Stephan Costabel

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.

Comparison and optimal parameter settings of referencebased harmonic noise cancellation in time and frequency domains for surface-NMR

The technique of surface nuclear magnetic resonance (surface-NMR) provides information on porosity and hydraulic conductivity that is highly valuable in a hydrogeological context. However, the applicability of surface-NMR is often limited due to a bad signal-to-noise ratio. In this paper we provide a detailed insight into the technique of harmonic noise cancellation based on remote references to improve the signal-to-noise ratio. We give numerous synthetic examples to study the influence of various parameters such as optimal filter length for time- domain approaches or the necessary record length for frequency-domain approaches, all of which evaluated for different types of noise conditions. We show that the frequency-domain approach is superior to time-domain approaches. We demonstrate that the parameter settings in the frequency domain and the decision whether or not to use separated noise measurement depend on the actual noise properties, i.e., frequency content or stability with time. We underline our results using two field examples.

Estimation of water retention parameters from nuclear magnetic resonance relaxation time distributions

[1] For characterizing water flow in the vadose zone, the water retention curve (WRC) of the soil must be known. Because conventional WRC measurements demand much time and effort in the laboratory, alternative methods with shortened measurement duration are desired. The WRC can be estimated, for instance, from the cumulative pore size distribution (PSD) of the investigated material. Geophysical applications of nuclear magnetic resonance (NMR) relaxometry have successfully been applied to recover PSDs of sandstones and limestones. It is therefore expected that the multiexponential analysis of the NMR signal from water-saturated loose sediments leads to a reliable estimation of the WRC. We propose an approach to estimate the WRC using the cumulative NMR relaxation time distribution and approximate it with the well-known van-Genuchten (VG) model. Thereby, the VG parameter n, which controls the curvature of the WRC, is of particular interest, because it is the essential parameter to predict the relative hydraulic conductivity. The NMR curves are calibrated with only two conventional WRC measurements, first, to determine the residual water content and, second, to define a fixed point that relates the relaxation time to a corresponding capillary pressure. We test our approach with natural and artificial soil samples and compare the NMR-based results to WRC measurements using a pressure plate apparatus and to WRC predictions from the software ROSETTA. We found that for sandy soils n can reliably be estimated with NMR, whereas for samples with clay and silt contents higher than 10% the estimation fails. This is the case when the hydraulic properties of the soil are mainly controlled by the pore constrictions. For such samples, the sensitivity of the NMR method for the pore bodies hampers a plausible WRC estimation. Citation: Costabel, S., and U. Yaramanci (2013), Estimation of water retention parameters from nuclear magnetic resonance relaxation time distributions, Water Resour. Res., 49, 2068-2079, doi:10.1002/wrcr.20207.

Despiking of magnetic resonance signals in time and wavelet domains

In this paper three different despiking methods for surface-NMR data are investigated and compared. Two of these are applied in the time domain: a threshold is determined that identifies and marks a spiky event. Afterward, the marked time sequence is substituted with zeros or with the mean value of the signal amplitude of the measurement repetitions for the same passage on the time axis. The third despiking approach takes advantage of the wavelet-like nature of spiky events. It isolates and eliminates spiky signals in the wavelet domain, i.e., after transforming a single record with the help of the discrete wavelet transform. The latter is able to reconstruct the original signal content in the (spike-caused) distorted time sequence to some extent. If the spiky noise in surface-NMR measurements consists mainly of single spiky events, the three despiking methods show very similar results and are able to remove spiky noise from data very effectively, as we can show with two real data examples. However, a synthetic study shows that, if a series of spikes within a relatively short period of time occurs, the wavelet-based despiking approach shows significant shortcomings. Because the NMR signal content cannot be restored completely in a single record, the fitting of the signal after stacking leads to underestimation of the initial amplitude up to approximately 10%. Nevertheless, we can show that, in principle, the processing of surface-NMR data in the wavelet domain works and can lead to the same results as straight-forward applications. Moreover, waveletbased strategies have some interesting properties and thus have some potential for further development regarding surface-NMR processing, which is discussed in detail.

Relative hydraulic conductivity and effective saturation from Earth’s field nuclear magnetic resonance – a method for assessing the vadose zone

Beside the water content, petrophysical nuclear magnetic resonance (NMR) techniques in the lab and in boreholes as well as in the field, provide estimates of the hydraulic conductivity of water saturated sediments and rocks. In the vadose zone, the hydraulic conductivity is a function of the water saturation. Regarding the characterization of the vadose zone, the magnetic resonance sounding (MRS) method is expected to have great potential. However, so far, the petrophysical relationship of the hydraulic properties under partial saturation conditions and the NMR parameters in the Earth’s magnetic field is not fully understood. In this study, laboratory NMR experiments in the Earth’s field (EFNMR) are performed in comparison to conventional high field NMR (HFNMR). Sand-filled columns were used to generate partially saturated conditions by simulating capillary fringes (grain sizes from fine to coarse). We investigate the ability of both NMR techniques to determine the residual water content and the dependency of the NMR relaxation times on the water saturation degree. We note that EFNMR measurements tend to underestimate the residual water content due to long measurement dead times. Furthermore, it shows that the HFNMR relaxation time T 2 , as a function of the saturation, behaves according to the Brooks-Corey model that describes the water retention function and thus allows for the prediction of the relative hydraulic conductivity K rel . The EFNMR relaxation time T 2 * as a function of the saturation degree differs from the Brooks-Corey expectation due to the influence of the dephasing relaxation rate that is, in general, responsible for the difference of T 2 and T 2 * . We assume that the dephasing relaxation rate itself, when induced by internal magnetic field gradients, depends on the water saturation. We introduce a model that accounts for this dependency with a weighting factor for the dephasing relaxation rate, given as a power law of the saturation degree. The model enables the description of T 2 * as a function of the water saturation and thus provides the estimation of K rel from T 2 * . We compare the NMR based K rel predictions with the K rel functions estimated from gravity induced outflow experiments at the columns. The results are in agreement within half a decade for every sand sample of the study. In principle, the suggested approach can be applied for estimating K rel in situ by MRS measurements in the vadose zone. We discuss the potential and limitations of this approach for MRS.

Relative hydraulic conductivity in the vadose zone from magnetic resonance sounding — Brooks-Corey parameterization of the capillary fringe

The magnetic resonance sounding (MRS) method is used for noninvasive one-dimensional assessment of aquifer structures, i.e., for estimating the vertical water content and hydraulic conductivity distributions in the saturated zone. So far, MRS interpretation schemes for estimating the hydraulic conductivity as a function of the saturation degree in the vadose zone have not been developed. In this study we developed a new inversion scheme for MRS to estimate the relative hydraulic conductivity Krel , which is the ratio of the unsaturated and saturated conductivities. The new approach is based on the Brooks-Corey parameterization (BCP) of the capillary fringe; that is, the water content distribution of the capillary fringe is considered to be equivalent to the water retention curve in equilibrium state. The BCP inversion directly provides estimates of Brooks-Corey parameters from MRS data, which allows for the calculation of Krel as a function of the saturation degree. In doing so, the water table must be gi...