The virtual source method: Theory and case study

We present a way to image through complex overburden. The method uses surface shots with downhole receivers placed below the most complex part of the troublesome overburden. No knowledge of the velocity model between shots and receivers is required. The method uses time-reversal logic to create a new downward-continued data set with virtual sources (VS's) at the geophone locations. Time reversal focuses energy that passes through the overburden into useful primary energy for the VS. In contrast to physical acoustics, our time reversal is done on a computer, utilizing conventional acquisition with surface shots and downhole geophones. With this approach, we can image below extremely complex (realistic) overburden - in fact, the more complex the better. We recast the data to those with sources where we actually know and can control the waveform that has a downward-radiation pattern that may also be controlled, and is reproducible for 4D even if the near-surface changes or the shooting geometry is altered slightly. To illustrate the method, we apply the VS technique to a synthetic, elastic example with extreme heterogeneity, where conventional approaches fail to image the data. A 4D fielddata example shows that the VS method (VSM) enables sensitive reservoir monitoring below a complex, time-variant near surface that is not achievable with surface 4D seismic or conventional 4D vertical seismic profiling (VSP).