Advancements in distributed near surface imaging, an application of multichannel analysis of surface waves (MASW) using Distributed Acoustic Sensing (DAS)

Abstract:

As Distributed Acoustic Sensing (DAS) has become an accepted technology for geophysical sensing, new applications are being investigated and are rapidly expanding. Here DAS data is used in a Multichannel Analysis of Surface Waves (MASW) analysis, a technique to determine properties of the near surface for environmental and geotechnical applications.

Fiber optic acoustic sensing consists of an interrogator that sends and receives laser light pulses and a fiber optic cable. The light traveling through the cable is scattered and measured. When a fiber optic cable is strained, this changes the phase and amplitude of the backscattered light and this difference is analyzed at distributed points along the fiber and converted into an acoustic signal. With DAS, it is possible to measure 10s of km of fiber, down to 0.25m spatial sampling, for 10s of years.

Investigations carried out at the Birmingham Institute of Forest Research (BIFoR) FACE facility yield promising results in developing applications of MASW using DAS arrays. Hammer shots were taken directly adjacent to three loops of cable buried at depths of 0.1m, 0.25m, and 0.40m. After pre-processing the data by doing spectral analysis, we generate dispersion curves of the shots and pick the Rayleigh wave fundamental mode arrival times to invert for shear wave velocity. The final shear wave profiles show a low velocity layer resolved at 1-2m deep and the contact between superficial deposits and the Sherwood Sandstone layer approximately 10m below the surface. These velocity profile results align with previous borehole data that indicates groundwater to be at depths between 0.5m and 9.4m. A comparison of the results at the three fiber depths shows deeper burial depths better resolve the seismic velocity changes and, with burial depths >10cm, the SNR of the data is higher.

The MASW technique used in conjunction with DAS technology presents a new method to provide repeatable near-surface geotechnical investigations. With fiber being a permanent installation, combining DAS with the MASW technique allows time-lapse surveys of the near-surface while reducing costs and deployment time without compromising on measurement distance or density compared to more traditional geophone surveys.