The complex gradient method of interpreting the magnetic anomalies due to long horizontal cylinders

Abstract

The resultant of the horizontal and the vertical gradients of the magnetic anomaly is a vector quantity and is defined as the complex gradient. the amplitude and phase of the complex gradient are used'to evaluate the parameters of the cylinder. The amplitude plot of the complex gradient of the magnetic anomaly due to a long horizontal cylinder, is a bell-shaped symmetric curve and its shape is independent of the magnetic inclination of the area and the strike of the body. The phase plot of the complex gradient is an antisymmetric curve with an offset value equal to the 'effective inclination' of the geomagnetic field vector which depends on the magnetic inclination of the area and the strike of the cylinder. A combined analysis of the amplitude and phase plots yields the depth to the centre of the cylinder, the effective inclination of the geomagnetic field vector and the magnetic moment of the cylinder. This method can be applied to the total, vertical or horizontal field components of the magnetic anomaly. Applicability of this method has been tested on a synthetic anomaly.