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.