Numerical Analysis of Distributed Acoustic Sensing Responses to Microseismic Sources with Varied Fiber Optic Layouts in Complex Media

Abstract Distributed acoustic sensing (DAS) is a new type of sensor, which can record elastic wave signals in many ways similar to traditional seismic geophones. Extensive research has demonstrated its response based on field experiments of DAS, and compared with that of geophones to find the consistence. The response of DAS is not only dependent on the layout form of fibers and the demodulation mode of the system, but also related to the elastic wavefield propagation with different source focal mechanism. We focus on the response characteristics of different configurations of fiber layout forms in the microseismic wave field. For this purpose, we develop a numerical solution for far-field elastic wave response of the DAS system based on phase-sensitive optical time-domain reflectometer to demodulate the phase difference with silica fiber, which is sensitive to axial strain. We investigate the relationship between the elastic wave field due to microseismic sources and the strain response of DAS with different fiber layout configurations (includes straight optical fibers and helically wound optical fiber cables). The response characteristics of DAS with different fiber layout configurations to different types of subsurface microseismic sources are analyzed through radiation pattern diagrams. The GPU-accelerated finite-difference time-domain method is used to numerically solve the DAS response of various fiber optic layouts. The correctness and effectiveness of the numerical solution are verified by comparing it with the analytical solution in homogeneous media. Based on this numerical simulation method, the microseismic monitoring simulation under complex media conditions is completed, and the characteristics of DAS receiving microseismic signals under various fiber optic layouts in complex media are discussed.