NOUVEL ASPECT DE LA SISMIQUE RÉFRACTION AU SAHARA: TIRS NON ENTERRÉS*

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.