Detection of proton NMR signal in the Earth's magnetic field at an urban laboratory environment without shielding

Nuclear magnetic resonance in the Earth's magnetic field was studied in an urban laboratory without shielding using a second-order low-T(c) SQUID (superconducting quantum interference device) gradiometer. Two different configurations were investigated, one with a prepolarization field perpendicular to the measurement field and the other one with the two fields parallel but a pi/2 AC pulse applied perpendicular to them. With a prepolarization field of 10 mT and a measurement field of similar to 29.1 mu T, the proton signal from 15 ml tap water was obtained in a one-shot measurement with high signal-to-noise ratio, and the signal was still discernible down to a sample volume of 2-3 ml. Spin-lattice relaxation time T(1) similar to 1.78 s was measured from the variation of signal magnitude on the prepolarization time. Spin-spin relaxation time T(2) was estimated to be around 0.9 s from the calculated FID signal by using a special method developed based on the principle of a lock-in amplifier.