The ratio vp/vs as a discriminant of composition for siliceous limestones

The ratio of the velocity of compressional waves, Vp, to the velocity of shear waves, Vs, is an important parameter for interpreting geophysical field data. Recent studies have emphasized the role played by pore geometry in controlling Vp/Vs in homegeneous rocks. We measured the carbonate content of a set of siliceous limestones of varying proportions of carbonate and silica and observed the pore structures of these samples using a scanning electron microscope. The range of Vp/Vs of individual samples during increasing confining pressure is consistent with crack-closure theory. However, the value of Vp/Vs within the sample set as a whole is dominated by its carbonate content. Variations in Vp/Vs due to total porosity and pore geometry are around 0.1, whereas the change due to composition is 0.4. Values of pore aspect ratios gained from comparison of the velocity-porosity-composition data with theory are in good agreement with the electron microscope observations.