Archaeologia Islandica - 01.01.2002, Page 31

Geophysical Techniques
addition to the more clearly defmed stone
features. Both these sets of evidence have
then been used to confirm the results of
the gradiometer survey for the detection
of structural remains, and demonstrate
the potential for this technique.
At this stage in the development of an
understanding of such methods in
Iceland, the results from Hofstaðir
demonstrate the necessity for both posi-
tive and negative geophysical results to
be backed up by trial excavation.
Discussion of results
Earth resistance
Earth resistance has been shown to posi-
tively identify buried features where no
surface indications remain, and to be
applicable in many situations although
some limitations caused by periglacial
phenomenon and wet ground conditions
need further investigation. The main
advantage of this technique over the flux-
gate gradiometer is that it is not severely
overwhelmed by geological effects.
At Skálholt and Hofstaðir this method
has been used to detect and correctly
identify known archaeological remains,
where the precise positions of buried fea-
tures was unknown. In addition, the
results from Gásir and other sites not
reported here demonstrate the potential
for this technique as an aid in the inter-
pretation of known features, where the
visible surface remains may be amor-
phous and unclear.
A problem of surveying on wet
ground was observed in the earlier sur-
veys, most likely due to 'geometric
effects', although it is not entirely under-
stood yet. Once this effect had been
recognised, earth resistance surveys were
conducted when the conditions were dry,
and successfully avoided these survey
defects.
Periglacial effects caused other limi-
tations, where a number of unusual
anomalies were recorded. Most influen-
tial were surface deformations such as
thufur. These produce intense localised
anomalies and create a noisy background
against which anomalies due to buried
features can become less clear.
The fíne sampling interval adopted in
Iceland, 0.5m x 0.5m, has allowed a high
level of detail to be recorded, however it
can be seen that a coarser interval of
l.Om x l.Om can still be used to success-
fully locate sites. This choice will depend
upon the aims of the survey, as well at
time constraints.
Magnetometry
Although in many cases the magnetome-
ter data is overwhelmed by the intense
background geological signal, it has been
demonstrated that it is often possible to
distinguish discrete rocks in an archaeo-
logical deposit from bedrock, even where
these natural anomalies dominate the
data.
At Gásir it is possible to identify
anomalies attributed to buried stone
walls, despite the near-surface igneous
geology producing strong thermorema-
nent effects. There is also a distinguish-
able variation produced by the booth
earthworks that might allow the extent of
the archaeological remains at the site to
be defíned. However, the much deeper
aeolian deposits overlying the bedrock at
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