Placer Exploration with Geophysics
For placer exploration we have refined the use of 2D Resistivity. After eleven years of surveying in the Western Canada and Alaska, resistivity has proven to be a reliable method for placer investigation. 2D Resistivity can detect the following subsurface features:
- 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
Resistivity surveys utilize the measurement of a material-dependent property. Resulting models provide insight into zones of specific geologic groupings based on their resistance to electric current. Resistivity is not a time domain method such as Radar and Seismic; therefore, the profiles can usually be well interpreted even in ground with disturbing influences (discontinuous permafrost, large boulders, clay, etc.)
Clients generally commission us to do a survey before drilling, where we provide details on areas of interest. If pre-existing drill holes are present, our resistivity profiles can be used to extend drilling information along a two-dimensional cross section. Once targets are verified, specific resistivity anomalies can be followed up or down valley. Alternately, surveys can rule out sections of the valley lacking evidence of a paleochannel. On virgin ground, a survey can help determine the feasibility of mining to bedrock or information on permafrost presence.
Paleoplacers, generally located in former channels, can be rapidly mapped with Magnetics. The measurability of gold placers depends on the ratio of depth of placer deposit to concentration of signal inducing accessory minerals (especially magnetite). The mapping of placer gold deposits with Magnetics will work at depths of 1m (3ft) to 8m (26 ft), therefore it is a useful method for prospecting in areas with shallow bedrock or shallow pay layers on false bedrock layers.
Combination: Resitivity with Magnetics
In unexplored areas, it is advisable to first use 2D Resistivity to detect bedrock depth and placer features, followed by a magnetic survey to correlate magnetic signatures. If anomalies are linked, potential placer gold deposits can be rapidly mapped along the valley using magnetics. Target areas can be further 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 economic sense to try magnetics as the primary geophysical method for locating gold placers.
Combined 2D Resistivity and Magnetics Survey