Relationships of bedrock fracturing to electrical conductivity and anisotropy measurements in a landfill, Victoria B.C.
Date
2003
Authors
Guy, Gordon Harold James
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Abstract
Ground conductivity (EM31) and ground-penetrating radar (GPR) surveys were carried out at Hartland Landfill located just north of Victoria, BC, Canada. The bedrock consists mainly of fractured gneiss overlain by a thin layer (up to 2 m thick) of till (Gartner Lee, 1998a). A ground and surface water monitoring program previously indicated that conductive groundwater is flowing south from the leachate collection and containment systems (Gartner Lee, 1998a). The current study was directed at mapping and characterizing the bedrock fracture system which is expected to control the flow of groundwater.
Vertical-dipole EM31 data were collected every 2 m along E-W oriented lines spaced 10 m apart over an area of approximately 195 x 240 m. Several approximately N-S conductive features about 10 to 20 m in width are visible. The conductivity of these features decreases with distance from the landfill ( 4 mS/m in the north near the Phase 1 landfill to 1 mS/m farther to the south). These N-S conductive features outline the possible direction and extent of conductive groundwater which is flowing away from the landfill. Ground-penetrating radar profiles also show discontinuities near these conductive features, again interpreted as fracturing. In addition, geochemical analyses show higher concentrations of leachate indicators ( e.g., conductivity, chloride, ammonia, and iron) closer to the landfill. However, these concentrations have diminished since 1997.
Vertical-dipole azimuthal conductivity data were collected at a number of stations along pre-existing EM lines by rotating the line joining the transmitter and receiver coil about a vertical axis and taking readings every 15°. Signal-to-noise was improved by using reciprocity and averaging data separated by 180 or 360°. Results indicate that azimuthal conductivity can vary by as much as 30% between maximum and minimum values. The azimuth of maximum conductivity orientation is approximately 11 ° ± 51 °. The range of maximum conductivities lies within the measured bedrock fracture directions in the area (generally NE-SW, SE-NW, and N-S, i.e. between± 45°). The EM azimuthal data can be combined with the traditional EM31 data and with GPR, geochemical, and geological sources of data to demonstrate a relationship between electrical anisotropy and subsurface bedrock fracturing.