音频大地电磁约束反演在地层识别中的应用:以哈密盆地为例

覆盖区地质填图是新时期地质填图的重要方向.音频大地电磁法（AMT）是覆盖区地质填图的重要地球物理方法之一，可以为地层和基岩面的刻画提供电性参数的约束.然而，常规的AMT反演无法精确刻画电性异常体的边界，且当缺乏其他地质与地球物理资料约束时，难以进行地质解译.针对此问题，基于数值模拟结果的可行性，利用哈密烟墩戈壁覆盖区的4条实测AMT剖面探讨了AMT约束反演在地层识别中的应用效果.相位张量分析指示研究区浅部（> 1 Hz）电性结构表现为二维特征，深部受三维结构影响；浅部为低阻，深部电阻率逐渐升高.采用不加约束二维反演获得了4条剖面的地下（< 3 km）电性结构.基于研究区的重力异常、地震解译结果、物性和钻孔资料，在二维反演结果上初步划分了渐新统-中新统和侏罗系地层的底界面；进而以这两个界面建立先验模型，并根据物性资料设置电阻率变化范围，进行AMT约束反演，获得了更优化的反演结果以及清晰可靠的渐新统-中新统和侏罗系的底界面.结果显示，研究区渐新统-中新统地层电阻率值略小于10 Ω?m，其底界面平均埋深为120 m；侏罗系电阻率值为10~100Ω?m，其底界面最深可达2 km.基岩面（侏罗系底界面）埋深整体上呈现为东南深、西北浅，这指示哈密烟墩地区中生代以来的沉积中心在东南部；此外，侏罗系地层与下伏的古生界地层存在角度不整合.研究表明，哈密烟墩地区新生代以来可能受到了近南北向应力挤压，在研究区中部形成近东西或北东东向的侏罗系隆凹相间的构造地貌格局.而渐新统-中新统地层底部的不整合面可能反映了后期褶皱构造的影响，这种隆凹作用相伴的褶皱构造可能具有同沉积性质. 

Constrained Inversion of Audio Magnetotelluric for Identifying Strata: A Case Study in Hami Basin

Geological mapping on covered area is a crucial research of new geological mapping. Audio magnetotelluric (AMT) is one of effective geophysical methods for geological mapping on covered area, which can provide the constraint of electrical parameters for depicting strata and bedrocks. However, conventional AMT inversion cannot accurately describe the boundaries of electrical anomalies, and it is difficult to perform geological interpretation when there is no other geological and geophysical data constraints. Based on the feasibility of numerical simulation results and four AMT profiles in the Gobi desert area of Hami Yandun, it tries to investigate the availability of constrained AMT inversion in strata identification. Phase tensor analyses indicate that the shallow (< 1 Hz) electrical structure is mainly two-dimensional (2D), while the deep is affected by 3D structure. The resistivity is low in the shallow and gradually increases in the deep. The underground electrical structures of four profiles were obtained by 2D unconstrained inversion. Based on gravity anomalies, seismic interpretation results, physical properties of rocks and borehole data in the research area, the bottom interfaces of Oligocene-Miocene and Jurassic strata were preliminarily divided from the 2D inversion results. Furthermore, a prior model was established using the two interfaces, where the resistivity change ranges were set according to physical property data. Then, constrained AMT inversion was carried out. The better inversion results and the clear and reliable bottom interfaces of Oligocene-Miocene and Jurassic were obtained. Research results show that the resistivity of Oligocene-Miocene stratum is slightly smaller than 10 Ω?m and the bottom interface is shallow with an average depth of 120 m. The resistivity of Jurassic stratum is about 10-100 Ω?m and the bottom interface is deep up to 2 km. The buried depth of bedrock surface (Jurassic bottom interface) is deep in the southeast but shallow in the northwest, which indicates that the sedimentary center since Mesozoic is in the southeast of Hami Yandun area. Moreover, an angular unconformity exists between Jurassic and underlying Paleozoic strata. The study supports that Hami Yandun area may go through the extrusion from nearly NS compression stress since Cenozoic, resulting in the morphotectonic pattern of Jurassic uplift and depression with nearly EW or NEE direction in the central part. Besides, the angular unconformity beneath the Oligocene-Miocene strata may reflect the influence of later fold structure. This kind of fold structure along with uplift and depression might have the synsedimentary property.