Edge Detection of Gravity Anomalies with Normalized Total Horizontal Derivative (NTHD)

There are several varieties of edge detection method. Edge detection and edge enhancement techniques play an essential role in interpreting potential field data. Some of common methods are: Tilt angle filter (TA), Total horizontal derivative of tilt angle filter (THDT), Theta filter, TDX filter, Hyperbolic tilt angle (HTA). These filters maximum values are when facing with the edge of an anomalous mass, and their minimum values are above the anomalous mass except tilt angle filter which is positive when over the source and passes through zero when over or near the edge. Local phase filters (edge enhancement methods) are based on the phase variation of the derivative quantities.
The mentioned filters have different advantages like flexibility in making new filters but a universal disadvantage of these methods is that they cannot display the large and small amplitude edges simultaneously. In this paper the ability of normalized total horizontal derivative (NTHD) method is shown and it compared with the other methods. The NTHD method is based on the ratio of the horizontal derivative to the maximum of nearby values which are in an arbitrary window. The maxima of the NTHD method are located on the edges of causative sources.
To determine the ability of NTHD method, it is applied on an artificial rectangular prism which is created with Matlab software. In order to find the stability of the method when facing a noisy data, a Gaussian noise created with randn command in Matlab area and added to the artificially rectangular prism and then the NTHD method applied on it. To evaluate the capability of this method with prevalent edge detection methods, a Matlab code has written and the numbered edge detection filters were applied on several artificially rectangular bodies which are in shallow and deep depths. The results showed that almost all filters delineated edges of shallow anomalies successfully but when they facing with the deeper anomalies their ability rising down and cannot detect the edges precisely. The excellence of the NTHD method in recognition of source edges is due to the fact that it can make the strong and weak amplitude edges visible simultaneously and can bring out more details. The edge detection technique (NTHD) was further applied on the Mobrun gravity anomaly which digitized from Grant and West (1965). The gravity data of Mobrun ore body consist of thirteen profiles. The distance between profiles is 60 meters and in each profile, data have space of 30 meters. In order to reduce the existent noise in these data, we upward data for a distance of 10 meters then we apply NTHD filter with a 1×1 window. The result of applying the NTHD method on Mobrun ore body are in concord with the prevalent results of exploration bore data and precisely detects the edges of anomaly. In order to examine the results of edge detection, we use the data of borehole that they dugout along the AB profile. Among the boreholes the BH2 borehole is near to the edge of Mobrun ore body and is in concord with the edge detection results of NTHD method.