The potential of a noise‐reducing antenna for surface NMR groundwater surveys in the earth's magnetic field¹

TLDR

Non‑invasive NMR in the earth's magnetic field is used for groundwater surveys to depths of 100 m or more, but high electromagnetic noise can compromise measurement accuracy. The study aims to mitigate noise in earth‑field NMR surveys by employing a figure‑of‑eight antenna composed of two 50‑m coils. A circular 100‑m wire loop excites and receives the NMR signal, and the figure‑of‑eight antenna with two 50‑m coils is added to reduce noise. Model calculations and experimental data using the two‑coil antenna agree, demonstrating its suitability for groundwater surveys up to 30–40 m depth, which is valuable for engineering geology.

Abstract

Abstract A method of non‐invasive NMR in the earth's field has been developed and is now used for groundwater surveys to depths of investigation of 100 m or more. A circular wire loop of diameter 100 m, laid out on the ground, is employed to excite and receive the NMR signal in the earth's field. However, in areas with high electromagnetic noise, the NMR measurements may be inaccurate. To overcome this problem, a noise‐reducing figure‐of‐eight‐shaped antenna, consisting of two touching coils each of diameter 50 m, has been utilized. Using this antenna, the NMR signal has been calculated for different depths of water‐saturated layers with various inclinations of the geomagnetic field. The model calculations and experimental data have been compared and found to be mutually consistent. The two‐coil antenna is shown to be suitable for studies at depths of up to 30–40 m, which is of practical importance for engineering geology.