Analytic signal method to estimate the magnetic source parameters of 2D structures



The analytic signal method is usually used to interpret the magnetic and gravity anomalies. The main advantage of using analytic signal method in interpretation of magnetic source parameters from magnetic anomalies is that the amplitude dip of the analytic signal is independent of directional parameters like magnetization and the dip of the source. Using the coefficient of the amplitude to estimate the depth can make the interpretation of the magnetic data easier. By this method the location depth and angle of the anomalies and even the dip thickness and the direction of its extension can be estimated. The amplitude of the signal is a symmetric bell-shaped function that the maximum of the curve is located exactly above the edge of the source and its width is proportional to the depth of the top of the magnetic source. This analytic signal characteristic is used to estimate the magnetic source parameters like depth location and the thickness.
We use these characteristics of the analytic signal function to estimate the depth, horizontal location and thickness of the magnetic sources. In this study, we use MATLAB in order to write codes for calculating the analytic signal of two dimensional anomalies, determine the geometrical parameters of the source and forward modeling of magnetic anomalies.
For evaluating the efficiency of the codes, first we applied them on the synthetic noise contaminated data. We then used analytic signal method to interpret the field magnetic data from Sorkh-Dizej, Zanjan. To estimate the depth and thickness of the magnetic source, we used 5 profiles perpendicular to the strike of the magnetic anomaly. These profiles are located in the longitude from 305986 m to 306176 m and latitude from 4051813 m to 4052013 m. Finally, we compared the results of analytic signal with the results of Euler method.
To consider the fact that the horizontal and vertical differentials are each other's Hilbert conversion, in each step of 2D analytic signal differentiation of the anomalies of 3D sources can be interpreted by extending 2D analytic signals.
The function of the analytic signal can be calculated easily in frequency region that its real and imaginary parts are horizontal and vertical differentials of the magnetic field in sequence. Another advantage of using analytic signal differential is the interpretation of magnetic and gravity data in common projects by Poisson equation.
The previous data in analytic signal method is not essential while in other methods the interpretation of the magnetic data needs some data such as the geology of the structure and other parameters. For example in Euler deconvolution method, that it is one of the depth estimation methods, knowing the structural index is vital and difficult.
In Sorkhdizaj in Zanjanin an iron mine with apatite content the results of analyzing data by analytic signal method show that the structure of the anomalies is too similar to a thin dike so that the method recognizes the thin dike with high accuracy and low uncertainty. Besides, it is shown that the depth of the top of the anomalies are changing from 10 to 55 m so that the results are too close to the ones obtained by Euler method calculated by Geosoft software. Also dikes with thickness less than 1 m to about 10 m have been estimated.