A real P-wave and its dependence on the presence of gas

Abstract

Attenuation of seismic compression waves leads to the real existence of a fast P₁ wave in rocks which are fully saturated with dropping fluid and a slow P₂ wave in the rocks containing gas in their pores. This accounts for the seismic blanking zones below the gas horizons for the P₁ waves. Oscillations of gaseous inclusions ensure the energy transfer to the dominant frequencies which are different for the cases of passive seismic (few Hz) and active source seismic (10–20 Hz). The intervals of dominant frequencies are determined from the negative attenuation of these low-frequency waves. According to the observations and the suggested equation, random noise amplifies the signal at these frequencies. Thus, the P₂ waves at the dominant frequency of the active source seismics are applicable for elaborating on the details of the saturation of the production layer by hydrocarbons. The relation to the AVO method (Amplitude Variation with Offset) and dilatancy effect during the preparation of an earthquake is noted.