| SQUID (Superconducting Quantum Interference Device) is the most sensitive sensor ever known in the field of measurement of weak magnetic. The sensitivity of magnetometer made by SQUID can reach as high as 10fT/Hz1/2 (range of white noise), which is over three orders higher than other magnetometers. With characteristics such as direct magnetic field measurement, low noise, wide frequency band, and good low-frequency effects, it can be utilized in the areas of MCG and MEG detection, nondestructive testing of metals, geophysical prospecting, and detecting submarine, etc. However, when it is employed in the wild magnetic prospecting or as the receiving probe of TEM doing deep metal mining, oil and gas exploration, the noise interference is so great in unshielded environment that it could seriously lead to inability of functioning, because its sensitivity is too high to the magnetic field. Meanwhile, calibration of magnetic diurnal variation is needed during a long period of data measurement, the angle of measurement must be compensated, and the measurement data can only reflect the existence and size of the magnetic anomalies, limited information when identifying the characteristics of magnetic anomaly sources and the details. To solve the problems mentioned above, in this paper, two SQUIDs are introduced to research on magnetic gradient measurements and to develop the equipment.The common mode noise outside can be suppressed effectively by measuring the magnetic gradient. Compared with the magnetometer, Gradient measurements can provide more information that reflects the characteristics and details of the sources of magnetic anomalies. This research is supported by the topic in development of full digital vector airborne magnetic prospecting system in airborne geophysical prospecting techniques system of the 863 Project, and it is also funded by the key technology of HTS SQUID magnetic gradiometer and development of instrument in Jilin University Graduate Innovation Program. Main contents and results are as follows:1. The method of high temperature superconducting magnetic gradient measurement has been studied and the high temperature superconducting magnetic gradiometer has been developed. Studying the principle and methods of magnetic gradient measurement, analyzing the differences between the measured value and the theoretical value, the advantages and feasibility of high-temperature superconducting magnetic gradient measurement are obtained through studying the influence on magnetic gradient measurement value made by different baseline distances. Measurement principle of SQUID and measurement method of HTS SQUID magnetic gradiometer has been studied and the design of the equipment has been realized.2. The HTS SQUID magnetic gradiometer probe has been developed. The high-vacuum multilayer insulated non-magnetic Dewar has been developed after studying the vacuum performance of Dewar and selecting proper Dewar material with non-magnetic. The special treatment of anti-radiation layer has been done to improve the insulation performance of Dewar, extend the Bandwidth, and reduce electromagnetic wave radiation effectively. The vacuum performance between inner and outer skin of Dewar is enhanced with suitable technology. The tests of daily evaporation rate have been carried out which indicate that it meets the requirements of magnetic prospecting in the wild. After studying the cross-talk between tow SQUIDs and analyzing the impact of the sensor baseline orientation on gradient measurement, a reasonable structure of magnetic gradient probe is developed and the HTS SQUID magnetic gradiometer probe is assembled.3. The research on the key signal detection technology of HTS SQUID magnetic gradiometer has been done, which mainly includes the RF SQUID detection technology based on flux-locked state, RF signal generation technology of adjustable frequency and power, weak RF signal amplification technology of RF SQUID, anti-jamming technology of electromagnetic shielding system, etc. Based on these key technologies above, a high stable signal detection with low noisy based on HTS SQUID is developed.4. The research of the key control system technology of HTS SQUID magnetic gradiometer has been carried out. By studying the low-noise data acquisition technology of adaptive variable sampling rate and adaptive-switching range, the technology of high-speed data transmission and data error correction, and the technology of RF SQUID operating point adjustment, a control system with the functions including adjusting the operating point automatically, measuring and saving dynamically is developed, which greatly facilitates the magnetic work in the field and significantly improves efficiency, compared with the traditional manual operating system.5. The indoor testing and calibration of HTS SQUID magnetic gradiometer has been carried out. The platform for indoor testing and calibration has been built. The methods for parameters test have been introduced and the results of static noise level, sensitivity, dynamic range, bandwidth, and slew rate have been obtained and analyzed. The balance adjustment and test methods of HTS SQUID magnetic gradiometer have been presented, and the calibration method of HTS SQUID magnetic gradiometer has also been studied, which provide technical support for further field experiments. The HTS SQUID magnetic gradiometer has been calibrated and tested. The results of tests show that the key parameter and performance index of the instrument is able to meet the demand of field work, which lay the technical support for field experiments.6. The field experiments of HTS SQUID magnetic gradiometer has been carried out. The noise level, impact of baseline offset, and the stability of the instrument has been tested. The results of the tests show that the HTS SQUID magnetic gradiometer can keep long time work stably. Compared with the magnetic field measure, gradiomentic measurement can suppress the common model effectively, eliminate the influence of geomagnetic diurnal change and interferences caused by light mechanical vibration or sloshing. The real performances of system working in field conditions are verified with setting known experimental models. Comparison experiments with other magnetic are carried out, and then the advantages of measurement by HTS SQUID magnetic gradiometer are obtained.The field experiment shows that the developed HTS SQUID magnetic gradiometer is able to achieve the purpose of the study.Innovative work in this paper is as follows.1. The HTS SQUID suitable for field work without any shielding has been developed. Aiming at the problems of field magnetic prospecting, the principle of magnetic gradient measurement has been studied, and the method of magnetic gradient measurements with two SQUIDs is researched, which is combined with the key technology of HTS SQUID, the HTS SQUID magnetic gradiometer for field unshielded environment containing HTS SQUID magnetic gradiometer probe, signal detection device, and control system has been developed. The field experiments of HTS SQUID magnetic gradiometer has been carried out and results indicate that the purpose of the study has been achieved, which lay the technical support for the further application research of the HTS SQUID and full tensor magnetic gradiometer in practical magnetic prospecting.2. A self-correcting algorithm for data transmission had been proposed. In the algorithm, the sum of rows and columns are considered as the parity. After locating the incorrectness data, finite cross iteration are introduced to realize the data self-correct according to incorrectness level in order to avoid the phenomenon of data errors or data packet loss caused by external interference during high-speed data transmission, which enhanced the reliability and stability of data transmission of HTS SQUID magnetic gradiometer.3. A method of adjusting operating point of SQUID automatically has been proposed. The range of frequency is determined with the Fourier Transform of modulation signal obtained by the SQUID. Amplitude and SNR are estimated via spectral analysis. The optimal operating point of the SQUID is obtained when the modulation signal has max amplitude and highest SNR. The automatic operating point adjusting function has been realized with the test-controlled software and the designed hardware circuit, which reduced the equipments required for filed adjusting by HTS SQUID magnetic gradiometer, lowered the degree of tedious of adjusting, and improved efficiency.Through this study, HTS SQUID magnetic gradiometer for unshielded environment field is developed, which solved the problems of field application of HTS SQUID magnetometer. By the field experiments, it can be seen that the developed HTS SQUID magnetic gradiometer contains the advantages of low noise, high stability, accurate measurement, etc. It provides an important technical foundation for the research and development of airborne tensor magnetic gradiometer in the future. Meanwhile, it has a positive impact on the underground iron, oil and gas exploration.
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