3D Compact Inversion For Gravity And Gravity Gradient Data

Gravity exploration is one of the important means to obtain the density distribution of underground target,which is widely used in oil and gas,metal and nonmetal mineral,engineering and environmental exploration.3D inversion is an important method and research hotspot of quantitative interpretation in gravity exploration.However,due to the complexity of the density distribution of underground target,the diversity of actual geological conditions and the multiple solutions of inversion method,there are still many shortcomings in the classical 3D inversion method.First of all,there are usually redundant structures in the inversion results of threedimensional inversion method with smooth model constraints,which can only approximate the center buried depth and boundary shape of the target body,and can not accurately reverse the sharp boundary information of the target body.Secondly,the work area usually obtains the accurate density value of the target through lithology collection or logging and other methods.However,the classical three-dimensional inversion method can't make full use of the known physical properties to obtain the calculation results of the residual density and high focus.Finally,compared with the gravity data,the gravity gradient data has higher resolution and is more sensitive to the change of surface density.However,how to make full use of the gradient data to carry out inversion calculation to obtain more accurate and stable inversion results remains to be studied.In order to solve the above three problems,this paper firstly studies the threedimensional compact inversion method of gravity data.Based on the minimization of the field source volume and the fitting of the abnormal data,the objective equation is established and solved,and the prior physical property information is regarded as the "objective density".Model tests show that the method can obtain inversion results with sharp boundary and accurate contour under the condition of accurate prior physical information.However,when the prior physical information is unknown or inaccurate,the inversion results are often not ideal,and there are some problems such as too sparse.In order to solve the problem of over dependence on "target density" in compact inversion,this paper also carried out the research on the compact 3D inversion method of gravity data with the combination of compact constraints and smooth constraints.Through the same model test,the improved method can accurately reverse the distribution of abnormal bodies without "target density".The improved method retains the advantages of compact inversion method and has wider applicability.At the same time,this paper also makes a comparative analysis of different inversion results of gravity and gravity gradient data in compact inversion method.Through comparative analysis,it is found that the inversion results of gradient data gxy are better than other data.Finally,the improved inversion algorithm is applied to the gravity data of Maobei area.The results show that the depth,strike and residual density of the abnormal rock mass obtained by inversion are consistent with the actual data.