Eiichi Arai

High Tc SQUID system for transient electromagnetic geophysical exploration

Transient electromagnetic (TEM) method using high temperature SQUID (HTS SQUID) for geophysical exploration has been developed. In terms of high sensitivity, wide bandwidth and its easy handling, HTS SQUIDs are anticipated for use with the coolant of the liquid nitrogen. The development was focused on high magnetic field sensitivity and high slew rate for better resolution and deeper depth of exploration. A highly sensitive large direct couple SQUID is made with step-edge junctions on a 20 mm by 20 mm substrate. The effective area is 0.52 mm/sup 2/ and the typical magnetic field noise is 50-100 fT/Hz/sup 1/2/. The flux locked loop circuit is a direct-readout type for the purpose of high frequency response up to 100 kHz and has low amplifier noise of 0.5 nV/Hz/sup 1/2/. The system achieved noise level of 200 fT/Hz/sup 1/2/ under the magnetic field environment and the slew rate of 7.3 mT/sec. We conducted field trials at several sites in Japan with the successful operation of the system. It was confirmed that the system has up to two orders improved sensitivity than the conventional system which enables the exploration of deeper regions. The reconstructed resistivity profile by 1-D inversion of the SQUID data agrees well with the results of the drilling survey.

Development of TDEM Data Acquisition System Based on High T~c SQUIDs for Mineral Exploration

The Time Domain Electromagnetic Method (TDEM) survey is one of the several geophysical exploration methods. In the conventional TDEM survey, an induction coil is used as the magnetometer. However, the measurement depth is limited to about 500 m. Using high Tc SQUIDs, there are expectations of large bandwidth and high sensitivity for the TDEM. We developed the high T c SQUID TDEM system. We have reduced the system noise by developing a 20 mm x 20 mm step-edge type direct coupled SQUID and a low noise direct readout flux locked loop (FLL) circuit. We have also improved the slew rate, optimizing the parameter of the FLL circuit. Consequently, the system noise of less than 0.2 pT/Hz 1/2 at 1 kHz was achieved in the earths magnetic field. The slew rate was 7.3 mT/sec. We conducted field trials and confirmed that the TDEM using high Tc SQUIDs obtains information of deeper region with high precision compared with the TDEM using induction coils.

Development of a TDEM data acquisition system based on a SQUID magnetometer for mineral exploration

We report on the research and development of a TDEM data acquisition system (SQUITEM) by JOGMEC using a highly sensitive high-temperature superconductor (HTS) SQUID vector magnetometer cooled by liquid nitrogen, which is suitable for mineral exploration. JOGMEC has achieved stable operation of all three channels during field tests. SQUITEM meets high requirements for slew rate (6.8 mT/s), dynamic range (100 dB) and bandwidth (DC - 100 kHz). It offers deeper penetration of depth than the conventional induction coil system because it can record the step response that decreases with time slower than the impulse response of the induction coil system. SQUITEM also performs horizontal gradient observations that can provide much better resolution of shallow conductive targets than conventional TDEM observations. We have obtained good reproducibility of SQUID data and correlation between the output signals of the reference induction coils and the derivatives of the SQUID signals in TDEM field tests. Keyword HTS SQUID, TDEM, mineral exploration