GEOPHYSICAL FACT FILE
ELECTROMAGNETIC METHODS 1
INTRODUCTION
Electromagnetic (EM) systems measure ground conductivity by the process of electromagnetic induction. The systems consist of transmitter and receiver coils spaced at standard configurations. The transmitting coil outputs a primary electromagnetic field which in turn induces a secondary field in the ground. The receiving coil measures both the magnitude of the secondary field (quadrature component) and the ratio between the primary and secondary fields (in-phase component). Quadrature fields are proportional to ground conductivity, which is sensitive to bulk changes in lithology, groundwater or ground contamination. Metal produces strong secondary fields making the in-phase component useful for finding buried metal targets.
OPERATION
During survey operations the equipment is suspended above the ground, rather than being in direct ground contact. This mode of operation makes EM surveys very rapid and cost effective in comparison to conventional resistivity surveys. Data is collected as a series of point readings taken at regular intervals along a grid of survey lines positioned over the site area. The spacing between grid-lines and reading stations will depend on the size of the targets under investigation, with smaller targets requiring more densely spaced readings.
APPLICATIONS
- Finds voids & solution features in soil and rock
- Locates former mineshafts, crown holes & subsidence features
- Detects bedrock discontinuities & mineralised veining
- Identifies former landfill site & associated leachate plumes
- Locates buried UST's and chemical waste drums
- Detects saline groundwater incursions & heavy metal concentrations
(C) 2008 SUBSURFACE GEOTECHNICAL