浅层不均匀体对大定源瞬变电磁响应的影响

Influence of the Shallow Heterogeneities to the TEM Response of the Large Fixed Loop

根据瞬变电磁法基本原理,当大定源发射回线下方存在局部不均匀体时,由于不均匀体在接收线圈中产生的二次场方向不同,导致衰减曲线与均匀大地响应曲线偏离。为了分析浅层不均匀体对衰减曲线的影响,采用大定源回线装置发射电流为梯形波,回线面积为150m×150m,电流波形为梯形波,幅值为10A,关断时间为176μs;对于时间常数分别为1、3、5ms的浅层不均匀体,利用多匝密绕的闭合线圈(即异常线圈模型)的电磁信号衰减规律,来模拟其响应衰减曲线。研究表明,当异常线圈位于发射回线下方或者是框边下方时,随着发射、异常、接收之间的相对位置的变化,三者之间的互感发生变化,引起异常线圈的响应可能为负值,致使接收到的综合响应衰减曲线出现过0情况。因此,大定源回线结构下存在不均匀体时,产生的负响应值在后续处理时需要引起注意。

Based on the knowledge of the TEM theory,the secondary field direction generated by the heterogeneities in the position of the receive loop is different when the local heterogeneities exist below large fixed transmit loop,which may result in attenuation curve deviating from the response curve of the homogeneous body.In order to analyze the impact of heterogeneities to the attenuation curve,we adopts large fixed loop equipment whose area is 150m×150mand emission current is trapezoidal wave with the amplitude of 10Aand turn-off time of 176μs.We use the electromagnetic signal attenuation rule of multi-turn and close-wound coil,namely anomaly loop to simulate the response curve of shallow heterogeneities whose time constant is respectively 1ms,3ms or 5ms.When the anomaly loop is located below the transmit loop or its frame edge,with the different relative position between the transmit loop,anomaly loop and receive loop,that will change the mutual inductance of them and cause the response of the anomaly loop negatively and lead to the integrated response attenuation curve of the receive loop appearing zero-crossing condition.So we need to attract attention of the negative response in the subsequent processing when the heterogeneities exist below the large fixed loop.