G-858BP Backpack Mag Operators Manual
Location of Small Objects within Associated Anomalies
When data has been acquired over an area and processed into map form, it is often necessary to
reacquire the location of each anomaly and dig to expose the ferrous object. The relocation of the anomaly
is relatively simple since the coordinates can be taken directly from the map produced. However, the exact
location of the object within the anomaly is often times difficult to identify and the convoluted magnetic
field can be very confusing.
The anomaly may be sharp and steep (high frequency) indicating a small object close to the
surface. It may be just one peak or it may be multiple peaks depending upon the object’s shape, its
orientation within the earth’s field, and whether the anomaly is largely due to permanent or induced effect.
It may appear as a dipole or monopole, and its shape on the map may be further distorted by the distance
between profiles, especially if this is large with respect to the object size. Each of these factors will also
affect the anomaly signature when the object is much larger in ferrous mass and/or is buried much deeper
with the resultant areal coverage of the anomaly much larger (low frequency).
As one sweeps the G-858BP sensor over these peaks, it is difficult to conceptually grasp their
significance, especially when using the audio output as a reference. To reduce confusion and to provide the
basis for a systematic approach, it is very helpful to produce a 3-D map showing each of the peaks and
valleys with the perspective of depth. Generally, a high and low pair (monopole) will stand out from the
rest of the peaks and if these peaks are relocated using the G-858BP, the object will be midway between
them.
When undertaking this exercise in the field, the audio tone should be turned down or even
disregarded with the visual display of the earth’s field on the front panel becoming the primary focus. By
slowly moving the sensor over the anomalous area, the exact high and low peak can be located and a spot
on the ground marked for each. A dipole, in the earth’s field inclinations of greater than 60° will have its
low North of the high peak, and in horizontal fields (inclination of 0°), the low will be in the center with a
high at the North and the South ends. The point midway between high and low will contain the highest
gradient and will be directly over the object or very close to it. In those cases where there is only one
strong peak, the object will be directly beneath the peak.
For very large anomalies the distance between high and low peaks may be two to ten meters or
even greater. To reduce the amount of digging, it is suggested that a short profile be run completely over
the anomaly, passing directly over each peak previously located and marked on the ground. Viewing this
profile on the G-858BP display will allow the estimation of the point of inflection of the curve between the
peaks indicating the point of maximum Gradient (which should be directly over the object), and the depth
of burial by means of the half width rule. (Refer to chapter V of Applications Manual for Portable
Magnetometers.)
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