Core Ideas
Evaporative drying of soil columns was monitored in high resolution using NMR.
Soil moisture data were used for validating a water, vapor, and heat flow model.
The Richards approach was ...insufficient under Stage II conditions.
NMR is a feasible technique for soil moisture measurements under Stage II conditions.
Evaporation from bare soil surfaces can be restrained to a great extent with the development of a dry layer at the soil surface where capillary hydraulic conductance ceases and water flow proceeds only by gas phase transport. Model calculations and preliminary experiments with model porous media have shown that this surface layer can be very thin. An accurate characterization of these processes is required, which is provided by noninvasive magnetic resonance (MR) methods. The evaporative drying of a silt loam and a sandy loam was monitored at high spatial resolution in laboratory experiments. The MR data were used to assess the performance of two numerical models: (i) the Richards equation, which considers isothermal liquid water flow, and (ii) a coupled soil water, heat, and vapor flow numerical model. The experimental results reveal two distinct drying regimes for both soil types where, at the onset of the second evaporation stage, a dry surface zone developed with increasing thickness over time. This layer revealed that water loss inside the soil coincided with a relatively low evaporation rate as the liquid continuity to the soil surface vanished. The modeling results clearly demonstrated the need to consider heat and vapor flow. It was shown, as a proof of principle, that MR relaxation time spectra may serve as a proxy to follow desaturation processes where spatially resolved transverse relaxation can reveal a secondary evaporation front.
The majority of low field Magnetic Resonance (MR) analyses rely on T
lifetime measurements. Modification of the T
measurement to include a T
dimension has made the T
-T
measurement a very powerful ...analytical technique. The T
-T
measurement is uniquely well suited to characterization of different spin populations in porous materials, such as fluid bearing reservoir rocks, and in soft biopolymer materials, for example foods. However, the T
-T
measurement is challenging or impossible if the T
relaxation lifetime, or a component lifetime, is short-lived and the signal unobservable in an echo measurement. This occurs in many important classes of materials. A short lifetime T
will not however, in general, preclude observation of a free induction decay with signal decay governed by T
*. As outlined in this paper a T
-T
* measurement is a useful analog to the T
-T
experiment. T
-T
* measurement enables one to differentiate species as a function of T
* in one dimension and T
in the other dimension. Monitoring changes of the T
-T
* coordinate, and associated signal intensity changes, has the potential to reveal structural changes in materials evolving in time. These methods may also be employed to discriminate and identify solid-like species present in static samples. The T
-T
* measurement is very general in application, but in this paper we focus on cement-based mortars to develop and illustrate the essential ideas. T
-T
* results show a multi-modal behaviour of the MR signal lifetimes, T
and T
*, in mortar samples under study, indicating at least two different water populations. The short T
* lifetime was assigned to interlayer water (water between C-S-H layers) where the associated T
is also short lived. The longer T
* lifetime was assigned to water in the pore space, where T
is also longer lived. In addition to mortar samples we also show application of the method to a crystalline organic species, o-phenylenediamine, which features Sinc Gaussian and exponential decays of transverse magnetization.
Display omitted
•T1-T2* measurement permits speciation of different spin populations when a T1-T2 measurement is impossible.•Tracking the T1-T2* coordinate, and associated intensity may reveal ...structural changes.•The T1-T2* measurement permits a simple assignment of T1 to different T2* populations.•The T1-T2* measurement will permit discrimination solid-like species in mixtures.
The majority of low field Magnetic Resonance (MR) analyses rely on T2 lifetime measurements. Modification of the T2 measurement to include a T1 dimension has made the T1-T2 measurement a very powerful analytical technique. The T1-T2 measurement is uniquely well suited to characterization of different spin populations in porous materials, such as fluid bearing reservoir rocks, and in soft biopolymer materials, for example foods. However, the T1-T2 measurement is challenging or impossible if the T2 relaxation lifetime, or a component lifetime, is short-lived and the signal unobservable in an echo measurement. This occurs in many important classes of materials. A short lifetime T2 will not however, in general, preclude observation of a free induction decay with signal decay governed by T2*.
As outlined in this paper a T1-T2* measurement is a useful analog to the T1-T2 experiment. T1-T2* measurement enables one to differentiate species as a function of T2* in one dimension and T1 in the other dimension. Monitoring changes of the T1-T2* coordinate, and associated signal intensity changes, has the potential to reveal structural changes in materials evolving in time. These methods may also be employed to discriminate and identify solid-like species present in static samples.
The T1-T2* measurement is very general in application, but in this paper we focus on cement-based mortars to develop and illustrate the essential ideas. T1-T2* results show a multi-modal behaviour of the MR signal lifetimes, T1 and T2*, in mortar samples under study, indicating at least two different water populations. The short T2* lifetime was assigned to interlayer water (water between C–S–H layers) where the associated T1 is also short lived. The longer T2* lifetime was assigned to water in the pore space, where T1 is also longer lived. In addition to mortar samples we also show application of the method to a crystalline organic species, o-phenylenediamine, which features Sinc Gaussian and exponential decays of transverse magnetization.
H relaxometry measurements of petroleum core plugs are commonly performed on low field magnets (<0.5 Tesla) to reduce the influence of magnetic susceptibility mismatch on measurements of the ...spin-spin relaxation time, T
. The Signal to Noise Ratio (SNR) of the MR signal, however, generally decreases with lower magnetic fields. Higher magnetic fields (>3 Tesla) are typically employed in small animal MRI studies to improve SNR and image resolution. For many rock core plug samples, susceptibility mismatch effects can be severe at these higher fields leading to decreased T
and T
*. In this work we seek an answer to the general question of what is the best field for MRI of rock core plugs, anticipating that it will be both sample and measurement method dependent. Free Induction Decay (FID) relaxation time measurements were undertaken to investigate the conditions under which the SNR in Centric Scan SPRITE (Single Point Ramped Imaging with T
Enhancement) MRI measurements is maximized. The image SNR benefits from greater signal at higher fields, but is negatively impacted by the correspondingly shorter signal lifetimes. Depending on the noise regime of the sample, the maximum SNR may be predicted for Centric Scan SPRITE MRI with T
* being B
field dependent. In this work we describe a series of simple experimental considerations to determine the optimal B
field for SPRITE MRI. Selection of the best field is aided by a new generation of superconducting magnets which allows the experimentalist to readily vary the field strength. Such magnets allow one to experimentally control sample magnetization for high sensitivity MRI measurements of core plug samples, while controlling the effect of susceptibility mismatch on the signal lifetimes.
Display omitted
•Variation of 1/T2* was linear with B0 field for a range of rock core plug samples.•The optimal field based on SNR in SPRITE MRI measurements was predicted.•Implementation of the best ...field is facilitated by a variable field magnet.•The optimal field may be determined in three simple experimental steps.
1H relaxometry measurements of petroleum core plugs are commonly performed on low field magnets (<0.5 Tesla) to reduce the influence of magnetic susceptibility mismatch on measurements of the spin-spin relaxation time, T2. The Signal to Noise Ratio (SNR) of the MR signal, however, generally decreases with lower magnetic fields. Higher magnetic fields (>3 Tesla) are typically employed in small animal MRI studies to improve SNR and image resolution. For many rock core plug samples, susceptibility mismatch effects can be severe at these higher fields leading to decreased T2 and T2*.
In this work we seek an answer to the general question of what is the best field for MRI of rock core plugs, anticipating that it will be both sample and measurement method dependent. Free Induction Decay (FID) relaxation time measurements were undertaken to investigate the conditions under which the SNR in Centric Scan SPRITE (Single Point Ramped Imaging with T1 Enhancement) MRI measurements is maximized. The image SNR benefits from greater signal at higher fields, but is negatively impacted by the correspondingly shorter signal lifetimes. Depending on the noise regime of the sample, the maximum SNR may be predicted for Centric Scan SPRITE MRI with T2* being B0 field dependent. In this work we describe a series of simple experimental considerations to determine the optimal B0 field for SPRITE MRI.
Selection of the best field is aided by a new generation of superconducting magnets which allows the experimentalist to readily vary the field strength. Such magnets allow one to experimentally control sample magnetization for high sensitivity MRI measurements of core plug samples, while controlling the effect of susceptibility mismatch on the signal lifetimes.
Soils are highly complex and heterogeneous porous materials, and thus measuring water distribution non-invasively with high accuracy and adequate spatial resolution still remains challenging. The ...first few centimeters of a soil surface control the vapor flux to the atmosphere justifying the need for high spatial resolution measurements of moisture content. The objective of this study was to compare and assess the feasibility of various high-resolution magnetic resonance (MR) methods to characterize an unsaturated porous system. We employed (1) a spin-echo, (2) three types of single-point imaging and (3) a unilateral three-magnet array to monitor
T
1
and
T
2,app
relaxation time spectra and the effective moisture saturation (Θ
MR
) of a silt loam under progressing desaturation with focus on an emerging unsaturated surface layer, which is predicted by theory. During the first stage of drying where evaporation occurred at the soil surface, all methods showed homogeneously distributed moisture. A decreasing Θ
MR
and a shift in the
T
1
and
T
2,app
relaxation time spectra to shorter values indicated the commencement of stage 2 evaporation coincided with an increasing unsaturated layer. At low water contents, the most suitable method to determine the extent of a desaturated surface zone with high accuracy was found to be single--point ramped imaging with
T
1
enhancement. As a simple and low-cost device the unilateral three-magnet array was feasible to monitor the drying process until the dry surface layer developed.
The movement of water or water carrying aggressive ions is one of the leading causes of deterioration of concrete structures worldwide. Moisture profiles during unsteady state wicking were determined ...by one dimensional magnetic resonance imaging of 10 cm mortar specimens. Bulk free induction decay and
T
2
*
lifetime mapping results show a bi-exponential behavior of the MR signal lifetime,
T
2
*
, in all samples, indicating at least two different water populations. From
T
2
*
mapping, the short
T
2
*
lifetime, assigned to interlayer water (water between C–S–H layers), and its associated amplitude are constant along the sample. The long
T
2
*
lifetime and its associated amplitude, related to water in the pore space (micro and macropores), change with local moisture content. To the best of our knowledge, this is the first time that interlayer water has been spatially resolved in MRI of cement-based materials. Gravimetric sorptivity measurements show two regimes of water absorption in the four samples studied. In the first regime, a capillary transport mechanism is dominant in filling the macropores. In the second regime, the interaction between water and solid matrix is postulated to cause swelling that results in a reduction of the water absorption rate. After swelling, diffusion controls pore filling. Water front penetration behavior observed with magnetic resonance imaging, and bulk free induction decay measurements showed changes in behavior that can be linked to sorptivity changes from an initial to a secondary regime. Inverse modelling was conducted to extract the transport properties, using the Hydrus program with the one-dimensional moisture content profiles. Modelling results showed a decrease in saturated hydraulic conductivity with water exposure time.
1D Centric Scan SPRITE magnetic resonance imaging measurements were undertaken to observe the water content distribution in 10 cm length mortar specimens with a steady state water content due to ...wicking and evaporation. Results show bi-exponential behavior of the magnetic resonance signal lifetime
T
2
∗
in all samples, indicating at least two different water populations. The short
T
2
∗
lifetime, related to interlayer water, and its associated amplitude are constant along the sample. The long
T
2
∗
lifetime, related to water in the pore space, and its associated amplitude, change with local moisture content. In the steady state the wet front was displaced deeper into the sample as the water to cement ratio increased. The transport parameters controlling the wicking and drying were extracted by fitting the experimental profiles to two literature models. In both cases the transport parameter values increase as the water to cement ratio increases.
PDF
