Ground electromagnetic experiments to investigate the possibility of using an airborne electromagnetic system to detect nuclear magnetic resonance associated with unbound water in the subsurface

Abstract

Ground-based geophysical experiments were undertaken to test the possibility of de- tecting groundwater using nuclear magnetic resonance from an airborne electromagnetic prospecting system. Two experimental surveys were conducted: One over an ice-covered lake, containing a large volume of freshwater, and the other over an equal volume of land, with little to no freshwater in the subsurface. If exposed to a radio-frequency pulse at the Larmor frequency, protons in water molecules create a magnetic eld oscillating at the Larmor frequency. Both surveys used a 120-second long frequency sweep from 2300 to 2400 Hz. It was initially hypothesized that as the frequency passed over the Larmor frequency, 2348 Hz, there would be phase shift between the transmitter and receiver signals. Phase measurements, using the heterodyne method, between the lake and land data proved to be inconclusive, although the lake data showed an oscillatory decay in the o -time, not equal to the Larmor frequency.