Non-remote reference noise cancellation - using reference data in the presence of surface-NMR signals

Surface-NMR measurements commonly suffer from low signal-to-noise ratios. In recent years, with the introduction of multi-channel surface-NMR instruments, the technique of remote-reference noise cancellation (RNC) was developed and significantly improved the applicability of surface-NMR. The current formulation of RNC requires a reference loop to be placed a remote distance from the transmitter loop such that no NMR signal is recorded. Reference loops placed at non-remote distances have been envisaged to provide both improved noise cancellation performance and field efficiency; however, the concept has not been previously applied because the theoretical framework was missing. In this paper, the theoretical framework is presented. It is demonstrated that reference loops placed at non-remote distances provide superior noise cancellation performance. Considerations for placing the reference loop relative to the transmitter loop are provided, and the theoretical framework is evaluated based on a semi-synthetic example using real field noise and synthetic surface-NMR data. © 202

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

Structurally coupled cooperative inversion of magnetic resonance with resistivity soundings

Hydrologic parameters, such as porosity, salinity, and hydraulic conductivity are keys for understanding the subsurface. Hydrogeophysical investigations can lead to ambiguous results, particularly in the presence of clay and saltwater. A combination of magnetic resonance sounding and vertical electrical sounding is known to provide insight into these properties. Structural coupling increases the model resolution and reduces the ambiguity for both methods. Inversion schemes using block models exist, but they have trouble resolving smooth or complex parameter distributions. We have developed a structurally coupled cooperative inversion (SCCI) that works with smooth parameter distributions and is able to introduce blocky features through the exchange of structural information. The coupling adapts the smoothness constraint locally in connection to the model roughness to allow for sharper model boundaries. We investigate the performance of the SCCI using blocky and smooth synthetic models that depend on two controlling coupling parameters. A well-known field case is used to verify the results with drilling core and well logs. Varying the coupling parameters results in equivalent models covering the bandwidth from smooth to blocky, while providing a similar data fit. The SCCI results are more consistent with the synthetic models. Structural coupling improves the resolution of the single methods and can be used to describe hydrogeophysical targets in more detail and less ambiguously

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

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

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

Der Boden als Wasserspeicher : Nuklear Magnetische Resonanz zur Bodenfeuchtebestimmung

Aktuell ausgelöst durch die ausgedehnten Trockenphasen der vergangenen Jahre gewinnen die in Böden gespeicherten Wasservorräte und deren Verfügbarkeit im Jahresverlauf erhöhte Aufmerksamkeit. Wissenschaftler vom Institut für Geologie, vom Institut für Bodenkunde, dem Leibniz Institut für Angewandte Geophysik (LIAG) sowie von der Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) forschen mittels innovativer Feldmethoden aus dem Bereich störungsfrei arbeitender geophysikalischer Messverfahren an der Bodenfeuchtebestimmung

Erweiterter Nutzen von Datensätzen der Magnetischen-Resonanz Sondierung (MRS) - QT Inversion und Studien zum Auflösungsvermögen

Die vorgelegte Dissertationsschrift stellt einen möglichst umfassenden Einblick in die inverse Aufgabe der Magnetischen-Resonanz Sondierung (MRS) dar. Im Besonderen wird ein neues Inversionsschema, die QT Inversion als natürlicher der Datensatzstruktur entsprechender Lösungsansatz, vorgestellt. Das inverse Problem wird dieser Zielstellung folgend, hinsichtlich Auflösungsvermögen und Stabilität sowohl im theoretischen Rahmen als auch mittels synthetischer und praktischer Beispiele. Dabei folgt die Arbeit mehreren Aufgabenfeldern. Zum Einen, werden die verfügbaren Lösungsansätze im Hinblick auf die Entwicklung eines verbesserten Schemas analysiert. Die Analyse beinhaltet dabei das Inversionsschema basierend auf extrapolierten Werte der gemessenen Zeitreihen zum Zeitpunkt null (Initial value inversion) und das Zeitschrittinversionsschema, d.h. die gemeinsame Interpretation von Inversionen zu definierten verschiedenen Zeitpunkten der Zeitreihen (Time step inversion). Der, auf dieser Analyse beruhende, verbesserte Ansatz wird als QT Inversion bezeichnet und benutzt erstmalig nativ die Abhängigkeit des Datensatzes von zwei Variablen, dem Pulsmoment q und der Aufzeichnungszeit t, direkt, um verbesserte Auflösungseigenschaften und Stabilität zu gewinnen. Zum Anderen, werden der Model- und Datenraum des Initial value Inversionsschemas analysiert. Die Analyse des Modelraums führt dabei auf die Evaluierung des Inversionsresultates in Abhängigkeit von Spulengröße, maximalem Pulsmoment und Rauschlevel. Die gewonnenen Erkenntnisse helfen dabei, dem Objekt der Untersuchung entsprechende, angepasste Messanordnungen zu wählen und durch Berechnung von Auflösungsmaßen das durch die Inversion gewonnene Model hinsichtlich seiner Vertrauenswürdigkeit zu untersuchen. Die Analyse des Datenraums ermöglicht die Wahl einer optimalen Verteilung von Pulsmomenten, dergestalt dass unter dem Gesichtspunkt der Effektivität der maximale Informationsgehalt extrahiert werden kann. Diesem Feld des experimentellen Designs wird damit die Analyse der Datenauflösungsmatrix hinzugefügt. Ergänzend werden die Eigenschaften der mono und multi-exponentiellen Datenanpassung untersucht, sowie die Möglichkeiten das Inversionsresultat mittels Transformationen, d.h. dem Einschränken des Model- und Datenraumes durch obere und untere Grenzen, zu verbessern, gezeigt. Dabei wird die Größe des scheinbaren Wassergehaltes eingeführt, um die Transformationen auch auf den Datenraum erweitern zu können. Abschließend werden alle Teilaufgaben in einem Feldbeispiel zusammengefasst, beurteilt und diskutiert.The present thesis provides a comprehensive insight into the Magnetic Resonance Sounding (MRS) inverse problem and introduces the QT inversion scheme as native solution scheme of the inverse problem. At this, the thesis analyses the inverse problem in terms of resolution and stability both in theory, synthetic and field examples, mostly restricted to 1D depth sounding (MRS) but with an outlook on 2D tomography like investigations (MRT), and follows several tasks. First and within the scope to develop an improved inversion scheme the currently available solution schemes are analysed. This analysis includes the initial value inversion using extrapolated initial values and the time step inversion using multiple inversions for several different record times. The resulting new approach, called QT inversion, incorporates directly the native data structure of two variables, namely the pulse moment q and the record time t, and provides improved resolution and stability. Second, the model and data spaces of the initial value inversion scheme are investigated. In order to assess the model resolution as a function of the loop size, maximum pulse moment and noise level, the model space is studied. On the one hand, the results of this study help to determine field settings appropriate to the target of investigation. On the other hand, the estimated model can be evaluated concerning its reliability using measures of resolution derived from the model resolution matrix. Besides model resolution the data space caries information on data dependencies. Consequently, analysing the data resolution matrix a sequence of optimal distributed pulse moments is derived. This optimal pulse moment distribution provides the maximum amount of information most efficiently. This analysis introduces the data resolution matrix rather than the model resolution matrix, as object of analysis into the field of experimental design. In addition, the properties of the mono or multi-exponential fitting are studied, and possibilities to improve the estimated model using transformation that restrict the model and data space with upper and lower boundaries are shown. At this, the pseudo water content is introduced to permit data space transformations. Finally, all subtasks are summarised, assessed and discussed at a field example

Schlussbericht zum Verbundvorhabens Entwicklung, Bau und Erprobung einer 1H-NMR-Bohrlochsonde zur Grundwassererkundung, -erschließung und -gewinnung, Teilprojekt 1: HYDRO-NMR-Bohrlochsonde: Geophysikalische Grundlagen

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