Numerical simulation of detecting seismic signals in DAS wells
(2.Functional Laboratory for Marine Mineral Resources Assessment and Prospecting, Qingdao National Laboratory for Marine Science and Technology, Qingdao, Shandong Province, China 266071)
(3.College of Science, China University of Petroleum (East China), Qingdao, Shandong Province, China 266580)
【Abstract】Owing to the Rayleigh scattering, distributed acoustic sensing (DAS) could detect seismic vibrations in optical fibers, which meanwhile also functions as the carrier for signal transmission. Thus, this system is suitable for borehole seismic acquisition with low cost, high resolution, and high performance of anti-electromagnetic interference. Based on the discrete Rayleigh scattering interference model, we use numerical simulation to model borehole seismic signals in the DAS system; we also discuss the impacts of source strength, pulse width, and spatial fiber sampling interval on the waveform and signal to noise ratio of DAS signals. In this process, we do not consider the influence of ambient pressure, temperature, and borehole wallfiber coupling. The results show that (1) DAS signals vary with source strength. A strong source may cause waveform distortion or increased side lobes; this may lead to signal distortion. (2) Small pulse width is usually associated with strong noises. In contrast, large pulse width may facilitate high-frequency noise suppression and improve signal to noise ratio; but the resolution will be inevitably sacrificed to some extent; (3) increased fiber sampling interval by multi-trace stacking may be useful to the improvement of signal to noise ratio. Thus, we may choose a proper sampling interval to improve signal to noise ratio and signal quality. DAS signals usually exhibit slightly higher frequencies than the original seismic signals; there are also inherent high-frequency noises.
【Keywords】 distributed acoustic sensing; Rayleigh scattering; source strength; pulse width; spatial fiber sampling interval; numerical simulation;
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