High-resolution surface NMR tomography of shallow aquifers based on multioffset measurements

Abstract

Conventional surface nuclear magnetic resonance (NMR) surveying based on 1D inversions of data recorded using a common (coincidence) transmitter and receiver loop provides only limited or distorted water-concentration information in regions characterized by strong lateral heterogeneity. We introduce a combined field-acquisition and tomographic-inversion strategy suitable for 2D surface NMR investigations of free (i.e., unbound) water stored in hydrogeologically complex regions. Using combinations of coincident and multioffset loops, we take advantage of the range of sensitivities offered by different loop configurations to variations in subsurface free-water concentration. The new tomographic scheme can invert data acquired with diverse loop configurations. Tests of the combined acquisition and inversion strategy on complicated synthetic and observed data demonstrate the substantially higher resolution information provided by combinations of loop configurations vis-à-vis that supplied by a standard coincident loop. A combination of coincident and half-overlapping loop data sets yields tomograms rich in detail, comparable to tomograms derived from a combination of all considered loop configurations. If resources are limited, surface NMR practitioners should consider the half-overlapping loop configuration as an alternative to the standard coincident loop configuration. For a four-station data recording campaign, the half-overlapping loop configuration with 50% more measurements and equal number of loop deployments and retrievals provides significantly higher resolution tomograms than a coincident loop configuration.