CH3 – Overburden Mapping – Southern Manitoba, Canada


This very simple example illustrates that airborne radioactivity surveys can be used to map variation in the types of unconsolidated surficial materials.

GamX CH3 Overburden-Soil Mapping Viden Manitoba

 

 

Figure 1: Reconnaissance Soil Map of the Virden Area, Manitoba1. The area is covered with “black earth soils”  developed on various lacustrine and outwash deposits, as follows:

     Legend

              Loams (fine-medium textured lacustrine deposits)

     Cl       loam, medium textured

     Ccl     clay loam, medium textured

     Bd      clay loam (lacustrine sediments over till)

     Sfsl    fine sandy loam

              Clay

     Hc      clay to silty clay

              Sand

     Slfs    loamy fine sand

 

 

Figure 2: Boundaries between loams, sands and clays are reproduced for clarity. Standing water and swampy areas are also shown, where the measured airborne radioactivity is low due to absorption of the gamma rays by water.

 

 

 

 

 

Figure 3: The above boundaries are overlain onto the airborne total radioactivity map of the same area, expressed as Exposure Rate in micro-Roentgen per hour. Correlation of the surface radioactivity with mapped soil types is striking, with low levels (~3.2 µR/h) over sandy areas, moderate radioactivity (~4 µR/h) over loams and higher values (~5 µR/h +) over clay. Without more definitive information such as the individual K, eU and eTh concentrations determined by a spectrometric gamma ray survey, we can only speculate that soil texture and moisture variations are less likely causes for the variations we see.  Rather, it appears that the observed variations are related to compositional differences. Typically, clays may contain more potassium-bearing phyllosilicates, and their lability supports increased uranium values. Sandy units in general may be arkosic (K-bearing), or simply quartz-dominated and with lower radioactive element concentrations, as in this example. Regardless, it appears than in the study area, unmapped clay soils are predicted by the airborne results in the northwestern part as indicated within the dashed white line boundary labelled “Clay ??”.  If useful to do so, the soil map could be improved using this evidence.

 

 

 

Many other surficial deposit mapping examples exist within Manitoba, where large arkosic sand deposits (kames) have been mapped and distinguished (by their higher K, lower eU) from nearby lacustrine clay deposits (lower K and higher eU)2. Those sand deposits offer economic value as sources of material for road building, construction, and related uses.  By establishing the normal radioactive element signature of known material on the earth’s surface (whether overburden or exposed bedrock) we can predict where those materials should occur, and detect anomalies within those subdomains which may be of special interest.

 

 

  1. Reconnaissance Soil Map of the Virden Area, Manitoba. Compiled from soils information supplied by the Manitoba Soil Survey, Dominion Experimental Farms Service, and the Manitoba Department of Agriculture, co-operating jointly with the Soils Department, University of Manitoba. Original map drawn and published by the Experimental Farms Service, Ottawa, 1956.

 

  1. Shives, R.B.K., 1996: Application of airborne multiparameter geophysical data (gamma ray, magnetometer, VLF-EM) to mapping and exploration in the Rusty Lake and Snow Lake areas; in EXTECH /:A Multidisciplinary Approach to Massive Sulphide Research in the Rusty Lake-Snow Lake Greenstone Belts, Manitoba, ( ed.) G.F. Bonham-Carter, A.G. Galley, and G.E.M. Hall; Geological Survey of Canada, Bulletin 426,p. 279-297, 382-386.