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Electromagnetic geophysical surveys measure ground conductivity by the process of electromagnetic induction. An EM system consists of a transmitter and a receiver coil spaced at a standard configuration. The main systems used for site investigation are the Geonics EM 31, EM 34-3 and EM 38. All of the systems work on similar principles but use different operating frequencies, providing a range of depth penetrations and resolutions for different applications.
Low frequency systems are utilised for investigating subsurface ground conditions, or finding large underground cavities such as caves and mine workings. Intermediate frequency ones are more useful for locating discrete features such as sinkholes or abandoned mineshafts. High frequency systems work best on small targets such as buried tanks, drums and other metal artifacts.
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Data is collected as point readings taken at regular intervals along a survey grid set out over the site area. The spacing between grid-lines and reading stations is dependant upon the target size. Generally smaller targets require closer survey lines and denser spaced readings .
The data readings acquired during sitework operations are recorded on a digital data logger, for later downloading to a PC for post-survey processing and interpretation. The most commonly used interpretation procedure is contouring, carried out with specialist interactive software to produce contour plans.
The contoured data is analysed in detail by our experts to identify anomalous features relative to the general background. Once identified, the anomalies are correlated with local ground conditions. Survey results are presented as plans tied in to site co-ordinates, in a readily understandable engineering compatible format.