Three-dimensional seismic response mechanism of straight and helically wound optical fibers
SHI Zhiwen1, YANG Jidong1, WANG Yangzhou2, HUANG Jianping1, QIN Shanyuan1, YANG Yonghong3
1. School of Geosciences, China University of Petroleum (East China), Qingdao, Shandong 266580, China; 2. Shandong Energy Group South America Co., Qingdao, Shandong 266580, China; 3. Research Institute of Exploration and Development, ShengLi Oilfield Company., SINOPEC, Dongying, Shangdong 257022, China
Abstract：Distributed Acoustic Sensing （DAS） measures axial strain or strain rate， and straight optical fibers receive a single component signal and are insensitive to the response of waves incident perpendicularly to the fiber. Helically wound optical fibers can receive multi-component signals， which can solve the problem of insensitive response. Firstly， the theoretical axial strain rate responses of P-wave， SV， and SH waves of straight optical fibers and helically wound optical fibers with winding angles of 35.3° and 54.7° are analyzed. Secondly， the axial strain rate response is simulated using the three-dimensional elastic finite difference method. Finally， DAS is deployed on the surface， horizontal wells， and vertical wells， and the seismic responses of straight optical fibers and helically wound optical fibers with winding angles of 35.3° and 54.7° are simulated using uniform models， double-layer models， and southwest shale gas models. The axial strain rate response of DAS is compared with the velocity z component and pressure component of conventional detectors. The results show that： ① Straight optical fibers receive single component information， but the response of waves received in the direction perpendicular to the optical fiber is weak， while helically wound optical fibers receive multi-component information， so the response of waves received in the direction perpendicular to the optical fiber is strong； ②there is no S-wave response in the helically wound optical fiber seismic record with a winding angle of 35.3°， and its waveform is similar to the pressure component of a conventional detector. The acoustic equation can be used for imaging.
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