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The EM option is of course
also active when measuring
3D Projects. The results of
the EM measurements can
be displayed as a Top view,
on the 3D Project 2,5D View
screen when selecting the
option EM Data.
The EM-option is mainly designed for detection of the leakage from AC
power lines. This leakage is highly dependent on how the cable is designed
i.e. a cylindrical twisted cable will have much less leakage than the cables
commonly put into plastic pipes in walls.
The leakage from power lines will result in a leakage field, which is
dependent on the distance between the “hot” cable and the return line as
well as of the current flowing in the lines and the distance from the coil to
the cable. On top of this the measured field is also dependent on the
location of the two lines with respect to the coils in the antenna.
The EM sensor system built into the antenna measures the vertical
amplitude of the leaked magnetic field from the cable.
There are EM-sensors on each side of the antenna and they detect the
magnetic field independently. The results from the left hand sensors are
presented as a red line and the right hand as a green line, see picture
above.
In the 2D operation mode the resulting two signals are plotted, using a
logarithmic scale. When used in the Grid project mode, the two signals are
averaged and plotted in a separate top view.
6.1 Resulting signals/images
The exact shape of the EM signal is difficult to predict since the number of
possible source geometries is so large. In general, though, the maximum
variation in the EM signal occurs around the cable. For a flat horizontal two-
wire cable, e.g. the signal has an almost anti-symmetric shape with a
positive peak on one side of the cable and a negative peak on the other.
These peaks are generally not symmetrically located and their height will
