Low Frequency Anomalies below Gas Reservoirs - Possible Explanation

We consider a possible explanation of the seismic low frequency anomalies using converted Fast-Slow- Fast P- waves in a thinly layered porous permeable fluid-saturated medium. Wave propagation in highly interbedded permeable gas reservoirs suggests significant and anomalous amount of mode conversions between Fast and Slow P-waves, which may be observed from surface seismic reflection data. Those converted P-waves experience high frequency dependent attenuation. In case if some converted waves propagated only a short fraction of their travel paths as Slow P-waves they will be notably delayed and attenuated relative to Fast P-wave reflections. A model of sandstone reservoir with typical parameters is used to estimate time delays of the converted Fast-Slow-Fast P-waves and their influence into total reflected P-waves energy at seismic frequencies.