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12.08.2021, 15:30
Archaeology
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The many geophysical techniques available on the market all have their individual strengths and weaknesses. However, the industry stereotypes have meant that Ground Penetrating Radar is somewhat misunderstood.
A study of an Iron Age hillfort in Hampshire, in which KB Surveys was involved, recently revealed a perfect opportunity to display the discrepancies between Magnetometry, EM and GPR surveys over the same area.
Magnetometry surveys have long been held as a fast and effective method for wide area geophysical investigation.
This technology
involves employing a magnetic gradient. This technology is used because
magnetic particles are produced/altered through different processes in nature.
Some of these processes are natural in origin e.g. bush fire, however some of
the processes have human origins e.g. Kiln ovens. Different activity causes
magnetic particles of varying strength to be deposited in the Earth after the
event. If this event is anthropogenic (human) in nature, then it would display
as intense ‘hotspots’ in the ground. Over time, these areas of hotspots wash
away into ditches or pits. Magnetometry can be used to map areas of human
activity as a by-product of heat generated magnetic variance in the soil.
Magnetometry is not, however, effective at mapping archaeological stratigraphy, discriminating between layers or use near the geomagnetic equator without in depth knowledge on how to compensate. It is also not good at discovering wall foundations unless the wall is composed of magnetic materials (Schmidt, et al., 2015).
Magnetometry survey
results are indications of certain features (burning or increased magnetic
responses) but do little to map other features or areas where the geology can
counteract the Iron elements and create negative results e.g. limestone
(Fassbinder, 2017).
Magnetometry also, of course, only works if there has been burning or a change in the magnetic field of certain areas.
Overall, Magnetometry is fast and effective at finding magnetic shifts, but does not produce the same resolution or discrimination as GPR. It cannot be used for small detail or stratigraphy.
GPR is often believed to be
useful only for the location of larger or geological features. However, it is
strangely also used for utilities and smaller targets. GPR is also often
considered complimentary to a survey, after magnetometry or EM surveys have
been conducted first (Schmidt, et al., 2015). Clearly there is a discrepancy
with how it is perceived and GPR’s full capabilities (in the right hands) have
been misunderstood.
EM surveys, otherwise known as Conductivity
Surveys, measure the ability of the soil to conduct an electric current (Clay,
2005). An EM device induces an electromagnetic field through its primary coil
and receives the returning EM field through its secondary coil. A comparison of
the transmitted and received data sets is converted into an EM conductivity map
of the area. As with many geophysical techniques, there are different devices
for different applications, but broadly speaking this technique is another wide
area method which lacks high resolution detail. EM is primarily used in search
of large area features such as landfills, buried metal objects and locating
water fractures deep underground.
Strengths of EM surveys are often considered to include speed of application and coverage, variety of ground conditions including wet and dry, it has the ability of magnetic susceptibility and can somewhat measure the vertical stratigraphy in the ground (Clay, 2005).
However, this technology is not without its
weaknesses. These include a sensitivity to a wide range of metals which could
cause interference, difficulty in depth discrimination, electromagnetic
interference and the technology is becoming less and less competitive compared
to the alternative survey techniques on the market. This is due to increased
technological advances which enhance the performance and capability of the
other techniques, whereas EM is relatively limited in its future development
because it is already at the limit of the technology.
info
Ground Penetrating Radar (GPR) has been employed in geophysical surveys for decades. GPR has the ability to be used in wide area and small-scale applications using a variety of frequencies to produce an array of resolutions, depth penetration and subsurface mapping requirements. The GPR transmits an electromagnetic signal into the ground (or wall) which then receives any reflected signal. The GPR essentially maps subsurface reflectivity of different features, from pipes to coal mines.
GPR can be used in
small spaces for near surface detection, e.g. Rebar, and wide-open areas such
as Archaeological prospection, utility surveys or void detection. This
technology can detect buried features such as pits, walls, voids, rebar,
stratigraphy/layers and much more.
Usually, GPR requires being in
contact with the surface (unless using an air-launched antenna) and certain
materials can cause interference, e.g. clay or metal. GPR is relatively
‘geologically dependent’, meaning that the velocity of the signal transmitted
and received varies greatly on subsurface conditions and mediums. For example,
a very dry concrete medium will induce faster speeds of signal transmission.
However, this velocity can be accounted for and factored into data processing
when being analysed. In fact, it is the features themselves which allow for the
speed calibration in most cases as a hyperbolic speed calibration is used to
detect and adjust data.
Of the techniques used on the site
mentioned at the beginning, it was clear that GPR produced the most detailed
results. Also, the application of the GPR on-site was done through towing
behind an ATV with GPS coupling so the survey was done quickly and accurately.
The data produced features which had never been seen in 25 years of geophysical
investigation on this site, resulting in a greatly improved understanding of
this site and the potential for further study.
This is just one example, but GPR is continually improving and being
developed in a highly competitive market to create incredible results and
capabilities. It is no longer an expensive, slow and inaccurate technology,
quite opposite in fact. GPR has so much versatility in its application that, as
seen here, it out competes the closest of geophysical competitors.
Bibliography:
Clay, R. B., 2005. Conductivity (EM)
Survey: A Survival Manual. [Online]
Available at: https://www.researchgate.net/publication/242118324 Conductivity
EM Survey A Survival Manual [Accessed 05 11 2018].
Fassbinder, J., 2017. Magnetometry for Archaeology. In: Encyclopedia of
Geoarchaeology. Munich: Springer Science and Business Media, pp. 499-514.
Green, F., 2018. Buckland Rings Project Report, Lymington: New Forest
National Park Authority.
Howard, B., 2018. Buckland Rings Archaeological Report, Southampton: KB
GPR Surveys Ltd.
Schmidt, A. R. et al., 2015. EAC Guidelines for the use of Geophysics in
Archaeology: Questions to Ask and Points to Consider. Belgium: Europae
Archaeologia Consilium.
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