Distributed Microstructured Optical Fiber (DMOF) Based Ultrahigh Sensitive Distributed Acoustic Sensing (DAS) for Borehole Seismic Surveys

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.