A Fast Three-Dimensional Inversion Method Of Magnetic Data With Equivalent Points Under Undulating Terrain And Exploration Of Burning Area

The structure of underground magnetic susceptibility can be obtained by three-dimensional inversion of magnetic data.Based on the inversion results,the underground target can be effectively interpreted quantitatively.Since the observation points remain unchanged in the undulating region,direct inversion without considering topographic relief will lead to deviation of inversion results.Therefore,it is of great significance to develop inversion methods with high precision and high efficiency for the undulating region for practical data interpretation.For undulating terrain areas,different grid division inversion methods are commonly used to fit undulating terrain and irregular geological bodies,thereby obtaining the magnetic characteristics of each division unit.Grid generation mainly includes unstructured grid generation and structural grid generation.The unstructured grid generation is flexible,but its calculation is complex and inefficient,while the structured grid generation is used to carry out inversion by constantly approaching the topography and the shape distribution of the geological body,which is simple and has high computational efficiency.In addition,under the condition of the observation surface and the topography level,The structural symmetry and translation equivalence can be used to reduce the double calculation.Based on this,this paper adopts hexahedral grid structure for subdivision.However,under the condition of topography and observation surface undulation,the subdivision unit no longer has structural symmetry and translation equivalence,but the corner points inside the subdivision are shared by eight hexahedral elements.Therefore,a kernel function calculation method based on equivalent points is proposed.Model tests show that the efficiency of kernel function calculation by this method can be improved by about 7 times.At the same time,the partition method and kernel function calculation method are not affected by terrain fluctuation and observation surface fluctuation.In the process of spontaneous combustion of coal seam,complex physical and chemical changes take place in iron bearing minerals such as hematite and pyrite in coal seam and surrounding rock at high temperature,and thermal remanency is obtained after cooling,forming burned rocks with significantly increased magnetic properties,showing high magnetic characteristics.Therefore,magnetic exploration can be used to effectively detect the burned area.In order to obtain higher resolution results,the UAV aeromgnetic survey usually adopts the earth-like flight mode along the rolling terrain.Moreover,due to the complicated topography of the burning area caused by coal mining,the proposed three-dimensional common point fast inversion method is adopted to invert the aeromgnetic anomaly data of the burning area of live Jirabbit well in Daliuta considering the terrain factors.The location of the combustion point in the burning area is determined by the inversion results.In order to further clarify the development trend of burning point,a simulation model was established based on the previous inversion results and the collected data of coal combustion direction and combustion velocity to simulate the magnetic anomaly changes in the burning area at different time intervals,so as to determine the shortest time interval for the effective magnetic anomaly changes in the burning area.The results show that the best time interval for aeromagnetic measurement in this area is 15 days.Based on this,a time-lapse aeromagnetic measurement technology system for the burning area is proposed,which regularly observes magnetic anomaly changes,and is applied to the detection of the burning area in the Daliuta Huojitu Well coal field.Through two inversion results,it is determined that there are two burning points A and B in the burning area.The horizontal combustion speed of the burning area A is 0.55m/day,showing a trend towards the northeast as a whole;The horizontal combustion rate in the fire zone B is 0.21m/day,showing a westward trend overall.