Two step inversion (tomography and pre-stack migration) for high resolution seismic imaging in crosshole or VSP surveys

Abstract

In the petroleum and the mining industries, more detail about underground structures is often needed than can be determined from surface seismic surveys. Crosshole surveys and vertical seismic profiles are commonly used techniques for determining the velocity distribution between two wells by traveltime tomography. Seismic migration is also a powerful tool for imaging the structures between wells, but it requires velocity information. Neither seismic tomography nor migration alone can achieve an ideal underground image in both velocity and structure. To get a high resolution image both in velocity and structure, we propose a two step inversion which is implemented by tomography and then pre-stack migration. The strategy uses the first arrival times to recover the low spatial frequency velocity components of the medium by tomography, then this velocity model is applied as a migration velocity to pre-stack migration to recover the higher spatial frequency components. As the result, high resolution images both of velocity and structure are obtained by this two step inversion process. For the pre-stack migration, a general Kirchhoff integral is used, based on Huygens' principle, which applies a Green's function by traveltime and amplitude mapping, using the finite difference method. This method handles an arbitrary velocity model (including high velocity contrasts and shadow zones) represented by a grid of velocities. For the tomographic technique, either a constant velocity or a gradient velocity background can be used, depending on the particular case. This two step inversion offers a very useful tool for the petroleum and the mining industry for getting a high quality seismic image. A numerical example which uses a strongly contrasting velocity model for imaging a low velocity anomaly from crosshole survey data is given here using this two step approach.