Exploration of sub-seafloor methanehydrate using artificial current source ocean-bottom electromagnetic method and its reservoir estimation

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1. A deep-towed electrical sounding system was developed for shallow ocean-bottom conductivity survey, especially for precise determination of the upper limit of hydrate layers. A sensitivity analysis was done in term of the parameters such as the depth, thickness and concentration of the hydrate layer using a electrical-field response modeling code. Optimum design of the observation system was investigated.2. Improvement of the artificial current source and the Ocean-Bottom ElectroMagnetic equipment (OBEM) was done in Kobe University. JAMSTEC developed smaller-size OBEMs.3. EM modeling and inversion codes were developed. Discontinuity in the conductivity structure was introduced in the 2-D inversion code, enabling to image sharp layer structures and faults. A 3-D magnetometric modeling code was developed to calculate anomalous magnetic field caused by any 3-D resistivity structure.4. In the first cruise (Off Sado, August 2005), the newly developed deep-towed electrical sounding system … More was tested at the depth of 1000 meters below sea-surface and successfully obtained electrical potential data. The electrical resistivity structure from 0 to about 100 meters below sea-floor was analyzed. It was proved that the resistivity structure was consistent to the visual inspection of the sea floor. The distribution of the hydrate was in columnar shape and was first imaged. MMR method using the artificial current source and the OBEMs revealed a low amplitude anomaly of magnetic field in the west of the survey area, suggesting the localized existence of the resistive hydrate in the area.In the second cruise (Off Tokai, February 2006), the operation of the deep-towed electrical system was again in success. The resistivity structure from 0 to about 100 meters below sea-floor was in good agreement with the result of the resistivity logging in the test drilling done in 1999.The two field test proved the reliability and effectiveness of the developed deep-towed electrical sounding system. Less