ABSTRACT
The theoretical behavior of P-wave reflection amplitude as a function of incidence angle (offset) indicates that diagnostic changes should be spatially observable when crossing boundaries between different formation fluids. In particular, when free gas is present in porous sand, Poisson’s ratios are known to be abnormally low (usually < 0.1-0.2), whereas for oil and water the ratios are usually much greater than 0.3. If the overlying layer has an impedance and Poisson’s ratio greater than the target layer, the reflection amplitudes will increase with offset, thereby producing bright spots in stacks.
The problems in analyzing seismic amplitudes are well known. The distortions produced in the recorded amplitudes owing to the effects of sources and receivers, instrumentation, processing, attenuation and absorption, layer tuning, multiple interference, and noise can render the measurements meaningless unless corrected for, which would be difficult. However, by using relative instead of absolute amplitudes and by applying suitable analysis techniques, most of these effects can be virtually eliminated. The resulting amplitude behavior can then be properly interpreted in terms of changes in formation properties.
