Ugur Yaramanci

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.

QT inversion — Comprehensive use of the complete surface NMR data set

The technique of surface nuclear magnetic resonance (surface NMR) is the only geophysical exploration method providing direct and nondestructive information on subsurface aquifer properties due to the method’s unique sensitivity to hydrogen protons. The method combines the information content accessible via nuclear magnetic resonance (NMR) measurements and the nondestructive approach to derive subsurface information from surface-based measurements. Because of this, surface NMR became a useful tool for hydrogeophysics during the last decade. Two different inversion schemes exist. The initial value inversion (IVI) extracts the water content distribution from the surface NMR information content by estimating a sounding curve from surface NMR data. The time step inversion (TSI) extracts the distribution of both water content and decay time by separating the surface NMR data into several time steps. Both solve the inverse problem using independent steps and by separating subdata sets from the complete data. In...

Use of block inversion in the 2-D interpretation of apparent resistivity data and its comparison with smooth inversion

Abstract The ability of a block inversion scheme, in which polygons are employed to define layers and/or bodies of equal resistivity, in determining the geometry and true resistivity of subsurface structures has been investigated and a simple strategy for deriving the starting model is proposed. A comparison has also been made between block inversion and smooth inversion, the latter being a cell-based scheme. The study entailed the calculation (by forward modelling) of the synthetic data over 2-D geologic models and inversion of the data. The 2-D structures modelled include vertical fault, graben and horst. The Wenner array was used. The results show that the images obtained from smooth inversion are very useful in determining the geometry; however, they can only provide guides to the true resistivity because of the smearing effects. It is shown that the starting model for block inversion can be based on a plane layer earth model. In the presence of sharp, rather than gradational, resistivity discontinuities, the model from block inversion more adequately represents the true subsurface geology, in terms of both the geometry and the formation resistivity. Field examples from a crystalline basement area of Nigeria are presented to demonstrate the versatility of the two resistivity inversion schemes.

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.

Assessment of the potential of a new generation of surface nuclear magnetic resonance instruments

The technique of magnetic resonance sounding (MRS) has shown several improvements in data processing, inversion and interpretation during the last years. Along with these improvements, detailed innovations on instrumentation have been demanded to support their use. Latest developments in surface nuclear magnetic resonance (NMR) instrumentation promise to fulfil these hardware requirements such as decreased dead time, improved digital signal detection, multi-channel capabilities and improved reference techniques with the second generation surface NMR instruments. In this paper, we compare data from two generations of instruments and assess the impact of the improvements on practical issues, i.e., the increased accuracy of data due to shorter dead times and new noise reduction approaches and the feasibility for efficient 2D measuring schemes. Well-known and documented test sites and synthetic considerations are used to evaluate these developments. First, the relaxation signals of different devices using the same loop match each other. The inversion results coincide within the range of data errors. Decay time estimation appears to be more stable for the new generation instrument. Second, the potential of shorter effective dead times (considering a relaxation of the protons during the pulse) is investigated using statistical analysis of synthetic data sets with different decay times and noise levels. The additionally measured data at early times significantly improve the scope and accuracy of the determined parameters initial amplitude and T 2 * time and thus extend the range of formations to be characterized. A field example comparing an effective dead time of 18 ms and 45 ms is presented. Two different reference techniques were successfully applied for noise cancellation at the very noisy test site Nauen. We observed an equivalent signal improvement using the software-based and hardware-based technique. However, software noise cancellation approaches are easily adaptable and extendable. Finally, considerations are given how to efficiently carry out 2D surveys using multi-channel instruments. A 2D field data set using the GMR demonstrates that 2D surveys can already be realized in moderate measuring times. The new generation of instruments provides comparable results and improved capabilities that will enable surface NMR measurements to be applied in a wider range of applications.

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.

Forward modeling and inversion of MRS relaxation signals using multi-exponential decomposition

The geophysical method of surface nuclear magnetic resonance (SNMR) or magnetic resonance sounding (MRS) allows a direct determination of the water-content (amplitudes) distribution in the subsurface. In addition, the MRS signal also contains information about the distribution of poresizes (decay times) and electrical conductivity (phases) in the ground. So far, the inversion of MRS data has mainly concentrated on the interpretation of the water-content and decay-time distributions. The inversion is performed by fitting the recorded relaxation curve for each excitation pulse with a single initial amplitude value and relaxation constant, i.e. decay time (mono-exponential fit). However, MRS relaxation signals inherently exhibit a multi-exponential behaviour that arises due to the superposition of signal contribution originating from layers or volume fractions that feature different decay-time properties. Another geological situation that contributes to the multi-exponential behaviour of MRS relaxation data is given by a possible multimodal decay-time distribution within volume units or layers in the subsurface. This can been countered in hard-rock material, e.g. sandstone, and occasionally in sediments. As a further consequence of the signal contribution from sources with different decay-time properties, the signal phase of MRS data is also affected. With the aim of a more quantitative and accurate aquifer characterization, in analogy with applications commonly used in borehole and laboratory NMR, we introduce a new approach to the inversion of MRS data taking into account the inherent multi-exponential behaviour of MRS relaxation data due to layering and non-uniform distribution of pores in the subsurface. Inversions carried out for synthetic data, i.e. the results of forward modelling, and MRS field data clearly show the advantages of such a comprehensive inversion (COIN) approach with respect to an improved determination of water content and decay times in the subsurface.

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.