Travel-time inversion for an inhomogeneous transversely isotropic medium applied to the SJ-6 seismic profile, south-central California and the TACT seismic profile, southern Alaska

Emine Aysun Boztepe, Purdue University

Abstract

A travel-time inversion technique is presented for determination of the 2-D horizontal and vertical velocities and depths to interfaces from seismic reflection/refraction (near vertical /mostly horizontal raypaths) data sets. The inversion technique is based on two major assumptions. First, the seismic velocity structure is two dimensional transversely isotropic. Second, the anisotropy can be approximated by an elliptical formula which is accurate for degrees of anisotropy up to about 20 percent. Synthetic test model studies demonstrate that the method is effective if raypath coverage within the model is adequate and indicate that horizontal velocities and depths to interfaces are better resolved than vertical velocities. The travel-time inversion technique presented in this study is applied to two seismic profiles: the SJ-6 seismic profile, south-central California and the TACT seismic profile, southern Alaska. In the derived SJ-6 velocity model, between SP5 and SP6, the resolutions of the model parameters for deeper parts of the velocity structure are low since shot point spacing (about 25 km) isn't sufficient to provide a complete raypath coverage. The results of anisotropic inversion for the TACT seismic reflection/refraction profile (South Richardson Highway) between SP19 and SP11 suggest that the upper 5 km of the crust is approximate transversely isotropic. The range of degree of anisotropy for the derived model is between 10 and 25 percent, consistent with the range determined in previous studies and from laboratory measurements. The cause of anisotropy is preferred mineral orientations in the foliation planes of the metasedimentary rocks. Both isotropic and anisotropic velocity model inversions were performed on the TACT travel-time data. Although field geological information and laboratory measurements of seismic velocity indicate that near surface rocks along the profile are anisotropic, the travel-time data are only slightly better matched by the anisotropic model. In conclusion, it is suggested that future anisotropic inversion studies will require particularly high quality data with dense raypath coverage in which multiple sources and receivers and both reflection and refraction data are utilized. In velocity structures, which are approximately transversely isotropic, such data can be used to accurately determine anisotropic and heterogeneous seismic velocity distributions.

Degree

Ph.D.

Advisors

Braile, Purdue University.

Subject Area

Geophysics

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