SOTEM响应特性分析与最佳观测区域研究

Study on the response and optimal observation area for SOTEM

电性源短偏移距瞬变电磁法(SOTEM)是目前研究和应用较为广泛的一种人工源时间域电磁法工作装置,对深部资源地球物理精细探测具有一定的实际意义.为了深入理解方法内涵并更好地进行推广应用,本文基于电性源瞬变电磁一维正演理论,研究了SOTEM地下感应电流扩散、多分量电磁响应平面分布、多偏移距衰减等特性,然后根据上述特性研究了SOTEM的最佳观测区域.研究结果表明:电性源在地下可以产生水平和垂直两个方向的感应电流.其中,水平感应电流又分为上部水平感应电流和下部水平感应电流(又称作返回电流),水平感应电流的极大值主要集中于发射源附近并垂直向下扩散;垂直感应电流极大值沿与地面呈45°角的方向向下、向外扩散,并且具有较低的振幅和较快的扩散速度.电性源激发的六个方向的电磁场分量都具有一定的探测能力,但是考虑到地面观测的方便性和各分量的传播、分布特点,大多数情况仅利用垂直磁场分量Hz(B/t)和水平电场分量Ex.其中,Hz仅对低阻目标体敏感,且敏感区域位于赤道向区域,并集中在发射源附近;Ex既对低阻体敏感也对高阻体敏感,对低阻体的敏感区域位于赤道向区域,而对高阻体的敏感区域位于轴向区域,并且敏感区域距发射源的距离与目标体埋深和围岩电性有关.

SOTEM is a kind of time domain electromagnetic method with great detecting depth and high resolution.In order to further understand and promote this method,studies on its distribution of electromagnetic field response and diffusion characteristics was conducted in this paper.Calculation results based on the 1Dforwarding theory of SOTEM indicate that grounded wire source can excite both horizontal and vertical induced current under ground.The horizontal current includes upper and lower parts（also called return current）.The maximum of horizontal induced current mainly focus on the area close to the source and diffuses downward vertically.The maximum of vertical induced current diffuses along the direction of 45 degrees with the ground surface with a weaker amplitude and faster speed than horizontal induced current.All the six EM components have the ability for geophysical detection.However,in consideration of thediffusion characteristics of each component and observation convenience,vertical magnetic component Hz（B/t）and horizontal electric component Ex are the mostly used in practical.Magnetic component（Hz）is more sensitive to low resistance body and it mainly focuses on the region close to equator area.Electric component（Ex）shows same sensitivity to both low and high resistance body,while low resistance sensitive area mainly focuses on the region close to equator and high resistance sensitive mainly focuses on axial region,and the distance between sensitive region and source is depended on the buried depth of targets and resistivity of overburden.