One of the more significant technologies that have emerged in the last forty years has been the use of the seismic shear wave (S-wave), including its combined use with the seismic compressional wave (P-wave). Combined and simultaneous interpretations of P-wave and S-wave data can improve subsurface images and estimates of reservoir and fluid properties. The improvement is possible because P-waves and S-waves respond differently to solids and fluids. However, technical development of this important technology is currently limited because the effectiveness of the techniques cannot be fully tested. This article proposes a redefinition of the Vp/Vs ratio that makes it more useful for exploration and research. In searching for this new definition, other interesting and new understanding of the Vs and Vp relationship emerged, which are presented herein.
The oil and gas industry witnessed a surge in the applications of combined P-wave velocity (Vp) and S-wave velocity (Vs) after Pickett (1963), using laboratory data, observed that the Vp/Vs ratio could be a lithology indicator. He suggested values of the ratio to be 1.9 for limestone, 1.8 for dolomite and 1.6 to 1.7 for sandstone. Pickett also stressed the effect of porosity changes on these values, especially for sandstone. Other potential applications that Pickett predicted for combined use of Vp and Vs included refinement of porosity predictions by the use of Vs, improvement of fracture detection due to a greater reduction in S-wave amplitude than P-wave amplitude by fractures, quality control of transit-time logs, and improvement of cement bond evaluations.
Industry’s response to Pickett’s work has been extensive research, concentrating efforts in the applications of the Vp/Vs ratio. More recent activities in the acquisition and applications of multicomponent data is also an indication of the importance the industry places in the use of combined Vp and Vs for reservoir characterization. Based on the work of subsequent geoscientists, it has become accepted that correlations exist between the Vp/Vs ratio, lithology and porosity (Tatham, 1982; Domenico, 1984). Others have adapted the Vp/Vs ratio in reflection seismic data for hydrocarbon identification and stratigraphic interpretations (Tatham et al, 1976; McCormack, 1985).
Other studies have reported general relationships of Vp and Vs for specific lithologies; for example, Han et al (1986) studied Vp and Vs relationships in clastic rocks, including the effects of clay. Rafavich et al (1984) showed relationships between Vp, Vs and the petrographic character of carbonate rocks. Research is continuing in the area of fracture prediction and detection by studying multi-component data. Among present users of the Vp/Vs technology, however, results have not been consistent or predictable. The Vp/Vs ratio cannot accurately determine lithologies and porosities. Attempts to predict pore fluids have not been consistently fruitful. These predicaments have caused some authors to caution against indiscriminate use of the Vp/Vs ratio because of considerable overlaps that exist in the range of Vp/Vs values reported in geophysical literature. .For the past 40 years that the industry has been engaged in research and applications of the Vp/Vs ratio, nobody has questioned what the Vp/Vs ratio really is.
