Distributed acoustic sensing for shallow seismic investigations and void detection

During the past few decades, fiber-optic-based distributed acoustic sensing (DAS) has emerged as an affordable, easy-to-deploy, reliable, and noninvasive technique for high-resolution seismic sensing. We have determined that fiber deployments dedicated to near-surface seismic applications, commonly used for the detection and localization of voids, can be used effectively with conventional processing techniques. We tested a variety of small sources in different geologic environments. These sources, operated on and below the surface, were recorded by horizontal and vertical DAS arrays. Our results and comparisons to data acquired by vertical-component geophones demonstrate that DAS may be sufficient for acquiring near-surface seismic data. Furthermore, we tried to address the issue of directional sensing by DAS arrays and use it to solve the problem of wave-mode separation. Records acquired by a unique acquisition setup suggest that one can use the nature of standard DAS systems as uniaxial strainmeters to record separated wave modes. Finally, we applied two seismic methods on DAS data acquired at a test site: multichannel analysis of surface waves (MASW) and shallow diffraction imaging. These methods allowed us to determine the feasibility of using DAS systems for imaging shallow subsurface voids. MASW was used to uncover anomalies in the S-wave velocity, whereas shallow diffraction imaging was applied to identify the location of the void. The results we obtained illustrate that by using these methods we are able to accurately detect the true location of the void.