Magnetic dipole inversion study of the lithospheric magnetic field in Changning area

Wu Xiaotao; Chen Bin; Dong Chao

doi:10.11939/jass.20240015

Abstract：The Changning region of Sichuan Province is located at the junction of the Sichuan Basin and the Yunnan-Guizhou Plateau in southern Sichuan,Since 2015,seismic activity has significantly increased,prompting scholars to conduct in-depth investigations into the funda-mental causes of this surge in seismic activity.The study of magnetic material distribution is of paramount importance for elucidating the geological evolution and underground medium struc-ture of the Changning region.Magnetic material localization is a key aspect of the investiga-tion,relying on the inversion of magnetic data obtained from detailed magnetic sensor measure-ments.This process helps to precisely determine the spatial coordinates of magnetic targets. Analysis of the spatial distribution of lithospheric magnetic field,acquired from 952 mea-surement points with an average spacing of 2 km in Changning,Sichuan,reveals pronounced disparities in magnetic field distribution between the eastern and western sectors of the Chang-ning backslope.Notably,these variations appear to correlate with areas of heightened seismic activity within the region.For instance,the inner segment of the backslope predominantly exhibits positive lithospheric magnetic field values,juxtaposed against negative values along the periphery.The most significant concentrations of negative magnetic fields are observed along the northwestern boundary of the backslope,while the northeastern sector predominantly manifests positive lithospheric magnetic fields,with negative values dominating the southern flank. This paper adopts the magnetic dipole theory as a foundational framework to explore the distribution characteristics of magnetic dipole structures within the Changning area.By inver-ting the magnetic moment information derived from equivalent magnetic dipole sources,the study aims to delineate the spatial distribution and properties of underground magnetic dipole structures. Based on the lithospheric magnetic field data,a technical approach for regional magnetic dipole inversion was established.The inversion technique used was as follows:the lithospheric magnetic field of the Changning Anticline after polarization was subjected to a fast Fourier transform to obtain the lithospheric magnetic field distribution corresponding to rock materials at different depths.On this basis,the magnetic moment distribution of magnetic dipoles at diffe-rent depths underground was inverted,and the magnetic dipole structure underground in the study area was reconstructed. The results indicate that magnetic materials in the shallow region of Changning are scattered and relatively weak.Magnetic materials are concentrated at depths below 6 km,mainly in the northwestern end of the Changning anticline,which is also a seismically active area,coinciding with the area where magnetic materials are concentrated.