عنوان مقاله [English]
ASMGT is a new method for the approximation of the depth and location of the magnetic sources.In this study, the horizontal location of the sources was estimated based on the position of the maximum values of themagnitude of the magnetic vector components (MMVC) and the analytic signals of the magnetic gradient tensor (ASMGT). Also, the depth of the magnetic sources was estimated from the ratio of the maximum of MMVC to the maximum of ASMGT.
ASMGT is an analytic signal method based on the measurement of the magnetic gradient tensor (MGT) of the magnetic field components. The measurement of the MGT data is rapidly becoming a new tool for geophysical explorations. The principal advantage of the MGT method over traditional magnetic surveys is that the local variations are enhanced well, making small and weakly magnetic bodies recognizable. Consequently, it is ideal for locating small, near surface anomalies, and is therefore useful in archeological sites. This new method is applicable on data along profiles and grids.
The MGT method was offered by Oruc (2010) over a space domain, whereas in this study, this method was implemented over a frequency domain on the synthetic magnetic data. In other words, taking the derivatives was performed over the frequency (wave number) domain by using the FFT style, while Oruc performed taking derivatives over the space domain.
Magnetic bodies are classified roughly into two categories, i.e. two- and three-dimensional bodies.For the first category, a line of dipoles is usually considered and for the second category a single dipole. The concept of the line of dipoles and point-dipole is often employed in the analysis of magnetic anomalies caused by geologic bodies whose geometric shapes approach to a thin horizontal cylinder and spheres, respectively. Theoretical examples have been carried out to compare the feasibility of the ASMGT method in obtaining the location and depth of a dipole and a line of dipoles over a wave number domain with space domain. The results showed more accuracy for the frequency domain than for the space domain, especially for a 2D structure.
In fact, one of the basic characteristic of the analytic signal of the 2D magnetic sources is that the shape and location of the analytic signal amplitude are independent of the magnetization direction, and amplitude of the signal is symmetrical. However, for a 3D source, magnetic dipole, the maximum value of the amplitude of the analytic signal is not located directly over the body. Consequently, the shape of the amplitude of the analytic signal depends on the direction of magnetization and its asymmety. Therefore, there will be some errors in determining the horizontal location and depth of the magnetic dipoles based upon the maximum value of the amplitude of MMVC and ASMGT. By using the reduction to the pole filter (RTP), the horizontal location and depth of the spheres are approximated with greater accuracy. Because of taking derivative in the MGT method, this method is sensitive to the noise; thus, upward continuation filter is applied to reduce the effect of noise.
Generally, the ASMGT enhances the magnetic response of point dipole and line of dipoles placed at shallower depth. Hence, this new method is useful in determining the depth and location of shallow magnetic bodies. All of the processing steps in this study were performed by using codes written in MATLAB.