Seismic Reflection

This is the most common marine geophysical tool. It involves a discharge seismic source, often an array of airguns and a receiver (Figures 3 & 4). Sediments that contain hydrate will normally show Vp higher than in normal oceanic sediment. Free methane accumulating in the pore spaces beneath the HSZ will cause a significantly lower Vp. Seismic velocity normally increases with depth so this velocity inversion, or negative acoustic impedance contrast, may exhibit the distinctive BSR reflection on seismic records. As the HSZ follows the shape of the seabed, at a near constant depth beneath the seafloor so does the BSR, which marks its base. The BSR is particularly well developed where free methane exists in the pore spacesbeneath the HSZ, the base of which is most likely to contain concentrated hydrate deposits. The BSR will not always be visible and it may vary in amplitude. The Ocean Drilling Program Leg 164 attempted to quantify this variation (Paull et al., 1996). Cementation of the sediment pore spaces also may exhibit seismic blanking on record sections as the inherent acoustic structure within the sediment becomes masked. Velocity analysis from multichannel reflection seismic data, especially where controlled by drilling, is the main interpretive method for estimating methane volume in oceanic hydrate (Holbrook et al, 1996).

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Figure 3. Standard seismic reflection configuration showing relationship between airgun array and streamer. Distances are in metres.

Figure 3. Standard seismic reflection configuration showing relationship between airgun array and streamer. Distances are in metres.

Single channel data may be adequate for the primary task of identifying the presence of hydrate through recognition of hydrate blanking and BSR distribution. In fact single-channel data may produce a more accurate graphic impression of the seismic structure of hydrate and associated methane deposits than processed multi-channel data. The principle advantages of single channel systems are that it is less expensive to operate, can be acquired by none multichannel specialist vessels and surveyed at higher speeds. It is also noted that many surveys of these data already exist for many continental margin areas. These data can be valuable for regional desk studies. Some data of this type is currently archived by the National Geophysical Data Center in Boulder and the European Commission SEISCAN project (Miles et al., 1997). Figure 4 shows a typical air gun source beam array being deployed. Gun sources can be used singly or as multiple arrays depending on the complexity of the desired source signature (frequency) and energy requirement for depth penetration.

Multi-channel seismic data enables velocity analysis for sequence depth, thickness estimates, and the potential location of hydrate concentrations. However, sedimentary sequence velocity measurements are most accurately measured using expendable sonobuoys - these provide wide angle information to 15 km rather than being restricted to the NMO dictated by the receiver array length. In practice multi-channel velocities are used for stacking and extrapolating sonobuoy results. Normal industry standard data has limited resolution along the shot line and is probably unable to identify significant horizontal velocity changes with a resolution of less than 0.25 - 0.5 km at best. This is because the separation angles between incident and reflected energy are very small in the 1-5 km water depths where hydrate occurs.

More complex acquisition (3D) and processing of multi-channel data, such as amplitude variation with offset (AVO) may also help estimate hydrate density and subjacent methane concentrations at shallow depths below sea level (Andreassen et al, 1997).

Figure 4. Three-airgun beam array being deployed (D. Booth, SOC).

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