Inversion of surface NMR data

Abstract

The main advantage of the surface nuclear magnetic resonance (NMR) method compared to other geophysical methods in the field of groundwater investigation is the ability to measure an NMR signal directly from the water molecules. An NMR signal stimulated by an alternating current pulse through an antenna at the surface, confirms the existence of water in the subsurface with a high degree of reliability. The NMR signal amplitude depends on the pulse parameter (the product of the pulse amplitude and its duration), bulk water volume, and water depth. Measurements are performed while varying the pulse parameter, and subsequent data processing reveals the number of water‐saturated layers, and data concerning their depth, thickness, and water content. One of the major problems in the practical application of the NMR method is the very weak signal (<3000 nV): hence the problem of signal to noise ratio (S/N). S/N can be improved by stacking the signal, but measurement time is increased. We have developed an algorithm that minimizes the number of measurements (number of different values of the pulse parameter) without a loss of inversion accuracy for a given S/N ratio, making it possible to determine a set of optimal pulses for the measurements. NMR measurements are also sensitive to the electrical conductivity of the subsurface; an electrically conductive subsurface causes variations in the depth of investigation and in the vertical resolution of the method. Experience gained from application of the method has proven that both the inversion algorithm and the analysis of the problem are efficient.