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- Volume 17, Issue 1, 2019
Near Surface Geophysics - Volume 17, Issue 1, 2019
Volume 17, Issue 1, 2019
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Automatic identification of fresh–saline groundwater interfaces from airborne electromagnetic data in Zeeland, the Netherlands
ABSTRACTIn a setting of predominantly saline surface waters in Zeeland, the Netherlands, the only available shallow fresh groundwater resource is present in the form of freshwater lenses floating on top of saline groundwater. This fresh water is vital for agricultural, industrial, ecological, water conservation and drinking water functions. An essential first step for managing the usable water properly is to know the location of the fresh–saline groundwater interface. Traditional salinity mapping with ground‐based vertical electrical soundings, electrical cone penetration tests or chloride measurements from groundwater samples is time‐consuming and, therefore, expensive to cover large areas. Airborne electromagnetics, which is fast and can cover large areas in short time, is an efficient alternative. Therefore, a consortium of BGR, Deltares and TNO conducted the project FRESHEM Zeeland during 2014–2017. An area of more than 2000 km² was surveyed using BGR's helicopter‐borne geophysical system totalling 9640 line‐km. The helicopter‐borne electromagnetic data, after inversion to resistivity–depth models, served as baseline information for further interpretation. Without information on lithology, however, an accurate discrimination between fresh and saline groundwater applying fixed resistivity thresholds would fail if clayey sediments exist. Therefore, a probabilistic Monte Carlo approach was developed within the FRESHEM project. This approach combines helicopter‐borne electromagnetic resistivities, 3D geological model (GeoTOP), laboratory results (formation factor and surface conductivity) and local in situ groundwater measurements for the translation of resistivity data to chloride concentration. As such detailed information is generally not available, another approach, which uses only helicopter‐borne electromagnetic results to derive the thicknesses of the freshwater lenses from smooth inversion models, is presented in this paper. The corresponding fresh–saline groundwater interfaces are derived from steepest resistivity–depth (log–log) gradients appearing within a certain resistivity range. The bounds of this range are defined by resistivity values, which predominantly correlate with fresh or saline water, nearly independent of the lithology type. The results of this approach are checked using both synthetic and field data. The latter are compared with electrical cone penetration test measurements, the common threshold approach and the 3D chloride distribution of the FRESHEM results, particularly in an evaluation area where the transition of fresh to saline groundwater is relatively sharp. The fresh–saline groundwater interfaces derived by all methods are quite similar on average with deviations in the order of a metre. Locally, however, greater deviations occur, particularly close to the coast and along creek ridges or dunes, where the elevation of the fresh–saline groundwater interface varies notably.
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Permittivity mapping in the VLF–LF range using a multi‐frequency EMI device: first tests in archaeological prospection
Authors Fançois‐Xavier Simon, Alain Tabbagh, Jamieson C. Donati and Apostolos SarrisABSTRACTIn the early years of near‐surface geophysical applications, measurements obtained using electromagnetic induction (Slingram or dipole–dipole) instruments were first interpreted in terms of electrical conductivity. Afterwards, studies began considering the importance of magnetic susceptibility. Until now, however, no attention has been paid to possible responses in dielectric permittivity. After reviewing the current state of knowledge regarding the expected theoretical responses in the very low frequency to low frequency (VLF‐LF) range (3–300 kHz) and the value range of this property for soils, we propose the use of multi‐frequency instruments to define a process allowing for the determination of the apparent conductivity and permittivity in the higher part of the frequency range above 20 kHz. We test this process through a series of experiments at archaeological sites in Greece using the GEM‐2 (Geophex Ltd) instrument. In our experiments, the soil permittivity values fall between several hundredths and several thousandths. The results indicate lateral variations different from the other properties with a significant influence of the stone content and of the ionic strength, while a decrease with frequency may also provide further information on soil dielectric behaviours.
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Preferential groundwater seepage in karst terrane inferred from geoelectric measurements
Authors Scott Ikard and Emily PeaseABSTRACTThe Ellenburger–San Saba aquifer discharges spring flows into the overlying Hamilton Creek bed in Burnet County, central Texas. The aquifer is susceptible to contamination from surface‐water reservoirs because of the presence of dissolution cavities that are hydraulically connected to the reservoirs in some locations. There is concern that preferential groundwater seepage from reservoirs into the aquifer in these locations might ultimately degrade the quality of the springwater that enters Hamilton Creek. To investigate preferential groundwater seepage patterns and hydraulic connectivity between surface‐water reservoirs and the Ellenburger–San Saba aquifer, geophysical reconnaissance surveys were completed between July 2017 and January 2018 to map dissolution cavities and locate preferential groundwater seepage within a specific region of the aquifer. Two‐dimensional electric resistivity tomography and self‐potential profiling were utilized, and a simplified, three‐dimensional finite‐element model of the field site was constructed to provide an interpretive aid. The self‐potential data indicated the occurrence of preferential groundwater seepage through a porous seepage conduit that was imaged by the electric resistivity tomography data but did not indicate the occurrence of groundwater seepage through two fluid‐filled dissolution cavities that were imaged by electric resistivity tomography data. Collectively, the surveying and modelling results demonstrate the efficacy of geoelectric methods for mapping the locations of dissolution cavities and preferential groundwater seepage in the electrically resistive karst terrane of the Ellenburger–San Saba aquifer.
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Time‐lapse laboratory tests to monitor multiple phases of DNAPL in a porous medium
Authors Luciana Orlando and Lucia PalladiniABSTRACTA soil contaminated by dense non‐aqueous phase liquids generally exhibits variation of electrical permittivity and resistivity over time. In this paper, we relate photo images with results of electrical resistivity tomography, induced polarization tomography, and ground‐penetrating radar survey in a laboratory‐controlled test, where a cell, filled with grained‐glass beads contaminated with HFE‐7100 (hydrofluoroether) and under saturated conditions, was used. The aim was to find robust links between geophysical and physico‐chemical parameters of a contaminated soil in order to reduce the ambiguities in data interpretation. The monitoring, conducted over 100 days, shows that each geophysical method may or may not be sensitive to the multiple phases of hydrofluoroether (pure, dissolved and gaseous phases) depending on the contaminant (chemical composition, fresh, mature, etc.) and on the host enviroment. We propose some empirical relationships that allow us to quantitatively monitor the water saturation variation induced by hydrofluoroether.
The photo images confirm that the pure phase of hydrofluoroether changes over time, which is probably due to biological or/and chemical variation of the contaminant. Electrical resistivity tomography results appear to be sensitive to the vast amount of pure phase of the hydrofluoroether. Induced polarization, measured in the extremely low frequency range (<25 Hz), allows detecting the pure phase of the contaminant, probably due to the decrease of water permeability and chemical degradation of the compound over time. The ground‐penetrating radar and time domain reflectometry methods based on measurements in very‐high frequency and ultra‐high frequency ranges are mainly sensitive to the water content and, as a consequence, to the replacement of water by pure and gaseous phases of hydrofluoroether.
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Comparison of a low‐cost magneto‐inductive magnetometer with a proton magnetometer: a case study on the Galali iron ore deposit in western Iran
More LessABSTRACTThe proton magnetometers, also known as the proton precession magnetometers, are among the most widely used instruments for magnetometry surveys, owing to their high resolution which is about 0.01 nT. As the economically attractive ferromagnetic deposits generate a magnetic field anomaly which exceeds 100 nT, this resolution is not needed for detecting such large signals. The recently introduced magneto‐inductive technology has led to the development of a low‐cost magnetometer that can measure the ground magnetic field up to a resolution of around 10 nT. These magnetometers are inexpensive, come in a very small size, and are lightweight compared to the more common magnetometers such as the proton precession type. In this research, a low‐cost magneto‐inductive sensor and a highly precise proton magnetometer are simultaneously utilized at the same profiles on the Galali iron ore deposit in the northwest of Iran. The discrepancy of two measurements is less than 400 nT, often less than 100 nT, while the changes in the total magnetic field on this anomaly are around 7500 nT, which is 11 times larger than the maximum difference between the two measurements. In addition, the value of the regression coefficient between the two measurements for all profiles is more than 0.97. Also, the slope of the fitted line for the two measurements for all profiles is close to one. These results demonstrate the applicability of the new low‐cost magneto‐inductive sensor in magnetic prospecting.
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Evaluation of traveltime tomography in estimating near‐surface velocity inversion
More LessABSTRACTNear‐surface velocity structures are generally inverted by ray‐based traveltime tomography in which the objective function is usually constructed using raypath traveltime residuals. However, inversion accuracy in these situations remains low due to the asymptotic assumption. On the contrary, phase and instantaneous traveltime tomographies invert frequency‐dependent traveltime rather than the asymptotic raypath version that is contained in the first‐arrival waveform for near‐surface velocities. They also take the finite‐frequency effect of the seismic wave propagation into account by using wavepaths instead of raypaths as sensitivity kernels. In this context, we firstly compare synthetic results between raypath traveltime, phase traveltime and instantaneous traveltime in this study before comparing their corresponding sensitivity kernels. We then analyse three sets of inversion results to prove that it is still an acceptable way to construct the objective function using raypath traveltime for tomography within the seismic exploration frequency band. In this way, time‐domain wave‐equation forward simulation can be avoided and therefore the computational cost can be dramatically reduced. We also demonstrate that a sensitivity kernel that takes the finite‐frequency effect into account is more important for inversion accuracy than traveltime used in tomography.
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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)