A laboratory study of NMR relaxation times in unconsolidated heterogeneous sediments

Geophysics ◽  
2011 ◽  
Vol 76 (4) ◽  
pp. G73-G83 ◽  
Author(s):  
Elliot Grunewald ◽  
Rosemary Knight
Keyword(s):  
Grain Size ◽  
Relaxation Time ◽  
Time Distribution ◽  
Relaxation Times ◽  
Nmr Relaxation ◽  
Unconsolidated Sediments ◽  
Relaxation Response ◽  
Relaxation Time Distribution ◽  
Near Surface ◽  
Heterogeneous Sediments

Nuclear magnetic resonance (NMR) relaxation-time measurements can provide critical information about the physiochemical properties of water-saturated media and are used often to characterize geologic materials. In unconsolidated sediments, the link between measured relaxation times and pore-scale properties can be complicated when diffusing water molecules couple the relaxation response of heterogeneous regions within a well-connected pore space. Controlled laboratory experiments have allowed us to investigate what factors control the extent of diffusional coupling in unconsolidated sediments and what information is conveyed by the relaxation-time distribution under varied conditions. A range of sediment samples exhibiting heterogeneity in the form of a bimodal mineralogy of quartz and hematite were mixed with varied mineral concentration and grain size. NMR relaxation measurements and geometric analysis of these mixtures demonstrate the importance of two critical length scales controlling the relaxation response: the diffusion length ℓD, describing the distance a water molecule diffuses during the NMR measurement, and the separation length ℓS, describing the scale at which heterogeneity occurs. For the condition of ℓS > ℓD, which prevails for samples with low hematite concentrations and coarser grain size, coupling is weak and the bimodal relaxation-time distribution independently reflects the relaxation properties of the two mineral constituents in the heterogeneous mixtures. For the condition of ℓS < ℓD, which prevails at higher hematite concentrations and finer grain size, the relaxation-time distribution no longer reflects the presence of a bimodal mineralogy but instead conveys a more complex averaging of the heterogeneous relaxation environments. This study has shown the potential extent and influence of diffusional coupling in unconsolidated heterogeneous sediments, and can serve to inform the interpretation of NMR measurements in near-surface environments where unconsolidated sediments are commonly encountered.

A laboratory study of NMR relaxation times and pore coupling in heterogeneous media

Geophysics ◽  
2009 ◽  
Vol 74 (6) ◽  
pp. E215-E221 ◽  
Author(s):  
Elliot Grunewald ◽  
Rosemary Knight
Keyword(s):  
Relaxation Time ◽  
Time Distribution ◽  
Heterogeneous Media ◽  
Relaxation Times ◽  
Nmr Relaxation ◽  
Micropore Volume ◽  
Heterogeneous Porous Media ◽  
Relaxation Time Distribution ◽  
Basic Parameters ◽  
Nmr Relaxation Times

Nuclear magnetic resonance (NMR) relaxation times of geologic materials are closely related to pore geometry. In heterogeneous media, however, the details of this relationship are poorly understood because of a phenomenon known as pore coupling, which arises when diffusing protons sample multiple pores before relaxing. Laboratory experiments allow us to explore whether surface geochemistry can influence pore coupling and how this process affects the observed relaxation-time distribution. Measurements of the NMR response for microporous silica gel packs, treated with varying amounts of surface-coating iron, demonstrate that samples with less iron exhibit stronger pore coupling than those with abundant iron. When pore coupling is strong, the relaxation-time distribution grossly misrepresents the underlying bimodal pore-size distribution of micropores and macropores. Specifically, the bimodal relaxation-time distribution becomes merged and the relative amplitude of the peaks fails to reflect the true macropore and micropore volume. A reduction in pore coupling, observed with increasing iron content, is attributed to a decrease in the distance protons are able to diffuse before relaxing. Basic parameters describing the shape of the relaxation-time distributions for this range of samples are well-predicted by a 1D analytical model. Experimental results conclusively demonstrate that surface geochemistry is an important factor determining the degree to which pore coupling occurs and illustrate how this phenomenon can affect the interpretation of NMR relaxation measurements in heterogeneous porous media.

scholarly journals Ultrafast energy relaxation in single light-harvesting complexes

2016 ◽  
Vol 113 (11) ◽  
pp. 2934-2939 ◽  
Author(s):  
Pavel Malý ◽  
J. Michael Gruber ◽  
Richard J. Cogdell ◽  
Tomáš Mančal ◽  
Rienk van Grondelle
Keyword(s):  
Relaxation Time ◽  
Time Distribution ◽  
Light Harvesting ◽  
Purple Bacteria ◽  
Relaxation Times ◽  
Energy Relaxation ◽  
Spectroscopic Techniques ◽  
Probe Type ◽  
Relaxation Time Distribution ◽  
Light Harvesting Complexes

Energy relaxation in light-harvesting complexes has been extensively studied by various ultrafast spectroscopic techniques, the fastest processes being in the sub–100-fs range. At the same time, much slower dynamics have been observed in individual complexes by single-molecule fluorescence spectroscopy (SMS). In this work, we use a pump–probe-type SMS technique to observe the ultrafast energy relaxation in single light-harvesting complexes LH2 of purple bacteria. After excitation at 800 nm, the measured relaxation time distribution of multiple complexes has a peak at 95 fs and is asymmetric, with a tail at slower relaxation times. When tuning the excitation wavelength, the distribution changes in both its shape and position. The observed behavior agrees with what is to be expected from the LH2 excited states structure. As we show by a Redfield theory calculation of the relaxation times, the distribution shape corresponds to the expected effect of Gaussian disorder of the pigment transition energies. By repeatedly measuring few individual complexes for minutes, we find that complexes sample the relaxation time distribution on a timescale of seconds. Furthermore, by comparing the distribution from a single long-lived complex with the whole ensemble, we demonstrate that, regarding the relaxation times, the ensemble can be considered ergodic. Our findings thus agree with the commonly used notion of an ensemble of identical LH2 complexes experiencing slow random fluctuations.

Studying Blended Cement Paste with Nuclear Magnetic Resonance Relaxation Time

2011 ◽  
Vol 492 ◽  
pp. 433-436 ◽  
Author(s):  
Dan Jin ◽  
Wu Yao ◽  
Hong Zhi Wang
Keyword(s):  
Relaxation Time ◽  
Pore Structure ◽  
Cement Paste ◽  
Time Distribution ◽  
Blended Cement ◽  
Nmr Relaxation ◽  
Mineral Admixture ◽  
Relaxation Time Distribution ◽  
Nmr Relaxation Time ◽  
Blended Cement Paste

The pore structure of cement paste has a relationship with its strength and durability. An appropriate method of measurement is a prerequisite to study the pore structure of cement paste. Among many test methods, Nuclear Magnetic Resonance (NMR) relaxation time is a novel testing methods to study pore structure of cement paste. Different from previous research object is limited to white cement, the test sample in this paper is the blended cement paste containing mineral admixture and has been widely used in practical engineering applications. The factors of pore structure are water to cementitious material ratio, kind of mineral admixture, and mineral admixture content. Measure the same sample at four different ages to obtain the relaxation time distribution to reflect the pore structure. The test results show that, in most cases, the distribution curves of the same kind of paste are in good agreement, and the change of relaxation time distribution of the blended cement paste with different ages can be interpreted as the characteristic of the mineral admixtures in cement paste. So the NMR relaxation time is suitable for study on the blended cement paste. However due to side effects caused by iron content and unsaturated water in gel pore, this method needs further improvement.

A laboratory study of the effect of clay, silt, and sand content on low-field NMR relaxation time distributions

Geophysics ◽  
2021 ◽  
pp. 1-34
Author(s):  
Yonghui Peng ◽  
Kristina Keating
Keyword(s):  
Relaxation Time ◽  
Relaxation Times ◽  
Nmr Relaxation ◽  
Clay Content ◽  
High Specific Surface Area ◽  
Unconsolidated Sediments ◽  
Sand Content ◽  
Kaolinite Clay ◽  
Nmr Study ◽  
Nmr Relaxation Time

We have developed a laboratory nuclear magnetic resonance (NMR) study to investigate the effect of clay, silt, and sand content on the NMR relaxation time distribution. Transverse NMR relaxation times ( T2) were determined for water-saturated unconsolidated sediment mixtures of 1%–60% kaolinite clay, 5%–85% silt-size glass beads, and 8%–94% quartz sand by mass. Nearly all of the mixtures were characterized by a unimodal T2 distribution. When clay is present in quantities greater than 10%, the clay content dominates the response. For these samples, the mean-log relaxation times ( T2ML) range from 0.03 to 0.06 s, regardless of silt or sand content. For mixtures with <10% clay, T2ML decreases with increasing clay content. When the clay content is kept the same, T2ML decreases with increasing silt content and increases with the increasing sand content. The strong effect of the clay content on the NMR response is due to the high specific surface area of the clay and the distribution of clay throughout the samples. These results will help improve the interpretation of NMR field data in soils and unconsolidated sediments.

The impact of pore-scale magnetic field inhomogeneity on the shape of the nuclear magnetic resonance relaxation time distribution

Geophysics ◽  
2016 ◽  
Vol 81 (5) ◽  
pp. EN43-EN55 ◽  
Author(s):  
Denys Grombacher ◽  
Emily Fay ◽  
Matias Nordin ◽  
Rosemary Knight
Keyword(s):  
Magnetic Field ◽  
Relaxation Time ◽  
Pore Size ◽  
Time Distribution ◽  
Pore Space ◽  
Relaxation Times ◽  
Magnetic Field Inhomogeneity ◽  
Field Inhomogeneity ◽  
Relaxation Time Distribution ◽  
Measured Relaxation

Measurements of the nuclear magnetic resonance (NMR) signal’s behavior with time provide powerful noninvasive insight into the pore-scale environment. The time dependence of the NMR signal, which is a function of parameters called relaxation times, is intimately linked to the geometry of the pore space and has been used successfully to estimate pore size and permeability. The basis for the pore size and permeability estimates is that interactions occurring at the grain surface often function as the primary mechanism controlling the time dependence of the NMR signal. In this limit, called the fast diffusion limit, and when each pore can be considered to be isolated, the measured relaxation times are often interpreted as representative of pore sizes. In heterogeneous media, where the NMR signal is described by a distribution of relaxation times, the measured relaxation time distribution is often interpreted as representative of the underlying pore-size distribution. We have explored a scenario in which an additional relaxation mechanism, which arises due to magnetic field inhomogeneity across the pore space, violates the assumption that interactions occurring at the grain surface are the dominant relaxation mechanism. Using both synthetic and laboratory studies, we demonstrate that magnetic field inhomogeneity can lead to a complex relationship between the measured relaxation time distribution and the underlying pore-size distribution. Magnetic field inhomogeneity is observed to lead to a spatially heterogeneous magnetization density across the pore space requiring multiple eigenmodes to describe the evolution of the magnetization within a single pore during the NMR experiment. This results in a breakdown of the validity of the interpretation of the relaxation time distribution as representative of the underlying pore-size distribution for sediments with high magnetic susceptibility.

Relaxation time distribution obtained from a Debye decomposition of spectral induced polarization data

Geophysics ◽  
2016 ◽  
Vol 81 (2) ◽  
pp. E129-E138 ◽  
Author(s):  
Andrea Ustra ◽  
Carlos Alberto Mendonça ◽  
Dimitrios Ntarlagiannis ◽  
Lee D. Slater
Keyword(s):  
Relaxation Time ◽  
Time Distribution ◽  
Relaxation Times ◽  
Induced Polarization ◽  
Complex Conductivity ◽  
Alternative Formulation ◽  
Conductivity Method ◽  
Relaxation Time Distribution ◽  
Spectral Induced Polarization ◽  
Conductivity Spectra

We have developed an alternative formulation for Debye decomposition of complex electric conductivity spectra, by recasting it into a new set of parameters with a close relationship to the continuous formulation for the complex conductivity method. The procedure determines a relaxation time distribution (RTD) and two frequency-independent parameters that modulate the complex conductivity spectra. These two parameters represent (1) the direct current contribution and (2) the conductivity range spanned by the low- and high-frequency limits. The distribution of relaxation times quantifies the contribution of each distinct relaxation process. Assuming that characteristic times with insignificant contributions can be ignored, a minimum set of characteristic relaxation times is determined. Each contribution can then be associated with specific polarization processes that can be interpreted in terms of electrochemical or interfacial parameters of mechanistic models derived from inverted parameters obtained from the proposed approach. Synthetic tests show that the procedure can fit spectral induced polarization (SIP) data and successfully retrieve the RTD. We have applied the procedure to laboratory SIP data from experiments with sand and oil mixtures undergoing microbial degradation of hydrocarbons. The RTD reveals evidence of a length scale at which a new polarization process takes place as a result of the biodegradation process.

A new approach to fitting induced-polarization spectra

Geophysics ◽  
2008 ◽  
Vol 73 (6) ◽  
pp. F235-F245 ◽  
Author(s):  
Sven Nordsiek ◽  
Andreas Weller
Keyword(s):  
Grain Size ◽  
Relaxation Time ◽  
Time Distribution ◽  
Induced Polarization ◽  
Model Parameters ◽  
Mass Percentage ◽  
Relaxation Time Distribution ◽  
Best Fitting ◽  
Cole Model ◽  
Polarization Spectra

Best fitting of induced-polarization (IP) spectra by different models of Cole-Cole type evidences discrepancies in the resulting model parameters. The time constant determined from the same data could vary in magnitude over several decades. This effect, which makes an evaluation of the results of different models nearly impossible, is demonstrated by induced polarization measurements in the frequency range between [Formula: see text] and [Formula: see text] on thirteen mixtures of quartz sand and slag grains. The samples differ in size and the amount of the slag grains. Parameters describing the IP spectra are derived by fitting models of the Cole-Cole type to the measured data. The fitting quality of the generalized Cole-Cole model, the standard Cole-Cole model, and the Cole-Davidson model is investigated. The parameters derived from these models are compared and correlated with mass percentage and grain size of the slag particles. An alternative fittingapproach is introduced, using the decomposition of observed IP spectra into a variety of Debye spectra. Four integrating parameters are derived and correlated with parameters of the slag-sand mixtures and Cole-Cole parameters, respectively. The alternative approach generally enables a better fitting of measured spectra compared with Cole-Cole type models. It proves to be more flexible and stable, even for complicated phase spectra that cannot be fitted by single Cole-Cole type models. The integrating parameters are well correlated with characterizing parameters of the slag-sand mixtures. The total chargeability well indicates the mass percentage of slag grains, and the mean relaxation time is related to the grain size. The relaxation time distribution can be displayed by cumulative normalized chargeability versus relaxation time, similar to granulation curves. Anologous to the latter, a nonuniformity parameter characterizes the width of the relaxation time distribution.

The relaxation time distribution in dielectrics

1992 ◽  
Vol 11 (14) ◽  
pp. 988-990 ◽  
Author(s):  
S. N. Al-Refaie ◽  
H. S. B. Elayyan
Keyword(s):  
Relaxation Time ◽  
Time Distribution ◽  
Relaxation Time Distribution
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