Noncontacting laser-based acousto-seismics at the laboratory scale: towards near-real-time monitoring of granular analogue models

Abstract

Summary In this work we present a novel, experimentally-efficient setup for performing noncontacting laser vibrometry on geologic materials and their analogues. We show it is possible to acoustically monitor a granular material experiment in real time compared to the typical timescale of analogue modeling experiments. We acquire noncontacting waveform data with consistently high signal-to-noise-ratio (SNR). Compared to previously used standard contacting transducers, the novel joint use of sources and receivers that are both laser-based resulted in measured signals with improved waveforms and temporal bandwidths. These data acquisition improvements, in our case where surface waves are prominent in the data, enable enhanced multichannel surface wave processing, e.g., in terms of reliable dispersion curve estimates. We find, given the high waveform fidelity of our acquisition system, that the observed surface waves are highly sensitive to relatively small changes in in the medium’s elastic properties, making them a demonstrably reliable to monitor any processes that affect elasticity in these models in near-real time. As a demonstration, we continuously monitor a scaled analogue model containing granular glass beads. By continuously monitoring - i.e., performing repeatable active-source acousto-seismic surveys at short time-lapse intervals - over a period of ten hours, we find that an increase of relative humidity of 10% can lead to as much as a factor of two increase in surface wave group velocities. Finally, we discuss future applications of the developed method by considering surface wave inversion for fault and stress monitoring during the deformation of a model.