عنوان مقاله [English]
Remnant magnetization causes a change in the direction and intensity of the magnetization vector. If inversion is performed regardless of remnance, in some cases it may have unreliable and misleading results. For inversion with respect to remnant magnetization, several solutions have been proposed so far, one of which is to convert the data of total magnetic field into data that is independent of the direction of magnetization. In this study, the transformation of Total Field Anomaly (TFA) into Total Magnitude Anomaly (TMA) is used. The inversion algorithm is based on improving compact inversion method and is just two-dimensional. In compact inversion, anomalies may concentrate on the surface of the earth, and thus the response is unreliable. To solve this problem, a combination of matrices and weighting functions have been used, including elements such as magnetic susceptibility and depth function. The resulting model can be smooth or compact (with sharp edges) based on changing compactness factor. The method has been tested using several synthetic and real data. The synthetic data are a 2D tabular prism, of which the top buried-depth is 20 m and the length and width are 40 to 20 m, a dipping prism with a vertical tabular nearby. The real example is magnetic data over Galinge iron-ore deposit in Qinghai province of China, and the data of four profiles have been considered for 2D inversion. Inversion even smooth or sharp, have been conducted with all models, and especially sharper models are consistent with the known geologic attributes of the magnetic sources.
In some magnetic observations, it can be assumed that there is no remnant magnetization or can be neglected. In these cases, the direction of remnant magnetization is assumed to be parallel with the direction of the Earth's magnetization, and some modeling can be done with this hypothesis. But there is remnant magnetization in most cases, which, if severe, causes uncertainty in inversion of magnetic data. Although this problem was present from the beginning of the magnetic measurement, it was not important for two reasons: first, in many magnetic studies, the amount of remnant magnetization was considered to be negligible; secondly, many magnetic data were interpreted qualitatively; It was performed only for the purpose of estimating depth, in which there was no need to know the direction of magnetization . However, remnant magnetization is severe and inevitable in many applications of magnetic method, including mineral exploration, regional explorations of the earth's crust and archeology. Given the above mentioned reasons, inversion result is unreliable without attention to the remnant magnetization; so many researchers have offered numerous ways to solve this problem. In general, there are three methods for inversion of magnetic data with respect to remnant magnetization: 1) inversion method with estimation for the direction of remnant magnetization, 2) inversion by converting magnetic data into the data that is independent of magnetization and 3) Magnetic Vector Inversion (MVI).