Seismic imaging of the shallow subsurface with high frequency seismic measurements.

摘要

Elastic wave propagation in highly heterogeneous media is investigated and theoretical calculations and field measurements are presented. In the first part the dynamic composite elastic medium (DYCEM) theory is derived for one-dimensional stratified media. A self-consistent method using the scattering functions of the individual layers is formulated, which allows the calculation of phase velocity, attenuation and waveform. In the low frequency limit the self-consistent formulation is consistent with the Reuss average and in the high frequency limit it yields the correct ray theory average velocity. The comparison with complete numerical solutions shows that the DYCEM theory predicts the coherent wave through randomly layered media. In the second part the DYCEM theory has been generalized for three-dimensional inclusions. The specific case of spherical inclusions is calculated with the exact scattering functions and compared with several low frequency approximations. Spectra and waveforms for materials with solid and liquid inclusions in a solid matrix are presented. The results show that the exact scattering functions are required to adequately describe wave propagation at all frequencies. In the third part log and VSP data of partially water saturated tuffs in the Yucca Mountain region of Nevada are analyzed. The anomalous slow seismic velocities can be explained by combining self-consistent theories for pores and cracks. The effective matrix velocities in the studied tuffs deviate strongly from the individual mineral velocities. This effect may be due to the presence of two dimensional inhomogeneities like cracks and grain contacts. The fourth part analyzes an air injection experiment in a shallow fractured limestone, which has shown large effects on the amplitude, but small effects on the travel time of the transmitted seismic waves. The large amplitude decrease during the experiment is mainly due to the impedance contrast between the small velocities of gas-water mixtures inside the fracture and the formation. The slow velocities inside the fracture allow an estimation of aperture and gas concentration profiles. The aperture estimates range from less than one millimeter to a few millimeters, which is comparable to previous tracer tests.