Luigi Sambuelli – DIATI – POLITECNICO DI TORINO Stefano Gallinaro – ST-POLITO S.c.ar.l. Michelangelo Grosso – ST-POLITO S.c.ar.l.
A PRELIMINARY TEST ON THE FEASIBILITY OF LOCATING AN IRON RESTORATION PIN IN A
STATUE BY MEASURING THE TMF ANOMALY WITH A TRIAXIAL MEMS MAGNETOMETER
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Sometimes ancient statue restoration works, carried out during the Renaissance, were done using iron pins inserted in the stone and today invisible.
Archaeologist‘s demands can be more complex than simply stating the presence or not of a pin. A pin located out of the axis, obliquely, within an arm, a leg, a neck, can seriously compromise the safe moving of the artifact.
To estimate the location and the length of an iron pin, at a satisfying approximation level, is a challenge for today’s NDT and micro-geophysics techniques.
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The “restored arm” analogue model. A 18 cm high cylinder of bioclastite having a diameter of 10 cm. An iron pin (9 cm long, 5 mm diameter) has been inserted and cemented obliquely and out of the sample axis
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X-ray scans of the sample at the maximum intensity and maximum exposition time available with a veterinary apparatus
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STMicroelectronics LSM303DLHC 3D Accelerometer and 3D magnetometer module
Magnetic full-scale selectable from 130,000 to 810,000 nT Maximum sensitivity of 90 nT/digit Digital output Size: 3x5x1mm – Power consumption: <400 μW.
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Sketch of the experimental setup. A vertical scanning arm and a rotating platform allow for measuring the three components over the whole lateral surface of the
sample.
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Actual realization: vertical scan 1 cm; radial scan 10°.
Measuring on the sample with no pin
Each component at each point measured 50 times at 50 Hz
0
5
10
15
20
25
30
35
40
0 50 100 150 200 250 300 350
Fie
ld [nT
]
Rotation angle [°]
V st. dev of the mean
R st. dev of the mean
T st. dev. Of the mean
Standard deviations of the means of the measured three components at 20 cm of elevation from the rotating platform
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0
10
20
30
40
50
60
70
80
0 10 20 30 40 50
Stan
dar
d d
evia
tio
n o
f th
e m
ean
[n
T]
Number of samples
Evolution of the standard deviation of the mean. Taking 50 samples a fairly stable value is obtained
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-40000
-30000
-20000
-10000
0
10000
20000
30000
40000
50000
60000
70000
0 50 100 150 200 250 300 350
Fie
ld [nT
]
Rotation angle [°]
V R
T TMF
Mean values of the measured three components and the modulus at 20 cm of elevation from the rotating platform
No Pin Pin
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bottom
up
Radial components at increasing height
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Rough location of pin ends from raw radial components
No Pin Pin
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Vertical component
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Tangential component
No Pin Pin
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Radial component
No Pin Pin
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Modulus
No Pin Pin
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On summary …….
In spite of the magnetically noisy environment (that however is likely similar to the one in a museum room) even a vectorial raw data representation gives some information on location and length of the iron pin. It must be kept in mind that the results do not represent the magnetic field due to the pin on the surface of the sample.
Other acquisition procedures, such as gradient or gradient tensor data gathering, would allow for a greater deal of information.
On purpose design algorithm for modeling the anomaly field of an iron pin would allow the making up of an inversion, so that the pin location would be less qualitative.
ACKNOWLEDGEMENTS ST-Microelectronics for providing the MEMS sensors and the data acquisition circuitry. Diego Franco for precisely rotating many many many times the sample platform.
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