- Copyright © 2003 Society of Exploration Geophysicists
In many practical situations when the objective is to differentiate between high and low gas concentrations, using P-to-P (PP) seismic data alone may not be enough to successfully complete the task. The abrupt reduction in P-wave velocity (VP) with the first few percent of gas controls the seismic response. Therefore, usually only the presence of gas, but not the saturation, can be detected with PP seismic. This well known physical phenomena can be modeled by Gassmann's equation, and was documented by Domenico in 1976. In contrast, density (ρ) varies more gradually and linearly with gas saturation, while S-wave velocity (VS) does not vary much. As noted by Berryman et al. (2002), the linear behavior of ρ with saturation makes seismic attributes that are closely related to density useful proxies for estimating gas saturation. Attempting to extract and to use information about rock density from AVO analysis or inversion has not been a successfully robust approach in many cases because of limitations in data quality and type of processing required.
The use of P-to-S (PS) converted waves has been proposed as a possible solution for distinguishing fizz water from commercial gas. Specifically, Wu (2000) and Zhu et al. (2000) suggest using P-to-S reflectivity, which is an interface attribute. Landro et al. (1999) derived the “shear wave elastic impedance” (SEI) that is an intervallic attribute, assuming weak contrast and small incidence angle, using a linear approximation of RPS (reflectivity of converted waves).
In this paper, we present a practical application of a PS converted waves “elastic” impedance (PSEI) formulation not limited to small incidence angles. We show how exploiting the PS AVO behavior, by combining near and mid-to-far offsets PSEI, it is possible to distinguish fizz water from commercial gas concentrations. Using statistical rock physics methods, we compare the …