In recent years geophysical prospection techniques
in archaeology have become routine for the detection of buried
structures. The development of these survey techniques in the
last half of the 20th century, in particular the advances made
in the last twenty years, and the very recent progress made in
data acquisition and computer imaging procedures have contributed
to this. The purpose of this paper is to provide a brief overview
of the current impact of these techniques on archaeological research
in the Tiber Valley where, as part of the British School at Rome's
Tiber Valley Project directed by Helen Patterson, a variety of
field survey methods are being applied to a range of Roman urban
centres in the area.
The general history of geophysical survey techniques is covered
elsewhere (Clark 1990, 11ff; Scollar et al. 1990, 371; 513). Through
all of these technical developments, the essential purpose of
geophysical prospection in archaeology has remained constant;
to detect and map buried features (Cammarano et al. 1997, 157).
This purpose was appreciated by Ward-Perkins as part of the British
School at Rome's South Etruria Survey in 1960, where resistivity
was carried out by Theodor Schwarz at Veii (Ward-Perkins 1961,
88-90), and has gained credence with the fact that traditional
archaeological excavation, whilst providing the most detailed
information on a particular structure or site, is completely destructive
in nature, and is both costly and time-consuming. Application
of prospection techniques provides a more efficient, less costly
and non-destructive method of exploring archaeological sites.
Whilst the use of any one geophysical prospection technique may
successfully provide the required information on buried structures,
to retrieve as much useful information as possible from a survey
requires set research objectives, and an integrated method of
geophysical prospection (Cammarano et al. 1997, 158; Piro 1998).
Furthermore, proficient application of these methods requires
training and expertise in the execution of the fieldwork, the
processing and interpretation of the data and the eventual dissemination
of the results in report or publication format (Schurr 1997).
Techniques of geophysical prospection are based on the premise
that variations in the soil can be recognised using equipment,
which measures a particular physical property, for instance the
magnetic field produced by buried materials, or resistance to
an electrical current. A number of methods are used in detecting
archaeological remains, from soil conductivity to techniques investigating
the thermal properties of soil. To date, geophysical survey in
the Tiber Valley has focused on the application of magnetometry,
resistivity and ground-penetrating radar (GPR), due to the relative
efficiency and widespread use of these techniques in the detection
of a range of buried features.
Magnetic prospection of soils is based on the measurement of differences
in magnitudes of the earth's magnetic field at points over a specific
area. In the presence of the earth's magnetic field all objects,
due to their magnetic susceptibility, generate their own field.
Variations in the earth's magnetic field associated with archaeological
features can be detected using specific instruments (Gaffney et
al. 1991). Fluxgate gradiometer magnetometers have been used on
most geophysical surveys in the Tiber Valley. They comprise two
fluxgates positioned vertically to one another on a rigid staff
(Scollar et al. 1990, 456; Patella 1991, 79) to reduce the effects
of instrument orientation on readings. When applied in the field,
a series of systematic readings in nanotesla (nT) of variations
in the local magnetic field are recorded across the survey area.
Archaeological features such as brick walls, hearths, kilns and
disturbed building material, as well as more ephemeral changes
in soil consistency, register as positive and negative responses
in the data.
Resistivity survey is based on the ability of sub-surface materials
to conduct an electrical current passed through them. All materials
will allow conduction of an electrical current (Scollar et al.,
307). The differences in the structural and chemical make-up of
soils mean that there are varying degrees of resistance to an
electrical current (Clark 1990, 27). In practical terms, an electrical
current is passed from probes into the earth, to measure variations
in resistance, measured in ohms (W), over a survey area. The apparent
resistivity, the resistance in ohm-metres (W/m) in a given volume
of earth, is then calculated. Higher resistance features are interpreted
as structures which have a limited moisture content, for example
walls, mounds, voids, rubble filled pits, and paved or cobbled
areas. Lower resistance anomalies usually represent buried ditches,
foundation trenches, pits and gullies.
Details of the principles and application of ground-Penetrating
radar (GPR) are explained in a number of recent publications (Conyers
& Goodman 1997; Finzo & Piro 1990; Piro 1998, 332ff.).
In principle a high-frequency electro-magnetic radio wave is passed
into the earth, and the time taken between transmission, reflection
from a buried feature and reception of the responding signal is
measured. Readings are taken at intervals across the survey area,
and at different depths depending on the frequency of the signal.
The profiles are then converted to 3D maps of the present buried
structures.
Although these techniques have been successful in locating buried
structures in almost every instance, some limitations do exist
in their application. In the case of magnetometry, the quality
of results can be badly affected by modern disturbance or infrastructure,
particularly on urban sites. Results can also be restricted by
the magnetic susceptibility of different forms of geology. Different
problems affect the application of resistivity. Primarily it is
not always possible to consistently obtain contact resistance
on certain soils, particularly on hard, compact or arid soil.
The modern topography of a survey area may hinder the placing
of probes, and produce misleading anomalies in the survey results.
Complex stratigraphy, for instance on urban sites, together with
large-scale modern disturbance will produce data that are difficult
to interpret, or results where archaeological features are completely
masked by responses to modern features.
A variety of geophysical prospection methods have been used in
the Tiber Valley to investigate archaeological sites. The work
of Salvatore Piro of the Consiglio Nazionale delle Ricerche (ITABC-CNR)
at Veii utilised magnetometer and georadar survey to locate structures
on the Piazza d'Armi (Cammarano et al. 1997). Since 1997, the
British School at Rome has applied these methods as part of the
Tiber Valley Project, to aid in the investigation of urban centres
whose existence is seen as pivotal to past systems of settlement
and interaction in the region (Patterson & Millett 1998, 7).
Geophysical prospection techniques are being carried out in different
locations in the Tiber Valley (Fig.
1); in the Sabine hills at Forum Novum, in South Etruria at
Falerii Novi and Vignale, at Veii, and at Seripola. The work
undertaken at Falerii Novi and Forum Novum in particular illustrate
the different approaches where prospection techniques have been
used.
The archaeological survey at Falerii Novi, directed by Simon Keay
and Martin Millett of Southampton University, demonstrates the
application of geophysical techniques over a large area of modern
farmland, to establish a plan of the structures and roads of the
ancient town. The survey, covering an area of 28 hectares, successfully
located the town plan within the extant defensive walls, including
the forum, theatre and insula blocks (Fig.
2), and the internal street plan (Keay et al. 2000).
This is in direct contrast to the more specific and integrated
use of prospection techniques at Forum Novum. At this site, modern
buildings are located in the vicinity of the Romanesque church
of Santa Maria in Vescovio. Excavations by the Soprintendenza
di Archeologia in the 1970's uncovered remains of Roman temples
and a forum. However, little was known about the archaeology of
the areas beyond the excavations.
Therefore, a programme of geophysical survey, together with survey
of the topography and standing structures, was initiated, directed
by Helen Patterson of the British School at Rome, Vince Gaffney
of Birmingham University and Paul Roberts of the Museum of London.
Primarily magnetometry and resistivity were applied to extensive
areas around the church and excavations. When compared, the results
showed a number of structures including a villa (Fig. 3), and a residential block associated
with the forum. Slight differences between each set of data added
to the detail of the survey, for example highlighting other rooms
within the buried structure of the villa. The results also located
more ephemeral anomalies possibly associated with structures to
the south of the excavations. Ground Penetrating Radar was then
applied in particular areas by Salvatore Piro of ITABC-CNR, Dean
Goodman of the University of Miami laboratory in Nakajima, Japan,
and Yasushi Nishimura of the Nara Cultural Institute, Japan. This
enabled high resolution location of archaeological structures
at different depths, and across areas where survey with resistivity
or magnetometry would have been impossible, for instance over
the tarmac of a modern car park. These results verified the location
of other structures, including an amphitheatre (Patterson et al.
2000; Gaffney et al. forthcoming).
These examples demonstrate that geophysical survey provides an
important method of investigation in an archaeological context,
which facilitates the location and mapping of buried structures.
The use of prospection methods must be centred on particular objectives,
integrating a number of techniques and utilising the expertise
of archaeologists and geophysicists. The use of such an approach,
as illustrated by its application on urban centres and settlements
in the Tiber Valley, can be of absolute benefit to the archaeologist.
Kris Strutt
The British School at Rome
Via A. Gramsci 61
I-00197 ROMA