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 .