Explosives for Seismic Prospecting

8/7/2019 Explosives for Seismic Prospecting

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EXPLOSIVES FOR SEISMIC PROSPEC TING

N. G. JOHNSON AN D G. H. SMITH*

ABSTRACTA test has been devised whereby explosives can be graded visually according totheir relative merit for blasting under high water pressures. Representative explosivesfor seismic prospecting are compared and analyzed by means of this test. One of thenewest developments in explosives for deep-hole shooting is described, and its supe-riority for this work is clearly demonstrated.The function of explosives in seismic prospecting is too well known

to require detailed discussion in this paper. It is sufficient to state thatthe reflection method, which is in most gene ral use at the present timerequires primarily that the explosive be sufficiently water-resistant todeliver full efficiency unde r dep ths of water up to 250 feet. An addi-tional important requiremen t, in certain types of reflection shooting,is that the explosive mus t be hard-packed and in a rigid container,so that it may be pushed down partially blocked drill-holes.

Stand ard grades of dynam ite are seldom used in depths of waterover 50 feet, such use comp rising the deepening of rivers and harbors,etc. The explosives industry was, therefore, placed at a serious dis-advan tage when the geophys ical prospector deman ded explosiveswhich wou ld perform consistently in depths up to 25 0 feet. Naturally,those grades know n to be satisfactory for the usu al type of subm arineblasting were first recomm ended. These were soon modified to meetcertain sp ecific requirements of reflection shooting, that is, they werepacked more firmly than usu al and were provided with special, extra-heavy, wrappers. On the whole, these explosives have been very suc-cessful. There have, how ever, been certain instances w here it appearedthat full efficiency was not being developed. Efforts to clarify thesesituations led to extensive investigations in the field, including care-fully supervised comp arisons in actual seismic prospecting. These datawere very difficult to interpret, however, because of the many varia-bles inherent in seismic work. It was then decided that an effort sho uldbe made to develop a test which would classify the various explosivesaccording to their relative merit for use und er high water pressures,independent of variables other than the explosives themselves. Sucha test has been developed, and it is the purpose of this article to de-scribe the work and to discuss the results obtained.

* Contribution No. ro from Eastern Laboratory, E. I. du Pant de Nemours&Company.

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EXPLOSIVES FOR SEISMIC PROSPECTING 22 9TYPES OF EXPLOSIVES USED FOR SEISMIC PROSPECTING

According to the usual practice in the industry, the du Pont Co.has adopted trade names for the se veral types of dynamites offered tothe seismic prospecting group. Beferem going firrther, it is believed~de-sirable to trace briefly the development of the standard grades, and todefine the seismograph types in terms of the latter.

It is generally known that No bel p erfected the comm ercial manu-facture of nitroglycerin, and ma de its use as a blasting agent prac -tical by his invention of dynam ite. In the broa d sense, the term dy-namite now covers any high explosive used for commercial blasting,and thus may include explo sives containing no nitroglycerin. Nobe lsdynamite, howe ver, comprised high percentages of nitroglycerinmixed with various absorptive m aterials. Such pow ders are known to-day as the Straigh t dynamites and contain nitroglycerin in anamount equal to their grade strength markings. The higher grades,such as 60% , are quite w ater resistant but are unsuited for seismicprospecting becau se of their relatively high sensitiveness to shock andfriction.

Som e years after his invention of dynamite, Nobel discovered thatcertain types of nitrocotton we re soluble in nitroglycerin, produc ing agelatin. His first com merc ial utilization of this discove ry was the pro-duction of Blasting gelatin, the composition of which is 91% nitro-glycerin, 8% nitrocotton, and 1% chalk. This is a tough, rubbery ma-terial resembling an Artgum eras er. Since Blasting gelatin is ex-pensive, and unnecessarily strong for m ost purpose s, it was soon di-luted with materials such as woo d pulp and sodium nitrate, to form aseries of explosives which are now generally known as the Straightgelatins. To reduce cost further, there was developed an additionalseries of gelatins, in which am monium nitrate replaced a portion ofthe nitroglycerin. Thes e, usually terme d the Special or Ammoniagelatins, are equal in strength to the corresponding Straight gelatins,but are somewh at lower in velocity (rate of detonation) and less waterresistant.

At the present time the explosive in most general use for seismicprospecting is 607~ Special (Ammonia) gelatin. While almost all ex-plosives manufacturers offer this grade, each has prepared modifica-tions especially for the geophysical trade. Du Pont Seismogels Aand B are both in this category, grade A being packed very hard andgrade B medium hard. B oth are cartridged in extra-heavy containers.

As mentioned previously, the Straight g elatins contain m ore nitro-



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230 IV. G. JOHNSON AND G. II. SMITH

glycerin than the Special gelatins and are consequently more water re-sistant and higher in velocity. ,4 peculiarity of gelatin d ynam ites,however, is that they have two velocities, a low of around 8,200 feetper seco nd and highs ranging from 13,000 to 20,000 or more dependingupon the explosive content of the grade involved. Development ofhigh velocity usually doe s not occ ur in the open, but will ordinarilyresult under close confinement in a drill-hole. High press ure, especiallywhen due to water, greatly reduces the tendency of gelatin dynamitesto assume their high velocities. Thorough investigation of the abovefacts led to develop men t of the du Pont Hi-Velocity gelatins. Theseare essentially the same as the corresponding g rades of Straight gela-tin, but because of a patented feature may be relied upon to developmaxim um velocity under all conditions of confinement or press ure.To a certain extent, the name Hi-Velocity may be misleading, inthat the normal velocities are no higher than those of the correspond-ing Straigh t gelatins. The point is that the Hi-Velocity gelatins willgive full efficiency under con ditions w hich will c ause inferior resultsin any other type of commercial explosive. Proof of this contentionis offered by the data presented in the following paragraph s.

TESTS AT HIGH WATER PRESSURESIt is obviously both difficult and exp ensive to carry out tes ts on

large quantities of explosive s under heav y confinement. For one thing,such work must be carried out in a large bomb-proof, or in some iso-lated locality whe re dama ge would be unlikely to result. C onse-quently, it has been the general practice to expose relatively smallquantities at the desired water pressures, and to remove the explosivefrom the pressure vessel before attempting detonation. These testshave the weaknesses, first, that the explosive is not tested under ac-tual working conditions and, second, that little information is ob-tained as to the efficiency of the shot.

In the present work, it was first propos ed that the explosives to beinvestigated should be shot, under w ater p ressure, in a heavy steelpipe standing upright on a lead or aluminum plate. Such a test wouldshow: (I) whe ther the detonation propag ated from cartridge to cart-ridge throughout the column of explosive, and (2) whether the ex-plosion took place at high or at low velocity. Preliminary experimentsalong the se lines are illustrated in Fig. I.

After preliminary trial, it wa s decid ed to use aluminum plates.These were 3 thick and 24 squ are. The pipe was extra-heavy



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EXPLOSIVES FOR SEISMIC PROSPECTING 231

steel, of 2 inside diameter for I: diameter powder, and of 3 insidediameter for 2 diameter powder. Four cartridges 8 in length wereused for each test, an electric blasting cap being placed in the last,or top cartridge in the pipe. Th e cap wires were led out through rubbergaskets between the flanges. All of these tests were carried out at IOOlbs. per sq.in. water pressure, which is equivalent to a depth of 231feet. Approxim ately 15 minutes w ere required to apply pressure,transport the assembly to the shooting groun ds, and return for theshot.

FIG. I. Preliminary aluminum plate testsDiscussing Fig. I, it will be seen that 6oyc Hi-Velocity, in the

2 diameter, propagated througho ut the four cartridges. That the de-tonation took place at extremely high velocity is evidenced by the6 diameter hole in the alum inum plate. Reduction in the diameter ofthis grade to I+ also resulted in complete detonation. Since only asmall depression w as found on the plate, it was apparent that highvelocity had not been attained. The same type of plate was obtainedwith Seismogel B in the 2 diameter, When Seismogel B was re-duced to I:, however, only the first, or primed cartridge detonated,the remaining three cartridges being recovered after the shot. Natu -rally, the plate corresponding to this test showed no ma rk. This pre-



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23 2 N. G. JOHNSON AND G. H. SMITHliminary work brings out two points; namely, the superiority of 60%Hi-Velocity over Seism ogel B, and the advantage o f the largeover the small diameter.

After the experiments w ith aluminum plates, it was sugges ted thatmore intelligible results wou ld be obtained by laying the pipe flat overlead cylinders spac ed evenly down the length of the powder column.For example, such an arrangement would sho w definitely whe ther ve-locity tended to increase or decrea se as the detonation wav e pro-gresse d from c artridge to cartridge. As finally adopte d, the lead blockassembly is shown in Fig. 2.

FIG. 2. Preliminary lead block compression tests

To obtain compression, rather than shattering, the blocks we replaced between a steel rail at the bottom and I'X~'X~' steel platesat the top. Lead block No. I rested directly under th e first, orprimed cartridge, while block No . 4 was directly below the lastcartridge. T hese blocks we re actually cylinders, 24 in diam eter and4" in height, weighing approximately IO lbs. each.

Results of preliminary lead block tests are also shown in Fig. 2.Confirming the aluminum plate tests , 60% Hi-Velocity gave com-plete propagation at low velocity, in the 12 " diameter at IOO lbs. persq.in. wa ter pressure. Similarly, Seism ogel B, under the sam e condi-tions, show ed low velocity detonation of the first cartridge, with fail-ure of propagation to the remaining cartridges.



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EXPLOSIVES FOR SEISMIC PROSPECTING 235

deterioration whatsoever was noted at this pressure after three hoursexposu re. Indications are, from this work , tha t gelatins are subjectto a critical pressure and that, below this pressure, the time of ex-posure is a relatively minor factor. In spite of this indication, however,good practice requires that the explosive be fired as soon after loadingas is possible.

As has been mentioned previously, the reflection method fre-quently requires that several shots be made in the same drill-hole.Thus rigid explosives are of advantage in that they may be moreeasily pushed dow n holes which have been partially blocked as a re-sult of previous shots. With this in mind, 6 0% Hi-Velocity gelatin

FIG. 5. Comparisonof regularand seismograph607~ Hi-Velocity gelatinwas modified to produce the stiffest possible produc t, especially foruse in seismic prospecting. This modification, designated new for-mula, is shown in Fig. 5 , in com parison with the softer form of 60%Hi-Velocity which is used in ordinary subm arine blasting. It willbe noted that the mod ification for seismic prospecting is fully as effi-cient as the older type.

Mention was made, in discussing Fig. I, that increase in the di-ameter of the explosive resulted in improved performance. Additionalwork was carried out to amplify this point, the complete data beingshown in Fig. 6. It is evident that the largest practicable diametershould be used.

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23 6 N. G. JOHNSON AND G. H. SMITHIn tracing the development of the various types of gelatin, the

statement was mad e that Seismoge ls A & B were merely modifica-tions of standard 60% Special (Ammonia) gelatin. Fig. 7 show s a

FIG. 6. Effect of variation in,diameter on performance of explosive

comparison of these grades . It will be seen that Seismoge l B gavevirtually the same results, while Seismoge l A proved som ewha t in-ferior.

FIG. 7. Comparison of 60 % Am monia gelatin and the SeismogelsAgain, it was stated that Hi-Velocity gelatins were the sam e as

the Straight gelatins, with the exception of the patented low density



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EXPLOSIVES FOR SEISMIC PROSPECTING 237feature. Fig. 8 shows a comparison of the two 60% grades. Theadva ntage of Hi-Velocity over Straight gelatin is particularly ap-parent in the tes ts at IOO lbs. per sq.in. (231 feet). Data on 80%

FIG. 8. Comparisonof 607~Straightgelatin,60% Hi-Velocitygelatinand 80% Ammonia gelatinSpecial (Ammonia) gelatin wa s included as a matter of possible in-terest. It will be noted th at this explosive is also inferior to 60% Hi-Velocity.

There has been som e comm ent that Hi-Velocity, because it is aStraight gelatin type, and, therefo re, has a higher nitroglycerin con-tent, might be more sensitive to shoc k than the usual Amm oniagrades . That such is not the ca se is shown by the da ta given in TableI. In this table, D stands for com plete detonation, P for partial (local)

TABLE I

detonation, and F for no detonation whats oever. T he tests we re car-ried out by dropping an 182 lb. iron ball a distance of 103 feet onto azX 2" section of powder resting upon a steel surface.

CONCLUSIONAs has been mentioned previously, the explosive in most common

use for seismic prospecting is 607~ Amm onia gelatin. Explosives of



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238 N. G. JOHNSON AND G. H. SMITH

this classification, whic h includes Seism ogels A Br B, have given riseto little comp laint during the pas t severa l years . Prior to the investiga-tion describ ed in this article, how ever, there have been available nodata on the relative performance of the various explosives for seismicprospecting w hen tested under working conditions. Base d on the evi-dence presen ted herein, it seem s very probab le that full efficiency isnot realized with v ery ha rd gelatins of the Seism ogel A type indepths of water over 35 feet, or with medium hard gelatins such asSeismogel B in depths over 60 feet. It has been shown that Straightgelatins beca use of their higher nitroglycerin contents, hav e inherentlyhigher velocity and water resistance than the Ammonia gelatins, andalso that the Straight gelatins will retain these qualities to a muc hgreater extent when exposed to water pressure. Hi-Velocity gelatinnot only includes the general advan tage of the Straight gelatins but,because of patented features, is much superior from the standpointof developing full efficiency under exces sive wa ter pre ssure s. In con-clusion, it app ears that the Hi-Velocity gelatins should be of in-terest to the geop hysical prosp ector for two reasons: first, as the great-est possible assurance of satisfactory results under all shooting con-ditions, and, second, on the basis that worth while econom ies maywell be available.

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Explosives for Seismic Prospecting

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