Edge detection of magnetic body using horizontal gradient of pseudo gravity anomaly



Mapping the edges of magnetized bodies is fundamental to the application of magnetic data to geologic mapping. Whether as a guide for subsequent field mapping or as a predictive mapping tool in areas of limited exposure, delineating lateral magnetization changes provides information on not only lithological changes but also on structural regimes and deformation styles and trends. Adding contact locations to maps of the magnetic field or enhanced versions of the field (derivatives, transforms, etc.) improves significantly the interpretive power of such products. Furthermore, this has recently become particularly important because of the large volumes of magnetic data that are being collected for environmental and geological applications. Hence, a variety of semi-automatic methods, based on the use of derivatives of the magnetic field has been developed to determine magnetic source parameters such as locations of boundaries and depths. Almost all methods that determine contact locations are based on calculating some function of the magnetic field that produces a maximum over a source body edge. Finding the maxima is then efficiently done with the curve-fitting approach of Blakely and Simpson (1986). Gravity and magnetic data are usually processed and interpreted separately, and fully integrated results basically are created in the mind of the interpreter. Data interpretation in such a manner requires an interpreter experienced both in topics concerning potential field theory and the geology of the study area. To simplify the joint interpretation of data, the automatic production of auxiliary interpreting products, in the form of maps or profiles, is useful to help a less experienced interpreter or when investigating regions with poorly known geology. Fortunately, a suitable theoretical background for the joint interpretation of gravity and magnetic anomalies is well established and can serve promptly in generating such products. Because of its mathematical expression, this theory is commonly referred to as the Poisson relation or the Poisson theorem. This theorem provides a simple linear relationship connecting gravity and magnetic potentials and, by extension, field components that are commonly derived from geophysical surveys. To validate this, an isolated source must have a uniform density and magnetization contrast. The relationship, however, is independent of the shape and location of the source. Therefore, the magnetic field can be calculated directly from the gravity field without knowing the geometry of the body or how magnetization and density are distributed within the body and Vice Versa.
Therefore, a magnetic grid may be transformed into a grid of pseudo-gravity. The process requires pole reduction, but adds a further procedure which converts the essentially dipolar nature of a magnetic field to its equivalent monopolar form. The result, with suitable scaling, is comparable with the gravity map. It shows the gravity map that would have been observed if density were proportional to magnetization (or susceptibility). Comparison of gravity and pseudo-gravity maps can reveal a good deal about the local geology. Where anomalies coincide, the source of the gravity and magnetic disturbances is likely to be the same geological structure. Similarly, a gravity grid can be transformed into a pseudo-magnetic grid, although this is a less common practice. Pseudo- gravity transformation is a linear filter which is usually applied in the frequency domain on magnetic data. This filter produces an applicable result because interpretation and quantifying the gravity anomaly is easier than magnetic anomaly.
Filtering (enhancement techniques) is a way of separating signals of different wavelength to isolate and hence enhance anomalous features with a certain wavelength. One of the enhancement methods in magnetic data filtering is Total Horizontal Derivative (THDR) designed to look at Maxima in the filtered map indicate source edges. It is complementary to the traditional filters and also first vertical derivative enhancements techniques. It usually produces a more exact location for faults than the first vertical derivative, but for magnetic data it must be used in conjunction with the other transformations e.g. reduction to pole (RTP) or pseudo-gravity. Computing horizontal gradient of the pseudo-gravity anomaly and mapping the maximum value of this causes edge detection of the magnetic causative body. In this paper this method is applied on synthetic magnetic anomaly and also on the magnetic anomaly from the Gol- Gohar area in Sirjan which demonstrate a 30m width body.