Geophysical methods use electric, magnetic, vibration (seismic) or gravity measurements to see what is under the ground.
These methods do not sample the earth, but provide images of it, just like an an X-Ray or ultrasound. They cannot show where gold is directly, but are used together with other observations to help guide sampling and drilling, with the aim of increasing the likelihood of success.
Geophysical surveys are run by professional Geophysicists. Only large mining companies employ full time Geophysicists. Junior companies rely on contractors to complete surveys as they are needed.
When are geophysics used?
To illustrate the concept, imagine a flat featureless desert. Within this desert, a gold deposit has already been found. The rest of the area is covered in wind blown sand, so surface sampling won’t give an indication of what lies beneath it. The exploration company decides to try a geophysical survey, using airborne magnetics. Magnetics are chosen over other methods because the known gold deposit is associated with rocks that have a strong magnetic signal. The company knows the magnetic signal of the gold-related rock because it used a handheld magnetometer on the deposit itself. In this scenario, a magnetic survey is done over the known gold deposit and the surround sand-covered areas. On viewing the completed survey, the known gold deposit would appear as a clear feature, for example an elongated shape matching the shape of the orebody. Any areas under the sand that showed a similar response would be assumed to be additional potential gold deposits, and tested with drilling.
Geophysics can be used at all stages of exploration. At the early regional scale, broad surveys over 100’s of square kilometers can be done using aircraft either manned or, increasingly, unmanned. The results would be used to help prioritise prospecting and surface sampling areas for first-pass work. At the prospect scale of several square kilometers, man-portable equipment provides more detailed data that can be used to help with drill targeting.
At all scales it is important that geologists view geophysical results of any survey in context. For example, a survey method may be very useful in one ground type but useless in another, owing the to the way different rock types relate in each area. This requires the geologist to interpret all surveys on a case by case basis. Also important is the geophysicist’s method of data processing. Algorithms are used to filter unwanted signals but can dramatically change what is finally seen, depending on the method used.
How are Geophysical surveys performed?
Geophysical surveys can be carried out from aircraft, for when large areas need to be covered, or by foot, when more detailed and larger scale surveys are required.
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