Important mechanical properties of rocks and soils can be calculated from the shear-wave velocity, V s , the compressional-wave velocity, V p , and the density or from (V s /V p ) ratios and the density. Therefore, determination of (V s /V p ) ratios is an important tool to assist with lithological identification and many associated applications. In most geologic settings, mixtures of different materials are quite common, and one sequence that is important to understand is the effect of thin layers on measured (V s /V p ) ratios. An equation is derived for (V s /V p ) ratios in transversely isotropic materials. These ratios are tested as a possible aid for identifying and assessing organic contamination in porous rocks. It is found that even small amounts of a second component, such as 10% shale or 10% oil-saturated sandstone, can change (V s /V p ) ratios significantly from that of a pure end member, such as clean sandstone. Various examples using real data from a variety of geological settings illustrate some practical applications to environmental problems dealing with organic contamination.