Advanced superconducting gradiometer/Magnetometer arrays and a novel signal processing technique

The Magnetic Field Gradiometer

“…A caution is needed, however, in interpreting the sensitivities too literally. As pointed out by Gillespie et al [1977], the sensitivites reported by Zimmerman and Frederick [1971] and by Wynn et al [1975] do not take adequate account of thc variation of the gradiometer noise with frequency. Gillespie et al [1977] carried out a careful investigation of the noise characteristics of their gradiometer, which was ultimately used to measure the magnetic field gradients associated with internal waves in the sea [Podney and Sager, 1979], and they found that intrinsic noise of its SQUID dominated the measured noise spectrum on quiet days for frequencies less than 0.…”

“…For example, Goodman et al [1973] measured a peak-topeak magnetic field gradient noise of less than 10' pT/m (bandwidth 0 40 Hz) with a 20 cm baseline single-axis gradiometer at a remote test site. Wynn et al [1975] reported a sensitivity of 3 X 10-2 pT/n for a single-axis gradiometer having sensing loops of 2.5 cm radius and a 30 cm baseline (equivalent to a sensitivity of 9 X 10-3 pT/m for a 1 m baseline) and, more 1: Ketchen et al [1977] obtained a sensitivity of about 2 pT/m (at frequencies near 1 Hz, in a I Hz bandwidth) with an innovative thin-film superconducting gradiometer having a baselength of the order of 2.4 cm (equivalent to a sensitivity of 5 X 10-2 pT/m for a I m ... baseline). These sensitivities appear to be close to the value of 3 X 10-3 pT/rn described as 'today's target' in 1971 by Nicol [1971].…”

“…If the number n is five or more, indicating that at least five independent simultaneous measurements can be made on orthogonal spatial variations of a magnetic field, and if provision is made to measure the actual components of the magnetic field as well (which is easy to do in a superconducting instrument), a new kind of magnetic field instrument is obtained. As was pointed out in increasing detail by Nicol [1971], Goodman et al [1973], andWynn et al [1975], such an instrument can provide from a single location and a single set of measurements: 1) direction to the source; 2) distance to the source; 3) magnitude of the source equivalent dipole; and 4) orientation of the source equivalent dipole [Goodman et al, 1973]. …”

“…The containers necessary for operation in tha conducting media also introduce perturbations into the measurements. The implications of these problems for gradient measurements in the sea or earth are unfavorable, and it appears unlikely, in particular, that the technique proposed by Wynn et al 1975] for deriving the location and strength of a magnetic dipole source will be effective in the sea except at very short ranges. I might conclude by observing that experimental data are badly lacking in this area, and the right experimental results could alter the negative picture I have drawn for measurements in conducting media.…”

“…in magnetic field gradiometer studies because its assumption forms part of the theoretical basis for a method of measuring the position and moment vectors of a magnetic dipole source [ e.g., Wynn et al, 1975].…”