Geophysical Survey in Archaeological Field Evaluation - HELM

unit, consisting of a separate transmitter and
receiver will be used, although these may be
enclosed within a common housing. Most midto
high-centre-frequency antennas will also be
shielded to minimise unwanted reflections.
More specialised antenna units designed for
specific requirements such as borehole surveys
or high-frequency air-launched systems for
road pavement analysis are also available. Of
greater interest to archaeological surveyors are
multiple parallel antenna arrays, which allow rapid
acquisition of densely sampled data-sets.
Electronic control unit These units provide
the driving signal to the antenna and sample
the received response at a sufficiently high
frequency. Modern systems digitise the receiver
data directly, enabling detailed post-acquisition
processing. Some units may apply an analogue
gain directly to the signal prior to digitisation,
to improve the discrimination of later reflections,
but it is important to avoid clipping the response
beyond the maximum amplitude value recorded
by the system. Older analogue instruments,
producing only a graphical record of the GPR
traces, are not appropriate for archaeological
surveys because it is not possible to apply
any post-acquisition processing or visualistion
to the data.
Increasingly, GPR systems offer multi-channel
operation where two or more sets of antennas
can be recorded in a near-simultaneous manner.
This might allow a site to be covered with a
range of centre frequencies, imaging both nearsurface
and deeper-lying targets, or a parallel
array of antenna units can be used for the
rapid acquisition of densely sampled data.
Data console The function of the data console
is to set the instrument parameters on the
control unit, to view the receiver output in real
time and to record the digitised data securely.
A laptop computer running suitable control
software can often suffice for this purpose,
using an internal hard disk drive for data storage
and a high speed transport bus to cope with
the large volume of data produced by the
GPR system. Integration with a co-located
GPS receiver or robotic EDM enables the
simultaneous collection of positional and
topographic data (eg Leckebusch 2005).
Power supply GPR systems require a
considerable power supply to function
adequately throughout the working day. This
power is usually supplied from a 12v lead acid
battery but a direct supply may be possible from
a vehicle mounted system. Lead acid batteries
can pose a Health and Safety risk because
of the weight of high power units and from
potential liquid acid leakage. Gel acid batteries
considerably reduce the risk of leakage.
System mounting More recently, integrated
GPR systems have been designed for single
user operation with all of the components
mounted on a compact, collapsible wheeled
cart. These systems are readily portable and
may be deployed on more confined sites
where the absence of trailing cables between
the various subunits can greatly speed the rate
of data acquisition. Transport of the antenna
units may be improved by mounting these in a
sledge with a flexible, plastic skid to ride over
uneven terrain while maintaining good coupling
with the ground surface. A GPR system may
also include an odometer wheel to automatically
trigger the unit at set distance intervals, although
these may require calibration when operated
over sites with uneven terrain.
1.4.3 Methodology
This section considers only the use of impulse
GPR operating in a common offset antenna
configuration. Alternative applications of GPR
are considered in section 1.7 below.
Initial field tests are recommended to confirm
that the equipment is functioning properly, and
that instrument parameters are correctly set.
Antennas of differing centre frequencies should
be trialled to determine an appropriate balance
between resolution and depth of penetration
(Fig 15). Operators should ensure that mobile
telephones and any other RF transmitters
in the immediate vicinity of an impulse GPR
antenna are switched off. The survey may have
to be conducted with more than one centre
frequency of antenna, either because of rapidly
changing site conditions (eg an increasing depth
of overburden) or the need to resolve targets
of differing physical size and depth of burial
(eg on a deeply stratified urban site).
If the instrument trials prove unsuccessful, or
suggest marginal data quality, then the survey
should be aborted at a pre-agreed fee. This
may be unnecessary for small surveys, where
data acquisition is unlikely to exceed a single
day in the field.
The requirement for the survey grid is similar
to other geophysical techniques but operation
on standing buildings may impose special
requirements for recording the position of
the antenna over the face of a wall or ceiling.
Survey transects should, where possible, be
positioned parallel to any surface irregularities,
for example kerb stones, to maintain good
antenna coupling with the ground surface.
Strong radar reflectors (eg metal fences, walls
or vehicles) present at the surface of the site
may produce spurious reflections in the data
caused by uncoupled energy leaking from the
transmitter. This may occur over sites with
uneven terrain where the antennas do not
make good physical contact with the ground
surface. Such air wave anomalies can be
distinguished in the data as characteristicly high
velocity (~0.3m/ns) and of limited attenuation
compared with sub-surface reflectors.
Table 9 Approximate values for the variation of GPR penetration depth and resolution with centre frequency for typical soils, encompassing a
range of values for dielectric constant and soil conductivities. The horizontal resolution will decrease with depth and is given for the maximum
penetration depth assuming a dielectric constant, εr = 15. These values are intended as a guide and may be improved when a more detailed
estimate of the site conditions and target parameters are available.
Centre Frequency Depth penetration Wavelength (λ) in Horizontal resolution –width of Fresnel zone Vertical resolution
(MHz) for typical soils (m) soil εr = 15 (m) at maximum depth (m) λ/4 (m)
1000 ~1.0 0.08 0.2 0.02
500 ~2.0 0.16 0.4 0.04
200 ~3.0 0.39 0.8 0.10
100 ~5.0 0.77 1.4 0.19
50 ~7.0 1.55 2.4 0.39
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