3: Rock Physics Measurements
To interpret geologic significance from P-wave and S-wave velocity variations within the earth an understanding of how individual rock properties each affect these parameters is necessary. There are several methods to obtain such information. The straightforward method is to take a sample of a rock into the lab and measure the rock's S- and P-wave properties. These measurements are typically made using crystal transducers which operate at frequencies from several tens of kilohertz to the megahertz range. Researchers have been making such measurements for decades, and an enormous body of literature describing the results exists. Many of the experiments, however, do not adequately examine details of the pore fluid properties or the morphology of the porosity and so are of limited use in exploration geophysics. Another important source of velocity information comes from well logging. Acoustic, electric, and nuclear logs of sedimentary rocks have been extensively studied. Well-logging sondes that measure formation velocities typically use sonic frequencies on the order of a few tens of kilohertz. Reliable S-wave velocity measurements in wells, however, have become available only in the last few years; consequently, the number of wells with both S- and P-wave data is rather limited. Finally, vertical seismic profiling (VSP) surveys relate P- and S-wave velocities and attenuations to rock properties at frequencies (tens of Hertz) similar to those used in surface seismic surveys. While VSP data are clearly an advantage in relating surface and borehole measurements, the low frequencies (and hence long wavelengths) limit resolution of fine details of the subsurface rock properties.
Virtually any change in the structure or composition of a solid will induce some change in the propagation characteristics of elastic waves. Figures 18 and 19 qualitatively depict the variations in P- and S-wave velocity, and their ratio, Vp/Vs, resulting from a change in composition or other property of the rock. One constituent or property is varied in each column of this diagram, and all other properties are assumed to remain fixed. Clearly, several kinds of variations can produce changes in Vp, Vs, and Vp/Vs of about the same magnitude or sign. Thus, the interpretative value of any given parameter is ambiguous when considered singly and independent of the other values. Hence, elastic wave velocities, and their ratio, are not unique as a single diagnostic parameter of rock properties. Further complications arise when, as has been noted, a change in one rock property often is accompanied by variations in one or more other rock parameters (e.g., porosity and permeability). Therefore, a comprehensive interpretation strategy must be developed that uses all available information. Some of the parameters that need to be considered are Vp, Vs, Vp/Vs, prestack and poststack reflection amplitudes, attenuation, anisotropy, well-log information, and regional geological data. A detailed discussion of the synthesis of these various data into an interpretation is deferred to Chapter 6.