The Apparent Density Mapping Approach In Spherical Coordinates And Its Application In Chinese Mainland

Apparent density mapping is an important approach of regional gravity data interpretation,which serves for the division of regional structural units,the delimitation of structural boundary,etc.,and provides an important basis for the study of regional geotectonics.The conventional apparent density mapping approach adopts Cartesian coordinate system,and assumes that the density layer and gravity observation surface are plane.However,for continental and global scales,the curvature of the earth should be taken into account,and the assumption of the conventional apparent density mapping method is not tenable.It is necessary to develop the new apparent density mapping approach in spherical coordinates.Unfortunately,the former gravity forward or inversion methods in spherical coordinates are only available for the field of high altitude observation surface,which results in large errors in the ground surface observation points.On the basis of previous studies,we present high-precision forward algorithm and apparent density mapping method of gravity anomaly in spherical coordinates,which are suitable for high-precision forward or inversion of regional ground observation surface and can provide technical support for regional geological structure research.Based on the systematic analysis of Taylor integral expansion method and Gauss Legendre method,the improved Gauss Legendre algorithm and adaptive subdivision algorithm are proposed to improve the gravity forward modeling accuracy of Tesseroid cell in the ground observation surface.Meanwhile,on the basis of the improved forward algorithm in this paper,the gravity forward algorithm flow and apparent density mapping algorithm flow in spherical coordinates of density layer are presented.In the aspect of tesseroid cell forward modeling,many tests of spherical shell model show that the improved forward algorithm in this paper is effective and applicable to the gravity high-precision forward modeling of ground surface observation.In the forward modeling and mapping of regional density layer,the theoretical model test shows that the results of the method are close to the theoretical values,and it is suitable for effective apparent density mapping of ground surface observation.The approach has been applied to the Chinese mainland and the crustal apparent density distribution of Chinese mainland is obtained.The results show that the crustal apparent density distribution of Chinese mainland is low in the west and high in the east,and increase gradually from the west to east,with a range of 2.45?2.81g/cm~3.In Western Area,the apparent density is lower than 2.67g/cm~3 due to the large thickness of the crust caused by the compression of the Eurasian plate and the Indian plate.The Middle Area is the transition area,and its apparent density mapping range is 2.52?2.75g/cm~3.The crust in the eastern region is thinner due to the subduction of the Pacific plate,and its apparent density is higher than 2.67g/cm~3.The results of cruatal apparent density mapping of Chinese mainlannd have a good correspondence with the block boundary and regional geological structure.