Repeat well logging using earthquake wave amplitudes measured by distributed acoustic sensors

Abstract

<jats:p> Well-based technologies for seismic subsurface monitoring increasingly utilize fiber-optic cables installed in boreholes as distributed acoustic sensing (DAS) systems. A DAS cable allows measuring linear strain of the fiber and can serve as an array of densely spaced seismic receivers. The strain amplitudes recorded by the DAS cable depend on the near-well formation properties (the softer the medium, the larger the strain). Thus, these properties can be estimated by measuring relative variations of the amplitudes of seismic waves propagating along the well. An advantage of such an approach to subsurface characterization and monitoring is that no active seismic source is required. Passive sources such as earthquakes can be utilized. A synthetic data example demonstrates viability of the approach for monitoring of small-scale CO<jats:sub>2</jats:sub> injection into an aquifer. Two field DAS data examples based on signal recordings from several distant earthquakes show that the relevant properties of the near-well formation can be estimated with an accuracy of approximately 5%. </jats:p>

Downhole distributed acoustic sensing reveals the wave field structure of the coastal microseisms

“…7b we may clearly identify the peaks and troughs corresponding to stiff and soft geological formations. Similar behaviour was observed in the analysis of amplitudes along CRC-3 of compressional waves from remote earthquakes (Pevzner et al, 2020a). Unlike the stress traction, vertical strain is discontinuous at the lithological boundaries, and hence its finescale variability reflects the variation of the stiffness of rocks along the borehole.…”

“…However, the phase interferometry approach implemented in iDASv3™ provides sufficient SNR starting from a few millihertz. Pevzner et al (2020a) showed that eq. (1) holds true for the DAS system in CRC-3 and the data still provides clear records of teleseismic earthquakes.…”

Quantification of DAS VSP Quality: SNR vs. Log-Based Metrics

“…This survey can be acquired with a cost-effective fiber installation as the detection of the signal does not require perfect coupling for precise amplitude extraction. On the other hand, if the primary goal of the survey is to analyze amplitudes [23][24][25] or conduct full wavefield inversion with resolution enhancement, reliable amplitude information is crucial, and one should choose a log-based amplitude metric.…”

Seismic Applications of Downhole DAS

“…Other subsurface properties can also be analyzed using earthquake records in DAS-VSP. By measuring DAS amplitudes and applying energy conservation principles, one can infer a velocity- and density-dependent quantity along the well [ 93 ]. Therefore, a joint analysis of these two methods can yield independent density values in addition to the velocity.…”

“…In addition, the high spatial density is not always directly useful, as recorded events underwent attenuation and thus have much longer wavelengths, for which the meter-scale resolution of DAS is not necessary. Oftentimes, earthquakes are not the primary target of downhole DAS deployments [ 80 , 93 ]. As a result, they are not necessarily studied for seismological purposes.…”