Vertical Seismic Profile First-Break Amplitude Study

There are several published papers on Vertical Seismic Profile (VSP) First-Break Amplitude (FBA) studies that attempt to relate seismic attenuation to depth. In this study, the basic principle is to normalize FBAs and then to transform them into the decibel (dB) domain. Thereafter, dB values are plotted in the depth-decibel domain rather than in the conventional linear depth-amplitude domain. The depth-decibel curve shows very good correlation with sonic logs. The similarity between a sonic log and a dB curve depends on the type of geophone used for VSP recording. A correlation can be achieved only when the accelerometer is sensitive to particle velocity acceleration. Acceleration and amplitude are high in unconsolidated lithology. Conversely, acceleration and amplitude are low in consolidated layers. Thus, the dB plot mimics a sonic log. Using this method, individual and cumulative attenuation gradients for varying lithological units can be defined and formation boundaries identified. Formation densities and interval velocities can also be computed using the FBA technique. It would appear that this methodology is not widely employed because most amplitude studies do not take into consideration the type of geophone used for VSP recording.