Trent Valley GeoArchaeology                                                                            

Ground penetrating radar surveys use pulses of Electromagnetic (EM) radio waves directed down into the soil profile from a transmitting antenna in order to investigate subterranean features. When discontinuities are encountered some of these radio waves are reflected back towards the surface, whilst other waves travel further down into the soil profile until they meet other discontinuities. At the surface a receiving antenna measures the reflected waves. By measuring the time taken between emission of the radar pulse and reception back at the antenna it is possible to measure the depth of a discontinuity in the soil profile. Within a floodplain context the boundaries between different geomorphological units will be seen as discontinuities, due to their different physical properties, e.g. clay and gravel.

Data are collected in single transects through pulling the GPR over the ground and collecting data either continuously or at set intervals. These GPR transects are calibrated for changes in surface topography, as recorded by a differential global positioning system (dGPS). The transects can then be viewed singly to give a vertical profile of the section or alternatively, several spatially referenced transects can be welded together to produce a solid cube. This cube can be sliced at set intervals producing plan views of the subsurface environment.

The primary aim of the GPR survey was to provide information relating to the subsurface stratigraphy of the floodplain, which could in turn be used to both classify different geomorphological units and to produce relative chronologies of different geomorphological units. This information would be of considerable benefit in ascertaining the potential palaeoecological and geoarchaeological resources which are present in these different sedimentary units. The surveys revealed that GPR penetration was considerably greater over gravel-dominated bodies (up to 4+m) than over palaeochannels where it could be as little as 1m or less. GPR therefore proved able to highlight the location of palaeochannels with high palaeoenvironmental potential, but the high water tables and clay-rich sediments often associated with these alluvial features meant that depth penetration was limited and the internal sediment stratigraphy was not recorded.

                                                                                                                                                                                               GPR depth slices combined with lidar intensity image

Electrical Resistivity Ground Imaging (ERGI)

Electrical Resistivity Ground Imaging (ERGI) uses multi probe measurement of soil resistivity to generate subsurface profiles and has previously proved effective at differentiating sediment units within fine-grained alluvial contexts. The system employed was the Syscal Junior resistivity system, a forty eight channel system that can be used in a variety of probe arrays, although principally Wenner (alpha, beta, gamma), Wenner-Schlumberger and dipole-dipole arrangements. The electrode spacing was altered between 0.25m and 1.0m to provide alter the depth penetration and resolution of the technique. The ERGI equipment was primarily used to look at the palaeochannel depth and composition, and to compare the effectiveness of the method against GPR. The results of the surveys demonstrated that ERGI provided considerably more detail in fine-grained sediments than GPR was capable of and was even able to identify within channel sediment architecture.