Near-surface model reconstruction using pseudo well-surface simultaneous travel time tomographic inversion

The static correction of a near-surface model may be improved by using travel time tomographic inversion. We discuss unfavorable factors in the inversion of surface seismic waves that have been analyzed by the first break. These factors show that sources and geophones arranged on the surface, or close to the surface, give a first break that only includes the direct wave and the up going wave from the down going to up going transition. These up going waves have weak directivity when they arrive at a geophone and so the rays passing through the grids have small directional differences and a narrow azimuth. Drawing lessons from the advantages of Vertical Seismic Profiling (VSP) acquisition mode we describe a pseudo well-surface simultaneous travel time tomographic inversion of a near-surface model. The well depth should be increased in the surface seismic study to produce a pure up going wave, to enhance the verticality of the rays and to increase the azimuth and shorten path length of the rays. Simulations of the effect of well depth on a pseudo well-surface simultaneous travel time tomographic inversion model are reported. The results show that the static corrections are improved significantly when the well depth extends below the weathered or sub-weathered layers. The root mean square error of the statics is 1.14 or 0.93 ms for these two situations, respectively.