有限元-无限元耦合法在复杂金属矿体电阻率/极化率正演模拟中的应用——以庐枞盆地泥河铁矿床为例

Application of finite-infinite element couple method in forward modeling of complex metal ore body——examples of Luzong,Nihe iron deposit

金属矿电法勘探由于受限于探测深度和复杂矿体形态的解释,在深部矿产资源勘探中难受重视.安徽庐江泥河铁矿床是长江中下游成矿带中最主要的矿床类型之一,矿区复杂的地质结构和矿体形态以及矿体的大深度赋存状态给地球物理勘探特别是电法勘探带来了很大的挑战.为了研究电阻率/极化率法在大深度复杂金属矿体探测中的应用效果,本文以泥河铁矿某勘探剖面为例,在实际矿区的地质模型基础上通过简化建立了相应的地球物理模型,采用偶极-偶极装置及二极装置,对矿区进行直流电阻率/极化率数值模拟.提出采用有限元-无限元耦合法以解决传统有限元法截断边界所引起的问题.理论和计算结果表明:有限元-无限元耦合法相比传统有限元法,能显著提高计算的效率和精度.偶极-偶极装置的视电阻率断面图较二极装置能更好的区分不同地层及磁铁矿异常,而根据铁矿高极化特性,探测深度相对较大的二极装置的视极化率断面图能够更好的反映出异常体位置及范围.

Due to the limitation of investigation depth and data interpretation for complicated ore body, electrical exploration is paid little attention in deep mineral resources prospecting. This paper takes Nihe iron deposit as an example to discuss the application of electrical method in complex electrical structure area. Nihe iron deposit, located at Anhui Lujiang, is one of the main deposit types in the metallogenic belt of middle and lower reaches of theYangtze River. Not only the deep location and complicated shape of mineral, but also complex geological structure in mine area bring great difficulty to geophysical exploration, especially to electrical exploration. In order to study the application efficiency of resistivity/IP method in the use of deep detection for complicated deposits, an exploration profile of Nihe iron mine, for instance, was studied in this paper. First, a reasonable geophysical model was established by simplifying the geological model of ore area. Then resistivity/IP forward modeling was carried with pole-pole array and dipole-dipole array. Finite-infinite element couple method was proposed to solve the problem of tradition FEM which result from truncation boundary, the 2. 5D mapping functions of infinite elements were derived. Then, a new type of shape functions was proposed. After that, we integrated infinite element method into conventional finite element method to replace the mixed boundary conditions, which made the electrical potential distribute continuously in half space and decay to zero at infinity. A more superior property of Finite-infinite element couple method is the invariability of the global system matrix with variant source positions which makes this method be able to alleviate the computational burden and speed up inversions. Theoretical and numerical results show that calculation with Finite-infinite element couple method is more efficiency and has higher accuracy. Compared with pole-pole array, pseudo-section map of apparent resistivity with dipole-dipole array could distinguish different layer and anomalous body better. With consideration of the high polarizability characteristics of iron ore, pseudo-section map of apparent polarization with pole-pole array, which has deeper exploration, could reflect the location and scope of anomaly better.