Since December 1992, CSIRO and BHP have been field trialing rf HTS SQUID magnetometers for mineral prospecting applications. Ten field trials in widely varying environments (from -16°C to +4OoC ambient temperatures) in mostly remote locations saw the development of a system which can be operated in many configurations including ground based and airborne Transient ElectroMagnetics (TEM). The magnetometer system has been developed to a point where, at late times in TEM applications, the SQUID system has a higher signal-to-noise level than the competing traditional coil technology. In some trials, a SQUID magnetometer detected anomalies at later times than were observed with the coil system, indicating an enhanced ability to detect highly conductive targets. This paper reviews development of our 3-axis SQUID magnetometer. SQUID systems as B field sensors have advantages over coils which are dB/dt type sensors. We will discuss the importance of these advantages for mineral prospecting in regions with a conducting soil cover or overburden typical of the Australian landscape.
Traditionally airborne time-domain electromagnetic (AEM) survey systems use induction coils as the sensor (receiver). We have replaced the induction coil in a transient electromagnetic (TEM) system with a liquid-nitrogen cooled superconducting quantum interference device (SQUID) magnetometer sensor. Using this prototype system, we aimed to improve performance in detecting conductive mineralization, particularly where the conductive mineralization of interest is covered by a conductive regolith. We successfully demonstrated one- and three-component SQUID sensors in airborne TEM surveying, and achieved performance comparable to the induction-coil systems. Implementation of the SQUID system required development of devices capable of operating in magnetically unshielded environments with low noise, high slew rate, and wide bandwidth. Operation of the SQUID sensor in the highly dynamic environment of a towed bird was also necessary, and this implies a high dynamic range and high level of noise associated with the motion in Earth's magnetic field. The high dynamic range of the SQUID response was handled by a combination of resetting the SQUID flux locked loop, reducing the bandwidth, and providing high-gain feedback in parallel with the flux locked loop. A digital stacking filter was used to eliminate low-frequency noise associated with sensor motion. Isolation of the sensor from motion at the TEM signal frequencies required development of a sophisticated suspension system. The SQUID systems were tested over two known conductive targets, and their performance compared with the induction-coil TEM system. A comparative performance measure is developed to take the different sensitivities of the SQUID magnetometer and induction-coil receivers into account. This measure indicates that the SQUID system has superior performance for responses over earth structures with decay time constants greater than ∼6 ms when compared with the induction-coil signals. We also estimate the performance in comparison with integrated outputs of the induction-coil system and show that, at the demonstrated levels of SQUID performance, it is expected to have poorer performance by a factor of two or more. This disadvantage will be reduced for lower frequency, wider channel width TEM configurations or by improvements in the SQUID devices.
Absiract-High quality reproducible junctions are mandatory in the construction of high-?', RF and DC SQUIDs using YBazCu307-, thin films. In this paper, we describe two methods used to produce step-edge junctions on MgO substrates suitable for SQUID devices. In both processes a titanium mask has been used to produce a straight, well defined step which assists, in minimizing the occurrence of multiple junctions. We describe two ionmilling processes which produce differently connected stepedge junctions as indicated by SEM micrographs, I-V and I,-B characteristics. Predictable relationships between the ratio of the film thickness t o step height and the junction critical-current density are demonstrated; these enable the creation of step-edge junctions with appropriate values of I, for particular SQUID designs.
A system has been designed for the characterizing of both low and high T, junctions and devices. Fully automateId measurements of resistance versus temperature, current versus voltage, and critical current versus applied field can be carried out on up to eight devices in one cooldown cycle. The system has been proven to reduce the repetitive nature and time involved in characterizing many junctions and devices, and greatly speeds up the analysis of changes in their parameters. Connections are made to the sample using spring contacts which eliminate the need for soldering to contact pads. One or two substrates with ten connections per substrate can be placed in the probe at the one time. A microcomputer with an IEEE488 interface, four standard instruments (two constant current sources, one digital multimeter, and one voltmeter), and a specially designed probe and software has been incorporated to provide a system which is very flexible and yet very simple to operate.
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