Distributed Acoustic Sensing in Geophysics

Determining the appropriate depth or location of each DAS channel is important for the accuracy of the resulting DAS products. One improved method is to use known locations as calibration points with the interleaving channels interpolated from them. The angle‐of‐incidence response of the fiber is different from that of geophones. It is important to first determine its effect, and then, where appropriate, remove it. DAS data quality is intrinsically lower than that from geophones; however, because there is frequently one or two orders of magnitude more density of information, the signal quality can be greatly improved. Finally, while the native measurement of a DAS system is phase, it can be converted to relative strain, strain rate, particle velocity, and an equivalent geophone response.

Figure 2.10 Relationship among the various products created from a DAS data set.

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Qizhen Sun1, Zhijun Yan1, Hao Li1, Cunzheng Fan1, Fan Ai1, Wei Zhang1, Xiaolei Li2, Deming Liu1, Fei Li3, and Gang Yu3

1School of Optical and Electronic Information, National Engineering Laboratory for Next Generation Internet Access System, Huazhong University of Science and Technology, Wuhan, China

2OVLINK Inc., Wuhan, China

3BGP Inc., China National Petroleum Corporation, Zhuozhou, China

Distributed acoustic sensing (DAS) can record acoustic or seismic waves along the optical fiber with advantages of long distance, short operation time, full well coverage, and cost saving, which has important significance in borehole seismic surveys. By designing and fabricating a distributed microstructured optical fiber (DMOF) with successive longitudinal microstructures, the signal‐to‐noise ratio (SNR) of the Rayleigh backscattering light is enhanced and random interference fading is greatly eliminated, which are beneficial to improve the sensing performance of the system. Combined with coherent detection and phase demodulation, a DMOF‐based fiber optic DAS system with a wide frequency bandwidth from 0.01 Hz to 60 kHz and an ultrahigh strain resolution of 3.4 pε/√Hz around 10 Hz was explored and demonstrated. By employing the DMOF‐DAS system as data acquisition (DAQ) equipment (interrogator), zero‐offset vertical seismic profile (VSP), offset VSP, and walkaway VSP test surveys were conducted in two oil fields in China, respectively, with DMOF cables deployed inside a water‐filled borehole and cemented outside the casing, respectively. The good quality VSP data with a high SNR, correct amplitude, and clear upgoing/downgoing waves proved that the DMOF‐DAS system could be a competitive alternative to geophone arrays for the acquisition of borehole seismic data.