M. Le Goff (1) , N. Warmé (2) , Y. Gallet (1) , and A. Genevey (3)
description
Transcript of M. Le Goff (1) , N. Warmé (2) , Y. Gallet (1) , and A. Genevey (3)
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 = 30
°I = 60
°
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 Measurement
Z
X
Y
TRM
± VRM
Samples with a Viscosity ratio v (VRM/TRM) > 10 % are discarded
North Lab direction
Viscosity ExperimentsViscosity Experiments (Thellier, 1981)
By Vectorial operation
VRM and TRM
2 - Samples in “Inverse” position during at least 2 weeks
Direct Measurement
1 - Samples in “Direct” position during at least 2
weeks
Two Examples of directional results with v=7%
Structure : Rungis 1277Mean TRM direction: D=-18.5°, I=64.7°
a95=0.34°, N=16, k=10860
Structure : Villeparisis 4069Mean TRM direction: D=5.4°, I=67.7°
a95=1.82°, N=8, k=743
VRM VRM
TRMTRM
Declination (°) Declination (°)
Incl
inat
ion
(°)
Incl
inat
ion
(°)
0
5
10
15
20
25
<0.2 < 0.4
< 0.6
< 0.8 < 1
< 1.2
< 1.4
< 1.6
< 1.8 < 2
< 2.2
< 2.4
< 2.6
< 2.8 < 3
< 3.2
< 3.4
ou plus..
.
Alpha 95 (°)
Freq
uenc
y (%
)
0
20
40
60
80
100
363 results ; 95% < 1.6°
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 5 10 15 20 25 30 35Number of Samples
Alp
ha 9
5 (°)
363 new results
0
2
4
6
8
10
12
14
16
18
20
<1 <2 <3 <4 <5 <6 <7 <8 <9 <10 <11 <12 <13 <14 <15 <16 >16Mean Viscosity (%)
frequ
ency
(%)
0
20
40
60
80
100
in which30 results
with v>10%were
discarded
393 new collected sites
Alpha 95histogram
Viscosityhistogram
-200 0 200 400 600 800 1000 1200 1400 1600 1800 2000
384 with mean date < 1000 AD 149 with mean date > 1000 AD
All Resultswith « true » or indicative
datation(533)
Some examples of other European databasesfrom archeolabs using standard methodology
All directionsreduced in Paris
Path 1300-1750 from Lübeck
363 new results from France
NEW RESULTS
THELLIER BUCUR
Know n DateUnknow n Date
Know n DateUnknow n Date
Léry, 28 results Marines
21 resultsVilliers le Sec
42 results
Saint-Pathus 59 results
The Geographical Distribution
is very uneven through-out the country.
The Ile de France region contains more than half of the sampling
locations
These 393 new sampled structures represent more than 4400 kg-sized specimens.
-200 0 200 400 600 800 1000 1200 1400 1600 1800 2000
-200 0 200 400 600 800 1000 1200 1400 1600 1800 2000
-200 0 200 400 600 800 1000 1200 1400 1600 1800 2000
130 dated(in Bucur 94)
163 new datedstructures
Resulting ina total of 293
dated structures
CONSIDERING NOW ONLY THE WELL DATED RESULTS...
50
55
60
65
70
75
80
0 200 400 600 800 1000 1200 1400 1600 1800 2000
-30
-20
-10
0
10
20
30
40
0 200 400 600 800 1000 1200 1400 1600 1800 2000
The updated French reference curve is calculated using the self adaptable, variable width sliding
windows (LeGoff et al, 2002)
Inclination
Declination
293 dated structures
20 < windows < 100 years
WHICH MODEL FITS ?
The most precise directions (small alpha95) are alsothe less scattered around the mean curve
82 data with Alpha95 < 0.75°
82 data with Alpha95 > 0.75°
(selection of age incertitude <= ±100 years)
CONCLUSION
• The Thellier’s methodology combined with suitable instrumentation provides highly precise directional data.
• The French reference database, which now contains 293 dated directions, is currently used as an accurate dating
tool for archeological purpose.
Our future article (in preparation) will bring more information !
Thank you
The Saint-Maur Laboratory
Examples of archeological sites with a long and continuous
occupation
Excavating a ditch Digging the kiln in silt
Heating the kiln Cooking meals
Abandonment of the kiln And digging a new one
and, now, before sampling
Haut Empire millieu Ier BC ; fin IIIème -50 -> 300
antiquités Tardive (ex Bas-Empire) déb IV, fin Vème (476, fin Emp Rom) 300 -> 500
Haut Moyen Age (HMA) Vème, Xème 450 -> 1000
dont Mérovingien Vème, VIIIème 450 -> 750
Carolingien VIIIème, Xème 750 -> 1000
Moyen Age Classique (ex Médiéval) XIème, XIIIème 1000 -> 1300
Bas Moyen Age (BMA) XIVème, XVème 1300 -> 1500 (1492)
Moderne fin XVème, fin XVIIIème 1500 -> 1800 (1789)
Contemporain fin XVIIIème -->> 1800 (1789) ->>>>>