Shear Wave Splitting Inversion in a Complex Crust

Abstract

Shear wave splitting (SWS) inversion presents a method whereby the upper crust can be interrogated for fracture density. It is caused when a shear wave traverses an area of anisotropy, splits in two, with each wave experiencing a different velocity resulting in an observable separation in arrival times. The current body of work on linear SWS inversion utilises an equation that defines the time delay between arriving shear waves with respect to fracture density. This equation makes the assumption that no fluid flow occurs as a result of the passing shear wave, a situation called squirt flow. This thesis shows that the assumption is not applicable in all geological situations. When it is not true, its use in an inversion produces a result which is negitively affected by the assuptions. This is shown to be the case at the test case of 6894 SWS observations gathered in a small area at Puna geothermal field, Hawaii. To rectify this situation, a series of new time delay formulae, applicable to linear inversion, are derived from velocity equations presented in the literature. The new formula use a ‘fluid influence parameter’ which indicates the degree to which squirt flow is influencing the SWS. It is found that accounting for squirt flow better fits the data and is universally applicable. The fluid influence factor that best describes the data can be identified prior to solving the inversion. Implementing this formula in a linear inversion has a significantly improved fit to the time delay observations than that of the current methods. In an extension to inverting for crack density alone, a method of jointly inverting for crack density and fluid influence parameter is proposed. This is tested on synthetic data and applied to the Puna data set, with the result that spatially independent models of each can be resolved, and a slightly better data fit achieved.