GPR surveys and excavation ground-truthing at the San Tau Backbeach Site, Hong Kong

This paper presents the results of an archaeological fieldwork project conducted by a pro-geophysics archaeologist (Atha) and two `archaeo-curious' GPR specialists (Chang and Lai) at the San Tau backbeach site in Hong Kong. Previous small-scale test pitting suggested that the site might be a locally unique Tang dynasty cemetery, with probable later (Northern Song) activity, but grave definition was problematic and the cemetery's wider extent remained unknown. However, the fine-grained, relatively homogenous background appeared ideal for GPR. A two-stage approach was used: a 400MHz antenna was applied in an initial prospection survey, while both 400MHz and 900MHz were used in a second campaign of higher-resolution intra-site analytical survey. The GPR results identified many `targets' and proved decisive in locating and discriminating both Tang dynasty graves and overlying Northern Song pits. Based on the results of GPR surveys and excavation ground-truthing, it seems the site may in total contain several hundred Tang burials, significant among which were several co-aligned `warrior burials' with iron weapons and tools. Reference to site records of object types (metallic or non-metallic) and sizes shows that the very different slice images by 400MHz and 900MHz GPR are best explained by the radar footprints in First Fresnel zone (FFZ), which is a function of object depth, antenna frequency, GPR wave velocity in soil, and two-way travel time of the objects' reflections. The findings indicate that GPR is in general highly effective on sandy coastal sites, and in particular can provide useful estimates of the size and character of buried archaeological features and artefacts. Based on our findings, more routine application in local archaeology is therefore strongly recommended.