Marian Hertrich

Aquifer characterisation using Surface NMR jointly with other geophysical techniques at the Nauen/Berlin test site

The quite new technique of Surface Nuclear Magnetic Resonance (SNMR) has been extensively tested on the test site Nauen near Berlin to yield the geometry, water content and hydraulic conductivity of the aquifer. The test site is composed of an unconfined aquifer consisting of Quaternary sands with glacial till beneath. It is a very favourable site for assessing the suitability and performance of joint geophysical methods for groundwater exploration. Complementary measurements to SNMR were conducted with Ground Penetrating Radar (GPR), 1D-complex resistivity soundings, i.e. Spectral Induced Polarisation (SIP), 2D-geoelectrics and refraction seismics. Laboratory measurements of porosities, grain size distributions and Nuclear Magnetic Resonance (NMR) decay times were carried out on core samples, and hydraulic conductivities were also derived in order to control and interpret the results of field measurements. The SNMR method allowed the detection of the aquifer beyond any doubt and the determination of the approximate aquifer geometry. The aquifer water content found by SNMR fits very well with the independent measurements on core samples. Hydraulic conductivities derived from decay times are well in range with those from laboratory measurements. GPR allowed a very reliable determination of the aquifer geometry. This information, incorporated into inversion of geoelectric data, led to an improved determination of aquifer electrical resistivity. The estimation of water content by GPR and geoelectrics, even under the favourable conditions in Nauen, is by far not as reliable as that by SNMR. Obtaining information about hydraulic conductivity is possible only with SNMR. Thus, in combination with other geophysical methods, SNMR allows a much more detailed and reliable assessment of aquifers than what was possible with other surface geophysical methods before. In fact, it is, by far, the only method that allows direct detection of water and reliable estimations about water content. It is expected that SNMR will turn out to be a valuable and powerful tool in applied geophysics for groundwater exploration.

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.

Joint inversion of Surface Nuclear Magnetic Resonance and Vertical Electrical Sounding

The method of Surface Nuclear Magnetic Resonance (SNMR) provides a very new technology to directly determine subsurface water distribution. The microscopic magnetization of water molecules is used to derive water content and pore size information from SNMR soundings. The observed similarity and agreement between interpreted aquifer structure from SNMR and resistivity distribution from Vertical Electrical Sounding (VES) has led to our objective to jointly invert both data sets using a generalized petrophysical model based on Archies Law. To perform inversion of both methods, the Simulated Annealing (SA) technique was applied. Since a very fast numerical solution is available for both geophysical methods, this kind of guided random search algorithm promises better performance than least square methods. The developed inversion algorithm has been applied on a number of different synthetic data to study its properties and prove its reliability. Investigations on well-known test sites where both methods were conducted finally proved the effectiveness of the joint inversion on real data. The interpretation of the subsurface model could be optimized beyond an enhanced spatial resolution to a quantitative interpretation of the ratio of mobile and adhesive water contents, leading to prediction of hydrological parameters from geophysical investigations.

Accounting for relaxation processes during the pulse in surface NMR data

Under favorable conditions, the surface nuclear-magnetic-resonance (NMR) technique can provide direct quantitative estimates of subsurface water variations and indirect information on pore size and hydraulic conductivity. The technique is based on various exponential relaxation processes that become measurable after the intrinsic spin magnetic moments of groundwater protons have been rotated out of equilibrium by a pulse of alternating electromagnetic (EM) field generated at the surface. An implicit assumption in previous surface NMR studies is that relaxation processes need to be considered only after the EM pulse has been extinguished. Although this approximation is valid for short EM pulses, neglecting relaxation during the pulse (RDP) can result in significant errors for the generally long pulses used in surface NMR investigations. Because the influence of RDP cannot be isolated and quantified using field-scale approaches, this study is based on sample-scale NMR experiments and numerical simulations (Bloch equations) that mimic field practices and conditions. The results demonstrate that standard surface NMR methods that ignore the effects of RDP may yield significantly erroneous estimates of water volume (RDP-related errors of 25% are possible) and the key relaxation parameter (RDP-related errors of 50% are possible) that supplies information on pore size and hydraulic conductivity. Fortunately, the study also demonstrates that relatively simple interpretational approaches can reduce the RDP-related errors to less than 2%.

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.

Off-resonance effects in surface nuclear magnetic resonance

Surface nuclear magnetic resonance (NMR) is a noninvasive geophysical tool used to investigate groundwater reservoirs. The relevant physical process in surface NMR is the nuclear spin of hydrogen protons in liquid water. Standard single-pulse surface NMR experiments provide estimates of water content in the shallow subsurface. Under favorable conditions, pore-structure and even hydraulic-conductivity information can be extracted from double-pulse surface NMR data. One crucial issue in surface NMR experiments is the resonance condition: the frequency of the excitation field should closely match the Larmor frequency of the protons, which is controlled by the local magnitude of the earth’s magnetic field. Although the earth’s field can be measured accurately by an on-site magnetometer, several effects impede perfect matching of the frequencies. These include temporal variations of the earth’s field, instrumental imperfections, and the magnetic susceptibility of the underlying rocks. We assess the impact of v...

Three-Dimensional Magnetic Field and NMR Sensitivity Computations Incorporating Conductivity Anomalies and Variable-Surface Topography

We have developed a numerical algorithm for computing the magnetic field distribution and the nuclear magnetic resonance (NMR) sensitivity function for arbitrary topography overlying a known 3-D conductivity structure. The magnetic vector potential is split into primary and secondary terms. The primary term is obtained using a thin-wire line integral equation that accounts for arbitrary loop shape and position. It allows the singularity of the source field to be effectively removed. The secondary potential is obtained by solving the second-order partial differential equations on an unstructured tetrahedral mesh using the finite element technique. We validate the results of applying our algorithm against an explicit infinite integral solution for circular loops on a layered earth and against the results of applying a commercial simulation tool. The spatially oscillating NMR sensitivity functions to hydrogen protons (i.e., unbounded water molecules) in the sub-surface are computed on a refined unstructured grid. We apply the numerical algorithm to a number of synthetic examples in surface NMR tomography of hydrological relevance.

Estimating the longitudinal relaxation time T1 in surface NMR

Surface nuclear magnetic resonance (NMR) is a noninvasive geophysical method that is primarily used in hydrological investigations of shallow aquifers. An important parameter in surface-NMR experiments is the relaxation time T1. Information on pore structure and even hydraulic permeability/conductivity may be inferred from accurate estimates of this parameter. Estimates of T1 are usually obtained by evaluating the spin response of groundwater molecules to excitation by two sequential electromagnetic pulses, the second of which is delayed and phase-shifted by π relative to the first. We have discovered that variations of the excitation field with distance from the transmitter and common imperfections in the transmitted pulses introduce considerable bias in estimates of T1 (e.g., errors as large as 50%). We assess the significance of these problems via numerical simulations based on the Bloch equation. As a result of this assessment, we propose a novel yet simple modification to the T1 acquisition method th...