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Direct Detection of Mobile Hydrocarbons – Converted Shear Waves

Kandiah Balachandran

B & M Geophysical Research Company, Inc., Kalamazoo, MI 49009-8015 and

Kalamazoo Valley Community College, Kalamazoo, MI 49003-4070

Converted shear waves are clearly seen at large offsets in conventional seismic reflection profiling using 3-component seismometers. However, at normal or near normal incidence it is unusual. One such observation where the horizontal component signal was more pronounced than the corresponding vertical component prompted this inquiry. Possible explanations are scattering and anisotropy. A different possibility considered here is the conversion due to lateral motion of fluids in the reservoirs. This lateral motion of fluids induces shearing forces on the matrix due to a combination of viscous drag and pressure differences.

Compressional waves were induced in a slab of aluminum bonded to a Berea sandstone block and reflected waves, both compressional and shear, were recorded for varying offsets. In the first run the rock was dry. In a repeat run the rock was wetted with water. In one of these experiments significant differences between the observed shear wave amplitudes and those predicted by theory were observed.

Another experiment investigated the influence of fluids on the transmitted wave. In this experiment compressional and shear motion in two orthogonal directions were induced on one face of the core sample and for each set-up all three motions were recorded on the opposite face. The data suggests that the converted shear is enhanced by the presence of fluids, in this instance water. These results are preliminary and further work is in the planning stages.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York