Elastic Time Reversal Mirror Experiment in a Mesoscopic Natural Medium at the Low Noise Underground Laboratory of Rustrel, France

TL;DR

This study demonstrates that seismic time reversal in a mesoscopic natural medium can effectively refocus energy at source locations, with hyper-focusing achieved due to medium heterogeneity, though mirror roughness affects the process.

Contribution

First experimental demonstration of seismic time reversal in a mesoscopic natural medium with detailed analysis of refocusing and hyper-focusing effects.

Findings

Seismic energy refocuses accurately at shot locations.

Hyper-focusing occurs due to medium heterogeneity.

Mirror roughness degrades reversal quality.

Abstract

A seismic time reversal experiment based on Time Reversal Mirror (TRM) technique was conducted in the mesoscopically scaled medium at the LSBB Laboratory, France. Two sets of 50 Hz geophones were distributed at one meter intervals in two horizontal and parallel galleries 100 m apart, buried 250 m below the surface. The shot source used was a 4 kg sledgehammer. Analysis shows that elastic seismic energy is refocused in space and time to the shot locations with good accuracy. The refocusing ability of seismic energy to the shot locations is roughly achieved for the direct field, and with excellent quality, for the early and later coda. Hyper-focussing is achieved at the shot points as a consequence of the fine scale randomly heterogeneous medium between the galleries. TRM experiment is sensitive to the roughness of the mirror used. Roughness induces a slight experimental discrepancy between recording and re-emitting directions degrading the quality of the reversal process.