Archaeomagnetic Study performed on Early Medieval Buildings from western France
A multiple dating study, involving a collaboration between specialists of dating techniques (thermoluminescence (TL) and radiocarbon), historians of art and archaeologists, has been carried out on several early medieval buildings from western France. The early medieval period is not well known especially in France where there is a lack of visible evidence that identifies pre-Romanesque architecture. The majority of buildings to have survived from this period are religious ones, considered important enough to be made of strong, non-perishable material such as stone or brick, as for example the churches of Notre-Dame-sous- Terre in the Mont-Saint-Michel or St Martin in Angers. Due to their significance in architectural history, it is imperative to position them accurately in the chronology of the history of art. Bricks are often used to build up round-headed arches or to reinforce the frame of a wall with bonding courses in those churches. TL dating and archeomagnetic analysis were performed on cores drilled within bricks while radiocarbon dating were undertaken on coals found within mortars. In order to increase the number of data during the early Middle Ages, archeointensity determinations using the classical Thellier technique with anisotropy of thermal remanence and cooling rate corrections were performed. Archaeomagnetic directions were used to recognize the firing position of bricsk during manufacture. Reliable and precise ages were obtained on the church Notre-Dame-sous-Terre; they indicate two phases of building in 950±50AD and 990±50AD. Mean archeointensities obtained on 17 (21) samples from the first (second) phases appears very closed 69.1±1.2 and 68.3±1.6 microTesla. Ages and archeomagnetic results obtained on 4 other sites will be presented and compared to the available data in western Europe.
Near Surface Magnetic Survey for Investigating the Cultural Relics in Suchon, Gongju, Korea
A magnetic study by the FM256 Fluxgate Gradiometer was conducted in Suchon, Gongju to measure the vertical magnetic gradient of the Earth's magnetic field and to give further details of the shallow section. The region was divided into two separate areas. The first study area measured 40m by 20m while the second study area was 20m x 20m. Each was subsequently divided into four grids of dimension 20m by 10m and 10m by 10m respectively. Measurements of the vertical magnetic gradient were conducted through successive zigzag traverses. The sample-interval and the traverse-interval were set to specifically record small anomalies at a high resolution. A total of 3200 readings were measured at the first study area and 1600 at the second study area. The data have been downloaded, presented and processed through the Geoplot software to remove the spikes, grid discontinuities, and traverses stripes, and also to enhance the display and smooth the data using the Gaussian low-pass filtering techniques. The vertical gradient of the processed data over the second study area ranges from -34nT to + 21nT, while it ranges from -36nT to + 62nT at the first study area. The gradiometer results defined several positive and negative magnetic anomalies, which revealed the existence of several subsurface features of different shapes and sizes. A comparison between the processed magnetic images suggest that the subsurface features may include a room structure (e.g. hut), a cave-shaped stone chamber tomb, an accumulation of potteries and porcelains common in the Baekje period in the ancient Korean history. The biggest anomaly (3 m in diameter) may illuminate a quartzite tomb chamber. As a result, the study area has great archaeological interest.
Magnetic Susceptibility of Ancient and Modern Potsherds Using a Fast, Cheap and Portable Probe
It has been estimated that there exist over 100 million ancient potsherds in various collections worldwide,
many of which have never been studied and for which the provenance is ambiguous or unknown. Indeed, many
collections are extremely badly catalogued or completely mixed-up. We have been using a novel portable
probe to measure the magnetic susceptibility and electrical conductivity of potsherds in the hope that this fast,
cheap and portable measurement can provide data that will help to sort similar looking potsherds into sets in a
manner which may help to define their provenance.
The probe, which resembles a firearm, uses the Hall effect to make a non-destructive measurement on the
potsherd. The probe is attached to an Dell Axim X51 PDA, which runs software that allows the measurement to
be carried out and logged. Each measurement, which is made by pressing a button on the gun, takes only a
few seconds. We have made measurements on three suites of ancient potsherds as well as a suite of modern
potsherds that were created by using a garden centre and a hammer! In each case a set of 5 stacked
measurements were taken on the inside and outside faces of the potsherd in two perpendicular directions.
Potsherds which were either (i) so flat that the inside and outside could not be distinguished, (ii) so curved
(radius of curvature less than 5 cm) that the probe tip could not approach the surface sufficiently closely, or (iii)
smaller than the probe tip, were excluded from the suite of measurements. Each suite contained over 50
measureable potsherds. All measurements were completed within one day.
In this pilot study we found that (1) each suite was represented by a normal distribution of magnetic
susceptibility values, (2) the four different suites could be distinguished statistically on the basis of their
magnetic susceptibility measurements, but (3) the distinction was not sufficiently powerful to separate all
potsherds (i.e., there was a significant overlap of the susceptibility distributions). This seems to confirm that
the method may be used to give additional information that can be used to help to provenance a potsherd, but
the susceptibility measurement is not sufficient on its own. In addition, we found that (4) the electrical
conductivity measurements depended upon the local conditions (mainly humidity) and was of no use in
distinguishing between suites of potsherds. However, most interestingly, we found that (5) there is a
statistically significant difference between the magnetic susceptibility measured on the inside face and that
measured on the outside face for all three ancient suites of potsherd, but not for the modern potsherds. The
reason for this is not currently known. One hypothesis is that the difference is due to the manufacturing style.
Further studies are being planned to extend our database.
Magnetic Fingerprinting of Central Mediterranean Obsidian Source Groups
The Central Mediterranean islands of Lipari, Palmarola, Pantelleria, and Sardinia are known to be the only sources of obsidian in that region (Tykot, 2002). The obsidian on each source island has a unique chemical composition (i.e., a fingerprint) that has allowed Tykot (2002) to provenance Neolithic obsidian artifacts from archaeological sites throughout the Central Mediterranean to these four sources. Thus, Tykot (2002) revealed material distribution patterns otherwise inaccessible to archaeologists. An exploratory study by McDougall et al. (1983) demonstrated the potential use of magnetic properties to distinguish Central Mediterranean obsidians. If each obsidian source group has a unique magnetic fingerprint, magnetic provenancing might serve as a quick, inexpensive, and non-destructive alternative to chemical provenancing. Our research continues the work started by McDougall et al.; we set out to characterize the Central Mediterranean obsidians and search for magnetic fingerprints that distinguish them. Our preliminary study of the magnetic properties of 30 geologic obsidian specimens from Sardinia A, B, and C subgroups and the three other Central Mediterranean island sources suggests that each source has a unique combination of magnetic properties that could allow magnetic provenancing of archaeological obsidians. For example, the combination of natural remanent magnetization (NRM; median [med] = 2.46 x 10-4 Am2/kg, interquartile range [IQR] = 9.77 x 10-4 Am2/kg) and magnetic susceptibility (low field, low frequency magnetic susceptibility [χlflf]: med = 27.0 x 10-6 m3/kg, IQR = 75.9 x 10-6 m3/kg) seems to have good potential to discriminate the groups. However, statistical analysis shows that there is significant overlap in the confidence intervals of these variables, a limitation which appears to be partially imposed at this point by the small sample size. Discriminant analysis of several magnetic variables also shows potential to assist in classification (i.e., provenancing) of Central Mediterranean obsidians. We have obtained discriminant functions that correctly classify over 80% of the obsidian specimens in leave-one-out cross validations using as few as three log- transformed magnetic parameters: NRM, χlflf, and saturation isothermal remanent magnetization (SIRM; med = 7.59 x 10-3 Am2/kg, IQR = 2.85 x 10-2 Am2/kg). Backfield coercivity of remanence, median destructive field, percent frequency effect, and saturation coercivity seem to contribute the least to group discrimination, yet their effects are statistically significant. Even though initial tests of discriminant analysis look promising, it should be noted that the small sample sizes and relatively large number of variables used in the discriminant analyses violate the test's assumptions and may make the resulting discriminant functions unusable; this will be addressed in future research. The results from our preliminary study suggest that the Central Mediterranean sources of obsidian might have characteristic and distinguishable magnetic fingerprints. However, many more geologic obsidian specimens must be measured to define the ranges of the source groups' magnetic properties and to further test this hypothesis.