Ground penetrating radar (GPR) is a popular and important subsurface imaging tool for environmental and engineering site investigations. Full-waveform inversion of crosshole data offers greatly improved resolution over standard ray-based tomography, but still suffers from limited angular coverage of the target and convergence problems due to the high degree of non-linearity of the forward problem. Here, we present a modification to the standard time-domain full-bandwidth inversion approach that mitigates the latter problem. We start with a low frequency filtered version of the radargrams to avoid getting trapped in a local minimum and progressively expand the bandwidth as the iterations proceed. This conveys stability and at the same time builds resolution as the shorter wavelength features of the permittivity and conductivity distributions are sequentially added. We illustrate the improved performance of this scheme over the traditional approach by means of two synthetic examples – the first involving two small embedded high/low permittivity and conductivity bodies in a uniform background and the second involving multiple low conductivity / low permittivity bodies in a layered background with superimposed stochastic fluctuations.


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