Explanation
Geophysical Diffraction Tomography (GDT) is a type of sonar adapted for detection of underground objects. A GDT device uses shotgun shells aimed into the ground to create sound waves which bounce off of underground layers and objects of certain densities. Microphones sunk into bore holes drilled around the potential find then pick up the waves and a computer is used to interpret the resulting data. Unlike underwater active sonar, in which the sound waves that bounce off of an object return and are heard by the receiver, the underground sound waves are picked up after having traveled in a direct line through the ground, with "holes" left in the image where an object of a different density blocked the sound. By measuring the phase shifts of the sound distance, a layer density can be determined, and with the help of computers, a three dimensional model of the underground objects can be mapped.
Uses in Archaeology
This technically is minimally invasive because of the slight disturbance to the site made by digging the holes for the microphones.
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