Lieven Verdonck

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.

3-D survey with a modular ground-penetrating radar system at the Roman town Mariana (Corsica)

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. Although gradually expandable modular systems, consisting of antennas which can be used independently, do not match the fast acquisition of detailed datasets by means of multi-channel arrays, they can help finding a compromise between increased acquisition speed and (limited) resources. In modular systems, the separation between transmitter-receiver pairs is often larger than the sampling distance prescribed by the Nyquist theorem. As a consequence, additional profiles have to be recorded in between, which requires a high positioning precision. As a completely identical response for the different antennas in an array is difficult to achieve, stripes can occur in the horizontal slices, especially when ringing occurs. This complicates the interpretation of features in the direction of the survey lines. In this paper, a threedimensional frequency-wavenumber filter is proposed, consisting in a combination of a circular filter and a fan filter. The application of this filter to GPR data collected at the Roman town Mariana (Corsica, France) showed a reduction of the stripe patterns, allowing a more reliable characterization of subtle archaeological structures.

Mariana (Corsica). Integrating GPR in Roman urban survey

In 2000 a new phase of archaeological field activities started on the abandoned city site of Mariana, located south of Bastia (NE-Corsica). Within the on-going international research project “Projet Collectif de Recherche: Mariana et la vallée du Golo” a joint team of the Universities of Cassino (I) and Gent (B) studies the topography of the ancient city in a diachronic framework. In this paper we will investigate the crucial role of GPR prospections integrated in a whole set of more traditional methods of non-invasive city survey and stratigraphic excavations.

Speed control strategy of geophysical measurement platform for archaeological prospection: A conceptual study

Shallow geophysics have become an important tool in archaeological prospection (i.e. the investigation of archaeological remains without recurring to excavations). This paper presents the conceptual study for speed control of geophysical measurement platform for archaeological prospection on challenging terrain using a fuzzy logic approach. A traversal-terrain speed behavior is introduced to have suitable platform movement on the interested terrain regions. The regional traverse-terrain behavior is complemented by following behaviors: traversal terrain behaviors, local obstacle avoiding and geophysical investigating behavior. The fuzzy based speed control strategy has several advantages. Firstly, the fuzzy logic rules that govern the autonomous geophysical measurement platform (AGMP) motion are simpler and more understandable since it can emulate the archaeologist drivers knowledge and experience. Secondly, the behavior-based strategy has a modular structure that can be extended to incorporate new behaviors. The experiments are implemented on a Lego robot to test the proposed approach.

GPR survey to explore social stratification in a pre-Islamic burial area at Mleiha, Sharjah (United Arab Emirates)

This paper presents an archaeological GPR prospection at Mleiha (Sharjah, United Arab Emirates), occupied between the 3rd century BC and the 4th century AD. The aim of the survey was to explore a necropolis near the eastern border of the site. Four parallel 500 MHz GPR antennas were towed behind a four-wheel drive vehicle. Positioning occurred by means of a robotic total station. Processing included de-striping (because of amplitude differences between the channels), migration and topographic correction. For the velocity determination, migration velocity analysis was used. Heavy rainfall during the prospection changed the moisture conditions, so varying wave velocities were used for the time-to-depth conversion. Several square tomb-towers were detected. All have marks that they have been plundered (a hole in the middle of the structure). Some small circular anomalies, initially interpreted as pit burials, after excavation proved to be the result of water absorption by plant roots. In general, the simple burials, located in the marl below a shallow loamy sand layer, were harder to detect by GPR.

Investigating the Impact of Roman Urbanisation on the Landscape of the Potenza Valley

This paper reports on a set of intensive interdisciplinary field operations by a Belgian team of Ghent University in 2007 in the Marche region of central Adriatic Italy. Most of the interventions, comprising geophysical prospections, geomorphologic observations, aerial photography, surface artifact surveys, excavations, topographic surveys and pottery studies, aim at a better understanding of the developing Romanisation of this part of Picenum and the rapid urbanization of the area from the late Republic onwards. Quite spectacular are the results of combined remote sensing work on such towns as the coastal colony Potentia and the interior municipium Trea , with unusually detailed mapping of the majority of public and private town structures. In Potentia these intrasite and peri-urban surveys are now also being checked in the field with focused excavations on a town gate and an amphora workshop. Also important are original contributions towards a better comprehension of the townlandscape nexus, involving the discovery of roads, cemeteries, aqueducts and quarries discovered near the four Roman cities. Finally new observations concerning the pre-Roman situation of centrally organized settlement and its links with the establishment of more Roman style towns, add much to the debate about the relatively late urbanization of this Adriatic region.