Research On Edge Depth Inversion Of Geological Body Based On Gravity And Magnetic Data

Gravity and magnetic exploration is based on the differences in physical properties(density,magnetism)between different rocks and ores,and there are obvious density or magnetic differences at the edges of different geological bodies.Therefore,the spatial position of geological body edges can be inferred according to gravity and magnetic anomalies.The edge of the geological body includes horizontal position and depth position.The horizontal position recognition method has been very mature in practical application,but the application effect of the edge depth inversion method is not ideal.Therefore,it is necessary to study the theory of the edge depth inversion method and improve its practical application effect.At present,the edge depth inversion methods of geological bodies mainly include Werner Deconvolution,Analytical Signal Amplitude(ASA),Local Wave-number,Tilt-depth method,Euler Deconvolution and Curvature Attributes methods.These methods have the problem of adaptability and stability.The adaptability of the edge depth inversion method refers to the adaptability of different types of input data to each edge depth inversion method.There are many studies on the adaptability of Euler deconvolution method,but there are few comparative studies on the inversion accuracy of different input data,and few studies on the adaptability of other methods.The instability of the solution mainly includes the instability caused by the input data containing noise and ill-conditioned inversion equations.The data noise problem is usually processed by filtering method,but the degree of filtering is difficult to control,which easily leads to the reduction of inversion accuracy.There are few studies on the illconditioned problem of inverse equations.Starting from the forward formula of edge depth inversion method,this paper analyzes the factors affecting the accuracy of edge depth inversion results(data source,difference between real model and theoretical model,condition number of coefficient matrix of inversion equations,etc.),and studies the influence of these factors on the accuracy of inversion results.It is found that the main factors affecting the adaptability of various edge depth inversion methods are the type of data source and the difference between real model and theoretical model.The main factor affecting stability is the condition number of coefficient matrix.By comparing the traditional edge depth inversion methods,it is found that among all the edge depth inversion methods,the Werner deconvolution method has the best stability and the highest accuracy of inversion results,followed by Euler deconvolution and curvature attribute methods;the accuracy of Tilt-depth method is the worst.The adaptability study of the edge depth inversion method show that Werner deconvolution and Euler deconvolution adapt to the most data types,and the curvature attribute and Tilt-depth method are the least.For gravity data,the analytical signal amplitude of vertical first derivative(VDR-ASA)is suitable for all methods,and the inversion accuracy is relatively high.This data has obvious advantages for inversion of steps with smaller thickness,but not for steps with smaller width.In addition,in the Werner deconvolution method,it is found that the horizontal derivative of the vertical first derivative(VDR-HDR)of gravity has higher inversion accuracy for narrow steps.The vertical second derivative(VDR-VDR)has higher inversion accuracy for steps with smaller thickness.Therefore,in order to further improve the adaptability of the Werner deconvolution method,considering the fusion of VDR-HDR and VDR-VDR,a new input data(aVDR-HDR+(1-α)VDR-VDR)is proposed,which can better invert steps with smaller width and thickness.The data source is called VDR-VHDR.The inversion results show that the new input data type takes into account the two factors of step width and thickness.According to the Poisson relationship of gravity and magnetic anomalies,there is also a similar law for magnetic anomalies,but it is not necessary to obtain the vertical first derivative of magnetic anomalies.The stability study of the edge depth inversion method show that for the Werner deconvolution method,the inversion results of VDR-ASA,VDR-VDR,and VDR-HDR are better,and the inversion results of the local wave number of VDR-k is the worst.For Euler deconvolution method,the stability of edge depth inversion results of VDR-ASA is the highest,followed by VDR-HDR,VDR-VDR,and VDR-k is the worst.Although the stability of inversion results of VDR-ASA for the curvature attribute method is strong,its practical application is limited due to the need to know the structural index and horizontal edge position.The stability of Tilt-depth method is the worst among all methods.The magnetic data source is similar to the gravity data source,but it does not need to calculate the vertical first derivative.In addition,by comparing the coefficient matrix condition numbers of different data sources,it is found that near the edge position,the coefficient matrix condition number of the VDR-ASA of the gravity anomaly reaches the minimum value.Therefore,this paper proposes a solution screening method based on the minimum value of the coefficient matrix condition number of the inversion equation set,and combines the normalized vertical derivative of total horizontal derivative(NVDR-THDR)to achieve the purpose of understanding and screening.The gravity and magnetic anomalies are used to infer the plane position of faults in China seas and adjacent areas,and the apparent depth of faults is inverted by using the improved Warner deconvolution method.The results show that the strikes of the faults in the China Sea and its adjacent areas are mainly NE and NW,followed by EW,and near-SN.Among them,the lengths of most ultra-crustal faults are in the range of 1 000-3 000 km,and their apparent depths lie between 20 km and 40 km.The lengths of crustal faults lie between 300 km and 1 000 km,and their apparent depths are between 10 and 30 km.Considering the distance between faults and oil-gas fields in the basin,the thickness of sedimentary layers and the apparent depth of faults,the key areas for oil and gas exploration were predicted as follows:the east of South North China Basin in the intracontinental rift basins;the southeast region of East China Sea Shelf Basin,the Taixinan and Qiongdongnan basins in the continental margin rift basins;Zhongjiannan Basin in the strike-slip pull-apart basins;the Liyue,Beikang,and the Nanweixi basins in the rifted continental basins.This work provides valuable insights into oil and gas exploration,mineral resource exploration,and deep geological structure research in the China Sea and its adjacent areas.