Preparation of the free-air gravity anomaly map in the land and seas of China and adjacent areas using multi-source gravity data and interpretation of the gravity field

Multi-source gravity data including measurements on land, shipboard surveys, altimetry, Bouguer gravity anomalies and EGM2008 global gravity model data are collected and analyzed for compiling the freei-air gravity anomaly map on the scale of 1 : 5000000 in the land and seas of China and adjacent areas. The data merging and integration method is established based on analysis of the accuracy and resolution of different sources of gravity data. Evenly distributed land gravity data could be gridded in flat land areas. A synthetic method using Bouguer gravity and high precision topographic data to calculate the free-air gravity data is proposed to fill the land areas where the gravity measurement is hardly or impossible to implement due to complex terrain. Data comparison between the synthetic free-air gravity data, measured data and EGM2008 data proves that using the synthetic data is a better choice for filling the areas lacking real data. This method is also applied to the continental and island areas outside mainland China where there are no real data collected. The STD analysis method is applied to the northern part of the South China Sea to compare the accuracy of three independent gravity data, the shipboard gravity data and two kinds of satellite altimetry derived gravity data, the SS series from Scripps, US and DNSC08GRA from DTU, Denmark. SS V18. 1 and DNSC08GRA data, which show very good consistence in terms of accuracy and resolution. The standard deviation of the two kinds of satellite gravity data in the SCS is less than 2mGal, which is a major improvement compared with the former SS V16. 1. With the rectification of shipboard gravity data, the wide coverage of satellite gravity data could be used for small-scale free-air gravity mapping. In the coastal zone, the EGM2008 global potential model is used as a reference field providing a seamless gravity transition from land to ocean. Wavelet transform and multiscale analyses are applied to decompose the free-air gravity field in China and its adjacent areas to help the gravity field interpretation. Eight anomaly areas are confined by three horizontal and four vertical first-order gravity gradient belts. Anomaly subareas and the second-order gravity gradient belts are delineated in the gravity field to provide evidence for establishing the tectonic framework in the block tectonic scheme.