Using integrated near-surface geophysical surveys to aid mapping and interpretation of geology in an alluvial landscape within a 3D soil-geology framework

摘要

An integrated geological, geophysical and remote sensing survey was undertaken as part of the construction of a high-resolution 3D model of the shallow subsurface geology of part of the Trent Valley in Nottinghamshire, UK. The 3D model was created using the GSI3D software package and the geophysical techniques used included ground-penetrating radar (GPR), electrical resistivity tomography (ERT) and automated resistivity profiling. In addition, the remote sensing techniques of light detection and ranging (LIDAR) and airborne thematic mapping (ATM) were used. The objective of the study was to assess the contribution of these techniques to improve the geological mapping and interpretation of terrace deposits and other geological features. The study site had an area of ~2 km2 and consisted of a Triassic mudstone escarpment, overlain first by a sand and gravel river terrace that extended to the modern floodplain of the River Trent. Automated resistivity profiling mapping proved to be the central tool in identifying and positioning geological features at a greater resolution than would be obtained through traditional geological mapping and borehole observation. These features included i) a buried cliff delineating the south-eastern limits of the incised Trent valley, ii) siltstone beds within the Gunthorpe Member of the Mercia Mudstone Group and iii) the variability of the sediments within the river terrace. A long ERT transect across the site successfully imaged the buried cliff and outcropping siltstone beds on the escarpment. Combined ERT and GPR transects revealed the depth of the sand and gravel deposits (Holme Pierrepont sands and gravels), whilst the GPR provided information about the depositional environment. Remote sensing using light detection and ranging proved essential in the original geological survey because it confirmed the absence of a second river terrace that had been previously thought to exist. This case study demonstrates the importance of combining geophysical techniques with traditional geological survey and borehole analysis, in order to create high-resolution 3D geological models, which are increasingly being used as a platform to understand and solve environmental problems.