Explosions and seismometers.
Basically, they’re shaking their birthday presents and guessing the contents based on the sound.
By setting up a grid of seismometers (sensors that measure ground motion), they are able to collect high sample rate data that can be processed together with the known geolocations of the sensors to determine the speed at which a signal travelled through the ground. These signals are triggered impulses from explosions, detonated at known geolocations.
By using a lot of fancy math and science, they can predict with relative precision the rough composition of the ground in the site they are exploring.
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