Geophysical Survey in Archaeological Field Evaluation - HELM

measurements have been used to successfully
determine significant anomalies beneath
surface deposits across wider landscapes
(eg Dalan and Banerjee 1996).
1.7 Other geophysical methods
Despite offering limited use for traditional
applications of archaeological evaluation a wide
range of additional geophysical techniques is
available that may, under certain conditions, be
applicable. Some of the techniques discussed
in this section are highly specific – for example
the use of micro-gravity for the detection of
buried voids – while other techniques propose
new means for obtaining data-sets comparable
with more traditional methods. Most of these
latter techniques are currently at a stage
of development between research and full
commercial deployment, but may well be
adopted as the technology matures in the
near future.
1.7.1 Capacitative arrays
These systems are designed for the rapid
acquisition of apparent resistivity data and use
a series of electrodes mounted on individual
insulating mats that may be towed rapidly
across a site without the need to obtain a
direct contact with the ground surface. The
electric potential produced by the charges
on the electrodes causes the movement of
charged particles in the ground resulting in a
brief capacitative coupling, continuing only until
an equal and opposite reverse potential has
been established in the subsurface. Use of a
sufficiently high frequency source will reverse
the flow of charged particles in the ground,
a) b)
producing an alternating current in the
subsurface. Similar dipolar pairs of insulated
electrodes are then used to measure the
potential created by the ground current.
Multiple potential electrodes can be towed
at different spacings behind the current
electrodes to simultaneously measure the
apparent resistivity at varying depths and can
be inverted to provide a pseudo-section of
the ground surface (see above, section 1.3.4).
However, at higher source frequencies,
attenuation of the signal may prove to be a
limiting factor and the depth of investigation
restricted by the electrical skin depth. Currently,
these instruments seem to be ideally suited to
rapid, large-scale, reconnaissance surveys for
the detection of more deep-lying archaeological
or geomorphological features, but may yet
challenge the quality of traditional earth
resistance data for very near surface targets.
Rough or uneven ground conditions can be
problematic, causing poor coupling between
the insulating electrodes and the subsurface.
1.7.2 Seismic methods
Seismic methods use low energy acoustic
waves transmitted by vibration through the
host medium and can be used effectively in
both marine and terrestrial environments.
Velocities of seismic waves vary from c 200m/s
in soil up to 7000m/s in solid geological units
and, at the frequencies deployed, can result
in relatively long wavelengths, generally >1m.
This restricts the archaeological application
of seismic methods to relatively large-scale
features. For shallow, terrestrial, investigations
Fig 20 (below) Area magnetic susceptibility survey (a)
showing increased response over an area of dense
magnetometer anomalies (b). Low responses to the NE
correlate with recent soil dumping, but some finer detail is
not represented, such as a cemetery (c).
Fig 21 (above) Bartington MS2 magnetic susceptibility meter
in use: (a) in the laboratory with collected 10g soil samples
and; (b) on site with a field coil.
c)
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