عنوان مقاله [English]
Magnetic survey is nowadays the most efficient non-destructive geophysical method. This technique puts emphasis on the measurement of anomalies of the Earth’s magnetic field, caused by the presence of archaeological remains in the ground. Magnetic survey is one of the oldest geophysical methods for airborne and land use. That is why there is a large volume of data that could be used for geological studies and deposits exploration.
In this method, the intensity of earth's magnetic field is measured. The method is used for metal objects exploration in archeology, engineering investigations, metal mineral exploration, oil and gas exploration, as well as the regional geological studies for the study of igneous basement. The magnetic techniques in mineral exploration are used for both of the magnetic minerals and non-magnetic minerals exploration associated with magnetic ones.
Magnetic and gravity surveys are usually based on potential field methods. In potential field data processing the edge enhancement is an important issue. Edge enhancement of potential field data has been widely used as a tool in mineral exploration prospects. Vertical derivatives of potential field data are in current approach to enhance observed probable anomalies. A well-known method of enhancing boundaries of underground structures corresponds to the zero contour of the second vertical derivative of gravity or reduced-to-the-pole of magnetic fields. However, as is well known, the estimated boundaries acquired by such technique are systematically shifted from the true position even for vertical-sided sources, and application of this method produces fairly complicated results in multi-source cases (Fedi and Florio, 2001). Cordell and Grauch (1985) also showed that the maximum of horizontal derivative is applied to the gravity or pseudo gravity anomalies localized above abrupt changes of density or magnetization.
Potential field data often contains anomalies with a wide range of amplitude, and while the weak anomalies may be considered as useful geological phenomenas, they can be difficult to recognize among the strong anomalies in the total horizontal derivative (THD) method (Ma, 2013). The other complexity in potential field data is the effect of adjacent anomalies. The THD method is not successful to enhance the boundary of these models. Analytic signal (Nabighian, 1972, 1974) is another method for processing and interpretation of magnetic field data. Analytical signal is combination of vertical and horizontal derivatives of magnetic field that can also be used to determine the location of the masses. The high values in this map indicate the location of the masses.
Recently, numerous edge-detection filters have been used which are based on the horizontal and vertical derivatives of potential field data and they display a balanced result. Miller and Singh (1994) proposed using the tilt angle to enhance the edges of the sources. Tilt angle is an effective method in balancing the amplitude of strong and weak anomalies, but it is not exactly an edge detection filter.
Wijns et al. (2005) introduced the theta map as an edge detection method that uses the amplitude of analytic signal relative to the total horizontal derivative. The maximum of the theta map are located over the edges of causative sources. The theta map displays the edges of the shallow and deep bodies simultaneously, but the edges of the sources are diffused.
In order to increase the resolution of theta map filter, Ma (2013) proposed second order of the theta map (STM). He also suggested a high resolution filter to enhance the edges of potential field data (improved local phase (ILP)), which consists of first order and second order horizontal derivatives.
In this paper we use and compare phase based methods (such as tilt angle and theta map) and improved local phase filters in magnetic data interpretation. To evaluate the capability of the methods, magnetic anomalies caused by synthetic bodies are examined. After acquiring satisfactory results, these techniques are applied on real data. Ultimately, magnetic anomaly of iron ore body belonged to Tighe Now Ab iron deposit located in the north-east of Iran is used. As a consequence, drilling borehole results are incorporated to validate the outcomes.