High resolution 3D multicomponent seismic for a better structural, facial and process-oriented characterization of glacially overdeepened valleys in the Alps
Creators: name: Buness, Hermann
name: Burschil, Thomas
Publisher: name: Leibniz Institute for Applied Geophysics, Stilleweg 2, 30655 Hanover, Germany
Year: 2019
Subjects: physics of the solid Earth geology
engineering geology
palaeontology

description: This project aimed at an improved seismic characterization of overdeepened Quaternary valleys and basins in formerly glaciated, mountainous areas using innovative seismic data acquisition, processing, and interpretation. We therefore carried out comprehensive seismic surveys in two overdeepened valleys, the 250 m deep Tannwald Basin (TB) in the Alpine foreland and the &gt;600 m deep inneralpine Lienz Basin (LB). The surveys comprised multicomponent and 3-D seismic experiments using P- and S- waves. In both basins, we were able to derive an excellent seismic imaging with P-waves, using a dense acquisition schema and pre-stack depth migration in processing. We could infer a seismic facies analysis, which lead to an interpretation of the whole set of profiles, thus giving a 3-D geological model of the survey areas, including several details, such as dislocated molasse blocks, eskers, onlap and foreset geometries, slumping, and alluvial fans. Due to the existence of an older research borehole in the TB, the erosional and depositional history could be revealed in this basin. In the LB, borehole information is not yet available, but expected to come in the course of the ongoing ICDP project. S-waves deliver generally a higher resolution, but less continuity of the reflections with respect to P-waves, and they depend strongly on the surface conditions, i.e. whether the surface is hard or soft. In both basins, they give a more detailed image of the uppermost 100 m, reducing the minimum depth of the seismic imaging to only 2-5 m below ground level. In the TB, the S-waves, excited using a small electrodynamic vibrator source, penetrate down to the base and show reflections, where P-waves cannot due to near-surface scattering. In the LB, a strong impedance contrast at a shallow facies change impedes deeper imaging. Signals from the base of the LB are still apparent, but could not be focussed into coherent reflectors. In addition, we compiled a selection of S-wave processing strategies enable focussing on various targets, e.g. a small-scale or coherency-enhanced image. We recorded 6-component data, i.e. the source being either inline or crossline oriented and the receivers recording 3-components. As expected, similar components (e.g. inline/crossline source and receiver) give the most coherent image. A considerable amount of energy on the mixed components indicates a high rate of wave conversion, imposing problems for filter techniques based on polarisation. Semblance analysis hint to anisotropy, but this subject need a further robust estimation of the errors of the method. Source tests revealed that a very small electrodynamic vibrator instead of a larger hydraulic one suffices to image the entire TB using P- as well as S-waves. This enabled a 3D multicomponent survey for an approximately 100 m x 100 m area with full coverage. The inline and crossline data were rotated into radial and transversal components showing most of the energy. The basin base as well as internal reflections became visible. A 3-D P-wave survey covering the same area was additionally shot as a benchmark study.