等值反磁通原理瞬变电磁法探测薄层能力

等值反磁通原理瞬变电磁法(OCTEM)的应用日益普遍.为了研究其探测能力,论文基于正演层状模型的瞬变二次磁场时间导数响应,以包含薄层的响应和不包含薄层的背景响应的相对误差作为评价指标,对比分析了不同埋深、不同厚度、不同电阻率薄层以及不同电阻率围岩等情况下的瞬变电磁响应相对异常.结果表明:OCTEM对低阻薄层较对高阻薄层更敏感,薄层与围岩电阻率差异越大,薄层厚度与埋深比越大,越容易被探测到.以响应相对异常值15%作为临界标准探讨了OCTEM方法的探测能力(薄层厚度与可探测埋深的百分比):对于低阻薄层,当与围岩电阻率比值为0.1时,OCTEM探测能力约为1.5%;对于高阻薄层,当与围岩电阻率比值为10时,OCTEM探测能力约为20%.相同情况下常规单磁源瞬变电磁法的探测能力分别为2%和25%,略低于OCTEM对薄层的分辨能力.

Detection capability of opposing coils transient electromagnetic method for thin layers

Opposing Coils Transient Electromagnetic(OCTEM) method has been widely used. To evaluate its detection capability for thin layers, the time derivative of the secondary magnetic field responses generated by models with conductive or resistive target layers have been computed, and the relative errors to the background uniform half space have been defined as an evaluation factor. Forwarding models results show that OCTEM is more sensitive to conductive layer than to resistive layer, and that it is more likely to detect the layer successfully when the layer’s resistivity ratio to host-rock is greater and its thickness-depth ratio is larger. Taking relative abnormal response value 15% as critical value, when resistivity ratio of thin layer to host rock is 0.1 or 10, the detectable ability(ratio of the thin layer thickness to its buried depth) is 1.5% or 20%, it is stronger than that of conventional TEM, of which it is 2% or 25%. The resolution for thin layer of OCTEM is higher than that of conventional TEM.