Archaeological Geophysics II
Presiding: T F Tchakirides, Department of Earth and Atmospheric Sciences, Cornell University; B B Ellwood, Department of Geology and Geophysics, Louisiana State University
NS32A-01 INVITED 10:30h
Integrated Geophysical Methods for Archaeological Prospections
Ground surface geophysical prospecting methods have been used as valuable techniques for non-invasive detection of shallow-depth bodies of archaeological interest. Generally the often limited size and depth of the archaeological remains, the presence of structures made with the same material as the host rocks, soil inhomogeneity and the environmental and anthropogenic disturbances, make it rather difficult to define the position and the extension because of the low signal-to-noise ratio (S/N). Multi-methodological surveys have been employed with the aim of detecting either sharp discontinuities (boundary of the cavity, fractures in the medium, etc) or volumetric variations (bodies with different physical properties), at different archaeological test sites. For the surveys a combination of passive and active methods (magnetic, GPR and dipole-dipole geoelectric method) has been employed. The aim of the present work is to demonstrate the advantages of integrating the results obtained using different geophysical methods. The Fluxgate Differential Magnetic (FDM), the Ground Penetrating Radar (GPR) and the Dipole-dipole Geoelectric (DDG) methods and their results, used to investigate different archaeological sites, have been integrated. With all methods a high resolution data acquisition method has been adopted with the aim of reconstructing a global vision of the investigated area. The enhancement of the processing technique towards the integration of different geophysical methods, enabling one better to define the location, depth and geometry of any archaeological bodies, has been adopted.
NS32A-02 10:50h
Assessing the Potential of Archaeological Prospection Techniques in Iceland
Since 1999 research has been undertaken to assess the potential of archaeological prospection techniques in Iceland, where a particular set of geomorphological and archaeological challenges are present. In addition to the intense thermoremanent effects of the igneous geology, other limiting factors include numerous tephra deposits, periglacial phenomena (including frost hummocks and stone polygons), and regions of active soil erosion or sand deposition. The nature of the archaeological remains themselves provides further difficulties. Up until the beginning of the 20th century most structures were largely built of turf, and once collapsed and buried only provide slight features for detection. This investigation has required the development of a methodology for assessing the use of archaeological prospection methods in a new environment. Some fifty high-resolution magnetometer and earth resistance surveys have been systematically conducted throughout Iceland to encompass the range of archaeological features and a variety of geological and geomorphological situations. The results of these surveys have been integrated with other sources of archaeological evidence to allow a proper assessment of their success for not only locating buried remains but also for their characterization and interpretation. Despite the limiting factors, the results demonstrate the potential of these techniques for archaeological prospection in many parts of the country, especially when undertaken as part of an integrated approach. New and previously known structures have been detected with both techniques, including those constructed of turf and stone or entirely from turf, and dating from the earliest Viking remains to recently abandoned farm sites. This work also highlights the benefits of undertaking a thorough evaluation of geophysical methods in new environments before they are routinely applied. Fieldwork methodologies may be refined and novel approaches to data processing and displaying can be investigated to maximize the archaeological interpretations that may be drawn. This research has been undertaken as a NERC-funded Doctoral Research project at the University of Bradford, UK, in collaboration with the Institute of Archaeology, Iceland (FSI) and the North Atlantic Biocultural Organization (NABO).
NS32A-03 11:05h
Geophysical investigation of the June 6, 1944 D-Day invasion site at Pointe du Hoc, Normandy, France
A near-surface geophysical survey at the D-Day invasion site atop the cliffs at Pointe du Hoc, Normandy, France was carried out using ground-penetrating radar, electromagnetic induction, and magnetic gradiometry equipment. The subsurface targets of investigation are predominantly buried concrete and steel structures and earthworks associated with the German coastal fortifications at this stronpoint of Hitler's Atlantic Wall. The targets are readily detectable embedded within the vadose zone of a weakly magnetic, electrically resistive loess soil cover. The radar and electromagnetic induction responses lend themselves to plan-view imaging of the subsurface, while the magnetics data reveal the presence of buried magnetic bodies in a more subtle fashion. Several intriguing geophysical signatures were discovered, including what may be the buried remains of a railway turntable, ordnance fragments in the bomb craters, a buried steel-reinforced concrete trench, and a linear chain of machine gun firing positins. Geophysical prospecting is shown to be a very powerful tool for historical battlefield characterization.
NS32A-04 11:20h
The Magnetosusceptibility Stratigraphy (MS) Applied as a Correlation and High Precision Relative Dating Tool in Archaeology: Application to Caves in Spain and Portugal
The magnetic susceptibility (MS) method, when carefully applied, can be used to correlatie between sediment sequences and to characterize the paleoclimate at the time the sediments were deposited in protected archaeological sites, such as within caves or deep rock shelters. This method works because the MS of sediments outside caves, that are eventually deposited in caves, is controlled by pedogenesis that in turn is driven by climate. Here we summarize the method and discuss ways designed to identify anomalous samples that should not be used in relative dating or for correlations. We will then present our results from Cueva del Conde located in the Province of Asturias, northwestern Spain, and compare those results with results from other caves from Spain and Portugal. Cueva del Conde was first excavated in 1915, with additional excavations and studies performed in 1962, 1965, and 1999. The current excavations began in 2001. This body of work identified a transitional sequence from Middle Paleolithic (Mousterian) to early Upper Paleolithic (Aurignacian) artifacts, including perhaps the earliest art known from the Upper Paleolithic, thus establishing Cueva del Conde as an important Paleolithic cave site. We collected a continuous series of 44 samples, each covering about 0.027 m of section, from an exposed 1.2 m sequence within the cave. This section has been excavated and studied by archaeologists working at the site and three 14C dates from charcoal have been reported. The MS for samples collected for this study were measured using the susceptibility bridge at LSU. The MS shows a systematic cyclicity that when constrained by the 14C ages can be correlated to our MS standard curve for Europe (Ellwood et al., 2001; Harrold et al., 2004), and thus to other sites in the region. This cyclicity we interpret to result from climate fluctuations. By comparison to our MS standard curves, we are able to assign MS relative ages to Cueva del Conde that extends the sequence from about 31,500 BP to greater than 36,000 BP (uncalibrated 14C ages). Our results show that the transition from the Middle to the Upper Paleolithic at this locality in northwestern Spain occurred during a time when climate was relatively cold. The implications of this work for correlation to equivalent age sites in Spain and Portugal will be discussed.
NS32A-05 11:35h
Comparison of Rapid and Precise Archaeo-magnetic Survey Techniques Over the Wroxeter Roman City Site, Shropshire, UK.
Magnetometer surveys have been carried out over a representative section of the Wroxeter roman city site to demonstrate the capabilities and limitations of current survey techniques. The site was chosen for this test because it is has an extensive literature of archaeological investigation, and has published blanket archaeomagnetic data coverage recorded with fluxgate gradiometer equipment. Repeated surveys of the chosen sample area, in excess of 2 hectares, tested the effects of; different makes of caesium vapour magnetometers, measurement configurations (total-field and gradient), navigation systems, and data processing techniques. All surveys were compared to the existing full coverage of the site obtained with differential fluxgate equipment. Specific contrasts are drawn between data acquisition techniques. English Heritage have recorded data along pre-located, gridded lines positioned by RTK DGPS navigation. Leicester data are recorded with navigation from real-time DGPS with no prior positional surveying, and survey lines decided by the physical features of the survey area and the incoming data, monitored in real time. Both vertical gradient data and total field data were acquired to contrast the relative merits of these alternative data types. The Leicester system is much more rapid and adaptable, but presents some more difficult and new challenges in terms of data processing. The two new data sets have been subject to alternative processing methods using different approaches and software. The results demonstrate the repeatability and integrity of the survey systems, such that archaeological anomalies are detectable by virtually all combinations investigated. Detailed analysis of the data provides useful guidance concerning the trade-offs that can be made to tune the survey technique for rapid survey of large areas, or best possible detail on specific areas.