US3059575A - Seismographic exploration - Google Patents

Description

Oct. 23, 1962 e. GRIFFITH ETAL 3,059,575 SEISMOGRAPI-IIC EXPLORATION Filed Aug. 19, 1957 2 Sheets-Sheet 1 WT. OF CHARGE VS. MAXIMUM DEPTH HT WHICH FIRING DOES NOT GIVE OBJECTIONRBLE SECOND- REY OSCILLQTIONS :r PROVED EXPLOISIVE UNIT CONV ENTIONFII. UNIT DEPTH OF CHARGE IN WATER, FEET S IO 20 3O 4O 50 LBS. OF CHARGE INVENTORS GEORGE L. GRIFFITH :1 4 I FBEYANKLIN B. WELLS QTTOENEV Oct. 23, 1962 G. L. GRIFFITH ETAL SEISMOGRAPHIC EXPLORATION Filed Aug. 19, 1957 2 Sheets-Sheet 2 INVENTORS Geoeas L. GElFF/TH QEHNKL/N 5. 'WELLS Jae-MW HTTOENEY .George L. Griffith, Coopersburg,

3,059,575 SEISMOGRAPHIC EXPLORATION and Franklin B. Wells, Allentown, Pa., assignors to Trojan Powder Company, a corporation of New York Filed Aug. 19, 1957, Ser. No. 678,836 7 Claims. (Cl. 102-22) This invention relates to seismographic exploration of geologic structures and more particularly to an explosive unit and process for minimizing or eliminating entirely the objectionable secondary bubble oscillations.

The art of seismographic exploration is highly developed. In submarine exploration, for example, a charge of explosive is lowered below the surface of a marsh, lake or sea, fired, and the reflected impulses are recorded on delicate instruments, all as a means of indicating the structure. In order that as much of the force of the explosion as possible may extend downward, it is desirable to lower the charge as far below the surface of the water as feasible. In so lowering the charge difliculties are encountered. Thus, charges fired too deep in the water, more than 6 ft. below the surface for one representative size of charge, create a secondary oscillation, called double bubble, which affects the instruments recording the shock waves reflected from various rock layers or strata and thus distorts the record. This double bubble is explained as being due to the gas first generated by the explosion, expanded by the very high temperature, subsequently undergoing contraction on cooling and thus causing rushing in of water producing a secondary shock.

To avoid this distorting factor, US. Patents 2,599,245, 2,619,186, and 2,771,961 propose the use of two or more charges fired in close sequence, so that the primary shock from one of the many charges eliminates the diffic-ulty from the secondary shock from the other charge.

We have discovered a single explosive unit and process of use that solves the problem.

Our single explosive unit and process make possible an increase of 50% or more in the depth of submergence at which the charge may be exploded, as compared with a conventional charge of the same size, with the secondary shock effects reduced to acceptable level. This superiority over the conventional charge and booster was not anticipated, was unpredictable on the basis of existing knowledge, and is ascribed to features of our unit that are lacking in conventional explosive charges or bombs for seismographic submarine exploration.

Furthermore, the unit described has safety factors which make it more satisfactory than the conventional seismographic explosive units. This additional safety is particularly important because in submarine seismography the operators literally sleep on a boat-load of the explosive units. The increased safety of our unit is shown by tests with a primer of kind that causes detonation of conventional nitro-carbo-nitrate explosive units in submarine exploration at 4-6 feet immersion depth. Our unit did not explode with this primer introduced into the booster well and fired. Also our unit was not detonated by three No. 8 caps inserted into the main charge of explosive and fired.

The invention will be first illustrated by description in connection with the attached drawings in which:

FIG. 1 is a side view of the explosive unit as submerged and supported by a float;

FIG. 2 is a vertical sectional view of the explosive unit on an enlarged scale;

FIG. 3 is a graph showing the increased depth to which explosive units of this type may be submerged without giving objectionable secondary oscillations as compared to the maximum permissible depth of submergence of conventional units of equal weights of explosive charge; and

FIGS. 4-7 are vertical sectional views of various forms of the explosive unit, each of which forms may be substituted for the explosive unit in the assembly of FIG. 1.

There is shown a body of water 10, such as ocean, lake, marsh, or river, in which the explosive unit is submerged and supported by a wire or other conventional means 12 attached to float 14.

The explosive unit includes the main explosive charge 16, booster 18, cap 20 such as an electric blasting cap with lead wires 22 for firing the cap, container 24, booster well 26, bail 28, and means 30 securing the bail pivotally to the top of the container 24.

In the modifications shown in FIGS. 4 and 5, the booster 31 is recessed at its lower end so as to focus somewhat the downward force of explosion of the booster. More specifically, the lower end of the booster is recessed, so as to provide, for example, a hollow cone, the term cone or conical being used herein to include a concavity in general that extends upwardly into the booster. Thus the recession may be in the form of a right cone, the side wall of the conical space making an angle of about 30 60 with the vertical.

In the modification shown in FIG. 7, the lower end of the booster charge terminates in effect in a stem 32 resting at its lower end on the bottom of the booster well, so that the upper and principal part 34 of the booster is thus spaced above the bottom of the booster well and at an appreciable distance above that part of the main charge below the booster.

The booster may be inserted approximately flush at its top with the top of the explosive charge 16 as illustrated in FIGS. 2 and 4 or may be raised somewhat thereabove as shown in FIGS. 5-7. In the arrangements of FIGS. 5 and 6, the cap is inserted into the booster through the side, as at a level about midway of the length of the booster.

Suitable dimensions for the stem 32 of the embodiment shown in FIG. 7 are diameter one-fifth to one-half that of the principal portion 34- of the booster and length onehalf to two times that of the said principal portion. Thus the stem may be about O.21 inch in diameter and 2-4 inches long for a main part 34 of the booster that is 2-5 times the diameter of the stem and /22 times as long.

The elevation of the booster sticks of FIGS. 5 and 6 above the upper level of the charge 16 is equal at least to a quarter of the length of the sticks.

In all cases the center of gravity of the explosive unit is below the bail 23 and suitably near the midpoint of the container as illustrated in FIG. 5.

The figures are in part diagrammatic and parts not shown in detail are conventional.

Materials of construction are those that are usual in this type of article. Thus the container and booster well may be constructed of styrene, other plastic, waterproof paper, metal such as terne plate, sheet iron, aluminum, or the like.

The float is suitable hollow plastic, a rubber balloon, or other float conventionally used for such purpose.

The blasting cap is any that is usual for detonating seismic charges, as for example one containing as the sensitive ingredient mercury fulminate, lead azide, or mannitol hexanitrate.

For best results the detonator, i.e. the booster, should have a rate of linear detonation at least about twice that of the said explosive charge 16. Thus the booster should have a detonation rate that is ordinarily at least 6000-7500 m./ sec. Examples of booster materials that meet the requirements and that may be used are pentolite, tetryl, or cyclotrimethylenetrinitriamine. The booster should be in stick form, the stick form including pressed or cast material. The booster stick may or may not be enclosed in a paper cartridge or other container of usual kind (not shown).

The main explosive charge that we have found particularly satisfactory is a composition consisting largely of ammonium nitrate with a small proportion of a liquid fuel such as fuel oil, paraflin oil, kerosene and the like. We use the ammonium nitrate in the proportion of 92-97 parts by weight for 100 of total weight of the nitrate and fuel, leaving 3-8 parts supplied by the fuel. Other conventional admixtures are unnecessary. Such explosive charge may for example have a bulk density of approximately 1 to 1.1.

In order to obtain an explosive of relatively low sensitivity and low brisance, which releases its energy at a relatively slow rate and thus reduces the suddenness of contraction of the gases generated during the explosion, we employ a coarse grade of ammonium nitrate. Thus we use granules of such size that at least about 15% by weight is retained on a 20 mesh screen and no more than about 50% will pass through 100 mesh.

In making the ammonium nitrate and liquid fuel explosive composition, the components in the proportions stated are mixed in a tumbler or other usual equipment for such purpose.

Other explosives that may be used, with some loss of economy or decreased effectiveness or both, as the main charge in our explosive unit are the following: (1) An ammonium nitrate explosive containing -10 parts of a dinitrotoluene, 80-90 parts of ammonium nitrate of the screen test specified hereinabove and 2-5 parts of paraffin wax, all blended and compounded in the usual manner for this type of explosive; and (2) a well mixed blend of 5-10 parts of nitrostarch, 80-90 parts of the coarse ammonium nitrate, and 1-2. parts of petrolatum or parafiin Wax.

In any of the explosive charges described there may be added to the conventional mixture aluminum powder, ferrosilicon, calcium silicide, and additional carbonaceous fuel materials.

As indicated in FIG. 3 the charges used for seismographic exploration and of the type to which this invention relates will range in weight between lbs. and 50 lbs.

The booster should be of relatively large mass, as for instance 0.2-1 1b., and for best results 0.2-0.4 lb. For each of the explosive units to be fired we use one such booster as in the form of elongated sticks such as those of diameter 1 to 3 inches and of length greater than the diameter and suitably 2 to 4 times the diameter. When the lower end of the booster is recessed, to focus the explosive wave, the ratio of length to diameter of the booster may be decreased below the ranges stated.

With boosters of this weight and these dimensions and the high ratio of rate of detonation as compared to that of the main charge, in the units described, we avoid the objectionable secondary shocks experienced heretofore with other seismographic explosive units. Furthermore we make possible the immersion of the unit to a distance below the surface to a depth of as much as somewhat more than 11 feet for an explosive unit having an overall weight of 37.5 lbs. of which weight approximately 2 lbs. are represented by the container and other mechanical parts. With a 20 lb. total weight, we are able to immerse the unit to a distance of 8 ft. with satisfactory seismographic results on firing. For comparison, a 20 lb. conventional seismographic explosive unit gave objectionably strong secondary oscillations when fired at a depth of 5 to 6 ft. and a 40 lb. unit at depths below 7 ft., all as charted in FIG. 3.

Because of this greater immersion made possible by our explosive unit, we are able to reduce the weight of explosive materials required for given intensity of seismographic effects to about 75% of that which is necessary with the conventional units, and at the same time, to obtain equally good or better seismographic records.

The mass and dimensions of the booster are selected, within ranges given, in accordance with the size of the main explosive charge employed. Thus we use mass and dimensions of the booster 18 in the upper part of the range stated for large main explosive charges 16 and in the lower part of the range for the smaller main explosive charges.

In effecting the explosion, we lower the explosive unit of the kind described below the surface of the water to the maximum practical depth determined by previous tests. Then we orient the unit with the booster stick in a generally upright (vertical) position, by supporting the unit by a plastic or other float on the surface of the water, the float being attached to the top of the unit. In such orientation, the end of the explosive charge in which the booster is inserted is uppermost. The force of the explosion is principally vertical.

It is to be understood that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purpose of illustration which do not constitute departures from the spirit and scope of the invention.

We claim:

1. An explosive unit for under water seismographic exploration comprising a water proof container which disintegrates when the unit is exploded, such unit weighing between about 10 lbs. and 50 lbs., 2. main explosive charge in said container, such charge comprising ammonium nitrate as the essential explosive ingredient, such ammonium nitrate being in the form of granules coarse enough so that at least 15% by weight is retained on a 20 mesh screen and not over 50% will pass through a mesh screen, such explosive charge being characterized by its relatively low sensitivity and low brisance and by the fact that it releases its energy at a relatively slow rate, a booster charge also in said container selected from the group consisting of pentolite, tetryl, and cyclotrimethylene trinitriamine, such booster being characterized by the fact that it has a mass of from 0.2 to 1.0 lb. for a total weight of the unit of between 10 lbs. and 50 lbs. and has a linear rate of detonation at least about twice that of the said main explosive charge and has a rate of detonation which is at least 6000 m./ sec. which will detonate the ammonium nitrate and rapidly form an initial bubble when detonated under water after which a second bubble of relatively large size generated by the ammonium nitrate will form relatively slowly, and a cap for detonating the booster charge.

2. A unit as specified in claim 1 in which the booster charge is pentolite.

3. A unit as specified in claim 1 in which the weight of the booster charge is between 0.2 and 0.4 lb. for a total weight of the unit of between 10 lbs. and 50 lbs.

4. A seismic prospecting explosive unit in accordance with claim 1 in which the main explosive charge consists essentially of from 92 to 97% ammonium nitrate and from 3 to 8% liquid organic fuel.

5. A seismic prospecting explosive unit in accordance with claim 1 in which the main explosive charge consists essentially of from 80 to 90% ammonium nitrate, from 5 to 10% dinitrotoluene and from 2 to 5% paraflin Wax.

6. A seismic prospecting explosive unit in accordance 5 with claim 1 in which the main explosive charge consists essentially of from 80 to 90% ammonium nitrate, from 5 to 10% nitrostarch and from 1 to 2% parafiin wax.

7. A seismic prospecting explosive unit in accordance with claim 1 in which the main explosive charge also in- 10 eludes from 1.5 to 2.5% water.

References Cited in the file of this patent UNITED STATES PATENTS 905,336

" 6 Kirst Feb. 26, 1935 Woodbury Dec. 8, 1936 Kirst et a1. Feb. 2, 1937 Pugh Aug. 14, 1951 Moses Oct. 15, 1957 Hradel Jan. 6, 1959 Hamilton Apr. 12, 960

FOREIGN PATENTS Great Britain Nov. 1, 1950 France Nov. 10, 1954 OTHER REFERENCES Military Explosives, US. Army Manual TM 9-1910,

Lheure Dec. 1, 1908 15 Page 42

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