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- Volume 9, Issue 2, 2011
Near Surface Geophysics - Volume 9, Issue 2, 2011
Volume 9, Issue 2, 2011
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Assessment of the potential of a new generation of surface nuclear magnetic resonance instruments
More LessABSTRACTThe 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 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.
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Practical limitations and applications of short dead time surface NMR
Authors David O. Walsh, Elliot Grunewald, Peter Turner, Andrew Hinnell and Paul FerreABSTRACTThere is increasing interest in the unique measurement capabilities of nuclear magnetic resonance (NMR) for hydrologic applications. In particular, the ability to quantify water content (both bound and free) and to infer the permeability distribution are critical to hydrologists. As the method has gained in acceptance, there has been growing interest in extending its range to near‐surface and vadose zone applications and to measurement in finer grained and magnetic soils. All of these applications require improved resolution of early‐time signals, which requires shorter measurement dead times. This paper analyses three physical/electrical processes that limit the minimum achievable measurement dead times in surface NMR applications: 1) inherent characteristics of electromechanical and semiconductor switching devices, 2) the effective bandwidth of the receiver and signal processing chain, 3) transient signals associated with induced eddy currents in the ground. We then describe two applications of surface NMR that rely on reduced measurement dead time: detection and characterization of fast decaying NMR signals in silt and clay and the detection of fast decaying NMR signals in magnetic geology.
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Development of field techniques for improving MRS quality in shallow investigations
Authors Jesús Díaz‐Curiel, Bárbara Biosca, Lucía Arévalo and Juan L. PlataABSTRACTThe application of magnetic resonance sounding (MRS) to near‐surface studies, especially in the vadose zone, requires a significant improvement in the signal‐to‐noise ratio. Small dimension loops with several turns must be used in order to achieve a high vertical and lateral resolution. This increases the loop impedance, limiting the current intensity and increasing the criticalness in the requirement to produce excitation pulses with a good shape.
Due to the high current intensity required to create the excitation field, the tuning system used in MRS equipment is achieved selecting the capacitance values amongst several fixed configurations in discrete steps that depend on the loop characteristics and the emission frequency. When the equipment works with high inductances, this procedure implies a wide range in the tuning frequency that corresponds to a given value of capacitance. For this reason, it is necessary to change other system characteristics in order to achieve an optimal resonance fitting to the Larmor frequency.
Moreover, a way to improve the excitation field in the very shallow zone of the subsurface and to achieve the dipolar approximation of the electromagnetic field of the loop in this zone, would be to increase the distance from the plane of the antenna to the surface of the ground.
In this paper, several processes that have been developed as an aid to optimize the recording parameters for shallow MRS investigations are presented: 1) a methodology to improve the tuning through an optimized configuration of the transmitter antenna is proposed, 2) a quantitative index has been derived to evaluate the quality of the excitation pulse, as an aid to decide whether the design of the antenna should be modified and 3) a portable and low‐cost device to lift the antenna above the ground has been designed and constructed in order to increase the excitation of shallower water.
These developments have been carried out using a NumisLITE Iris Instrument. Field experiments have shown the validity of the proposed theoretical and field techniques.
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Reliability and limitations of surface NMR assessed by comparison to borehole NMR
Authors Mike Müller‐Petke, Thomas Hiller, Rolf Herrmann and Ugur YaramanciABSTRACTThe measurements of nuclear magnetic resonance (NMR) parameters to investigate petrophysical properties related to fluid (e.g., water) storage and transport processes provide unique insights compared to other geophysical methods and have become a very useful tool for geophysicists during the last decades at the laboratory scale and as a borehole tool.
We investigated, at a groundwater test site in the desert of Abu Dhabi, the reliability and limitation of surface NMR, a new but establishing technique that measures the NMR parameter from the surface by comparing its results to borehole NMR logs. Surface NMR or magnetic resonance sounding measurements (MRS) were conducted along a profile, close to several boreholes. The available borehole NMR logs were used to i) evaluate the potential of surface NMR derived results comparing them with borehole NMR measurements and to ii) extend the hydrogeological knowledge of the groundwater site.
Firstly, we show how to carefully handle short relaxation signals of surface NMR data. The most significant steps during this process are: i) broad‐band filtering to preserve the short decaying NMR signals, ii) correction for relaxation during pulse effects and iii) QT‐inversion to extract reliable subsurface parameter distribution.
By comparing surface NMR results with borehole NMR logs we found the following limitations: i) surface NMR is not able to detect borehole NMR measured water content related to decay times lower than decay time. This reduced detectable water content of surface NMR is due to an instrumental dead time of 40 ms, measured relaxation times and a lower Larmor frequency of 2 kHz and ii) borehole NMR has significantly higher vertical resolution. Taking this into account, surface NMR is in good agreement with borehole NMR.
Secondly, on a profile of 1.3 km length 11 MRS measurements were carried out to map the lateral aquifer structure. The obtained results show that surface NMR provides unique lateral information of demanded aquifer properties complementary to e.g., transient electromagnetic.
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Characterizing aquifers when using magnetic resonance sounding in a heterogeneous geomagnetic field
Authors J.M. Vouillamoz, A. Legchenko and L. NandagiriABSTRACTIt has previously been reported that the heterogeneity of the geomagnetic field disturbs the currently‐measured free induction decay signal of magnetic resonance sounding (MRS). To overcome the limitation of MRS in a non‐homogeneous geomagnetic field, we adapted the spin‐echo methodology usually used at the laboratory scale and in boreholes. We present examples of measurements carried out in a sandy aquifer in southern India. The 15–25 m thick sand deposit overlays a gneissic basement. Two sources of geomagnetic field heterogeneity have been identified at this site, both affecting the geomagnetic field within the sandy aquifer: the gneissic bedrock and an intruded dyke into the bedrock. Spin‐echo and free induction decay signals have been recorded at six locations. We found that the groundwater content, the thickness of the saturated aquifer and its transmissivity calculated with free induction decay measurements are underestimated compared to those derived from spin‐echo measurements. The closer to the dyke the higher the underestimation. Time‐domain electromagnetic measurements indicate that the aquifer is rather homogeneous at the site scale, as suggested by spin‐echo results. We also found that a small heterogeneity of the geomagnetic field can go unnoticed, thus leading to an unknown mis‐estimate of aquifer properties when using free induction decay measurements. Thus spin‐echo measurements can be used to improve the accuracy of aquifer characterization when using MRS in geological contexts where geomagnetic field heterogeneity exists.
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The detectability of water by NMR considering the instrumental dead time – a laboratory analysis of unconsolidated materials
Authors Marie Boucher, Stephan Costabel and Ugur YaramanciABSTRACTThe geophysical technique of magnetic resonance sounding (MRS) is directly sensitive to groundwater but a part of groundwater is not detected by MRS because of an instrumental dead time, during which the signal is not recorded. In order to estimate the influence of physical and hydrous properties of the porous media on the amount of water that cannot be detected because of this instrumental dead time, laboratory nuclear magnetic resonance (NMR) measurements on synthetic samples have been performed. The laboratory measurements allow recording the signal without dead time and offer the possibility of easily controlling physical properties and hydrous conditions of the media. The effect of a dead time similar to that of the common MRS instrumentation (NUMIS equipment) has been modelled on the signals and the amount of undetectable water has been estimated for different grain size distributions, different clay contents and different saturation degrees.
Results show that the amount of the dead‐time‐related undetectable water is controlled by the distribution of the decay time Both the average and the uniformity of influence the percentage of undetectable water. Variations of median grain size, sorting of grains, clay content and/or saturation degree cause variations in decay time distributions. Less water is missed when median is long and/or when the distribution of is narrow. Generally the fraction of undetectable water decreases when enlarging or better sorting the grains, when decreasing the clay content, as well as when increasing the saturation degree. However, a median grain size lower than 0.2 mm or clay contents higher than 20% may also lead to decreasing fractions of undetectable water in comparison to respectively higher median grain size and lower clay contents because of the narrowing of the decay time distribution.
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Relative hydraulic conductivity and effective saturation from Earth’s field nuclear magnetic resonance – a method for assessing the vadose zone
Authors Stephan Costabel and Ugur YaramanciABSTRACTBeside 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 , 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 ,. The EFNMR relaxation time 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 and . 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 as a function of the water saturation and thus provides the estimation of from . We compare the NMR based predictions with the 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 in situ by MRS measurements in the vadose zone. We discuss the potential and limitations of this approach for MRS.
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The effect of pore size and magnetic susceptibility on the surface NMR relaxation parameter
Authors Elliot Grunewald and Rosemary KnightABSTRACTSurface nuclear magnetic resonance (NMR) is a non‐invasive geophysical method that can provide valuable information about aquifer properties related to groundwater flow and storage. Our ability to extract such information from surface NMR data, however, is limited by an insufficient understanding of the relaxation parameter governing the decay rate of the surface NMR signal in Earth’s magnetic field. In this study, we use a combination of numerical and laboratory experiments to systematically explore the effect of two key geologic properties, pore size and magnetic susceptibility, on the relaxation process. A one‐dimensional numerical model is developed and parametrized to simulate the surface NMR response for a wide range of geologic materials. These simulations illuminate the processes controlling relaxation and identify conditions under which exhibits varied sensitivity to pore size. For materials with low magnetic susceptibility, is highly sensitive to pore size; however, as susceptibility increases, this sensitivity diminishes and becomes dominated by complex dephasing effects, particularly when pores are large. Laboratory Earth’s field NMR experiments complement the numerical simulations. Measurements on water‐saturated quartz sands show that for weakly magnetic materials, can be sensitive to pore size and thus could provide useful information about aquifer properties.
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Experimental study of domestic waste material using magnetic resonance measurements
ABSTRACTIn this paper, we present results of a laboratory and in situ study of a domestic waste landfill using magnetic resonance measurements. For our study, we used a laboratory Earth’s field nuclear magnetic resonance (NMR) instrument developed at LTHE and a large‐scale commercial magnetic resonance sounding (MRS) system NUMISLITE from IRIS Instruments. We show that NMR could be a tool for investigating different processes in water‐saturated waste samples.
Our results show that domestic waste material contains ferromagnetic or paramagnetic particles that perturb the homogeneity of the geomagnetic field at a microscopic scale and render an NMR signal short. Consequently, only the spin echo technique can be applied for measuring. At a macroscopic scale, waste and different buried objects may also perturb the natural geomagnetic field. While investigating the landfill, we observed that magnetic anomalies (±2500 nT) are localized around some cells. This is probably linked to the presence of a higher percentage of metallic objects within the waste disposal.
Our first appraisal of the possibility of investigating water‐saturated waste in a laboratory using an Earth’s field NMR instrument shows that, with existing instruments, waste samples can be studied when the dry density of waste is less than approximately 450 kg/m3.
Because the relaxation times of magnetic resonance signals in landfill may be short ( and ), existing large‐scale MRS instrumentation is not adapted to the investigation of domestic waste landfills.
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The contribution of MRS and resistivity methods to the interpretation of actual evapo‐transpiration measurements: a case study in metamorphic context in north Bénin
Authors M. Descloitres, L. Séguis, A. Legchenko, M. Wubda, A. Guyot and J.M. CohardABSTRACTA quantitative budget estimate of actual evapo‐transpiration is a key issue for enhanced hydrological modelling in northern Bénin. Actual evapo‐transpiration is estimated using large aperture scintillometer equipment, devoted to sensible heat flux measurements. However, a previous study reported that the actual evapo‐transpiration cycle is not fully understood. Indeed, the actual evapo‐transpiration depends strongly on several factors such as climate, vegetation pattern, soil water storage and human activities. The respective contributions of the aquifer and vadose zone to the actual evapo‐transpiration budget are not known. When using piezometric variations of the water table, the aquifer contribution is not easy to quantify since the specific yield may vary in the investigated area, located in a metamorphic rock environment. In the present study, we investigate whether significant differences in the aquifer’s specific yield could exist within the large aperture scintillometer measurement area, leading to different actual evapo‐transpiration water losses. We use joint frequency electromagnetic resistivity mapping, geological surveys and magnetic resonance sounding (MRS) to delineate the effective porosity of the regolith around the scintillometre measurement area. Thirteen MRS soundings implemented in key areas reveal a clear classification of the main geological units on the basis of their water content. The MRS water content varies between 1.5–3% for amphibolite and micaschists formations to more than 12% for quartzitic fractured formations, whereas the MRS relaxation time is less discriminating (150–250 ms), indicating a small variation in pore size. Then, as a first modelling exercise, we assumed that the MRS water content (the effective porosity) maximizes the specific yield. The actual evapo‐transpiration budget given by a previous study (Guyot et al.2009) is then re‐interpreted using geophysical data: we found that a) the measured water table depletion can explain the actual evapo‐transpiration value providing enough water for the transpiration process and b) the significant discrepancies in actual evapo‐transpiration signals observed between the eastern and western parts of the watershed can be explained by the respective effective porosity of the geological units. Even if further research is needed to link MRS water content to the specific yield and to evaluate a possible role of the deep vadose zone, the hydrogeophysical mapping presented in this study highlights the role of the MRS method for providing relevant information to understand hydrological processes in this complicated geological context of north Bénin.
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Investigating water distribution in the Luitel Lake peat bog using MRS, ERT and GPR
ABSTRACTIn this paper we present the results of a geophysical study of water distribution in the peat bog at Luitel Lake. The goal of the study was to determine water distribution within the bog, which is part of a protected nature preserve. The small peat bog (17 ha) provides a good test site for developing and testing surface geophysical methods. For this study we used magnetic resonance sounding (MRS), electrical resistivity tomography (ERT) and ground‐penetrating radar (GPR). Because the water distribution in the bog is a 2D target for MRS, we had to develop a measuring procedure and 2D inversion routine for MRS. The fieldwork consisted in establishing seventeen MRS stations and conducting three ERT profiles and one GPR line. The MRS, ERT and GPR results on the reservoir geometry correlated well with each other. Pine and birch trees cover most of the bog surface but they have not yet populated the centre of the bog, the location where the maximum water content was observed. This result agrees well with vegetation distribution in the study area: at the centre of the bog, vegetation is typical of a swampy environment but outside the centre the vegetation is typical of a forest. According to MRS, the water content of the peat formation at the centre of the bog is 60–70%, whereas GPR estimated the water content to be between 64–70%. Outside the centre, MRS showed the water content of the peat to be about 30%.
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Comparison of transmissivities from MRS and pumping tests in Denmark
Authors M. Ryom Nielsen, T.F. Hagensen, K. Chalikakis and A. LegchenkoABSTRACTIn Denmark, hydrogeophysical mapping is performed intensively in accordance with the Danish Government’s environmental plans of ensuring a high quality drinking water supply. Previously the magnetic resonance sounding (MRS) method has been proven to improve the hydrogeological characterization from geophysical surface measurements in Denmark. In order to implement hydraulic parameters estimated from MRS together with hydraulic parameters obtained from pumping tests, it is necessary to analyse and compare the hydraulic parameters from the two data sources. These parameters are determined very differently with uncertainties arising from very different sources and the comparison requires an analysis in each specific case with attention on the investigated volumes.
Several comparisons of transmissivities from MRS and pumping tests in Denmark show that the pumping test transmissivity value corresponds to the MRS transmissivity accumulated over depth intervals determined by the screen position depth interval in the borehole. When specific determinations of these depth intervals are performed, very good correlation is obtained between transmissivities from MRS and pumping tests.
Comparisons of transmissivities from MRS and pumping tests will also lead to the determination of MRS calibration coefficients. MRS calibration is essential for quantitative use of MRS transmissivities. Sixteen calibration coefficients in different survey areas have been calculated. If these calibration coefficients can be categorized according to the different Danish survey locations and hydrogeological conditions in which they are obtained, these categorized calibration coefficients could be used in future MRS surveys without the need of calibrating with pumping tests in each survey. However, only a slight tendency of increasing calibration coefficient with increasing transmissivity and increasing grain size is observed. The present study shows that in general only little dispersion is observed around an average calibration coefficient for the different Danish survey locations and hydrogeological conditions.
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The potential of the qualitative interpretation of MRS data: hydrogeophysical study of the Lagoons of Estaña (Spain)
Authors Juan L. Plata and Félix M. RubioABSTRACTThe Lagoons of Estaña, at the Spanish pre‐Pyrenees, constitute a set of natural water ponds the origin of which is not well‐known, though a karstic formation is one of the hypotheses. A project is being developed with the objective of understanding the hydrological functioning of the lagoons related to underground waters. The lagoons are in Keuper and Muschelkalk rocks in contact with the main water‐bearing structure of the zone, formed by a syncline of Cretaceous limestone. All the area is affected by a strong tectonics. In order to know the hydrogeological limits of this contact and to map the subsurface Triassic structures several resistivity profiles and eighteen magnetic resonance soundings (MRS) are made connecting the syncline with the lagoons and around them. Because of space limitations in the field the geographical distribution of the measurements is not the most appropriate one to detect possible karstic structures. A special strategy for the interpretation, mainly qualitative, is designed based on the pattern of the and curves, including a detailed verification of the geological situation of the MRS measurements to prevent errors that could occur in case of 1D modelling is not appropriate. Part of the area is influenced by a high geomagnetic gradient due to the presence of basic rocks but good MRS measurements are obtained and it is demonstrated that this magnetic anomaly does not affect the lagoon area. The joint use of MRS and resistivity sections allows a correct interpretation and to establish the geometry of a hydrogeological model.
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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)