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Surface nuclear magnetic resonance (NMR) is emerging as a competitive method for aquifer exploration due to its direct sensitivity to subsurface water, but the method still has several shortcomings, for example, a signal-to-noise ratio that is often poor, long survey times, and bulky equipment. We have developed Apsu, a new surface NMR system designed for near-surface groundwater investigations. It provides several features such as a compact transmitter unit, separated, small receiver coils, wireless connections between multiple receivers, quasi-zero dead time, and robust phase determination. The transmitter unit is powered by a lightweight generator, and it drives a triangular current in an untuned [Formula: see text] transmitter coil. The peak current of the triangular waveform is up to 145 A, with an effective peak current of 105 A at a Larmor frequency of 2 kHz, corresponding to a 30 m depth of investigation. The frequency and amplitude in each half-oscillation of the transmit pulses can be modulated independently, which gives great flexibility in the pulse design. The receiver uses low-noise preamplifiers and multiple receivers linked to a central unit through Wi-Fi. The use of small receiver coils and wireless connections to multichannel receivers greatly improves the layout configuration flexibility and survey efficiency. The performance of the system under field conditions is demonstrated with high-quality data collected near Silkeborg, Denmark, using on-resonance and numerically optimized modulation pulses.

Geophysical methods show promise for detecting the spatial variability of subsurface clay content and its effect on subsurface hydraulic properties. We have developed a laboratory study that examines the influence of clay content and distribution on the relationships between hydraulic conductivity [Formula: see text] and the physical and geophysical properties of the media. Two geophysical methods are investigated: spectral induced polarization (SIP) and nuclear magnetic resonance (NMR). We used synthetic sediment mixtures of sand and up to 10% kaolinite clay by mass; the clay was homogeneously mixed or was present as large (approximately 5 mm) clusters distributed through the sample. The [Formula: see text] varies moderately well (normalized root-mean-square error [[Formula: see text]rms error] = 0.393) with the pore-volume normalized surface area [Formula: see text] a proxy measure of clay content, in the homogeneous samples and poorly ([Formula: see text]rms error = 0.507) when the clustered samples are included in the fit. The SIP parameters indicate moderately good to excellent fits with [Formula: see text] for homogeneous samples ([Formula: see text]rms error = 0.0783–0.139) and moderately good to good fits for clustered samples ([Formula: see text]rms error = 0.140–0.336), and the coefficients describing the polarizability of the samples depend on the clay distribution. NMR parameters vary moderately well with [Formula: see text] in the homogeneous samples ([Formula: see text]rms error = 0.341–0.412) and poorly ([Formula: see text]rms error = 1.08–6.04) in the clustered samples. The SIP parameters vary moderately well with [Formula: see text] ([Formula: see text]rms error = 0.301–0.466); however, the relationship between the SIP parameters and [Formula: see text] is compromised by the nonnegligible polarization of the clay clusters. NMR parameters indicate good to excellent fits with [Formula: see text] ([Formula: see text]rms error = 0.0789–0.116). For SIP and NMR, fitting homogeneous and clustered samples together does not compromise the fit quality. These results suggest that the geophysical measurements are better predictors of [Formula: see text] in heterogeneous porous media than bulk measures of pore geometry such as [Formula: see text].

Envelope detection is an integral step of the surface NMR data processing workflow to estimate these parameters. We present a new method for retrieving NMR signal envelopes using spectral analysis and the subsequent inversion scheme with the new data space. By exploiting the fact that the spectral magnitude at the Larmor frequency is proportional to the product of the initial amplitude and relaxation time, a high-SNR complex envelope can be extracted by Fourier transform for a number of sliding windows. However, SA estimated envelopes are weighted by the NMR relaxation time during processing and the envelopes have units of volt-seconds. We propose to modify the surface NMR forward model such that it predicts data directly in the voltage-time data space. Field data inversions are presented to demonstrate advantages of pairing the SA envelope detection scheme with a forward model that works in the voltage-time data space.

The Rutgers-Newark near-surface geophysics research group is dedicated to advancing the use of our science both at home and abroad. Our chapter has been involved in a remarkable number of international projects across the globe. Here we briefly discuss three ongoing projects in which students from our group traveled abroad to four countries, each on a different continent, to pursue a research project. These projects have given our students unique insight into how geophysical methods can be used to solve problems locally and globally.