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- Volume 35, Issue 8, 2017
First Break - Volume 35, Issue 8, 2017
Volume 35, Issue 8, 2017
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Multi-component digital-based seismic landstreamer and boat-towed radio-magnetotelluric acquisition systems for improved subsurface characterization in the urban environment
It is estimated that urban life will be the norm for around 60% of the world’s population by 2040, leading to a more centralized distribution of people and making the city as the main place of residence (Whiteley, 2009). This population centralization inherently implies rapidly expanding cities and imposes the need for more infrastructure within, around and between the present city boundaries. However, infrastructure projects nowadays have to follow strict civil engineering standards that require detailed knowledge of subsurface conditions during different stages of the construction processes. Since direct methods conventionally used for site characterization (e.g., drilling and/or core testing) are still relatively expensive the focus in the last two decades has been on non-invasive, geophysical methods. However, geophysical site characterization in urban areas is not an easy task owing to numerous challenges and various types of noise sources. Challenges such as electric/electromagnetic (EM) noise, pipelines and other subsurface objects (sometimes even unknown or undocumented), the inability to properly couple sensors because of pavement, traffic noises and limited space are common in urban environment. Since geophysical surveys need to be done with the least amount of disturbances to the environment, residents and traffic, new geophysical techniques for fast, non-invasive and high-resolution site characterization are needed. To overcome some of these challenges, a nationwide joint industry-academia project was launched in 2012 TUST GeoInfra, www.trust-geoinfra.se). As a component in the project, Uppsala University developed two new data acquisition systems. These are a fully digital MEMS-based (Micro-machined Electro-Mechanical Sensor) three component (3C) seismic landstreamer and a boat-towed radio-magnetotelluric (RMT) acquisition system. Both systems were specifically designed to address urban environments with the RMT system particularly aiming at efficient and cost-effective geophysical surveying on shallow-water bodies, which constitute 7% of Scandinavia. In this article, we will describe the two systems and present two case studies illustrating their potential. A number of published accounts are now available from the two systems showing what type of problems they can address (e.g., Bastani et al., 2015; Brodic et al., 2015; Malehmir et al., 2015a, 2015b, 2016a, 2016b, 2017; Dehghannejad et al., 2017; Maries et al., 2017; Mehta et al., 2017; Brodic et al., 2017).
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Pros and cons of 2D vs 3D seismic mineral exploration surveys
Authors Alireza Malehmir, Gilles Bellefleur, Emilia Koivista and Christopher JuhlinWhile the economic downturn in the mineral industry is improving, exploring for economically feasible deposits to sustain our economy and the global growth in the long term remains a great challenge. Exploring giant deposits (> 30-50 Mt) at depth is believed to be a solution. However, the answers are only likely to be found using a multi-disciplinary approach involving improved field geological mapping, improved conceptual models (e.g., mineral system approach) for deep targeting, and a combination of physical property measurements together with 2D and 3D geophysical surveys. Most metallic deposits have favourable physical properties to be targeted using various geophysical methods (Figure 1), but many of these methods do not have sufficient sensitivity and resolution at great depth (> 500 m). Encouraging examples of the use of surface seismic methods for deep mineral exploration and mine planning are available (e.g., Eaton et al., 2003 and references therein; Malehmir et al., 2012 and references therein; Buske et al., 2015 and references therein).
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Mapping the fresh-saltwater interface in the coastal zone using high-resolution airborne electromagnetics
A major part of the drinking water in the Netherlands is derived from groundwater stored in coastal and inland aquifers. The water occurs through natural processes, but in some areas artificial infiltration of lake and river water in the coastal sand dunes is necessary to support the demand for clean drinking water in the densely populated country. In fact, artificial infiltration along the Dutch coast has been common practice for decades and is an essential part of the drinking water supply. The sustainability of this approach demands detailed knowledge of the fresh–saltwater balance to avoid salinization of the aquifers. The availability and quality of the groundwater are typically estimated using groundwater models, which can be used to forecast the behaviour of groundwater systems to external stresses, such as climate change. The reliability of groundwater models hinges on measurements such as hydraulic heads, stream flow rates, permeability of the subsurface and in coastal regions the boundary conditions for the coastal edge of the model. Here, a limiting factor is often the lack of offshore data since it is logistically difficult and costly to drill in the seabed. Airborne electromagnetics are increasingly being used to support groundwater management through high-resolution largescale mapping of aquifer properties. The method provides strong sensitivity to important hydrogeological units such as the fresh-saltwater interface and clay layers (which often constitute the base of the aquifers). Hence, AEM has been widely used to address issues such as mapping saltwater intrusion (Gunnink etal., 2012; Jørgensen et al., 2012) and aquifer delineation (Chandra et al., 2016; Schamper et al., 2013). One of the key advantages of the method is that it is airborne, allowing areas, which would otherwise be difficult to access, to be mapped in a cost-effective manner. As demonstrated in the present study, the fresh-saltwater interface and coastal boundary conditions of the groundwater model can be determined by combining onshore and offshore airborne electromagnetics.
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Delineating hazardous material without touching — AEM mapping of Norwegian alum shale
Alum shale is a particular type of metamorphic clay that occurs in Cambro-Ordovician metasediment units throughout southern Scandinavia. Owing to its geochemical composition, it is considered a large environmental challenge in Norway (Endre and Sørmo, 2015) and poses a significant site hazard for infrastructure projects (Endre, 2014). Swedish alum shale has such a high Uranium content, that is was mined as a Uranium ore during the mid-1900s (Dyni, 2006). Norwegian alum shale typically contains more than 15 g/kg sulphides and 60-200 mg/kg Uranium. It is consequently a source for radon gas posing a risk to human health. The sulphides oxidize to produce sulphuric acid with pH down to 2-1 once the shale is exposed to air and water, posing a hazard to concrete and metal constructions and the environment. Finally, the most critical geotechnical risk connected to alum shale is its intense swelling owing to oxidation that has damaged properties in the vicinity of building sites that exposed alum shale units and consequently initiated swelling. Prior knowledge of the existence and extent of this hazardous shale is consequently a major risk management factor for underground building activities in areas prone to alum shale. The state of practice for identifying alum shale and other acid producing black shales classified by the Norwegian Environment Agency (Endre and Sørmo, 2015), is primarily based on geochemistry and an understanding of the local geology. Geochemical identification is based on the balance between acidification- and neutralisation potential (Pabst et al., 2016) the geological risk for projects in alum shale prone areas is consequently high. In the worst case, a project may have to be abandoned, for example if the excavated shale volume exceeds the capacity of special waste landfills in the area. Geophysical delineation can decrease the geological risk and increases the efficiency of detailed volume assessments. Since 2014, we have been studying how sulphide and uranium content of various black shale types relate to electrical properties. Sulphide-rich alum shale has very low resistivity (~0.1 Ωm) (Lysdahl et al., 2015) and a strong induced polarization signature (Lysdahl et al., 2016). In the following we show the relationship between geochemically classified shale types and resistivity measurements directly on drill cores as well as surface electrical resistivity tomography (ERT) data. Two case histories show black shale units even at depths exceeding 80 metres based on airborne electromagnetic (AEM) surveying. Here we only focus on resistivity, for further studies on the polarization behaviour of alum shale please see Bazin et al. (2015) and (2017).
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Integrated scanning for quick clay with AEM and ground-based investigations
Near shore, high latitude lowlands throughout Canada, Norway, Sweden, Finland and Russia are prone to a particular geohazard – quick clay. The key geotechnical properties of quick clay, also called Leda clay in Canada, are a very small remoulded shear strength (< 0.5 kPa in Norway after NGF, 2011; and <0.4 kPa in Sweden after Rankka et al., 2004) and consequent high sensitivity (= undisturbed/remoulded shear strength). These properties stem from the fact that this formerly marine clay has been leached by ground water owing to postglacial uplift, losing salt ions that stabilized the flocculated clay structure. When quick clay fails, it liquefies and leads to retrogressive landslides that have caused massive damage and claimed lives both in North America and Europe. Quick clay can be found in sedimentary areas close to the coast and below the marine limit, areas that are attractive for human settlements. Two of Norway’s four most populated urban areas (Oslo and Trondheim) are located in quick clay areas. Detailed study and mapping of areas prone to quick clay slides relies on geomorphology, quaternary geology and point information from geotechnical boreholes and laboratory tests. This is a labour intensive and long lasting process that, in Norway, has been going on since the 1980s and only the most vulnerable parts of the country are mapped so far. The detailed hazard mapping continues, at a pace of a hand full of municipalities a year (Figure 1).
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Building site investigation by joint shear wave reflection seismic and geotechnical drilling at Tønsberg hospital area, eastern Norway
Authors U. Polom, J.S. Rønning, G. Tassis, J. Gellein and G. DruivengaIn northern hemisphere countries such as Norway, Sweden, Finland, Russia, Canada and Alaska (USA), so called quick-clays seriously affect the safe building of settlements, and depth to bedrock is a prerequisite for safe building foundation. Such clays show a mineralogical structure where the stability is dependent on the ionic content in pore water. The composition is sensitive to leaching by low mineralized water. Originally deposited in a marine or brackish environment, clay formations composed of silt and clay are exposed to freshwater owing to the isostatic uplift of nearly 200 m (Bjerrum et al. 1967, ca. 180 m in our study area Tønsberg, www.ngu.no) above sea level after deglaciation. This may have caused leaching to low salinity depending on the time and volume of fresh water inflow, which may destabilize the formation up to a sudden liquefaction collapse. The detection of safe building ground e.g. bedrock and the knowledge of the internal soil structure above it is therefore essential in areas prone to quick-clay. Typically, quick-clays are not exposed to the surface and covered by other lithological units, which makes it difficult to map their area in the subsurface. The administration of the central hospital of Tønsberg (Sykehuset I Vestfold, SIV), Norway, planned to expand the hospital with new buildings in an area prone to quick-clay (Figure 1). Past borehole investigations indicated an undulating bedrock topography below soil, with clay, silt, and anthropogenic infills estimated up to 25 m thick and that a dense borehole grid would be needed for accurate depth-to-bedrock knowledge. Ground Penetrating Radar (GPR) was tested, but failed owing to high electric conductivity in marine sediments. Geoelectric and electromagnetic methods previously applied in other locations by Long et al. (2012) and Solberg et al. (2016) were considered, but were discarded owing to lots of buried hospital infrastructure e.g. pipes, cables and underground transportation tunnels, and the disturbing urban environment. Seismic refraction could not provide the resolution required and was also limited in application owing to the restricted space, the nearby buildings and the asphalt pavement at the surface. Therefore NGU, as geophysical project leader, advised SIV to provide shear wave reflection seismic surveying prior to a focused drilling campaign. Because of the lack of competence of this research in Norway, NGU established a joint research expertise enabling the full range from shallow reflection seismic acquisition and geotechnical analysis towards geological model building for construction site planning.
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Integration of 3D seismic attributes for preliminary shallow geohazard identification in deep water exploration area with no well data
Authors Sigit Sukmono, Vladimir Machado, Ria Adelina and Donasita AmbarsariDrilling in the deep water is very challenging as the safety of drilling rigs and other installations could be threatened by the presence of shallow geohazards. These hazards are a global problem, and while the industry has matured in handling these problems, significant losses owing to the improper assessment of the geohazards prior to drilling have been widely reported (Campbell, 1999). In the Gulf of Mexico (GoM) alone, it was estimated that the associated losses continue to be more than $1.7 million per well (Dutta et al., 2010). ISO 17776 defines a hazard as a ‘potential source of harm’, thus shallow geohazards in the context of offshore drilling activities can be defined as local and/or regional shallow geological features having potential to cause loss of life or damage to health, environment or assets. The geohazards characteristics vary from place to place depending on the regional geology, tectonic history and sedimentation pattern. The US Department of Interior classify the hazards into two categories: 1. Sea-floor geohazards which include fault scarps, gas vents, hydrate mounds, unstable slopes, slumping, active mud gullies, crown cracks, collapse depressions, furrows, sink-holes, mass sediments movements, surface channels, pinnacles and reefs. 2. Subsurface geohazards include faults, gas-charged sediments, abnormal pressure zones, gas hydrates, shallow water-flow sands and buried channel.
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Modelling time-lapse shear-wave velocity changes in an unsaturated soil embankment due to water infiltration and drainage
Authors Atsushi Suzaki, Shohei Minato, Ranajit Ghose, Chisato Konishi and Naoki SakaiThe partially saturated vadose zone, located between the Earth’s surface and the water table, is made of soil particles, water and air. The water in the vadose zone can be transient percolating water which moves downward to join the phreatic water below the water table or the capillary water held above the water table by surface tension (internal pore pressure less than the atmospheric pressure). The distribution and transport of fluids in the vadose zone have a significant influence on the human life and the environment. For example, the dynamic transportation of fluids in the vadose zone is an important factor which controls the pollution at a near-surface, hazardous waste site (Mercer and Cohen, 1990), affects the desertification in arid/semi-arid areas (Scanlon et al., 2003), and determines the sensitivity of water resources to the climate change (Green et al., 2011). The distribution of water in the vadose zone also affects the microbial processes, e.g., biodegradation, which is necessary in assessing agricultural sustainability (Holden and Fierer, 2005). Last but not the least, the dynamic fluid transportation in the vadose zone causes dynamic changes in the yield strength of the unsaturated soil. Therefore, it is critically important in estimating the stability of earth-retaining structures (e.g., river dykes and embankment dams) and natural slopes (e.g., Collins and Znidarcic, 2004). Soil suction and saturation play an important role in controlling the hydraulic and mechanical properties of unsaturated soil in the vadose zone. Depending on the degree of saturation (Sr), unsaturated soils show different values of suction (s). The s-Sr curve, known as the soil-water characteristics curve (SWCC), is the most important piece of information that characterizes the unsaturated soils. Figure 1 shows a typical plot of SWCC. While the suction is zero at the fully saturated condition, it increases as the degree of saturation decreases. Depending on soil texture, SWCC shows different trends. At the same degree of saturation, clayey soils show larger suction values than sandy soils (Figure 1). This is because clayey soils represent smaller pore sizes (capillary radii) than sandy soils, thus creating a larger capillary pressure (Fredlund et al., 2012).
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Volumes & issues
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Volume 42 (2024)
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Volume 41 (2023)
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Volume 40 (2022)
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Volume 39 (2021)
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Volume 38 (2020)
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Volume 37 (2019)
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Volume 36 (2018)
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Volume 35 (2017)
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Volume 34 (2016)
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Volume 33 (2015)
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Volume 32 (2014)
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Volume 31 (2013)
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Volume 30 (2012)
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Volume 29 (2011)
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Volume 28 (2010)
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Volume 27 (2009)
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Volume 26 (2008)
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Volume 25 (2007)
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Volume 24 (2006)
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Volume 23 (2005)
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Volume 22 (2004)
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Volume 21 (2003)
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Volume 20 (2002)
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Volume 19 (2001)
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Volume 18 (2000)
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Volume 17 (1999)
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Volume 16 (1998)
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Volume 15 (1997)
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Volume 14 (1996)
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Volume 13 (1995)
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Volume 12 (1994)
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Volume 11 (1993)
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Volume 10 (1992)
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Volume 9 (1991)
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Volume 8 (1990)
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Volume 7 (1989)
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Volume 6 (1988)
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Volume 5 (1987)
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Volume 4 (1986)
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Volume 3 (1985)
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Volume 2 (1984)
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Volume 1 (1983)