Enhanced ground-penetrating radar data analysis through multi-trace coherence imaging

Abstract:

Ground-penetrating radar (GPR) surveying can be a powerful method to image natural and human-made structures in the shallow subsurface in three dimensions. Commonly, GPR data are either visualised as 2D vertical amplitude sections (aka GPR profiles or B-scans) showing the positive and negative reflection amplitudes of the individual recorded GPR traces plotted versus the two-way travel-time of the GPR pulse, or horizontal amplitude maps representing GPR time- or depth-slices (aka C-scans). The amplitude maps can either show positive and negative reflection amplitudes, or, when thick maps are created, amplitude data after envelope trace calculation using the Hilbert transformation. While GPR reflection amplitudes can successfully reveal anomalies caused by structures in the subsurface, such as strongly reflecting buried remains of walls, stone floors, compacted interfaces, or utilities (pipes and cables), as well as strongly absorbing fillings of infilled trenches, pits or postholes, more subtle subsurface changes do not necessarily cause noticeable changes in reflection amplitude.

In exploration seismology, seismic trace attribute analysis has been developed and successfully used to extract other than merely amplitude information from the data. In particular, multi-trace coherence, discontinuity or semblance analysis has proven to be under suitable conditions highly useful and complementary to traditional seismic imaging. We have implemented and applied this approach to the imaging of similarity in extensive high-resolution GPR data sets, using different algorithms. Particularly single-phase Scandinavian archaeological sites, comprising numerous subsurface discontinuities in form of refilled postholes, pits and gullies, seem to be well suited for coherence imaging.

Through fusion of traditional GPR amplitude maps with coherence/discontinuity maps using different colour scales it becomes possible to extract more information on buried archaeological and geological structures from the data. The fused data images permit enhanced understanding of the present situation and thus an improved interpretation of the data. In case of extensive high-resolution GPR surveys, multi-trace discontinuity imaging should become a standard procedure alongside traditional amplitude mapping.