3-D high-resolution reflection seismic imaging of unconsolidated glacial and glaciolacustrine sediments: processing and interpretation

Shallow 3-D seismic reflection techniques have been used to map glacial dep osits in a Swiss mountain valley. A dense distribution of source and receiv er positions resulted in a small subsurface sampling of 1.5 m x 1.5 m and a high fold of >40. Common processing operations that included pseudotrue am plitude scaling, deconvolution, and band-pass filtering successfully enhanc ed shallow reflections relative to source-generated noise. Careful top muti ng helped avoid erroneous stacking of direct and guided waves. Azimuth-depe ndent velocity analyses proved to be unnecessary. Three-dimensional (3-D) m igration of the slacked data yielded the final high-resolution images of th e shallow subsurface(15-170 m). Because most reflections and diffractions w ere migrated to their correct subsurface locations, confident interpretatio ns of 3-D structures were possible. Time slices and cross-sections along ar bitrary directions proved to be powerful analysis tools. Even small-scale f eatures (<20m wide), such as subglacial channels and troughs, could be mapp ed. Five major lithologic units separated by four principal reflecting boun daries were distinguished on the basis of their characteristic seismic faci es. The principal reflecting boundaries were semiautomatically tracked thro ugh the 3-D data volume. Borehole information allowed the uppermost boundar y at 15-27 m to be identified as the top of a 68-80-m-thick sequence of bas al and reworked tills characterized by high-amplitude discontinuous to quas i-continuous reflections. Low reflectivity of seismic units above and below the till units was associated with finely layered or massive glaciolacustr ine clay/silt deposited during and after two principal phases of glaciation (Wurm at 28000 to 10000 and Riss at 200000 to 100000 years before the pres ent). Top of Tertiary Molasse basement was delineated by prominent east-dip ping reflections at variable depths of 85-170 m.