NEUTRON TOMOGRAPHY AS A VALUABLE TOOL FOR THE NON‐DESTRUCTIVE ANALYSIS OF HISTORICAL BRONZE SCULPTURES

This paper explains, with the example of a Roman object (the ‘Mars from Oberweningen’, Switzerland), why neutrons can be used preferentially for the non‐destructive analysis of metallic objects of relatively large size. This method is superior to the conventional X‐ray tomography method due to the higher penetration ability in metals such as copper, tin and even lead. With this method, differing materials can also easily be differentiated. In this way, the inner and outer structures of objects can be observed with the help of tools based on mathematical algorithms. Slices at arbitrary positions and segmentation of different material clusters aid the understanding of manufacturing processes and can describe the present preservation status. This helps in selecting optimal and additional conservation measures in museums for further preservation. Beside the qualitative overview on structure and inner properties of the objects, quantitative information can be derived about material content and composition. This method and the subsequently needed facilities for this method are available at PSI for similar studies on demand.     

THE WRITING'S IN THE WALL: A REVIEW OF NEW PRELIMINARY APPLICATIONS OF ELECTRICAL RESISTIVITY TOMOGRAPHY WITHIN ARCHAEOLOGY

Natural processes are known to cause significant damage to archaeological monuments. In fact, the key to understanding the decay of building materials is the internal movement of water through the mineral matrix, which influences the distribution of chemical, physical and biological deterioration processes. Electrical resistivity tomography (ERT) was traditionally used as a surveying tool within archaeology, but a new high‐resolution technique that accurately traces the movement of moisture in building materials could provide a vital tool for understanding the decay of many archaeological monuments. This paper considers current progress, the shift of ERT from soil to rock research and the impact that this development could have on future conservation, using Hertford College (Oxford) and Neolithic rock art (Golden Gate Reserve, South Africa) as case studies.     

THE DISCOVERY OF AN ANGLO‐SAXON GRUBENHAUS AT NEW BEWICK,NORTHERN UK USING ELECTRICAL SURVEYING AND PREDICTIVE DECONVOLUTION

Twin‐probe and 33‐fold multiplexed Wenner electrical resistivity surveys were carried out at New Bewick, northern UK to examine the extent of crop marks and potential Grubenhäuser (sunken‐featured buildings, sunken‐floored buildings or SFBs). The twin‐probe method was faster, but provided data with a lower spatial resolution. However, the Wenner array data was affected by characteristic ‘M’‐ or ‘W’‐shaped responses over filled excavations such as those expected to represent a Grubenhaus. The raw Wenner array data have been analysed using one‐dimensional and two‐dimensional predictive deconvolution in order to remove these artefacts. The deconvolution was carried out using an inverse matrix element method. The filtered results indicate the presence of anomalies consistent with the presence of at least six Grubenhäuser and other anomalies concurrent with the linear crop‐marks. One particular anomaly measured about 5 m by 4 m and with a pit depth of 0.6 m below 0.3 m of topsoil. This anomaly was subsequently excavated and a Grubenhaus was discovered at the site. The excavated Grubenhaus measured 4.7 m by 3.9 m with a pit depth of 0.5 m below the base of the topsoil, confirming the electrical survey results.     

A COMPARATIVE ELECTRON MICROPROBE STUDY OF LERNA III AND IV CERAMICS AND LOCAL CLAY-RICH SEDIMENTS*

This study uses electron microprobe analysis of ceramic artefacts and local day-rich sediments and lithics to help address questions concerning the nature of the cultural change implied by ceramics from Lerna III and IV. It redresses the previous lack of a comparative collection of clay-rich sediments that adequately represents the local variation in clay and temper for the area surrounding Lerna. The resulting comparisons indicate that although they produced stylistically different ceramics, potters from both Lenta phases acquired their clays from the local Lerna area.     

AN APPRAISAL OF THE PROBLEMS INVOLVED IN THREE-DIMENSIONAL GROUND PENETRATING RADAR IMAGING OF ARCHAEOLOGICAL FEATURES

There are a number of problems that are due to the physics involved in the ground penetrating radar (GPR) technique which result in the recorded data giving a distorted representation of the true subsurface structure. Many of these problems are reduced by the experience of the person interpreting the data. This paper is an attempt to remove some of the subjectivity from this process by considering how the distortion in the data could be reduced by applying a migration routine. The amount of data, and the time needed to collect it, place a restriction on the use of the migration process to two dimensions only and result in a compromise approach to the use of GPR. This compromise may, however, result in better integration of GPR with the other geophysical techniques available.     

Site Characterization by Geophysical Methods in The Archaeological Zone of Teotihuacan, Mexico

In the present investigation, a geophysical study was carried out in the eastern flank of the Pyramid of the Sun to define potential continuations in this direction of a tunnel discovered beneath the western main entrance of this building. This man-made structure is one of the many extraction tunnels hollowed by the ancient Teotihuacans to obtain construction materials to build their city. Total field and high-resolution vertical gradient magnetic surveys were carried out. The spectral analysis of the total magnetic field enabled us to estimate the thickness of the alluvial cover over the basaltic flow as 3·3 m. It was also found that the main contribution to the observed magnetic field comes from the pyroclastic flow that covers the area of study. The horizontal gradient filter was applied to the low-pass filtered magnetic field to enhance magnetic contacts and structural boundaries. Inferred magnetic trends were related to fracture patterns within the basalts and pyroclasts, and low gradients provided an evidence of voids or tunnels. The Euler deconvolution method was applied as an attempt to confirm the above results. Using a structural index S=0, we determined the boundaries of main magnetic contacts as well as the interface between the basaltic flow and Las Varillas tunnel. Euler depths were found to range between 3 to 6 m, which represent the mean thickness of the basaltic flow.Two parallel ground probing radar (GPR) profiles were surveyed in the NW-SE direction. One passes on top of the known location of a tunnel (Las Varillas) and a second one 10 m to the north, approximately. The tunnel's roof is well outlined at depths between 3·5 and 4 m. On the other hand, little evidences of other buried tunnels or extensions of the known one were found. The second profile depicts a more complex morphology for the pyroclastic sheet. Anomalies related to basaltic flow or eruptive centres are clearly observed. Both profiles depicted the sedimentary base at 3 m, on average. A resistivity profile was undertaken along the first GPR profile. A resistivity image was obtained, that showed the vertical and lateral distribution of the true resistivity. High resistivity values were associated with the tunnel location (Las Varillas). Its geometry could also be inferred, its top is found at about 4 m and extends 20 m in the profile direction to the west. Unfortunately, the depth to its base could not be estimated, since profile length was too short. The sediment-pyroclastic flow interface could also be delimited at a depth of 3 m.Finally, a vertical magnetic field profile taken along the same surveyed line (GPR and resistivity) was inverted applying a two-dimensional algorithm. The initial model was estimated from GPR and resistivity interpretations. A simple model of Las Varillas tunnel was computed, which reasonably well satisfied geological and geophysical considerations.     

INVESTIGATION OF THE CONTENT OF ANCIENT TIBETAN METALLIC BUDDHA STATUES BY MEANS OF NEUTRON IMAGING METHODS

Many important cultural and religious objects from Asia consist of outer metallic shapes, usually bronze, which fully enclose inner contents made of organic materials such as wood, bark, paper, textile, plants and others. Bronze and other metallic materials, such as copper and silver, are generally more transparent to neutrons than to X‐rays. However, organic materials are less transparent to neutrons than to X‐rays and therefore organic materials, enclosed by metallic materials, can be made visible with neutrons. Therefore, neutron imaging (radiography and tomography) was found to be an ideal tool for the inspection of objects that consist of metal outside and organic materials inside. This has been successfully demonstrated here with four metallic Tibetan Buddha statues, providing archaeometry with a powerful new tool. The first successful applications of this novel technique are described in this article. Further possible and useful applications of neutron imaging of cultural objects are outlined.     

DETECTING TRENDS IN THE PREDICTION OF THE BURIED PAST: A REVIEW OF GEOPHYSICAL TECHNIQUES IN ARCHAEOLOGY*

Geophysical survey techniques are a highly visible part of the scientific toolkit that is now used by archaeologists. In this paper, the history of the use of geophysical techniques in archaeology will be discussed, as will significant research themes associated with the most widely used prospecting devices. It is apparent that while the use of geophysical techniques is at an all‐time high, there are many key areas where prospecting is rapidly developing. Some of the advances relate to fundamental aspects of the techniques, while others dictate how we undertake survey in the future. There is a movement away from pre‐gridded survey areas towards real‐time GPS for navigation. This allows greater integration, or fusion, of disparate data sources using visualization techniques derived from associated disciplines. The analysis of landscapes has become a major component of the application of new technology and there are many challenges to be tackled, including how to analyse and interpret significant archaeology within large‐scale, data‐rich, multi‐technique investigations. The reflective nature of the review acknowledges the important role of Archaeometry in the development of archaeological geophysics.     

FIXED‐FREQUENCY RADIO‐WAVE IMAGING OF SUBSURFACE ARCHAEOLOGICAL FEATURES: A MINIMALLY INVASIVE TECHNIQUE FOR STUDYING ARCHAEOLOGICAL SITES*

In memoriam Tony Clark Sadly, this research marked the last field visit to the test site that the authors were able to share with their colleague Tony Clark. His enthusiasm, experience and encouragement inspired us to complete this project that, to many, must have appeared as grown men at play in a sand pit. A new approach to subsurface exploration has been developed, based on fixed‐frequency radio‐wave illumination from a buried transmitter. Data are collected from a buried, continuous wave source with a phase‐coherent surface‐scanning receiver and recorded in a digital archive from which images of the archaeological features are subsequently produced. An important feature of this approach is the opportunity to separate the data collection and archive functions, which form an uncompromised record of the site, from the more subjective image formation function. This paper reports the results of a feasibility assessment programme and discusses the future application of the method to real archaeological sites.