覆盖层影响下典型地-井模型瞬变电磁法正演

Transient Electromagnetic Method Modeling in Ground-Borehole Model with Overburden Influence

地-井瞬变电磁法是用于地质找矿的有效方法之一,开展该方法的三维正演研究能对资料处理与解释提供帮助。在时间域有限差分算法的基础上,建立了三维地质模型和覆盖层模型,模拟了该模型下的地-井瞬变电磁响应;引入了参数EA,分析了方形低阻体和覆盖层的响应特征以及覆盖层影响因素。结果显示:低阻体位于发射场源下方时,其响应曲线呈现"双高一低"的极值特征,高峰值分别对应低阻体上下边界,低峰值对应低阻体中心;低阻体位于发射场源旁边时,其响应曲线呈现单极值特征,该特征与覆盖层响应特征相同。当覆盖层电阻率较低时,"趋肤效应"作用使覆盖层的响应强度强于低阻体;但当发射场源离钻孔足够远时,覆盖层影响可以忽略,低阻体响应强度更与覆盖层-低阻体电阻率的比值有关。孔中发射能有效降低覆盖层影响。

As one of the effective methods used to geological prospecting, three dimensional modeling of ground-borehole transient electromagnetic method can offer help for data processing and interpretation. Based on time domain finite difference algorithm, ground-borehole transient electromagnetic response was simulated under three dimensional geological and overburden model. Ground-borehole transient response characteristics of conductive body and overburden, and influenced factors of overburden were analyzed by using the parameter EA. The results indicate that, ground-borehole transient response curves appear two high and one low extreme value when the conductive body locating below transmitter loop. The two high values occur at the depths of upper and down boundary of the body, while the low value occurs at depth of the center of the body. The curves show one single extreme value when the body locating beside loop, which is similar to overburden. Transient field value of overburden is higher than that of conductive body when the body has a lower resistivity because of skin effect. However, the value of overburden is comparatively lower making its influence can be ignored when the distance between transmitter loop and borehole is far enough. Conductor response is related to the ratio of overburden and body. Overburden effect can be effectively reduced.