ALBERT

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 shape of the seismic signal generated by an explosive charge is assumed to be governed by shock-wave phenomena in a non-linear region around the shot hole.The dimensions of this shock-wave region are shown to depend on the weight of the explosive charge and the properties of the medium in which the shot is fired.The presented theory enables us to elucidate the power law relationship between the amplitude of the seismic pulse and the weight of the charge fired, in the literature often denoted by A = c.Qn, in which A is the pulse amplitude, Q the charge weight and c a constant.Experiments have been made to check the theory and the results are promising.

Notes: Let Oxyz be a system of rectangular axes with origin at the earth's surface and with the z axis pointing vertically downwards. If a body B lies wholly] between the planes z=h, z=I then for all x, y and for n= 1, 2, 3 it is proved that 〈displayedItem type="mathematics" xml:id="mu1" numbered="no"〉〈mediaResource alt="image" href="urn:x-wiley:00168025:GPR607:GPR_607_mu1"/〉 where a=h/d, β=l/d and K is the gravitational constant. Dn are very easily computed from the Bouger anomaly and Jn are tabulated in this paper.

Notes: Distinguishing between diffractions and true reflections is often difficult and may lead to misinterpretations. In the Molasse zone of Upper Austria, numerous faults were established by seismic surveying. Diffractions were observed at several antithetic faults but not at synthetic faults. As an example, a seismic record section of the Steindlberg structure is shown. The reflections from the base of Tertiary arid from the Cretaceous-Jurassic contact run parallel over long distances, and so do the less important reflections lying above and between. If, contrary to the general trend, the reflection from the base of the Tertiary approaches the underlying reflection from the Cretaceous-Jurassic contact, or if the latter diverges from the former, this is considered a criterion for a diffraction.

Notes: In some mines of the Siegerland, besides dilatational and shear waves, an additional direct wave has sometimes been found the velocity of which was somewhat lower than that of the shear wave. Investigations were carried out with the object to establish the directions of oscillations and the velocities of all three kinds of waves. It was the purpose to draw conclusions as to the character and the causes of the appearence of the third wave.By these investigations the wave having the lowest velocity can be identified as a second shear wave. This points to the fact that double refraction of the shear wave is taking place, the wave of higher velocity being a pure transverse wave and the wave of lower velocity being a quasi-transverse wave.As a result of these measurements the examined medium is shown to have a transverse isotropy due to stratification. It is, however, not possible to decide whether there exists also an anisotropy due to schistosity.Some further calculations show that the wave of lowest velocity can not be explained as a surface wave propagated on the walls of the gallery.

Notes: An example is given from the Gulf of Aden of a total intensity magnetic anomaly which cannot be explained by a body assumed to be magnetized in the direction of the Earth's present magnetic field. It is enferred that the natural remanent magnetization is much greater than the induced magnetization and computations have been made which suggest the presence of igneous rocks with a reverse magnetization. The example illustrates that it is important to know the ratio of remanent to induced magnetization and the direction of remanent magnetization for the correct interpretation of magnetic anomalies.

Notes: By means of demonstration material, the author reports on the successful application of a shooting technique for seismic reflection operations, which has proved to work satisfactorily in those parts of the North-West German prospective oil area which are unfavourable to reflections. The shooting technique is characterized by uniformly linear shotpoint arrangements which are parallel with the geophone spread and reach from one end of the spread to the opposite. All charges are detonated simultaneously.Technical details of the method are discussed, particularly the problem of the most favourable geophone spreads in continuous profiling as well as the position of the uphole geophone. The difference in quality of reflections obtained by the normal pattern shooting and the linear shotpoint arrangement is demonstrated by seismograms. The special problems involved in the interpretation of the seismic records are referred to.

Notes: This paper is intended as a basic introduction to synthetic seismogram techniques. Their principles, uses and limitations are discussed and the relative advantages of the different methods are compared.