Controlled-source electromagnetic data processing based on gray system theory and robust estimation

We propose a novel method that combines gray system theory and robust M-estimation method to suppress the interference in controlled-source electromagnetic data. We estimate the standard deviation of the data using a gray model because of the weak dependence of the gray system on data distribution and size. We combine the proposed and threshold method to identify and eliminate outliers. Robust M-estimation is applied to suppress the effect of the outliers and improve the accuracy. We treat the M-estimators of the preserved data as the true data. We use our method to reject the outliers in simulated signals containing noise to verify the feasibility of our proposed method. The processed values are observed to be approximate to the expected values with high accuracy. The maximum relative error is 3.6676%, whereas the minimum is 0.0251%. In processing field data, we observe that the proposed method eliminates outliers, minimizes the root-mean-square error, and improves the reliability of controlled-source electromagnetic data in follow-up processing and interpretation.

3D joint inversion of controlled-source audio-frequency magnetotelluric and magnetotelluric data

Different geophysical exploration methods have significant differences in terms of exploration depth,especially in frequency domain electromagnetic(EM)exploration.According to the definition of skin depth,this difference will increase with the effective detection frequency of the method.As a result,when performing three-dimensional inversion on single type of EM data,it is not possible to effectively distinguish the subsurface geoelectric structure at the full scale.Therefore,it is necessary to perform joint inversion on different type of EM data.In this paper we combine the magnetotelluric method(MT)with the controlled-source audio-magnetotelluric method(CSAMT)to study the frequency-domain three-dimensional(3D)joint inversions,and we use the unstructured finite-element method to do the forward modeling for them,so that the numerical simulation accuracies of different electromagnetic methods can be satisfied.By combining the two sets of data,we can obtain the sensitivity of the electrical structure at different depths,and depict the full-scale subsurface geoelectric structures.In actual mineral exploration,the 3D joint inversion is more useful for identifying subsurface veins in the shallow part and blind mines in the deep part.It can delineate the morphological distribution of ore bodies more completely and provide reliable EM interpretations to guide the mining of minerals.

Precision of meshfree methods and application to forward modeling of two-dimensional electromagnetic sources

Meshfree method offers high accuracy and computational capability and constructs the shape function without relying on predefined elements. We comparatively analyze the global weak form meshfree methods, such as element-free Galerkin method （EFGM）, the point interpolation method （PIM）, and the radial point interpolation method （RPIM）. Taking two dimensional Poisson equation as an example, we discuss the support-domain dimensionless size, the field nodes, and background element settings with respect to their effect on calculation accuracy of the meshfree method. RPIM and EFGM are applied to controlled- source two-dimensional electromagnetic modeling with fixed shape parameters. The accuracy of boundary conditions imposed directly and by a penalty function are discussed in the case of forward modeling of two-dimensional magnetotellurics in a homogeneous medium model. The coupling algorithm of EFG-PIM and EFG-RPIM are generated by integrating the PIM or RPIM and EFGM. The results of the numerical modeling suggest the following. First, the proposed meshfree method and corresponding coupled methods are well-suited for electromagnetic numerical modeling. The accuracy of the algorithm is the highest when the support-domain dimensionless size is 1.0 and the distribution of field nodes is consistent with the nodes of background elements. Second, the accuracy of PIM and RPIM are lower than that of EFGM for the Poisson equation but higher than EFGM for the homogeneous medium MT response. Third, RPIM overcomes the matrix inversion problem of PIM and has a wider selection of support-domain dimensionless sizes as compared to RPIM.

New methods of controlled-source electromagnetic detection in China

As an important branch of geophysical exploration method,the electromagnetic method with artificial source has advanced rapidly in the past decade.These methods are classified as airborne electromagnetic method,ground-air electromagnetic method,ground electromagnetic method,and marine electromagnetic method.Over the years,researchers in China have made significant improvement to the fundamental theory,forward modeling and inverse for series of electromagnetic detection methods.Conversely,significant progress was made in the development of corresponding equipment.The researched techniques and their developed equipment have been successfully utilized to detect underground targets as deep as 10 km.However,there is increasing need for deep resources exploration,urban subsurface study,and prediction,monitoring and detection of geological hazards.To meet the increasing need and catch up with the advanced international level of exploration technologies and developed equipment,there is urgent necessity and requirement to continue developing geophysical methods and the corresponding equipment.