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.
Distributed acoustic sensing (DAS) is a rapidly evolving fiber optic technology for monitoring cement curing, perforation performance, stimulation efficiency, and production flow and, more recently, for performing vertical seismic profiling (VSP). VSP data can be acquired and processed to determine velocity models that are used in surface seismic imaging for reservoir characterization, or for microseismic monitoring of hydraulic fracturing operations. The limitation of conventional VSP data acquisition has been well accessibility, with wireline-conveyed tools deployed during openhole or casedhole logging campaigns before well completion or during workovers. Fiber optic cable conveyance by coiled tubing (CT) expands the opportunity for VSP data acquisition during planned CT interventions. This paper presents an example of a CT DAS VSP acquisition. The processing steps are shown to overcome some of the noise challenges inherent in CT DAS data, such as persistently strong borehole tube waves induced from the surface operations activities. A case study is shown for the depth tie between surface seismic data and the CT DAS VSP derived corridor stack image, demonstrating the viability of CT deployed fiber to acquire DAS VSP data.
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