=Paper=
{{Paper
|id=None
|storemode=property
|title=Geophysical Prospection: a Powerful Non-destructive Research Method for the Detection,
Mapping and Preservation of Monuments and Sites
|pdfUrl=https://ceur-ws.org/Vol-806/paper1.pdf
|volume=Vol-806
|dblpUrl=https://dblp.org/rec/conf/aquileia/Fassbinder11
}}
==Geophysical Prospection: a Powerful Non-destructive Research Method for the Detection,
Mapping and Preservation of Monuments and Sites==
https://ceur-ws.org/Vol-806/paper1.pdf
B-1
Geophysical Prospection: a Powerful Non-destructive
Research Method for the Detection, Mapping and
Preservation of Monuments and Sites
Jörg W.E. Fassbinder
Bavarian State Department of Monuments and Sites, Munich, Germany
joerg.fassbinder@blfd.bayern.de
Abstract. Geophysical Prospection has yielded remarkable results to the research in
Archaeology. Besides the recognition of places, also the earlier uses of a site can be
reconstructed. Therefore, the geophysical techniques help to understand the layout of a site and
raise new research questions. In this paper, we summarize the main geophysical prospection
methods for archaeology: magnetic, radar and resistivity-based techniques. We also present
results of combined methods that have been employed for the survey on the Bavarian sites of
the “Obergermanisch-raetischer Limes”, frontiers of the Roman empire.
Keywords: Geophysics, Archaeology, Bavarian Limes, Remote Sensing,
Magnetometry.
1 Introduction
Geophysical science offers a large range of prospecting methods that were adapted to
the detection and description of archaeological structures beneath the soil.
Three main methods are nowadays very popular and widely used for non-
destructive archaeological surveys. Magnetometry, resistivity and radar prospecting
allow the creation of precise and detailed, but nevertheless large scale maps of all
structures that are hidden beneath the ground. The choice of technique to be used in
search of the buried archaeology depends very much on the particular situation and on
the different landscapes that have to be surveyed.
Magnetometry is one of the most successful methods for inexpensive but detailed
survey on large sites. On nearly all archaeological sites magnetometry can
successfully detect structures, regardless if there are stone walls in the adjacent soil,
soil marks of wooden structures, fireplaces, pits, ditches or traces of wooden
palisades. However, magnetometry is a passive method, and whenever there are some
technical constructions of iron or electric power lines nearby, the application of
magnetic techniques is highly disturbed or utterly impossible. In these cases active
prospecting methods can overcome such problems and resistivity and radar
prospecting can be more than supplementary methods. Where stone structures are
present, the prospecting results of radar and resistivity will definitely surpass the
quality of the magnetometer data.
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2 Geophysical prospection
It is already more than 50 years ago, when resistivity and magnetic prospection were
first applied to detect archaeological features beneath the ground (Atkinson, 1953;
Aitken et al. 1958; Clark, 1996; Aspinal et al. 2008; Conyers, 2004).
The first results of such methods were displayed as a simple profile plot, or a
contour map. A milestone in the development of geophysical data processing was the
introduction of digital image processing techniques developed by Scollar and
Krückeberg (1966). Since then, the application of geophysical methods and their
interpretation was no longer restricted to geophysicists, but could be understood by
non-specialists and archaeologists (Fassbinder & Gorka, 2009).
Today’s most common geophysical prospecting techniques are:
Magnetometry 2 – 4 ha per day, sampling rate 25x50 cm;
Resistivity 1/4 – 1 ha per day, sampling rate 50x50 cm;
Radar 1/4 – 1 ha per day, sampling rate 2x25 cm.
Depending on the spatial resolution such methods can cover large areas. However, the
processing and interpretation of these data exceeds 10 or 20 times the time required
for data collection.
The following special applications are only rarely applied for archaeological
purposes:
Magnetic susceptibility, e.g. to discriminate archaeological layers;
Electromagnetic methods used for metal detection;
Sonar prospecting, e.g. for underwater archaeology;
Seismic methods, for the detection of layers in great depth;
Gravity prospection, for the search of cavities;
Thermal prospecting, for the detection of heat flows.
2.1 Magnetic prospection
Magnetic prospection is a passive geophysical method, which measures the anomalies
of the Earth’s magnetic field, based on the:
- Enrichment of magnetic minerals in the top soil and archaeological structures;
- Natural Remanent (NRM) and ThermoRemanent (TRM) magnetization of soil and
archaeological features.
The signal is a magnetogram, with the relative intensity of the magnetic anomaly as a
plan-view image, typically in grey-shade.
Magnetic prospection is the most widespread, successful and cheap method in
Archaeology. The origin of magnetic anomalies on archaeological sites can be
ascribed to two reasons. First, the enrichment of ferrimagnetic minerals such as
Geophysical Prospection… B-3
magnetite, maghemite and greigite in top soils, and hence in all archaeological layers.
Such enrichment occurs worldwide in almost all soils by pedogenic or biological
processes, as well as the use of fire through ancient settlement activities (Le Borgne,
1955; Fassbinder, et al. 1990). Second, a further part of the magnetic anomalies is due
to the natural remanent (NRM) or thermoremanent magnetization (TRM) of soils,
sediments or archaeological layers (Le Borgne, 1960; Fassbinder & Stanjek, 1993).
Understanding the magnetic properties of minerals in the soil and rock, sometimes
allow us a very detailed and fundamental interpretation of the different archaeological
features.
2.2 Radar prospecting
Radar is an active geophysical method, which is based on the transmission and
reflection of electromagnetic waves in the ground. Frequencies typically range from
10 MHz – 2 GHz; 400 MHz is most suitable for archaeological prospecting. Runtime
is proportional to the penetration depth of the archaeological structure. The signal
output is a radargram consisting of reflected signal amplitude versus runtime, forming
a section view in grey-shade.
2.3 Resistivity prospecting
This is an active geophysical method, which measures the apparent resistivity of soil.
It is based on the contrast of electric resistivity between the soil, the sediment and the
rock. Penetration depth depends on the distance between the electrodes. The signal is
a resistogram, consisting of a grey-shade image of the apparent electric resistivity.
3 Case history: the “Obergermanisch-raetischer Limes” in Bavaria
by geophysical prospection
The Roman Limes with a length of 550 km is the largest archaeological site in Europe
as well as the largest monument of the Roman period (Fig. 1). In July 2005 it was
decided that the Limes and its interrelated archaeological sites, together with
Hadrian’s Wall in England, would be a component of a "Trans-National World
Heritage Site" taking the name "Frontiers of the Roman Empire". From that point, it
was necessary to minimize or avoid archaeological excavations. Further research is
therefore limited to the application of non-destructive techniques (Fassbinder, 2010)
and geophysical prospecting methods turned out to be highly suitable. Four examples
that allowed us the verification and completion of old maps of the Reichs-Limes-
Kommission will be shown here; these projects exemplify the potential of geophysical
prospecting on the Bavarian Limes.
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Fig. 1. Geographic map of the Obergermanisch-raetischer Limes.
3.1 The fort of Theilenhofen
At Theilenhofen we were able to complete the map of the fort with all fortification
ditches and the water supply; to verify the troop level and confirm the former fort on
which are superimposed the traces of the Roman vicus (Fig. 2, Fig. 3).
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Fig. 2. Magnetogram of the Roman fort of Theilenhofen. The ground plan of the Roman fort
constructed on stone foundations (right-hand side). The older fort consisting of wooden
barracks, which were later overbuilt by the civil settlement (left-hand side). Dynamics +/- 20
Nanotesla, in 256 grey levels; grid size 40x40 meters, sampling rate interpolated to 25 x 25 cm.
Fig. 3. Theilenhofen. Archaeological map of the magnetometer data, combined with the
excavation data by the Reichs-Limes Kommission. AutoCAD-Map 6930/006.
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3.2 The fort of Oberhochstatt
The fort is situated about 500 m N of the village, at the border of the Jura platform. In
spite of the early reports in 1833 of a Roman fort, almost nothing was known until
recently. The archaeological community first knew of the location in 1897, when a
farmer reported four bronze letters found on the field. In 1920 the
“Streckenkommissar” F.Winkelmann made test excavations on the site the letters had
been found. Unfortunately, the trench was made in such an awkward way, that only
an empty foundation ditch was found that possibly had contained an earlier wall. The
construction of the highway 2228 in 1979/80 went directly through the centre of the
fort, but it was never studied by the archaeologists, although farmers again reported
seeing structural remains. After the misinterpretation of an aerial photo of 1983, the
fort seemed to be located about 200 m from the highway. Finally, the compelling
evidence of the location of the fort given by the geophysical prospection performed in
2009.
Fig. 4. Oberhochstatt. Aerial photo of the site combined with magnetic prospections; bottom is
North. Archive No. 6932/119-1Ds09253, photo K. Leidorf, 19.08.2008.
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3.3 The Roman camp of Eining-Unterfeld
Radar prospection. The results allowed the relief of structures down to three meters
underground (Fig.5).
Fig.5. Eining-Unterfeld. Aerial photo of the site, combined with magnetometer and radar
measurements. Archive No. 7136/075b-10i-18, photo O. Braasch.
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3.4 The fort of Ruffenhofen
The resistivity survey and its potential for archaeological investigations are
documented by the Roman fort of Ruffenhofen (Fig. 6). The results show a map of all
the constructions that were made of stone. Even some parts of the former stone wall,
that tumbled down into the fortification ditch, are visible. However, these findings
also make clear that, besides the stone structures, no other traces become visible by
resistivity measurements alone.
Fig. 6. Ruffenhofen. Aerial photo of the site combined with the resistivity measurements.
Archive 6928/074-8941-3, photo K.Leidorf, resistogram H.Becker.
4 Conclusions
Geophysical prospection for archaeology delivers not only information about wooden
and stone structures, ditches and pits, but, moreover, results in detailed archaeological
maps. Besides the tracing and recognition of places, the former use of an
archaeological site can be reconstructed. Therefore, the geophysical results not only
help to understand the layout of a site, but also raise new research questions. In 2005
the Roman Limes and all associated constructions were declared a UNESCO World
Heritage site. Therefore, archaeological excavation is no longer a suitable tool for
research on these archaeological sites and monuments. Geophysical prospecting,
combined with aerial archaeology and airborne laser scanning methods, remain the
only effective and non-destructive techniques to recover, understand and pursue
archaeological research beneath the ground.
Geophysical Prospection… B-9
References
1. Aitken, M.J. Webster, G. Rees, A.I., Magnetic prospecting. Antiquity, 32, pp.270-
271 (1958).
2. Aspinall, A. Gaffney, C. Schmidt, A., Magnetometry for archaeologists. Altamira
Press, Lanham (2008).
3. Atkinson, R.C.J. , Field archaeology. 2nd ed. Methuen, London (1953).
4. Clark, A.J., Seeing beneath the soil 2nd ed. Batsford, London (1996).
5. Conyers L.B., Ground penetrating radar for archaeology. Altamira Press, New
York (2004).
6. Fassbinder, J.W.E. Stanjek, H. Vali, H., Occurrence of magnetic bacteria in soil.
Nature, 343, pp. 161-163 (1990).
7. Fassbinder, J.W.E. Stanjek, H., Occurrence of magnetic bacteria in archaeological
soils. Archaeologia Polonia 31, pp.117-128 (1993).
8. Fassbinder, J.W.E. Gorka, T., Beneath the desert soil - archaeological prospecting
with Caesium magnetometer. In: New technologies for archaeology. Multidisciplinary
investigations in Palpa and Nasca, Peru. Reindel, M. Wagner,G.A. (eds), pp.49-69,
Springer, Berlin-Heidelberg DOI 10.1007/978-3-540-87438-9 (2009).
9. Fassbinder, J.W.E., Geophysical prospection of the frontiers of the Roman Empire
in Southern Germany, UNESCO World Heritage site. Archaeological Prospection, 17,
pp. 129-139 (2010).
10. Le Borgne, E., Susceptibilité magnetique anormale du sol superficial. Annales de
Géophysique 11, pp. 399-419 (1955).
11. Le Borgne, E., Influence du feu sur les propriétés magnétique du sol et sur celles
du schiste et du granit. Annales de Géophysique 16, pp. 159-195 (1960).
12. Scollar, I. Krückeberg, F., Computer treatment of magnetic measurements from
archaeological sites. Archaeometry, 9, pp. 61-71 (1966).