Fig. 84 Archaeological GPR survey in Jordan. Image source .

Ground penetrating radar (GPR) is an electromagnetic method used for very near surface exploration; metres through 10s of metres. It is sometimes referred to as echo-sounding. GPR exploits contrasts in electromagnetic properties in order to define the physical boundaries of shallow structures (both natural and man-made). Physical properties important to GPR include: Dielectric Permittivity, Electrical Conductivity and Magnetic Permeability. Ground penetrating radar is used for a variety of applications. These include:

• Investigating concrete structures and roads
• Mapping the thickness of peat-bogs and other sedimentary layers
• Obtaining the depth to groundwater or permafrost
• Locating buried infrastructure such as irrigation pipes, tunnels and electrical wires
• Finding land mines and unexploded ordnance

Fig. 85 Schematic of a zero-offset GPR setup.

During GPR surveys, a source ($$Tx$$) is used to send a pulse of high-frequency electromagnetic waves (radiowaves) into the ground. As the radiowaves propagate through the Earth, they are distorted as a result of the Earth’s electromagnetic properties. At boundaries where the subsurface electromagnetic properties change abruptly, radiowave signals may undergo transmission, reflection and/or refraction. Because of this, much of the understanding of seismic methods can be applied to ground penetrating radar.

Fig. 86 Radargram example of two underground tunnels (hyperbolas). Gray-scale shows the amplitude of the returning signal.

Sensors ($$Rx$$) at the Earth’s surface measure the amplitudes and travel times of signals returning to the surface. These data from each trace (Tx-Rx pair) are amalgamated and represented using a radargram (similar to a seismogram). The horizontal axis (distance) is used to represent the location of the receiver relative to the source for a particular sounding. The vertical axis shows the travel times of measured signal. By assuming some a-priori knowledge of the radiowave velocity, the vertical axis is sometimes represented by an approximate travel distance. The gray-scale denotes the amplitude of the returning signal at each time and location. It is from radargrams that we will do the majority of our interpretation of GPR data.

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