GDT is an imaging technology pioneered by Alan Witten, a researcher from the Oak Ridge National Laboratory.
A GDT device essentially consists of a shotgun aimed at the ground, several microphones placed in surrounding bore holes, and a laptop computer for real-time data analysis.
By mapping sound waves generated by the shotgun blasts, 3d images of the underlying area can be generated.
This technology was first put to use in 1988. Witten used it on the sandstone cliffs of New Mexico, uncovering a dinosaur now labeled Seismosaurus.
Since then, Witten's GDT has been used for various paleontology and archaeology projects, including the excavation of a lost city under the desert in Isreal.
Altogether impressive, considering the technology was intended to locate underground sludge.