Stratigraphic architecture and fault offsets of alluvial terraces at Te Marua, Wellington fault, New Zealand, revealed by pseudo-3D GPR investigation,

International audiencePast earthquake slips on faults are commonly determined by measuring morphological offsets at current ground surface. Because those offsets might not always be well preserved, we examine whether the first 10 m below ground surface contains relevant information to complement them. We focus on the Te Marua site, New Zealand, where 11 alluvial terraces have been dextrally offset by the Wellington fault. We investigated the site using pseudo-3D Ground Penetrating Radar and also produced a high-resolution digital elevation model (DEM) of the zone to constrain the surface slip record. The GPR data reveal additional information: (1) they image the 3D stratigraphic architecture of the seven youngest terraces and show that they are strath terraces carved into graywacke bedrock. Each strath surface is overlain by 3-5 m of horizontally bedded gravel sheets, including two pronounced and traceable reflectors; (2) thanks to the multilayer architecture, terrace risers and channels are imaged at three depths and their lateral offsets can be measured three to four times, constraining respective offsets and their uncertainties more reliably; and (3) the offsets are better preserved in the subsurface than at the ground surface, likely due to subsequent erosion-deposition on the latter. From surface and subsurface data, we infer that Te Marua has recorded six cumulative offsets of 2.9, 7.6, 18, 23.2, 26, and 31 m (± 1-2 m). Large earthquakes on southern Wellington fault might produce 3-5 m of slip, slightly less than previously proposed. Pseudo-3D GPR thus provides a novel paleoseismological tool to complement and refine surface investigation

Three-dimensional stratigraphy view from ground penetrating radar attributes for soil characterization

The Ground Penetrating Radar (GPR), a geophysical technique that uses non-destructive testing to detect objects and structure beneath the soil was a huge contribution in survey and engineering, especially in underground utility. GPR has been used since 1970 and the method is still undergoing upgrade alongside the sophisticated processing software. Nevertheless, soil is the principal medium which interferes with the signal penetration of GPR due to its physical and electrical properties. Thus, a study using soil stratigraphy is a prerequisite to understanding GPR radargram. In this study, a test bed was constructed to simulate different soil layers which consist of existing clay, sand, small stone, and crusher run stone. The GPR instrument with frequencies of 100, 250, 400, 750, and 900 MHz was used to collect the data. The processing was carried out using reflex software for image interpretation and three-dimensional (3D) visualizations. This study is expected to help surveyors in understanding the measurement, for example, soil composition, problems related to GPR underground surveying