Gas hydrates are found worldwide and many studies have been carried out to develop an efficient method to identify and quantify them using various geophysical as well as other anomalies. In this paper, we model P- and S-wave velocities of gas hydrates and associated gas, and generate synthetic seismic response for different models by 2D wave equation technique. Various patterns in synthetic seismograms related to gas hydrates and associated free gas are analyzed. Datasets from different regions of the world along with synthetic modeling results are used to evaluate our observations. The results reveal that bottom simulating reflector (BSR), enhanced seismic reflections below BSR, seismic chimneys, mounds, and amplitude blanking are the key seismic indicators for gas hydrates.
Gas hydrates are crystalline minerals composed of water and certain gases. Gas hydrates feature high on energy, wide in distribution and large in size, and are is thought to be a vital source of energy for the twenty-first century. It is estimated that 90% of the ocean area is favorable for generation of hydrates (Kvenvolden et al., 1993). As the supply of the available energy sources tends to get tight globally, seeking alternate sources of energy becomes an urgent affair. Therefore, it is important to carry out research on hydrate exploration. Gas hydrates lie in marine sediments with solid state structures. The distribution of hydrates has its own characteristics. Gas hydrates have been described extensively (Collett, 1995, Ecker et al., 1996; Howell, 1993; Kvenvolden, 1993a, b; Max, 2000) Max and Lowrie, 1992, 1996) and are known to occur in the Arctic and in sediments underlying the world’s oceans (Fleischer et al., 2001, Kvenvolden 1993). The presence of hydrates can be inferred from seismic evidence such as bottom simulating reflectors (BSRs) or changes in seismic velocity (e.g., Hovland and Judd, 1988), or not (Lowrie et al., 1997). Gas hydrates have also been found where there is no BSR (Holbrook et al., and Shipboard Scientific Party 1996).
The presence of gas hydrate in sediment pore space elevates interval velocity in the gas hydrate stability zone (GHSZ) and may significantly reduce interstratal acoustic impedance contrasts, causing a marked decrease in seismic amplitude above the BSR (Dillon et al., 1993), a seismic anomaly known as amplitude blanking. Gas hydrate-bearing sediments can also create bright spots as seen in the Mallik gas-hydrate field (Bellefleur et al., 2006). Free gas below the GHSZ increases acoustic impedance contrasts between sedimentary layers, enhancing seismic amplitude below the BSR. The free gas can act as a low-frequency filter and thus the BSR often marks a sharp interface between high frequency reflections above and low-frequency reflections below (Vanneste et al., 2002). Seismic chimneys or vertical seismic wipe-outs are also seen in many oceanic gas hydrate provinces. These seismic reflection anomalies are often accompanied by pockmarks or craters (Ginsburg, 1998) or mound-like vents (Sager et al., 1999) on the seafloor, suggesting localized expulsion of gas, originating from below the GHSZ (Hyndman et al., 2001; Wood et al., 2002).