A simplified workflow for accurate 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. The final stacked seismic volume, if imaged in the depth domain, is stretched to time with the migration velocity, or a smoothed version of it (Jones, 2009), before redepthing with the calibrated velocity model. There are many different techniques used routinely in the industry (see Etris at al., 2001, for a detailed summary), by scaling either 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 because the derivation correction functions 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, interpolated between wells, they could produce unrealistic interval velocity in the presence of thin layers.