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- Volume 16, Issue 4, 2018
Near Surface Geophysics - Volume 16, Issue 4, 2018
Volume 16, Issue 4, 2018
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Re‐parameterisations of the Cole–Cole model for improved spectral inversion of induced polarization data
Authors Gianluca Fiandaca, Line Meldgaard Madsen and Pradip Kumar MauryaABSTRACTThe induced polarization phenomenon, both in time domain and frequency domain, is often parameterised using the empirical Cole–Cole model. To improve the resolution of model parameters and to decrease the parameter correlations in the inversion process of induced polarization data, we suggest here three re‐parameterisations of the Cole–Cole model, namely the maximum phase angle Cole–Cole model, the maximum imaginary conductivity Cole–Cole model, and the minimum imaginary resistivity Cole–Cole model. The maximum phase angle Cole–Cole model uses the maximum phase φmax and the inverse of the phase peak frequency, τφ, instead of the intrinsic charge‐ability m0 and the time constant adopted in the classic Cole–Cole model. The maximum imaginary conductivity Cole–Cole model uses the maximum imaginary conductivity instead of m0 and the time constant τσ of the Cole–Cole model in its conductivity form. The minimum imaginary resistivity Cole–Cole model uses the minimum imaginary resistivity instead of m0 and the time constant τρ of the Cole–Cole model in its resistivity form.
The effects of the three re‐parameterisations have been tested on synthetic time‐domain and frequency‐domain data using a Markov chain Monte Carlo inversion method, which allows for easy quantification of parameter uncertainty, and on field data using 2D gradient‐based inversion. In comparison with the classic Cole–Cole model, it was found that for all the three re‐parameterisations, the model parameters are less correlated with each other and, consequently, better resolved for both time‐domain and frequency‐domain data. The increase in model resolution is particularly significant for models that are poorly resolved using the classic Cole–Cole parameterisation, for instance, for low values of the frequency exponent or with low signal‐to‐noise ratio. In general, this leads to a significantly deeper depth of investigation for the , , and parameters, when compared with the classic m0 parameter, which is shown with a field example. We believe that the use of re‐parameterisations for inverting field data will contribute to narrow the gap between induced polarization theory, laboratory findings, and field applications.
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Estimation of hydraulic transmissivity from MRS by varying the porosity exponent in detrital aquifers of the Iberian Peninsula
More LessABSTRACTTo this date, estimation of hydraulic transmissivity with magnetic resonance sounding (MRS) requires transmissivity values from pumping tests in nearby wells in order to determine a calibration constant from the relationship between both types of data. The use of this technique is limited due to the fact that reported values of this calibration constant are highly variable, even within the same type of aquifer. In order to minimize this shortcoming, a new methodology, which has been developed and presented here, is used to estimate the transmissivity of detrital aquifers with heterogeneous behaviour based only on the results obtained by MRS. The exponents applied to decay time and free porosity used in the expression to compute MRS transmissivity have conventionally been fixed values. In contrast, the work presented here aims to improve the applicability of this technique using a variable exponent for free porosity in each type of aquifer, obtained only from MRS results. In order to achieve this goal, previously published MRS data from four detrital aquifers in the Iberian Peninsula have been used and a relationship between the porosity exponent and a function of decay times and free porosities has been developed. This approach eliminates the need to drill and perform pumping tests in additional boreholes, which has cost and time implications, once the correct expression is determined for a given type of aquifer. The range of values thus obtained for the porosity exponent is similar to that reported in the literature. The deviation of the results obtained with this methodology regarding the results of pumping test is similar to the deviation with conventional process. This suggests that the proposed methodology is appropriate for the aquifers considered, and the approach may be expanded to study other types of aquifers. The drawbacks of the proposal related to the conventional equivalence are explored.
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Investigating a firn aquifer near Helheim Glacier (South‐Eastern Greenland) with magnetic resonance soundings and ground‐penetrating radar
ABSTRACTWe apply the magnetic resonance sounding (MRS) method to investigate a firn aquifer in the south‐east region of the Greenland ice sheet. Our study aims to delineate and estimate the volume of the recently discovered water stored within the firn (compacted snow) that remains liquid throughout the year. We develop and test successfully a methodology for joint use of MRS and ground‐penetrating radar (GPR). This non‐invasive geophysical approach is particularly well‐adapted to glacier conditions and has a promising future for in situ investigation of water distribution in glaciers. At our field site, MRS showed an aquifer located at variable depths between 20 and 30 m beneath the ice‐sheet surface. At the monitoring site, both MRS and GPR show an increase in the water volume stored between April 2015 and July 2016. MRS estimates suggest that the volume increased by approximately 28%.
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Monitoring of moisture and salinity content in an operational road structure by electrical resistivity tomography
Authors H. Rasul, L. Zou and B. OlofssonABSTRACTMoisture dynamics in road systems significantly affect road structure design and maintenance. This study analysed moisture dynamics in a cross‐section of motorway (the E18) in Sweden during a 1‐year period through in situ monitoring using electrical resistivity tomography (ERT). The monitoring methodology was assessed since resistivity can provide a good proxy for monitoring moisture in the road structure. Monthly electrical resistivity was calculated by inverting resistivity data along a pre‐installed electrical resistivity line beneath the surface asphalt layer of the road at the test site. The electrical resistivity data were then statistically analysed and correlated with local climate data, that is precipitation and temperature, and with ground parameters such as moisture content. The results showed high variation in resistivity in the road surface layer and road shoulders depending on weather conditions, water flow and other surface activities. In general, negative correlations between electrical resistivity and precipitation were observed. The results also indicated possible retardation of de‐icing salt after accumulating in the top layer during winter. These findings advance understanding of the moisture dynamics in roads and can help improve pavement design in response to future climate change.
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What exists beneath the place where Conrad Schlumberger carried out the first (1912) electrical prospection experiment: the Val‐Richer Abbey
More LessABSTRACTThe Val‐Richer in Normandy is recognised as the place of origin of the resistivity method in applied geophysics. There, during the summer of 1912, Conrad Schlumberger tested for the first time a new method designed to map out the electrical resistivity of the sub‐surface. A well‐known, hand‐drawn blueprint made by Conrad Schlumberger describes this experiment. It shows the voltage distribution observed at the ground surface together with a hand‐written comment describing the difficulties encountered and the solutions retained. This test led the Schlumberger brothers to develop this type of prospection and to later create, first, an engineering office in 1920 and, then, companies among the largest international ones in the oil service industry.
In 2014, 102 years after this founding experiment, a research project was initiated with the main objective to delineate the remains of the Val‐Richer Abbey, which were destroyed during the French Revolution. A resistivity survey was conducted complemented by an electromagnetic survey over the area where the abbey's religious buildings were expected to be located, the same place where Conrad Schlumberger's measurements were carried out. The interpretation of the geophysical survey, in the light of archival sources and current knowledge of the Cistercian abbeys of Normandy, has completely altered our previous vision of the Val‐Richer Abbey and made it possible to establish an accurate outline of this monastic settlement. The knowledge about sub‐surface electrical resistivity authorises a renewed analysis of the 1912 blueprint and associated drawings and a new assessment of the solutions then adopted by Conrad Schlumberger, whose major idea was the injection of DC rather than AC current to avoid induction effects.
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Processing considerations and improved interpretation for ground‐penetrating radar imaging of a relict archaeological excavation unit
Authors Peter Lanzarone and Daniel BigmanABSTRACTA ground‐penetrating radar survey was carried out at The Great Temple Mound (Mound A), Ocmulgee National Monument, Georgia, USA to determine the location of buried archaeological features. Mound A is the largest pre‐historic mound at the site and its function remains unclear. High quality data and sufficient spatial sampling with a 500‐MHz centre frequency antenna facilitated the generation of two‐dimensional profiles and amplitude depth slices. The analyses presented here describe a processing sequence used to improve the image fidelity and the visualization of the remains of a relict archaeological excavation unit. Additionally, we employ depth slice overlay analysis for enhanced visualization of this feature. The results demonstrate a new processing methodology that helps in delineating the lateral extent of the excavation and its evolution. The results also highlight the implementation of a new visualization strategy in GPR surveys aimed towards detecting graves, burial tombs, and other rectilinear or cubic features. We conclude that even with excellent GPR imaging, there may be significant positioning errors in defining the boundaries of buried features.
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Using broadband seismic networks to optimize microgravity survey strategy in the United Kingdom
Authors Yuriy Goncharenko, Daniel Boddice, Anthony Rodgers, Philip Atkins, Nicole Metje and David ChapmanABSTRACTMicrogravity measurements are a useful tool for detecting subsurface features, especially deep targets or those in conductive ground which lie outside the capabilities of other methods based on electromagnetic signal transmission. However, the method is limited by a range of noise sources including vibrational noise from the environment, one source of which being the microseism noise due to ocean waves. This noise travels through the bedrock and manifests itself in the data. It varies as a function of time and location. The effect of the wave noise on microgravity measurements in the United Kingdom was assessed for the first time using a field gravimeter (Scintrex CG5) and a link was demonstrated between the noise from microgravity measurements and those from a broadband seismometer. As a result, a new method for assessing the impact of this noise on microgravity measurements in the United Kingdom is proposed using readily available data from the continuously monitoring seismic network run by the British Geological Survey to create an accurate nowcast. Knowledge of this noise on the day of survey in conjunction with an approximate signal strength of the expected targets can be used to significantly improve survey planning in terms of the optimal observation time at which surveys for particular targets should be conducted, saving time and money on failed microgravity surveys.
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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)