The prediction of hydraulic conductivity [Formula: see text] from nuclear magnetic resonance (NMR) measurements has been performed primarily in sandstones. In hydrogeological applications, however, unconsolidated material is more prevalent. Compared to sandstones, unconsolidated sediments can show pore sizes up to several millimeters. The known (semi-)empiric relations to estimate [Formula: see text] from NMR have been applied on this material, but the underlying assumptions are not valid for large pores. We formulated a new model, called the Kozeny-Godefroy model. It is based on capillary pores with a single pore radius, and accounts for bulk water relaxation and relaxation in porous media under fast- and slow-diffusion conditions. The bulk-water relaxation and slow-diffusion conditions significantly affect the NMR measurements on coarse material. If the impact of the bulk-water relaxation is well known and small, a maximum [Formula: see text] can be derived from NMR measurements by accounting for the slow-diffusion case. The model replaces the empirical factors in known relations with physical, structural, and intrinsic NMR parameters. Focusing the calibration on material-specific NMR parameters improves the prediction of [Formula: see text] for similar material. Measurements on well-sorted glass beads and natural sands with different grain sizes are used for evaluation. These measurements confirm the applicability of the new model and, for coarse material, show the limit of the fast-diffusion-based Seevers and Schlumberger-Doll-Research equations. The application of our model is limited to (1) simple pore geometries, and (2) materials with a small range of pore sizes.

The technique of magnetic resonance sounding (MRS) has shown several improvements in data processing, inversion and interpretation during the last years. Along with these improvements, detailed innovations on instrumentation have been demanded to support their use. Latest developments in surface nuclear magnetic resonance (NMR) instrumentation promise to fulfil these hardware requirements such as decreased dead time, improved digital signal detection, multi‐channel capabilities and improved reference techniques with the second generation surface NMR instruments. In this paper, we compare data from two generations of instruments and assess the impact of the improvements on practical issues, i.e., the increased accuracy of data due to shorter dead times and new noise reduction approaches and the feasibility for efficient 2D measuring schemes. Well‐known and documented test sites and synthetic considerations are used to evaluate these developments. First, the relaxation signals of different devices using the same loop match each other. The inversion results coincide within the range of data errors. Decay time estimation appears to be more stable for the new generation instrument. Second, the potential of shorter effective dead times (considering a relaxation of the protons during the pulse) is investigated using statistical analysis of synthetic data sets with different decay times and noise levels. The additionally measured data at early times significantly improve the scope and accuracy of the determined parameters initial amplitude and T2* time and thus extend the range of formations to be characterized. A field example comparing an effective dead time of 18 ms and 45 ms is presented. Two different reference techniques were successfully applied for noise cancellation at the very noisy test site Nauen. We observed an equivalent signal improvement using the software‐based and hardware‐based technique. However, software noise cancellation approaches are easily adaptable and extendable. Finally, considerations are given how to efficiently carry out 2D surveys using multi‐channel instruments. A 2D field data set using the GMR demonstrates that 2D surveys can already be realized in moderate measuring times. The new generation of instruments provides comparable results and improved capabilities that will enable surface NMR measurements to be applied in a wider range of applications.

No abstract

The pore-size distribution (PSD) of geologic materials is an important rock parameter to understand the flow of water in the subsurface. PSDs can be obtained from sieving analyses, mercury porosimetry measurements, and imaging techniques, but none of these methods is available for in situ measurements. Nuclear magnetic resonance (NMR) measurements are controlled by rock parameters such as the surface-area to porevolume ratio. NMR is available for in situ measurements. State-of-the-art NMR relaxation time measurements need a calibration of the surface relaxivity ρ to extract pore-size information. State-of-the-art NMR diffusion measurements avoid the calibration of ρ but are limited to small pores. We developed an approach that estimates the average pore size without calibrating ρ by means of incorporating higher order modes into the signal interpretation of NMR relaxation times. We conducted forward-modeling studies using an analytic solution for cylindrical tubes, 2D finite-element simulations to incorporate fractal pore spaces, and laboratory experiments on synthetic and natural samples. Our experimental data indicated that relaxation can occur outside the fast-diffusion regime not only for coarse-grained materials, but also for fine-to medium-grained unconsolidated sandy materials due to high surface relaxivities. We found that the rock-fluid interface's roughness had a significant impact on the diffusion regime and led to an apparent increase in ρ, which may cause intermediate or slow diffusion. The methodology was limited to materials with a narrow PSD and uniform distribution of ρ because we assumed multiexponential decay due to diffusion in single isolated pores.

SUMMARY Surface nuclear magnetic resonance (SNMR) is a well-established technique for the hydrogeological characterization of the subsurface up to depths of about 150 m. Recently, SNMR has been adapted to investigate also the shallow unsaturated zone with small surface loop setups. Due to the decreased volume, a pre-polarization (PP) field prior to the classical spin excitation is applied to enhance the measured response signal. Depending on the strength and orientation of the applied PP-field, the enhancement can often reach several orders of magnitude in the vicinity of the PP-loop. The theoretically achievable enhancement depends on the assumption of an adiabatic, that is perfect, switch-off of the corresponding PP-field. To study the effect of imperfect switch-off, we incorporate full spin dynamics simulations into the SNMR forward modelling. The affected subsurface volume strongly depends on the chosen PP switch-off ramp and the geometry of the loop setup. Due to the imperfect switch-off, the resulting SNMR sounding curves can have significantly decreased signal amplitudes. For comparison, the signal amplitudes of either a 1 ms exponential or linear switch-off ramp are reduced by 17 and 65 per cent, respectively. Disregarding this effect would therefore yield an underestimation of the corresponding subsurface water content of similar magnitude.