Active Remote Sensing for Archaeology Surface and subsurface clues

Radar Image – Lost city of Ubar on the Arabian Peninsula (Omam) Occupied from 2000 BC to 300 AD. Remote desert outpost where traders assembled caravans for transporting frankincense. Dunes Wadi (stream bed) Ubar Trails leading to Ubar Limestone bedrock This Radar image uses 2 bands with 3 polarizations to make a color image. Used to find Ubar in 1992

Great Wall of China

Applications of radar for archaeology Characterization of surface can yield archaeological information not apparent in optical imagery Surface roughness can relate to past activities Ability to see through clouds (e.g., tropics) Ability to see through tree canopies (e.g., tropical forest) Radar can penetrate into dry soil and detect subsurface features Airborne – a few cm to a couple of meters Ground penetrating radar – a few meters to 50 m

Airborne imaging radar for archaeology Airborne radar can survey large areas quickly Creation of detailed topographic data that can reveal archaeological features (e.g., pyramids, temples, burial mounds) Can penetrate slightly into the surface and see buried walls, etc. Can penetrate tropical forest canopies and clouds (e.g., Inca ruins in Peruvian jungles) Can sense surface differences associated with trails and roads that aren’t spectrally visible

Case Study: Angkor Wat, Cambodia Complex of 60 temples that date to 9th century Once had population of 1 million Today is buried in dense rainforest Usually cloudy Scientists used Shuttle Imaging Radar to map this site, including unexcavated structural remains

Ground Penetrating Radar (GPR) Long wavelength radar can penetrate into dry soil Large dielectric constant of water inhibits penetration into wet soil Works particularly well in dry sand Physical and chemical changes in the ground affect radar returns and can give clues about buried materials Wherever there is a change in the dielectric constant of materials the radar is reflected. Strongest reflection where materials very different. Thanks to Lawrence Conyers, University of Denver (http://mysite.du.edu/~lconyers/SERDP/GPR2.htm) for much of the GPR info in this lecture!!

Why GPR for Archaeological Surveying? GPR data are inexpensive and easy to acquire. GPR is non-invasive, leaving archaeological artefacts intact. GPR has the potential to image the subsurface with a higher resolution than any other geophysical technique. It’s a valuable part of a geophysical dataset – excavation can be completely avoided if enough high quality are collected. Slide and photo from Adam Booth, Leeds University, England

How do you collect GPR data? Radar instrument is moved along closely spaced transects Radar returns are measured every 2 – 10 cm Can plot strength of return with time for round trip on y-axis Plotting all traces on one graph and coloring strong returns darker and weak returns lighter give a “picture” of the subsurface

GPR data collection at Petra, Jordan (Photo by Dr. Lawrence Conyers)

Trade-offs Long wavelength radar can penetrate more deeply into the soil but… Long wavelength radar has low spatial resolution (can only sense large objects) Minimum object size that can be detected is about 75% of wavelength So…have to strike a balance between penetration depth and resolution Also must consider the radar “footprint” when planning transect spacing so that all objects are illuminated by the radar pulses

Kansas Cemetery Black squares are tombstones Colored areas are graves

Civil War Bunkhouse – Ft. Garland, Colorado

Soil moisture can have strong affect Kiva before and after rain – rain obscures ability to see the structure Figures from Lawrence Conyers

Kiva-like structure, Grand Gulch, Utah

References L.B. Conyers and D. Goodman, 'Ground Penetrating Radar, An Introduction for Archaeologists' (AltaMira Press, 1997)