Ground penetrating radar uses very low frequencies to bounce off objects and surfaces under the earth's surface. One of the advantages of ground penetrating radar is that with a fairly flat site, a person who is pushing the cart the unit is mounted on can survey the entire site in a day. Unfortunately, on uneven ground you usually can't use the cart. In addition, because of the low frequencies used, the images that result are usually very low resolution, even with computer enhancement.
While metal detectors are most often thought of as something coin collectors use, archaeologists also use them on sites where they expect to find metal. Obviously, at a Stone Age site, the detectors are useless. Metal detectors generate a current that, when it encounters metal objects, creates a return signal. Standard metal detectors can detect coins at a few inches and larger metal objects, such as a shield, at a couple of feet. The size of the detector's coil has an effect on the depth you can reach, and very large coils can sometimes detect large metal objects as far down as ten feet.
In some circumstances, such as bedrock or stone walls, radar and other such approaches don't work. These sites are often found in places like Egypt, where tombs were sometimes cut into solid rock. For sites like this, archaeologists often use equipment designed to send out high frequency sound waves that then reveal openings or passages beyond the wall in question.
One very useful tool for archaeologists when they are looking for new sites, or trying to determine the extent of an existing site, is aerial photography. Infrared cameras that are using dual lenses to take stereo images can often reveal features that are hidden from the ground. For example, what looks like just a slight rise from the ground can turn out to be part of a city wall or moat.