ALBERT

Notes: Reflected refractions are considered as the limiting case for ordinary steeply dipping reflections. Particularly, reflected refractions may be constructed (migrated) according to the same principles as ordinary reflections. In view of the inherent inaccuracy of the velocity data, there is no discrimination between reflected refractions and other steep events by methods of construction. Velocity determinations from reflection data also show a continuous transition from steep reflections to reflected refractions. I.e., it is impossible to discriminate by seismic ray methods between reflected refractions and other steep events.If disturbing, steep events may be attenuated to a great extent by special shot and geophone set-ups. Also in this connection it is unnecessary to distinguish between reflected refractions proper and steep reflections.It seems to be more rational to consider steep events as ordinary steep reflections in the beginning of a survey, because otherwise real structural indications might possibly be thrown away.

Notes: Measurements have been made of amplitudes of reflected seismic waves, in order to investigate their change as a function of the shotpoint-seismometer distance and of the position of the reflection point. The change with shotpoint-seismometer distance, after correction for geometrical divergence, was usually small. This indicates either a rather strong difference between the values of Poisson's ratio for adjacent strata, or, more likely, a rather small value of the absorption coefficient. In one instance a very strong increase in amplitude with shotpoint-seismometer distance was observed. This may be attributed to incidence near the critical angle. In another instance a pronounced increase in amplitude with the position of the reflection point was observed over a very short distance. This may be due to an elongated depression of the reflecting boundary.

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: In the last two years several double-receiver Velocity Logs (V-Logs) have come into service. Theoretically, in this type of log the effect of the travel time in the mud and in the invaded or altered formations, responsible for the “Lag” (or delay times) in single receiver V-Logs, could be eliminated. Thus the integration of the interval times would give the “real” vertical time as a function of depth. In practice, several authors have already noted that even for V-logs with two receivers the integrated times differ from the results of the conventional well surveys.We consider here some of the geometrical, geological and instrumental factors that affect the time measurement of double receiver Velocity Logs, as well as the possibility that the discrepancy between the V-logs and the conventional well survey could in some cases be due to faults, dip, folding, or the intrusion of high velocity layers near the well. Some examples are given, and the question of the polarity of first arrivals in well surveys is discussed.At present the discrepancies between the integrated times of even double receiver V-Logs and the results of the well surveys remain too great to permit us the economy of eliminating the well survey. Furthermore, for the study of these discrepancies we must continue to shoot a considerable number of calibration points. We have, however, succeeded in reducing greatly the cost of the conventional survey by reorganising our well shooting methods. These methods, as well as the problems of surface corrections and the choice of datum plane they entail, are discussed.

Notes: Lavas and intrusives are often permanently magnetized in a direction different from the direction of the present geomagnetic field and this can make the interpretation of a magnetic intensity map difficult. However, the profiles over a permanently magnetized body are identical with the profiles over an inductively magnetized body of the same shape provided the direction and dip of the geomagnetic field in the induced case is appropriately changed. In which case, the shape of the body can be deduced by the well-known methods of interpretation of induced anomalies. Examples of the application of this method to Australian Kainozoic basalts, Tertiary basalts from Australia and Tasmania, and the classical Pilansberg dykes have been worked out. This article stresses the fact that the apparently confused patterns obtained by magnetic surveys over volcanic rocks can often be explained with little difficulty as the juxtaposition of normal and reversely magnetized rocks.

Notes: Some tests carried out on a Worden “Educator” gravimeter (n. E 302) showed that pressure variations had a significant effect on the readings.The changes of pressure cause changes in readings of the same sign. Corrections are given which may be applied to the instrument readings to compensate for this effect.

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: Book reviews in this article Introduction to Geophysics, by B. F. Howell, jr. Introduction to the Theory of Sound Transmission, with Application to the Ocean. C. B. Officer Tectonic Scetch Map of N. America, by W. T. Thom, Jr.

Notes: Seismic work near Paris first began in 1954. Two major difficulties characteristic of this region rapidly appeared:〈list xml:id="l1" style="custom"〉1) The existence of silicified limestone beds in the plateaus: these high-velocity beds, hard to drill, make it difficult to obtain surface corrections and carry strong “ground-roll” noise waves.2) The feeble structural variations call for a very accurate control of time measurements.Drilling difficulties, and the need to eliminate parasite waves, have led to the adoption of pattern shooting from hammer-drilled holes.The most effective elimination of ground roll has been obtained by shooting with a lateral offset of 600 m. This method has made it possible to work in regions where originally no reflections could be obtained.The difficulty of obtaining surface corrections has induced us to shoot up-hole velocity surveys in deep holes at particularly important points. This work is done by a special crew. At other shot points, surface corrections are calculated from structural maps of surface strata and from velocity-depth curves. The accuracy of these corrections, checked by drilling and velocity surveys, appears sufficient.The determination of lateral velocity variations by surface velocity shooting (X2/T2) is not feasible in this part of the Paris basin, as the accuracy of this method is insufficient.The control of reflection times must take into account the deformation of the signal as a function of the type of recording equipment and filters used, and also the modifications of the character of the reflection due to lateral stratigraphic variations. These variations can be observed by comparing the different velocity logs.

Notes: The object of this paper is to present 19 synthetic records recorded on 12 wells in the North Sahara by different methods.A comparaison is made of the synthetic records obtained by different methods:by analogy: United Geophysical Corporation Seismograph Service Corporationby computation: Institut Français du Pétrole Compagnie Générale de GéophysiqueThe method originated by Mr Baranov of the C.G.G. is studied in two cross sections: the first on four wells in the Hassai-Messaoud field and the second on eight wells in the Anngueur region. The influence of internal and surface multiples on the correlations is very important and explains the reasons for marked variations in the quality of reflections. The problem of the origin of the basis of the stratigraphic correlations furnished by the geologists can be a source of errors.Finally, the comparison between the synthetic and field records indicates that the best results are not always obtained with a synthetic record including multiples.In conclusion, on the North Sahara, where the importance of multiples is preponderant and not readily apparent, and where interpretation is primarily based on continuity, the study of synthetic records including multiples on all exploration wells would appear to be indispensable.