导电薄层下三维岩脉的CSAMT响应分析

为了研究探测3D导电岩脉状目标体的可控源音频电磁法（CSAMT）的最佳观测装置，我们用压缩积分方程法数值模拟了赋存于导电薄层下的3D导电岩脉的电磁场响应.计算结果表明，最有效的源的位置是把发射源沿岩脉的走向放置.相对于其他几种装置，这种观测装置得到的异常场与主场的比值最大.利用这种最佳的观测装置，通过将异常场E_ya 和 H_xa与噪音作对比，研究了当岩脉的埋深和高度发生变化时岩脉的可探测性问题.当岩脉的顶部埋深增加时，导电岩脉的可探测性降低.岩脉高度的变化对异常场幅值的影响要小于岩脉顶部埋深所产生的影响.所以通常情况下，即便应用了最有效的CSAMT装置，也很难探测到岩脉的高度，应用钻孔中的探测可能会解决这个问题.

THE ANALYSIS OF CSAMT RESPONSES OF DYKE EMBEDDED BELOW CONDUCTIVE OVERBURDEN

Three-dimensional (3D) electromagnetic (EM) fields of a conductive dyke with a conductive overburden are simulated numerically using 3D contraction integral equation (CIE) method to discuss the optimal survey configuration of the controlled-source audio-frequency magnetotelluric (CSAMT) method in order for the exploration of conductive dyke-type target.The calculation results show that the most effective configuration of the transmitter is the case that the direction of the grounded-wire transmitter is along the strike of dyke. In this configuration, the ratio of anomalous fields to primary fields becomes larger than those of any other survey configurations.Using the optimal survey configuration, we also investigate the detectability of the conductive dyke. We calculate the anomalous fields E_(ya) and H_(xa), which are the responses of the conductive dyke only, and compare them to the noise level for the cases that the depth and the height of the conductive dyke are variable. When the depth of top of the dyke increases, the detectability of the conductive dyke decreases. Also, the effect of the height of the dyke to the amplitude of the anomalous fields is much smaller than that of the depth, so that it is usually difficult to detect the height of the dyke by general CSAMT measurement even with the optimal survey configuration. The measurement using borehole can be helpful to address this problem.