Boulder tracing and geological mapping of the bedrockGeophysical surveys
Regional geophysical surveys from airplanes or helicopters provide the foundation for mapping the geology of a large area - often several hundred square kilometres. The results lead prospectors to places with promising geology for mineral discoveries. Geophysical ground surveys are carried out when a promising area for exploration is delineated. Many different characteristics of the ground are measured. The intensity of the magnetic field is measured, and this magnetic field is used, for example, for orientation. This magnetic field reveals how the bedrock is built up, including areas where the bedrock is completely covered with soil. Sometimes the resistivity to the electric current in the ground is measured; this is called a resistivity survey, and many ore minerals have low resistance. Aside from the two exploration methods just named, there are several others to choose between, depending on the information that is sought after.
A work team prepares the geophysical surveys in the interesting areas. The first step is to set out stakes or paper strips, with enough distance from each other so that the survey technicians can easily orient themselves while carrying out the survey. Sometimes the implementation of the grid system requires the removal of trees for better visibility along several important lines that hold up the grid system. However, modern GPS technique has somewhat reduced the need for clearance.
When surveying, one or more surveying technicians enter the terrain, carrying surveying equipment (figure 3). Only one person carrying a magnetometer is required when the magnetic field is measured. The surveying technician walks along a line, which should be as straight as possible - stopping every 5th or 10th metre to take a measurement. When measuring the strength of the electrical conductivity, the surveying group may consist of 3 to 4 people.
The surveying technicians and the geophysicists control the quality of the data during the evening and then send the data via email to a central processing unit where the data is interpreted. From the results of the surveys, the geophysicists can tell how the bedrock is built up and also, sometimes, where mineralisation can be found. These indications of mineralisation are often called geophysical anomalies. The anomalies are typically investigated by excavating or drilling.
Geophysical surveying is a relatively cost effective technique, considering how much information it provides over a large area. The cost depends on the surveying method. The cost of surveying magnetic fields is rather low while surveying methods that are labour-intensive have a higher cost. Surveying the magnetic field within a square kilometre generally costs under SEK 100,000.
Fig 3. Measuring the resistivity to the electrical conductivity in the ground.