Understanding sensitivity of distributed acoustic sensing integrated with velocity data

Abstract:

Distributed acoustic sensing (DAS) systems measure axial strain (or strain rate), which has significantly different sensitivity to seismic waves than particle velocity or acceleration measured by geophones, nodes and many other sensing technologies. These changes affect polarity and amplitude responses, and the changes differ for longitudinal and shear wave motions. We see clear effects of this sensitivity in ambient noise interferometry results from DAS arrays, particularly making the detection of Love waves more challenging. However, at corners of arrays, we can reconstruct velocity-equivalent data from DAS data in two directions, enabling horizontal rotations of velocity. Additionally, some experiments may have a limited number of multicomponent seismometers available which could be integrated with DAS data. Factors such as gauge length of the DAS system, wave length of the signal, direction of the signal and orientation and distance between DAS channels and the multi-component velocity measurement control the response of interferometry between these data. Based on these models, we can optimize rotation angles (pre- or post- correlation) to enhance certain wave modes between sensor pairs. These simple models and the flexibility enabled by having some multi-component velocity sensors can help us improve survey design prior to deployment, although there are still some limits on detecting wave modes between certain sensor pairs.