On the precision of noise correlation interferometry

TL;DR

This paper develops a theoretical expression to distinguish true wave speed changes from apparent waveform dilations caused by source variations in noisy waveforms, validated with ultrasonic and seismic data.

Contribution

It introduces a precise method to differentiate actual wave speed changes from source-induced effects in noise correlation measurements.

Findings

The derived expression accurately separates wave speed changes from source effects.

Validation with ultrasonic data shows high precision in detecting true wave speed variations.

Seismic data analysis confirms the method's applicability to real-world geophysical monitoring.

Abstract

Long duration noisy-looking waveforms such as those obtained in randomly multiply scattering and reverberant media are complex; they resist direct interpretation. Nevertheless, such waveforms are sensitive to small changes in the source of the waves or in the medium in which they propagate. Monitoring such waveforms, whether obtained directly or obtained indirectly by noise correlation, is emerging as a technique for detecting changes in media. Interpretation of changes is in principle problematic; it is not always clear whether a change is due to sources or to the medium. Of particular interest is the detection of small changes in propagation speeds. An expression is derived here for the apparent, but illusory, waveform dilation due to a change of source. The expression permits changes in waveforms due to changes in wavespeed to be distinguished with high precision from changes due to other reasons. The theory is successfully compared with analysis of a laboratory ultrasonic data set and a seismic data set from Parkfield California.