Clay-rich till plains cover much of the U.K. Such sites are attractive locations for landfills, since clay aquitards lower the risk of landfill leachate entering groundwater. However, such tills often contain sand and gravel bodies that can act as leachate flow routes. Such bodies may not be detected by conventional site investigation techniques such as drilling boreholes and trial pitting. A method of b'llided inversion, where a priori data are used to construct reference models for use in inverting electrical resistivity tomography (ERT) data, was proposed as a tool to improve the detection of sand and gravel bodies within clay-rich till sequences. Following a successful 2D guided inversion synthetic modelling study, a field study was undertaken. Seven parallel Venner 2D ERT lines, 11 resistivity cone penetrometry (RCPT) bores and electromagnetic (E1'.I) induction gTOlllld resistivity data were collected over a 72001112 site at Holmpton on the East Yorkshire coast, where at least two sand and gravel lenses were known to exist from cliff exposures. A 41-electrode section of one ERT line that passed over a sand and gravel body 20 m wide and 1m thick at a depth of 5m was chosen to test the guided inversion approach. The EM data determined the width of the body in the reference model and individual RCPTs determined the background resistivity, and the body resistivity and thickness. It was shown for the first time that the best solution model produced by 2D inversion of one dataset with a range of reference models could be determined by using the l2 model misfit between the solution modeL,> and associated reference models (reference misfit) as a proxy for the l2 misfit between the solution models and the synthetic model (true misfit). For a 2.5D study based on the same field data, the reference misfits and true misfits were poorly correlated. This is possibly because the true 3D geoelectrical structure of the field study site is not sufficiently well known to enable comparisons between the solution models and the true geoelectrical structure of the ground. The 2D methodology developed here is applicable in clay-rich till plains containing sand and gravel bodies throughout the U.K.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:485203 |
| Date | January 2007 |
| Creators | Catt, Lucy May Louise |
| Publisher | University of Leeds |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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