Transcript Archaeological Prospecting - Are you sure you want to look at this?
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Archaeological Prospecting
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What have you got? Where do you look? Where do you dig???
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*Aerial photography
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*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 , 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
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Want to use electrical conductivity:
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Good insulators–dry brick, non-porous stone, glass, voids, dry soil
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Good conductors–metals
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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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+ + +
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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? =????
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Higher conductivity
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Higher moisture content
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More dissolved minerals
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Metal artifacts Lower conductivity
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Lower moisture
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Masonry
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Voids
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Conductivity array
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Conductivity data 20 meter grid
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Magnetometry
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How to look for magnetic materials?
Simplest of magnetometers is a compass
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Earth’s field
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Magnetometry Magnetometer Signal Surface Buried object
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Difference Signal Earth’s field Magnetic gradiometer Gradiometer difference 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
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Most obvious–iron artifacts Recall colors of unglazed pottery
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Red–hematite–Fe 2 O 3 –non-magnetic–“oxidized” Black–magnetite–Fe 3 O 4 –magnetic–“reduced” Magnetite containing soil
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Soil replaced by stone masonry or red brick
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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
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Reduction of iron in fire hearth
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Destruction of building by major fire
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6-element gradiometer
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Real data Conductivity Magnetometry
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Columned hall
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Aerial photo
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100 meters AA&A Spring 2002 Magnetic survey 40
Ground Penetrating Radar = GPR
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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
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Distance = (travel time)/(twice velocity of light) 10 microseconds
2 nanoseconds
1 mile 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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