Abstract: Methods and related apparatus are described for determining lithological characteristics, such as formation material type and porosity, of an underground formation by utilizing compressional velocity (V.sub.p) and shear velocity (V.sub.s) data. One described method utilizes the plotting of V.sub.p and/or V.sub.s versus a Seismic Parameter, such ad K/.rho. or the Bulk Velocity, and boundaries of velocity values of certain formation material types to define a field of data points. By comparing the position of the data points to the boundaries of the formation material types, the formation material type and, later, porosity can be determined. A method is described for determining the presence of a hydrocarbon gas at a particular location in an underground formation utilizing the above methods. Also, a method is described for determining V.sub.s from V.sub.p data and obtained ratios of V.sub.p and V.sub.s for at least one formation material type.