3-D High-Resolution Seismic And Georadar Reflection Mapping Of Glacial, Glaciolacustrinel And Glaciofluvial Sediments In Switzerland

Abstract

During the past decade significant progress has been made in the development of geophysical techniques designed to resolve problems associated with the unconsolidated sedimentary layers of the shallow subsurface. Two of these techniques, the high-resolution seismic reflection and the ground-penetrating radar (georadar) methods, have the potential to define the character, size, and geometry of surficial sedimentary units. In an attempt to determine optimum recording and processing strategies, seismic and gwradar reflection profiles have been recorded across a variety of glacial, glaciolacustrine, and glaciofluvial deposits in Switzerland. At one test location we have also conducted a pilot 3-D high-resolution seismic reflection survey and at another we have collected an experimental 3-D gwradar data set. Results of the 3-D surveys are striking images of the shallow subsurface that provide many more details and much more reliable information on the 3-D heterogeneities of the shallow subsurface than are afforded by conventional 2-D data sets; at both 3-D test sites critical geological features would have been misinterpreted had only limited profile information been available. INTRODUCTION Mankind is placing increasing demands on the surficial geology in formerly glaciated terranes in connection with groundwater, resource exploration and engineering (including waste disposal) activities. Switzerland’s vast glacial and glaciofluvial deposits contain more than two-thirds of the country’s aquifers and include large concentrations of industrial minerals (e.g. sand and gravel). Because the most densely populated centres are built upon these unconsolidated deposits, the underlying groundwater aquifers are susceptible to contamination from chemical spills, leaky waste disposal sites and polluted rivers. It is well known that groundwater flow is influenced strongly by the nature of the surficial geology (i.e. the distribution of lithofacies) and recent investigations have shown that predictions of contaminant transport require reliable information on the 3-D distributions of the highly permeable sedimentary units. Sands and gravels have by far the highest hydraulic conductivities, so knowledge of their distribution and physical characteristics is key to an improved understanding of groundwater transport and containment. Variations of glacial lithofacies and associated order-of-magnitude changes of hydraulic conductivity may occur over such short distances that even a relatively dense network of boreholes and outcrops may not resolve the geometries and sizes of the different geological units the information would be “spatially aliased”. The task for engineering and environmental geophysicists is to further develop non-invasive and non-destructive methods that allow the character (lithofacies), size, geometry and 3-D distribution of important unconsolidated sedimentary units to be determined. As a contribution to this goal, high-resolution seismic and georadar reflection data have been recorded across a variety of Swiss glacial deposits. In this paper we describe preliminary results of the following: n high-resolution 2-D and 3-D seismic reflection surveys conducted across a sequence of till and ouhvash overlying glaciolacustrine sediments (Stetten survey area); H high-resolution seismic reflection profiling performed over end moraine sediments (Suhretal survey area); n 2-D and 3-D georadar surveys conducted across glaciofluvial sediments (Rafzerfeld survey area).