ALBERT

Notes: The author describes the responsibilities of the forester both for the production of timber and the preservation of natural resources, and shows how aerial photographs can assist in supplying the information required for organising the management of forest areas. On limited experience with infrared and colour films, the author compares them with panchromatic film for aerial photography of forest country; he considers that in most cases good definition is the most desirable quality in such photographs, although special contrast effects may be useful for particular purposes.

Notes: A brief description is given of the history of the stereocomparator, followed by a comparison of the major features of five modern instruments: the Wild STK 1, Nistri TA 3 and TA 3/A, S.O.M., Zeiss PSK 1 and Hilger and Watts stereocomparators. Special attention is paid to the production model of the Hilger and Watts stereocomparator, which differs in several important respects from the prototype.

Notes: Three unrelated matters are discussed. Firstly, the recent rapid advances in the application of precision engineering, electronics and digital computers to photogrammetric surveying operations are shown to be likely to result in a change of emphasis; from the need for great skill in making each particular map, to the need for even greater skill in making machines which, once made, will make many maps, largely automatically. This in turn is likely to have far-reaching effects on the type and training of persons employed in the profession. Secondly, an example is quoted which suggests that the value of photogrammetric methods, as an aid to the work of professions other than land surveying, is not sufficiently recognised by the teaching faculties of some of our universities. Ways and means of improving the situation are considered. Thirdly, after a brief historical survey of the birth and growth of the Photogrammetric Record, arguments for and against some possible changes in editorial policy are stated and certain conclusions reached.

Notes: Book reviews in this article:GEODESY. By Brigadier G. Bomford.INTERNATIONAL YEAR BOOK OF CARTOGRAPHY (Vol. II, 1962). Editor, Eduard Imhof.MODERN COMPUTING METHODS. National Physical Laboratory: Notes on Applied Science.SCIENTIFIC PHOTOGRAPHY. Editor, H. Sauvenier.

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.