High resolution time-to-depth conversion using 3D Grid Tomography

Abstract

Time-to -depth conversion is the process where seismic velocities are scaled accordingly to check shots and well logs so that geological markers and seismic horizons match in depth. Seismic data, if in depth, are stretched to time with the migration velocity before re-depthing with the calibrated velocity model. There are many different techniques that are routinely used in the industry either by scaling average velocities or interval velocities with some derived correction functions. These functions can be simple scalars, depth variant scalars or more complex time and space variant functions which need to be interpolated between wells. In any case the depthing process is quite laborious since the derivation of correction function for each well is time consuming. The application of these correction functions may produce results that look somehow discontinuous, with jumps in velocity at the horizons where the corrections have been applied, and, when working on average velocities, they could produce unrealistic interval velocity in the presence of thin layers. Since most of today seismic projects are run through PSDM, an elegant way to derive the velocity volume for time-to-depth conversion is to use the same tool used for PSDM velocity model building, the 3D Grid Tomography. In this paper I show how this technique greatly simplify the depthing process producing a velocity model that ties all the markers for all wells within a velocity depth sample.