In recent years considerable attention has been focused on the use of ground-penetrating radar (GPR) to detect a variety of pavement problems. The results to date have been mixed. Information on the electrical properties of highway materials is limited as is the ability to model the propagation of electromagnetic waves in a pavement system. A forward model capable of simulating the signature of GPR waveforms is proposed. A monostatic 1 GHz GPR is used in this study. The pavement system is modeled as a layered medium comprising flat parallel layers ofpavement materials laminated together. Physical laws governing electromagnetic wave propagation inside the layered medium are used to calculate the attenuation, dispersion, reflection, and transmission encountered by the pulse. Major reflection paths and some multiple reflection paths are selected that begin from the open tip of the antenna, penetrate into the pavement, and reach the pavement surface again. The analysis is performed in the frequency domain. The transmitted pulses are traced through each of these paths one at a time. The resultant echoesat the pavement surface are then positioned and superimposed together accordingto the time required for the pulse to travel each of the selected paths. A synthetic waveform is thus formed. This process is called forward modeling. The forward model is tested on data collected on experimental pavements of known layer thicknesses and types. Reasonable agreement was achieved between theoretically calculated and field-measured GPR traces. In predicting the amplitude of the wavesreflected from layer interfaces average errors of less than 9 percent were calculated. The error in estimating the time delays between peaks was less than 2.5 percent. More work is required, particularly in the area of measuring the complex dielectric properties of paving materials under a range of operational temperature and moisture conditions. (A)
Abstract