486 ENGINEERING GEOLOGY/Geophysics Figure Examples of different display formats (A) Plot of residual magnetics data showing ferrous targets, draped on an aerial photograph of the 80 brownfield site (B) Modelled seismic velocity cross section for proposed pipeline route (C) Three dimensional visualization of the geometry of the base of a landfill cell (green) and the top of leachate (blue mesh) derived from a combined two dimensional resistivity imaging and time domain electromagnetic survey method or methods being proposed For example, in the case of electromagnetic and magnetic methods, signal degradation or geophysical ‘noise’ may be introduced by the presence of surface metallic structures and overhead power lines For microgravity or seismic surveys, noise may result from traffic movements or wind and waves Where the level of noise exceeds the amplitude of the anomaly due to the target and where this cannot be successfully removed, the target will not be detectable The best way to assess the likely influence of site conditions is to visit the site at the design stage and/or carry out a trial survey The design of a survey can be aided by the use of powerful two-dimensional and three-dimensional forward-modelling geophysical software With information on the expected target size, depth, and composition, a geophysicist can evaluate the feasibility of a particular method, including likely error bars on modelled size or depth, and can determine optimum survey design parameters such as the sampling interval and the optimum configuration for the method (e.g transmission frequency for groundpenetrating radar and electromagnetic surveys or the geophone spacing in a seismic survey) Figures and illustrate the results of a feasibility study for a tunnelling project in London, UK Figures 6A and 7A are the client’s conceptual model of a Victorian well thought to be associated with a risk of subsidence caused by tunnelling at 20 m depth The feasibility analysis facilitated an early appreciation of