Efficient application of the prospection methods requires in the first place an exchange of opinion among the various experts involved. Only thus is it possibile to optimize the prospection results and to garantuee that a preliminary research becomes a time and costs saving element within the archaeological project.
The geotechnical prospection techniques are manyfold. Only some of the most important will be treated here schematically, which are: the method of geoelectrical resistivity, the magnetic method and the georadar method. For each method will be treated: the aims, the working method and the advantages.
The aim of this method is the detection of lithoid elements contained in the soil, voids at shallow depth and sedimentary variations.
The method of electrical resistivity consists in putting up a quadripole on the ground, with electrodes of iron and copper, and in energizing with a current generator. The current flow produces an electric field within the quadripole, with a tension proportional to the electrical resistance of the crossed layers.
This is not an ifficient method for broad surveys. Resistivity surveying should be favoured where strong electrical contrasts are to be foreseen, such as in the case of building foundations and ditches. The advantages of the method are: the simple operativity, the possibility to detect features below the groundwater table and the insensibility to most antropogenic interferences (vibrations, metal masses).
The magnetic method consists in the evaluation of the earth's magnetic field. Since the earth's magnetic field is influenced by the magnetic properties of the rocks, it is possible, through the study of the anomalies in a restricted area, to evaluate the contrasts between magnetically susceptible rocks.
The rock's magnetic properties are related to their mineralogical composition, structure and texture. Volcanic rocks have a high magnetic susceptivity, whereas sedimentary rocks are characterized by moderate values. Very low values are typical of organic sediments.
In an archaeological context, this methods allows a good distinction of infills and especially of metal working areas. Where it's use is possibile, the magnetic method offers a high degree of precision. In urban environments it is practically to be excluded.
The aim of the method is the detection of stratigraphic contacts, buried structures and voids. Georadar uses the reflection of the electromagnetic waves to explore the subsoil. The survey is realized by means of an antenna, emitting very short impulses of electromagnetic energy. When these impulses meet any discontinuity, part of the energy is reflected. The reflected waves are captured on the surface by a measuring instrument that converts them into the electrical impulses to be registered.
The advantages of the method are: great velocity, simple operativity and the production of sections instead of point values. But the penetration is influenced by humidity; it is anyhow impossible to measure below the groundwater table. Operative difficulties are met in urban environments. Moreover, the antenna must scour on a well leveled surface, which may create working problems in the field.
The perforation method allows a highly precise reconstruction of the stratigraphcal column extending below a point (normally to a depth of 10 or 20 meters). The perforation is executed by a machine able to operate a drilling tube. The advantages are: precision of the results and availability of the samples and materials.
Recently, in the area of the Archaeological Park of Trajan, on request of the "Soprintendenza Archeologica" of Ostia, 7 core drillings have been carried out, with the aim to verify the local stratigraphy and to allow the installation of several piezometers for monitoring the water table. The drillings, carried out by Geoplanning, were concentrated in the entrance channels to the Trajan Harbour, nowadays completely dry.
In all drillings, the base level of the roman excavations was detected, sited at an average depth of 7 meters below actual sea level. Considering that in roman times the sea level is supposed to have been 1 meter lower than present, the central parts of the entrance channels must have allowed a ship draught of about 6-7 meters and the marginal parts of at least 5 meters.
In the channel infill, the marine influence was clearly in the borings, in the form of sand with shells and algae.
The final part of the natural infil is gray and loamy, and seems therefore deposited during the Tiber floods, probably from the beginning of the XVIIth century on. The arguments for this thesis are: the correspondence between the fresco of the Trajan Harbour carried in 1582 and the base levels of the loamy sediments encountered in the drillings; the known reclamation of the channel connecting the Trajan area with the Tiber at the beginning of the XVIIth century; and finally, the likeness between the sediments in the drillings and those generally exposed in the Tiber valley.
The aim is to verify the presence of archaeological material or layers in a large area, using a bulldozer to excavate the trenches. Normal depth is about 2 meters.
Important operational aspects are the spatial distribution of the trenches, as well as their direction and depth. There is a tendency to design the lay out in relation to known structures or at regular intervals. However, if trenches are laid out on the basis of other prospection techniques (surface survey) or preliminary studies (aereal photographs, geological or soil study), their effiency may become much higher, with less disturbance of the stratigraphy.
The main advantage is the direct visibility of the local stratigraphy and the materials.
In an area of the Tiber valley (Pt. Galeria), it was possible, on the base of hundreds of trenches, to create not only a detailed (1:5.000) soil map, but also a soil map of the roman surface, now buried below ca. 1 m. of alluvial sediment. Only the soil distribution of roman times turned out to be coherent with the position of the roman acquaduct: along the border of a marshy area.
Antonia Arnoldus-Huyzendveld & Angelo Pellegrino (in print): "Development of the Lower Tiber Valley in historical times"; in Proceedings of the Conference on Geotopes, held in Rome, may 1996.