Geophysical investigations in the European Alps

Joint interpretation of magnetotelluric and geomagnetic depth sounding data in the western European Alps offer new insights into the conductivity structure of the Earth's crust and mantle. This first large scale electromagnetic study in the Alps covers a cross-section from Germany to northern Italy and shows the importance of the alpine mountain chain as an interrupter of continuous conductors. Poor data quality due to the highly crystalline underground is overcome by Remote Reference and Robust Processing techniques. 3d-forward-modelling reveals on the one hand interrupted dipping crustal conductors with maximum conductance of 4960 S and on the other hand a lithosphere thickening up to 208 km beneath the central western Alps. Graphite networks arising from Paleozoic sedimentary deposits are considered to be accountable for the occurrence of high conductivity and the distribution pattern of crustal conductors. The influence of huge sedimentary molasse basins on the electromagnetic data is suggested to be minor compared with the influence of crustal conductors. In conclusion, electromagnetic results can be attributed to the geological, tectonic and palaeogeographical background. Dipping direction (S-SE) and maximum angle (10.1°) of the northern crustal conductor reveal the main thrusting conditions beneath the Helvetic Alps whereas the existence of a crustal conductor in the Briançonnais supports theses about its palaeographic belonging to the Iberian Peninsula.

Tab. 1: Abflußwerte der Pegelstation Vernagtbach

This paper describes the extraordinary difficulties met in gauging glacier streams and establishes criteria that have to be fulfilled for a correct determination of glacier run off. Discussion of the hydraulic principles of gauging in tranquil and rapid flow shows why the latter is to be favored for glacier streams.The concept of the station at Vernagtbach, the original stream bed, considerations ofproper dimensioning and characteristics of the construction of the gauging channel aredescribed in detail. A brief history is given of planning, organization and technology involved.The water level is recorded by two floats and one pneumatie gauge which together provide uninterrupted and trouble-free records. The rating curve was determined with current meters and shows an unambiguous relation between water level and discharge with little scattering of calibration values. Temporary changes of flow conditions upstream of the station do not have any noticeable effect on the water level in the channel.The discharge records of the summers 1974-1976 are presented and discussed. The maximum mean summer discharge was 1.33 m**3/s in June-September 1975; extreme mean monthly discharge was found in July 1976 with 2.51 m**3/s, the highest daily mean was 4.76 m**3/s and the maximum hourly mean was found at 7.23 m**3/s. The discharge conditions of the summers of 1974 and 1975 are very similar, while in the summer of 1976 they differed completely as far as seasonal and mean daily hydrographs are concerned: in 1975, 42 % of summer discharge was recorded in June and July compared to 76 % in 1976. The analysis of the hydrographs gives valuable clues to the mass and heat balances of the glacier.