low ratio of stress drop to crustal strength suggest that both fluid pressure perturbation and aseismic deformation play important roles in inducing the earthquakes. The comparison between stress drops of induced earthquakes in the experiment and in the central United States indicates a dependency of stress drop on crustal shear strength.

Long Period Seismicity at Mammoth Mountain, California

AbstractLong period (LP) seismicity is a ubiquitous feature at Mammoth Mountain, California. Phase‐based locations exhibit source depths ranging from ∼8 to 25 km. Between September 2011 and October 2013, over 200 LP events were added to the Northern California Earthquake Data Center catalog. Three typical LP events were used as master events, and continuous waveform correlation was performed over the 2‐year period, with the results indicating that thousands of LP events were present in the continuous seismic data. Using phase‐weighted stacking, representative network stacks for the three events were produced. Full waveform inversion of the representative events using Green functions that adequately represent the topography of Mammoth Mountain and the high‐resolution velocity structure produced through local earthquake tomography suggest that all three event types are located within ∼100 m of the surface in the vicinity of the Chair 12 tree kill and CO2 emission area and within ∼1 km of the Mammoth Mountain Ski Area base lodge. The source mechanisms are purely volumetric and can be attributed to a subhorizontal hydrothermal crack excited into acoustic resonance by the rapid discharge of CO2 gases trapped in the crack. An accompanying vertical force that is opposite in phase to the volume change in the crack can be attributed to pressure and momentum changes in an adjacent fluid‐filled conduit linking the crack to the surface. The implication of this work is that LP seismicity at Mammoth Mountain occurs almost continuously and in direct contradiction to the traditional view—at very shallow depths.

Earthquake source parameters in Norway determined with empirical Green’s functions

Norway experiences low to intermediate seismicity, mainly caused by ridge push of the Mid-Atlantic ridge. To gain a better understanding of local tectonics and fundamental physics, we used the empirical Green’s function (EGF) method to obtain earthquake source parameters in Norway. We validated our findings against earthquake source parameters obtained from spectral analysis. Between January 1990 and May 2018, we found 263 earthquake pairs eligible for the EGF method. The local magnitudes of the 107 master events range between 1.3 and 3.4. Based on the Brune (J Geophys Res 75(26):4997–5009, 1970) source model, we obtained stress drops between 0.4 and 355 bar. We observed an increase of stress drop with earthquake size in southern Norway and the Svalbard archipelago. Calculated fault radii between 80 and 320 m suggest that larger magnitudes are caused more by increase in slip than by increase in fault dimension. Hence, our results indicate that smaller earthquakes in southern Norway and the Svalbard archipelago deviate from the assumption of self-similarity. Stress drops obtained for northern Norway show only weak correlation with seismic moment and therefore fit into the theory of earthquake self-similarity.

Application of Double Difference Tomography Method to Determine The 3D Seismic Wave Velocity Structure of GoLF Geothermal Field

GoLF geothermal eld is located in South Solok Regency, 150 km SE of Padang city, West Sumatra.Geology, geochemistry and geophysical surveys had been conducted since 2008. Geophysical survey which had been performed including microseismic and magnetotelluric surveys. Seismic velocity structure modelling need to be conducted in order to characterize geothermal reservoir.This study uses microseismic data recorded from 36 seismometers which installed in two time recording time ranges; from September 2010 to April 2011 and from September 2012 to December 2013, with microseismic events recorded respectively 135 and 2692 events. To maximize the result of picking waveform, the data is processed using the Master Event Cross Correlation method to update the catalog data and get more accurate arrival time. Furthermore, the author used TomoDD software to produce hypocenter relocation and the 3D velocity structure under GoLF's geothermal reservoir. The results of the 3D velocity model can be used to determine the structure and phase of the fluid under GoLF geothermal field.

Detection, Estimation of Magnitude, and Relative Location of Weak Aftershocks Using Waveform Cross-Correlation: The Earthquake of August 7, 2016, in the Town of Mariupol

The study of the stress-strain state of a medium in seismically quiet areas is difficult because of the absence of strong events. Under such circumstances, each earthquake, even relatively weak, is of high importance. In this case, all possible information on tectonic stresses and their dynamics, e.g., information on time, location, and magnitude of aftershocks, should be obtained from available seismic data. The earthquake near the town of Mariupol which occurred on August 7, 2016, had a body wave magnitude of 4.5–4.9 from the data of the different seismological centers. We detected 12 aftershocks that occurred within 5 days after the main shock using two seismic arrays (AKASG and BRTR) and one three-component station (KBZ) of the International Monitoring System, as well as two array stations of the Institute of Geosphere Dynamics, Russian Academy of Sciences. For six aftershocks, signals were found at three or more stations. The other aftershocks were detected from the data at two out of three nearest stations. Signal detection and association with aftershocks of the main shock, as well as estimation of magnitude and relative location of the found aftershocks, were carried out using the method of waveform cross-correlation (WFCC). The signals from the main shock that acted as the only master event (ME) for the WFCC method were used as waveform templates. To increase the signal-to-noise ratio and to determine the exact onset time of regular seismic waves from aftershocks, we used waveform templates of different length, from 10 to 180 s depending on the wave type and distance to the station, as well as band filtering in narrow frequency bands. The highest sensitivity of the detector and accuracy of the P-wave onset time estimates were reached when a waveform template included all regular waves from P to L g . Association of signals with aftershocks was based on back projection of signal arrival times to origin times using the travel time from a master event to the station, which was measured with a very low error, being equal to almost half of the digitization step length. To develop a seismic event hypothesis, the origin times at two or more stations should be spaced within a 2-s interval.

Microseismic Monitoring of Stimulating Shale Gas Reservoir in SW China: 1. An Improved Matching and Locating Technique for Downhole Monitoring

AbstractWe introduce an improved matching and locating technique to detect and locate microseismic events (−4 &lt; ML &lt; 0) associated with hydraulic fracturing treatment. We employ a set of representative master events to act as template waveforms and detect slave events that strongly resemble master events through stacking cross correlograms of both P and S waves between the template waveforms and the continuous records of the monitoring array. Moreover, the residual moveout in the cross correlograms across the array is used to locate slave events relative to the corresponding master event. In addition, P wave polarization constraint is applied to resolve the lateral extent of slave events in the case of unfavorable array configuration. We first demonstrate the detectability and location accuracy of the proposed approach with a pseudo‐synthetic data set. Compared to the matched filter analysis, the proposed approach can significantly enhance detectability at low false alarm rate and yield robust location estimates of very low SNR events, particularly along the vertical direction. Then, we apply the method to a real microseismic data set acquired in the Weiyuan shale reservoir of China in November of 2014. The expanded microseismic catalog provides more easily interpretable spatiotemporal evolution of microseismicity, which is investigated in detail in a companion paper.