冀东沉积变质型铁矿床瞬变电磁响应研究

Transient Electromagnetic Response of the Sedimentary-Metamorphic Iron Deposit in Eastern Hebei Province,China

本文介绍瞬变电磁方法在冀东沉积变质型铁矿勘查中的应用。根据冀东地区成矿地质背景,以及BIF型铁矿的矿体形态、产状、赋存部位、规模大小及含矿岩层特征,建立了层状模型、透镜体模型与向形模型三种地质模型。将地质模型转化为可处理的地球物理模型,按实际地质情况改变模型参数,完成了瞬变电磁的时域二维正演计算。分析了三种模型在多种情况下(低阻覆盖层、不同倾角、不同深度、发射源特征和不同电阻率)的瞬变电磁响应特征。结果表明,低阻覆盖层对于瞬变电磁场的扩散有一定的屏蔽作用,因而减小探测深度;随着矿体倾角的逐渐增大,瞬变电磁响应强度逐渐减弱,水平板状矿体比垂直板状矿体的响应要更为明显;勘探效果随着地质体埋深的增加而逐渐减弱;发射磁矩的大小对于瞬变电磁场的强弱产生直接影响,在实际应用中需要结合多方面因素,选择最佳的发射边长来达到最佳的勘查效果。利用冀东司家营地区进行的瞬变电磁勘察数据,研究了该地区铁矿床的瞬变电磁响应特征,验证了上述模型的结论。

This paper presents the application of the transient electromagnetic method( TEM) to prospecting of sedimentary metamorphic iron deposits in eastern Hebei Province. Considering the geological setting and features of the orebodies in this area,three geological models are established,i. e.,the laminated model,lens model and syncline model. Then,these geological models are transformed into geophysical models with different geophysical properties. A 2D forward calculation is accomplished by the time-domain finite difference method based on the geo-electric parameters. With the forward method,transient electromagnetic responses for the three models are analyzed under complex conditions such as,conductive overburden,different angles and depths,different resistance and sources. The results show that the conductive cover can screen the diffusion of the TEM field to some extent,thus lowering the exploration depth. With the increasing angle of dip angles of the ore body,the TEM response strength becomes weaker. The signal of the horizontal tabular model is stronger than that of the vertical tabular model. The exploration effect will decline with growing depth of the ore body. The emission magnetic moment determines the signal strength to a certain degree,and many factors should be taken into consideration in practice. Using the data from the TEM survey at the Sijiaying iron deposit of eastern Hebei,this article analyzes TEM response of this mine and proves the model conclusions above.