三维AMT正反演技术对贵州马坪含金刚石岩体探测的精细解释

Fine Interpretation of the exploration results of diamond-bearing rock masses in Maping area,Guizhou using the 3D AMT forward modeling and inversion technologies

贵州镇远马坪“东方一号”岩体是我国首次发现的含金刚石原生矿母岩,马坪地区发现的岩体属于金伯利岩浆体系的浅部相,其深部可能存在规模较大的隐伏岩管或岩筒。为揭示马坪地区深部含金刚石隐伏岩管或岩筒的空间展布特征,在区内开展了80 m×40 m高密集网度的音频大地电磁勘探工作;利用三维正演技术模拟研究区纯地形响应并在实测数据中去除,得到的定性解释结果在一定程度上恢复了被静态效应扭曲的AMT阻抗相位不变量分布形态;使用AR-QN拟牛顿反演方法对数据进行三维反演,根据研究区岩性统计结果设定地下单元的电阻率变化区间,获得了可靠的三维电性结构;最后依据地表发现的岩筒、钻孔揭露的多条岩脉等地质资料对该地电模型进行精细解释,勾画出了隐伏岩管(岩筒)的形态,为区内下一步金刚石原生矿找矿方向及预测提供了地球物理依据。

The"Dongfang No.1"rock mass in the Maping area,Zhenyuan County,Guizhou is the parent rock of a primary diamond-bearing deposit discovered in China for the first time.Studies have shown that the rock mass found in the Maping area is of the shallow facies of the kimberlite magmatic system,and large-scale concealed rock pipes or buckets may exist in the deep part.To reveal the spatial distribution of deep diamond-bearing concealed rock pipes or buckets in the Maping area,this study carried out the audio-magnetotelluric(AMT)data acquisition in the area using a high grid density of 80 m×40 m.Then it simulated the pure terrain response in the study area using the 3D forward modeling and deducted the pure terrain response from the measured data.The obtained qualitative interpretation results restored the distribution morphology of AMT impedance phase invariants to some extent that was distorted by static effects.Afterward,this study performed the 3D inversion of the data using the AR-QN quasi-Newtonian inversion method.Based on the lithologic statistical results of the study area,the resistivity variation intervals of the underground units were set during the inversion,obtaining a reliable 3D electrical structure.Finally,this study carried out a fine interpretation of the geoelectric model of this area based on geological data such as rock tubes found on the surface and multiple dikes revealed by boreholes,outlining the morphology of concealed rock pipes or buckets.This study will provide a geophysical basis for the future prospecting and prediction of primary diamond deposits in this area.