内蒙古白云鄂博主东矿三维地电模型与短偏移距瞬变电磁三维正演分析

3D electric model and short-offset transient electromagnetic forward modeling analysis of the main and east mines of Bayan Obo deposit, Inner Mongolia

内蒙古白云鄂博大型稀土-铌-铁矿床历经近70年开采,浅部铁矿资源已开发殆尽,亟需寻找深部资源.利用电磁法摸清主矿和东矿深部矿体连接状态及赋存特征对进一步认识白云鄂博成矿模式和寻找深部接替资源意义重大.然而,该区复杂的构造、地层、岩性、地形等条件给电磁法勘探带来极大挑战.采用数值建模与模拟手段对关键观测要素进行分析有助于勘探设计和数据解释.本文以白云鄂博主东矿及其结合部位为主要研究区域,基于已有地质、钻孔等资料,构建了主东矿三维地电模型,利用时域矢量有限元算法开展了电性源短偏移距瞬变电磁法(SOTEM)三维数值模拟,并重点分析了矿坑地形对观测数据的影响程度以及不同收发装置下的深部矿体的三维电磁学响应特征和探测能力.结果表明矿坑地形会导致SOTEM电磁场响应值升高,在主矿西侧布置发射源,更有助于实现对连接部位下方铁矿体的有效探测.该工作可为今后野外实际观测中工作参数优选和数据解释提供依据.

The Bayan Obo large REE-Nb-Fe deposit in Inner Mongolia has been exploited for nearly 70 years. The shallow iron ore resources have been exhausted, so it is urgent to find deep resources. Using electromagnetic methods to find out the connection state and occurrence characteristics of deep orebodies in the main mine and the east mine is of great significance for further understanding the metallogenic model of the Bayan Obo deposit and searching for deep resources. However, the structure, strata, lithology and topography of this area are relatively complex, which brings great challenges to electromagnetic exploration. The analysis of the key observation elements by means of numerical modeling and simulation is helpful for exploration design and data interpretation. Based on the existing geological and borehole data, the three-dimensional (3D) geoelectric model of the main and east mine and their junction area in Bayan Obo deposit is constructed, and then the 3D numerical simulation of the grounded-wire source short-offset transient electromagnetic method (SOTEM) is carried out by using the time-domain edge-based finite element algorithm. The influence of pit topography on observation data and the 3D electromagnetic response characteristics and detection ability of deep orebody under different transmitter-receiver devices are analyzed. The results indicate that the mine pit can lead to an increase in the response value of the SOTEM electromagnetic field. Placing a source on the west side of the main mine is more conducive to achieving effective detection of the orebody below the connection area. This work can provide a basis for the optimization of acquisition parameters and data interpretation in the field observation in the future.