岩溶隧道勘察接地源半航空瞬变电磁三维响应规律研究

Study on the Three-dimensional Response Law of Semi-airborne Transient Electromagnetic of Grounding Source for Karst Tunnel Investigation

半航空瞬变电磁探测方法将长导线发射源布设于地面,发射阶跃脉冲电流激发瞬变电磁信号,使用无人机在空中吊载接收线圈来接收瞬变电磁二次场信号,最终实现探测.然而与传统地面瞬变电磁相比,半航空瞬变电磁的不同之处在于其正演计算时需考虑飞行高度、接收偏移距等因素.针对上述问题,开展半航空瞬变电磁数据的三维时域有限差分正演理论研究,推导了加入接地源电流项Js的Maxwell方程组时域有限差分格式,并进行三维正演模拟.首先,通过均匀半空间模型和多种层状模型,对比分析感应电动势衰减曲线形态特征以及不同电阻率参数和层厚参数对衰减曲线的影响;然后,建立包含高阻异常体、低阻异常体和高-低阻混合异常体的三维模型,对比分析接收高度、异常体电阻率、规模和偏移距对感应电动势衰减曲线的影响特征,研究三维地下目标对半航空瞬变电磁探测的影响因素和响应规律,并开展试验验证,为后续半航空瞬变电磁观测设计提供了参考.

Semi-airborne transient electromagnetic receives the transient electromagnetic secondary field signal by the receiving coil equipped on a drone.The signal is excited by grounded long wire source, which injects the pulse current.However, semi-airborne transient electromagnetic is different from the traditional transient electromagnetics in the aspects of flying height, receiving offset, etc.Aiming at the above problems, the Maxwell equations finite-difference time-domain format of the grounded source current term Js is derived based on the three-dimensional finite-difference time-domain method.And the three-dimensional forward modeling of the semi-airborne transient electromagnetic is simulated.Firstly, through the uniform half-space model and various layered models, the morphological characteristics of the induced electromotive force attenuation curve and the influence of different resistivity parameters and layer thickness parameters on the attenuation curve are compared and analyzed.Besides, high-resistance anomalies, low-resistance anomalies and high-low resistance anomalies are established.By comparing and analyzing the influence characteristics of the receiving height, abnormal body resistivity, scale and offset on the induced electromotive force attenuation curve, the influencing factors and response laws of the semi-airborne transient electromagnetic detection underground three-dimensional target are obtained.