Digital impulse radar surveys were undertaken on two stone masonry arch bridges: a single span low rise arch and a twin span arch. It was found that on highly attenuated arch bridge fill, conventional impulse radar reflection surveys proved ineffective. In the cases examined, transmission radar surveys proved more effective. The second aspect of the work involved using tomographic imaging based on fuzzy logic to gain greater insight into the structure. It was found that three-sided tomography (i.e. upstream-wing-wall/abutment/downstream-wing-wall or upstream-spandrel-wall/arch-intrados/downstream-spandrel-wall) would give better coverage of the sections investigated and better reconstructed image resolution. The greater the angle of the ray paths connecting the radar transmitting antenna and the receiving antenna improved the defect location by enhancing the "time-depth" of any anomaly present. Variations in the composite fill of arch bridges were discussed in relation to the general equation of electromagnetic wave velocity taking account of conductivity and frequency effects. For the covering abstract see ITRD E111699.