Application of the improved filter based on the Theta method to edge detection of potential field data: an example from the Luzong ore district

Edge detection plays an important role in interpretation of potential field data, which has been widely used to image geologic boundaries such as contacts and faults. The outlines of the geologic boundaries can be determined by tracing the enhanced analytic signal. Many filters are employed to detect and enhance the edges. The horizontal and vertical derivatives are commonly used to enhance edge features, but they can only outline the edges of large-amplitude anomalies. In order to display large and small amplitude anomalies simultaneously, some balanced filters have been proposed. We define new filters based on the Theta map method, using second-order horizontal and vertical directional derivatives, and display large and small amplitude edges simultaneously. These methods were tested on synthetic gravity data, and compared with other traditional filters. The results show that the new filters can achieve better results and reveal more details. The method has been applied to gravity and magnetic data acquired in the Luzong ore district in the Middle and Lower Yangtze River Valley mineralization belt. Based on the analysis of relations between lithology, density and magnetic susceptibility, calculated results are analyzed. Analysis results on gravity and magnetic data accurately depict the location of the Tanlu fault zone. A boundary feature in the regional gravity to the north of the Yangtze River is interpreted to be part of the Yangtze River fault. The identified boundaries from the magnetic data are consistent with the margins of the Luzong basin, and indicate that the bounding structures are normal faults that dip toward the basin. Some circular or approximately circular closed boundaries occur around the periphery of the Luzong basin. Recent exploration results confirm that there are intrusions hidden beneath the enclosed area, where skarn with iron copper mineralization is present. The results of this study provide significant guidance for further exploration.