A MAGNETIC SURVEY OVER LA MAJA,AN ARCHAEOLOGICAL SITE IN NORTHERN SPAIN

A Roman archaeological site in northern Spain (La Maja, province of La Rioja) includes pottery workshops, part of an important ceramic industry that developed from about the first century BC to the first century AD. A geophysical study was planned to determine the extent of the industrial complex in the vicinity of an excavated kiln. Magnetic observations were done on a grid of 30 × 36m, with a sampling interval of 1m. Two dipolar-shaped anomalies were found: one in the north-east and the other in the south-west of the area. Low-pass and band-pass filters were used in the Fourier domain to enhance the characteristics of the magnetic data and to remove noise and undesirable shallow features. Three-dimensional polygonal prisms were used to model the magnetic anomalies based on palaeomagnetic measurements performed over an uncovered kiln that helped to constrain the induced and remanent parameters of the magnetizing fields.     

NON‐DESTRUCTIVE PORTABLE GAMMA RAY SPECTROMETRY USED IN PROVENANCING ROMAN GRANITOID COLUMNS FROM LEPTIS MAGNA,NORTH AFRICA*

Portable gamma ray spectrometry (PGRS) provides a non destructive means to analyst quantitatively large artefacts, such as building stones, for the radioelements K. U and Th. Nine Raman granitoid columns at the Leptis Magna Ruins in Windsor Great Park, London, were measured in situ by PGRS. Corrections for the environmental background contribution to the gamma ray flux measured, and for the shape and size of the columns, are described Comparison of the PGRS data with a radioelement data base for Roman granite sources indicates that most of the columns originated in the Troad area of Turkey. Two columns could not be unambiguously provenanced using PGRS alone because there is insufficient difference between radioelement concentrations in certain sources. However, non‐destructive measurements of magnetic susceptibility, used in conjunction with PGRS data, suggest that these two columns originated in the Kozak Daǧ, also in Turkey.     

MAGNETIC SUSCEPTIBILITY USED IN NON-DESTRUCTIVE PROVENANCING OF ROMAN GRANITE COLUMNS

Magnetic susceptibility provides a rapid, cheap and non-destructive method of in situ characterization of archaeological artefacts containing magnetic minerals, and can be used as an aid to geological provenancing. Three hundred and sixty-three Roman granite columns were measured by this method and results show clear groupings and similarities with potential granite sources in Italy, Turkey and Egypt. Magnetic susceptibility measurements must be made on representative, unweathered surfaces of rocks and artefacts, and corrected for object size and surface relief according to manufacturers’recommendations. In addition, corrections for column curvature have been derived for use with measurements made on columns.     

MAGNETIC SUSCEPTIBILITY THICKNESS CORRECTIONS FOR SMALL ARTEFACTS AND COMMENTS ON THE EFFECTS OF‘BACKGROUND’MATERIALS*

Correction factors for magnetic susceptibility measurements on thin (<c. 50 mm thick > artefacts have been determined experimentally for a KT‐5 Exploranium G S. instrument using prepared blocks of Whin Sill dolerite. The cor rection factor is large (> 1.4) for samples less than 10mm thick, and reduces to 1 01 for samples of 50mm thickness. Measurements on thin samples can also be affected by the backing or substrate material on which they are measured. ‘Background’material, for example, soil or plaster, can contribute significantly to recorded measurements on artefacts, particularly for thin artefacts with low susceptibilities     

The Construction Process of the Angkor Monuments Elucidated by the Magnetic Susceptibility of Sandstone*

The Angkor monuments in Cambodia are mainly constructed of grey to yellowish‐brown sandstones. No differences in the constituent minerals and in the chemical composition of the sandstones have been confirmed among the monuments. However, we have found their magnetic susceptibility a useful parameter by which to distinguish them. The principal monuments of Angkor Wat, Ta Prohm, Preah Khan, Banteay Kdei and Bayon, constructed from the Angkor Wat period to the Bayon period (from the beginning of the 12th century to the beginning of the 13th century ad ), were investigated in detail using a portable magnetic susceptibility meter. We succeeded in dividing the periods of construction into stages. This elucidated the enlargement process of the monuments and correlated their construction stages.     

GEOPHYSICAL SURVEY AT BODEN VEAN,CORNWALL, INCLUDING AN ASSESSMENT OF THE MICROGRAVITY TECHNIQUE FOR THE LOCATION OF SUSPECTED ARCHAEOLOGICAL VOID FEATURES*

A geophysical survey was conducted at Boden Vean, St. Anthony Meneage, Cornwall, over the site of a buried chamber thought to be the remains of a souterrain or fogou. A combination of geophysical techniques was successfully applied including an experimental microgravity survey over the location of the buried chamber itself. Magnetometer survey revealed a complex palimpsest of archaeological activity extending throughout the surrounding landscape, centred on a rectangular ditched enclosure containing the fogou. A series of gravity anomalies were recorded in the vicinity of the latter which were consistent with the collapsed section of the feature recorded by the Cornish Archaeological Unit. Further gravity anomalies suggested the presence of additional void features, possibly related to the extended passages of the fogou.     

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPIC AND ISOTOPIC ANALYSIS OF CARBONIZED RESIDUES FROM SUBARCTIC CANADIAN PREHISTORIC POTTERY

Late prehistoric pottery is found in abundance at archaeological sites around Southern Indian Lake. Black residues, found on the two dominant vessel forms, flat plates and round pots, are presumed to be the remains of prehistoric meals. ¹³C cross-polarization magic-angle-spinning nuclear magnetic resonance spectroscopy (CPMAS NMR) and¹³C and¹⁵ N isotopic ratios and C/N ratios are used to reconstruct prehistoric diet and to shed light on possible uses for the plates. Samples of foods were cooked in clay pots, on a wood fire, to simulate the conditions of burning that could have produced the residue. Decomposition of carbohydrates, protein, and fat during cooking is studied with ¹³C CPMAS NMR, and the effect of cooking on isotopic and C/N ratios documented. Predominantly fish and fat were cooked in the pots, whereas the residues from plates contain a greater proportion of fat and could have been used as frying pans or possibly as fat-burning lamps placed on the ashes of a wood fire.     

An assessment of the magnetic response of an iron‐smelting site*

The magnetic response derived from an iron‐smelting site was investigated by comparing magnetometry and magnetic susceptibility geophysical survey data and laboratory analyses of the magnetic characteristics of the furnace and slags. Magnetic analysis and microscopy (optical and SEM) of samples from the furnace lining and the slag deposits demonstrated the heterogeneity in the magnetic, morphological and mineral compositions of both materials. The comparison of the magnetic characteristics of the material with the geophysical survey data illustrated the importance of using both magnetometry and magnetic susceptibility survey techniques to maximize the information from an iron‐smelting site. The furnace was dated archaeomagnetically to the 14th century ad . The results demonstrate that the magnetic analysis of iron‐smelting sites is highly valuable, both to characterize sites and to improve the understanding of early iron‐working technology.     

CHARACTERIZING THE SEDIMENTARY HISTORY OF CAVE DEPOSITS,USING ARCHAEOMAGNETISM AND ROCK MAGNETISM,ATAPUERCA (NORTHERN SPAIN)

We use a combination of rock magnetism (anisotropy of magnetic susceptibility, AMS) and magnetic polarity to characterize cave deposits and as a proxy for sedimentary fabric. In three localities at the Atapuerca archaeological site (Galeria, Gran Dolina and Sala de los Cíclopes), magnetic foliation (K_max/K_int) is always greater than lineation (K_int/K_min), consistent with a primary, depositional, sedimentary fabric. Our results, although preliminary, reveal a higher degree of anisotropy in autochtonous deposits compared to allochthonous deposits, possibly indicative of a higher hydrodynamic regime in the former. At two localities the magnetic lineation (K_max) defines a cluster, which is thought to be antipodal to the palaeocurrent direction. Hence we are able to retrieve palaeoflow directions in deposits that otherwise lack any other sedimentary structure. We conclude that AMS is a powerful tool for determining the hydrodynamic character of depositional environments in cave sediments at the Atapuerca archaeological site. A better understanding of the depositional environment and how sedimentation occurred allows reconstruction of the karst evolution and ultimately a better definition of human interaction with the environment.     

MEAN SHIFT: A NON‐PARAMETRIC ALGORITHM FOR THE SEGMENTATION OF ANOMALIES IN GEOPHYSICAL IMAGES OBTAINED FROM MAGNETIC PROSPECTION DATA

This paper studies the applicability of the Mean Shift algorithm as support in interpreting geophysical images produced, on this occasion, from magnetic prospection data. The data obtained from a magnetic survey carried out in Gilena (Seville province, Spain) by the La Rábida Archaeophysics Group will be used for the research. Its applicability is illustrated by comparing, on the one hand, some reduction‐to‐pole algorithms and on the other, the (well‐known) k‐means algorithm. Finally, the paper shows the results obtained by applying the Mean Shift algorithm as an alternative method to ‘unsupervised clustering’ of anomalies that appear in images obtained from geophysical data, in which the a priori knowledge of the number of classes is difficult or impossible.