MRS multi‐exponential decay analysis: aquifer pore‐size distribution and vadose zone characterization

ABSTRACT (the NMR signal decay‐time constant) spectra are analysed for two purposes: (1) to determine the pore‐size distribution below the water table for lithological and flow parameter purposes and (2) to determine the moisture content as a function of water‐film thickness/water‐drop size to characterize water storage and fluxes in the unsaturated zone. Unsaturated‐zone hydraulic conductivity versus moisture‐content relationships are non‐linear and normally have a memory effect (hysteresis). Such relationships are dependent on the partitioning of moisture into pores of different sizes, which are wetted and drained in an order that depends on the soil composition and texture. Magnetic resonance sounding (MRS) multi‐exponential decay analysis may supply a unique insight in this component of water‐resources quantification through the differentiation of the moisture content into different film/droplet sizes. Field examples from two sites, one in The Netherlands and the other in Botswana, are shown, in which the free induction decay NMR signal is analysed in terms of three spectral components.