AA&A Spring 2002
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Archaeological Prospecting
• What have you got?
• Where do you look?
• Where do you dig???
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*Aerial photography
*Electrical resistivity
*Magnetometry
*Ground penetrating radar (GPR)
Electromagnetic induction
Seismic probes (sonar)
Gravimetry
Gamma “radiography”
IR emission imagery
Archaeology without a shovel
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What are the clues?
Shadowing
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Soil marking
Filled ditchesMore moisture
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Roman roadsPaving-less moisture
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Crop or vegetation marking
Roman villa
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Familiar territory
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Less familiar
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Zoom out
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What is it?
Nazca, Peru
200 BCE-600 CE
Rocks removed to reveal
lighter colored subsoil
No rhyme or reason to
complex array of lines
photo
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Aerial photo-hummingbird
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Spider
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Sketches of others
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Geometric patterns
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Kerkenes, Turkey: ~585-545 BCE
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Kerkenes-aerial view
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Closer aerial photo-interior
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How to see without digging?
Activities of man-----subsurface inhomogeneities
• Want to use electrical conductivity:
– Good insulators–dry brick, non-porous
stone, glass, voids, dry soil
– Good conductors–metals
– In between (a tremendous range)–moist
brick or soil, water
• A way to measure conductivity of what’s
underground???
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Currents in homogeneous soil
+
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= local high conductivity
+
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Where is it?
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How deep is it??
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Sensitive to depths ~ 1.5 x separation
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What do you see?
=????
• Higher conductivity
– Higher moisture content
– More dissolved minerals
– Metal artifacts
• Lower conductivity
– Lower moisture
– Masonry
– Voids
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Conductivity array
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Conductivity data
20 meter grid
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Magnetometry
• How to look for magnetic materials?
• Simplest of magnetometers is a compass
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Magnetometry
Magnetometer
Signal
Earth’s
field
Surface
Buried
object
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Magnetic gradiometer
Difference
Signal
Gradiometer
difference
Earth’s
field
Surface
Buried
object
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Magnetometry
Sense variations in Earth’s magnetic field
need high sensitivity
killed by fluctuations in time
Solution:
magnetic gradiometer
pair of sensors
look only at difference
Sensitivity of the best (not
the one illustrated):
10-8 of Earth’s field!
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What do you see?
Local variation in magnetism
• Most obvious–iron artifacts
• Recall colors of unglazed pottery
– Red–hematite–Fe2O3–non-magnetic–“oxidized”
– Black–magnetite–Fe3O4–magnetic–“reduced”
• Magnetite containing soil
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Soil replaced by stone masonry or red brick
Ditch or earthworks filled in by non-magnetic soil
Alignment of grains in clays disturbed by working
Soil disturbed (e.g., a grave) allowing oxygen access
• Hematite containing soil
– Reduction of iron in fire hearth
– Destruction of building by major fire
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6-element gradiometer
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Real data
Conductivity
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Magnetometry
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Columned hall
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AA&A Spring 2002
Aerial photo
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100 meters
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Magnetic survey
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Ground Penetrating Radar
= GPR
• How did we see below the surface of a
painting?
• Used Infra-red light to penetrate surface layer
and see how it interacts with deeper layers.
• Radar uses “light” of MUCH longer
wavelength to penetrate soil.
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Electromagnetic waves
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Radar
Distance =
(travel time)/(twice velocity of light)
10 microseconds  1 mile
2 nanoseconds  1 foot
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Can energy really go
from here to there???
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WE SUGGEST THAT YOU
TURN OFF YOUR LAPTOPS
AND WIRELESS DEVICES
(INCLUDING CELL
PHONES)
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