Surface nuclear magnetic resonance signal contribution in conductive terrains

Abstract

To correctly invert and interpret Surface Nuclear Magnetic Resonance (SNMR) data collected in conductive terrains, an accurate estimate of subsurface conductivity structure is required. Given such an estimate, it would be useful to determine, before conducting an SNMR sounding, whether or not the conductivity structure would prevent groundwater being detected. Using SNMR forward modelling, we describe a method of determining the depth range from which most of the SNMR signal originates, given a model of subsurface conductivity structure. We use the method to estimate SNMR depth penetration in a range of halfspace models and show that for conductive halfspaces (< 10 ?.m) the depth of penetration is less than 50 m. It is also shown that for these halfspaces, increasing coincident loop size does not significantly improve depth penetration. The results can be used with halfspace approximations of more complicated 1D conductivity structures to give a reasonable estimate of the depth range over which signal is obtainable in conductive terrains.