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: Master diagrams for electromagnetic responses of nine dipole dipole systems are presented for various depths of a vertical infinitely conducting vein. The YY system gives the minimum anomaly for both the inline and broadside arrays. Among other inline systems, it is difficult to decide the clearcut superiority of one system over the other, whereas the XX system in broadside array gives maximum anomaly.

Notes: The Laboratory of Geophysics of the University of Arizona was presented with an exploration problem by Falconbridge, Ltd. of Canada. Massive mineralized hemispherical “pods” are in the vicinity of a tuff layer of high conductivity and induced polarization response, covered by large thicknesses of resistive volcanics. The initial approach was to utilize electrolytic tank modeling. The extreme resistivity and IP contrasts proved to be difficult to recreate. Two dimensional modeling was attempted next with conductive paper, using copper and silver paint for anomalous masses. This method also proved inadequate. Finally, mathematical equations were solved which could model any arbitrary anomalous body in any steady state electrical field. Plane waves as well as point current sources producing non-plane waves are possible. Finite difference equations were derived for the non-linear partial differential equations under consideration. The equations were solved using a digital computer. Initially, the boundary conditions had to be satisfied at the boundaries of resistivity changes, severely restricting possible geometric shapes for anomalous bodies. The final and successful solution was to apply numerical techniques to obtain solutions of equations which require only that the relative resistivities through the area be specified. The Falconbridge problem and its solution are analyzed.

Notes: For the two and three layer cases geo-electrical sounding graphs can be rapidly and accurately evaluated by comparing them with an adequate set of standard model graphs. The variety of model graphs required is reasonably limited and the use of a computer is unnecessary for this type of interpretation.For more than three layers a compilation of model graphs is not possible, because the variety of curves required in practice increases immensely. To evaluate a measured graph under these conditions, a model graph is calculated by computer for an approximately calculated resistivity profile which is determined, for example, by means of the auxiliary point methods. This model graph is then compared with the measured curve, and from the deviation between the curves a new resistivity profile is derived, the model graph of which is calculated for another comparison procedure, etc. This type of interpretation, although exact, is very inconvenient and time-consuming, because there is no simple method by which an improved resistivity profile can be derived from the deviations between a model graph and a measured graph.The aim of this paper is, on the one hand, to give a simple interpretation method, suitable for use during field work, for multi-layer geo-electrical sounding graphs, and, on the other hand, to indicate an automatic evaluation procedure based on these principles, suitable for use by digital computer.This interpretation system is based on the resolution of the kernel function of Stefanescu's integral into partial fractions. The system consists of a calculation method for an arbitrary multi-layer case and a highly accurate approximation method for determining those partial fractions which are important for interpretation. The partial fractions are found by fitting three-layer graphs to a measured curve. Using the roots and coefficients of these partial fractions and simple equations derived from the kernel function of Stefanescu's integral, the thicknesses and resistivities of layers may be directly calculated for successively increasing depths.The system also provides a simple method for the approximative construction of model graphs.

Notes: The theoretical horizontal resistivity profiles over an outcropping vertical dyke for various parameters-electrode spacing, vein-width and resistivity contrast—with inline alpha, beta and gamma-Wenner electrode systems are described. The resistivity profiles present a most bewildering variety of shapes as compared to those in resistivity soundings. The analysis of type curves suggests suitable electrode configuration for detection of wide, moderately wide, and thin veins. The negative apparent resistivity values on the gamma anomalies for resistive vein over certain vein-widths of higher positive values of resistivity reflection factor contradict the concept of apparent resistivity hitherto held.

Notes: Il est maintenant courant d'employer l'ordinateur pour interpréter les sondages électriques. Cependant, le problème à résoudre en prospection électrique n'admettant pas une solution unique, le but des traitements automatiques consiste simplement à fournir plusieurs hypothèses qui aident à bâtir une interprétation géologique des mesures géophysiques. C'est, d'ailleurs, ce qu'on recherche aussi par la comparaison visuelle des courbes de terrain avec des abaques précalculés. Mais, tandis que l'interprétation manuelle, pour conduire à des solutions qui respectent les mesures faites sur le terrain, exige un temps très long, le traitement automatique fournit immédiatement des solutions compatibles avec les courbes àétudier.Les procédés employés s'adaptent aux divers besoins de la prospection: ils peuvent traiter aussi bien la station isolée que les profils de sondages électriques. C'est ce qu'on se propose de montrer, à l'aide de quelques exemples tirés d'études récentes.

Notes: Various experiments are described in designing two-dimensional magnetic interpretation algorithms using computer curve fitting techniques.For a single anomaly the position of the anomaly maximum and the half-width of the anomaly give good initial estimates of the plate position and thickness. A nomogram and formulae for improving these estimates is given. Curves and estimates for the effects of finite depth extent of a plate show the limits, when the lower surface of the plate can be neglected in curve fitting.The combined anomaly of two parallel plates can be separated into partial anomalies with no common points using the horizontal derivative of the anomaly. The changes of the anomaly maxima and changes in anomaly half-widths are studied as a function of plate separation. The position of the maxima and the half-widths can be corrected before applying the one-plate procedure for obtaining initial estimates of plate positions and thicknesses.The performance of standard optimization methods of Powell, Davidon, and Marquardt in improving the values of the plate parameters are compared. The Powell method seems to be the most reliable for both single and multi-plate anomalies. All methods become unacceptably slow when the number of plates is greater than 2 or 3. In these cases feasible interpretation times are obtained using the partial anomalies and sequential parabolic search of the parameter values as tailored specially to the thick plate model.Experiments with three different error norms, the classical least squares, weighted least squares and minimax, show that the first norm gives the best overall performance in automatic interpretation. The behaviour of the classical least squares norm as a function of the plate parameters is also briefly described.