In the GPR scanning process electromagnetic energy (in the form of radio waves) is pulsed into the substrate (ground, concrete, etc.) via an antenna at a certain frequency. These pulses bounce off of buried objects (such as pipes and conduits) and are then captured by a receiver antenna.
Computer software within the GPR equipment records the strength of the reflected signal (amplitude and frequency) and the time delay between transmitting the pulse and receiving the signal. The time delay allows the system to calculate the depth to the object. Detected items are revealed in real-time on the computer screen as the GPR equipment moves along. In some cases it can even be determined if the object is metallic or non-metallic.
Since the equipment uses electromagnetic pulses, the depth of the scan is determined by the electrical conductivity of the material being scanned. Material with higher conductivity achieves greater depths, and vice versa. The greater the depth of the scan, the less accurate it becomes. Because of this, GPR is often conducted using high-frequency waves that may only penetrate several feet into the concrete, but offer a much higher resolution and a clearer image of what’s inside.
Since GPR spreads out as it travels into the ground, as opposed to a straight beam, the antenna can “see” the buried object even if it is not directly above it. The reflected radio waves create an image where the buried objects show up in the form of peaks and hyperbolae.
The shape of the hyperbola, whether narrow or broad, depends on the electromagnetic velocity and size of the subsurface material. If the signal finds nothing to reflect off of, it will gradually dissipate as it passes through the material.