G-858BP Backpack Mag Operators Manual
be large in amplitude and extend for many miles. Usually it is more effective to complete the survey at
night when these noise fields are greatly diminished.
Survey Accuracy
Commercial survey specifications may allow an “off line” error of up to ±20% (or more) of the
line separation. For a magnetic object having an anomaly extending over several lines, this amount of
location error does not prevent the object’s detection but does distort the anomaly’s shape, its peak-to-peak
amplitude and its true location. Large changes in speed along the profile line will have a similar effect but
can be prevented by the use of intermediate waypoints. (The worst case condition would be “off line” by
+20% in one direction and "off line” by -20% in the opposite direction with each line having a 10% change
in speed.) Location errors of this magnitude will not seriously change the overall correctness of the final
map, considering that this type of survey is primarily for detection/location. This is not the case, however,
if the location errors substantially exceed ±20%, e.g., off by one or more line separations. This amount of
error may cause targets of interest to go undetected, or target anomalies to be shifted in location, resulting
in, at worst, an erroneous map or at best an untrustworthy one. Careful layout and accurate tracking along
the line will avoid these problems.
Survey Credibility
How does a client or survey manager know that a survey has been conducted properly and that the
results are correct and believable? He examines the finished contour map for gross errors in data fit,
location of target signatures, and overall map quality.
1) Selected anomalies that have been detected are re-acquired to see if they are in the proper location.
2) The raw data are examined to insure that line numbers are correct, data corrections have been properly
executed, etc.
3) Selected tests are made on the finished data, e.g., a plot of “stacked profiles” to determine that
start/stop points are correct, speed changes are not excessive, there are no data gaps, etc.
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