Stability of Monitoring Weak Changes in Multiply Scattering Media with Ambient Noise Correlation: Laboratory Experiments

TL;DR

This study demonstrates that passive monitoring of weak changes in scattering media via ambient noise correlation is feasible without reconstructing the Green function, relying only on stable background noise.

Contribution

It shows that Green function reconstruction is not necessary for passive monitoring, expanding the applicability of ambient noise correlation techniques.

Findings

Temperature changes can be detected without Green function reconstruction.

Stable background noise sources are sufficient for monitoring.

Passive correlation methods are effective in laboratory scattering media.

Abstract

Previous studies have shown that small changes can be monitored in a scattering medium by observing phase shifts in the coda. Passive monitoring of weak changes through ambient noise correlation has already been applied to seismology, acoustics and engineering. Usually, this is done under the assumption that a properly reconstructed Green function as well as stable background noise sources are necessary. In order to further develop this monitoring technique, a laboratory experiment was performed in the 2.5MHz range in a gel with scattering inclusions, comparing an active (pulse-echo) form of monitoring to a passive (correlation) one. Present results show that temperature changes in the medium can be observed even if the Green function (GF) of the medium is not reconstructed. Moreover, this article establishes that the GF reconstruction in the correlations is not a necessary condition: the only condition to monitoring with correlation (passive experiment) is the relative stability of the background noise structure.