Placer Exploration with Geophysics
For placer exploration we have perfected the use of 2D Resistivity. After seven years of intensive measuring practice in the Northwest, Resistivity has proven itself to be a most reliable, powerful method to investigate placer ground. With 2D Resistivity we detect the following subsurface characteristics:
- Bedrock depth and topography: channels, reefs, benches, terraces
- Sedimentary stratigraphy: muck, gravel types, sand, silt, clay
- Permafrost and groundwater table
- Mining history: former pits, settling ponds, technogenic gravels
In the Resistivity model the different underground zones are material-dependently differentiated according to their electrical conductivity. Because Resistivity is not a time domain method (such as Radar and Seismic) and is measuring a material property, the profiles can usually be well interpreted even in ground with disturbing influences such as discontinuous permafrost, large boulders, or clay etc.. In over 90% of our Resistivity profiles which have been proven by drilling, trenching or mining, the bedrock depth interpreted by our team has been confirmed.
In the case of existing drill holes, our Resistivity profiles can be used to convert the drilling information into a two dimensional view. But usually our customers first commission us to do a survey whose results are then verified by drilling or trenching at appropriate spots in the profiles. A few drill holes are usually sufficient to confirm the high information content of a series of 2D profiles along a valley. Furthermore, 2D Resistivity profiles lead to appropriate drilling locations where complications can be avoided and good samples are expected. An auger drill, for example, will be guided to spots with little groundwater or much permafrost.
Paleoplacers, mostly located in former channels, can rapidly be mapped with Magnetics. The measurability of gold placers depends on the ratio of the depth of the placer deposit to the concentration of signal inducing accessory minerals, especially magnetite. The mapping of placer gold deposits with Magnetics usually works fine in depth of 1m to 8m. Therefore this method is only suitable for prospecting in areas with shallow bedrock.
Combination: Resitivity + Magnetics
In unknown areas it might be advisable to first use 2D Resistivity to detect the depth to bedrock as well as potentially placer bearing features such as paleochannels, and clay layers ("false bedrock"). Then it can be checked, if those features produce increased magnetic data. If this is the case, the potential placer gold deposits can be mapped effectively with Magnetics along the valley: The magnetogram indicating the possible course of gold placers along the valley can secondarily be investigated with 2D Resistivity to reveal the depth of the deposits.
In areas where bedrock or "false bedrock" is known to be shallow and associated with magnetite, it makes sense to try Magnetics as primary geophysical method for finding gold placers.