Joint 3D inversion of SWD and surface seismic data

In this paper, we present some results obtained in Italy, in a region characterized by complex geological structures. We jointly inverted direct and reflected arrivals of a SWD survey. The proposed approach provides two important benefits: first, the local resolution and reliability increases in the whole illuminated area; secondly, the error sensitivity in the reflector depth is smaller than in the interval velocity field of the adjacent layer. In this way, we get a better estimate of the lithologic properties in the rocks around the well.

The joint inversion of direct and reflected arrivals provides a reliable tomographic image in a walk-away VSP, in the depth interval where the ray paths of both wave types intersect the well. In a SWD survey, this information can be used to predict major lithological changes ahead of the bit. In the depth range where the SWD survey is missing or partial, we can improve the resolution and reliability of the estimated depth model by jointly inverting the SWD data and some conventional surface profile. The reflected arrivals are necessary for deciding the interfaces structure, and increase also the illuminated area. The null space energy, obtained from the singular value analysis of the tomographic matrix, is a minimum when all available wave types are inverted together. We processed a real SWD and a surface profile from Southern Italy. To get a quantitative estimate of the velocity-depth errors, we carried out an 'a posteriori' evaluation of the tomographic image. We found that the errors in the reflector depths are smaller than those in the velocity field, because of the averaging approach we adopted for estimating the layer interfaces.