f Geophysical investigations of a possible impact structure near Raabs/Austria

A circular depression of about 400m diameter is located in the vicinity of Raabs/Austria (Fig.1). The question arose whether this structure could have been caused by an impact. The structure is very flat, about 5m of depth, whereas impact structures of this diameter should have a depth of at least 40m. However the circular shape of the flat central plane and the steep flanks resemble a crater that could have been filled by sediments at a later time. There are no rock exposures in the near vicinity, therefore shocked quartzes to prove the impact could not be found. The surrounding rock material is gneiss and amphibolite. Impact craters are associated with a low density zone of destroyed rock in the centre caused by the impact shock wave. This low density area can be recognised by different geophysical methods. At the Raabs site refraction-seismic, gravimetric, magnetic and geoelectric studies have been made. The refraction-seismic were carried out at two perpendicular profiles and indicated virgin rock with velocities of up to 6000m/s no deeper than 8m below the surface in the centre. Gravimetric and magnetic studies were carried out at stations on a regular grid of 50m mesh size covering the structure and extending to the outside region. In the centre measurements were taken on a more dense grid with 10m of mesh size. One line striking N-S was used for a comparison of all geophysical methods that had been applied. It was found that the gravity anomalies did not follow the shape of the surface. The Bouguer anomalies shown in fig. 2 are dipping about from SW to NE without any relation to the topography. 2-D modelling along the profiles was carried out. The models suggested that the amphibolite was dying out toward N beneath the structure. The depth to the solid rock found by the models fit the depth found by the seismic studies. Fig.3 shows the free air anomalies along the N-S profile passing the centre and the geological interpretation. A small micro-gravity high located near the centre is well defined and also shown. A correlation with a nearby magnetic high was suspected but could not be verified. EM34 measurements were taken along two profiles crossing the whole structure, whereas multielectrode measurements were made on a small profile in the centre only. The conductivities found with the EM34 showed an anomaly in the centre which could not be correlated with the seismics carried out on the same line. The Schlumberger measurements showed low resistivities (142 Ohmm) at the surface and high resistivities (1200 Ohmm) at a depth of 8m indicating solid rock. Very high magnetic anomalies have been found within a small region in the centre. This zone could not be identified by the other methods. The conclusion drawn from the measurement is, that the structure cannot have been caused by an impact. The comparison of the methods with each other shows a general agreement concerning the main structure. However, some differences and details could not be recognised by all methods.