A Nazari - M. Sc. Student of Geophysics, Institute of Geophysics, University of Tehran, Iran
M. A Riahi
H Shahbazi

The nature of the frequency response depends strongly on the AVO behavior at an interface. In order to understand and predict the seismic response to fluidsaturation for Class IV sands, we performed Synthetic geological models with a hydrocarbon-bearing zone .The synthetic seismic sections were produced byperforming offset acoustic ray-tracing in GX Technology’s GX-II 2D forward modeling package based on existing well log information close to thestudy area, which can effectively model the characteristics of attenuation and frequency-dependent reflection. Then transform the seismic data centeredin a target layer slice within a time window to the time-frequency domain using S-Transform timefrequency signal analysis and sort the frequency gathers to common frequency cubes. Strong lowfrequency energy beneath a hydrocarbon reservoir(seismic low-frequency shadow) can be used as a hydrocarbon indicator (Taner et al., 1979). we observe the characteristics of the seismic low-frequency shadow in the common frequency cubes. Reservoir simulations reveal that the main mechanism of seismic low frequency shadows is attributed to highattenuation of the medium to high seismic frequency components caused by absorption in the hydrocarbonfilled reservoir. After modeling and validation of the technique with synthetic data, seismic analysis supported with SD technologies was performed to evaluate prospects located in real data. Results from a practical example of seismic low frequency shadows show that it is possible to identify the reservoir by the low-frequency shadow with high S/N seismic data. This study is useful to identify reservoir and non reservoirs and the results of this study are great importance in making plans for developing the reservoir