Peter O'Connor

Multi-method geophysical mapping of quick clay

Marine clay deposits in coastal, post-submarine areas of Scandinavia and North America may be subjected to quick clay landslides and hence significant efforts are being taken to map their occurrence and extent. The purpose of this paper is to assess the use of a number of geophysical techniques for identifying quick clay. The investigated area, Smorgrav, located in southern Norway has a history of quick clay sliding, the most recent event occurring in 1984. Geophysical techniques that are used include electromagnetic conductivity mapping, electrical resistivity tomography, seismic refraction and multichannel analysis of surface waves. These results are compared to geotechnical data from bore samples, rotary pressure soundings and cone penetration testing. A number of these approaches have proved promising for identifying quick clay, in particular electrical resistivity tomography and electromagnetics, which delineated a zone of quick clay that had previously been confirmed by rotary pressure soundings and sampling. Seismic refraction was useful for determining the sediment distribution as well as for indicating the presence of shallow bedrock whereas the multichannel analysis of surface-waves approach suggested differences between the intact stiffness of quick and unleached clay. It is observed that quick clay investigations using discrete rotary pressure soundings can be significantly enhanced by using, in particular, electrical resistivity tomography profiles to link together the information between test locations, perhaps significantly reducing the need for large numbers of soundings.

Geophysical Mapping of Quick Clay - A Case Study from Smørgrav, Norway

Marine clay deposits in coastal, post-submarine areas of Scandinavia and North America may be subjected to quick clay landslides. Quick clay may be described as highly sensitive marine clay, deposited in a marine environment during the last glaciation. In Norway some of the most densely inhabited areas, such as the areas around Oslo and Trondheim are located in potential quick clay areas and hence significant efforts are being taken to map its occurrence and extent. In this paper Electromagnetic (EM-31), Electrical Resistivity Tomography (ERT) and Multichannel Analysis of Surface Waves (MASW) methods were tested on a site known to contain quick clay. The site under investigation, Smorgrav, has a history of quick clay sliding, the most recent event occurring in 1984. A number of these approaches have proved promising, in particular ERT, which delineated a zone of quick clay that had previously been confirmed by rotary pressure soundings and borings.

Multi-method high resolution geophysical and geotechnical quick clay mapping

Quick clay is highly sensitive, marine clay with an unstable mineral structure due to post glacial heaving and consequent leaching of saline pore fluids by surface- and groundwater. Extended 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 small scale quick clay mapping at a research site close to Oslo, Norway. By scrutinizing results from Electric Resistivity Tomography (ERT) and Controlled Source Radiomagnetotellurics (CSRMT) and integrating them to geotechnical borehole data with the help of a resistivity logging tool (RCPT) we confirm the value of this 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 boreholes alone. Our resistivity data from ERT, RCPT and lab measurements are consistent and appear isotropic.

Geophysical Characterisation of Glacio-fluvial Gravels - A Case Study from Cork, Ireland

Extensive glacio-fluvial deposits occur within Cork city and its surrounds. The glacio-fluvial gravels are of mixed provenance and thickness due to the complex geology of the area. In this paper, Electrical Resistivity Tomography (ERT) and Multichannel Analysis of Surface Waves (MASW) methods were tested simultaneously on a site with existing geotechnical and Standard Penetration Test (SPT) data. The combined use of both methods has potential, with ERT approximating material type recorded in boreholes and shear wave velocities from MASW predominantly corresponding to densities identified by SPT.

Relationship between shear wave velocity and undrained shear strength of Irish glacial tills

A large part of the city of Dublin it is underlain by a glacial deposit known as Dublin boulder clay (DBC). In engineering terms this lodgement till is characterised as being very dense / hard, of very high stiffness and of low permeability. The use of “Geobore S” rotary coring and Multichannel Analysis of Surface Waves (MASW) shear wave velocity profiling has improved our ability over recent years to adequately characterise this material. The main objective of this work was to explore the link between high quality laboratory strength tests on Geobore-S cores and MASW derived shear wave velocity measurements of Dublin boulder clay. A reasonably clear relationship was observed between these tests for three boulder clay sites.

Application of 3D Statics Technology and Seismic Interferometry to Engineering Geophysics

This paper presents the application of 3D statics used in oil exploration industry to Engineering Geophysics, and an attempt to prove the unicity of inverted solutions with an analysis of amplitudes of refracted first arrivals. The principle of one of the most renowned method, the Plus-Minus method (Hagedoorn (1959)) , has been improved by Dereck Palmer (1981) with the General Reciprocal method (GRM). Later on, the same author (reference needed) outlined a problem of non-unicity of inversions which can be supported with the following question; is the interface of a refractor dipping or is the velocity varying laterally? Palmer (2001) proposed a solution through the analysis of refracted amplitudes, with a technique alled Refraction Convolution section, or RCS. Different cases show that the smaller is the amplitude, the higher is the impedance contrast between layers on top and beneath a same refractor. If the amplitude remains constant along a delineated refractor, change in time-depth is related to a change in depth of the refractor. On the other hand, if amplitude varies laterally along a same delineated refractor, the change in time-depth is related to a lateral change in velocity.