String magnetic gradiometer system: Recent airborne trials

A novel sensor design for measuring magnetic gradient fields has been developed by Gravitec Instruments Ltd, New Zealand. The sensor features a single string element that reacts only to the gradient field, ignoring the much stronger total-field of the Earth. Laboratory tests of the system show the room temperature noise floor is down to 0.1 nT/m over the measurement bandwidth of DC?1 Hz. For the current sensor design and room temperature operation, thermal noise is the dominant noise source. The signal to noise ratio can be enhanced further either by cooling the sensor down (~ (T/300)1/2 fold noise decrease, where T is the operation temperature), or by increasing its length ((L₀/L)3/2 fold noise decrease, where L₀ = 250 mm is the current length of the string). The sensor is being tested in the field on-board a geophysical survey aircraft to determine the noise floor outside the lab and to measure the gradient from the geological target at Gingin, Perth Australia.