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DC SQUID magnetic field sensors

(Data sheet: Model 581 - pdf format)

2G Enterprises magnetometers employ DC SQUID magnetic field sensors to measure rock magnetism. The SQUID sensors are mounted inside the rock magnetometer vacuum jacket near the bottom of the internal helium reservoir.

The DC SQUID sensors are fabricated using thin film technology which has proven to be very reliable. The DC SQUID sensor has also demonstrated to have a very low noise of less than 10 ^-5 flux units ( 1 flux unit =2 X 10^ -8 G-cm^2). When coupled to a pickup coil which transfers the rock magnetism signal to the SQUID sensor, the DC SQUID has a total magnetic moment noise level of less than 10^-9 emu (10^-12 A-m^2).When expressed in terms of magnetization, the DC SQUID based rock magnetometer has a noise level of less than 10^-7 A/M for a 10 cm^3 volume rock.

In rock magnetometer systems, the SQUID sensor is used to measure the current induced in superconducting pickup coils placed at the center of the measurement region. There are three sets of such pickup coils, two for transverse moment measurement and one for axial moment measurement. These pickup coils have a large volume of uniform response to a small magnetic dipole.

When a rock is inserted into the measurement region, persistent currents are generated in all three pickup coils. These persistent currents do not decay because they are generated in a closed superconducting wire. The pickup coil outputs are routed from the measurement region to the input coupling coils on 3 DC SQUID sensors mounted near the bottom of the helium reservoir.

To prevent magnetic noise from being picked up from sources other than the rock inserted in the system, both the pickup coil and coil structure and SQUID sensors have superconducting shields placed around them.