aa&a spring 20021. 2 archaeological prospecting what have you got? where do you look? where do...
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AA&A Spring 2002 1
AA&A Spring 2002 2
Archaeological Prospecting• What have you got? • Where do you look? • Where do you dig???
– *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 ditches-More moisture
Roman roads-Paving-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-aerial view
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Closer aerial photo-interior
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How to see without digging?
• 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???
Activities of man-----subsurface inhomogeneities
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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??
+ _ _+
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
Earth’s field
Buried object
Surface
SignalMagnetometer
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Magnetic gradiometer
Earth’s field
Buried object
Surface
DifferenceSignal
differenceGradiometer
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MagnetometrySense variations in Earth’s magnetic field
need high sensitivitykilled by fluctuations in time
Solution:magnetic gradiometerpair of sensorslook 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– 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 dataConductivity Magnetometry
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Columned hall
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Aerial photo
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100 meters
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 mile2 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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