Not extinct yet: innovations in frequency domain HEM triggered by sea ice studies

-The last 15 years have brought major innovations in helicopter towed time domain electromagnetics (EM), while few further developments have been made within the classic frequency domain segment. Operational use of frequency domain EM for sea ice thickness mapping acted as a driving force to develop new concepts such as the system under our consideration. Since its introduction we have implemented new concepts aiming at noise reduction and drift elimination. We decreased signal noise base levels by one to two orders of magnitude with changes to the signal transmission concept. Further, we increased the receiver coil dynamic range creating an EM setup without the need for primary field bucking. Finally, we implemented control signals inside the receiver coils to potentially eliminate system drift. Ground tests demonstrate the desired noise reduction and demonstrate drift control, leading to essentially drift free data. Airborne field data confirm these results, yet also show that the procedures can still be improved. The remaining quest is whether these specialised system improvements could also be implemented in exploration helicopter EM (HEM) systems to increase accuracy and efficiency

Snow thickness profiling on Antarctic sea ice with GPR—Rapid and accurate measurements with the potential to upscale needles to a haystack

Snow thickness on sea ice is a largely undersampled parameter yet of importance for the sea ice mass balance and for satellite-based sea ice thickness estimates and thus our general understanding of global ice volume change. Traditional direct thickness measurements with meter sticks can provide accurate but only spot information, referred to as “needles” due to their pinpoint focus and information, while airborne and satellite remote sensing snow products, referred to as “the haystack,” have large uncertainties due to their scale. We demonstrate the remarkable accuracy and applicability of ground-penetrating radar (GPR) snow thickness measurements by comparing them with in situ meter stick data from two field campaigns to Antarctica in late winter/early spring. The efficiency and millimeter-to-centimeter accuracy of GPR enables practitioners to acquire extensive, semiregional data with the potential to upscale needles to the haystack and to potentially calibrate satellite remote sensing products that we confirm to derive roughly 30% of the in situ thickness. We find the radar wave propagation velocity in snow to be rather constant (± 6%), encouraging regional snow thickness surveys. Snow thinner than 10 cm is under the detection limit with the off-the-shelf GPR setup utilized in our study.publishedVersio

Combining airborne electromagnetic and geotechnical data for automated depth to bedrock tracking

-Airborne electromagnetic (AEM) survey data was used to supplement geotechnical investigations for a highway construction project in Norway. Heterogeneous geology throughout the survey and consequent variable bedrock threshold resistivity hindered efforts to directly track depth to bedrock, motivating us to develop an automated algorithm to extract depth to bedrock by combining both boreholes and AEM data. We developed two variations of this algorithm: one using simple Gaussian or inverse distance weighting interpolators, and another using ordinary kriging and combined probability distribution functions of input parameters. Evaluation shows that for preliminary surveys, significant savings in boreholes required can be made without sacrificing bedrock model accuracy. In the case study presented, we estimate data collection savings of 1,000 to 10,000 NOK/km (c. $160 to$1,600 USD/km) would have been possible for early phases of the investigation. However, issues with anthropogenic noise, low signal, and uncertainties in the inversion model likely reduced the comparative advantage that including AEM provided. AEM cannot supersede direct sampling where the model accuracy required exceed the resolution possible with the geophysical measurements. Nevertheless, with the algorithm we can identify high probability zones for shallow bedrock, identify steep or anomalous bedrock topography, and estimate the spatial variability of depth at earlier phases of investigation. Thus, we assert that our method is still useful where detailed mapping is the goal because it allows for more efficient planning of secondary phases of drilling

In situ detection of sensitive clays – Part II: Results

Sensitive and quick clays are typically found in Norway, Sweden and Canada, and are characterised by a remoulded undrained shear strength considerably lower than the undisturbed shear strength. In geotechnical engineering, the presence of sensitive clays poses a major challenge. The landslides at Rissa in 1978, and more recently at the Skjeggestad bridge in Norway, are devastating reminders of the potential threats related to such soils. In a construction project it is hence important to 1) determine if there is sensitive clay present and 2) clarify the extent of the quick clay deposit. This is currently done based on interpretation of soundings and to some extent geophysical methods such as electrical resistivity measurements. However, for verification of quick clay, sampling and laboratory testing must be performed. Here, a set of updated and new guidelines for classification of sensitive clays from in-situ measurements are presented. The aim is to provide the geotechnical engineer with a practical classification tools where all available information is utilized and combined efficiently. The classification tools are based on results from methods such as conventional soundings, CPTU with measurement of total force, electrical field vane testing in combination with geophysical methods such as R-CPTU, 2D resistivity profiles (ERT) and airborne electromagnetic measurements (AEM). The methods, and how they are utilized in investigation strategies for detection of quick and sensitive clays, have been described in another paper to this conference. An extensive database of Norwegian test sites forms the basis for the work. The results from this study show that the above mentioned site investigation methods holds information that complements each other, to form a solid basis for detection of sensitive clays. In turn, this opens for more efficient site investigations where all available data are interpreted in a systematic manner to produce a reliable map of sensitive clay deposits.publishedVersio

Glacio-marine clay resistivity as a proxy for remoulded shear strength: correlations and limitations

In geotechnical engineering in Norway, Sweden and Canada the presence of sensitive and/or quick clays poses a major challenge. Formation of these clays involves the leaching of salt from the pore fluid. Thus it has been recognized that electrical resistivity measurements could be useful in delineating leached and unleached clays. This paper seeks to assess the applicability, repeatability and reliability of the various geophysical techniques in the study of sensitive clays. It also attempts to understand the factors that control the measured resistivity and in particular to determine the limitations of directly obtaining the remoulded shear strength from the resistivity measurements. It was found that borehole, surface and airborne resistivity measurements are accurate and compatible. For the 30 Norwegian sites studied it was found that resistivity is primarily defined by the porewater salt content, with minor additional influence by clay content and plasticity, and porosity. A relationship exists between resistivity and remoulded shear strength but this is limited to material deeper than the dry crust and a surface weathering zone of about 7.5 m thickness. High resistivity (>10 Ω m) may indicate quick or weathered clay but low resistivity (<10 Ω m) conclusively points to stable, unleached clay.acceptedVersio

In situ detection of sensitive clays – Part I: Selected test methods

Sensitive and quick clays are typically found in Norway, Sweden and Canada, and are characterized by a remoulded undrained shear strength that is considerably lower than the undisturbed shear strength. In geotechnical engineering, the presence of sensitive clays pose a major challenge. The landslides at Rissa in 1978, and more recently at the Skjeggestad bridge in Norway, are devastating reminders of the potential threats related to such soils. For a geotechnical engineering project it is hence important to 1) determine if there is sensitive clay present and 2) clarify the extent of the quick clay deposit. This is currently done based on interpretation of soundings and to some extent by geophysical methods such as electrical resistivity measurements. However, for verification of quick clay, sampling and laboratory testing must be performed. Here, a set of methods for classification of sensitive clays from in situ measurements are presented. The aim is to provide the geotechnical engineer with practical and rational methods, from which all available information is utilized and combined efficiently. The methods presented herein include conventional soundings, CPTU with measurement of total force, vane shear testing in combination with geophysical methods such as R-CPTU, 2D resistivity profiles (ERT) and airborne electromagnetic measurements (AEM). An extensive database of Norwegian test sites forms the basis for the work. This paper describes the methods utilized in this study and how they may be combined in a strategy for detecting deposits of quick and sensitive clays. The major results from the study are presented in another paper presented to this conference.publishedVersio

Atmospheric mercury observations from Antarctica: seasonal variation and source and sink region calculations

Long term atmospheric mercury measurements in the Southern Hemisphere are scarce and in Antarctica completely absent. Recent studies have shown that the Antarctic continent plays an important role in the global mercury cycle. Therefore, long term measurements of gaseous elemental mercury (GEM) were initiated at the Norwegian Antarctic Research Station, Troll (TRS) in order to improve our understanding of atmospheric transport, transformation and removal processes of GEM. GEM measurements started in February 2007 and are still ongoing, and this paper presents results from the first four years. The mean annual GEM concentration of 0.93 ± 0.19 ng m&lt;sup&gt;−3&lt;/sup&gt; is in good agreement with other recent southern-hemispheric measurements. Measurements of GEM were combined with the output of the Lagrangian particle dispersion model FLEXPART, for a statistical analysis of GEM source and sink regions. It was found that the ocean is a source of GEM to TRS year round, especially in summer and fall. On time scales of up to 20 days, there is little direct transport of GEM to TRS from Southern Hemisphere continents, but sources there are important for determining the overall GEM load in the Southern Hemisphere and for the mean GEM concentration at TRS. Further, the sea ice and marginal ice zones are GEM sinks in spring as also seen in the Arctic, but the Antarctic oceanic sink seems weaker. Contrary to the Arctic, a strong summer time GEM sink was found, when air originates from the Antarctic plateau, which shows that the summertime removal mechanism of GEM is completely different and is caused by other chemical processes than the springtime atmospheric mercury depletion events. The results were corroborated by an analysis of ozone source and sink regions

Settlement monitoring using space-borne radar interferometry, in the context of large infrastructure projects

Geodetic data acquisition in urban areas and along linear infrastructure are major cost drivers in big infrastructure projects. Modern remote sensing methods help us to manage data acquisitions with high effectiveness during the different phases of such projects. Here, we show how we apply space-borne radar interferometry to monitor settlements. We showcase two examples: a) the new E18-highway development project in Bærum, Norway; and b) the Follo Line railway project between Ski and Oslo. Monitoring is done by exploiting very high resolution TerraSAR-X data (ca. 1.5 x 1.5 m spatial ground resolution) in the E18 case, and high resolution Radarsat-2 data (ca. 7 x 7 m spatial ground resolution) in the Follo Line case. In both cases

Detektering av kvikkleire - sluttrapport

Rapporten oppsummerer arbeidet innen NIFS for detektering av kvikkleire/ sprøbruddmaterialer. Det gis anbefalte strategier for bruk av konvensjonelle geotekniske metoder (sonderingsmetoder, CPTU, vingeboring og prøvetaking med laboratorieundersøkelser) også kombinert med geoelektriske målinger på overflaten (ERT) og luftbårne målinger (AEM) samt resistivitetsmålinger utført nedhulls (R-CPTU) for detektering. Rapporten er støttet av NGF og gir grunnlag for en NGFveilede