عنوان مقاله [English]
Magnetic survey data are generally used to map faults, geologic contacts and magnetic ore bodies. The spatial distribution of magnetic sources will be determined during the mapping process. Variation in depth, magnetization and geometrical parameters generate magnetic anomaly waveform. Direction of the remanent and induced magnetization vector will also affect the shape of these waveforms. A magnetic anomaly waveform includes amplitude, phase and wavelength. Putting these parameters altogether makes the interpretation of the magnetic data a difficult task. There are different useful methods for interpretation of a magnetic map. Generally, these methods are based on reduction data to a simpler form, so that the edges and center of the causative bodies will be determined easily.
In recent years many methods have been used to balance the difference between various anomaly amplitudes. Each method is designed to determine a specific parameter of the magnetic anomalies. Local phase filters are commonly used in potential field data interpretation. They are high-pass filters based on horizontal and vertical derivatives, such as total horizontal derivative, tilt angle, theta map, etc.
Edge detection of a magnetic structure is one of the most important issues in the interpretation of magnetic data. In the present study we have used two local phase filters for this purpose: Tilt angle (TDR) and Total Gradient of Tilt angle (TAHG). Although the tilt angle filter is used to determine the boundary of anomaly sources, but it is relatively less sensitive to the source depth, so it can resolve shallow and deep sources as well. As the tilt angle is a function of vertical derivatives normalized by horizontal derivatives of magnetic field intensity (THDR), it does not contain information on the strength of the geomagnetic field nor the susceptibility of the causative bodies. The tilt angle amplitudes depend strongly on magnetic field inclination. Their maximum occurs at the center of the magnetic sources and they disappear over the anomaly edges.
Another enhancing method employed in this study to determine the structure boundaries, is the tilt derivative of horizontal gradient. It is defined by taking the arctangent of the vertical derivative of the THDR, divided by the modulus of the horizontal gradient of THDR:
TAHG equalizes the signals obtained from shallow and deep sources. This method has two notable features: I- it produces maximum amplitudes over the edges of the sources, II- it gives suitable resolution and is less dependent on the structure depths. However, like the TDR, this method depends on the inclination of magnetic field.
We applied these methods for synthetic noise-free and noisy data. Magnetic responses of synthetic models as well as calculations of different edge detection methods have all been done in MATLAB. In comparison with common methods like horizontal gradient and analytic signal, it delineates the edges of sources more efficiently and accurately. Furthermore the TAHG method has better resolution in determining the boundaries of deeper sources than TDR method.
We applied the TAHG method for the aeromagnetic dataset from Zanjan region. The Zanjan depression is a narrow and continuous igneous basin, located in the north western Zanjan province. There are many young active and basement faults in the study area. Total magnetic anomaly map of the region, shows two major structural trends in NW-SE and NE-SW, respectively. Applying different edge detection algorithms we obtained the hidden boundaries of the basement which is not detectable in the geological maps because of the thick sedimentary covers. The results show that TAHG method is suitable for determining the basement faults and boundaries, as well as mapping the contacts of magnetic units.