ALBERT

Notes: Book reviews in this article:THE CHESA PHOTOGRAMMETRIC EXPERIMENT (Technical Publication

Notes: The salient features of a method of analytical triangulation operating directly on single photographs are described, and the close analogy with geodetic triangulation computation is demonstrated.Any combination of photographs, whether in stereo pairs, stereo triples, strips, sub-blocks or blocks, is handled by forming the set of observation equations for each individual photograph in turn, and then taking all these sets together and solving them simultaneously by the method of least squares.Special procedures for dealing with very large numbers of variables are discussed, and certain applications of the method are indicated.

Notes: This second part deals with a subject analogous to the one developed in the first part, but related to surface irregularities with a non-cylindrical structure. In this part we have taken into consideration both the effects due to potential electrodes and the effects due to current electrodes.The study of these cases has been made with the help of mathematical theory, but it has been necessary to extend this theory from the case of cylindrical bodies to the case of non-cylindrical ones.Finally, as conclusion to both parts of the paper, some pages have been dedicated to the details of the calculation procedure, in order to demonstrate its practical htness for the study of this problem.

Notes: The usual computation methods in refraction seismic in the Sahara often involve considerable shot distances. Therefore, in addition to a high consumption of explosives, difficulties of operation and interpretation arise.In order to cut down the shot distances, it was necessary to study with great care the refracted waves having the shortest possible traveling path along the marker. Such paths correspond to the zone of great relative energy resulting, on one hand, from the sudden increase of reflected energy beyond the critical angle (total reflection) and, on the other hand, from the sudden appearance of refracted energy. The proposed method is founded on correlations of traces located in this “critical zone” on the different records shot on a profile; in order to get around certain difficulties due to insufficient knowledge of the marker velocity, the interpreter is led to select a constant correlation distance, at least for sections of the profile.The paper describes in detail the setting up and utilization of the proposed method, analyses a few examples and gives an estimate of the advantages and the restrictions of this method.

Notes: 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.

Notes: The vertical component of the attraction exerted by a horizontal homogeneous polygon is obtained as the sum of terms proportional to the difference, for each of the vertices, between the external angle and its spherical perspective. A set of computation charts is presented, each for a fixed value of the angle of the polygon (22° 30′, 30°, 45°, 67° 30’ and 90°). These charts give the difference between the external angle and its spherical perspective as a function of the quotient of the distance from the sides to the vertical through the station over the depth. The chart for the right angle is an alignment chart, permitting a more accurate reading; the others are graphs in rectangular coördinates.For a remote station the result would be obtained as a small difference between relatively large quantities, and consequently with a not quite satisfactory precision. For these stations it is preferable to assume the entire mass to be concentrated in the centre of gravity.