Martina Braun

Study on complex inversion of magnetic resonance sounding signals

Magnetic resonance sounding (MRS), also known as surface nuclear magnetic resonance (SNMR), is used for non-invasive direct groundwater determination and aquifer characterization. Among other parameters, the electrical conductivity of the subsurface causes a complex-valued MRS signal. We show in our study that the real and imaginary parts of the signal evolve from different depth volumes, and so they contain complementary information. Generally, the imaginary part is more sensitive to deep structures than the real part of the signal, i.e. in conductive media, signals arising from deep layers have a significantly greater imaginary part than an equivalent signal from shallow depths. Statistical analyses of the inversion result of synthetic data show the advantages of a complex inversion scheme for determining the water content in the subsurface: model ambiguities are significantly reduced and the depth resolution is increased. Also the investigations on artificially noise-perturbed data show a clear improvement in the stability and resolution of model boundaries. For field data, amplitude and complex inversion schemes yield significantly different results. However, the various influences on the phase are still undergoing investigation, and at present the complex inversion is limited to selected (explainable) field data sets.

High-resolution surface NMR tomography of shallow aquifers based on multioffset measurements

Conventional surface nuclear magnetic resonance (NMR) surveying based on 1D inversions of data recorded using a common (coincidence) transmitter and receiver loop provides only limited or distorted water-concentration information in regions characterized by strong lateral heterogeneity. We introduce a combined field-acquisition and tomographic-inversion strategy suitable for 2D surface NMR investigations of free (i.e., unbound) water stored in hydrogeologically complex regions. Using combinations of coincident and multioffset loops, we take advantage of the range of sensitivities offered by different loop configurations to variations in subsurface free-water concentration. The new tomographic scheme can invert data acquired with diverse loop configurations. Tests of the combined acquisition and inversion strategy on complicated synthetic and observed data demonstrate the substantially higher resolution information provided by combinations of loop configurations vis-a-vis that supplied by a standard coincident loop. A combination of coincident and half-overlapping loop data sets yields tomograms rich in detail, comparable to tomograms derived from a combination of all considered loop configurations. If resources are limited, surface NMR practitioners should consider the half-overlapping loop configuration as an alternative to the standard coincident loop configuration. For a four-station data recording campaign, the half-overlapping loop configuration with 50% more measurements and equal number of loop deployments and retrievals provides significantly higher resolution tomograms than a coincident loop configuration.

Surface Nuclear Magnetic Resonance Tomography

Groundwater is the principal source of freshwater in many regions worldwide. Expensive drilling, borehole logging, and hydrological testing are the standard techniques employed in groundwater exploration and management. It would be logistically beneficial and cost-effective to have surface-based nonintrusive methods to locate and quantify groundwater occurrences and to estimate other key hydrological parameters. Surface nuclear magnetic resonance (SNMR) techniques, which are based on the spin magnetic-moment precession of protons in the hydrogen atoms of water, offer the possibility of achieving these goals. Current SNMR practices are based on 1D inversion strategies. These simple strategies impede applications of SNMR techniques in hydrologically complex areas. To address this issue, we introduce a very fast 2D SNMR tomographic-inversion scheme and apply it to four series of measurements simulated for a perched water-lens model. Whereas the new 2D scheme correctly reconstructs all important characteristics of the original model, 1D strategies produce highly inaccurate/misleading results.

Magnetic resonance soundings with separated transmitter andreceiver loops

Triggered by an extended mathematical formulation of the response signal for magnetic resonance soundings (MRS), which allows the treatment of individual transmitter and receiver loops, we make a comprehensive evaluation and assessment of the emerging new possibilities of the technique. Based on a reformulation of the basic equation, we indicate and interpret the evolving effects. The influence of loop separation on MRS sounding curves in terms of offset and direction is also assessed as is the corresponding sensitivity to depth and lateral spin variation. Interpretation of field data measured with separated loops using the extended formulation is found to fit the predicted response extremely well. From the encouraging results, we derive new aspects of two-dimensional investigation of groundwater resources. Furthermore, new perspectives of future developments of the MRS technique, with individual loops for optimized field measurements and hydrological applications, are discussed.

Simultaneous inversion of magnetic resonance sounding in terms of water content, resistivity and decay times

Magnetic resonance sounding (MRS) or surface nuclear magnetic resonance (SNMR) is used for direct groundwater exploration and for an improved aquifer characterization. Currently, it is the only geophysical method that is capable of directly determining the free water content and estimating the pore sizes of the aquifer in the subsurface. However, MRS is basically an electromagnetic method. Therefore, it is sensitive to the resistivity of the subsurface. The water content is the main target of investigation, therefore first inversion routines focused on the water content. Later on, inversion routines determining water content and decay times became available. Very recently, MRS inversion for water content and resistivity has been realized. We present here a simultaneous inversion of MRS in terms of determining the three inversion parameters – water content, resistivity and decay time – within one single inversion routine. Within the iterative inversion scheme, the extrapolated initial values are determined on the basis of the physical effective decay times in the subsurface, which are estimated within the inversion scheme. Due to an instrumental dead time, the initial values for amplitude and phase, which are related to water content and resistivity, cannot be measured directly. Therefore, the initial amplitude must be extrapolated using the decay time of the signal. The standard approach is a mono-exponential decay curve; implicitly, the phase is assumed to be time-invariant. However, multi-exponential signals are natural when considering relaxation behaviour in the underground. It originates from multi-modal pore size distributions or simply a number of differently relaxing signal contributions from the various lithological units.

Evaluation of the Influence of 2-D Electrical Resistivity on Magnetic Resonance Sounding

Magnetic resonance sounding (MRS) or surface nuclear magnetic resonance (SNMR) is used for groundwater exploration and aquifer characterization. The method performs NMR measurements with large surface coils (diameter range of 10‐100 m ) to measure the induced decay of the spin of protons of water molecules. The main property of interest is water content. For a reliable analysis of the measurement, the electrical resistivity in the subsurface must be taken into account. The extent of resistivity influence on MRS depends on the size of the coil. So far, the influence of the resistivity has been considered only for stratified 1-D subsurfaces. We explore the influence of 2-D resisitivity models on the MRS signal by comparing the signal calculated for 2-D models to the signal calculated for layered 1-D models. The results indicate that a 1-D approximation is valid if the midpoint of the loop is at least one diameter away from the 2-D structure, or if the extension of the 2-D structure is twice as wide as the l...

1D Inversion of Resistivity and Water Content of Magnetic Resonance Sounding

Magnetic Resonance Sounding (MRS, or Surface Nuclear Magnetic Resonance, SNMR) is used for groundwater exploration and aquifer characterization. It provides valuable information about geometry of the aquifer, water content and hydraulic conductivity. In MRS, since the excitation magnetic field depends on the resistivity of the subsurface, it has to be taken into account in the inversion. Currently, resistivity is provided by a priori information from supplementary geoelectrical measurements or by assumptions. We developed an inversion scheme for MRS to invert also for the resistivity directly from the MRS data. Results for synthetic data show that the water content, layer boundaries and resistivity can be derived using only the amplitude of the MRS data. However, using amplitude and phase of the MRS signal improves the inversion result. The feasibility of the new inversion scheme has been demonstrated by the successful implementation of field data. The resistivities derived from SNMR are comparable with those derived from conventional electromagnetic methods.

Inversions of surface-NMR signals using complex kernels

Surface Nuclear Magnetic Resonance (SNMR) is used for groundwater exploration and aquifer characterization (e.g. [1]). The NMR-Experiment is conducted by a coincident transmitter and receiver loop at the surface exciting the protons of water molecules underground with the Larmor frequency of the earths magnetic field. Performing a SNMR measurement with increasing excitation intensity (pulse moment) yields a complex sounding curve, where the amplitude of the relaxation signal is determined by the numbers of protons i.e. the water content. The subsurface electrical conductivity affects both the amplitude and especially the phase [2,3].

Complex Inversion Of Surface-Nmr Signals - Extending The Limits Of Model Resolution

The technique of Surface Nuclear Magnetic Resonance (SNMR, also known as MRS - Magnetic Resonance Sounding) is used for direct groundwater determination and aquifer characterization. Among other influences, the electrical conductivity of the subsurface leads to a complex-valued signal. However, the standard interpretation scheme uses only the amplitude of the signal for determining the water content. But real and imaginary parts of the signal are sensitive to different depth volumes. Generally, the imaginary part is more sensitive to deep structures than the real part of the signal, i.e. in conductive media, signals arising from deep layers have a significantly greater imaginary part than an equivalent signal from shallow depths. For real data, physical effects additionally to the electromagnetic phase delay have to be considered and adequately quantified to use the phase information for an enhanced data interpretation. This study assesses the complex inversion using real and imaginary parts of the signal. Analyzes of synthetic and real data with sufficient data quality show that the complex inversion is more reliable in terms of determining deep structures, equivalence errors are reduced, and the depth resolution is increased.