Obtaining reliable S-wave velocity depth profile by joint inversion of geophysical data: the combination of active surface-wave, seismic refraction and electric sounding data

The inversion of active surface-wave data is highly affected by the non-uniqueness of the solution. While a deterministic approach is generally chosen due to certain advantages, there is the risk of getting trapped in a local minima, especially when no a priori information is available about the sub-surface geometry since the layer thickness is assumed fixed to a priori. The fixed-layer thickness in a deterministic inversion of the active surface-wave raises significant issues, such as the relevance of the initial model geometry (the thickness of the intermediate layer and the total depth of the initial model) and the equivalence problems. Thus, the inversion result is inherently not reliable, even in the case of the normal dispersion medium, and the result could be unachievable in challenging sub-surface situations. These issues could be reduced by using a joint inversion approach. The present paper first presents examples of issues through four case histories in Trabzon, Turkey. Then, two joint inversion approaches based on local search are carried out to handle the issues concerning individual inversion. The first approach combines active surface-wave data with electric sounding data and the second includes travel times from seismic refraction data. In addition, an independent inversion is carried out with a neighbourhood algorithm for a global search to compare against the joint inversion results. The joint inversion schemes clearly reduce the ambiguities of the individual inversion of the active surface-wave data, and the dependence on the initial model regarding the layer thickness is also mitigated. Moreover, the joint inversion approach provides an estimate of the complementary model parameters, namely electrical resistivity and the compressional wave velocity. It is shown that the proposed joint inversion approaches provide consistent results with former boreholes, seismic tomographic profiles and the known geologic features of the study area even in the absence of any a priori information.