unterwasser-metalldetektoren uwex® erobern den tauchsport - Secon

occur when – as a result of magnetic rock, stronger
than usual remanent magnetism and/or the fact that
an unexploded ordnance device is located unfavourably
– the latter cannot be detected by purely magnetic
detection methods so that active electromagnetic probing
represents an appropriate alternative.
These interrelationships are also made clear taking
as an example again the survey of the landfill. Fig. 5
shows on the left the result of the frequency electromagnetism
survey with the twin coil frequency electromagnetism
device shown in Fig. 2 over a central part of
the landfill and – on the right – the data from the magnetic
field survey of the same area. It is easy to see that
numerous FEM-detected anomalies correspond with
magnetically-detected anomalies and thus are likely to
represent ferrous metal bodies. However there are also
significant differences and the electromagnetic signature
is overall clearer than the magnetic one. This results
from the very different measurement parameters
(magnetization and electrical conductivity) and could
be interpreted in the sense of differentiation between
materials.
The perhaps most impressive images from the geophysical
survey of the landfill are those obtained with the
transient or pulsed electromagnetic method (TEM, Fig. 6).
However this method is also the most expensive one
in terms of equipment. For better understanding but
expressed in very simplified terms one can say that
the electromagnetic signal transmitted with the pulses
Abb. 6. Aktive Impulssondierung (TEM) in drei ausgewählten Zeit-
fenstern mit der oberen Schleife im Array der Abb. 4. Fläche
wie in Abb. 5. Das Messsystem erlaubt die Registrierung in
insgesamt sechs Zeitfenstern synchron in beiden Schleifen
(unten, schematisch).
Fig. 6. Active pulsed probing (TEM) in three selected time frames with
the upper loop shown in the array in Fig. 4. The area surveyed is
the same as the one surveyed in Fig. 5. The measuring system
permits registering in a total of six time frames synchronously
in both loops (at the bottom, schematically).
wanders downwards (and also laterally) with time so that it gives its „answer“ in the
form of a signal to the reception coil in a continuous manner. Thus in a certain way
the substrate is imaged in respect of a time period of the order of microseconds and
milliseconds. The measurements at two different distances above the surface of the
ground mean that surface factors of influence and deeper areas can be addressed
and the differentiation between the signals from the upper and lower loops (similar
to the differentiation with the Fluxgate gradiometers) comes close to the so-called
vertical gradient which is characterized by a higher resolution. The selection shown
here for just three channels (Fig. 6) makes very clear how much the field structure
changes in successive time frames.
Time-bombs: The problem of historic wastes, that is becoming ever more urgent,
forced geophysicists 20 – 30 years ago to adjust their measurement systems, which
up to this time had been used primarily to look down to much greater depths, to the
new situations requiring investigation. Explosive ordnance disposal methods have
followed this trend only in a very dilatory manner or not at all. This article is intended
to indicate that different procedures are also possible.
Abb. 4. Ausschnitt aus der Deponievermessung mit der Magnetik.
Originaldaten und berechnete Felder spezieller Bearbeitungsprozeduren.
Fig. 4. Section from the landfill survey by magnetic field measurements.
Original data and the fields calculated with special processing methods.
Abb. 5. Ausschnitt aus der Deponievermessung mit der aktiven Frequenz-Elektromagnetik (links) und
ein Vergleich mit der passiven Magnetik auf derselben Fläche.
Hier ist im Vergleich mit den Feldern in Abb. 5 eine nochmals veränderte Filterung und Farbskalierung
gewählt worden, um bestimmte Merkmale dem Auge besser sichtbar zu machen.
Fig. 5. Section from the landfill survey data with the active frequency electromagnetism data (on the
left) and for comparison the passive magnetism data obtained from the same area (on the right).
Here, in contrast to the fields in Fig. 5, the filtering has been changed again and colour-scaling
has been selected in order to make particular characteristics more visible to the human eye.
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