Compressional- and shear-wave studies of distributed acoustic sensing acquired vertical seismic profile data

Abstract

Understanding the strengths and limitations of rapidly advancing distributed acoustic sensing (DAS) technology used for recording vertical seismic profile (VSP) data is achieved by comparing DAS and geophone data sets using both compressional-wave (P-wave) and shear-wave (S-wave) VSP data and their corresponding geophysical answer products. We validate the kinematics (time) and dynamics (amplitude) of DAS VSP data by examining the extracted slowness values, response-to-incident angles, corridor stacks, and common-depth-point (CDP) transforms. For kinematics validation, the slowness values computed from P- and S-wave components of DAS VSP data agree with the geophone slowness values. For dynamics validation, we confirm the cos2 θ response of the fiber to the incident angle of the seismic wavefield for P-waves and sin 2θ for S-waves. The amplitudes of the P-wave corridor stacks are comparable; the S-wave corridor stacks are similar for shallow events and differ for later events due to the limited response of the fiber to S-waves at near-vertical angles of incidence. High-quality CDP transform images are obtained for P- and S-waves. These analyses indicate that properly acquired DAS VSP data sets are reliable for the kinetics and dynamics of both P- and S-waves. Once the fiber-optic cable is installed in the well, VSP acquisition costs are greatly reduced because DAS data may be acquired with no additional well intervention. The extensive spatial coverage obtained using fiber-optic cables, the ease of acquiring time-lapse (4D) VSP data, and the reliability of the resulting DAS VSP data sets are making DAS technology an extremely important VSP acquisition tool.

Surface Wave Imaging using Distributed Acoustic Sensing Deployed on Dark Fiber: Moving Beyond High Frequency Noise

“…DAS technology enables acquisition of high-resolution seismological data at frequencies from the kHz to mHz for long distances (tens of km) and at very dense spatial samplings (down to 1 m) in contexts in which the use of conventional sensors is intricate and/or costly (Daley et al, 2016;Becker et al, 2017). DAS is incrementally being adopted in the field of applied geophysics and a variety of studies have demonstrated its suitability for a range of applications such as vertical seismic profiling (Daley et al, 2013;Mateeva et al, 2013;Wu et al, 2015;Wu et al, 2017), time-lapse monitoring of near-surface properties (Dou et al, 2017;Ajo-Franklin et al, 2019), earthquake detection (Lindsey et al, 2017;Ajo-Franklin et al, 2019;Li & Zhan, 2018), and hydrological monitoring (Becker et al, 2017).…”

A field experiment of walkaway distributed acoustic sensing vertical seismic profile in a deep and deviated onshore well in Japan using a fibre optic cable deployed inside coiled tubing

“…In Figure 9(b), the amplitude of the direct P-wave arrival changes with depth. The DAS measurement has directional sensitivity (Wu et al 2017); that is, the signal amplitude varies with the incident angle of the wave relative to the FO axis. The DAS amplitude for compressional waves decreases as the wave propagation direction becomes perpendicular to the FO, and the sensitivity to shear waves reaches its maximum at an incident angle of 45 degrees.…”

“…; Humphries, Vidal and de Dios ; Wu et al . ) that had been published as of the end of 2017. In most of the existing field experiments, FOs were deployed via the behind casing or along tubing methods.…”

“…Although considerable ringing noise or cable slapping noise can be seen in the acquired data, subsurface imaging results with a sufficient signal-to-noise ratio (SNR) and a higher frequency than 2D surface seismic results were obtained after careful data processing. Figure 1 summarizes the DAS-VSP survey specifications of the abovementioned field experiments and others (Barberan et al 2012;Humphries, Vidal and de Dios 2015;Wu et al 2017) that had been published as of the end of 2017. In most of the existing field experiments, FOs were deployed via the behind casing or along tubing methods.…”

Using Dark Fiber and Distributed Acoustic Sensing for Near-Surface Characterization and Broadband Seismic Event Detection

“…17, 33 used a shallow hydrogeologic pump test in a well with a fiber-optic cable to show that DAS has sensitivity to 9.4 × 10 −3 Hz (period = 1080 seconds) oscillations in strain induced by the variable confining pressure, presumably due to Poisson effects. This subject is complicated by the known directionality of DAS cables ( 34,35 ), which for the horizontal geometry of telecommunications dark fiber cables is theoretically insensitive to vertically-incident compressional motion (P-waves).…”