Precision and Accuracy of surveys
ENVI-MAG Manual B2-3
Planning
Precision and Accuracy of surveys
Once you have determined the line and station spacing, you should also take into
consideration the sources of noise (both natural or man-made), as well as the
best tuning method. Once you have made a decision regarding the accuracy and
precision of the data you require, then you can consider the method of surveying
best suited to give those results.
Noise
The most commonly encountered sources of noise in the data are the time-based
(diurnal) variations we had discussed in “Variations in the Earth’s magnetic field”
on page B1-7. These are natural variations and can be removed using either base-
station corrections, tie-line corrections or by measuring the vertical gradient of
the magnetic field. The procedures to carry out these correction methods will be
discussed in the next chapter.
Of the man-made sources of noise, or cultural noise, the most prevalent are
electromagnetic and electrical fields. These sources, if strong enough, can
seriously hamper any magnetometer survey. Therefore, surveying directly under
power lines, since they will disturb the magnetic field you are trying to measure,
can be problematic. Other sources of cultural noise in typical waste dumps are
the many buried ferrous objects near the surface. Some of these objects, such as
tin cans, bed springs, appliances, etc., can introduce unwanted magnetic spikes
in the overall results.
As previously mentioned, you may want to remove the natural time-based
variations from your data. You may choose to perform these corrections in the tie-
line mode or base-station mode or perform a gradiometer survey. However, each
of these methods has a cost associated with it. The tie-line method is less accurate
than the base-station, especially if a long period of time has elapsed between tie-
points. The base-station method, on the other hand, requires the use of two
magnetometers and this may not be a feasible alternative in certain cases. The
gradiometer approach is intermediate in cost, as only one unit is required, but
you are compromising your ability to detect deep and subtle anomalies, since
you are only measuring the rate of change of the field.
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