The Inversion Of Magnetic Anomaly Of Thin Plate On The Undulating Topography Directlyby Characteristic Points And Its Application

The actual ground magnetic exploration is often measured on the undulating observation surface,especially in mountain areas of western China.When the undulating degree of the topography is large or the underground magnetic body is close to the surface,the magnetic anomaly curve shape tends to become complicated or even distorted,then the topography has a great influence on the inversion and interpretation of the magnetic anomaly.Therefore,in order to highlight the useful anomaly information and interpret anomaly characteristics more accurately,there are two general approaches,one is to seek direct interpretation method on the undulating topography,the other is interpretation after the topographic correction(conversion of curved-surface data into horizontal-plane data).In response to this situation,the characteristic point method to directly interpret the magnetic anomaly of the thin plate on the undulating topography is discussed in this paper,with a view to acting a certain guiding role in the relevant production practice.The direct interpretation method of infinite extension thin plate on the undulating topography(characteristic point method)is proposed by a teacher named Guo Shaoyong(1980)of Chengdu University of Technology.It is used to inverse magnetic anomaly Za of thin plate directly on the undulating topography(single slope,ridge,valley and arbitrary valley or ridge terrain).In this method,the ratio of the minimum and the maximum on the Za curve of the main section of thin plate on the undulating topography and the distance between the extreme points are used to obtain the angle ?,the depth h and the location of the center of the top of thin plate(the projection point of the top center on the ground),so as to achieve the purpose of inversion.In this paper,based on previous studies,the characteristic point method is extended to the magnetic anomaly ?T of thin plate.It is found that the ratio of the minimum and the maximum and the distance between the extremum points in the ?T curve on the undulating topography(single slope,ridge,valley and arbitrary valley or ridge terrain)also can be used to obtain the angle ?,the depth h of the center of the top ofthin plate and the location of the projection point of the top center on the ground.In this paper,the formulas of inversion for magnetic anomaly of thin plate directly by characteristic point method on the complex slopes and arbitrary complex slope topography are also deduced.Then by summarizing the characteristic point inversion method of magnetic anomalies of thin plate on horizontal topography,we can get the formulas of direct inversion for the magnetic anomaly of thin plate on the arbitrary topography(six terrains)and add new contents for the characteristic point method of inversion.The accuracy and validity of the method has been proved by the model verification and inversion calculation under six topography conditions(horizontal terrain,single slope,complex slope,ridge,valley and arbitrary undulating terrain)and the applicable conditions of this method was discussed.After analysis,it is found that the direct inversion by the characteristic point method is also effective for the thick plate satisfying certain conditions.The degree of undulation of the topography also affects the accuracy of the inversion result to a certain extent.In the actual data processing of this paper,there are two magnetic anomaly sections in the magnetic field data(?T and Za)of a magnetite mining area in Sichuan Province,and the anomaly characteristics are similar to finite extension thick plates and infinite extension thin plates,respectively.The inversion results of these two magnetic anomaly sections directly by characteristic point method are similar to the results by using gravity and magnetic 2.5-D human-computer interaction inversion method.It shows that the characteristic point method described in this paper has a practical value for the preliminary interpretation of magnetic measurement data.