Characteristics of microseismic data recorded by distributed acoustic sensing systems in anisotropic media

Abstract

<jats:p> Fiber-optic distributed acoustic sensing (DAS) cables are now used to monitor microseismicity during hydraulic-fracture stimulations of unconventional gas reservoirs. Unlike geophone arrays, DAS systems are sensitive to uniaxial strain or strain rate along the fiber direction and thus provide a 1C recording, which makes identifying the directionality and polarization of incoming waves difficult. Using synthetic examples, we have shown some fundamental characteristics of microseismic recordings on DAS systems for purposes of hydraulic fracture monitoring in a horizontal well in anisotropic (vertical transverse isotropy [VTI]) shales. We determine that SH arrivals dominate the recorded signals because their polarization is aligned along the horizontal cable at the near offset, although SV will typically dominate for events directly above or below the array. The amplitude of coherent shear-wave (S-wave) arrivals along the cable exhibits a characteristic pattern with bimodal peaks, the width of which relates to the distance of the event from the cable. Furthermore, we find that S-wave splitting recorded on DAS systems can be used to infer the inclination of the incoming waves, overcoming a current limitation of event locations that have constrained events to lie in a horizontal plane. Low-amplitude SV arrivals suggest an event depth similar to that of the DAS cable. Conversely, steep arrivals produce higher amplitude SV-waves, with S-wave splitting increasing with offset along the cable. Finally, we determine how polarity reversals observed in the P and SH phases can be used to provide strong constraints on the source mechanisms. </jats:p>

Seismic Applications of Downhole DAS

“…They simultaneously utilize spatiotemporal patterns across hundreds or thousands of channels and can thus overcome the lower SNR of individual channels. DAS can also be used for focal mechanism [99,111] and magnitude [14,105] estimation. For both, the directional measurement of DAS poses a significant challenge, as not all components of the seismic wavefield excited by the source can be recorded.…”

“…Therefore, the most widespread microseismic analysis methods, based on identifying distinct seismic phases at each receiver, underperform when applied to DAS data. Finally, microseismic information is most valuable in near-real-time [ 99 ]. The amount of data generated by DAS imposes significant computational difficulties to the timely delivery of results.…”

Application of machine learning to microseismic event detection in distributed acoustic sensing data

“…DAS data provides only a single-component recording (the fiber is sensitive to changes in strain along the fiber, but insensitive to changes broadside to the cable) whereas standard geophones and seismometers can provide three-component recordings. Nevertheless, processing techniques are advancing to determine microseismic event locations (Webster et al, 2016;Verdon et al, 2020) and source mechanisms (Cole et al, 2018;Baird et al, 2020) from DAS data. The technology is also being explored for microseismic monitoring in other industrial settings, for example at geothermal sites (Mondanos and Coleman, 2019), geological CO 2 storage sites and for volcano monitoring.…”

“…We then convert particle velocity along the cable direction to DAS strain-rate by differencing over the gauge length and dividing by gauge length (Miller et al, 2016). For further details on the synthetic data modeling see Baird et al (2020).…”

“…To train YOLOv3 on DAS data, a training dataset of example images of microseismic events and background noise are prepared. The synthetic data are prepared as described above in the Data section and in Baird et al (2020) and 2000 microseismic event examples with a single event in each file are created. Two thousand examples were used to gain a good distribution of event magnitudes, locations and source mechanisms while restricting the number to minimize training times.…”