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- Volume 19, Issue 5, 2021
Near Surface Geophysics - Volume 19, Issue 5, 2021
Volume 19, Issue 5, 2021
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Using geophysical survey results in the inference of aquifer vulnerability measures
Authors Niels B. Christensen and Anders V. ChristiansenABSTRACTThe physical parameter derived from the inversion of electromagnetic surveys, the distribution of subsurface conductivity, is interesting in itself only in very few instances. In most cases, the conductivity distribution will have to be interpreted in terms of the target properties of the survey, for example: a geological interpretation of lithology; a hydrogeological interpretation of hydraulic conductivity; a biohazard/geotechnical interpretation of polluted/not‐polluted ground; and/or an archaeological interpretation of manmade/natural finds. The parameters of interest in these categories are often called derived products, indicating that the parameter of interest is not the same as the parameter whose distribution is found in the inversion process of the geophysical data. The interpretation process can be done in a wide variety of ways; from a predominantly cognitive approach based on professional experience, to an application of rigorous quantitative relations found from scientific endeavours.
In most practical situations, the number of locations with independently measured information on the derived product is considerably smaller than the number of geophysics locations. It is precisely this sparsity of primary information on the derived product that encourages the use of geophysical inversion results as a sort of qualified interpolator through a formulation of a correlation between a geophysical parameter and the parameter characterising the derived product. In this paper, a general, quantitative approach to deriving the parameter of interest is presented using statistical analytic measures and an advanced use of an interpolation method that takes uncertainties into account. The approach is demonstrated in a field example from Ølgod, Denmark, where the cumulated clay thickness in the upper 30 m is estimated using a combination of borehole drilling records and an airborne transient survey.
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A machine learning–based approach to regional‐scale mapping of sensitive glaciomarine clay combining airborne electromagnetics and geotechnical data
ABSTRACTSensitive glaciomarine clays, often referred to as ‘quick clay’, commonly occur in many countries at high, northerly latitudes, causing frequent and occasionally devastating landslides. The salt content of quick clay is strongly correlated to both its shear strength and electrical resistivity. Hence, it can be mapped using electromagnetic methods more efficiently than traditional intrusive methods, the latter of which can often be slow and costly. However, the resistivity signature of quick clay is non‐unique, leading to ambiguous, imprecise interpretations of geophysical models. In this study, we present an improved method for predicting the probability of quick clay using airborne electromagnetics. Using machine learning algorithms, we combine geophysical models with geotechnical data to address the issue of their non‐unique resistivity signature. Beyond resistivity values, the machine learning algorithms use spatial derivatives of resistivity and spatial attributes. We evaluate the performance of this method using data collected from a road construction project in central Norway. Results show that this method is able to make plausible and accurate predictions of quick clay occurrence using as few as 10 boreholes across an area of 14.8 km2, and that it outperforms a simple interpretation based on resistivity intervals alone. In addition to a ‘best guess’ categorical classification, these algorithms output probability estimates, and we demonstrate that they are a reliable indication of uncertainty. The accuracy of these predictions also tends to increase as more geotechnical data are included as training data, helping compensate for the limited resolution of the airborne electromagnetics data. Given that the petrophysics of the clays at this test site are consistent with observations in other regions, we expect this method has the potential to make quick clay hazard mapping more efficient by offering valuable early‐phase insights, leading to large time and cost savings for both infrastructure planning and regional hazard mapping.
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Ambient temperature and relative humidity–based drift correction in frequency domain electromagnetics using machine learning
ABSTRACTElectromagnetic instrument responses suffer from signal drift that results in a variable response at a given location over time. If left uncorrected, spatiotemporal aliasing can manifest and global trends or abrupt changes might be observed in the data, which are independent of subsurface electromagnetic variations. By performing static ground measurements, we characterized drift patterns of different electromagnetic instruments. Next, we performed static measurements at an elevated height, approximately 4 metre above ground level, to collect a data set that forms the basis of a new absolute calibration methodology. By additionally logging ambient temperature variations, battery voltage and relative humidity, a relation between signal drift and these parameters was modelled using a machine learning (ML) approach. The results show that it was possible to mitigate the effects of signal drift; however, it was not possible to completely eliminate them. The reason is three‐fold: (1) the ML algorithm is not yet sufficiently adapted for accurate prediction; (2) signal instability is not explained sufficiently by ambient temperature, relative humidity and battery voltage; and (3) the black‐box internal (factory) calibration impeded direct access to raw data, which prevents accurate evaluation of the proposed methodology. However, the results suggest that these challenges are not insurmountable and that ML can form a viable approach in tackling the drift problem instrument specific in the near future.
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The role of electromagnetic induction in development planning: the case of the Grands Philambins enigma
Authors Frank Muller and Michel DabasABSTRACTSpatial planning aims at constantly improving the distribution of people and activities within available space, especially near cities on ancient agricultural or industrial areas. However, a proper schedule of the development work necessitates the assessment, as precisely as possible, of the difficulties that would be encountered in terms of geotechnical abilities of the terrain and hazards resulting from previous use. In the Grands Philambins site, three different geophysical techniques were combined: multi‐depth resistivity; magnetic gradiometry; and electromagnetic induction. The aim of this study was to map, in a few days and with high measurement densities, all the electromagnetic properties of the terrain and to evidence metallic features. The acquired data were compared with prior documentation, mainly the aerial photos acquired since the middle of the 20th century. The terrain structure is complex, at the junction of geological formations composed of limestones and marls in different proportions, which were characterized by the geophysical survey. It also exhibits metallic features, among which an enigmatic 70 m diameter radially striped disc object, which was identified as non‐ferrous and corresponding to an ancient antenna earthed socket. This case study illustrates the relevance of the use of electromagnetic induction measurements in land planning studies as this method can map three independent underground properties and identify metallic features.
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Laboratory study of the electrical properties of Lutetian limestones in the 100 Hz to 10 MHz frequency range
Authors Blaise Souffaché and Alain TabbaghABSTRACTLutetian limestones have been widely used in historical monuments within the Paris Basin during the course of the medieval and modern periods. Among the physical properties that can be used to assess the evolution of the limestones in situ in the buildings and their present health, the complex effective permittivity in the 10–100 kHz frequency range is easy to measure and reflects the internal structure of the stone along with the dependence on the water content. To improve our knowledge about this property, a laboratory study on four samples collected in the relevant quarries has been undertaken using measurements in the 100 Hz–10 MHz frequency range. Except close to zero water content, the observed results exhibit a quasi‐absence of variation of the real effective permittivity with the water content. The frequency variation fits fairly well with a model taking into account a Jonscher's decrease, a direct current conductivity, a high‐frequency dielectric permittivity and losses, and a relaxation phenomenon. When fitted by a Cole–Cole model, the magnitude of the corresponding relative permittivity change always stays close to 30, but the time constant varies from 1.0 μs to 0.1 μs as the water content increases.
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Collapsed and non‐collapsed ice‐marginal glaciodeltaic morphosequence structure mapped with ground penetrating radar in central Connecticut
Authors El Hachemi Bouali and Charles SweeneyABSTRACTGlacial ice from the last major advance covered the state of Connecticut and reached its glacial maximum on Long Island ∼21,000 years ago. As glacial ice melted and retreated northward, meltwater streams deposited sediments, like gravel, sand, silt and clay, in low‐lying areas. Many periglacial lakes formed in central Connecticut as meltwater accumulated into valleys that were dammed by ice or glacial sediments. The Cromwell delta was deposited into glacial Lake Middletown. Previous studies have identified seven types of morphosequences, which are sedimentary facies that were deposited contemporaneously. The Cromwell delta is an ice‐marginal deltaic morphosequence, with proximal deposits laid atop stagnant glacial ice and distal deposits built up as free fronts in open waters of glacial Lake Middletown. Proximal deposits experienced collapse as stagnant ice blocks melted. Thus, each ice‐marginal deltaic morphosequence within the Cromwell delta includes two sections bounded by an east‐west‐trending topographic high delineating a previous ice‐margin position: north of this boundary are collapsed deltaic deposits dipping northward; south are non‐collapsed deltaic deposits dipping southward. Ground penetrating radar was used at River Highlands State Park to map sections of the Cromwell delta. The purpose of the study is threefold: (1) to demonstrate the capabilities of ground penetrating radar for identifying deltaic structures and locating previous ice‐margin positions; (2) to provide insight into the collapsed delta formation by examining the areal extent and geometry of collapsed structures; (3) to show how ground penetrating radar can be used in an exploratory manner, with limited access to direct subsurface data, by combining ground penetrating radar with shallow auger samples and results from previous studies in areas near River Highlands State Park, the Cromwell delta and other contemporaneous deltas. Results show that ground penetrating radar can be used to map deltaic morphosequences and ice‐marginal positions to provide a more detailed understanding of the Cromwell delta history at River Highlands State Park.
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Archaeogeophysical exploration in Neuss‐Norf, Germany using electrical resistivity tomography and magnetic data
Authors Ismael M. Ibraheem, Rainer Bergers and Bülent TezkanABSTRACTA combination of electrical resistivity tomography (ERT) and magnetic gradiometry was selected to examine a hypothesis concerning the presence of remains of one of the oldest archaeological churches in the Rhineland, located in Neuss‐Norf, Germany. The gradiometer survey was carried out to measure the vertical gradient of the magnetic field using a proton precession magnetometer along several profiles. The magnetic data were reduced to the magnetic pole; then analytic signal and power spectrum techniques were applied. The ERT survey was based on the magnetic results, and both Wenner and dipole–dipole configurations were employed to collect the apparent resistivity data along 12 ERT profiles. The field ERT data from these two arrays were merged into one dataset to form a non‐conventional mixed array. The robust (blocky) inversion technique was applied to the resistivity data in order to derive the two‐dimensional resistivity distribution of the subsurface. Despite the noisy surroundings, the magnetic survey was able to give an indication of potential walls of the ancient church in addition to several subsurface magnetic sources. Moreover, highly resistivity anomalies were observed within the first 1–2 m of the subsurface soil and were interpreted as possible remains of man‐made structures. This depth range was also confirmed by the spectral analysis of the magnetic data. A strong consistency between the two methods was observed in some locations of the site. In addition, the ERT measurements confirm and complement most of the magnetic results. We successfully detected anomalous zones that could be associated with the walls of at least one ancient church building in addition to several possible archaeological structures in the survey area.
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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)