A new distributed charge (1981)

Abstract A new type of distributed charge intended to be a seismic source in geophysical prospecting is described; it uses a number of lump charges, located in a shothole at uniformly spaced separations, connected by a time-delay fuse. The time-delay fuse consists of appropriate lengths of linear detonating explosive (Primaline) in contact with a nondetonating linear deflagrator (Nonel). Nonel is a nonelectrical delay detonator invented by Nitro Nobel AB of Sweden and licensed to Ensign Bickford for manufacturing and marketing Nonel-based products.Field tests performed near Tulsa, Oklahoma, determined that (1) Nonel deflagration was both consistent and reliable for pressures from atmospheric to 150 psi, (2) Nonel did not cross-detonate even when 'bundled,' and (3) distributed charges using Nonel as an element in the timing fuse detonated 26 days after being loaded in a shothole.Field tests of distributed charges using Nonel performed in central Louisiana showed that (1) distributed charges increase the signal-to-noise (S/N) ratio relative to that yielded by concentrated charges by reducing shot-generated surface noise, (2) distributed charges increase the usable frequency content over that yielded by concentrated charges, and (3) a reliable and efficient distributed charge can be constructed with Nonel Primadet delay cord. BACKGROUND AND PAST PERFORMANCE OF DISTRIBUTED CHARGESA distributed charge is an explosive assembly of elongated form with a velocity of detonation in the direction of its length that is approximately equivalent to that of seismic waves in the surrounding medium. In the case of land prospecting, this assembly is ordinarily detonated in a water-filled section of a shothole, and the velocity of the surrounding medium is that for the appropriate type of wave traveling in the rock forming the walls of the shothole. If one were to use such an assembly in a marine environment, the speed of detonation would be equivalent to that of compressional waves in water. We show (Figure 1) a series of idealized snapshots of the wavefronts of a distributed charge with progressing time. The results of this sequence are the reinforcement of the downward energy and the tendency to cancel the energy in all other directions.An article entitled 'Broomstick Distributed Charge' by Martner and Silverman (1962) describes the background and general use of distributed charges in seismic surveying and the types of charges then available. The greatest success obtained with this charge was achieved by wrapping a linear explosive cord known as Primacord (a tradename of the Ensign Bickford Company) in a helix around a length of material such as wood, plastic, or rubber hose. The explosive material was the Primacord alone, and the choice of size of explosive and size and length of the mandrel was made so that the explosive detonation in an axial direction was about the same speed as the velocity of the generated seismic waves inFIG. 1. Idealized wavefronts radiate from points representative of a concentrated charge detonated sequentially in a manner that reinforces the wavefront in the downward direction. the adjacent walls of the shothole. The article also showed that it was possible to place lump explosives (also called concentrated charges) along the helix to increase the pressure of waves going into the shothole walls.However, it was sometimes found that the performance of the distributed charge was poor when lump explosives were introduced into the assembly. Results were outstanding using the broomstick charge when the required charge size (on the order of 5 lb) permitted the use of Primacord alone. Once the lump explosives were inserted at intervals along the broomstick charge to achieve additional charge weight, however, the performance was much less satisfactory. Evidently, poor coupling occurred for reasons that are not altogether understood. There are many areas that require charges larger than 5 lb to obtain adequate depth penetration. In the Wyoming Rockies area, for example, charges of 50 lbs and more are almost a necessity.In the Gulf Coast area, formation velocities are 6000 ft/sec and less. This velocity required, if broomstick charges were to be used, that the Primacord be wrapped at a low angle of pitch. When this was done, these charges often cross-detonated along the axial length of the charge at velocities higher than desired. Because of this problem, distributed charges of this type were seldom used in Gulf Coast area operations. There is a definite need for distributed charges there because of the interest in obtaining high frequencies necessary for stratigraphic trap exploration. Use of distributed charges in this area is further complicated since it is often necessary to shoot 15 lb or more of explosive in order to obtain sufficient depth penetration.In addition, it was found difficult to waterproof such a distributed charge to the point that the charge could be left in a shothole for periods of up to or greater than 24 hours before detonation. Since seismic surveying practices frequently necessitate keeping the charge in a water-filled shothole longer than this period, there was considerable likelihood of incomplete detonation. CONCEPT OF NEW DISTRIBUTED CHARGEGeneral descriptionIn this section we describe a new type of distributed charge for use as a seismic source in geophysical prospecting. It makes use of a number of lump charges of explosive located in a shothole at predetermined distances apart, preferably uniformly spaced. Charges are connected by time-delay fuses. The distance between the charges is chosen to correspond to the time delay of the fuses in detonation in the following manner: Each succeeding jump charge is detonated after that above it at a time which permits at least approximate matching of the effective velocity of detonation (in the direction of the length of the elongated charge) to the seismic velocity of appropriate type waves in the surrounding medium. The medium is usually the rock formations around a water-filled section of a shothole drilled in subsurface formations.Each charge at its bottom end is connected through a blasting cap to a short but effective length of detonating cord, such as Primacord. Below this, and connected to it, is a nondetonating linear deflagrator with a propagation velocity considerably below that of the detonating cord. It in turn terminates in another blasting cap inserted in the next lower lump charge.In our early work, we predetermined the distance between adjacent lump charges by a physical framework which can be a separate strain wire or cable. This takes the strain off the Primacord and deflagrator unit while holding the lump charges at the proper interval apart. Obviously, this framework is chemically inert during detonation.In this embodiment, the entire distributed charge is suspended from a strain wire or cable. The upper end of the top lump charge is provided with a seismic blasting cap supplied with electric insulated leads up to the blaster.;We now use rigid tube members enclosing the delay assemblies between the lump charges, as shown by the sketches in Figure 2. Such a design is accomplished by using flow-molded NACO cases patterned after the standard 2 1/4 X 5 lb, 60 percent high-velocity dynamite cases. The effective length of each case is 29 3/8 inch. When two cases are threaded together, they are 58 3/4 inch, i.e., the length of two-case sections is less than 5 ft. Additional sections may be connected to achieve any desired spacing consistent with the dimensions of the sections. The standard delay element consists of a pair of the two-case sections. This combination allows 5-lb lumps to be spaced approximately 10 ft apart. In calculating the lengths of Primaline and Nonel necessary to accomplish the required transit time between charges, we allow enough slack so that each two-case section can be disconnected and laid side by side in the shipping box. Note that the ends of the NACO cases are modified by a small hole so that the Nonel/