The French archeomagnetic directional database for the past two Millennia and its application in geomagnetism and archeology M. Le Goff (1) , N. Warmé (2) , Y. Gallet (1) , and A. Genevey (3) (1) Institut de Physique du Globe de Paris, CNRS UMR 7154, Laboratoire de Paléomagnétisme, Paris, France, (2) Institut National de Recherches Archéologiques Préventives, France, (3) Centre de Recherche et de Restauration des Musées de France, CNRS UMR 171, Paris, France
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The French archeomagnetic directional database for the past two Millennia and its application in geomagnetism and archeology M. Le Goff (1), N. Warmé (2),
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The French archeomagnetic directional database for the past two Millennia
and its application in geomagnetism and archeology
M. Le Goff (1), N. Warmé (2), Y. Gallet (1), and A. Genevey (3)
(1) Institut de Physique du Globe de Paris, CNRS UMR 7154, Laboratoire de Paléomagnétisme, Paris, France, (2) Institut National de Recherches Archéologiques Préventives, France,
(3) Centre de Recherche et de Restauration des Musées de France, CNRS UMR 171, Paris, France
The last article of Pr Thellier (1904-1987), in PEPI-1981, shows the directional variation curve of the Earth Magnetic Field, in France, between
0 and 1800AD, obtained with only 63 dated structures selected in a set of more than 200
ovens and 50 sets of bricks .
13 years later, Ileana Bucur, in PEPI 1994, upgraded the French data base with 110 new data ; 67 being archeologically dated.
Including the Thellier results, she computed this French reference curve with more than 120 dated directions.
Since her retirement, in 1997, a young team has continued the
directional data acquisition in the Saint-Maur Laboratory, with until now, nearly 400 new structures
measured
Over the past two millennia, the directional variations stand in a small
aperture cone of less than 20°. Recovering these variations thus requires
a high precision in measurement
N
EW
I = 0
I = 3
0°I = 6
0°
N
The expected parasitic magnetizations are :- partial or total TRM due to refiring- IRM due to lightning effect- and most often, significant VRM acquisition
TRM directions can be biased by magnetic anisotropy
Mechanical effects, such as structure dislocation or tilting, contribute to disperse the directional results
ABOUT ACCURACY AND RELIABITY
The methodology used in our laboratory,
not « standard », follows the Thellier’s one
Extensive sampling of the fired structures, avoiding as much as possible walls, or clay mixed with pieces of bricks or tiles.
Big samples (12cm in size) precisely oriented, without further cutting or induration.
Systematic measurement of magnetic viscosity.
AF demagnetization in selected cases.
« Big » sample inductometer
Sampling and Magnetic measurements
« Big » sample AF demagnetizer
Sampling Domestic Ovens in JOSSIGNY
Horizontal surface and sun orientation
12 cm
Inverse Measurem
ent
Z
X
Y
TRM
± VRM
Samples with a Viscosity ratio v (VRM/TRM) > 10 % are discarded