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The world's offshore continental margins contain vast reserves of gas hydrate, a frozen form of natural gas that is embedded in cold, near-seafloor strata. Published estimates suggest that the energy represented by gas hydrate may exceed the energy available from conventional fossil fuel by a factor of 2 or more. Understanding marine hydrate systems has become critical for long-term worldwide energy planning. Groups in several nations are attempting to evaluate the resource and to define seafloor stability problems across hydrate accumulations. Affordable, reliable, remotely based methodologies for evaluating deepwater gas-hydrate systems have been slow to develop. Four-component ocean-bottomcable (4-C OBC) technology offers an option for remote, detailed evaluation of deepwater, near-seafloor geology. Increasing use of marine multicomponent seismic technology by oil and gas companies now allows marine gas-hydrate systems to be studied over areas of many square kilometers and the geomechanical properties of the strata that confine these hydrates to be analyzed. Data acquisition and calibration. Marine multicomponent seismic data acquisition requires a surfacebased airgun source and long lines of ocean-bottom sensors that record three-dimensional vector motion of the seafloor. Using this combination of surface source and seafloor receivers, standard-frequency (roughly 10-100 Hz) compressional (P-P) and converted shear (P-SV) wavefields can be acquired that backscatter from nearseafloor strata. We used these wavefields to image seafloor strata over distances of several kilometers across the Green Canyon area of the Gulf of Mexico. For calibration purposes, high-frequency, chirp-sonar data were acquired along the same seafloor profiles using an autonomous underwater vehicle (AUV). This AUV system traveled at a height of 40 m above the seafloor and illuminated near-seafloor strata with a 2-8 kHz chirp-sonar signal. The backscattered, high-frequency, P-P data acquired with this system imaged geology to a depth of a few tens of meters below the seafloor.

Suggested reading. "Direct measurement of in-situ methane quantities in a large gas-hydrate reservoir" by Dickens et al. (Nature, 1997). "Methane hydrate and free gas on the Blake Ridge from vertical seismic profiling" by Holbrook et al. (Science, 1996). "Rock-physics of a gas hydrate reservoir" by Dvorkin et al. (TLE, 2003). "Compressional and shear wave velocities in uncemented sediment containing gas hydrate" by Yun et al. (Geophysical Research Letters, 2005). The Rock Physics Handbook by Mavko et al. (Cambridge, 1998). "Framework for AVO gradient and intercept interpretation" by Castagna et al. (GEO-PHYSICS, 1998). "Elastic-wave velocity in marine sediments with gas hydrates: Effective medium modeling" by Helgerud et al.

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We show that SV-P reflectivity closely matches P-SV reflectivity; thus, in concept, an SV-P image should be as informative and as valuable as a P-SV image for seismic interpretation purposes. If the dip of rock layering is not severe, the length of the SV raypath involved in SV-P imaging is approximately the same as the length of the SV raypath in P-SV imaging; thus, the important lithology-sensitive V P ∕V S velocity ratio determined with SV-P data should be approximately the same as the V P ∕V S velocity ratio determined with P-SV data. We compare velocities used in P-SV imaging and SV-P imaging to emphasize the equivalence of P-SV and SV-P stacking velocities, and therefore seismic-derived V P ∕V S velocity ratios, obtained with both converted-wave modes. We compare images of P-SV and SV-P data to illustrate the high-quality images that can be made with a SV-P mode. The SV-P data used in these comparisons are recorded by vertical geophones, whereas the P-SV data are recorded by horizontal geophones. In the real-data examples we present, the energy sources that produced the downgoing SV wavefield are vertical-force sources, not horizontal-force sources. A vertical vibrator is used in the first case, and shot-hole explosives are used in the second case. The interpretation technology described here thus introduces the option of extracting valuable S-wave information and images from legacy P-wave data generated by a vertical-force source and recorded with only 1C vertical geophones. We discuss several principles involved in constructing SV-P images from VSP data because of the importance that VSP technology has in calibrating depth-based geology with surface-recorded SV-P data. We emphasize that cautious and attentive data processing procedures are required to segregate SV-P reflections and P-P reflections in VSP data.