P-WAVE TRAVEL-TIME AND POLARIZATION TOMOGRAPHY OF VSP DATA

The presence of anisotropy requires that tomographic methods be genera lized to account for anisotropy. This generalization allows geological structure to be correctly imaged and allows the anisotropic parameter s to be estimated. Use of isotropic inversion for imaging anisotropic structures gives systematic trends in the traveltime and polarization residuals. However, due to the limited directional coverage, the trave ltimes alone may not be sufficient to study the anisotropic properties of the structure. Polarizations can provide independent information o n the structure. Traveltime and polarization inversion are applied to synthetic examples simulating VSP experiments. Transverse isotropy and 1-D structure are assumed. Plots of traveltime and polarization resid uals are an important tool to detect the anomalies due to the presence of anisotropy. For receivers located in anisotropic layers, polarizat ion residuals display consistent anomalies of several degrees. The syn thetic examples show that even the simple 1-D problem is difficult, wh en using direct arrivals only. Large a posteriori errors in anisotropi c parameters are obtained by traveltime inversion in layers where avai lable incidence angles are less than 45 degrees. Resolution of the tom ographic image of VSP data is greatly improved by a combination of tra veltime and polarization information. In order to obtain accurate inve rsion results, the measurement error of polarization data should be ke pt to within a few degrees.