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- Volume 16, Issue 2, 2018
Near Surface Geophysics - Volume 16, Issue 2, 2018
Volume 16, Issue 2, 2018
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The competing effects of stress and water saturation on in situ Q for shallow (< 1 m), unconsolidated sand, evaluated with a modified spectral ratio method
Authors James M. Crane, Juan M. Lorenzo, Jie Shen and Chris D. WhiteABSTRACTA publicly available seismic dataset from a lab experiment shows the dependence of quality factor simultaneously on water saturation and stress in unconsolidated sand. Large gradients (e.g., > 10 m−1) necessitate a spectral ratio method modified to assume that Q changes with each ray path, thereby eliminating false values (e.g., < 0). Interval values increase the most with depth and stress in dry sand and the least in partially saturated sand ( and ) where attenuation created by local fluid flow reaches a maximum. Expected values can be extrapolated from and are bounded by of the dry and partially saturated media (e.g., ). deviations outside this range may be explained by changes in effective stress, attenuation mechanism, or sediment composition. Field estimation of seismic attenuation in natural settings may be helped by these constraints, although attenuation remains subject to careful consideration of other factors, e.g., grain size, sorting, and shape.
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Measurement of dielectric and magnetic properties of materials by means of a TDR probe: a preliminary theoretical investigation in the frequency domain
Authors Raffaele Persico and Massimiliano PieracciniABSTRACTThis paper is focused on the measurement of both dielectric and magnetic characteristics of a penetrable material at radio frequency through a time‐domain reflectometry probe. The goal is to prove that a time‐domain reflectometry probe can offer the possibility to discriminate the dielectric permittivity from the magnetic permeability of the material. This is due to the fact that the time‐domain reflectometry datum depends both on the propagation velocity of the electromagnetic waves in the probed medium and on the intrinsic impedance of the probe. However, the possibility to attain such a clear discrimination is bound by the condition that the reflection coefficient is measured (or calculated) along the probe at the air–soil interface in the frequency domain. Generally, time‐domain reflectometry probes measure the total (incident plus reflected) field in the time domain, and subsequently, the datum is needed to meet the condition that such claimed purpose is not just the Fourier transform of a datum collected by means of a common time‐domain reflectometry probe. Rather, either a devoted hardware should be implemented or a very accurate knowledge of the incident field should be guaranteed in order to separate the reflected wave from the incident one. In this preliminary work, our study has been restricted to a theoretical investigation in the frequency domain. In particular, our focus is set on the lossless case, and the attention is devoted to the issue of possible multiple solutions to demonstrate that this obstacle can be overcome by making the frequency step narrower or, alternatively, by narrowing the length step of the probe. Simulation results based on a bifilar transmission line model are shown.
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Substrate‐sensitive relationships of dielectric permittivity and water content: implications for moisture sounding
Authors Frank Owenier, Jens Hornung and Matthias HindererABSTRACTDielectric permittivity is a fundamental parameter for all investigations based on electromagnetic waves, particularly for moisture sounding. However, the physical principles of dielectric permittivity in natural materials and its influence on propagation velocity, the reflection coefficient, and the decay of the electromagnetic waves have been rarely addressed for variable substrate compositions or frequencies. Hence, this paper aims to fill this gap by systematically investigating quantitative relationships between dielectric permittivity and soil water content for a continuum of the three most common soil components in moderately temperate humid latitudes (i.e., illite, carbonate, and quartz‐rich sand). We carried out frequency‐dependent permittivity measurements for a large variety of artificial and natural substrates with an impedance analyser at frequencies ranging from 10 MHz to 1 GHz. Based on these measurements, we selected 200 MHz as the most commonly used frequency value for geological, ground‐penetrating radar applications and developed a set of empirical equations from third‐degree and ternary factorial diagrams to quantify the relationship between dielectric permittivity and volumetric moisture content of pure sand, clay, carbonate, and mixtures. The results show systematic trends, which can be explained by competing electrophysical properties of minerals and their wetting behaviour. This investigation shows that different wetting affinities of 1:1 and 2:1 clay minerals strongly control permittivity. Thresholds and non‐linearity effects were identified, where specific mineral properties and/or wetting processes became dominant in complex mixtures of soil materials. Bulk densities and porosities, respectively, were shown to have only minor influence compared with the impact of water. Consequently, the presented method is applicable to both undisturbed and disordered samples and, hence, to mobile devices or permanent installations in the field after compositional laboratory or onsite analyses. The permittivity of natural soils was measured in two field studies. The results show a good correlation between calculated and measured moisture contents. Our study confirms that the widely used Topp equation is only valid for sandy soil materials, i.e., the soil types it was originally designed for. For soils with a clay content >30% and/or a moisture level >20%, the new equations reduce the error in the level of permittivity measurements from up to 20% (using the Topp equation) to 5%.
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Integration of upward GPR and water content reflectometry to monitor snow properties
ABSTRACTWe adopt upward ground‐penetrating radar (up‐GPR) and water content reflectometry sensors to monitor the seasonal behaviour of snow density. Upward ground‐penetrating radar permitted observation at a single fixed station the time‐lapse response of the electromagnetic signal at the main frequency of 1500 MHz, with the antenna radiating upward from the soil toward the snow surface. Measurements have been performed at a test site on the Italian Alps (at an elevation of about 2100 m above sea level) during the 2014–2015 winter season at an interval of 30 minutes. The data processing of radar data involved the travel‐time picking and the conversion into snow depth and density. Water content reflectometry measurements have been useful in order to calibrate the radar response and retrieve information on the presence of liquid water content. The integration of upward ground‐penetrating radar and water content reflectometry technology allows us to infer snow high and layering, snow density changes during the winter season, and a preliminary estimate of the liquid water content. For snow in dry condition, we are able to estimate density values through mixing rules or polynomial formula. Snow density varies during the season in a range between 250 kg/m3 and 450 kg/m3; the results are in good agreement with the results of the ground truth. For snow in wet condition, the residuals of the electrical permittivity, after a trend removal on the original water content reflectometry data, permitted to estimate liquid water content in the range between 3% and 5%, during some periods of the winter season, according to warmer climate condition.
Snow layering and densification processes are monitored by the response of upward ground‐penetrating radar: fast phenomena such as wetting front infiltration can be also pointed out even if they appear challenging if other observations are not available (e.g., monitoring with water content reflectometry).
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Terrestrial and marine electrical resistivity to identify groundwater pathways in coastal karst aquifers
Authors Yvonne O’Connell, Eve Daly, Tiernan Henry and Colin BrownABSTRACTGroundwater movement in karst aquifers is characterised by high‐velocity fissure and conduit flow paths, and in coastal karst aquifers, these act as pathways for saline intrusion and freshwater discharge to the sea. This paper examines groundwater movement in two neighbouring catchments in the west of Ireland that represent canonical coastal karst aquifers dominated by discharges in the intertidal zone and at offshore submarine springs. Terrestrial and surface‐towed marine electrical resistivity tomography, coupled with ancillary hydrogeological data, identifies the influence of faulting and conduits on groundwater egress/saltwater ingress. The on‐shore and off‐shore subsurface geometry of major fault zones is identified, and the tidal influence of seawater and ground‐water flow is demonstrated in these zones and karst springs. Imaging of these sub‐surface structures is a pre‐requisite for numerical modelling of current and future climate‐driven freshwater–seawater interactions in karst coastal aquifers.
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Geophysical and geochemical characterisation of a site impacted by hydrocarbon contamination undergoing biodegradation
Authors Mohamad Abbas, Abderrahim Jardani, Nadine Machour and Jean‐Paul DupontABSTRACTHydrocarbon contamination, which can alter the physiochemical and biological properties of the subsurface, can be monitored by using geochemical analyses and integrated geophysical techniques. Electrical resistivity tomography, induced polarization, ground‐penetrating radar, and self‐potential methods were used in this work to characterise the hydrocarbon contamination and investigate the geoelectrical properties of a site impacted by an aged hydrocarbon plume. Throughout the investigation, geoelectrical measurements supported the conductive plume model and consistently recorded low‐resistivity anomalies and high‐chargeability values in the contaminated aquifer. Furthermore, the low‐resistivity anomalies were found to be coincident with regions of ground‐penetrating radar attenuated reflections and significant negative self‐potential anomalies associated with oxidation–reduction processes. These findings were supported by the geochemical measurements, which revealed depleted concentrations of terminal electron acceptors (TEAs) and elevated amounts of ions and total dissolved solids, which could be attributed to bacterial biodegradation of hydrocarbons. The study showed that biological alterations of hydrocarbon‐induced subtle changes in the pore water biogeochemistry, which consequently modified the geophysical properties of the contaminated sediments. Based on these observations, the extent of groundwater contamination was delineated according to the geophysical contrast between the contaminated and clean zones. The combination of different geophysical methods constrained by geochemical point measurements provided insight on the different processes that might have modified the soil and groundwater biogeochemical properties.
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Modelling of high‐resolution aeromagnetic data to decipher structural deformation in parts of Kaladgi basin, Peninsular India
Authors Sridhar Muthyala, Harsha Yalla, Ashwini K. Maurya and Anand K. ChaturvediABSTRACTWe show the effectiveness of forward and inversion modelling of high‐resolution magnetic data in deciphering the geological framework in a polydeformed Proterozoic Kaladgi basin. The Meso to Neo Proterozoic Kaladgi basin exposes platformal sediments in the northern margin of Dharwar Craton, Peninsular India. The study of high‐resolution magnetic data over Deshnur locality suggests two prominent trends NW–SE and NE–SW, followed by two minor trends of E–W and N–S. Analysis of the magnetic anomalies aided in understanding the succession of deformation events and their impact over sedimentation. The NW–SE trending Nalur shear zone marks the western contact between the Chitradurga Schist belt and Peninsular gneisses that are traced beneath the Badami sediments. The forward model suggests that the NE–SW trending block faulting resulted in generating a series of Horst and Graben structures. Three‐dimensional compact inversion of circular features bearing remanent magnetisation indicates elliptical‐shaped pipe‐like bodies. The three‐dimensional inversion of magnetic data implied thicker sediments within these Graben structures. The basement configuration depicted as elevation of magnetic basement corroborates these three‐dimensional inversion results. The derived results are validated by drill holes, and the intercepts substantiate the inferred structural setup over the study area. Available drill hole and magnetic data interpretation are combined with field information to reconstruct the tectonostratigraphy and the architecture of the Kaladgi basin around Deshnur locality.
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Studies of parameter correlations in surface NMR using the Markov chain Monte Carlo method
Authors Kristoffer R. Andersen, Ling Wan, Denys Grombacher, Tingting Lin and Esben AukenABSTRACTSurface nuclear magnetic resonance is a technique capable of providing insight into subsurface aquifer properties. To produce estimates of aquifer properties (such as the spatial distribution of water content and parameters controlling the duration of the nuclear magnetic resonance signal), an inversion is required. Essential to the reliable interpretation of the estimated subsurface models is an understanding of the uncertainty and correlation between the parameters in the estimated models. To quantify parameter uncertainty and correlation in the surface nuclear magnetic resonance inversion, a Markov chain Monte Carlo approach is demonstrated. Markov chain Monte Carlo approaches have been previously employed to invert surface nuclear magnetic resonance data, but the primary focus has been on quantifying parameter uncertainty. The focus of this paper is to further investigate whether the parameters in the estimated models exhibit correlation with one another; equally important to building a reliable interpretation of the subsurface is an understanding of the parameter uncertainty. The utility of the Markov chain Monte Carlo approach is demonstrated through the investigation of three questions. The first question investigates whether the parameters describing the water content and thickness of a layer exhibit a strong correlation. This question stems from applying concepts known to electromagnetic surveys (that the layer thickness and layer resistivity parameters are strongly correlated) to the surface nuclear magnetic resonance inversion. A water content–layer thickness correlation in surface nuclear magnetic resonance would not have large effects for quantifying total water content but would affect the ability to identify layer boundaries. The second question examines whether the parameter controlling the duration of the nuclear magnetic resonance signal exhibits a correlation with the water content and layer thickness parameters. The resolution of surface nuclear magnetic resonance typically does not consider the duration of the signal and focuses primarily on the distribution of current amplitudes that form the suite of transmit pulses. It is common to treat regions with short‐duration signal with greater uncertainty, but it is important to understand whether the signal duration controls resolution for medium to long duration signals as well. The third question explores if the parameter uncertainty produced by the Markov chain Monte Carlo approach is consistent with that produced by an alternative approach based upon the posterior covariance matrix (for the linearised inversion). The ability of the Markov chain Monte Carlo approach to more thoroughly explore the model space provides a means to improve the reliability of surface nuclear magnetic resonance aquifer characterisations by quantifying parameter uncertainty and correlation.
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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)