Full-waveform reconstruction of microseismic events via observations of acoustic pressure in the surrounding fluid

Abstract In this work, we explore the possibility of reconstructing microseismic events in a solid body, in terms of both event locations and respective seismic moment tensors, using full-waveform observations of acoustic pressure fluctuations generated in the surrounding fluid. Our developments are motivated by the prospect of locating and characterizing, via hydrophone measurements, reaction-driven cracking in (coastal or seabed) geologic formations underpinning mineralization-driven, permanent carbon storage in the subsurface. On representing the microseismic events as dipole and double-couple point sources in the solid, the prototype (two-dimensional) analysis is reduced to an inverse source problem that deploys (i) time-harmonic forward map for the solid–fluid system; (ii) combinatorial search over a prescribed grid spanning the source region; (iii) global and local grid refinements catering for the stability and veracity of source reconstruction, and (iv) multi-frequency components of the acoustic pressure traces observed in the surrounding fluid. Numerical simulations demonstrate the feasibility of reconstructing multiple (contemporaneous) microseismic events, in terms of both their locations and respective seismic moment tensors, using a modest number of acoustic pressure observations in the fluid. The simulations highlight the importance of observation aperture on the quality of source reconstruction, an item that features two modalities: (a) spatial—manifested via the number and location of pointwise sensors (hydrophones), and (b) temporal—synthesized via the number and linear independence of multi-frequency observations. The two aperture modalities are found via numerical simulations to work in unison in that the scarcity of one can, to a degree, be compensated by the abundance of another. In a physical setting, (a) is controlled by the sensing setup, while (b) hinges on the spectral bandwidth of microseismic events.