Magnetic properties abstracts from AGU

The last few abstracts from these two session on geophysics and archaeology, looking at magnetic properties at two type of archaeological materials.

The image is unrelated, other than it is a cool representation of magnetic properties of thin films.

GP11I-03

Magnetic Susceptibility of Ancient and Modern Potsherds Using a Fast, Cheap and Portable Probe
Glover, P W (paglover@ggl.ulaval.ca), Université, Département de géologie et de génie géologique, Québec, QC G1V 0A6, Canada

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.
http://www.ggl.ulaval.ca/personnel/paglover/Home.htm

GP11I-04
Magnetic Fingerprinting of Central Mediterranean Obsidian Source Groups
Weaver, I (isaacweaver@alumni.fandm.edu), Department of Earth and Environment, Franklin & Marshall College, PO Box 3003, Lancaster, PA 17604-3003, United States
Sternberg, R (rob.sternberg@fandm.edu), Department of Earth and Environment, Franklin & Marshall College, PO Box 3003, Lancaster, PA 17604-3003, United States
Tykot, R H (rtykot@cas.usf.edu), Department of Anthropology, University of South Florida, 4202 East Fowler Avenue, SOC 107, Tampa, FL 33620-8100, United States

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 [χ_lflf]: 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, χ_lflf, 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.

Geophysical archaeoprospection abstracts from AGU

Here are the abstracts on geophsyical prospection at archaeological sites from the session I co-organized at the just completed American Geophysical Union Joint Assembly:

GP11I-02
Near Surface Magnetic Survey for Investigating the Cultural Relics in Suchon, Gongju, Korea
Islam, M R (mislam62@uwo.ca), Department of Earth Sciences, University of Western Ontario, 40,Summit Ave. Unit# 50, London, ON N6H4S3, Canada, et al.

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.


GP22A-05 INVITED
The Donegal Sign Tree: A Local Legend Confirmed with Holographic Radar and 3-D Magnetics
Bechtel, T (bechtelt@sas.upenn.edu), Dept. of Earth & Environmental Science, University of Pennsylvania, Philadelphia, PA , United States, et al.

A tree at a crossroad in Historic Donegal, PA (founded 1722) bears unusual burls. Two are similar in size, and lie on opposite sides of the trunk at a height of six feet. Locals say that the tree engulfed an old road sign, and the geometry of the burls gives this appearance. However, the trunk between these two burls bears no welt where it sealed after swallowing the sign. In addition, there are other burls farther up the tree, which are not consistent with engulfed signs. Although the locals all know the legend of the swallowed sign, none ever actually saw the sign; not even an octogenarian who has lived at the crossroad his entire life, and recalls the tree as a child just as it is today. In order to test the veracity of the legend, this study performed subsurface imaging of the tree using holographic subsurface radar (Rascan), and 3-D measurements of the magnetic field about the tree using cesium vapor sensors. The Rascan system used is a continuous wave subsurface radar that operates at 5 discrete frequencies between 1.5 and 2.0 GHz. Reflections from subsurface objects are recorded as the phase difference pattern between an internal reference signal, and the reflected signal. Thus, it is a microwave analogy for optical holography. Rascan records reflections with two receiving antennae - parallel and perpendicular to the transmitter - so a single set of scans provides ten images; five frequencies at two polarizations. This ensures that an object at arbitrary depth will produce a strong phase difference in one of the images. As a consequence, elongate objects that are angled from the plane of scanning (e.g. a dipping sheet) produce "zebra stripes" of contrast values that vary cyclically with depth. The presence of stripes, and their relative positions in the different frequency images (the movement of which has been dubbed the "zebra shift") is useful for determining the relative depth of different portions of a dipping planar, or curved subsurface object. Rascan images of the tree revealed a reflector that produces a zebra shift pattern reminiscent of a curved reflector. However, given the curvature of the tree trunk, the zebra shift is more likely to represent a flat reflector beneath a curved scanning surface - consistent with the presence of the sign. As an independent confirmatory method, the tree was also subjected to a magnetic survey. First, the tree was swept with a magnetic locator - which indicated a magnetic target within the tree. In order to determine the configuration of this target, magnetic total field measurements were collected at the nodes of a 3-D grid surrounding the tree. The geometry of this survey is quite different from traditional archaeological prospection magnetometer surveys and, despite the relatively high latitude of Donegal PA, the vertical orientation of the suspected target mimics the common difficulties with magnetic surveys at low magnetic latitude. Therefore, the analytic signal was calculated to provide an easily interpreted magnetic anomaly that, together with the Rascan images, suggests that the story of the swallowed Donegal road sign may be true.


GP22A-06
3-D Modelling of Magnetic Data from an Archaeological Site in Northwestern Tlaxcala State, Mexico
Chavez, R E (exprene@geofisica.unam.mx), Instituto de Geofisica UNAM, CD Universitaria Circuito Exterior, Mexico, DF 04510, Mexico, et al.

In Archaeology, geophysical methods had been applied usually in a qualitative form, limited only to the use of filters that enhance the data display. The main objective in this work is the implementation of a modeling technique that allows us to reconstruct the geometry of buried bodies and the determination of their depths. This is done by means of the estimation of the magnetic moments of archaeological objects using a three- dimensional mesh of individual magnetic dipoles using the least squares method and the singular value decomposition of a weighted matrix to solve the linear problem. The distribution and shape of the underlying archaeological remains can be inferred. This methodology was applied to an archaeological site called Los Teteles de Ocotitla, in the state of Tlaxcala, Mexico. A high-resolution magnetic prospection was carried out in three selected areas (terraces). The most important total field anomalies found on each area were inverted, obtaining results that were corroborated by archaeological excavations. This investigation demonstrates the potential of quantitative geophysical methods for the characterization of archaeological structures, in extension and in depth.


GP22A-07
Archaeometric Prospection Using Electrical Survey Predictive Deconvolution (ESPD)
Glover, P W (paglover@ggl.ulaval.ca), Université Laval, Département de géologie et de génie géologique, Québec, QC G1V 0A6, Canada

Once upon a time archaeological prospection was carried out mainly using electrical techniques. These days magnetic techniques and GPR are used by preference. However, we have shown that electrical surveying combined with the technique of predictive deconvolution is very effective at finding buried features where the shape of the feature can be predicted in advance. One such type of feature is the Grubenhaus (or sunken-featured, sunken-floored building, or SFB). Grubenhaüser exist in the archaeological record as individual well-defined oblong pits that have been filled and buried with other material. Aerial photographs at New Bewick in Northumberland, northern England (UK Grid reference NU061206) showed quasi-rectangular features similar to those on aerial photographs at the nearby Anglo-Saxon palace of Milfield (NT941339) which had been confirmed by excavation to be Grubenhaüser. Several electrical resistivity surveys were carried out over the area with an ABEM Mk II Terrameter and a multiplexing box serving 31 electrodes in line at any given time. Both double-dipole and Wenner configurations were used with an electrode spacing of 1 m. Data was acquired in blocks of 30 m by 30 m during a period of dry summer weather while the field was under young winter wheat. The Wenner array produces a characteristic 'M' or 'W' shaped response over filled in excavations such as those expected to represent a Grubenhaus. While this seems a disadvantage in the first instance, it can be used to improve the data. Such anomalies were present in the raw New Bewick data. The resulting data were analysed using 1D and 2D predictive deconvolution in order to remove the Wenner response. The deconvolution was carried out using an inverse matrix element method. The filtered results indicated the presence of an anomaly that is consistent with a Grubenhaus measuring about 5 m by 4 m and with a pit depth of 0.6 m below 0.5 m of topsoil. The results also showed broader areas of increased resistivity which have been attributed to compaction resulting from human and animal movement. Following the geophysical study the site was excavated (T. Gates and C. O'Brien "Cropmarks at Milfield and New Bewick and the Recognition of Grubenhaüser in Northumberland." Archaeologia Aeliana 5th series, Vol XVI, 1988, 1-9) 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. The deconvolved Wenner data performed better than the double-dipole resistivity survey but was marginally slower.

Archaeomagnetism abstracts from AGU, Toronto

Here are the abstracts on archaeomagnetism from the session I co-organized at the just completed American Geophysical Union Joint Assembly:

Archaeomagnetic Study performed on Early Medieval Buildings from western France
Chauvin, A (Annick.Chauvin@univ-rennes1.fr), Géosciences-Rennes, Université Rennes 1, CNRS UMR 6118, Campus de Beaulieu, Rennes, France et al.

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.
News story on Chauvin presentation


European Archaeomagnetism: Progress and Problems
Evans, M E (evans@phys.ualberta.ca), Institute for Geophysical Research, University of Alberta, Edmonton, AB T6G 2G7, Canada et al.

Much progress has been made since the seminal work of Giuseppe Folgheraiter (1856-1913) in the late 19th century. So much so that recent advances now make it possible to draw up complete isogonic and isoclinic maps for Europe and adjacent areas spanning the last three millennia (Pavon-Carrasco et al., 2009). Results based on multiple independent studies, with high precision and good age control are crucial and should be recognized as "anchor points" (e.g. Pompeii). On the other hand, the nagging problem of outliers persists. Among the possible causes are magnetic refraction, physical distortion, and inadequate chronological control. Some examples, drawn from our own investigations over the last 30 years, will be discussed in detail. These include previously unpublished data from a detailed study (more than 100 samples) of a kiln in southern Italy, and an apparently good (but aberrant) archaeodirection from a kiln in southern Spain.


Regional Archeomagnetic Model for Europe for the Last 3000 Years: Application to Dating

Pavón-Carrasco, F (fjpavon@fis.ucm.es), Grupo de Paleomagnetismo. Dpto. de Geofísica y Meteorología, Universidad Complutense de Madrid., Avda. Complutense, s/n., Madrid, 28040, Spain et al.

Recently a new regional archeomagnetic model in Europe for the last three millennia has been proposed. This model, SCHA.DIF.3K (Pavón-Carrasco et al., 2009, Geochem. Geophys. Geosyst., doi:10.1029/2008GC002244, in press), is based on a Spherical Cap Harmonic Analysis (SCHA) for spatial representation and sliding windows method in time. The model provides information of both directional and intensity variation of the Earth's Magnetic Field for the last 3000 years in the European region. One of the immediate applications of SCHA.DIF.3K regional model is its use as tool for archeomagnetic dating. So far the PalaeoSecular Variation Curve (PSVC) determined for a region has been used for archeomagnetic dating. The limitation of this application is the distance from the dating point to the location of the reference curve (the relocation error). In addition it must be borne in mind that the PSVC are individually generated for each region, so there is no consistency enforced between curves from neighboring areas. The use of the SCHA.DIF.3K model as a tool for archeomagnetic dating represents an improvement for several reasons. First of all, the regional model has been generated considering all elements of the geomagnetic field (declination, inclination and intensity). Second, the regional model is built with an in situ archeomagnetic database. Furthermore, the database covers the whole time period from 1000 BC to 1900 AD, while the database used in the PSVC has gaps of data for any time interval. Finally, and more important, we can generate a PSVC at the location of the archeological structure, avoiding in this way the relocation error associated with traditional PSVC. To demonstrate the utility of the regional SCHA.DIF.3K model, we have used it to date several archeological structures and we have compared results with the archaeological information and/or archeomagnetic dating provide by the use of the PSVC.


The Mesoamerica Secular Variation Curve. A continuous research since 1999
Soler-Arechalde, A M (anesoler@geofisica.unam.mx), Universidad Nacional AUtonoma de Mexico, Instituto de Geofisica, Ciudad Universitaria, Del. Coyoacan, Mexico, DF 04510, Mexico et al.

Since 1999 we have been working on improving the Mesoamerica Secular Variation Curve. We re-sampled some sites that Wolfman initially used in its first proposal of this Curve, such as Teotihuacan, Teopancazco and Tula. Wolfman 's curve only had 4 radiocarbon dates directly associated with the sampled sites; the other dating were actually stratigraphic and ceramic correlations. More than 28 radiocarbon dates have been incorporated from AD 60 to 560 from samples associated to Teotihuacan civilization, 13 more from Xochicalco and 10 from Tula. More than 1000 specimens, from 12 twelve sites were fully processed with alternated field demagnetization We have been working during the excavation campaigns and training the archeologist to get their samples. A 100-year moving window was employed to get the average poles. A Bayesian statistic has been employed in order to improve our curve. We still denote a lack of data from two time intervals: from 0 to 200 and from 1000 to 1600. We are now working on it, collecting samples from these periods, such as those from La Joya, Ver, which are now been processing and from which some preliminary results will be reported.


Dating Post-Medieval Archaeology: Which Global Geomagnetic Field Model to use?
Lodge, A (a.lodge@liv.ac.uk), University of Liverpool, Department of Earth and Ocean Sciences, Jane Herdman Laboratories, 4 Brownlow Street, Liverpool, L69 3GP, United Kingdom et al.

The scientific dating of Post-Medieval archaeology (16th Century onwards) is problematic as most methods cannot provide any better resolution than may be apparent from contextual or stylistic considerations. As high resolution global geomagnetic field models exist for this period, archaeomagnetism offers the possibility of bi-decadal dating of burnt in situ structures, with implications for the management of cultural heritage. The question arises as to which global geomagnetic field model is most appropriate for this dating? Should the high resolution historical field model, gufm (Jackson et al., 2000, Four centuries of geomagnetic secular variation from historical records, Phil. Trans. Roy. Soc. Lond. A, 358, 957- 90.) which covers the period 1590-1990 AD and is based on data from ship's logs be used, or should an archaeomagnetic model such as GMADE2K.2 (Lodge & Holme, 2008, Developing a global geomagnetic field model for archaeomagnetic dating in Europe for the last 2000 years (updating GMADE2K.1 to GMADE2K.2), Geophys. Res. Abstr., 10, Abstract EGU2008-A-03470) be used? In general a higher accuracy can be expected from the historical model, but the modeling strategy for gufm is aimed at investigating the magnetic field evolution at the core-mantle boundary, whilst GMADE2K.2 is developed to serve as an archaeomagnetic dating tool. If we compare secular variation curves in Europe for declination at this time, then the two models agree very well. For inclination however, there is a discrepancy pre-1800 AD between the two models, with the historical model tending to higher inclinations. Here we study the possible causes of this discrepancy: How reliable are the early historical inclination data? How reliable is the historical model at this time - is the inclination being affected by the domination of declination data? Finally, are the archaeomagnetic data systematically low, possibly caused by undetected magnetic refraction? The advantage of constructing global geomagnetic field models is that the inter-dependence of the components is taken into account. However, if we cannot reconcile the archaeomagnetic models and data with the historical model and data, then dating remains problematic.