On-line complex permittivity measurements for ground penetrating radar

One of the major problems for a subsurface radar to predict is the true target distance and maximum likely penetration depth in the media under investigation. These radar parameters can be determined from the complex permittivity of the media. Since the permittivity of the surface under examination is usually unknown, most subsurface radars are calibrated in time units only: actual subsurface distances have to be inferred from other knowledge of the soil/rock. This paper presents a technique to provide on-line, complex permittivity measurements for the ground penetrating radar, to help predict these important distance parameters. The system consists of an electrically short monopole antenna backed by a ground plane; the input impedance of the antenna is a function of the complex permittivity of the surrounding media. Using a model for the complex input impedance of the antenna, the electrical properties of the media can be determined. To test the performance of the system, measurements were taken to determine the electrical properties of air, salt water and pure water, since these properties are known accurately via Debye´s formula. Measurements of the complex permittivity of sand as a function of frequency and water content, as well as the permittivity of the saline solutions are presented. This system was then integrated into a stepped frequency continuous wave ground penetrating radar. Tests using targets buried at known depths in a large sand box were used to verify the performance of the system and its limitations. Results of field trials are presented