Development of Magnetic Prospecting System with HTS SQUID Gradiometer for Exploration of Metal Resources

We have developed a magnetic prospecting system with HTS SQUID gradiometers for exploration of metal resources. The SQUID gradiometer consists of a flux transformer chip made of a YBCO thin film and a SQUID gradiometer chip which is stacked on the transformer chip. The SQUID gradiometer chip was fabricated by using an HTS multilayer and ramp-edge junction technology at ISTEC. Their effective volume and the balance estimated by using a Helmholtz coil were approximately 3 x 10-9 m3 and 1/500, resulting in the gradiometric field noise of 7 pT/m/Hz1/2 at 10 Hz. Two assembled SQUID gradiometers which measure dBz/dx and dBz/dy field gradients were cooled with liquid nitrogen in a cryostat of the magnetic prospecting system. The cryostat was suspended from the frame of the system and its attitude was self-controlled by gravity. The magnetic prospecting system with the HTS SQUID gradiometers, flux-gate sensors, a GPS module, and a gyro sensor was also tested in a field near an old mine.