Analysis of noise coherence in airborne magnetic gradients for UXO detection

In boom-mounted magnetometer systems, the primary noise falls into two main categories: maneuver noise and rotor noise. The forrner is relatively low Frequency, while the latter is relatively high frequency. Frequency filters of various types have been used with success, but fall short where the noise is within the bandwidth of the signal. Such is often the case when searching for unexploded ordnance (UXO) and other shallow targets at rapid flight speeds. Under these conditions, helicopter noise can be reduced by making use of the fact that much of it is coherent between sensors. In this paper, we examine the coherence of various types of helicopter noise and demonstrate that measured vertical gradients can offer raw signal-tonoise improvements of 4:1 or more over comparable total field measurements.