Sara Bazin

Seismic imaging of the three‐dimensional architecture of the Çınarcık Basin along the North Anatolian Fault

The Cinarcik Basin is a transtensional basin located along the northern branch of the northern North Anatolian Fault (NAF) in the Sea of Marmara, the eastern half of which has been identified as a seismic gap. During the SEISMARMARA (2001) experiment, a dense grid of multichannel seismic reflection profiles was shot, covering the whole Cinarcik Basin and its margins. The new seismic images provide a nearly three-dimensional view of the architecture of the basin (fault system at depth and sedimentary infill) and provide insight into its tectonic evolution. Along both northern and southern margins of the basin, seismic reflection data show deep-penetrating faults, hence long-lived features, which have accommodated a large amount of extension. There is no indication in the data for a single throughgoing strike-slip fault, neither a cross-basin fault nor a pure strike-slip fault running along the northern margin. Faster opening is presently observed in the eastern part of the basin. The Cinarcik Basin seems to have developed as a transtensional basin across strike-slip segments of the northern NAF for the last few million years.

The Mw = 6.3, November 21, 2004, Les Saintes earthquake (Guadeloupe): Tectonic setting, slip model and static stress changes

On November 21, 2004, a magnitude 6.3 earthquake occurred offshore, 10 km south of Les Saintes archipelago in Guadeloupe (French West Indies). There were more than 30000 aftershocks recorded in the following two years, most of them at shallow depth near the islands of the archipelago. The main shock and its main aftershock of February 14, 2005 (Mw = 5.8) ruptured a NE-dipping normal fault (Roseau fault), mapped and identified as active from high-resolution bathymetric data a few years before. This fault belongs to an arc-parallel en echelon fault system that follows the inner edge of the northern part of the Lesser Antilles arc, accommodating the sinistral component of oblique convergence between the North American and Caribbean plates. The distribution of aftershocks and damage (destruction and landslides) are consistent with the main fault plane location and attitude. The slip model of the main shock, obtained by inverting jointly global broadband and local strong motion records, is characterized by two main slip zones located 5 to 10 km to the SE and NW of the hypocenter. The main shock is shown to have increased the Coulomb stress at the tips of the ruptured plane by more than 4 bars where most of the aftershocks occurred, implying that failures on fault system were mainly promoted by static stress changes. The earthquake also had an effect on volcanic activity since the Boiling Lake in Dominica drained twice, probably as a result of the extensional strain induced by the earthquake and its main aftershock.

Towards geophysical and geotechnical integration for quick-clay mapping in Norway

Quick clay is a known hazard in formerly-glaciated coastal areas in e.g., Norway, Sweden and Canada. In this paper, we review the physical properties of quick clays in order to find a suitable, integrated and multi-disciplinary approach to improve our possibilities to accurately identify the occurrence of quick clay and map its extent both vertically and laterally. As no single geophysical method yields optimal information, one should combine a variety of geophysical methods with geotechnical data (in situ measurements using Cone Penetration Testing (CPTU), Seismic CPTU (SCPTU) and Resistivity CPTU (RCPTU); laboratory tests) for an in-depth quick-clay assessment at a given site. In this respect, geophysical data are used to fill the gaps between geotechnical boreholes providing ground-truth. Such an integrated and multi-disciplinary approach brings us closer to 2D or pseudo-3D site characterization for quick clays and as such, an improved assessment of the potential hazard they pose. The integrated approach is applied in practice on two Norwegian quick-clay sites. The first site, Hvittingfoss, was remediated against potential landslides in 2008 whereas the second one, Rissa, was the scene of a major quick-clay landslide in 1978, quick clays being still present over a large area. The collected data and preliminary site characterizations illustrate the high diversity as well as the complexity and clearly emphasize the need for higher resolution, careful imaging and calibration of the data in order to accomplish the assessment of a quickclay hazard.

An Integrated Approach to Quick-Clay Mapping Based on Resistivity Measurements and Geotechnical Investigations

Quick clay is highly sensitive, marine clay with an unstable mineral structure due to post-glacial heaving and subsequent leaching of saline pore fluids by surface- and groundwater. Quick-clay layers pose a serious geo-hazard in Scandinavia and North America and need to be delineated in detail. Geophysical methods, especially resistivity methods, have been tested for quick-clay mapping at several sites across Norway. By scrutinizing results from Electric Resistivity Tomography (ERT) and integrating them with geotechnical borehole data including Resistivity Cone Penetrometer Testing (RCPT), we confirm the value of an integrated study for quick-clay hazard zonation. ERT is an ideal tool to interpolate limited borehole results and thus to provide a more cost efficient and detailed result than with borehole data alone. Our resistivity data from ERT, RCPT and lab measurements are generally consistent and appear isotropic. Geochemical analysis confirms that changes in resistivity are directly related to changes in clay salt content and secondarily related to sensitivity. It’s a challenge to resolve small contrast in resistivity (to distinguish unleached from leached clay), in close vicinity to drastic changes in earth resistivity, for example the transition from clay to bedrock. To cope with this we improve results by means of constrained ERT inversion approaches based on drilldata and/or seismic refraction bedrock data. Though ERT is no silver bullet solution to detailed quick-clay mapping, it can provide a significant contribution to improve the risk assessment at comparably lower costs than extensive drilling campaigns. Remaining methodological ambiguities need to be handled by integration with further data.

Towards Joint Inversion/Interpretation for Landslide-prone Areas in Norway - Integrating Geophysics and Geotechnique

Quick clay may be described as highly sensitive marine clay that changes from a relatively stiff condition to a liquid mass when disturbed. Extended quick clay layers account for a lot of geo-hazards in Scandinavia and North-America and hence their occurrence and extent need to be mapped. Geophysical methods have been tested for small scale quick-clay mapping at a research site (Valen) close to Oslo, Norway. By scrutinizing results from Electric Resistivity Tomography (ERT) and Multi-channel Analysis of Surface Waves (MASW) and integrating them with geotechnical borehole data with the help of a resistivity logging tool (RCPTu), we confirm the value for such integrated studies in for quick-clay hazard zonation. Geophysical investigations allow indeed interpolation in between limited borehole results and thus provide a more cost-efficient and extended result than with boreholes alone.

Towards Using AEM for Sensitive Clay Mapping - A Case Study from Norway

An AEM survey has been carried out supporting a highway construction project in Norway in order to obtain information about possible sensitive clay (leached marine clay, also called quick clay). In addition, an ERT profile was acquired along a flight line in an area where geotechnical boreholes indicate a layer of sensitive clay. AEM and ERT data agree very well, indicating only small changes in electrical resistivity throughout the sediment layer. The AEM data show more structures than ERT, demonstrating that resolution and accuracy of AEM are high enough to resolve subtle changes in resistivity charecterizing different clay units. Based on our results, AEM can be used to target zones of possible sensitive clay which can subsequently be investigated in more detail through geotechnical analyses like boreholes and sampling. Thus, using AEM to design the geotechnical program has the potential to provide significant cost reductions for similar projects.

Geophysical Data Integration for Quick-Clay Mapping: The Hvittingfoss Case Study, Norway

Quick-clay landslides are a known hazard in formerly glaciated coastal areas. Some of Norway’s most densely populated areas are located in potential quick-clay zones and, hence, large efforts are devoted to map the distribution of quick clays. Here, we focus on one particular Norwegian site (Hvittingfoss, 100 km south-west of Oslo), which was remediated against potential sliding in 2008. A set of geophysical methods including Electrical Resistivity Tomography, P-wave seismic refraction tomography, S-wave seismic reflection profiling, and Ground Penetrating Radar, were jointly analysed and complemented with laboratory data and in-situ geotechnical measurements (i.e., CPTU, SCPTU and RCPTU) in order to establish a suitable, integrated and multi-disciplinary approach to map the extent of the quick-clay zone. Through careful integration and interpretation of the different data, the main deposits were identified. Both the clay deposit and the overlying sand layer were precisely imaged and their lateral variations were determined. The underlying moraine deposit and the bedrock were also identified, thereby yielding an idea of the preferential leaching paths. Considering the inherent complexity of quick-clay mapping, the collected data illustrate the benefit of an integrated approach, and emphasise the need for high resolution, proper imaging, calibration and ultimately joint inversion of the different data.

Comparison between 2D and 3D ERT inversion for engineering site investigations – a case study from Oslo Harbour

Excavation and piling works related to seafront development in Oslo’s historic harbour area need to mitigate the risk of damaging buried archaeological objects. In the Bjorvika harbour in Oslo, Norway, electrical resistivity tomography was performed to detect structures with potential archaeological value. A 2.5 dataset consisting of four equally spaced parallel lines was collected, trimmed, and systematically processed with both 2D and 3D inversion routines. The results were in good agreement with known underground features, and for the present dataset, an iteratively reweighted least squares 2D inversion was clearly preferable over a 3D inversion. This conclusion is based on differences in model resolution, data processing costs, and the value of the final product for engineering decision-making.

Helicopter Electromagnetic Scanning as a First Step in Regional Quick Clay Mapping

Identification of sediment types and in particular delineation of leached, possibly sensitive marine clays is of crucial importance for geotechnical design of infrastructure projects in Norway. Since leached clays normally have a lower salt content than intact marine clays, the electrical resistivity is consequently higher, and thus clay characterization may be based on data from high-resolution airborne electromagnetics (AEM) collected from helicopter. However, the resistivity difference between leached and unleached clays is small compared to the transition to bedrock and may furthermore vary locally. Therefore, indication of leached clays based on resistivity data has so far been done by manual interpretation. Here, we present a new procedure to calculate the likelihood of possible sensitive clays directly from AEM data. Geotechnical ground investigations are used to locally determine the expected resistivity of sensitive clay. The computation results are compared with well-known quick clay zones. The procedure is not intended as a simple solution to delineate quick clay, but to evaluate an area’s likelihood of sensitive clays that can be used as a cost-saving tool to efficiently place geotechnical investigations.

From Manual to Automatic AEM Bedrock Mapping

ABSTRACT An extensive airborne electromagnetic (AEM) survey was carried out in Norway with the primary purpose to obtain information of depth to bedrock in areas with little or no prior geotechnical knowledge. We present different approaches to extract a bedrock model from the high-resolution time-domain AEM data, including both automated and manual procedures. It was found that in the area of investigation a user-driven approach of manual bedrock picking was the most suitable, taking into account the strongest vertical resistivity gradient and geological information as additional information. A semi-automatic, statistical method, called Localized Smart Interpretation (LSI), is also presented and discussed. This method, while not included in the original bedrock model for the entire area, showed promising results while using less time compared to the fully manual approach. It is recommended that LSI be considered in future projects of similar scope.