Seeing Beneath the Surface: How Ground-Penetrating Radar Works
Ground-penetrating radar (GPR), a technology developed for gas and oil exploration, allows archaeologists to identify the possible presence of subsurface features without digging. In the photo above, an antenna transmitting impulses and receiving echoes is dragged over the site by author Yonathan Mizrachi (right) and an assistant. It sends high-frequency electromagnetic impulses into the ground. When the signal hits the interface between two subsurface layers, such as the surface of a buried wall or a cavity beneath the ground, part of the signal is reflected back. The rest continues still deeper to be reflected by other layers or objects (see diagram below).
After transmitting the outgoing pulse, the antenna detects the reflected signals. These signals are transformed into a visual display that provides an immediate view of the data. Below, Dr. Bernard Yudowitz (lower left) and Schashar Magali (right) work with the ground-penetrating radar processing unit. A stylus on a graphic recorder sweeps across a paper at a uniform speed. The reflected signals cause the paper to be darkened at points in proportion to the strength of the reflection. A large feature or empty space would produce a distinct area of difference on the paper. Because the antenna is being pulled forward slowly, each pass of the stylus represents a slightly different antenna position, and thus a slightly different underground “view.” As the recorder paper advances, the stylus generates a continuous cross-section of reflections from under the surface. From these patterns, a trained person can identify anomalies that suggest where to excavate. The tomb chamber within the cairn was located by this means.
