Comparisons Between Array Derived Dynamic Strain Rate (ADDS) and Fiber‐Optic Distributed Acoustic Sensing (DAS) Strain Rate.

Distributed acoustic sensing (DAS) strain rate and particle velocity can be compared through approximate scaling with medium velocity. We instead performed a direct comparison between array derived dynamic strain (ADDS) rate and DAS strain rate for six frequency bands. The PoroTomo project at Brady's Hot Springs, Nevada, deployed a 240‐geophone 3C array co‐located with fiber‐optic DAS system and 8.7 km of buried cable. We selected subsets of the geophone array to create four smaller arrays and computed ADDS. The horizontal components of the ADDS were rotated into the direction of the fiber‐optic cable and then compared with the observed DAS strain rates. From three example regional earthquakes of local magnitudes 2.9, 4.1, and 4.3, the ADDS are found to be coherent with DAS for frequencies ≤1 Hz. For frequencies >1‐Hz, this correlation decays quickly. Small differences between linear and areal dynamic strains at 1‐Hz suggest poor signal‐to‐noise or localized strain that is perturbed by shallow heterogeneities compare to the average strain propagating across the geophone array. The implication is that around 1‐Hz, straight fiber DAS is measuring axial strain along the fiber and can provide good approximations to translational particle motions. However, above 1‐Hz, DAS becomes more sensitive to shallow velocity gradients that can be beneficial for geophysical imaging yet becomes a limitation for traditional seismic analysis methods depending on absolute amplitude and phase from translational particle motions. Plain Language Summary: Seismological measurements including ground displacement are based on the motion at a point in 3‐dimensional space through time. Fiber‐optic distributed acoustic sensing (DAS) instead measures strain, a component of deformation that is the difference in motion between two points in space through time. We know that strain is much more sensitive to shallow Earth structure beneath the receiver and surrounding topography than traditionally measured particle motions. We examine the variability of surface strain along a line versus the average strain over an area and also how these variabilities change over both frequency and spatially across the extent of an array. We show that the strain over an area versus axial strain measured from DAS varies in amplitude and phase for several frequency bands. These results have implications on how methods developed in seismology based on traditional seismometers can be used with fiber‐optic DAS data analysis. Key Points: Array derived dynamic strains (ADDSs) are in agreement (correlation coefficients ∼0.9) with distributed acoustic sensing (DAS) strain rate for f ≤ 1 HzFor f > 1 Hz, the correlation between DAS and array derived strains decays quicklyThe root‐mean‐square amplitude ratios between ADDS rate and DAS strain rate is ∼1 below f ∼ 2 Hz but increases with frequency [ABSTRACT FROM AUTHOR]