Time-lapse travel time change of multiply scattered acoustic waves

Abstract

Existing techniques in correlation spectroscopy, such as coda wave interferometry and diffusing acoustic wave spectroscopy, determine the average motion of scatterers or change in the propagation velocity from the temporal change of multiply scattered sound. However, neither of them gives an indication of the spatial extent of the change in the medium. This study is an extension of the technique coda wave interferometry, where multiply scattered waves are used to determine the change in the wave field due to a localized perturbation in the propagation velocity. Here, the propagation of multiply scattered sound is described using the diffusion approximation, which allows the cross-correlation function of the unperturbed and perturbed wave fields to be related to the localized change in the propagation velocity. The technique is tested numerically for two-dimensional ͑2D͒ acoustic waves using synthetic seismograms calculated using finite-differences before and after a small perturbation in the propagation velocity has been introduced. Despite the relatively small size and magnitude of the change, multiple scattering greatly amplifies small perturbations, making changes in the phase or travel time of the wave field visible in the later-arriving waveforms. Potential applications of this technique include nondestructive evaluation of inhomogeneous materials and time-lapse monitoring of volcanoes and highly heterogeneous reservoirs.

Field observations of seismic velocity changes caused by shaking-induced damage and healing due to mesoscopic nonlinearity

“…We calculated 2-D autocorrelation sensitivity kernels according to Pacheco & Snieder (2005) assuming an average S velocity of 1000 ms −1 for the near surface and 500 m for the scattering meanfree path. The transport mean-free path characterizes the scattering strength of the medium and is based on studies in volcanic areas (Wegler & Lühr 2001;Yamamoto & Sato 2010) that we think are suitable for the type of material in the shallow subsurface at PATCX.…”

Local time in diffusive media and applications to imaging

“…The achieved sensitivity is so strong that a single change on one scatterer among thousands of others can be detected. Small relative velocity changes, of the order of 10 −5 , have also been measured using a technique based on the diffuse part of acoustic records [3,4] or the late part of seismograms, the seismic coda [5,6]. These results have been obtained by measuring the time shifts between waveforms that maximize their correlations.…”

“…For elastic waves, this holds when waves are in the equipartition regime, which is a consequence of diffusion. The coda wave interferometry technique (CWI) uses time stretches on the wave field records to measure small velocity changes [5]. The LOCADIFF technique uses the correlation losses and produces images of the changes of scattering cross-section, that is to say the structural changes [7,8].…”

MIIC: Monitoring and Imaging Based on Interferometric Concepts

“…The only theoretical approaches to the problem of locating velocity variations via travel time perturbations in the seismic coda were presented by Pacheco and Snieder (2005) for diffuse waves and by Pacheco and Snieder (2006) for single scattered waves. Brenguier et al (2008b) used a heuristic regionalization concept that is not based on the physics of wave propagation to assign detected travel time changes to some region.…”