Development Of HTS RF SQUID Magnetometer

The high temperature radio frequency superconducting quantum interference device(HTS RF SQUID) magnetometer is characterized by direct measurements of magneticfields, low noise, broad bandwidth and excellent low frequency response. Theconventional induction coil could be replaced with the HTS RF SQIUD as receiving sensorfor Transient Electromagnetic ( TEM ) meters, therefore realizing direct measurements ofmagnetic fields in TEM surveys. A systematic study on the development of HTS RFSQUID magnetometer to TEM method has been carried out, including the development ofthe HTS RF SQUID magnetometer, field comparison tests between sensors by respectivelyusing HTS RF SQUID and conventional induction coil, field magnetic data acquisition,magnetic data analyses and processing, influence of Dewar bandwidth on measurements,as well as setting parameters as the HTS RF SQUID sensor is connected to a TEM meter.The achievements made through the study are as following:A HTS RF SQUID magnetometer, featuring low noise, broad bandwidth, high slewrate and high stability, has been developed, which is suitable for measuringsingle-component magnetic field in TEM surveys. The critical problems relating totransmission, attenuation and amplification of high-frequency signals have beensuccessfully solved by adopting shielding technique. A preamplifier, the key unit for theHTS RF SQUID magnetometer, has been developed, making the stability of themagnetometer significantly improved. By separating the preamplifier from the controlunit, the magnetometer has been made fewer interferences and more practical. Thespecifications of the single-component HTS RF SQUID magnetometer developed throughthe study are as 1. magnetic noise – 100 fT /Hz;2. slew rate – 1 mT/S;and 3.bandwidth – 0 ~ 30 kHz.A prototype of three-component HTS RF SQUID magnetometer has also beendeveloped, which can be adjusted either manually or automatically. With such techniquesas optical coupling isolation, shielding and staggered frequencies adopted for the threechannels, the interferences for both digital and analogue circuits and the crosstalk amongthe three channels have been significantly reduced, so making the vector HTS RF SQUIDmagnetometer more stable. By adopting techniques of microcomputer controlling andA/D and D/A converting, the automatic control for the vector HTS RF SQUIDmagnetometer has been realized, resulting in the increase of field survey efficiency.A high-frequency non-magnetic Dewar has been developed, so filling in the gap inthis field in China. The heat-isolating metal film was cut in strips to avoid vortex excitedby the primary field of TEM, and thus the influence of Dewar on measurements waseliminated.A lot of field experiments show that the HTS RF SQUID magnetometer is of highstability, meet the requirements of TEM surveys and with its detection depth increased.To further make the HTS RF SQUID magnetometer practical would provide a new tool forexplorations for deep concealed deposits and oil and gas, making this advanced hightemperature superconducting technique turn into productive force in geophysicalexploration.