Magnetic surveys are probably the most common geophysical survey used in mineral exploration to help define geologic structure. The sensitivity of modern magnetometers is about +/- 0.01 nanoTesla (nT) which is approximately 1 part in 6 million with respect to the magnetic field strength of the earth. This sensitivity is sufficient to look for very small iron-bearing objects that may be just below the surface. Magnetic surveys are normally a one to two person operation with the surveyors making measurements along pre-established survey lines. The operator must be magnetically clean in the objects he is wearing/carrying or these sources may distort the measurements. Steel toed boots and iron clips on clothing can cause trouble and should be avoided.
The earth’s magnetic field strength is approximately 60,000 nT over much of Canada and is inclined at about 70 degrees from the horizontal. The earths main field is also compressed on the daytime side of the earth by the solar wind. This compression is highly irregular in time and space and causes the magnetic field strength at a fixed location to vary over time. During maxima of the 11-year sun spot cycle this buffeting by the solar wind can be so powerful that man-made electric power-grids experience difficulties. During these storms the local magnetic field can vary 1000’s of nT in minutes. Radio transmission in northern countries is also affected during these solar storms.
Figure 1. This photograph shows a magnetic survey in progress on the side of a lake.
These surveys are light and fast. The modern instruments all use microprocessor technology to automatically record the field strength and the operator grid coordinates and any observations about the local topography.
A base-station magnetometer is often set-up to record the diurnal variation of the earths field at a fixed location at regular time intervals during a survey. When the diurnal variations are removed from the field survey record the resulting map more accurately represents the influence of local geological units. Serious earth science can only really be done using a base station for most Canadian applications. A solar maximum in sun-spot activity is expected in the years 1999 - 2001 and is predicted to be moderately intense.
Figure 2. A base station setup during the winter.
Figure 3. The picture above shows a two-magnetic sensor system separated by a vertical distance of 0.5-m. This vertical gradiometer mode is insensitive to diurnal variations so the survey can be completed without the need of a base station. The vertical magnetic gradiometer mode is very sensitive to nearby iron bearing objects and is often used to screen industrial/brownfield sites prior to remediation.
Figure 4. Mag survey during the winter.
Figure 5. Another magnetic survey during the winter. Winter work on a lake can be very cold due to the wind.