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

1.研制了一套深海拖曳电测深系统，用于海底浅层电导率测量，特别是水合物上限的精确确定。利用电场响应模拟程序对水合物层的深度、厚度和浓度等参数进行了敏感性分析。对观测系统的优化设计进行了研究。神户大学对人工电流源和海底电磁设备(OBEM)进行了改进。JAMSTEC开发了较小尺寸的OBEM。开发了电磁场建模和反演程序。在二维反演码中引入了导电性结构的不连续性，从而能够成像尖锐的层状结构和断层。开发了用于计算任意三维电阻率结构引起的异常磁场的三维地磁建模程序。在第一次巡航(佐渡外海，2005年8月)中，新开发的深海拖曳电测深系统…在海平面以下1000米深处对MORE进行了测试，并成功地获得了电位数据。分析了海床以下0~100m范围内的电阻率结构。经验证，其电阻率结构与海底目视观测结果一致。水合物呈柱状分布，首次成像。使用人工电流源和OBEM的MMR方法在调查区西部发现了一个低幅度的磁场异常，表明该地区存在局部的阻性水合物。在第二次巡航中(2006年2月)，深海拖曳电力系统的运行再次成功。海底0~100m范围内的电阻率结构与1999年试验钻井的电阻率测井结果吻合较好。两次现场试验证明了所研制的深海拖曳电测深系统的可靠性和有效性。较少

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