1887
Volume 50, Issue 4
  • E-ISSN: 1365-2478

Abstract

A high‐resolution reflection seismic survey was carried out in the southern part of the Bavarian Molasse Basin in 1998 and 1999. The survey aimed to investigate the near‐surface structure of the complicated transition from the unfolded Foreland Molasse to the Folded Molasse, and the Folded Molasse to the internally complicated thrust systems of the Helveticum, the Ultrahelveticum and the Rhenodanubian Flysch. The study is linked to the TRANSALP seismic project, and the results help to fill the gap between the surface and the upper 300–500 ms two‐way traveltime (TWT), typical of deep‐reflection seismic experiments. The environmental conditions encountered in the study area required that particular attention be paid to the acquisition parameters for the three seismic lines (each about 4 km long). The energy source was a small vibrator; the geophone spread, spacing and frequency range were adjusted to image reflectors, which were expected to dip steeply southwards.

In general, the unprocessed field records did not show signals that could be attributed to specific reflectors. Individual trace processing considerably improved the data quality, taking into account the influence of the Quaternary cover and also the strong lateral velocity variations of the shallow subsurface. The effects of the various processing steps, such as muting, refraction statics, residual statics and velocity analysis, are discussed. To assess the NMO velocities, the qualitative analysis of the seismic energy in a common‐shotpoint gather offered advantages over an analysis in a common‐midpoint gather or in a stacked section, and proved to be very effective. As demonstrated along the Miesbach 9801 line, low‐velocity zones extend locally down to about 400 ms, adjacent to zones of extremely high velocities close to the surface, reflecting steeply dipping strata.

Besides the Quaternary cover on top, the Miesbach 9801 and Miesbach 9802 lines exhibit many horizontal reflections, in places down as far as 1400 ms TWT, indicating the sedimentary sequences of the unfolded Foreland Molasse. The southern part of both lines is dominated by southward‐dipping reflection bands, indicating units of the Folded Molasse. The reflection pattern shown by the Miesbach 9901 line suggests that there is almost no Quaternary cover. Southward‐dipping elements reflect the internal structure of the Folded Molasse, whereas a rather diffuse reflection signature may be attributed to Rhenodanubian Flysch units.

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