Application of adiabatic pulses for magnetic Resonance Sounding – Pulse shapes and resolution

Magnetic Resonance Sounding (MRS) can image the spatial distribution of hydrologically relevant parameters in in the subsurface. However, the application of MRS is often limited by its low signal-to-noise ratio. The use of adiabatic excitation pulses show promising features to overcome this limitation. In this work, we study practical considerations when applying adiabatic pulses for MRS, i.e. calculation of the sensitivity kernel for varying pulse shapes and vertical resolution. The pulse shape is crucial for the performance of adiabatic pulses. We investigate the shapes of adiabatic pulses recorded during a MRS and observe small systematic deviations from the theoretical predicted pulse shape and variations between different pulse strengths. We show that the overall impact on the obtained sounding curve and inversion result was small. This enables to limit the time consuming modelling of the spin dynamic to one representative pulse shape, which significantly speeds up the calculation of the sensitivity kernel, necessary for the interpretation of MRS. Additionally, we show that on-resonance excitation generally outperforms adiabatic excitation concerning vertical resolution and depth of investigation (both up to a factor of two). This is true for a wide range of noise conditions. For a very shallow depth interval compared to the loop size, however, adiabatic excitation features improved imaging capabilities. © 2020 The Author

Utilizing pre-polarization to enhance SNMR signals -- effect of imperfect switch-off

Surface nuclear magnetic resonance (SNMR) is a well-established technique for the hydrogeological characterization of the subsurface up to depths of about 150m. 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, i.e. 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 modeling. 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 1ms exponential or linear switch-off ramp are reduced by 17% and 65%, respectively. Disregarding this effect would therefore yield an underestimation of the corresponding subsurface water content of similar magnitude.Comment: preprint submitted to Geophysical Journal Internationa

Evaluation of single-sided nuclear magnetic resonance technology for usage in geosciences

Because of its mobility and ability to investigate exposed surfaces, single-sided (SiS) nuclear magnetic resonance (NMR) technology enables new application fields in geosciences. To test and assess its corresponding potential, we compare longitudinal (T 1) and transverse (T 2) data measured by SiS NMR with those of conventional geoscientific laboratory NMR. We use reference sandstone samples covering a broad range of pore sizes. Our study demonstrates that the lower signal-to-noise ratio of SiS NMR data generally tends to slightly overestimated widths of relaxation time distributions and consequently pore size distributions. While SiS and conventional NMR produce very similar T 1 relaxation data, unbiased SiS NMR results for T 2 measurements can only be expected for fine material, i.e. clayey or silty sediments and soils with main relaxation times below 0.05s . This limit is given by the diffusion relaxation rate due to the gradient in the primary magnetic field associated with the SiS NMR. Above that limit, i.e. for coarse material, the relaxation data is strongly attenuated. If considering the diffusion relaxation time of 0.2 s in the numerical data inversion process, the information content >0.2s is blurred over a range larger than that of conventional NMR. However, our results show that principle range and magnitudes of the relaxation time distributions are reconstructed to some extent. Regarding these findings, SiS NMR can be helpful to solve geoscientific issues, e.g. to assess the hydro-mechanical properties of the walls of underground facilities or to provide local soil moisture data sets for calibrating indirect remote techniques on the regional scale. The greatest opportunity provided by the SiS NMR technology is the acquisition of profile relaxation data for rocks with significant bedding structures at the μm scale. With this unique feature, SiS NMR can support the understanding and modeling of hydraulic and diffusional anisotropy behavior of sedimentary rocks

Feasibility study on prepolarized surface nuclear magnetic resonance for soil moisture measurements

In the past few years, small-scale (2 m) prepolarized surface nuclear magnetic resonance (SNMR) has gained increasing interest in the research community. As recent studies demonstrated, the application of a strong prepolarization field enhances the SNMR signal of coils with a footprint <1 m2 up to a level that even enables investigations in urban areas. In particular, it is expected that this noninvasive method provides the soil moisture distribution in the upper 2 m of the subsurface in the near future. However, until now all field experiments have been carried out on water reservoirs only, in an approach to test and implement this rather new technique into the field of SNMR applications. We present the first prepolarized SNMR measurement on a real soil and demonstrate the general feasibility of this technique to qualitatively and quantitatively detect soil moisture in the upper first 0.5 m. Our soil moisture measurements are validated by independent time domain reflectometry data. To complement the field experiments with numerical simulations, we adapted the underlying SNMR spin dynamics simulations and account for prepolarization switch-off effects in the forward modeling of the SNMR excitation

Charakterisierung von Aquiferen mit Hilfe der nuklear magnetischen Resonanz-Methode

Wasser ist nicht nur die Quelle des Lebens, der Zugang zu Trinkwasser ist auch ein anerkanntes Menschenrecht. Die Methode der Kernspinresonanz (NMR) erlaubt es detaillierte Informationen über Ausdehnung und hydraulischen Eigenschaften von Aquiferen zu sammeln. Dies stellt eine wesentliche Voraussetzung für deren nachhaltige Nutzung dar. Die wichtigsten Ziele dieser Arbeit sind (i) die verbesserte Ableitung der hydraulischen Leitfähigkeit aus NMR-Messungen, insbesondere an groben Lockermaterial, und (ii) die Entwicklung eines Inversionsansatzes für Oberflächen-NMR-Messungen zur Ableitung der 2D-Verteilung von Wassergehalt und NMR-Relaxationszeit im Untergrund. Das Kozeny-Godefroy-Modell erlaubt eine verbesserte Ableitung der hydraulischen Leitfähigkeit aus NMR-Messungen. Es ersetzt die empirischen Faktoren in bekannten Gleichungen mit physikalischen, strukturellen und NMR-spezifischen Parametern und berücksichtigt den Relaxationsprozess von Protonen in freiem Wasser und in diffusionskontrollierten Porenräumen. Der neue Inversionsansatz zur Auswertung von 2D Oberflächen-NMRMessungen berücksichtigt den kompletten Datensatz in einem Arbeitsschritt und bestimmt daraus die 2D-Verteilung von Wassergehalt und NMRRelaxationszeit im Untergrund. Herausragenden Eigenschaften des Inversionsansatzes sind dessen 2D-Fähigkeit und erhöhte Widerstandsfähigkeit gegenüber verrauschten Daten. Zudem ermöglicht die NMR-Relaxationszeit die Unterscheidung verschiedener Lithologien. Die Ergebnisse einer umfangreichen hydrogeophysikalischen Untersuchung am Teststandort Schillerslage, bestehend aus Oberflächen-, Erdfeld-, Bohrloch- und Labor-NMR-Messungen, werden vorgestellt, verglichen und diskutiert. Aus den gewonnenen NMR-Parametern werden die 1D- und 2D-Verteilungen der hydraulischen Leitfähigkeit im Untergrund abgeleitet. Als Fazit der Arbeit kann gesagt werden, dass das Kozeny-Godefroy-Modell die Ableitung der hydraulischen Leitfähigkeiten in Aquiferen aus NMR-Messungen verbessert und der neue Inversionsansatz den Anwendungsbereich von Oberflächen-NMR auf 2D-Ziele vergrößert. In Kombination erlauben beide Neuerungen die Abbildung der 2D-Verteilung der hydraulischen Leitfähigkeit im Untergrund.Not only is water the source of life, but access to clean drinking water is acknowledged to be a human right. Nuclear magnetic resonance (NMR) can help to gather detailed information about the extent and hydraulic properties of an aquifer which are essential information for a sustainable use. The main objectives of this thesis are (i) to revise the prediction of hydraulic conductivities including coarse-grained unconsolidated sediments using NMR and (ii) to develop a robust inversion approach for surface NMR to estimate the 2D distribution of water content and NMR relaxation time in the subsurface. The Kozeny-Godefroy model is introduced which allows for an advanced prediction of hydraulic conductivity from NMR measurements. The model replaces the empirical factors in known relations with physical, structural and NMR intrinsic parameters. It additionally accounts for the relaxation of protons in the bulk water and in pores which are controlled by diffusion limited conditions. A new sophisticated inversion approach for 2D surface NMR surveys is presented. It considers the entire recorded data set at once allowing to determine the 2D distribution of water content and NMR relaxation time in the subsurface. The outstanding features of this new inversion approach are its 2D capability, robustness in spite of noisy data and the potential to distinguish aquifers of different lithology due to their NMR relaxation times. Finally, the results of an extensive hydrogeophysical study at the Schillerslage test site, including surface, Earth’s field, borehole and laboratory NMR measurements, are presented, compared and discussed. The obtained NMR parameters are used for the prediction of 1D and 2D distributions of the hydraulic conductivity in the subsurface. In conclusion, this thesis demonstrates that the estimation of hydraulic conductivities in aquifers using NMR can be improved using the Kozeny-Godefroy model. The presented new inversion approach increases the range of application for surface NMR to 2D targets. This allows obtaining 2D images of the hydraulic conductivity distribution in the subsurface

Aquifer Characterisation Using Nuclear Magnetic Resonance

Wasser ist nicht nur die Quelle des Lebens, der Zugang zu Trinkwasser ist auch ein anerkanntes Menschenrecht. Die Methode der Kernspinresonanz (NMR) erlaubt es detaillierte Informationen über Ausdehnung und hydraulischen Eigenschaften von Aquiferen zu sammeln. Dies stellt eine wesentliche Voraussetzung für deren nachhaltige Nutzung dar. Die wichtigsten Ziele dieser Arbeit sind (i) die verbesserte Ableitung der hydraulischen Leitfähigkeit aus NMR-Messungen, insbesondere an groben Lockermaterial, und (ii) die Entwicklung eines Inversionsansatzes für Oberflächen-NMR-Messungen zur Ableitung der 2D-Verteilung von Wassergehalt und NMR-Relaxationszeit im Untergrund. Das Kozeny-Godefroy-Modell erlaubt eine verbesserte Ableitung der hydraulischen Leitfähigkeit aus NMR-Messungen. Es ersetzt die empirischen Faktoren in bekannten Gleichungen mit physikalischen, strukturellen und NMR-spezifischen Parametern und berücksichtigt den Relaxationsprozess von Protonen in freiem Wasser und in diffusionskontrollierten Porenräumen. Der neue Inversionsansatz zur Auswertung von 2D Oberflächen-NMRMessungen berücksichtigt den kompletten Datensatz in einem Arbeitsschritt und bestimmt daraus die 2D-Verteilung von Wassergehalt und NMRRelaxationszeit im Untergrund. Herausragenden Eigenschaften des Inversionsansatzes sind dessen 2D-Fähigkeit und erhöhte Widerstandsfähigkeit gegenüber verrauschten Daten. Zudem ermöglicht die NMR-Relaxationszeit die Unterscheidung verschiedener Lithologien. Die Ergebnisse einer umfangreichen hydrogeophysikalischen Untersuchung am Teststandort Schillerslage, bestehend aus Oberflächen-, Erdfeld-, Bohrloch- und Labor-NMR-Messungen, werden vorgestellt, verglichen und diskutiert. Aus den gewonnenen NMR-Parametern werden die 1D- und 2D-Verteilungen der hydraulischen Leitfähigkeit im Untergrund abgeleitet. Als Fazit der Arbeit kann gesagt werden, dass das Kozeny-Godefroy-Modell die Ableitung der hydraulischen Leitfähigkeiten in Aquiferen aus NMR-Messungen verbessert und der neue Inversionsansatz den Anwendungsbereich von Oberflächen-NMR auf 2D-Ziele vergrößert. In Kombination erlauben beide Neuerungen die Abbildung der 2D-Verteilung der hydraulischen Leitfähigkeit im Untergrund

Advanced surface coil layout with intrinsic noise cancellation properties for surface-NMR applications

A recent study demonstrated that in small-scale prepolarized surface nuclear magnetic resonance (SNMR-PP) measurements with a footprint of a few square meters, customized PP switch-off ramps can serve as an efficient excitation mechanism, eliminating the requirement for a conventional oscillating excitation pulse. This approach enables the detection of short relaxation signals from the unsaturated soil zone and can, therefore, be used to directly provide soil moisture and pore geometry information. Because ultimately such small-scale SNMR-PP setups are intended for a mobile application, it is necessary to develop strategies that allow for speedy measurement progress and do not require noise cancellation protocols based on reference stations. Hence, we developed a new concentric figure-of-eight (cFOE) loop layout that combines the direction independence of a circular loop with the intrinsic noise cancellation properties of a classical FOE-loop. This approach significantly decreases the measurement time because suitable signal-to-noise ratios are reached much faster compared to a classical circular loop and will bring us one step further toward fast and non-invasive soil moisture mapping applications

Improving the accuracy of 1D surface nuclear magnetic resonance surveys using the multi-central-loop configuration

Surface nuclear magnetic resonance is a near-surface geophysical method for characterizing the spatial distribution of liquid water in the top 100mof the subsurface. The recovered water content models are obtained through the solution of an ill-posed inverse problem that is a function of acquisition parameters, including location and shape of the transmitter and receiver coils. In this paper, we introduce the multi-central-loop acquisition and inversion strategy where one or several smaller receivers coils are placed in the center of the larger transmitter loop and where all the data sets synchronously recorded through each loop are inverted simultaneously. We investigate the attributes of this acquisition and inversion strategy including the ability to provide improved resolution, accuracy and reduced uncertainty on the estimated subsurface models compared to single channel acquisition methods. Using widely-adopted inversion methods and introducing a new data interpretation technique called Bayesian Evidential Learning 1D imaging, we show that the multi-central-loop configuration provides improved recovery of synthetic models and reduced levels of inverted parameter uncertainty. A field case is also presented where the multi-central-loop results appear to better match the lithologic knowledge of the area compared with single channel configurations, again providing smaller uncertainties