Roald Docter

2D and 3D ground-penetrating radar surveys with a modular system: data processing strategies and results from archaeological field tests

Recently, the use of ground-penetrating radar (GPR) arrays with a large number of antenna elements in a fixed configuration has become more common. The investment needed for these systems is significant. In order to reduce the recording time in the field, an alternative is the use of several single GPR antennas in parallel (a ‘modular system’). Although this does not match the fast acquisition of detailed data sets by means of multi-channel arrays, a system consisting in single antennas can gradually be expanded and investment can be spread over time. This paper presents a 2D and a full-resolution 3D survey, conducted with a modular GPR instrument. A characteristic of these systems is that the cross-line separation between transmitter-receiver pairs is larger than the sampling distance prescribed by the Nyquist theorem. As a consequence, for 3D data collection, profiles have to be acquired between previously recorded ones, which requires high positioning accuracy. A completely identical response for different single GPR antennas is difficult to achieve. For the system tested, on less favourable soils this resulted in striping in the horizontal slices. Several methods (3D frequency-wavenumber filtering, eigenimage filtering, mean profile filtering and filtering based on discrete wavelet transform, discrete ridgelet transform and linear Radon transform) were applied to two data sets exhibiting different kinds of linear noise and their capability to suppress artefacts was assessed. Although overall a reduction of the stripe patterns was achieved, mostly it was impossible to fully eliminate the noise in the time-slices without low-pass filtering in the cross-line direction. For the 2D data, low-pass filtering caused loss of some of the archaeological response and therefore was not applied. Mean profile filtering allowed the most reliable characterization of the archaeological structures.

Ground-penetrating radar survey at the Roman town of Mariana (Corsica), complemented with fluxgate gradiometer data and old and recent excavation results

This paper presents the results of a GPR survey carried out at the Roman town of Mariana (Corsica, France). Excavations (1959-1965 and 2000-2007) yielded a Roman street with houses and shops, an early mediaeval cathedral and a mediaeval bishops palace. When compared with the hypothetical town limits derived from aerial photography, old cadastral maps and the location of two cemeteries, the excavations are in an eccentric position. The principal aim of the geophysical survey was to shed more light on the Early Imperial town centre (1st-2nd century AD), which was to be found further north. The results from a fluxgate gradiometer survey demonstrated the presence of buildings with an orientation corresponding to the excavated street. In the GPR data, an orthogonal street system became noticeable and most of the buildings can be identified as private dwellings. In several parts of the town, there are indications for more than one occupation phase. For example, in the north-western insula of the main survey area, the GPR results show a large building complex with deep foundations, as well as shallow walls of poor construction quality in the courtyard of this building. A trial excavation confirmed this dichotomy: it revealed solid walls with brick facings originating in the 1st or 2nd century AD, as opposed to alignments of loose boulders, not older than the 3rd century. On the basis of the GPR results, two small excavation trenches from the 1930s, one of which contained the remains of a bathhouse, can now be located exactly.

Rural Malta : first results of the joint Belgo-Maltese survey project

The paper presents the first interdisciplinary results of a joint survey project in the northwest of Malta, with finds ranging from the Prehistoric till the Early Modern period. Three permanently inhabited sites were encountered dating to at least the late 6th or early 5th century BCE, with a clearer attestation in the Hellenistic/Roman and Late Antique periods. The resulting reconstructed settlement pattern of the Phoenician/Punic period suggests a managed landscape that seems to be a good reflexion of what is happening in North Africa and elsewhere in the central and western Mediterranean. At least from the Roman period on, these sites seem to have specialised on the production of olive oil.