Magnetic targets
ENVI-MAG Manual B1-11
Overview
3. Perform a vertical gradient survey. Since you are measuring the rate of
change between two sensors, any changes in the background field will
apply to both sensors and you will not see any of these noise effects.
This technique is quite effective for near-surface anomalies. Though
only one instrument is required, you will need to have the gradiometer
options installed on it. Further, you will need to staff-mount the sensors
while doing this type of survey in the continuous WALKMAG mode—
depending upon the site, this may not be as convenient as the back-
pack mounted total-field WALKMAG survey mode.
The different correction methods such as the base-station and tie-line
methods are explained extensively in the next chapter of this manual. Most
environmental applications do not require the use of a base station, as the
grids are relatively small and can be covered in a very short time. In this case
tie-point line or loop methods are adequate.
Magnetic targets
What do drums, pipelines and sheet metal look like “magnetically”? Westphalen
and Rice (1992) have shown that a single 55 (U.S.) gallon steel drum buried at a
depth of 3 metres (10 feet) will give rise to an anomaly of 10 nT. Also, Benson et
al., 1982 have calculated that the total field response in nT for different target
distance and mass. This is show in the form of a chart presented in (Figure B-11).
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