Sub-μm Sized Josephson Junctions Based LTS SQUID Receiver for Transient Electromagnetic Surveys

Superconducting Quantum Interference Device (SQUID) magnetometers combine low noise, wide bandwidth, and high tolerance to magnetic field slew rates. These characteristics significantly enhance the depth and accuracy of transient electromagnetic (TEM) surveys. In this paper, we developed a high-performance superconducting TEM receiver using low temperature superconductors (LTS) SQUID chips fabricated via sub-μm Josephson-junction integration. The design boosts sensor output amplitudes from tens of microvolts (μV) to over 200 μV. The SQUID sensors achieve a noise floor below 5 fT/√Hz, and the system tolerates magnetic field slew rates exceeding 5 mT/s. Side-by-side field tests comparing the system against a commercial Phoenix V8 multipurpose network electrical instrument equipped with an MTEM-AL induction coil showed approximately 30-fold improvement in effective late-time resolution. Additional validation surveys were conducted at two sites in China: a gold deposit concealed beneath conductive overburden in Anhui Province, and a deep copper deposit in Heilongjiang Province. The TEM results from both sites correlated well with borehole logs, demonstrating that SQUID-based TEM receiver can effectively explore targets beneath conductive cover and/or at greater depths.