Relations entre la réponse acoustique (5 kHz) et la nature lithologique des fonds marins

Sub-bottom profilers which were worked out in the sixty years did not first come into general use. These last few years, some new devices have been developed with most sediment penetration, which are to be more systematically used in sedimentological prospecting. After reviewing the sub-bottom profilers development, with laying stress on their originality, the different points of view of the interpretation are considered: layers geometry, lithological correlations, relations between the reflected picture and the depositional dynamics. In a bounded area, about 15 km², 90 bucket dredged samples and 12 core samples allowed a detailed mapping of the facies. This map is compared with different data from the seismic reflection investigation (5 kHz). A first approach concerns the distribution of some typical sismofacies selected on the whole seismic prospection. Thus five echotypes were picked out. By comparing, we can make a correlation between the character and surface feature of the sediment, and the corresponding seismic echo. In a second stage, the reflectivity change is considered in relation with the different sediment facies. The relative reflection coefficients are measured, either on the first peak, or on a longer time interval (1 ms), by integrating the seismic pulse. Both ways result in two maps of the relative reflection coefficient distribution.     

The Rotational Parameters Of Hale–Bopp (C/1995 O1) From Observations Of The Dust Jets At Pic Du Midi Observatory

We observed comet Hale-Bopp (C/1995 O1) at Pic du Midi Observatory in 1997 from February 2.24 UT to March 31.89 UT with the 1.05-m telescope equipped with a CCD camera and broad- and narrow-band IHW filters. A total of 30000 images were acquired both during night- and day-time. The images were automatically reduced and all the images obtained within 10 min. were co-added to give a set of ∼1000 images used during the analysis. We can identify two jets on the images. The position angle of the brightest jet from February 2.24 UT to March 5.22 UT is measured using an automatic routine which searches for the averaged position angle of the maximum of brightness at a projected distance of 3200–6100 km from the optocenter. A preliminary model of nucleus rotation is used to fit the data and retrieve the rotational parameters of the nucleus. The best fit is found for a source located at a latitude of 64 ± 3°, a sidereal rotation period of 11.35 ± 0.04 h and a right ascension and declination of the North pole of 275 ± 10° and -57 ± 10°. This preliminary analysis shows no evidence for a precession. Grains with velocities of 450–600 m s⁻¹ and radii <;∼ 1 μm dominate the optical scattering cross section in the jets. This revised version was published online in July 2006 with corrections to the Cover Date.