Relationships between compressional and shear wave velocities were established in the classic paper by Castagna et al. (1985). In particular, linear trends for correlating the compressional and shear wave velocities are presented, including the Castagna mudrock line derived from in-situ sonic and seismic measurements. The work of Brie et al. (1995) extended the work of Castagna et al. (1985) to include predicting gas saturation from a Vp/Vs vs. compressional traveltime crossplot. In this crossplot, Brie proposes one non-linear trend for water-wet sands and another non-linear trend for shales. In addition, the Brie Vp/Vs vs. compressional traveltime crossplot has a greater sensitivity to low velocities or large compressional traveltimes.
This paper compares some of the more famous rock physic trends, including the Castagna mudrock and Brie water-wet sands and shales, in both the Vp vs. Vs crossplot and the Vp/Vs vs. compressional traveltime crossplot. It also presents plots of laboratory data from the Bakken, Bazhenov, Barnett, Eagle Ford, Haynesville, Monterey, and Niobrara organic shales and descriptions of departures from the expected trends to the presence of kerogen and anisotropy.
The paper also includes an investigation of in-situ well log data from the Eagle Ford, Haynesville, and Barnett formations in the context of the Castagna, Brie, and additional crossplots. The Eagle Ford data satisfies the carbonate Vp vs. Vs trend predicted by Castagna, and the Haynesville data clearly satisfies the hydrocarbon/kerogen effect predicted by the Brie Vp/Vsvs. compressional traveltime crossplot.
