The 13th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2009)

High resolution 3D seismic survey, “Tokai-oki to Kumano-nada”, was conducted for methane hydrate exploration in the eastern Nankai Trough by METI in 2002. Our study focuses on zigzag-shaped specific reflectors on BSR margins on the 3D data. We call the reflectors “Foldback Reflectors (FBRs)” in this study. From the edge of BSR, the 1st FBR generally extends down to lower formation below the BSR crossing sedimentary horizons. The following FBRs (often the 2nd, sometimes 3rd and those of higher order) extend down from the edge of the upper FBR forming accordion-like shape. The 1st FBR indicates normal polarity (antiphase of BSR), and the following FBRs change their polarities alternately. FBRs are mostly developed in the well-stratified formation but not in the area of frequent fractures and the area of major lateral lithological change. The estimated dip directions of each FBRs are probably controlled by the dip direction of crossing formation. FBR generally corresponds to lateral seismic facies boundary between BSR distribution area and outside of the BSR area. The formation beneath the BSR shows dimmed facies characterized by relatively low amplitude and lack of high frequency components in contrast to outside of the BSR area with normal facies. Seismic velocity analysis suggests that FBRs correspond to velocity boundaries, where the dimmed faceis below the BSR coinsides with relatively low velocity. The polarities of FBRs are also consistent with such velocity changes. Such dimmed facies with low velocity and low amplitude anomaly suggests effects of gas components in the formation water. The lowest FBR does not cross major unconformities, which often exhibit negative polarity suggesting fluid bearing strata. In this case, the lowest FBR with negative polarity merges to the negative reflection of the unconformity. In addition, high amplitude layers are sometimes recognized at foldbacks convex to the outside of the BSR area. These high amplitude layers probably having higher permeability are interpreted as conduits of gas-related fluid from the BSR distribution side to the outside of the BSR area. From these facts, FBR can be regarded as an important proxy indicating migration front of gas-related fluid.

To understand stress perturbations associated with minor faults and fractures, relationships between the faults, fractures, lithologic boundaries and stress changes in hole B of Taiwan Chelungpu-fault Drilling Project (TCDP) were investigated. Here, we reported four patterns of stress changes in the vicinity of faults, fractures and lithologic boundaries found in TCDP hole B: (i) the stress orientation (breakout azimuth) rotates abruptly (discontinuously) in the vicinity of the faults or fractures; (ii) the orientation rotates gradually; (iii) breakouts are suppressed at faults, fractures, or lithologic boundaries; and (iv) the orientation does not change across faults or fractures.

Frequency domain electric dipole-dipole controlled-source electromagnetic (CSEM) sounding methods, first developed to study the oceanic lithosphere in the 1980’s, have been successfully applied to the exploration of oil, gas, and gas hydrate. In the case of hydrocarbon exploration, several companies have been offering commercial services for several years. However, considerable research remains to be carried out, and the marine EM group at Scripps is active in the development of software, instrumentation, and field techniques for CSEM studies. New instrumentation includes a continuously towed, three-axis electric field receiver, a highly sensitive long-antenna electric field gradiometer, and a novel long baseline acoustic navigation system for accurately locating the deep-towed transmitter. This equipment was used on two recent field campaigns, one to study gas hydrate in the Gulf of Mexico, and one to study a gas field off the Northwest shelf of Australia.

We conducted three-dimensional travel time tomography in and around the source region of the 1984 Western Nagano Prefecture Earthquake to investigate the generation process of the mainshock and swarm activity near the source. As many as about 250,000 travel time data of good quality (1 ms error) from a dense network were compiled and we obtained hypocentral distribution and three-dimensional P wave velocity structure with high accuracy and resolution. Most of the estimated hypocenters were aligned in lines or planes, not in the form of masses. We found the hypocenter distribution corresponding to the mainshock fault plane, and it was sandwiched by high-velocity regions on both horizontal sides. A low-velocity region spreading horizontally at the bottom of this hypocenter alignment was also seen. In the northeastern side of the mainshock fault, where there is swarm activity, we detected several other alignments of hypocenters, most of which may be located at the boundaries of geological strata. A low-velocity region was also found at the bottom of some of these alignments. This low-velocity region may be due to fluids from below the seismogenic zone. The generation of both the mainshock and the swarm activity might be related to fluid intrusions from the lower side of the hypocenter alignments.

A high frequency deep-tow seismic survey was carried out in the Nankai Trough area in 1996. The objective of the survey was to obtain high resolution seismic sections and velocity profiles of the methane hydrate zone, inferred from the strong BSR events seen on conventional seismic data. A special feature of the survey is that both the source and the streamer cable are towed close to the seabed. This special acquisition geometry requires special data processing to handle the varying source and receiver depths. A CMP floating datum processing sequence was designed which led to high quality sections of the shallow geology. The processing sequence was applied to a number of lines, totaling 200 km. The quality of the final stack sections was highly variable. It was found during the processing that the positions of source and receiver caused problems to harm the quality of the final sections. We classify the positioning problems as (1) streamer cable feathering, (2) source and cable depths, (3) relative shotpoint position and (4) absolute shotpoint position. The floating datum processing with static corrections could fix the problems (1) and (2). Post stack correction techniques are applied to fix (3) and (4) using 3D marine seismic. We are planning to use deep-tow seismic survey for hydrothermal deposits. Hydrothermal deposit is specially limited and higher resolution is necessary than methane hydrates. The very high frequency of the data is considered to make the data processing much more sensitive to errors or approximations. In this paper the navigation problems are reviewed to design the new deep-tow seismic system.