Near-source effects on DAS recording: implications for tap tests

In the immediate vicinity of a source, there are strong gradients in the seismic wavefield that are tamed and modified in distributed acoustic sensing (DAS) recording due to combined effects of gauge-length averaging and local stacking on the local strain field. Close to a source broadside propagation effects are significant, and produce a characteristic impact on the local DAS channels. In the presence of topography, of surface or cable, additional effects are introduced that modify the expected signal. All these influences mean that the results of tap tests used to calibrate the channel positions along a DAS cable may give a distorted view of the actual geometry. Such effects can be important for detailed mapping of faulting processes and comparable features.