Edge detection plays an important role in interpreting potential field data, and is widely used to delineate geologic boundaries and structures. Geologic boundaries can be determined by tracing the enhanced analytical signal, and many filters are developed to detect and enhance the edges. 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 2 nd 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-magnetic data acquired in the Luzong ore district in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB, Eastern China). Based on the relations between lithology, density and magnetic susceptibility, the calculated results were analyzed. The edge detection results accurately depict the location of the Tanlu fault zone. A gravity boundary 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 faults dip towards the basin. Some ring-like closed boundaries occur in the periphery of the Luzong basin. Recent exploration results confirm that there are buried intrusions beneath the gravity-magnetic anomaly bodies, where Fe-Cu skarn mineralization is present. The results of this study provide significant insights for regional deep-level Fe-Cu exploration.

The propagation characteristics of F-(flexural) wave and the reflection from vertical cracks in pipe-piles subjected to an eccentric vertical excitation were numerically investigated using elastodynamic finite integration technique (EFIT). The accuracy of the self-written code was validated by comparing computed results against the measured data. The snapshots of circumferential, radial and vertical velocities in intact and cracked pipe-piles, as well as the velocity histories of the pile-top were presented. It is found that not only L-(longitudinal) wave (corresponding to axisymmetric mode) but also F-wave (corresponding to nonaxisymmetric mode) can be excited in pipe-piles subjected to an eccentric vertical excitation. F-wave causes motion in all directions, while L-wave only causes vertical motion. The three-dimensional (3-D) interference is weak at 90 • away from the excitation position for circumferential velocity response, while at 45 • or 135 • for radial velocity response. The reflected F-wave (RFW) from the pipetoe has multiple peaks due to the dispersion effect of F-wave. Reflection occurs when F-wave encounters vertical cracks, while L-wave is not sensitive to vertical cracks. The intensity of RFW from vertical cracks varies with excitation-andsensor modes. For shallow vertical cracks, the RFW strength varies with the sensor positions; while the intensity of the RFW caused by deep vertical cracks varies little with the sensor positions. With the increase of crack length, the strength of the first arrival RFW due to the vertical crack has no obvious change; however, the strength of RFW from the pile-toe decreases.

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A decreasing radius iterative method in spatial domain is presented for regional-residual separation of potential field data. A new eight-point circumference average formula is derived by arithmetical average of potential field values at eight points along the circumference of a circle of given radius, which can be seen as a filter for calculating regional anomaly from gravity or magnetic data. The transfer function of the filter has a main lobe and multiple side lobes. When the radius becomes large, the number of the side lobes increases, and the filter characteristics become bad. The product of the transfer functions for various values of the radius from large to small is constructed, which is defined as decreasing radius iterative transfer function herein, with the largest radius as its parameter. The decreasing radius iterative transfer function is similar to the low-pass filter, and the cut-off wave number is inversely proportional to the largest radius. Based on the decreasing radius iterative transfer function, the decreasing radius linear iterative method in spatial domain is presented for separating regional anomaly, and the residual anomaly is obtained by subtracting the regional anomaly from the gravity or magnetic data. Furthermore, by constructing the nonlinear correction coefficient, the linear iterative formula of the decreasing radius linear iterative method is transformed into the nonlinear iterative formula, and the decreasing radius nonlinear iterative method in spatial domain is proposed. The decreasing radius nonlinear iterative method is tested with synthetic data from model and a field data set from the Nihe iron deposit in Anhui Province. The results show that the proposed method effectively suppresses false anomaly and high frequency interference, reduces anomaly distortion, and separates regional anomaly and residual anomaly from the gravity and magnetic data.