Keynote Address: Joint geophysical imaging for fractured reservoirs

Abstract

Seismic and electromagnetic wave fields in the accessible (drillable) earth both respond to changes in rock properties and structure, yet are not usually combined into a single subsurface map that reflect these common changes. Variations in layer thicknesses, folds, faults, fault-related offsets, porosity, fluids, and saturation create anomalies in both fields. The presence of oriented fractures and fabric adds anisotropic responses to both as well. Ideally, both fields would be used to create a map that combines their responses to a sought after property, say porosity, in a single ?joint geophysical image?. The members of the Institute of Earth Science and Engineering are working toward such JGI maps, progress in which is reported on in this presentation. A simple example of JGI is the inversion of high-resolution seismic refraction and magnetotelluric data collected over a simple layer-over-basement structure. Here the common factor is the layer thickness, the value of which is most accurately found forcing the seismic velocity and apparent resistivity models to give the same number. A less simple example is the combined use of seismic travel times and MT resistivity converted to seismic velocity to locate microearthquakes. An even more complicated example is the inversion of shared S-wave-splitting and MT-polarization effects from zones of oriented and fluid-filled fractures. Some of theoretical and practical aspects of these three cases will be discussed, including: (a) data gathering techniques, (b) physical models of the shared properties, especially in the case of fractures and anisotropy, and (c) quantitative methods for combining measurements.