Detecting seismic sources by focusing wavefronts in depth

What is it about?

In controlled-source seismology, receivers are available in abundance and collective properties of the recorded wavefield can be identified and traced in the form of wavefronts. In passive-source seismology, dense receiver networks likewise become more and more available, which makes the concept of wavefronts likewise usable in situations where the source location and time of excitation are unknown. Based on coherence measurements adapted from controlled-source seismology, we suggest a fully unsupervised strategy that allows for a joint source location, excitation time, and velocity inversion with the only requirement of dense seismometer / geophone coverage at the surface.

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Why is it important?

Passive seismic event detection is commonly performed on individual traces and the location of a source is estimated by assuming to at least have a crude idea about the in-situ seismic velocity structure. The presented strategy systematically investigates how waveforms and traveltimes change over an ensemble of traces by means of coherence arguments and, thereby, allows for a velocity-independent estimation of the excitation time of the passive seismic source. Inspired by geometrical optics (the traversed subsurface acts as a lens), the source location and the velocity structure can jointly be inverted for by back-projecting and focusing the detected wavefronts in depth.