and receiver. The GPR transmits pulses of electromagnetic energy at various frequencies into the pavement system. The pulses are reflected back to the receiver by the interface of the various pavement layers. The dielectric constant of the various pavement layers is determined by coring the pavement and calibrating the GPR. The GPR can be used to determine pavement layer thicknesses, locate voids, and locate areas with high moisture. Use of the GPR has been standardized in ASTM D-4748.