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- Volume 12, Issue 2, 2014
Near Surface Geophysics - Volume 12, Issue 2, 2014
Volume 12, Issue 2, 2014
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Despiking of magnetic resonance signals in time and wavelet domains
Authors Stephan Costabel and Mike Müller‐PetkeABSTRACTIn 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, wavelet‐based strategies have some interesting properties and thus have some potential for further development regarding surface‐NMR processing, which is discussed in detail.
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Comparison and optimal parameter settings of reference‐based harmonic noise cancellation in time and frequency domains for surface‐NMR
Authors Mike Müller‐Petke and Stephan CostabelABSTRACTThe 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.
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Estimating storage properties of aquifer with magnetic resonance sounding: a field verification in northern Cambodia of the gravitational water apparent cutoff time concept
Authors J.M. Vouillamoz, P. Sophoeun, O. Bruyere and L. ArnoutABSTRACTMagnetic Resonance Sounding (MRS) has already showed its capability for estimating some aquifer properties, but very little work has been carried out concerning the storage properties used for calculating groundwater reserve and recharge. For unconfined aquifer, this storage property (i.e., the specific yield) is often estimated from the MRS water content. However, a recent study carried out in a poor sandstone aquifer of Northern Cambodia found that the MRS pore‐size related parameter is probably more appropriate because the MRS water content can be dominated by water which does not contribute to the specific yield. A main output of this study is that a threshold value (so‐called apparent cutoff time ACT) of the MRS decay time can be defined for discriminating gravitational water from capillary and bound waters. Since measuring such short MRS signal is quite common, a validation of this result is compulsory to avoid the misuse of the MRS water content.
This paper presents an experiment that we set up to check the validity of the ACT approach. We compared two MRS carried out at the same location in a clayey‐sand layer 15 metres thick. The first MRS was carried out with a static water level located at 1.7 metres below the ground level. It revealed a layer with a water content of 3.8% and . According to the ACT approach, this short value of indicates water that is not part of the specific yield and that can not be released by pumping. Thus, we carried out a second MRS after pumping and lowering the water level to an average of 15 metres. As predicted by the ACT approach, we observed that the second MRS signal was the same as the one obtained before pumping.
To cross‐check our observation, we carried out a pumping test at the same location. We found a very low value of specific yield, confirming that the main part of the MRS signal is generated by water which was not drained by the pumping. We calculated that the volume of water removed from the aquifer by the pumping was far too low to generate a measurable magnetic resonance signal.
From our experiment, we conclude that the ACT approach can be successfully used to estimate the specific yield of poor aquifers, and to avoid a misuse of the MRS water content which can lead to strong overestimates of aquifer reserve and recharge.
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A numerical study of the relationship between NMR relaxation and permeability in sands and gravels
Authors Katherine Dlubac, Rosemary Knight and Kristina KeatingABSTRACTThe accuracy of NMR‐derived permeability estimates in sands and gravels are examined through simulations on numerical grain packs composed of uniform spherical grains. The packs consisted of randomly packed grains, with grain sizes set to represent a range corresponding to sands and gravels. The material properties for each pack were quantified through numerical analysis and the NMR response was simulated for a range of surface relaxivity values. The agreement between the numerically‐derived permeability estimates and the permeability estimates derived using the Schlumberger‐Doll Research (SDR) and Seevers equations was evaluated. Use of the SDR equation assumes that the relaxation of the bulk pore fluid can be neglected. The NMR‐derived permeability estimates were calculated using each equation for the cases where relaxation was assumed to occur in one of the two major diffusion regimes. We found that permeability is most accurately estimated in all packs through use of the Seevers equation with the empirical constant set equal to 1. We showed that the contribution of bulk fluid relaxation should be accounted for in materials with grain radii greater than 1.2e‐4m (fine sand) and surface relaxivity values less than 1.0e‐3 m s−1. In practice, this range of surface relaxivity values and grain sizes corresponds to situations where the measured relaxation time is greater than approximately one‐third the value of the bulk fluid relaxation time .
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Two‐dimensional distribution of relaxation time and water content from surface nuclear magnetic resonance
Authors Raphael Dlugosch, Thomas Günther, Mike Müller‐Petke and Ugur YaramanciABSTRACTDevelopment in instrumentation and data analysis of surface nuclear magnetic resonance has recently moved on from one‐dimensional (1D) soundings to two‐dimensional (2D) surveys, opening the method to a larger field of hydrological applications. Current analysis of 2D data sets, however, does not incorporate relaxation times and is therefore restricted to the water content distribution in the subsurface.
We present a robust 2D inversion scheme, based on the qt approach, which jointly inverts for water content and relaxation time by taking the complete data set into account. The spatial distribution of relaxation time yields structural information of the subsurface and allows for additional petrophysical characterization. The presented scheme handles separated loop configurations for increased lateral resolution. Assuming a mono‐exponential relaxation in each model cell, using irregular meshes, and gate‐integrating the signal, the size of the inverse problem is significantly reduced and can be handled on a standard personal computer.
A synthetic study shows that contrasts in both the quantities – water content and relaxation time – can be imaged. Inversion of a field data set outlines a buried glacial valley and allows the distinguishing of two aquifers with different grain sizes, which can be concisely interpreted together with a resistivity profile. The impact of the anisotropic weighting factor and subsurface resistivity on the inversion result are shown and discussed. A comparison of the results obtained by the previously used initial value and time‐step inversion approaches illustrates the improved stability and resolution capabilities of the 2D qt inversion scheme.
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A laboratory study to determine the effect of surface area and bead diameter on NMR relaxation rates of glass bead packs
More LessABSTRACTA laboratory study was conducted to explore the relationship between pore size, pore surface‐area‐to‐volume ratio and NMR relaxation rates and to determine which geometric parameter best predicts the average NMR relaxation rate. NMR relaxation measurements were collected on water‐saturated glass beads with controlled sets of bead diameters and surface areas. Four sets of beads were used with average diameters ranging from 55‐1125 m. The surface areas of the glass beads were altered by chemically treating the beads with a weak acid, a strong base and a cream commonly used to etch glass surfaces. Following the chemical treatments, the surface areas of the beads were quantified with krypton BET gas adsorption measurements. It was found that, for the range of bead diameters used in this study, relaxation did not strictly occur in the fast diffusion regime and, as such, the relaxation time associated with the peak of the largest mode in the distribution was found to more accurately represent the pore‐scale geometry than the mean log relaxation time. Using the relaxation time associated with this peak, the results from this study show that the pore surface‐area‐to‐volume ratio is a significantly better predictor of the surface relaxation rate than the mean grain radius ( = 0.014).
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Integrating MRS data with hydrologic model ‐ Carrizal Catchment (Spain)
More LessABSTRACTMagnetic resonance sounding (MRS) provides quantitative hydrogeological information on hydrostratigraphy and hydraulic parameters of subsurface (e.g., flow and storage property of aquifers) that can be integrated in distributed hydrologic models. The hydraulic parameters are typically obtained by pumping tests. In this study, we propose an MRS integration method based on optimizing MRS estimates of aquifer hydraulic parameters through hydrologic model calibration.
The proposed MRS integration method was applied in the 73 km2 Carrizal Catchment in Spain, characterized by a shallow unconfined aquifer with an unknown aquifer bottom. 12 MRS survey results were inverted with Samovar 11.3, schematized and integrated in the transient, distributed, coupled, hydrologic, MARMITES‐MODFLOW model. As the aquifer bottom was unknown, the aquifer was schematized into one unconfined layer of uniform thickness. For that layer, MRS estimators of specific yield and transmissivity/hydraulic conductivity were calculated as weighted averages of the inverted MRS layers. The MRS integration with hydrologic model was carried out by introducing multipliers of specific yield and transmissivity/hydraulic conductivity that were optimized during transient model calibration using 11 time‐series piezometric observation points. The optimized multipliers were 1.0 for specific yield and 3.5*10‐9 for hydraulic conductivity. These multipliers were used, and can be used in future MRS investigations in the Carrizal Catchment (and/or adjacent area with similar hydrogeological conditions), to convert MRS survey results into aquifer hydraulic parameters.
The proposed method of MRS data integration in the hydrologic model of Carrizal Catchment not only allowed us to calibrate the model but also to confirm the functional capability of MRS in quantitative groundwater assessment. Most importantly however, it demonstrated that if pumping tests are not available, the use of MRS integrated in distributed coupled hydrological models, or even in standalone groundwater models, provides a valuable aquifer parameterization alternative.
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Surface NMR instrumentation and methods for detecting and characterizing water in the vadose zone
Authors David O. Walsh, Elliot D. Grunewald, Peter Turner, Andrew Hinnell and Ty P.A. FerreABSTRACTA commercially available surface NMR instrument was modified to address the challenges of using earth’s field surface NMR to detect and characterize water in the unsaturated (or vadose) zone. The modified instrument incorporates faster switching electronics to achieve an instrument dead time of 2.8 ms, and higher output power electronics to enable a maximum coil voltage of 8000 volts and coil current of 800 amps. The instrument was used to collect and interpret surface NMR data at several active vadose zone investigation sites in the western US. A 6‐week surface NMR experiment was conducted at a managed aquifer storage and recovery facility in Arizona, to explore the measurement capabilities and limitations of the instrument, during a managed infiltration event. The resulting time lapse surface NMR data were used to map zones of held water prior to the flood event, image the influx of water through the top 15 metres of the subsurface during and after the event, quantify the spatial and temporal distribution of infiltrating water throughout the event, and characterize the distribution of water in different relative pore sizes throughout the event. Data obtained at pseudo‐static vadose zone investigation sites indicate that the surface NMR instrument can detect and image some forms of water held in unconsolidated vadose zone formations, at depths up to 30 metres. Complementary NMR logging data indicate that the surface NMR instrument does not detect all of the water held in these pseudo‐static formations, but that the non‐invasive surface NMR data may yield valuable information nonetheless.
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Using Nuclear Magnetic Resonance and Transient Electromagnetics to characterise water distribution beneath an ice covered volcanic crater: the case of Sherman Crater Mt. Baker, Washington
More LessABSTRACTSurface and laboratory Nuclear Magnetic Resonance (NMR) measurements combined with transient electromagnetic (TEM) data are powerful tools for subsurface water detection. Surface NMR (sNMR) and TEM soundings, laboratory NMR, complex resistivity, and X‐Ray Diffraction (XRD) analysis were all conducted to characterise the distribution of water within Sherman Crater on Mt. Baker, WA. Clay rich rocks, particularly if water saturated, can weaken volcanoes, thereby increasing the potential for catastrophic sector collapses that can lead to far‐travelled, destructive debris flows. Detecting the presence and volume of shallow groundwater is critical for evaluating these landslide hazards. The TEM data identified a low resistivity layer (<10 ohm‐m), under 60 m of glacial ice related to water saturated clays. The TEM struggles to resolve the presence or absence of a plausible thin layer of bulk liquid water on top of the clay. The sNMR measurements did not produce any observable signal, indicating the lack of substantial accumulated bulk water below the ice. Laboratory analysis on a sample from the crater wall that likely represented the clays beneath the ice confirmed that the controlling factor for the lack of sNMR signal was the fine‐grained nature of the media. The laboratory measurements further indicated that small pores in clays detected by the XRD contain as much as 50% water, establishing an upper bound on the water content in the clay layer. Forward modelling of geologic scenarios revealed that bulk water layers as thin as ½ m between the ice and clay layer would have been detectable using sNMR. The instrumentation conditions which would allow for sNMR detection of the clay layer are investigated. Using current instrumentation the combined analysis of the TEM and sNMR data allow for valuable characterisation of the groundwater system in the crater. The sNMR is able to reduce the uncertainty of the TEM in regards to the presence of a bulk water layer, a valuable piece of information in hazard assessment.
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Glimpse into the design of MRS instrument
Authors A. Legchenko and G. PierratABSTRACTThe Magnetic Resonance Sounding method (MRS) was developed in the former USSR in the late 1970s. Nowadays, available MRS instruments are more compact and reliable, and enormous progress has been made in electronics, computers and materials. Therefore, we can hope that it may be possible to increase the maximum depth of water detection and to improve the resolution of the method by using a larger current in the loop. Quite naturally, the questions arise: what are the practical limits of the MRS method and how much should be transmitting power to get the maximum depth of investigation? In this paper, we analyse the depth of groundwater detection and the vertical resolution of the MRS assuming different loops and different power levels of the current generator. The originality of our approach consists of a joint analysis of the maximum depth of investigation using accept able loop voltage and the modifications in the instrument design necessary for the improvements. We show that even under very favourable conditions it would be difficult to get significant improvement in the depth of investigation using currently available instruments. For example, under favourable noise conditions when rocks have low electrical conductivity and are non‐magnetic, a 20 m thick water saturated sand layer can be detected at a depth of about 325 m using an existing instrument (4 kV of the loop voltage) with a 400 × 400 m2 square loop. A 20% increase in the detection depth (390 m instead of 325 m) requires more powerful electronic equipment (16 kV instead of 4 kV) thus rendering the MRS system larger and heavier. However, using a 16 kV instrument allows us to increase the resolution depth by about 80% (from 120 m to 215 m). When rocks are electrically conductive, the screening of the MRS signal limits the depth of investigation and allows for only minor improvements even with a much more powerful current generator.
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Exploiting the MRS‐phase information to enhance detection of masked deep aquifers: examples from the Netherlands
Authors Jean Roy and Maciek W. LubczynskiABSTRACTSeveral magnetic resonance soundings (MRS) in the Netherlands showed a monotonous single peak anomaly on the amplitude versus excitation moment sounding pattern, which were interpreted as a single aquifer when using an amplitude‐only mode MRS data inversion. However, in all these soundings, borehole logs documented the presence of two or three aquifers separated by clay‐rich aquitards in the first 100 m below ground surface. Such environments were electrically conductive so a phase excursion was noticeable on the MRS soundings. Multi‐aquifer systems, in a conductive environment, may show interference among signals originating from different parts of the systems including amplitude masking or destructive interference. A new version of an off‐the‐shelf MRS forward modelling and inversion tool (Samovar 11.3) allowing complex amplitude and phase inversion was used to detect and parameterize deep, MRS‐masked second aquifers at two selected sites in the Netherlands, one near Delft and one near Waalwijk. At the Delft site, the proposed strategy was effective in the detection and characterization of a second previously missed aquifer at 45 m below ground surface, while at the Waalwijk site, the second aquifer was not detected because of a considerably deeper aquifer at 85 m and too small excitation (6000 A ms). However, forward modelling showed that with a larger excitation moment (e.g., 13 000 A ms), detection and parameterization of the second aquifer would become possible when using both amplitude and phase.
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A numerical assessment of the use of surface nuclear magnetic resonance to monitor internal erosion and piping in earthen embankments
Authors Trevor Irons, Meghan C. Quinn, Yaoguo Li and Jason R. McKennaABSTRACTThe ability to non‐invasively detect and monitor internal erosion and piping in earthen embankments is of great value to government agencies and municipalities responsible for maintaining large numbers of civil structures. Internal erosion can be a difficult failure mode of dams and levees to assess using traditional techniques. Surface nuclear magnetic resonance is a promising technology for monitoring these structures due to its direct sensitivity to liquid water. This is a new application for the method, and requires the formulation of a robust modelling framework capable of incorporating three dimensional water content models and topography. Adaptive octree meshes represent an efficient means to perform these calculations, as they are able to adapt and refine in areas where the kernel function is changing rapidly. Given the non‐trivial challenges associated with the development of realistic water content models, we focus on first‐order effects and present six 2.5D flow models representing seepage through a loaded earthen embankment under varying levels of erosional piping. Simulation results show that the employment of multiple surface nuclear magnetic resonance transmitter and receiver loops can provide early detection and effective monitoring of piping under the assumption of a 2.5D water content model. Therefore, the surface nuclear magnetic resonance technique has promise as a means for detection and/or monitoring of internal erosion and piping in earthen embankments.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)