High Resolution In Saturated Sediments - A Case For Shear Wave Reflection

“High Resolution Reflection” all too often carries with it the implied meaning of “High Frequency<br>Reflection.” As the resolving power of a reflected wave is a function of wavelength and not merely frequency, high<br>frequency in and of itself does not exactly define high resolution. While Poisson Ratio (u) in sedimentary and hard<br>rock masses hovers in the 0.25-0.33 range, within saturated sediments, (T approaches 0.45-0.49. What this means,<br>in effect, is that the ratio of compressional (P) to shear (S) wave velocity in such loose media is in the range of 5: 1<br>up to 10: 1. In terms of wavelengths for the w frequency input wavelet, an S wave will have wavelengths of<br>from l/10 - l/5 that of a P wave in the same medium. This is a direct increase in resolution by a factor of 5-10.<br>Under many circumstances, time is better spent using shear waves for reflection than in developing and employing<br>exotic higher frequency compressional sources which may only increase resolution a factor of two over more<br>standard seismic sources.<br>Example S wave reflection studies relating to stratigraphic delineation and growth fault studies from the<br>US Gulf Coast, Southeast US, and Eastern US areas illustrate the type of resolution obtained with minimal,<br>additional field effort.