Long Baseline Axial Gradiometer Based on the Fundamental Mode Orthogonal Fluxgate

Abstract

Based on the fundamental mode orthogonal fluxgate (FM-OFG), long base-line axial gradiometers are fabricated for the detection of buried ferromagnetic objects. A pair of FM-OFG sensor heads are installed inside a meter long aluminum pipe. The sensor head is made of a hairpin shaped amorphous wire core of 30 mm in length and has two sensing coils on it. An outer sensing coil on one sensor head is connected to other’s counterpart in subtractive direction, in the same way inner sensing coils are also connected but in additive direction; the outer sensing coils are used to make a gradiometer and the inner ones are used to make a magnetometer. The axial gradiometer should be designed so that the gradiometer can detect a tiny magnetic field anomaly in the presence of the earth’s magnetic field. Therefore, the axis of the sensitivity of two sensor heads should align on the axis of the casing pipe and their sensitivity should be exactly the same. Two methods are presented to meet the above requirements in this paper; one is to use a magnetic shield consisting of several short magnetic rings placed outside of the sensor heads with air gaps between rings and the other is to use the output from the magnetometer to compensate the sensitivity mismatch of the gradiometer sensor heads. Experimental results obtained in a laboratory and the design of the multi-ring shield are presented.