Application Of 3D Density Inversion Algorithm In Western Sichuan And Sanhe-pinggu Region

Gravity inversion is an important method to research density structure.The accuracy of inversion results is one of the important indexes to measure the inversion algorithm.In order to make an increase in the accuracy of the inversion results,the iterative idea is combined with the gravity density structure inversion algorithm of growth method.The calculation program is rewritten based on Python to realize iterative inversion.Firstly,a synthetic model was established to test the effect of iterative inversion.Then the bouguer gravity anomaly data in western Sichuan and Sanhe-Pinggu area were used for iterative inversion.The density structure of the crust in the study area is obtained.Finally,GMT and 3D visualization program based on Python were used to map the inversion results.The density structure is analyzed and explained with seismic activity and geological structure results.The following results are obtained:（1）The iterative inversion of the synthetic model shows that density bodies can better reflect the position of the synthetic model,and the error between the forward gravity anomaly and the observed gravity anomaly is further reduced.It shows that the iterative calculation of the gravity density inversion can increase the accuracy of the inversion results.（2）In western Sichuan,the density contrast ranges from-150 kg/m3 to 150kg/m3.The longquanshan fault zone has high-density bodies at a depth of about 17 km,and its distribution pattern is nearly consistent with the strike of the Longquanshan fault zone.The Dujiangyan-Dayi County area has low-density bodies at a depth of about 15 km,and its distribution pattern is almost parallel to the strike of the Longquanshan fault zone.It is inferred that this phenomenon is caused by the eastward extrusion of songpan-Ganzi block,the obstruction of Yangtze block,the strata depression of Dujiangyan-Dayi County,and the ancient strata uplift of Longquanshan area.（3）In the Sanhe-Pinggu,the density contrast ranges from-300 kg/m3 to300kg/m3.The density structure results show that the density contrast between the two sides of the fault zone is apparent,indicating that the fault zone controls the development of crustal depression and uplift.At the depth of 18 km,the density contrast between the east and west sides of the Xiadian fault zone is still apparent.The seismic activity of Xiadian fault zone is frequent.With the increase in depth,the earthquakes are more concentrated in Mafang-Qixinzhuang.It is indicated that stress energy accumulation still exists at the depth of 18 km.（4）The 3D visualization program based on Python can better display the results of gravity density inversion,which is helpful to analyze the distribution characteristics of density bodies in the region.