US2992611A - Seismic prospecting device - Google Patents

Description

July 18, 1961 R. w. FELCH 2,992,611

SEISMIC PROSPECTING DEVICE Filed June 27,- 1958 LWLM ATTORNEY United States Patent O I 2,992,611 SEISMIC PROSPECTING DEVICE Robert W. Felch, Wenonah, N.J., assignor to E. I; du Pout de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed June 27, 1958, 'Ser. No. 745,002

- 4 Claims. (Cl. 102-22) The present invention relates to an improved method of seismic prospecting and to assembly particularly adapted for use in this method.

In a method of seismic prospecting as described by Johnson et al., US. Patent 2,823,609, in which a number of explosive charges are spaced in a borehole and are detonated progressively at intervals to obtain an overall velocity less than the detonation velocity of the explosive, short period delays are provided between the individual charges for eifecting the required interval. The shortperiod delays are interposed in lengths of detonating fuse, and the explosive charges are spaced sufficiently to avoid sympathetic propagation of detonation.

Proper spacing of the charges and delay connectors within a borehole may be a very diflicult and time-consuming task if the charges cannot be suspended freely within the borehole. Too frequently the charges must be pushed down through such materials as mud, quicksand, and gravel which makes the proper separation of the charges and connectors practically impossible.

Accordingly, it is an object of this invention to provide an assembly adapted for use in a method of producing earth vibrations by progressively firing a plurality of explosive charges in sequence and at precise delay intervals.

Another object of this invention is to provide a seismic prospecting assembly particularly adapted for use in boreholes filled with materials detrimental to fast and eflicient loading.

A further object of this invention is to provide a seismic prospecting assembly wherein a plurality of explosive charges and delay connectors may be easily positioned the desired distance apart in boreholes filled with mud or quicksand or any other such material.

A still further object is to provide a highly precise delayproducing unit for use in seismic prospecting.

I have found that the foregoing objects may be achieved When I provide as a unitary assembly a rigid, elongated tubular element having at least two perforations and enclosed therein a delay unit. For the 'delay unit, I may use one as described in US. Patent 2,823,609 in which two lengths of detonating fuse are crirnped into a tubular shell having centrally positioned therein an explosive priming charge, an encapsulated exothermic non-explosive mixture, an empty lead tube, an air gap, and an enclosed heatsensitive detonating mixture, or one as described in copending application Serial No. 612,756, filed September 28, 1956, by G. A. Noddin and assigned to the present assignee, in which length of connecting cord having a core of from 0.5 to 5 grains per foot of a high-velocity detonating explosive encased Within a metal sheath and an outer protective sheath and having the ends of the cord capped by a metal shell containing an initiating charge is coiled or folded to a length sufiicient to require from 1 to 5 milliseconds for the detonation to travel from one end to the other.

A preferred embodiment of the invention is to enclose within a rigid, elongated tubular element having at least two perforations a delay unit comprising a length of connecting cord having the ends of the cord capped by metal shells containing an initiating explosive; a priming charge of a high-velocity detonating explosive at one end of the tubular element, andmeans to hold the tubular element adjacent to the priming charge or to the main explosive charges. I

In order to more fully describe the preferred embodiment of the present invention, reference is now made to the accompanying drawings, in which FIGURE 1 represents a borehole into which charges have been positioned in accordance with this invention, and FIGURE 2 represents a detailed view of the rigid delay connecting assembly used to transmit detonation from one charge to the next. Referring now to the figures in greater detail, 1 represents a conventional seismograph explosive charge, for example, a nitroglycerin-free explosive A in a metal container having internal B and external B threaded ends, 2 represents the rigid delay connecting assembly for connecting the individual charges in sequence at the desired spacing and is illustrated in detail in FIGURE 2, 3 represents the borehole into which the charges 1 and the rigid delay connecting assembly 2 have been pushed, and 4 represents an initiating means, such as an electric blasting cap having wires leading from the top of charge 1 to the surface of the borehole.

The delay connecting assembly 2 as illustrated in FIG- URE 2 includes a rigid tubular element 5 having at least two perforations 6 and having positioned therein a length of connecting cord 7 having a folded portion and having the ends of the cord 7 capped by a metal shell 8 containing an initiating explosive, the shell 8 being retained on the ends of the cord by the peripheral crimps 9 and being held by a suitable means, for example, by metal clips 10 and, if desired, by a congealing material, adjacent to the base of the initiating charge at one end and at the other to the priming charge 11 of a pressed high-velocity detonating explosive, for example, RDX or PETN, or a commonly known priming charge for a canned nitroglycerinfree explosive.

The tubular element which is critical to our invention must have sufficient rigidity to enable the delay unit and charges to be positioned with relative ease in boreholes filled with mud or water or any other such substance which would otherwise interfere with the proper spacing of the charges and to withstand pressures and abrasive action of tamping materials, such as sand. The rigid, tubular element may be constructed from such materials as paper, metals, glass, plastics, or any other material capable of providing resistance to the above substances frequently found or used in boreholes. Also, besides being of a rigid nature, the tubular element must have at least two perforations to avoid collapse because of unequalized internal and external pressure, for example, when water is present in the borehole, and to allow the tubular element to fill with fluid so it will sink in a water-or mud-filled borehole.

In the preferred embodiment of the invention, the lowenergy connecting cord comprises a core of a high-velocity detonating explosive, for example, PETN or RDX, an inner metal sheath, for example, of lead, and a protective outer sheath, for example, of synthetic polymeric compositions, such as polyethylene. Any cheap filling material, for example, cotton, rayon, glass wool, or impregnated paper or the like, may be used between the inner metal sheath and the outer protective sheath to obtain the desired outer diameter of the cord. Although the explosive core of the connecting cord used in the assembly of this invention may contain from 0.1 to 2 grains per foot of length of high-velocity detonating explosive, the preferred load is from about 1 to about 2 grains per foot of length of the high-velocity detonating explosive. I

As previously indicated, PETN or RDX is suitable as the high-velocity detonating explosive core for the connecting cord. Other high-velocity detonating explosives such as nitromannite, HMX, lead azide, picryl sulfone, etc., can also be used. As the metal sheath for the core, s lead or a tin-lead alloy is preferred, but other ductile metals and alloys can be used. As the outer protective 2,992,611 A a a a sheath, water-impermeable materials such as rubber or synthetic polymeric materials are preferred; however, the use of a waxed or asphalted fabric coating is also feasible. Additional sheathings, such as wire reinforcing for tensile strength may also be used as desired.

The velocity of detonation of this cord can be precisely determined; for PETN and RDX, the velocity will be approximately 7000 meters per second, or approximately 22,000 feet per second, depending upon the exact loading. For example, a 5-foot length of this cord would require approximately 0.25 millisecond to propagate detonation from one end to the other, whereas a. 110-foot length would require approximately 5.0 milliseconds to propagate detonation from one end to the other. The connecting cord having a core loading of 2 or less grains of explosive per foot of length has so little brisance that detonation is not propagated across adjacent strands even under confinement; therefore, the long length can be coiled or folded to provide the requisite length for the desired delay and still require only a relatively short total length for the assembly. Inasmuch as the cord itself h-as a relatively small diameter, 0.25 inch as a maximum, obviously the space required to hold a substantial length of cord is not great. For example, a length of cord coiled to a 2-inch diameter (axial distance) would contain approximately 2 feet of cord per inch of coil. By using a double coil, i.e., an outside coil about an inner coil, approximately 3 /2 feet of cord could be coiled in a 2-inchdiameter tube for every 1 inch of length. Thus a S-millisecond delay period requiring 110 feet of cord would require less than 32 inches of space in a 2-inchdiameter tube. Obviously, even more cord can be placed in the same space by increasing the number of coils Within the outer coil. Cords have been successfully tmted coiled as tightly as to have a /2-inch diameter (axial distance). Essentially the same loading can be obtained by folding the cord as illustrated in FIGURE 2 as by coiling the cord.

Whether the cord is coiled or folded, that portion of the cord will be positioned within the tubular element a suflicient distance from the initiating main charge so that no portion of the coil or fold will be destroyed by the detonation of the charge. The same distance will be sufiicient to prevent the initiation of the explosive in the core by the shock wave from the main initiating charge. Inasmuch as the velocity of detonation in the uncoiled portion of the cord, i.e., the portion between the coil or fold and the initiating main charge, will equal or exceed that of the shock wave from the initiating main charge, a constant velocity of the delay element throughout its full length will thus be assured. Proximity of the coil or fold to the receptor charge is of no consequence since the coil or fold will be fully consumed prior to the detonation of the receptor charge and since the cord alone is not sufliciently brisant to initiate either the priming charge or the receptor charge.

When a delay connector of the type illustrated in US. Patent 2,823,609 is used, it will be positioned similarly within the tubular element to insure proper functioning.

The capped ends containing an initiating explosive insure both the initiation of the connecting cord from the detonation of the initiating main charge in the sequence and the initiation of the priming charge adjacent the receptor charge in the sequence. In addition, the caps insure against damage to the ends of the cord and maintain a waterproof seal at the ends. In the preferred embodiment of the invention, the cap contains 2 grains of finely divided PETN above a pressed base load of 6 grains of cap-grade PETN. However, any shell containing a suit able initiating explosive may be used.

To insure initiation of the receptor charge by the capped connecting cord, a priming charge of a pressed, high-velocity detonating explosive, for example, a pellet of a pressed waxed RDX or a disk of pressed PETN or a primingchar'ge commonly used for a nitroglycerin-free explosive, must be pwitioned between the capped end of the connecting cord, and adjacent to the receptor charge.

The ends of the capped connecting cord are held adjacent an initiating main charge or a priming charge by any suitable means known in the art. Metal clips or prongs, for example, are well-known means for holding an electric blasting cap adjacent to an explosive charge. The rigid delay assembly and the main explosive charges may be firmly joined by any number of connector arrangements known in the art. For example, when a nitroglycerin-free explosive in a metal container is used, the rigid delay assembly may be provided with threaded metal ends to engage with the threaded portion of the canned explosive. Also a connector arrangement such as is found in dynamite couplers is suitable. In this type of arrangement, a tubular element having internal gripping means used. The sensitiveness is determined by the distance the detonation stimulus from the initiating charge will initiate the receptor charge in an end-to-end relationship. The ability to propagate over a gap is primarily a function of the sensitiveness of the receiving cartridge to initiation by detonation by influence or to impact from particles from the initiating charge. For the above-mentioned canned, nitroglycerin-free explosive, the detonation will not be propagated over a l-foot distance; however, to insure proper delay, the charges are usually placed approximately five feet apart. A more sensitive explosive, such as a seismograph gelatin dynamite, will propagate over approximately a 40-inch distance. Accordingly, a greater distance between changes must be provided to insure proper delay.

The series of main charges and delay connector assemblies are joined in any desired sequence, for example, a main charge may alternate with a delay connector assembly, or two or more main charges may be joined together and then joined with a delay connector assembly,

' or when additional time interval is desired, two or more delay connector assemblies may be joined together.

The accuracy of the sequential initiation by the assembly of the present invention is illustrated by the following examples.

Example 1 Acne-pound charge of a cap-sensitive ammonium nitrate composition and a one-pound charge of a cap-insensitive ammonium nitrate composition, both in metal containers, were positioned 60.inches apart by a rigid, tubular delay connector assembly placed between and connected to the charges. The delay connector assembly was assembled in the following manner:

Into a cardboard, elongated tubular element 5 feet long and having aZMz-inch OiD. was placed a 23-foot, 5- inch length of connecting cord folded in such a manner: as to fit the 5-foot tubular element so that the folded portion was a distance (about 18 inches) from the initicapped with a shell containing 2 grains of finely divided PETN above a pressed base load of 6 grains of cap-grade,

(The r PETN. A IS-gram, pressed waxed RDX pellet was positioned between and adjacent the capped end of the connecting cord, near the main charge to be initiated. The measured velocity of the connecting cord was 7,100 meters per second. In three trials, the intervals between detonations of the initiating and receptor charges were 1.003, 0.998, and 0.997 millisecond. When two of the lmillisecond delay connector assemblies described above were placed in series (no main charge in between), the time interval between detonation of the initiating and receptor charges was 1.993 milliseconds.

Example 2 The procedure for Example 1 was followed except that the length of the connecting cord was 5 feet, 4 inches. In five trials, the intervals between detonations of the initiating and receptor charges were 0.249, 0.249, 0.251, 0.244, and 0.252 millisecond.

The invention has been described in detail in the foregoing. However, it will be apparent that many variations may be introduced without departure from the scope of the invention. I wish, therefore, to be limited only by the following claims.

I claim:

1. A device adapted to rigidly maintain two high explosive charges in fixed spatial relationship and to initiate the second at said charges a predetermined interval following the detonation of the first charge comprising a rigid tubular casing having at least two perforations in its curved surface, explosive cord of a length greater than the length of said casing contained entirely within said casing, said explosive cord having from 0.1 to 5 grains of a high-velocity detonating explosive per foot of length encased within a metal sheath, a metal shell at each end of said explosive cord containing a charge of an initiating explosive, and connecting means at each end of said casing for connecting an explosive charge thereto, one of said connecting means including a priming charge of a detonating explosive, one end of said explosive cord being retained by one of said connecting means and the other end by the opposite connecting means.

2. A device as claimed in claim 1 wherein the length of explosive cord is such that an interval of from 0.25 to 5 milliseconds is required for detonation to propagate from one end to the other.

3. A device as claimed in claim 1 wherein a portion of said explosive cord is in a convoluted form.

4. An assembly for producing earth vibrations for seismic prospecting which comprises a plurality of high explosive charges within a borehole and interposed between at least two of said charges a rigid tubular casing of a length greater than the distance over which one of said high explosive charges will be initiated by sympathetic detonation from the detonation of the other of said explosive charge, said casing having at least two nonaxial perforations, explosive cord of a length greater than the length of said casing contained entirely within said casing, said explosive cord having from 0.1 to 5 grains of a high-velocity detonating explosive per foot of length encased within a metal sheath, a metal shell at each end of said explosive cord containing a charge of an initiating explosive, and connecting means at each end of said casing for connecting an explosive charge thereto, one of said connecting means including a priming charge of a detonating explosive, one end of said explosive cord being retained by one of said connecting means and the other end by the opposite connecting means.

References Cited in the file of this patent UNITED STATES PATENTS 701,173 Doyle May 27, 1902 2,224,565 Flude Dec. 10, 1940 2,655,993 Spencer Oct. 20, 1953 2,707,439 Hamilton May 3, 1955 2,724,452 Sorge Nov. 22, 1955 2,796,834 McCafirey et al June 25, 1957 2,808,894 'Eisler et al. Oct. 8, 1957 2,823,609 Johnson et al. Feb. 18, 1958 2,891,475 Dolan et al. June 23, 1959 FOREIGN PATENTS 493,862 Canada June 23, 1953 OTHER REFERENCES Publication: The Progressive Detonation of Multiple Charges in a Single Seismic shot, by Lorenz Shock; Geophysics Magazine, vol. XV, No. 2, April 1950, pages 208-218.

corrected below UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent, No. 2,992.611 July 18, 1961 Robert W. Felch It is hereby certified that error appears in the above numoered pats Patent. shouldread as ent requiring correction and that the said Letter Inthe grant/only linel, for "Robert W. Flech" read Robert W. Felch Signed and sealed this 2nd day of January 1962.

( SEA L) Attest:

DAVID L. LADD ERNEST w. SWIDER I Attesting Officer Commissioner of Patents USCOMM-DC- "corrected below. 7

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,992,611 July 18, 1961 Robert W. Felch It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent. should read as In the grant only, line 1, for "Robert W. Flech" read Robert W. Felch Signed and sealed this 2nd day of January 1962.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Commissioner of Patents Attesting Officer USCOM M DC-

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