GPR

Ground Penetrating Radar (GPR)

GPR surveys are impulse systems that transmit short duration EM pulses into the ground from an antenna near the surface. These EM pulses are reflected from interfaces with contrasting electrical properties back to the receiver section of the antenna connected to the control unit for processing and display. Contrasts in electrical properties of materials in the earth cause reflections of the radar signal. These reflections occur at different soil strata, soil/rock interfaces, rock/air interfaces (voids), fractures, manmade objects (drums, underground storage tanks (USTs), trenches, pits), or any interface that can create a contrast in the dielectric properties. Digging a trench and filling it, for example, can create a difference between the dielectric properties of the disturbed earth and the undisturbed earth.

The distance to reflectors on GPR profiles is determined by the echo delay time. The time can be converted to apparent depth: 1) if the velocity of the radar pulse can be determined; 2) if an object is at a known depth, the depths can be extrapolated; 3) the hyperbolic geometry of echo curves can furnish an estimate of depth; and 4) common depth point sounding can determine pulse velocity with separated transmitter and receiver antennas. Radar pulse velocities vary with position and depth and have a strong dependence on the earth's moisture content.

GPR profiles are continuous and repeatable during different times of the same profile. GPR produces digital data that can be processed and filtered to produce data enhancements for maximum penetration depth.

Compared to other geophysical techniques, GPR has a shallower exploration depth. GPR does not usually exceed 5 feet in clays, although it has recorded penetrations of over 100 feet in sands. GPR profiling depth is greatest in materials having a high resistivity such as coarse sands and gravel; it cannot profile through salt water.