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Crosshole seismic experiments were carried out using two, 1160 m observation wells (JAPEX/JNOC/GSC et al. Mallik 3L-38 and 4L-38), each located 42.5 mfrom the central, 1188 mproduction research well (JAPEX/JNOC/GSC et al. Mallik 5L-38). These provide images of a gas-hydrate-bearing interval between 900 m and 1100 m depth, and portions of the surrounding sedimentary sequences. Following a baseline survey, three time-lapse monitor surveys were carried out during a production experiment to evaluate the seismic detection of gas hydrate dissociation. Initial, 1-D and 2-D studies of the P-wave arrivals indicate that the highly saturated gas-hydrate-bearing sediments are associated with high velocities, strong attenuation, and some seismic anisotropy. Modelling studies indicate that the dissociation caused by the production test will introduce only very weak changes in the seismic data. Advanced processing will be required to detect these effects.

A one‐dimensional gas hydrate accumulation model is developed to simulate the existence of hydrate in the Nankai Trough margin, where the total organic carbon content is extremely low. In order to study the accumulation of gas hydrate along a seismic profile, our model assumes an initial 1000 m thick turbidite sedimentation at the trench axis, and then the sediment experiences tectonic uplift to form the Nankai accretionary prism. Methane generated by microbial processes during the sedimentation is partly trapped in the pore space and partly migrated upwards due to compaction and subsequent porosity decrease at deeper levels. Upward methane gas flux from the deeper part (below 1000 m) is predicted by the model because the maximum thickness of the accreted sediment exceeds ten kilometers in the Nankai Trough accretionary prism. The basic geological parameters input to the model are those obtained from the ODP site 808, which was drilled at the lower inner trench slope. The methane hydrate stability zone is essentially controlled by bottom water temperature, regional heat flow and hydrostatic pressure. The simulation shows that methane hydrate will not accumulate in the Nankai Trough margin if only microbial methane generation is considered because the organic carbon content in the Nankai Trough area is as low as 0.75%. However, if we assume an upward flux of 5 kg of methane per square meters per 10,000 years, it will cause approximately 10 to 25% hydrate saturation under current physical conditions. The origin of the methane may be thermogenic and/or microbial. The resultant distribution of methane hydrate shows very good correlation with the distribution of the BSR.

Abstract: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 in the area. 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. A key step in the processing was devising a residual statics technique to compensate for errors in the measured depths.The processing sequence was applied to a number of lines, totaling 200 km. The final data quality was highly variable. Some lines produced high quality sections and others, much poorer sections with few interpretable events. Conventional seismic data in the area also shows variation in the data quality so part of the reason is a variation in the sub-sea geology, but the deep-tow data is much more sensitive to change in conditions than conventional data. With the current acquired data and processing sequence the deep-tow system offers most advantages when 1) the water depth is around 1 km or greater, 2) the seabed and underlying geology is not too complex, and 3) the acquisition proceeds smoothly with regular shotpoints, slowly varying depths, and with accurate positioning.

JOGMEC/MOECO carried out an onshore seismic survey with explosive sources in 2012 in Cambodia.The project was executed under the Basic Agreement for the Study and Survey Program in Block XVII onshore Cambodia between JOGMEC and CNPA. This was the first experience of seismic data acquisition with explosives in Cambodia.Therefore, there were a lot of issues to be resolved before the field work was launched. One of most serious issues was the implementation of safe operations in a Mine/UXO contaminated eld. Other issues were a lack of regulations in Cambodia, education of non-experienced Cambodian people including both government staffs and local residents, explosive management and so on. Firstly JOGMEC/MOECO hired excellent Cambodian national staffs and further educated on seismic surveys. Then, they seriously and actively worked for communications with both government staffs and local residents. Such efforts resulted in good cooperation with all relevant parties even though unexpected procedures caused delays in the schedule. Finally, the seismic survey was successfully completed and no lost time for injury was recorded. A lot of knowledge on the seismic survey has been accumulated for all relevant parties through this experience. The procedures established by JOGMEC/MOECO through this survey will be one of the standards for onshore seismic data acquisitions in Cambodia.