Dual wavefields from distributed acoustic sensing measurements

Abstract

Distributed acoustic sensing (DAS) using fiber optic cables is an emerging seismic acquisition technology for the oil and gas industry, geothermal resource exploration, and underground fluid-storage monitoring. This technology offers the advantage of improving seismic acquisition by enabling massive arrays for monitoring of seismic wavefields at reduced cost with respect to conventional methods. In general, it is accepted that this method provides acoustic signals comparable with conventional seismic data, however, without the multicomponent directional information typical of geophones. We have developed a modified data extraction method and found that, as a result of the dense spatial distribution of recording points along the optic cable, DAS can provide two linked wavefield components in the axial direction, even when using a single 1D cable line. These signal pairs consist of dual components that are related to native strain rate (or strain) and particle acceleration (or velocity) fields at a given recording location. These dual signals are easily usable for wavefield separation purposes simply performing a trace-by-trace combination by appropriate scaling coefficient. The analysis performed with borehole data from linear and helically wound cables demonstrates the effectiveness of polarity recovery and dual-wavefield separation. We show real examples in which the data can be combined to provide separation of up- and downgoing wavefields. The ratio of the dual components provides information on local slowness properties in the formation.