Geomagnetic field variations as determined from bulgarian archaeomagnetic data

The archaeomagnetic determinations obtained from the collections of archaeological materials, dated in the last 2000 years are summarized. The review describes the materials used, the methodology applied, the local database organization, the principles of data processing and smoothing procedures. Errors evaluation at different levels of analysis of the experimental results are described. Special attention is paid to the archaeointensity determinations. Some rock magnetic properties studied in connection with the difficulties in the Thellier method are summarised. The curves of geomagnetic field elements variations for the last 2000 years for Bulgaria are shown. The Bauer plots, VGP path and VDM curve are also discussed, the latter confirming the general decrease in the strength of the geomagnetic field from the beginning of this era to the present. Westward drift of the geomagnetic field non-dipole part cannot be considered proved yet for the time interval 7th to 13th Centuries AD from the Bulgarian directional data.     

Distribution analysis of errors due to relocation of geomagnetic data using the 'Conversion via Pole' (CVP) method: implications on archaeomagnetic data

Secular variation 'master' curves are built up using geomagnetic historical observations or archaeomagnetic data from a limited area and their use is usually restricted to regions of around 1000 km radius. Relocation of data within this distance is a common practice to enable comparison of data, although the errors due to such process are rarely taken into account. A detailed analysis of the distribution of relocating geomagnetic data has been done using three popular sets of geomagnetic models (IGRF-9, GUFM and CALS7K-2). This study improves the error analysis of relocating geomagnetic directions made up to date and expands it to geomagnetic intensities. Maximum errors correlate with the non-dipole to dipole field ratio. Archaeomagnetists could use this analysis to valuate the error introduced by reducing data.     

Origin of deviations between the remanent magnetisation and inducing geomagnetic field direction in kilns and implications on archaeomagnetic dating

Standard secular variation curves of the geomagnetic field direction for the last few millennia for different countries in Europe, based mainly on baked materials discovered in archaeological sites, are reaching sufficient accuracy and reliability that archaeomagnetic dating within this timespan in these areas is becoming feasible now. Orientation errors during sampling and sample preparation can be kept to a minimum and measurement errors of baked materials are in general very small. Nevertheless, even when the secular variation in the site to be dated is similar to that in the reference site, systematic or regular deviations between the remanence vector and geomagnetic field direction in kilns, due to internal and external factors, may result in biased average magnetisation directions and consequently in diverging archaeomagnetic dates. In more or less circular kilns, important non-random deviations of the remanence direction in the walls, in function of the relative azimuth as seen from the centre of the kilns, have been noticed in spite of precise and highly reliable average kiln magnetisation directions. The effect was revealed in the walls of the combustion chambers of roman pottery kilns and of mediaeval brass melting and working kilns, of different shape, lined with refractory bricks. Several hypotheses for the origin of these deviations were examined and anisotropy and/or magnetic refraction are unlikely to be the principal causes. The regular component of these deviations may be responsible for a strongly biased average magnetisation direction when only part of the kiln wall is preserved or when sampling is limited to certain parts of it and consequently result in unreliable dates when applying the archaeomagnetic dating method.