VARIATION DES VITESSES EN RAPPORT AVEC L'AGE EN LA LITHOLOGIE APPLICATION DE LA LOI DE FAUST*

It was felt that Faust's first law was the best guide in classifying data concerning velocities and their correlation with lithology, even if, on a local basis, other laws fit the observed values better.

Data have been re‐examined which were provided chiefly by borehole measurements, both conventional and continuous, but also by some particularly reliable velocity profiles. “Faust's coefficient” has been computed for a series of geological formations, i.e. the ratio of true velocity to the value resulting from Faust's law, according to the age and present depth of the formation.

Some graphs are shown, in the first place concerning the Sahara, and especially the Palaeozoic. It can be seen that the only formation showing values notably less than I disappears.

On the other hand, values greater than I can be noticed for the quartzitic Ordovician of limited thickness.

This comparison has been continued on a broader basis, using, in particular, data published by Faust and Wyrobek. The data concerning the very homogeneous Westphalian result in an average value slightly less than I.

The Aquitanian basin has been studied in greater detail, owing to the closer spacing of the measurements.

Generally speaking, the Middle and Upper Jurassic, the Cretaceous and the Eocene‐Oligocene show values notably higher than the Palaeozoic in the Sahara. This seems to be connected with the higher proportion of limestones and dolomites. Values as high as 1.7 are observed in compact dolomites.

Four maps are given, concerning the Oligocene, the Eocene‐Paleocene (one of them excluding the Danian) and the Upper Cretaceous (exclusive of Danian), and a cross‐section showing the link with lithology.

Some discrepancies with regard to data from borehole samples are discussed.

The question may be raised as to whether such studies can be carried out at the very beginning of exploration of a basin, on the basis of only a few logged wells. In the Aquitaine basin, data given by velocity profiles are in close agreement with those from wells as regards the Tertiary; but as regards the Mesozoic, information is meagre. In basins of the Sahara type, synthetic records have shown that interference of true and multiple reflections with each other is a major drawback. But on the other hand, the possibilities of a combined use of reflection and refraction in order to define lateral changes in velocities may not have been fully appreciated. Some computed examples are shown, since the results of a survey at present being carried out on this basis cannot yet be published.

Finally, mention is made of the part played by the determination of anisotropy, and an appeal is made for more coöperation between geophysicists in this sphere.