ALBERT

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.

Notes: In the laboratories of the agip mineraria a new apparatus has been developed which, utilizing reflected energy collected by several groups of seismometers, provides on a single trace an indication of the total energy level. This indication is controled in time according to an exponential characteristic, thus compensating for the attenuation which is effected in the subsurface. The apparatus enables indications of successive energy levels to be obtained for different shots, thus furnishing a time section even during the routine field work.

Notes: The factors affecting the response of a velocity-sensitive well geophone are discussed and it is shown that the geophone movement in response to a seismic pulse is primarily due to the pressure difference between its ends. Several practical examples are calculated to show that the first arrival waveform recorded by a geophone depends on the distance from an interface and that these changes in recorded waveform may give errors of up to 3 msec in well-velocity surveys. These errors may be minimised by using pressure geophones or by avoiding measurements at geological interfaces.

Notes: Surface charges in seismic refraction work in the sand dunes of the Sahara have been necessitated by the many problems involved in shot hole driving. These include lost circulation, unconsolidated surface sands and gravels, caving, poor shot efficiency etc.Tests were conducted in September 1958 in an attempt to evaluate some of the problems involved in surface shooting. This method is complicated by the large number of variables, 8 major ones, influencing the shot pattern.It is difficult to evaluate each of such a large number of variables by an exhaustive and systematic test procedure due to the prohibitive number of shots that this would involve. However, two series of tests contributed greatly to an understanding of the problem, although they left some points to be cleared up.Firstly shots were fired using small charges (less than 25 kg). These gave interesting results, however, they are not applicable to the heavy charges currently in use (500-1500 kg). These results refer primarily to the use of suspended charges which gave superior results to those obtained from surface charges layed on the ground. They also tend to indicate the influence of the weight of the charge. This is manifested by a saturation in seismic energy, as the size of the charge increases.The results obtained from the shooting (200 kg) of several individual charges are not as pronounced. The following conclusions are drawn:Nitrate explosives, in current use in the Sahara for several years, give good results in refraction.– The use of suspended shots does not result in sufficient gain in energy over the ground shots to justify their more complicated utilization: preparation, partial destruction of supports, safety etc. Hence further shooting of suspended charges has been abandoned.– The detonation of individual charges by use of primacord layed parallel to the line, in preference to simultaneous electrical detonation, gave superior results from the energy point of view (especially for high frequency arrivals) and simplified lay-out procedures. Hence this method was adopted.– As for the small charges the relation between seismic energy and weight of the individual charges shows a saturation effect which is less pronounced.–The distance between the individual charges appears to be the most important variable. The relation, seismic energy-distance between charges, indicates an increase in a linear manner for distances of 5 to 20 meters. It would appear as though the beneficial effect for greater distances diminishes beyond this point.Finally, it would appear as though it would be preferable for a given overall charge to reduce the weight of the individual charges and increase the distance between the individual charges. Obviously this rule has certain practical and economic limits. If the cost of the unit of seismic energy is evaluated it is preferable to utilize large distances between charges (20 m) but also relatively large individual charges (about 25 kg).The testing program was followed by nine months of surveying in different types of terrain. The parameters were etablished from the results of the testing program.Economically the following conclusions may be drawn: The average explosives consumption increased from 1000 kg for the shot–hole shots to 1500 kg for the surface shots. This increase, costwise, represents almost exactly the cost of shot hole drilling at the present price of explosives. Efforts are being made to have the price of explosives reduced and should this realize the net result will be a definite cost advantage in favor of surface shooting.In addition, the introduction of magnetic recording in refraction work to the Sahara in 1959 appears to indicate a substantial reduction in the size of the charges needed, hence, a further advantage to surface shooting.

Notes: The purpose of the automatic gain control in seismic amplifiers is to balance the slow decrease of the signal in such a manner that its energy does not exceed the capacity of the recording system. At the same time the interesting seismic content of the signal has to be preserved as undistortedly as possible. At first, the technical data of AGC circuits are calculated. The characteristic curves of circuits with one, two, and three time constants are computed. In circuits with two and more time constants an oscillating of the AGC is possible. In order to guarantee aperiodic operation, the ratio of the time constants must not fall below a certain value. As long as these technical conditions are complied with, the free choice of the largest time constant remains open and can be accommodated to the geophysical conditions.From the endeavour to balance the slow decrease of the signal as well as possible, an upper limit for the choice of the time constants can be derived. A lower limit is obtained from the condition that the seismic content of the signal has to be preserved. The determinant time constant should lie between the two limits and differ from them as much as possible.Finally, the problem of the amplifier “squeezed” by energy pulses is touched.

Notes: Book Review in This Article:P. F. Shokin, Gravimetry (Apparatus and Methods for Measuring Gravity)

Notes: The paper deals with quantitative seismic measurements near underwater explosions. The blastings were carried out in an artificial small pond, the records were taken in adits, situated vertically below the charge. The range of charges extended from one detonator to 200 g of Ammon-Gelit explosive.Conventional seismic scaling by comparing amplitude and size of charge does not yield a simple power law, the exponent being larger for smaller charges and vice versa. This result is due to the different frequency-spectra of the seismic pulses, as shown by harmonic analysis. In contrast to this, the seismic energy as calculated from the records is directly proportional to the size of charge over the whole range.During the Lago Lagorai experiments in 1962 some quantitative seismic measurements near the shot point were also carried out. The relevant results are compared with the preceding ones and some differences are discussed.Some conclusions can be drawn:〈list xml:id="l1" style="custom"〉1The normal methodes, investigating the amplitude only, gives in many cases no clear description; this is especially true for nearby measurements with widely varying frequencies. Therefore it is suggested to extend conventional seismic scaling by including information on the frequency-spectrum and the kinetic energy of the seismic signal.2In order to get clear experimental data for theoretical investigations, it is necessary to extend the recording over a half-space, i.e. in adits or boreholes.3Influences of shape and size of the lake, in which the shots are fired, should be taken into account, too.