Towing an antenna along the ground surface, while radar pulses are transmitted into the ground.  Here is a 400 MHz antenna collecting data at the Petra site in Jordan

At the CATS test site in Illinois the 400 is rigged up with a survey wheel to better keep track of distance and the spacing of reflections along a traverse.

There are many different antenna frequencies.  For most archaeological applications we use the 400 MHz, as it has good resolution and depth penetration from about 50 cm to 3 meters, in some soils.  For more resolution, but less depth penetration the 900 MHz antenna is great.  Here is a picture of the 900 MHz:

For really deep penetration, but much less resolution we have used the 40 MHz antenna, which is very bulky, and can be moved around the ground surface only with one or two people.  We have gotten energy to more than 10 meters with this antenna, but resolution is quite poor (only things about the size of a car are visible!)