Nuclear Magnetic Resonance Imaging Of Water Content In The Subsurface

The technique of nuclear magnetic resonance (NMR) imaging of subsurface water<br>content is described and critically evaluated from both experimental and theoretical<br>points of view. We describe data from soundings conducted in different parts of Colorado<br>and New Mexico, and describe constraints on the technique imposed both by<br>the present state of the instrument technology and by field conditions, such as cultural<br>noise, the subsurface geolectric section, pore saturation and pore size distribution, and<br>magnetic impurities in the soil. We describe also a newly derived general formula for<br>the NMR response voltage, valid for arbitrary transmitter and receiver loop geometry<br>and arbitrary conductivity structure of the medium in which the nuclear spins reside.<br>It is shown that in cases where the conductivity is large enough such that the electromagnetic<br>skin depth at the Larmor frequency is of the same order or smaller than<br>the measurement depth, there are diffusive retardation time effects that significantly<br>alter the standard NMR response formula used in the literature. Large quantitative<br>differences are found between conducting and insulating cases in physically relevant<br>situations.