Analysis and application of low frequency shadows based on the asymptotic theory for porous media

Low-frequency shadows beneath gas reservoirs can be regarded as a time delay relative to the reflection from the reservoir zone, but they cannot be reasonably explained by the high-frequency attenuation or velocity dispersion observed in normal P-waves. According to the new asymptotic theory for porous media, seismic P-waves undergo multiple conversions between fast and slow modes during seismic waves passing through layered permeable reservoirs at low frequencies, and changes in the velocity and amplitude (i.e., energy) of slow P-waves can lead to low-frequency shadows. In this study, a forward analysis was performed on the dispersion and attenuation of fast and slow P-waves within the seismic frequency band based on the asymptotic theory for porous media; the results revealed that fast P-waves do not undergo frequency dispersion and attenuation within the seismic frequency band and that slow P-waves are the primary contributor of dispersion and attenuation. In addition, methods used to calculate the frequencies at which low-frequency shadows occur were analyzed and are discussed. Finally, S-transform time-frequency analysis method was used to calculate and analyze the low-frequency shadows of three-dimensional seismic data acquired from work area M in Sichuan. The low-frequency shadow anomalies determined by this method were found to be highly consistent with those identified based on the data acquired from wells in the target reservoir. These results indicate the good application performance of this method.