Interpretation of magnetic anomalies due to dikes: The complex gradient method

Abstract

A method to interpret the magnetic anomaly due to a dipping dike using the resultant of the horizontal and vertical gradients of the anomaly is suggested. The resultant of both the gradients is a vector quantity and is defined as the “complex gradient.” A few characteristic points defined on the amplitude and phase plots of the complex gradient are used to solve for the parameters of the dike. For a dike uniformly magnetized in the earth’s magnetic field, the amplitude plot is independent of [Formula: see text], the index parameter, which depends upon the strike and dip of the dike and the magnetic inclination of the area. The phase plot of the complex gradient is an antisymmetric curve with an offset value equal to [Formula: see text]. For a dike whose half‐width is greater than its depth of burial, two maxima at equal distances on either side of a minimum value appear on the amplitude plot. For a dike whose half‐width is equal to or less than its depth of burial, the amplitude plot is a bell‐shaped symmetric curve with its maximum appearing directly over the origin. In the case of a thin dike, the amplitude function falls off to half its maximum value at the same point on the abscissa where the phase function reaches, i.e., [Formula: see text]. A combined analysis of the amplitude and phase plots of the complex gradient yields all the parameters of the dike. The method is applicable for the magnetic anomaly in either the total, vertical, or horizontal field. A field example is included to show the applicability of the method.

Integrated studies for characterization of lineaments used to locate groundwater potential zones in a hard rock region of Karnataka, India

“…Since dykes have fairly high signatures in a low magnetic granite host medium (Atchuta Rao et al 1981; Haricha- The total magnetic field was measured as 3.95 × 10 −5 -4.2 × 10 −5 Tesla (T). The magnetic susceptibility measured for dolerite dykes varies from 2.6 × 10 −2 to 5.1 × 10 −2 Ampere/meter (A/m), and for granite gneiss, 3.0 × 10 −3 to 5.0 × 10 −2 A/m respectively.…”

Magnetic mineral exploration using ground magnetic survey data of Tajimi Area, Lokoja

“…Atchuta Rao et al, (1981) and Roest et al, (1992) used the anomaly width at half the amplitude to derive the depths of magnetic sources from the Earth surface. Atchuta Rao et al, (1981) also used other characteristics of the analytic signal, which they refer to as complex gradient to help resolve the effect of overlapping edges (Macleod et al, 1993). The following equations were used; The individual profile interpretation of the magnetic field plots [figures 2(a-o)] shows variable anomalies which is an indication of susceptibility contrast of the rock types across the study area.…”

Magnetic and DC resistivity investigation for groundwater in a complex subsurface terrain

“…The horizontal derivatives can be computed using differences or spline in the space domain while the vertical derivative is computed in the wave-number domain using, for example, the Fourier transform. The analytic Table 1 Statistical results of the detailed total magnetic intensity data signal method does not require knowledge of the magnetization direction and was successfully applied to data (Nabighian 1972;Atchuta Rao et al 1981; and many others). When the analytic signal applied to the observed magnetic field, the method generally generates good horizontal locations for contacts and sheet sources regardless of their geologic dip or geomagnetic latitude (Philips 2000).…”