R. A. Binks

Field trials using HTS SQUID magnetometers for ground-based and airborne geophysical applications

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/spl deg/C to +40/spl deg/C 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.

Operation of a geophysical HTS SQUID system in sub-Arctic environments

Transient ElectroMagnetic geophysical prospecting using SQUID sensors has demonstrated potential for improved target detection at late response times compared to conventional coil sensors. We have developed a three-axis, rf SQUID sensor system which has been extensively operated in sub-Arctic conditions by a geophysics contractor. Due to the harsh environmental and operating conditions, the system is designed to operate in sub-zero temperatures (as low as minus 40/spl deg/C) and is ruggedly packaged whilst still remaining quite portable. Auto-tuning of the rf electronics has been implemented by adjusting the rf SQUID control parameters via a microprocessor controller. After a small amount of training, regular field crews have operated two of these systems in the field continuously for periods of months at a time. An example, comparing SQUID B field data to coil dB/dt data, is presented in this paper.

Airborne TEM surveying with a SQUID magnetometer sensor

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 t...

Geophysical exploration using magnetic gradiometry based on HTS SQUIDs

Magnetic tensor gradiometry provides gradient components of true potential fields which enables unique depth estimates and improves analytic signal methods as well as providing a number of other advantages. A high temperature SQUID (HTS) gradiometer can provide measurements of the components of the earths field tensor creating a new tool for mineral exploration. A successful comparison between a HTS SQUID gradiometer and a Cs-vapour gradiometer under survey conditions has been conducted. Both instruments were configured vertically. The HTS gradiometer measured the B/sub zz/ component of the gradient tensor, while the Cs-vapor gradiometer measured the vertical gradient of the total magnetic intensity. The HTS gradient measurement was the difference in output between two coaxial SQUID sensors. Effective noise levels achieved were 0.16-0.3 nT/m RMS, compared with 0.1-0.5 nT/m RMS for the Cs-vapor system. The SQUID noise was dominated by vibration with additional contributions from the multiplexed sampling between the SQUIDs. This paper reports on the system development, design issues, trial results and the implications for geophysical exploration.

Axial high-temperature superconducting gradiometer with a flexibleflux transformer

An axial first-order gradiometer is formed by coupling the input coil of a flexible high-temperature superconducting flux transformer inductively to a directly coupled superconducting quantum interference device magnetometer. The transformer is patterned in a single-layer YBa2Cu3O7−x film on a flexible Hastelloy tape. The tape is bent such that the two outer pickup loops of the transformer are facing each other while perpendicular to the magnetometer plane resulting in a gradiometer baseline of 35mm. A superconducting shield is mechanically adjusted to reduce the gradiometer response to uniform fields applied perpendicularly to both the magnetometer plane and the plane of the transformer pickup loops, by a factor of typically 7000.

Application of high-temperature superconductor SQUIDs for ground-based TEM

Superconducting quantum interference devices (SQUIDs) are intrinsically very sensitive detectors of magnetic flux. Flux sensitivities of one millionth of a flux quantum per root Hz (1μφ0/√Hz) may typically be realized in low-temperature superconductor (LTS) materials, while sensitivities of ∼5μφ0/√Hz may be realized in high-temperature superconductor (HTS) materials. LTS devices are typically cooled with liquid helium (4 K) while HTS devices are typically cooled with liquid nitrogen (77 K). Coupling the magnetic field into a SQUID via a flux-transformer can result in a very sensitive magnetometer with, depending on the type of superconducting material used and the effective area of the flux-coupling transformer, achievable magnetic field sensitivities ranging from fT/√Hz to pT/√Hz over typical bandwidths that span hundreds of kHz. SQUID applications include NDE, biomagnetism and magnetic microscopes.

Highly balanced long-baseline axial gradiometer based on high-T/sub c/ superconducting tape

The improving quality of high-T/sub c/ superconducting (HTS) tape with critical current densities larger than 1 MA/cm/sup 2/ creates the possibility to construct high quality flexible superconducting electronics operating at liquid nitrogen temperatures. We patterned a symmetric flux transformer into a 700 nm thick YBa/sub 2/Cu/sub 3/O/sub 7-x/ film on a 70 /spl mu/m thick, 85 mm long Hastelloy tape. The center loop of the transformer was coupled to the pickup loop of a SQUID magnetometer in a flip-chip configuration. The two outer pickup loops of the transformer were bent such that they were facing each other perpendicular to the magnetometer plane. The resulting axial gradiometer has a long baseline of 35 mm and a gradient sensitivity of 7.3 nT/(cm/spl Phi//sub 0/). A superconducting shield was used to reduce uniform magnetic fields applied perpendicular to the magnetometer plane. Common-mode rejection ratios less than 10/sup -4/ were achieved in the best case. The noise-limited gradient field resolution was approximately 330 fT/(cm/spl radic/Hz) at a frequency of 10 Hz. This resolution was mainly limited by the flux noise level of our dc-SQUID magnetometer of approximately 45 /spl mu//spl Phi//sub 0///spl radic/Hz. In an unshielded laboratory environment, external noise contributions at 50 Hz were reduced by a factor of approximately 10/sup 3/.

Issues relating to airborne applications of HTS SQUIDs

Airborne application of HTS SQUIDs is the most difficult environment for their successful deployment. In order to operate with the sensitivity required for a particular application, there are many issues to be addressed such as the need for very wide dynamic range electronics, motion noise elimination, immunity to large changing magnetic fields and cultural noise sources. This paper reviews what is necessary to achieve an airborne system giving examples in geophysical mineral exploration. It will consider issues relating to device design and fabrication, electronics, dewar design, suspension system requirements and noise elimination methods.

Three component spinner magnetometer featuring rapid measurement times

We describe the fabrication and performance of a spinning rock magnetometer for measuring the intensity and direction of the remanent magnetisation of rock specimens collected in palaeomagnetic surveys. The strength of these fields is measured using a high temperature superconductor (HTS) rf SQUID. Samples are rotated around two orthogonal axes to facilitate the calculation of the three-component remanence vector with a minimum of operator intervention. Measurements of remanence values ranging from 10/sup 1/ to 10/sup -5/ A/m, with errors of 1/spl times/10/sup -5/ A/m, have been achieved in measurement times of 20 s, while operating the instrument in a typical geophysical laboratory environment. In this paper we discuss the design requirements and data processing necessary to achieve a user-friendly system.

77 K SQUIDs operating in the Earth's magnetic field

Weak magnetic fields like the Earths field (/spl sim/50 /spl mu/T) can adversely affect the operation of 77 K thin-film high-T/sub c/ SQUIDs through flux penetration into either the junction or the body of the SQUID. Josephson vortex penetration into the junction causes suppression of the critical current, increased white noise and possible cessation of normal SQUID operation. We have used various techniques to overcome this problem, including development of narrow (/spl sim/1 /spl mu/m wide) step-edge junctions with high critical current density. The adverse effects of Abrikosov vortex penetration into the SQUID include magnetic hysteresis and increased low-frequency noise. Film quality (J/sub c/) is found to be important In minimising these effects.