Application of the three-dimensional normalized full gradient method for multiple-object identification in gravity exploration: a case study of the Dachang tin-polymetallic ore district

Abstract The three-dimensional normalized full gradient (3DNFG) method was developed to identify multiple objects in complex geology. Two synthetic models- one with a single object and the other with multiple objects- were designed to evaluate the performance of 3DNFG. Through the analysis and study of these synthetic models, it was found that the planar position of targets can be easily determined using the 3DNFG response, regardless of whether the model is simple or complex. To accurately estimate the depth of targets, it is crucial to determine the optimal harmonic coefficient. For the simple model, commonly used methods such as the maxima method are feasible. For the multiple-object model, the local optimal harmonic coefficient induced by each target should be identified separately, after which the depth of each target can be determined using the corresponding harmonic coefficient. The 3DNFG methos was then applied to process gravity data collected from the Dachang area in Guangxi. The analysis revealed that the upper interface of the concealed granite at Longxianggai is approximately H = -530 m. Furthermore, the 3DNFG response indicates that the granite intrusion is shallow in several areas, including Longxianggai, Ruomacun, Dafulou, Yangjiaojian, and Tongkeng. These findings are in close agreement with the results obtained from inversion using the controlled random search method. Both synthetic models and the case study demonstrate that the 3DNFG performs well in identifying multiple objects in complex geological environments.