Date of Final Oral Examination (Defense)

5-2012

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Geophysics

Department

Geosciences

Major Advisor

Kasper van Wijk, Ph.D.

Abstract

Seismologists continually work to improve images of the Earth's interior. One new approach is seismic interferometry, which involves cross-correlating the seismic wave field recorded at two receivers to generate data as if one of the receivers was a source. Over the past decade, seismic interferometry has become an established technique to estimate the surface-wave part of the impulse response between two receivers; however, practical limitations in the source-energy distribution have made body-wave recovery difficult and causes spurious energy in the estimated impulse response. Rather than suppress such spurious energy, it can be useful to analyze coherent spurious events to help constrain subsurface parameters.

With this in mind, we examine a particular spurious event we call the virtual refraction. This event comes from cross-correlating head-wave (or critically refracted) energy at one receiver with reection and refraction energy at the other receiver. For this particular spurious event, we find that, similar to surface waves, the important part of the source-energy distribution is readily available. The sources need to be at or past the critical offset from both receivers. In a horizontal, two-layer subsurface model, the slope of the virtual refraction defines the velocity of the fast layer (V2). Furthermore, the stationary-phase point in the correlation gather defines the critical offset, a property that depends on the thickness (H) and velocity (V1) of the slow layer.

A two-layer numerical example is presented to illustrate the origin of the virtual refraction. After estimating the refractor velocity, a semblance analysis can be used to estimate H and V1. In field data from the Boise Hydrogeophysical Research Site, the virtual refraction alone is used to the estimate H, V1, and V2. This is an improvement over methods that rely on several wave types to fully characterize seismic properties above and below an interface. An exploration-scale active source seismic data set illustrates how we can use the method to build near-surface seismic models that can then be used for statics estimation in standard reection processing. Finally, we investigate multi-component seismic interferometry for the virtual refraction, a technique that has recently been developed to more accurately estimate the surface-wave impulse response with higher signal-to-noise than traditional single component estimates. We find that using multi-component correlations to estimate shear wave virtual refractions also improves signal-to-noise, but with a dependence on the incidence angle of the incoming wavefield.

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