ABSTRACT
Gas Hydrates are detected on seismic section, primarily through identifying an anomalous reflector know as bottom simulating reflector (BSR) that mimics the shape of the sea floor, cuts across the dipping strata and has opposite polarity with respect to the sea-floor event. Third is important manifestation in identifying the BSR is the polarity reversal of the wavelet. The abrupt change in the velocity pattern from high velocity zone associated with the gas-hydrates to lower velocity water saturated or gas filled sediments just below the BSR produces a polarity reversal. The velocity configuration produces characteristic reduction in amplitude in the reflector above the BSR. Wave attenuation is an important seismic attribute that contains significant information about physical rock Properties. Seismic reflection techniques are the most important tool for locating gas hydrate zones. Hydrates have very strong effect on the acoustic reflection because of high acoustic impedance contrast, since the cementation of grains by hydrates produces a high velocity velocity (3.3 km / sec), deposit. Sediments below the hydrate-cemented zone if saturated with water will have low velocity and if gas is trapped in these se-
diments the velocity of the underlying layer will be still lower (1.5-1.7 km / sec). Because the strength of the reflected signal is proportional to the change in acoustic impedance, the base of hydratecemented zone produces strong reflections. The degree to which amplitudes are diminished depends on the amount of hydrates present. The phenomenon of amplitude reduction is termed as 'blanking' –a characteristic feature for identifying gas-hydrates or BSR. It is to be mentioned here that sometimes the transparency is caused by homogenization of lithology. Three main manifestations (BSR, Blanking zone, Polarity reversal) have been used to recognize the presence of hydrates in the seismic section. The velocity build-up above the BSR can be used to quantify the amount of hydrates present above BSRs.
