Research On Integrated Application Of Geophysical Information On Geological Mapping Of Barkol Basin

Geological mapping in covered areas is a key domain of geological survey at home and aboard since the new century.The goal of this work is to obtain the geological information of the overburden and the underlying bedrock,including the basement relief,the geological structure of the bedrock surface and the inner composition of the overburden.How to carry out this work has always been a hot topic in the geoscience field.The pilot work of geological mapping in the covered area is conducive to exploring and innovating modern geological mapping theories and methods,forming a set of technical standards and methodology system for geological surveys in covered areas.Geophysical exploration is an indispensable tool in geological mapping in covered areas.This paper is based on the goals and tasks of 1:50 000 Banfanggou,Xiaoliugou,Yiwujunmachang,Koumenzi geological mapping pilot project from China Geological Survey.Based on various geophysical data acquired in Barkol Basin area,research of the extraction,fusion and application methods of geophysical information were conducted around the Cenozoic overburden structures,bedrock properties,unground water resource within the basin and so on.The paper used the information of strata and structure obtained from surface geological investigation,as well as the results of petrophysics statistical analysis to analyze the possible causes of geophysical anomalies.Based on the tectonic setting and evolution history of the Barkol Basin,regional gravity data,regional magnetic data and magnetotelluric(MT)sounding data revealed the tectonic framework.The tectonic trend is in NW direction,and Cenozoic sedimentary layers with large thickness are developed both in the south and north sag regions.Large-scale magmatic rocks are located deep in the bedrock.The main work and results of this paper include the following aspects:(1)The distribution of faults within the basin was determined through the identification and extraction of the linear structures using gravity and magnetic anomalies with the combination of results of MT,electrical resistivity tomography(ERT),and shallow seismic exploration.The faults were classified as main faults,basement faults and Cenozoic active faults.It was suggested that the mains faults were related to the formation of the basin.The basement fault distribution had a high correlation with the structural characteristics of bedrock exposed areas.The Cenozoic active faults reflect the sedimentary characteristics of the Cenozoic sedimentary layers.(2)Thickness information of the Quaternary and Palaeogene-Neogene sedimentary layers were extracted from the MT results.Based on the statistical petrophysical data and disclosure of drilling cores,two-dimensional forward model of the gravity anomaly was carried out to obtain the density information of bedrock strata.Based on the stratigraphic and tectonic distributions of the bedrock outcrops in the north and south of the basin,it was inferred that the underlying bedrocks of the Cenozoic overburden were mainly Ordovician and Carboniferous strata,and the lithology was dominated by sandstone and conglomerate.(3)The characteristics of the Quaternary and Palaeogene-Neogene layers in the Barkol Basin are the focus of this study.Controlled by the thickness information revealed by MT,ERT,shallow seismic exploration and drilling cores,the thickness of Quaternary layer was estimated by the gravity data.Then the thickness distribution of the Quaternary layer was obtained.On this basis,the gravitational response resulting from the density difference of the Quaternary layer relative to the Palaeogene-Neogene layer was removed to simplify the density model of the study area.The effective density contrast was obtained by 2D forward modelling of gravity with the constraint of MT results.Finally,the thickness of the Palaeogene-Neogene layer was obtained by inversion.The distribution of Cenozoic sedimentary layers was mainly controlled by the regional leftlateral strike-slip tectonic movement,which reflected the multiple Cenozoic uplift in the study area.(4)Based on the hydrogeological information about the aquifer,the shallow electrical structure revealed by the ERT method and well information,was used to evaluate the distribution of the aquifer and the depth of the water table.(5)2D and 3D processing of the aeromagnetic and surface magnetic data based on multiple methods was performed.The spatial distribution and magnetic characteristics of the concealed rock were determined with the combination of the MT results.From the spatial relationship between the concealed rock and the surrounding outcrops,it could be inferred that the concealed rock may be classified as the early Permian intrusion.The difference of the magnetic susceptibility can be reasonably explained by the tectonic evolution history of the study area.Gravity-magnetic data and profile geophysical data can complement each other and verify each other so as to comprehensively reveal the geological structure and texture information of the overburden and bedrock.It has the certain reference significance for carrying out geological mapping work in the similar covered area.The information from outcrops,drillings and wells can constrain the information from seismic profiles,electrical-electromagnetic profiles,etc.Then they can both constrain the information from gravity exploration,magnetic exploration,etc.Finally,it is constrained by prior information such as petrophysical properties and geological theory,which can improve the quality and efficiency of geological mapping and reduce the cost.