Mark N. Keene

Detection of mobile targets from a moving platform using an actively shielded, adaptively balanced SQUID gradiometer

Tests have been performed with a moving HTS dc SQUID gradiometer mounted on a nonmetal motion table. The system operated with sensitivity of 80 pT/m//spl radic/Hz (1 Hz) and 1 pT/m//spl radic/Hz (white) whilst undergoing pitch, roll and yaw motions of /spl plusmn/5/spl deg/ in an outside environment. Gradient anomalies of /spl ges/ 1 nT/m were detected with 12 dB signal-to-noise ratio and a measurement bandwidth of 1 kHz. In this paper, we describe the performance of the gradiometer at measuring remote moving ferromagnetic targets whilst it is in motion. These measurements have an important bearing on understanding the viability of SQUID-based systems as magnetic anomaly detectors, when mounted on airborne or seaborne platforms. In one early test, a target of approximately 300 Am/sup 2/ travelling at 2-5 m/s was detected at 25 m range with a signal to noise ratio of 4:1, whilst the gradiometer was undergoing motions of /spl plusmn/5/spl deg/ in pitch roll and heading.

Actively shielded, adaptively balanced SQUID gradiometer system for operation aboard moving platforms

Extremely high dynamic range is required if magnetometer SQUIDs are to be operated while in motion in the earths field. We have developed an HTS SQUID gradiometer system that uses active shielding and an adaptive signal processing algorithm to achieve the necessary dynamic range. An array of four thin-film SQUIDs with flux transformers is configured to form two orthogonal magnetometers and a single-axis gradiometer with a baseline of 10 cm. External field changes are cancelled by a three-axis Helmholtz coil set, driven by integral feedback from the SQUIDs, which surrounds the array. A real time adaptive balancing algorithm corrects for gradient offset, electronics scaling errors, sensor misalignments and uniformity errors in the Helmholtz coils. For outside use, light RF shielding is provided by thin layers of Al foil. In laboratory tests, intrinsic noise levels of 20 pT/m//spl radic/Hz (1 Hz) and 0.3 pT/m//spl radic/Hz (white) were measured. In field trials, the Helmholtz coils provided a shielding factor, for uniform fields, of 50-60 dB (0.1-10 Hz) during roll, pitch and yaw rotations of up to /spl plusmn/5/spl deg/. Adaptive balancing reduced gradient noise to the ambient background level of 80 pT/m//spl radic/Hz (1 Hz) and 1 pT/m//spl radic/Hz (white), whilst in motion.

HTS SQUID magnetometers with intermediate flux transformers

We have modeled and fabricated a novel HTS magnetometer with a d.c. SQUID which couples to an on-chip flux transformer via an intermediate flux transformer. This configuration allows high efficiency coupling between a 65 mm/sup 2/, 28 nH pickup loop and a low inductance (<15 pH) SQUID. The small SQUID inductance is required for low noise and high /spl part/V//spl part//spl Phi/ so that direct readout schemes may be used. The intermediate flux transformer was flip-chipped with a sub-micron separation to the main substrate. The magnetometer makes use of CAM junction technology, double thickness (/spl ap/0.7 /spl mu/m) YBa/sub 2/Cu/sub 3/O/sub 7/ layers, and a groundplaned SQUID slot. The measured responsivity at 77 K was 0.57 mm/sup 2/ (3.6 nT//spl Phi//sub 0/) with /spl part/V//spl part//spl Phi/=98 /spl mu/V//spl Phi//sub 0/ which is close to that predicted by our model. No resonances, which degrade the performance of similarly configured LTS devices, were observed. We attribute this to the use of PrBa/sub 2/Cu/sub 3/O/sub 7/ as the isolation barrier. Excess white noise, probably due to Johnson noise from the PrBa/sub 2/Cu/sub 3/O/sub 7/, limited the sensitivity in our first device to 33 fT//spl radic/Hz at 77 K.

Thin film HTc SQUID construction and characterisation

The authors describe the process for manufacturing DC superconducting quantum interference devices (SQUIDs) using both step junctions and 45 degrees -rotated grain boundaries. An assessment of the noise performance and flux-to-voltage ratio for fourteen devices of various inductances, operating temperatures, and junction technologies is given. Systematic experiments on the technology steps required to construct fully integrated magnetometers and gradiometers are described. These include crossovers, superconducting interconnects, and film growth on undulating surfaces. For each of these, test structures have been constructed, and their properties have been measured. A simplified three-layer fully integrated DC SQUID magnetometer design is described. The authors highlight the need for improved reproducibility and demonstrate that, for a number of devices, there is no obvious relation between the temperature and the magnetic flux noise. The process for constructing interlayer superconducting contacts is shown to work, and the insulator has sufficient resistance for flux transformer applications.<<ETX>>

Synthetic magnetic soils for landmine detector testing

This study aims to move towards a reliable method for synthesizing artificial soils that emulate the effect of real soils on pulse-induction metal detectors. The signals resulting from some mineralized soils can greatly impair the effectiveness of mine detectors. We analysed a sample of one such soil from Cambodia, using resistivity measurement, X-ray diffraction, magnetic susceptibility measurement and electron microscopy. The physical origin of the soil signal was found to be high proportions of small grained magnetite. By mixing a U.K. topsoil with finely crushed magnetite, a synthetic soil with similar magnetic susceptibility to the real Cambodian soil was created. Other problematic soils from around the world were also synthesized. Comparative tests with a pulse induction metal detector showed decay characteristics for the artificial soils to be within 13% of those for the real soil. We achieved close susceptibility matching between artificial and real soils, either for a 2kHz measurement or a pulse-induction measurement. In our experiments it was not possible to match with both measurements at once. We propose that this discrepancy is due to differences in the average properties of magnetic grains between those in real soil and the magnetite concentrate used for the synthesis tests.

YBa/sub 2/Cu/sub 3/O/sub 7/ thin film Josephson junctions

YBa/sub 2/Cu/sub 3/O/sub 7/ thin film step edge junctions have been studied using a magnetic field applied perpendicular to the substrate. The authors outline a theory to describe the magnetic field dependence of the critical current in the step edge junction geometry, and show that it is in broad agreement with the results of experiments.<<ETX>>

A novel locating and discriminating metal detector

We present the first report of a new experimental metal detector that is able to locate an underground metal object in three dimensions with an accuracy of millimeters and measure a signature to provide discrimination against frag (chaff). The ability to pinpoint the metal means that the physical excavation of the target can be conducted more quickly and safely. This detector consists of a single transmitter coil, an array of 40 receiver coils and a computer to control soil rejection and data inversion. An inversion algorithm returns the 3D location of a target with respect to the sensor head and the signature of the metal object that is largely independent of the geometry of the measurement. Tests were conducted in air, in sand and in soil using various surrogate mines and cartridge cases. Location accuracy was generally found to be very good. Several samples of a range of mine surrogates had their signatures recorded, and all samples of each type were found to have a signature falling in a very narrow band. Most of these bands are well separated, leading us to conclude that there is considerable potential for discrimination against frag. During a blind test 80% of the mines were correctly identified. We conclude that this experimental detector can accurately locate metal objects in three dimensions and provide useful information for discriminating frag from mines. This paper reports on the technology within the new detector and the early results of the performance tests conducted against surrogate mines in test lanes.