CLOSING THE GAP BETWEEN THE HYDRATE STABILITY ZONE AND PRODUCTION RESERVOIRS

As part of a program to image features within the hydrate stability zone, a seismic profiling technique has been developed that provides very high resolution in the shallow sub-sea-floor as well as relatively deep penetration into the sea bottom. It is called the surface-source-deep-receiver (SSDR) technique because it uses a seismic source towed at the sea surface and a hydrophone towed deeply enough directly beneath the source that the seismic wave front approximates a vertically propagating plane wave. One advantage of this geometry is that the direct wave constitutes a far-field source signature that can be used to enhance greatly the resolution of bottom and sub-bottom reflections. A disadvantage is that the depth of usable seismic penetration is dictated not by the strength of the source but by the arrival of so-called “ghost” reflection from the sea surface. This limits usable sea-floor penetration to approximately the depth at which the receiver is towed beneath the surface, even though the source may be energetic enough to generate much deeper reflections. During recent years, the SSDR technique has been used to image regions of known hydrate occurrence in the northern Gulf of Mexico. Conventional pneumatic sources were used and the hydrophone was towed approximately 400m below the surface. In water depths ranging from 800m to 1500m, decimeter-scale shallow layers and meter-scale layers hundreds of meters below the sea floor were resolved. Relatively high-amplitude ghost reflections arrived at times corresponding to about 400m penetration and strongly overprinted primary reflections of comparable, and greater, travel times. This contribution describes progress of an effort that is currently underway to develop a processing technique by which ghost reflections can be eliminated from SSDR profiles. Such processing would extend the usable penetration of SSDR data, perhaps far enough that it would overlap the shallower portions of industry-standard seismic information, thus closing the gap that exists between high-resolution and exploration seismic data. Optimally, this technique will result in images continuous from the hydrate stability zone down to the deep production reservoirs that supply the gas to form hydrates.