Geoscience Reference
In-Depth Information
fIGURe 8.2
Example of the “despiking” process to remove a large magnetic field anomaly shown in top plot,
thereby enhancing some of the more subtle magnetic field features that are present as shown in bottom plot.
Additional defects needing attention are drift, edge matching, striping, positional errors, and
periodicity due to walking gait. Drift is caused by thermal changes within the sensor and appears
as a slow temporal increase or decrease of the baseline magnetic field. Edge-matching is the pro-
cess of matching the magnetic baseline at the end of one survey unit to the beginning of the next.
Because there is some time delay between surveying each unit, the magnetic baseline changes due
to natural fluctuations caused by the Sun (Figure 8.3). Both of these defects can be removed in the
field by surveying in gradient mode or eliminated postsurvey using software. Striping appears dur-
ing bidirectional surveys with every other transect having a slightly higher or lower magnetic base-
line when compared to its neighbor (Figure 8.3). This defect becomes large if the magnetometer
operator accidentally carries ferrous metals on their person. As the operator walks in one direction,
the magnetometer is located in front of the operator and sits in one end of the operator's magnetic
dipole. (The operator will have a magnetic field similar to a bar magnet.) When the operator turns
around, his or her dipole does not rotate with him or her due to the operator's magnetic field being
induced by and aligned with the direction of Earth's magnetic field. As the operator walks in the
opposite direction, the magnetometer sits in the other side of his or her magnetic dipole. Because
the Earth's magnetic field is not parallel with the ground surface, the magnetometer will record a
different magnetic baseline when in the operator's north pole compared to the south pole. Positional
errors arise due to a range of sources such as changing walking speed, reaction time, and computer
timing. These errors give the data a zig-zag look. Periodicity is caused by the natural up and down
motion when walking and appears as highs and lows in the magnetic baseline on a regular interval
related to the operator's stride (Gaffney and Gater, 2003; Scollar et al., 1990).
