A landslide located on the Quesnel River in British Columbia, Canada is used as a case study to demonstrate the utility of a multi-geophysical approach to subsurface mapping of unstable slopes. Ground penetrating radar (GPR), direct current (DC) resistivity and seismic reflection and refraction surveys were conducted over the landslide and adjacent terrain. Geophysical data were interpreted based on stratigraphic and geomorphologic observations, including the use of digital terrain models (DTMs), and then integrated into a 3-dimensional model. GPR surveys yielded high-resolution data that were correlated with stratigraphic units to a maximum depth of 25 m. DC electrical resistivity offered limited data on specific units but was effective for resolving stratigraphic relationships between units to a maximum depth of 40 m. Seismic surveys were primarily used to obtain unit boundaries up to a depth of >80 m. Surfaces of rupture and separation were successfully identified by GPR and DC electrical resistivity techniques.
Permeability, porosity, formation factor, mercury porosimetry, and stress‐strain measurements were made on 10 shale samples taken at depths between 4500 m and 5600 m in three wells on the Scotian shelf. The purpose was to obtain shale permeability values for quantitative sedimentary basin modeling and to investigate the reasons for the very low permeabilities, less than [Formula: see text] (10 nD), exhibited by many tight shales. Permeabilities of [Formula: see text] ([Formula: see text]) and porosities of 0.9–9.2 percent were measured. The results suggest that the extremely low permeabilities occur because the flow path consists of a network of very tortuous pores (true tortuosity = 3.3) with small diameters, of the order of 8–16 nm. Presence of calcite and dolomite apparently is associated with reduced porosity, possibly a result of blocking of the pores, while kaolinite shows the reverse trend.
Landslides in the Thompson River valley, British Columbia have the
potential to adversely impact vital national railway infrastructure and
operations, the natural environment, cultural heritage features,
communities, public safety and the economy. To better manage geohazard risks
in the primary national transportation corridor, government agencies,
universities and railway industry partners are focusing research efforts on
the Ripley Landslide, 7 km south of Ashcroft. The internal composition and
structure of this very slow-moving landslide as revealed by geophysical
surveys and terrain mapping provides contextual baseline data for
interpreting slope stability monitoring results and guiding geohazard
mitigation efforts. Terrestrial and waterborne geophysical surveys were
undertaken using subsets of the following methods: electrical resistivity
tomography, frequency electromagnetic conductivity, ground penetrating
radar, primary-wave refraction and multispectral analysis of shear-waves,
natural gamma radiation, induction conductivity and magnetic susceptibility.
Small and irregular anomalies, areas of complex subsurface geometry and
groundwater-rich zones are resolved along all terrestrial geophysical survey
lines. Terrain mapping and geophysical surveys indicate a high relief
bedrock sub-surface overlain by a 10 m to >30 m thick package of complex
fine-grained sediments containing groundwater. Planar sub-surface features
revealed in surface exposures, borehole logs and geophysical profiles
include tabular bedding and terrain unit contacts. Profiles also show
discrete curvilinear features interpreted as rotational-translational
failure planes in clay-rich beds in the main body of the slide beneath the
rail ballast and retaining wall. Integrating data from surficial geology
mapping and an array of geophysical methods provided significantly more
information than any one technique on its own.
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