A geophysical survey was carried out in March, 1992 in the vicinity of the Town of South Bay,
Ontario.  The  purpose  of  the  survey  was  to  detect  the  presence  of  contaminants  and  abandoned
structures at the South Bay mine site. The work was authorized by M. Kahn of Boojum Research
Limited

Boojum Research Limited

Geomar Geophysics Limited

LU|ZONE|UL

EM SURVEYSOUTH BAY RECLAMATION PROJECTSOUTH BAY, ONTARIOPREPARED FOR:BOOJUM RESEARCH LIMITEDPREPARED BYGEOMAR GEOPHYSICS L-IDMISSISSAUGAONTARIOAPRIL, 19921.0 INTRODUCTIONA geophysical survey was carried out in March, 1992 in the vicinity of Town of South Bay,Ontario. The purpose of the survey was to detect the presence of contaminants and abandonedstructures at the South Bay mine site. The work was authorized by M. Kahn of Boojum ResearchLimited.2.0 SURVEY.DESCRIF’TION2.1 GeneralIn its natural (uncontaminated) state, ground water acts as a relatively poor electricalconductor. However, the presence of inorganic contaminants in the ground water can increase theground water electrical conductivity and thus the electrical conductivity of the saturated soil. ’Generally, ground conductivity depends principally on (McNeil, 1987):0 soil structure (coarser structure and smaller porosity producing lower conductivity)0 clay content (increasing clay fraction) producing higher conductivity0 soil moisture content (increasing moisture producing higher conductivity)0 conductivity of included pore water.The conductivity of a water (electrolyte) is proportional both to the total number of ions in thesolution and their mobility. The mobility is different for different ions since it depends on theirdiameter. Values for some common ions are given in table 1.1Table 1. Mobility of common ions at 25°C(after Keller and Frischknecht, 1966)Ion Mobility(m’/sec V)H+ 36.2 x 10”OH- 20.5 x 10”SO, 8.3 x 10”Na’ 5.2 x 10”Cl 7.9 x 10”K’ 7.6 x 10”NO, 7.4 x 104Lit 4.0 x 10”HCO, 4.6 x 10”It has been shown that for average unconsolidated soil, an increase of approximately 25 ppmof total dissolved solids (IDS) of sodium chloride (NaCl) to soil water will increase the saturatedbulk soil conductivity by 1 mS/m (McNeill, 1987).In order to best delineate the extent of contamination, a method that can resolve relativechanges in the subsoil conductivity caused by increases/decreases in the concentration ofcontaminant in terms of total dissolved solids can be employed. For this reason a Fixed FrequencyElectromagnetic (EM) Profiling technique were employed for this survey. The basic principles ofthe technique are briefly outlined below.2.2 Fixed Freauencv EM ProfilingIn the EM method, eddy current flow is induced in the ground by a time varying magneticfield of a vertical or horizontal magnetic transmitter dipole operating at a fixed frequency. This2eddy current flow induces a secondary magnetic field which, together with the primary field, issensed by a similar receiver dipole. The ratio of the primary field and secondary ~fields is relatedto the conductivity of the subsurface.(The instrument configuration, frequency and coil separation, are selected so that operationcan be described by the low induction number approximation (range in which the true conductivityI is linearly proportional to the apparent conductivity indicated by the instrument, McNeill,  1980)over a relatively large range of terrain conductivities. In this sense, each induced eddy current loopis independent of the others and the measured signal can be thought of as a linear superpositionof the responses of strata within the exploration range of the array used. The effective explorationdepth of the EM equipment can be varied by changing one or more of loop spacing, looporientation (vertical or horizontal), or height above the ground. Figure 3.1 shows relative responsesfor vertical and horizontal dipoles (where z is the depth normalized by intercoil  spacing s). ’Frequency domain EM profiling can also lx used for down-hole geophysical investigations.In this case the tool is lowered down a drill hole. Then, with properly selected parameters of thearrangement (such as, the tool length, frequency, etc.), the measurements will be referred to theconductivity of the subsurface in the vicinity of the plastic-cased (or open) borehole. Theinstrument is essentially insensitive to borehole fluid conductivity. This technique is used in the casewhen the detailed monitoring of the vertical distribution of the formation conductivity is required.2.3 InstrumentationThe Fixed Frequency EM measurements were carried out using Geonics EM31-DL andEM34-3  terrain conductivity meters.3Figure 3.1 Relative responses for vertical and horizontal dipoles.The measurements with EM31-DL  were taken with vertically oriented magnetic dipoles athip IeveL The instrument in this configuration has a depth of penetration up to 6 m. The distancebetween the measurement points along the survey line.5 varied from 1 to 2 m.The approximate exploration depths for EM34-3 instrument at various intercoil spacing aregiven in Table 2. The relative contribution from material at different depths to the conductivityindicated by the instrument meter is discussed in detail io literature (McNeiU, 1980).4Table 2. Approximate Exploration Depths for EM34-3  at various inter&l spacings.Intercoil  Spacing(meters)Exploration Depth (meters)Horizontal Dipoles Vertical Dipoles10 7.5 1520 15 30The measurements with EM34-3 were taken at a distance of 10 m along survey lines. Theintercoil  spacing for the horizontal dipole mode was 10 and 20 m, while for the vertical mode it was20 m.It should be understood that there are other factors, besides the presence of contaminants,which can affect conductivity of the subsurface. The major ones are lithology and water content.Factors affecting the soil conductivity were discussed above, and they are described in detail inliterature (McNeill 1980, and 1987). Those factors were taken into account in the description ofthe survey results.3.0 LOGISTICSThe survey was carried out by a two man crew. Both the crew and equipment weremobilized from Mississauga to Town of Ear Falls by air. Local commercial facilities were used toaccommodate the crew. The crew travelled daily to the mine site by truck.5IIIIIIII!IThree grids were surveyed in six days. Out of those, two days were spent for laying out thegrids, one day for EM31 measurements, and three days for EM34 measurements. The presenceof strong atmospheric noise and the weather conditions precluded the acquisition of EM34 data forall grids.4.0 RESULTSThe results of the survey are present in the form of coloured  maps for each reading andeach grid. A number of areas of anomalous conductivity can be clearly seen on each grid. Theanalysis of readings to different exploration depth shows that the anomalous objects occur as a ruleat shallow  depths. The linear anomaly detected near Confederation Lake at the mine site presentsan exception. The amplitude of the anomaly is the greatest in EM34 10H reading.5.0. CONCLUSIONS AND RECOMMENDATIONSA number of anomalies of increased conductivity were detected at all grids. The anomaliesappear to be in agreement with available information regarding the distribution of contaminants andlocation of abandoned structures.Regarding future studies, the following recommendations can be made:Since many of the anomalies are produced by targets of small lateral extent, tighter surveygrid can be strongly recommended. This would help to outline the extent of anomalies moreaccurately and clarify their nature.6I The technique used is very efficient in mapping lateral variations in the subsurfaceIconductivity. The use of a sounding technique can be recommended for delineating these variationsin section view.IMany drill holes are available at the site. It will be beneficial to perform downholegeophysical measurements. This would allow for accurate determinations of lithology and groundI water properties and provide valuable information for the analysis of surface data.BIBLIOGRAPHY1. G.V. Keller, F.C. Frischknecht (1966) “Electrical Methods in Geophysical Prospecting”.Ch.1. Pergamon Press, N.Y.2. J.D. McNeil (1980) “Electrical Conductivity of Soils and Rocks” Geonics Limited,Technical Note TN-S, 22 p.3. J.D. McNeil (1980) “Electromagnetic Terrain Conductivity Measurements at LowInduction Number”. Geonics Limited, Technical Note TN-6, 15 p.4. J.D. McNeil (1987) “Advances in Electromagnetic Methods for Groundwater Studies”.In: Proc. of Exploration  ‘87, Ontario Geological Survey, Special Volume 3. 678 - 702.7Respectfully submittedPerGeomar  Geophysics LtdJ. Pawlowski,  M.Sc.,PresidemG. &enberg, M.Sc.8Maps: 1A to 1C; 2A to 2C and 3A have no PDF formats in PDF report database. They are paper copy only, which is in Laurentian Library in Sudbury. 

EM survey, South Bay Reclamation Project, South Bay, Ontario.

https://zone.biblio.laurentian.ca/bitstream/10219/3042/1/SB018.pdf

EM survey, South Bay Reclamation Project, South Bay, Ontario.

Abstract

Similar works

Full text

Available Versions

LU|ZONE|UL