V p V s and lithology

Published laboratory investigations suggest an association exists between the ratio of seismic compressional and shear-wave velocities (V p /V s ) and sedimentary rock lithology. Comparisons of some theoretical models with these laboratory studies suggest that crack, or pore, geometry has a stronger effect on observed V p /V s values than elastic constants of the minerals comprising the matrix. Further, it can be inferred that the observed association between lithology and V p /V s is a result of an association between lithology and distribution of pore and crack shapes. Direct observation of crack shapes for a variety of lithologies is a next step in strengthening these inferences. The present study reviews the empirical relations between V p /V s and lithology and examines two published theoretical models of cracked elastic media. The models suggest that seismic velocities of sandstones may be controlled by cracks and pores with aspect ratios in the range of 10 (super -1) to 1, dolomite in the range of 10 (super -2) to 10 (super -1) , and dense limestones, of generally low porosity, in the range of 10 (super -3) to 10 (super -2) . Direct observations of the aspect ratio of cracks and pores in sedimentary rocks would test these inferences and offer a basis for physical and geologic insight into lithologic interpretations of V p /V s ratios.