Diffraction stack imaging

The aforementioned location methods all rely on picked arrival times of seismic phases. This means specific seismic phases should be identified and associated to a certain seismic event. This can be a very time consuming process when many seismic events exist in the recorded data. In addition, when the recorded data is contaminated by strong noise, phase picking will be very difficult or impossible. All these factors make the picking and arrival time based location methods not favourable for real time and automated seismic location determination. For automated seismic location methods,

automatically utilizing the recorded seismic waveforms is important. Kao & Shan

(2004) first proposed a source-scanning algorithm (SSA) to locate seismic events with- out picking. This method automatically stacks the recorded waveforms according to calculated traveltimes and uses the stacked waveforms as an indicator to identify seis- mic event location and origin time. This source location strategy is also referred to as diffraction stack imaging (Gajewski et al., 2007), backprojection imaging (Beskardes et al., 2017), waveform migration or stacking (Gharti et al., 2010, Grigoli et al., 2013a, Langet et al., 2014) and Kirchhoff reconstruction (Baker et al., 2005). As an automated seismic location method, the waveform migration can save human efforts involved in seismic location, such as arrival time picking. Utilizing seismic waveforms has the abil- ity to identify more seismic events and complete the seismic catalogue. As shown in

Figure 1.6, the waveform migration method has identified 7 times more events than traditional location method and also moves the detection threshold 0.8 lower in magni- tude (Beskardes et al., 2017). The located events can better delineate the complexity of the fault zone.

The diffraction stack imaging (DSI) is based on recorded seismograms. As Figure 1.7 a1-a2 shows, the generated seismic waves propagate through the Earth and are recorded by monitoring stations. The recorded seismograms contain information of the source, such as source location and origin time. After applying a simple transformation to remove the polarity of seismic waves (as shown in Figure 1.7 a3), such as calculating the envelope or absolute values, the transformed seismograms can be used to perform automated source location using DSI. The detailed processes of the DSI method are summarized as follows. The traveltimes from each imaging point in the subsurface to the station can be calculated efficiently using e.g. an Eikonal solver (Podvin & Lecomte, 1991). Assuming a trial origin time of the seismic event, the arrival time of a seismic event at a trial imaging point and a trial origin time can be calculated

using Tik = tik + t0, where tik is the traveltime from imaging point i to station k

and t0 is the trial origin time. Then, waveforms are stacked along different stations

at the calculated arrival times. The stacked/imaging value at the trial imaging point

can be expressed as W (xi, yi, zi, t0) =

PN

k=1S(Tik), where N is the total number of

stations, S(t) are the transformed seismograms and xi, yi, zi are spatial coordinates of

the imaging point i. At the same imaging point, the stacking process is performed along different trial origin times to search for a potential correct source origin time. Figure 1.7 b1-b4 shows the stacking process of DSI as a trial imaging point and three different trial origin times. The stacking process is then conducted on each imaging point in the subsurface (as shown in Figure 1.7 b-d). Therefore, the final obtained migration volume is a 4D data volume with 3D in the space domain (source location) and 1D in the time domain (origin time). Because the recorded waveforms are most coherent/consistent across all the stations at the correct source location and origin time, the stacked values will reach the maximum at the correct source location and origin time (as shown in Figure 1.7 c3). Thus, the source location and origin time can be identified from the final migration volume by finding the maximum imaging values:

xs, ys, zs, t0s= maxt0∈[t1,t2]{W (x, y, z, t0)} (as shown in Figure 1.7 e1-e4).

Chambers et al. (2010b) investigate the imaging ability of DSI and surface arrays for microseismic activity using perforation dataset. In their studies, perforation shots with known locations and origin times are located using DSI to test the ability and accuracy of surface arrays to detect and locate microseismic events. Since the surface arrays are deployed at surface and away from the perforation shots at depth, the SNR of the seismic records is low and signals from the perforation shots are not visible in the data. However, after stacking and migration clear source energy focuses appear, which reveal the locations and origin times of the perforation shots. Their studies show that surface

§1.3 Developments in local seismic location techniques 21

Figure 1.6: Hypocenter locations of earthquakes following the 2011 Virginia earthquake. The top panel shows map view of location results. Middle and bottom panel show cross sec- tions along AA’ and BB’. (a-c) Hypocenters located by automated waveform migration. (d-f) Hypocenters located by picking and arrival time based method using traditional aftershock network. Figure from Beskardes et al. (2017).

a1 a2 a3

b1 b2 b3 b4

c1 c2 c3 c4

d1 d2 d3 d4

Trial origin time t1 Trial origin time t2 Trial origin time t3

e1 e2 e3 e4

Stacked volume at different trial origin times

Figure 1.7: Schematic diagram showing the waveform stacking process. The star shows the location of an assumed seismic event. The triangles show the locations of surface stations. Figure a2 represents the recorded seismograms containing the direct P- and S-waves. Figure a3 represents the characteristic functions of the recorded seismograms after transformation. Figures b1-b4 and d1-d4 show the stacking processes at different trial origin times for a imaging point which is away from the exact source location. Figures c1-c4 show the stacking processes at different trial origin times for a imaging point which locates just at the exact source location. Figures e1-e4 show the stacking profiles at different trial origin times. The maximum stacking value appears at the correct source location and origin time. Blue solid lines show the calculated arrival times of P-waves and orange solid lines show the calculated arrival times of S-waves. The yellow diamonds represent the imaging points which have already gone through the stacking process. The red diamond represents the current imaging point. Figure modified from Cesca & Grigoli (2015).

§1.3 Developments in local seismic location techniques 23

arrays are capable of imaging microseismic events by utilizing DSI method, even when SNR is down to 0.1 and the signals are not visible in individual traces. Chambers et al. (2010a) apply DSI to image microseismic events induced by hydraulic fracturing. They also explore the influence of event magnitude, SNR, source mechanisms and velocity uncertainties to the seismic event location using synthetic tests.

In contrast to controlled explosive sources, natural earthquakes have complex source mechanisms, and thus show different source radiation patterns. For natural earth- quakes, many seismic events are caused by fault dislocation and therefore have double- couple source mechanism. For many seismic events induced by hydraulic fracturing, complex source mechanisms such as non-double-couple source time functions are often

observed as well (ˇS´ılen`y et al., 2009). This means that the signals recorded at differ-

ent stations will have different phase polarizations. Therefore, due to complex source radiation patterns, the waveforms may be stacked destructively and the maximum stacked value may not appear at the exact source location. Zhebel & Eisner (2014) show that several maxima will appear around the true source location when stacking the waveforms of pure double-couple sources. Therefore, directly stacking recorded seismograms is not effective and will cause location errors when imaging sources with complex radiation patterns. Instead of directly utilizing recorded waveforms, DSI is often performed on various characteristic functions of recorded waveforms. Charac- teristic functions are simply some kind of transformation of the original waveforms in order to remove source radiation patterns and obtain non-negative traces. Stacking of characteristic functions can remove the effect of source radiation patterns and obtain an accurate source location. Different characteristic functions have been proposed and used in the DSI, such as the absolute amplitude (Kao & Shan, 2004, 2007), the enve- lope (Baker et al., 2005, Gharti et al., 2010), the short-term-average/long-term-average (STA/LTA) (Grigoli et al., 2013a,b, Drew et al., 2013) and the kurtosis (Langet et al., 2014) of the waveforms. As shown in Figure 1.8, different characteristic functions can exhibit different imaging resolutions and noise resistance.

Different phases, such as the direct P- and S-phases, can be utilized alone or in

combination to image the source. Normally, if the SNR of recorded data is high,

jointly using different phases can improve location accuracy. In theory, any phase such as multiples, reflected waves and converted waves can all be used in DSI to locate the source. However, the traveltimes of these additional phases depend more strongly on the adopted velocity model than the primary phases. When the subsurface velocity model is poorly known, utilizing those non-primary phases can cause location deviations.

The techniques developed in other location methods can also be incorporated into the DSI framework. Grigoli et al. (2016) propose a master-event waveform stacking method to locate the source, which combines the master-event location method and the DSI method. By utilizing the techniques developed from relative location methods, this combined method can mitigate the negative effect of uncertainties of the velocity

Figure 1.8: Migration profiles of a magnitude 1.4 seismic event for different characteristic functions. Different columns show migration profiles in different directions. The color represents stacked/migration values. Large migration value (red color) represents high possibility of the existence of a seismic event. Different rows show results for different characteristic functions used in migration: (a) the energy, (b) the STA/LTA, (c) the envelope, (d) the kurtosis. Figure from Cesca & Grigoli (2015).

§1.3 Developments in local seismic location techniques 25

Figure 1.9: Hypocenter locations of synthetic events using a wrong velocity model. The synthetic seismic events are denoted by circles and are all located in the dashed lines. The tri- angles show locations of stations. Left panel: hypocenter location results of standard migration method using a wrong velocity model. Right panel: hypocenter location results of master-event migration method using the same wrong velocity model. The used master event is highlighted by the yellow circle. Figure modified from Grigoli et al. (2016).

model to seismic location results and reduce the dependence of DSI on velocity models. As shown in Figure 1.9, the master-event waveform stacking/migration method can re- move the effect of velocity model outside the source area, and obtain relatively accurate seismic locations even with a wrong velocity model. For finding the global maximum in a migration volume, global optimization algorithm can be more efficient than the full grid search method. As a global optimization technique, the differential evolution algo- rithm can be used in the nonlinear grid search methods to obtain the global maximum efficiently. Differential evolution is essentially an evolutionary algorithm for global op- timization, and it uses a differential scheme to generate new trial source locations and approaches the correct source location iteratively (Gharti et al., 2010). Gharti et al. (2010) applied the differential evolution technique to the DSI framework and improved the efficiency of the DSI algorithm.

Although DSI has many successful applications in automated seismic location, this method still needs further developments, such as handling ultra-low SNR data, esti- mating location uncertainties and effectively identifying seismic events in a migration volume. For surface monitoring which is common for passive seismic monitoring, the stations are normally deployed away from the seismic events. Therefore, the recorded signals can be very weak due to attenuation and geometric spreading. In addition, sur-

face recordings can easily be contaminated by various kinds of noise, reducing the SNR of the recorded data. Although migration-based methods can utilize the coherence of all stations and have the ability to image source events with low SNR, locating mi- croseismic events in ultra-low SNR scenario is still very challenging. For microseismic events, the recorded waveforms can be highly contaminated by noise and the signals are invisible against background noise, thus pose a great challenge to all location methods. For arrival time based location methods, the location error can be assessed using statis- tical approaches (Tarantola & Valette, 1982, Lahr, 1999, Lomax et al., 2000). However, the uncertainty estimation for the waveform migration based location methods is not straightforward and is difficult to conduct. The uncertainty is not only related to the raw data quality but also the characteristic functions. In addition, because of data aliasing, it is also difficult to identify different seismic events which occur at the same time or at adjacent time from the migration data volume.