电性源短偏移距瞬变电磁场扩散规律三维数值模拟

电性源短偏移距瞬变电磁法(SOTEM)采用近源探测形式,极大地提高了信号强度,目前对电性源短偏移距瞬变电磁响应特征的研究主要基于一维模型,且主要讨论电场E_x分量和磁场H_z分量的响应特征和探测能力,不利于复杂目标体的精细探查。基于以上问题,采用三维时域有限差分法,对电性源短偏移距瞬变电磁场在地层中的扩散规律进行了模拟,并研究了三维异常体对电性源短偏移距瞬变电磁场扩散的影响。结果表明:电场E_x分量的正极值区域主要集中在发射源附近,并随着时间的推移逐渐向下扩散; 电场E_y分量正极值区域和负极值区域分别位于发射源两端,并随着时间的推移逐渐向下、向外扩散; 电场E_z分量在地层分界面产生跃变; 磁场H_x分量正极值区域和负极值区域分别位于发射源两端,并随着时间的推移逐渐向下、向外扩散; 磁场H_y分量上部为负值区域,下部为正值区域,说明H_y分量也会受到返回电流影响; 磁场H_z分量极值区域主要位于发射源下方,并随着时间的推移逐渐向下移动; 对于三维异常体,电场E_x分量和磁场H_y分量的灵敏区域位于异常体上方,而电场E_y分量和磁场H_x分量的灵敏区域位于异常体外部,且分布在异常体四周,磁场H_z分量的灵敏区域位于异常体两侧(靠近发射源和远离发射源位置),说明电性源短偏移距瞬变电磁场的5个分量对三维异常体的灵敏区域不同,不同的分量应选择不同的区域进行观测。

Three-dimensional Numerical Simulation of Diffusion Law of Short-offset Grounded-wire Transient Electromagnetic Field

Short-offset grounded-wire transient electromagnetic method(SOTEM)has considerably improved the signal intensity by using the form of near-source detection. The present researches on SOTEM responses are mainly based on 1D models and explore the response characteristics and detection abilities of two components: electric field E_x and magnetic field H_z, which are not conducive to the accurate exploration of complex targets. Based on the above-mentioned problems, a three-dimensional(3D)finite-difference time-domain(FDTD)was employed to simulate the diffusion pattern of SOTEM field in strata and the influence of 3D anomalous body on the diffusion of SOTEM field. The results show that the positive extreme value area of electric field E_x component mainly appears nearby the transmitting source, and diffuses gradually downward with time; the positive extreme value area and negative extreme value area of electric field E_y component are mainly located at two ends of the transmitting source, and diffuse gradually downward and outward with time; the electric field E_z component undergoes jump at strata boundary; the positive extreme value area and negative extreme value area of magnetic field H_x component are respectively located at two ends of the transmitting source, and diffuse gradually downward and outward with time; the upper part and lower part of magnetic field H_y component are negative value area and positive value area, respectively, indicating that H_y component will also be affected by the return current; the extreme value area of magnetic field H_z component is mainly below the transmitting source, and gradually moves downward with time; for the 3D anomalous body, the sensitivity areas of electric field E_x and magnetic field H_y components are rightly above the anomalous body, while the sensitivity areas of electric field E_y and magnetic field H_x components are outside the anomalous body and distribute around it, and the sensitivity areas of magnetic field H_z component are on both sides of the anomalous body(close to transmitting source, and distant from transmitting source, respectively). It shows that the five components of SOTEM field have different sensitivity areas to the 3D anomalous body, and different components should be observed in different areas.