UXO survey using vector magnetic gradiometer on autonomous underwater vehicle

Vector magnetic gradiometer measures the gradient components of ferromagnetic objects and with appropriate processing, such as inversion, is possible to derive the range, bearing and magnetic moment of the target of interests. This paper presents the progress of the development of magnetic gradiometer using autonomous underwater vehicle (AUV) for UXO survey. A prototype magnetic gradiometer was developed for magnetic detection of underwater objects onboard an AUV. The prototype magnetic gradiometer is housed in a plastic nose cone of consisting of four fluxgate magnetometers. Rotation calibration is performed to correct offset, scaling and axis orientation errors of magnetometers with reference of a scalar magnetometer. The performance of magnetic noise compensation is evaluated with simulated magnetic noise sources, typically present in an AUV. Experimental tests have been conducted using a 3-axis rotation table to simulate motions to evaluate the performance of the magnetic gradiometer for mobile operation. This paper also discusses an iterative searching approach to determine the object detection range, bearing and magnetic moments by magnetic gradients matching with a volume magnetization model.