Development of a high sensitive receiver system for transient electromagnetics

Abstract

The search for metals and rare elements is an everlasting concern of our society. Therefore, in the past many techniques evolved aiming for the detection of natural mineral deposits. The transient electromagnetics (TEM) enables to detect deep lying deposits in the subsurface by means of electric conductivity. In this thesis dc Superconducting Quantum Interference Devices (dc-SQUIDs) as one of the most sensitive magnetic field sensors were developed and implemented in a receiver system for this method. Throughout this development special emphasis was given on reproducibility and a high reliability of the developed fabrication process, the sensors, and all sub-components of the entire system. Conditions to provide an adequate unshielded operation of the sensors within the Earth’s magnetic field are deduced and discussed. They result in an advanced technology for the fabrication small area and hence small capacitance Josephson junctions. The total capacitance of these sub-micrometer cross-type junctions could be reduced up to a factor of 30 compared to the conventional window-type technology. The behavior of the developed SQUID magnetometer is investigated theoretically and experimentally. With a SQUID inductance of about 130 pH they typically exhibit usable voltage swings of about 170 µV. The intrinsic energy resolution of these sensors amounts to 7 h at 4.2 K, corresponding to a magnetic field noise spectral density of 3 fT/Hz1/2. The experimentally demonstrated field stability of these sensors shows excellent agreement with theoretical predictions. Tolerable magnetic background fields of up to 6.5 mT during cool-down have been achieved. In operation mode they recover completely from magnetization pulses with amplitudes of up to 76 mT. The low-frequency noise performance of these sensors is discussed respect to recent theories. The development of a family of magnetic field sensors covering a wide range of effective areas moreover allows for the investigation of the observed low-frequency noise with device dimensions. The results reveal that main low-frequency noise contributions are due to magnetic flux noise, probably originating from fluctuating spins on the surface of the thin film superconductor. The composition and optimization of the geophysical receiver system is discussed and experimental results demonstrate a system dynamic range of ± 520 0 together with a white magnetic field noise of 12 fT/Hz1/2. The simulated frequency dependent slew rate is compared to experiments, which show a maximum measured value of 66 M0/s at about 25 kHz. A characterization method to demonstrate the intrinsic systems noise in an unshielded environment is introduced together with results obtained on a multi-purpose SQUID receiver system. The excellent system performance and reliability even in remote operation is shown on results from a ground based TEM field application. Moreover, the impact of the developed fabrication process and SQUID magnetometer on other application fields in superconducting electronics is briefly discussed. First results show e.g. white noise current sensitivities of down to 3 fA/Hz1/2 in SQUID current sensors, or first steps towards nanoSQUIDs based on Josephson tunnel junctions for the investigation of small spin systems. Finally, concepts for next generation receiver systems are discussed.