Electric field responses of different gas hydrate models excited by a horizontal electric dipole source with changing arrangements

The exploration of marine controlled source electromagnetic survey for gas hydrate generally consists of a dynamic horizontal electric dipole source towed in the sea water and a static receiver array allocated on the seafloor. In this paper, two orthogonal horizontal components of electric field excited by a horizontal electric dipole source upon one-dimensional horizontal-layered seafloor are calculated to study the effect of different arrangements and gas hydrate models on the response of electric field. There are several dominant factors determining the variation of induced field, including transmitting frequency of the source, distance between the source and the seafloor, water depth, thickness of the overburden layer, as well as physical and chemical structure of the gas hydrate model. Modeling results suggest that the amplitude of received electric fields can reach up to 10-16–10-10 V /(A·m2) by adjusting transmitting frequency; keeping the source being towed several tens of meters above the seafloor can improve the survey efficiency but not decrease the quality of collected data; deep water environment and shallow thin overburden layer are favorable for the survey because of their attribution to signal-to-noise ratio; the porosity and saturation of gas hydrate have significant influence on medium resistivity-the higher resistivity the medium has, the bigger the amplitude of electric field response will be. Forward modeling can provide valuable information for the development of instruments and the design of survey to detect gas hydrate.

摘 要海底天然气水合物电磁勘探采用动态水平电偶源发射与静态阵列式海底偶极接收的测量方式。基于一维地电模型，求解海底水平层状介质的电偶源电场响应，得到海底两个水平正交电场的传播表达式，据此研究天然气水合物电场响应的影响因素。对供电频率、电偶源离地高度、海水深度、覆盖层厚度以及天然气水合物地层自身的物理化学结构等因素与归一化电场幅值的关系分别进行了计算分析，结果显示：调整供电频率可使归一化电场幅值变化于10－16～10－10 V/(A·m2)；将电偶源施放至离海底数十米高度进行拖曳可获得较好的作业效果，且不改变信号采集的质量；深水及浅薄覆盖层属于有利的探测环境，该环境下的感应场源具有较高的信噪比；天然气水合物的孔隙度、饱和度影响介质电阻率值，其电阻率越高，电场响应幅值越大。正演模拟结果可对天然气水合物探测设备研制及海洋作业提出切实可行的设计方案。图7参22