t s t c ratio in sand formations and the t s t c ratio in shale formations equation 4 can be rewritten in a material balance form as where v sh represents the volume of shale in the formation and 1 v sh represents the volume of sand in the formation the shaliness indicator v sh can be obtained from logging measurements well known in the art such as from a natural gamma ray instrument of the type generally illustrated in u s pat no 4 271 356 which is incorporated herein by reference in normally compacted shaly sediment sequences shale bulk density and porosity increase with depth of burial and the compressional wave to shear wave travel time ratio decreases in value relating these behavior in a linear fashion they may be expressed as where u03c6 is the formation porosity as provided from conventional logging data such as well known methods of acoustic logging and u03c1 sh is the bulk density of the shales as provided from conventional logging data such as well known methods of density logging as illustrated in u s pat no 3 321 625 which is incorporated herein by reference studies detailed in 34 acoustic character logs and their applications in formation evaluation 34 authored by g r pickett and published in.