Time-lapse Monitoring with Coda Wave Interferometry

Alexandre A. Grêt

Alexandre A. Grêt, Time-lapse Monitoring with Coda Wave Interferometry, Ph.D. Thesis, Colorado School of Mines, Golden, Colorado, 2004.

The coda of seismic waves consists of that part of the signal after the directly arriving phases. In a finite medium, or in one that is strongly heterogeneous, the coda is dominated by waves which have repeatedly sampled the medium. Small changes in a medium which may have no detectable influence on the first arrivals are amplified by this repeated sampling and may thus be detectable in the coda. We refer to this use of multiple-sampling coda waveforms as coda wave interferometry. We have exploited ultrasonic coda waves to monitor time- varying rock-properties in a laboratory environment. We have studied the dependence of velocity on uni-axial stress in Berea sandstone, the temperature dependence of velocity in granite and in aluminum, and the change in velocity due to an increase of water saturation in sandstone. Furthermore, We applied coda wave interferometry to seismic data excited by a hammer source, collected at an experimental hard rock mine, Idaho Springs, CO. We carried out a controlled stress-change experiment in a pillar and we were able to monitor the internal stress change. We used coda wave interferometry to monitor temporal changes in the subsurface of the Mt. Erebus Volcano, Antarctica. Mt. Erebus is one of the few volcanoes known to have a convecting lava lake. The convection provides a repeating seismic source producing seismic energy that propagates through the strongly scattering geology in the volcano. There are many other possible applications of coda wave interferometry in geophysics, including dam and nuclear waste deposit monitoring, time-lapse reservoir characterization, earthquake relocation, stress monitoring in surface mining and rock physics.


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