Joint inversion of refraction and gravity data for the three-dimensional topography of a sediment-basement interface

The inversion of traveltimes from refraction experiments sometimes encounters the ray coverage problem. Irregular station distribution and ray shadow zones may lead to poor ray coverage in some regions of a complex underground structure. To overcome this problem, we propose a joint inversion of refraction and gravity data since gravity surveys are carried out densely and homogeneously. An interface separating homogeneous sediments and basement rocks is parametrized by a set of knot points with Lagrange interpolation. Densities are related to slownesses through experimental formulae. Model regularization is used to avoid oscillatory artefacts in the solution and to minimize the non-uniqueness of the solution. The validity of the formulation is verified using synthetic data for a valley-like basin with pseudo-random noise. The proposed method is successfully applied to actual data obtained in and around the Osaka Basin, Japan. To prevent solution instabilities owing to a lack of data beneath Osaka Bay, the interface under the bay is constrained to the depth interpretation of seismic reflection profiles. The result shows good agreement with the geological setting in this region.