It is shown that, up to a certain limiting weight, the seismic amplitude from underground explosions increases in direct proportion to the weight of charge fired. Above this limit the amplitude soon becomes closely proportional to the cube root of the charge weight. It follows that if weights greater than this limit, which is commonly 200–300 lb, are to be fired, it is more efficient to divide the charge into several portions, each well separated from its neighbor. This applies both to surface and to buried charges.
The limiting weight depends on the elastic properties at the shotpoint and on the predominant frequencies in the observed refracted pulse. It may be found by experiment, or less precisely, by calculation. The calculated limit for buried shots is obtained from the equation W1/3 = Vo/(2πKf), where W is the limiting weight in pounds, Vo is the velocity at the shotpoint in ft/sec, f is the highest frequency of interest in cps (say, twice the predominant frequency), and K is a constant which depends on rock type and is usually between 3.5 and 4.5.
It is shown why underwater explosions are usually more efficient than underground explosions, and Table 2 gives these efficiencies as a function of charge weight and frequency band.
Because surface charges generate a large amount of shear-wave energy they may cause the wave SPP to be recorded with large amplitude and may, therefore, present an interpretation hazard.
Figures & Tables
Seismic Refraction Prospecting
The seismic method is divided into reflection and refraction techniques, based on whether or not a wave undergoes a reflection at the extent of its travel. Thus, while most refracted events have not been reflected, most reflected events have been refracted, because a refraction occurs across any velocity interface in accordance with the simple and basic Snell’s law. This law states that the sine of the angle of incidence is to the sine of the angle of refraction as the velocity on the first side of the interface is to the velocity on the second side of the interface.
Where the refraction angle is large, and not near to zero as it is in the case of reflection work, there are many considerations concerning the geometry of the raypath that have to be made in refraction interpretation. Basically, the papers in this volume describe various techniques for separating out special raypath solutions and making approximations that give us a structural geologic picture from the study of these approximations or specializations.
The following factors are of extreme importance in refraction surveying’ 1) Distance’ Surveying must be accurate in order to make correct depth determinations of the refractor by the use of the refraction method. 2) Velocity’ The velocity of the various horizons, through which the refracted wave passes, must be known if an accurate structural picture is to be determined. Many of these velocities can be determined from the refraction data, and, in fact, the refraction method is a good means of establishing many of the velocities needed for these calculations. 3) Time’ Accurate time information is a prerequisite, although this is no more the case in refraction than in reflection work. In most instances, refraction information is to be recorded to the nearest 1/1,000 sec for exploration purposes.
The distance parameter will be discussed first. In many surveys the distance between the shot and receiver may be extremely long (25 to 50 miles), and the requirement for accuracy is just as vital as if this distance were very short (a few hundred feet). Because of the differential velocities involved, distance errors can cause errors in depth greater than the distance errors themselves. For somecases, in the experience of the editor, the depth error may be three times the distance error. The velocity is very critical in refraction information. Of particular important is the refractor velocity, which is often used to determine the time to be subtracted from the total time path to determine that amount of time which is near verticalor can be converted to a vertical path time.