A combined dynamite - Vibroseis 3-D survey in the subalpine Overthrust zone of Austria

Unfavourable surface conditions and geological complexities have often prevented seismic exploration for oil and gas in areas of potential hydrocarbon reservoirs. An area in this category is located adjacent to the northern outskirts of Vienna. It is the so-called Overthrust F1ysch Zone. The mountainous topography, forests, dense population and a railway are all rather obstructive for a conventional 2-0 seismic survey. However, previously shot Vibroseis lines had revealed a structure which was subsequently drilled, and a natural gas reservoir of considerable economic value was found. The geology encountered in the weil Höflein 1 was from top to bottom as follows: F1ysch- Eocene to Upper Cretaceous to a depth of about 2000 m. The flysch layers dip steeply towards the south at angles up to 40°. Molasse- Miocene sediments dipping gently towards the south. The molasse has been overthrusted by the Ilysch nappes. Mesozoic- was encountered from 2500 to 3250 m. The gas was found in a Jurassic series called 'doloquartzarenite' at a depth of 2730 m. The overlying carbonates with an interval velocity of approximately 6000 m sec -1 give rise to a good reflection horizon which can be followed across the whole area. Basement- was hit at 3200m depth. It is composed of gneisses belonging to the Bohemian massif. In order to develop this gas field the decision was made to carry out a 3-D survey. However, the above mentioned surface conditions did not allow the use of a regular grid of source and receiver points, nor could a unique source, dynamite or Vibroseis, give uniform coverage over the area to be investigated. Thus, we were forced to the conclusion that it would be necessary to combine Vibroseis and dynamite to give an adequate practicallay out for use in the field. The project had to be regarded as a pilot survey because we know of no previous survey of this type. The survey was planned and performed by ÖMV geophysical department.