Gravity gradiometric data or gravity data transformed into vertical gradient can be efficiently processed in three dimensions for delineating density discontinuities. Model studies, performed with the combined use of maxima of analytic signal and of horizontal gradient and the Euler deconvolution techniques on the gravity field and its vertical gradient, demonstrate the superiority of the latter in locating density contrasts. Particularly in the case of interfering anomalies, where the use of gravity alone fails, the gravity gradient is able to provide useful information with satisfactory accuracy.
In February 1998, a regional-scale, airborne gravity survey was carried out over the French Occidental Alps within the framework of the GéoFrance 3-D research program.The survey consisted of 18 NS and 16 EW oriented lines with a spacing of 10 and 20 km respectively, covering the whole of the Western French Alps (total area: 50 000 km2; total distance of lines flown: 10 000 km). The equipment was mounted in a medium-size aircraft (DeHavilland Twin Otter) flowing at a constant altitude of 5100 m a.s.l, and at a mean ground speed of about 280 km h−1. Gravity was measured using a LaCoste & Romberg relative, air/sea gravimeter (type SA) mounted on a laser gyro stabilized platform. Data from 5 GPS antennae located on fuselage and wings and 7 ground-based GPS reference stations were used to determine position and aircraft induced accelerations.The gravimeter passband was derived by comparing the vertical accelerations provided by the gravimeter with those estimated from the GPS positions. This comparison showed that the gravimeter is not sensitive to very short wavelength aircraft accelerations, and therefore a simplified formulation for computing airborne gravity measurements was developed. The intermediate and short wavelength, non-gravitational accelerations were eliminated by means of digital, exponential low-pass filters (cut-off wavelength: 16 km). An important issue in airborne gravimetry is the reliability of the airborne gravity surveys when compared to ground surveys. In our studied area, the differences between the airborne-acquired Bouguer anomaly and the ground upward-continued Bouguer anomaly of the Alps shows a good agreement: the rms of these differences is equal to 7.68 mGal for a spatial resolution of 8 km. However, in some areas with rugged topography, the amplitudes of those differences have a striking correlation with the topography. We then argue that the choice of an appropriate density (reduction by a factor of 10 per cent) for computing the ground topographic corrections over the highest mountains, results in significantly reducing the differences between airborne and ground upward-continued Bouguer anomalies, which shows that some of the misfit stems from errors in the ground data.
AbstractThe hypothesis, proposed by the late Gunter Klein, that the autonomous nyctinastic movements of bean leaves are related to the tidal force exerted by the Moon was tested. Using data collected by Dr Klein, a close correspondence was found between the time at which leaves initiated a sudden downward turning movement and the time at which the tidal force changed from either a minimum ('low tide') or a maximum ('high tide'). In many cases the data sets of leaf movements and the changing tidal force gave evidence of correlative patterns of behaviour. Evidence was also adduced for a minimal 6.2-h interval, or multiples thereof, between successively executed leaf movements. Data from an earlier study by A Kleinhoonte were also examined and have been found to show even closer correspondences than do Klein's data between bean leaf-movements and changing tidal forces. Thus, Klein's hypothesis of coherence between leaf and Moon is upheld.
P308 MAGNETIC FEATURES OF THE SARDINIAN BLOCK (WESTERN MEDITERRANEAN) - AN OVERVIEW S. FAIS 1 E.E. KLINGELE 2 and L. LECCA 3 Foreword The Sardinian-Corsican Block represents a domain with magnetic continental crust type anomalies interposed between the magnetic domain of the Western Mediterranean and the Tyrrhenian Basin. The Sardinian-Corsican Block a residual element of the Oligo-Miocene geodynamic evolution of the Western Mediterranean is made up of a continental crust that dates back to the Hercynian orogeny. Subsequently this crust underwent rifting processes during the Triassic and the Jurassic and was finally involved in the Africa-Europe convergence processes during the
