Distributed acoustic sensing for near-surface seismic applications

Abstract

Distributed acoustic sensing (DAS) has been used in the oil and gas industries for vertical seismic profiling (VSP) for approximately the past ten years. Its use for near-surface seismic surveys has also been investigated. This study relates to these types of applications. DAS consists of a buried optical fiber cable with one of its fibers connected to electro-optical instrumentation, i.e., an optical interrogator. The interrogator injects pulses of coherent light into the fiber and receives backscattered light. Seismic disturbances deform the cable, strain the interrogated fiber, and change the fiber’s optical path length. The resulting phase changes of the backscattered light are recorded by the interrogator. These phase change measurements, proportional to strain, are probed through time at spatially sequential (i.e., “distributed”) segments along the length of the fiber. The length of each segment is referred to as the gauge length. A single fiber is thus a nominal array of sensors with the gauge length defining the spatial separation. Here, we present DAS signals from ground vibrations caused by impact-hammer and electro-magnetic-shaker sources. The signals were recorded simultaneously by geophones for comparison. Interrogator modifications allowed selectable gauge lengths, 10, 4 and 2 meters. The experimental results demonstrate the benefits of higher resolution.