It has been found that by treating seismic data as damped oscillatory responses of earth layers in the form of resonating layer bars of rocks, data values, termed alpha values, can be obtained which yield an indication of subsurface composition. Seismic data are accordingly processed by obtaining an indication of the frequency-amplitude spectrum of each successive portion or data window of a seismic trace, determining the half-power points of each such spectrum for the trace, obtaining from the half-power points the alpha value for each trace and recording or plotting the alpha values. The alpha values obtained according to the present invention have been found to be useful in lithologic identification and in obtaining porosity information. The present invention is a calculator or digital data processing computer for obtaining such alpha values from field seismic data.