Estimation of depth, structural index and location of the magnetic sources by using combined method of AN-EUL
AN-EUL is a new automatic method for simultaneous estimation of depth, location and geometry of magnetic and gravity sources. The principle of this method is a combination of both the analytic signal and Euler Deconvolution methods. The derivation of the main equations of this method is based on the substitution of the appropriate derivatives of the Euler homogeneous equation into the expression of the analytic signal of the potential field. Location of source (Epi-centre) can be approximately estimated based on the position of the maximum value of the amplitude of the analytic signal, and the formulas of depth and structural index (SI) estimation is calculated at this point. This new method is applicable on data along profiles and/or girds. It is one of the basic characteristics of the analytic signal applied on the responses of the two dimensional magnetic sources, such as dike and infinitely long horizontal cylinders, that the shape of the signal amplitude and its location are independent of the magnetization direction. For these types of sources, the shape of the amplitude of the analytic signal is symmetrical, whereas for 3-dimensional sources, like spherical sources, the maximum value of the amplitude of the analytic signal is not always located directly over the body, and, for these sources, the shape of the amplitude of the analytic signal depends on the direction of magnetization and is asymmetric. Therefore, there will be some errors in determining the location of the magnetic source based upon the location of the maximum value of the amplitude of the analytic signal for these types of sources. An important advantage of the AN-EUL method is that it is not restricted only to idealized sources (i.e. having integer structural index). This wider applicability means that SI can be a fractional number that describes sources with arbitrary shapes. Because of the existence of high order derivatives in the AN-EUL method formula, this method is very sensitivity relative to noises and shallow sources; thus, the effects of noises and shallow sources can be reduced by applying an upward continuation filter. To study the resolution of the AN-EUL, the method has been applied on synthetic data generated from various magnetic models, including a thin dike, a magnetic sphere and a drum shape source. In the next stage, the simulation of real cases, the data were contaminated by random noise. For all of these models, with regard to the models parameters, the results have good accuracy. Finally, the method was applied to an aeromagnetic data set acquired over an area in Sweden to estimate the depth, location, and shape (structural index) of some of the anomalies. According to the geological studies in this region, there exists a granite intrusive body with certain fractures in which Diabase veins have penetrated. Results of this study show the nature of anomalies very well and give good estimations of the depth and shape of the magnetic sources causing these anomalies. The results agree well with the geological information found by other methods (e.g. MT, Gravity, field observations). All of the processing steps in this paper were performed by using codes wrote in Matlab.
Iranian Journal of Geophysics, Volume:4 Issue:2, 2011
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