Using Gradient Ratio Method in Interpretation of 2_D Magnetic Dyke Anomalies

Summary In the 1970s, with the appearance of digital systems and the collection of large amounts of gravity and magnetic field data, a new era was started in developing interpretation techniques of potential field geophysical data. New automated inverse techniques were widely adopted for interpretation of gravity and magnetic profile data based on two-dimensional (2-D) models such as the thin sheet, thick dike, geological contact, polygonal bodies and dike. The tilt angle (TA) and its horizontal derivative are termed alternatively in the literature as local phase and local wavenumber, respectively, as these methods are closely associated with analytical signal definition of a potential field anomaly. In this paper, we use derivative ratio method to estimate the depth and location of magnetic dyke anomalies. This method has been applied on synthetic magnetic data as well on real magnetic data from a mining region in Sirjan area.
Introduction Tilt angle, a relatively newly defined attribute of potential field data, is becoming increasingly popular among geoscientists for interpreting potential field, especially, the magnetic data. The power of the tilt angle in delineating body edges and, especially, the linear structures from a high resolution magnetic anomaly image in the tilt angle domain has been demonstrated by Thurston and Smith (1997), Salem et al. (2007, 2005). They have explained that high resolution attained in a tilt angle image is mainly due to a high dynamic gain attributed by the natural property of the arctangent function, the value of which ranges between −90° and 90°. A buried fault is a common geological structure of interest in many geophysical applications. Geophysical signature of the fault is often subtle in total magnetic intensity images, where TA, which is a derivative field, should be an important tool for interpretation. Both normal and reverse magnetic anomaly of a fault can be modeled using a dike model. One of the most important specifications of such models is the variation of the vertical and horizontal derivatives directly on top of the body. Estimation of the depth and horizontal location of the anomaly play an important role in magnetic data interpretation. There are varieties of methods which can be used for definition of anomalies. Many of these methods are model-dependent, namely the estimation equation is dependents on individual geometrical models.
Methodology and Approaches Dykes are thin sheet-like intrusions of igneous rock that can have lateral extents of up to several 100 km. They are often strongly magnetic, and as a result, are clearly visible in images of aeromagnetic data. Depending on the situation, detailed modeling may be necessary to determine their locations and dips, and this can be time-consuming. A semiautomatic method for the interpretation of magnetic dyke anomalies, based on ratios of the gradients of the anomaly, is presented in this paper. The method is applicable to dykes with dips of multiples of 45 degrees.
Results and 1. The ratio between vertical and horizontal gradients of the magnetic anomaly related to dykes can be used to estimate depth and location of the subsurface bodies.
2. The problem of noise enhancement in the computation of the gradients can be solved via two approaches: a. acquiring the vertical and horizontal gradients in the field by magnetic gradiometer or b. calculating the vertical derivatives (amplifying noise more) by means of Hilbert transform. The Hilbert transform is more stable than fast Fourier transform (FFT) and the results are more consistent.
3. The method is applicable to thin dykes with dips of multiples of 45 degrees. Obviously, if any of the assumptions inherent in the method are invalid (i.e. the anomalies are not from thin dykes, the dip angle is not a multiple of 45 degrees or the magnetization vector is not vertical) then the results will be incorrect.
4. The sensitivity of the method to the dip of the dykes and to the departure of the geomagnetic field from the vertical has been investigated.
5. The method has been tested on synthetic magnetic data and on real magnetic data from Sirjan mining region.