An electromagnetic survey for groundwater exploration in St. George, New Brunswick
University of New Brunswick
An electromagnetic survey was conducted near the Town of St. George, New Brunswick in order to explore the area for a possible location for a production well. The survey was conducted in an area south of Lake Utopia where one production well was already located. The survey was conducted using a Geonics EM34 terrain conductivity metre. The metre was used in two modes for measuring which allowed for at least two different penetration depths along each line of the survey. Different penetration depths allow more or less material underground to be studied depending on the orientation of the coils. A theoretical conductivity of clean sand expected to be measured in the field was calculated as being 5.40 mS/m. The survey generated data that indicated where there are areas of high and low electrical conductivities within the 20 to 30 metres of unconsolidated sediments that overly the bedrock in the study site. Areas of low conductivities indicate good locations for water production wells as they are representative of water-yielding sands and gravels. Contour maps were generated for apparent conductivities measured in the horizontal and vertical dipole mode for coil separations of 10 m and 20 m. Apparent conductivities measured ranged from less than 1 mS/m to approximately 13 mS/m. Areas of low conductivities were found to be between lines 15 and 20 of the survey. Onedimensional forward and inverse modelling were carried out at selected locations in order to determine how true electrical conductivity varied with depth. Modelling results agreed within reason with the measured apparent conductivities and the geological information. An upper layer of fine-grained clayey or silty material was observed as thickening as one heads towards the lake along line 20. This fine material may be blocking sand and gravel pores decreasing porosity and leading to a poor hydraulic connection between the aquifer and the lake in this area. It was recommended to the hydrogeologist that, from a lithological viewpoint, the best location in which to drill for a new production well would be between survey lines 15 and 20.