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NON DESTRUCTIVE GEOPHYSICAL SURVEYES:
ARCHAEOLOGICAL FEEDBACK
M. Cim inale and D. Gallo
Department of Geology and Geophysics, University of Bari, Campus Universitario 70125 Bari, Italy - marci@geo.uniba.it,
danilo_ 13@libero. it
KEY WORDS: Geophysics, Surveying, High resolution, Mapping, Detection, Anomaly, Feedback, Archaeology
ABSTRACT:
In this work are introduced three different magnetic surveys and a geoelectric one performed in Turkey and in Apulia (southern
Italy). The geophysical prospection was required by the archaeologists to guide excavation programmes and to rescue a site destined
to be partially destroyed. A cesium gradiometer magnetometer and a dipole-dipole array were used for collecting data. Despite the
presence of perturbing contributions all the surveys had a remarkable archaeological feedback that largely satisfied archaeologists’
expectations. At the same time for the geophysicists arose the possibility to provide, in retrospect, complete information of the
sources of signal (geometry, location in the space, composition, physics properties). This allowed to verify under best conditions, the
results of the employed methodologies, the interpretative aspects, the techniques of elaboration and visualization of the data. Some
detected anomalies were critically compared to the corresponding structures brought to the light by the following excavations.
1. INTRODUCTION
In the last 50 years geophysical techniques have been offering a
relevant contribution for archeological research. The possibility
to obtain quick information on buried archaeological structures
in a non-destructive way constitutes a precious advantage for
planning and optimizing excavations in already known sites.
Moreover by opportunely integrating the information obtained
with remotely sensed images, it is possible to offer a
considerable help to the investigation and the location of those
archaeological sites which are partially known and of those
which have not been discovered yet (Ciminale and Ricchetti,
1999). Finally, in many cases, geophysical survey proved to be
essential for the knowledge and the study of those areas of
archaeological interest which were bound to be extensively
modified or even destroyed by modern development.
Figure 1. Location of the surveyed archaeological sites.
The four geophysical surveys discussed below, which were
performed in Turkey and in Apulia (Figure 1), constitute
meaningful examples of the effectiveness of the magnetic and
electric methods in answering archaeologists’ demands. Despite
the difficult environmental conditions it was possible to locate
and recognize the archaeological structures. Digital Image
Processing (DIP) was used to enhance visualization (Mather,
1999) of the high resolution geophysical data making easier
interpretation and identification of archaeological features.
Each survey is presented explaining the archaeological context
in which it was performed and its main purpose. The
characteristics of the recorded signal are analyzed using the
information derived from the excavations. The comparison
between the highlighted anomalies and the recovered structures
has outlined the good correspondence both in their mutual
position and in the likeness of their shapes.
2. THE MAGNETIC AND ELECTRIC SURVEYS
The magnetic and the d.c. electric surveys are among the most
largely employed methods in archaeological research (Scollar et
al„ 1990).
The first one is based on the analysis of the anomalies of the
earth’s magnetic field, produced by the contrast of
magnetization and/or of magnetic susceptibility between the
bodies present in the subsoil (object of the research) and the
material which englobes them. Fundamental aspect in the
application of the magnetic method to the archaeological
research is the high resolution with which the signal is recorded.
Both the techniques of acquisition of data and instruments have
been conceived to allow a survey with a very narrow sampling
step in less time. The modern magnetometers are able to acquire
up to 10 data s’ 1 , with a sensitivity of a picotesla (Becker, 1995).
Therefore it is possible, by using proper field procedures, to
detect also the highest frequencies of the magnetic signal,
improving identification and location of the buried structures.
However, magnetic data are very often affected by noisy
contributions (due to systematic and non-systematic errors),
which may degrade the magnetograms rendering the correct
interpretation of the anomalies uncertain (Eder-Hinterleinter et
al., 1996). It is essential for this reason to restore the real value
and shape of anomalies before the interpretative phase. In
favorable conditions objects of remarkable archaeological
interest such as kilns, hearths, bricks, pottery, iron objects,
etc..., endowed with an elevated remanent magnetization, can
produce strong magnetic anomalies easily identifiable.
With the d.c. electric method it is possible to detect with high
sensitivity slight differences of electric resistivity between
bodies inside the subsoil. The most useful technique for
archaeological aims is a combination of sounding and profiling
which delineates variations in resistivity across the vertical
plane through the direction of the survey. Data can be visualized
as electric pseudosections obtaining an immediate transversal
view of the subsoil. This allows for an easy first estimate of the
