US3500949A

US3500949A - Marine seismographic prospecting

Info

Publication number: US3500949A
Application number: US691395A
Authority: US
Inventors: Stewart Paterson
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1965-08-02
Filing date: 1967-12-18
Publication date: 1970-03-17
Anticipated expiration: 1987-03-17
Also published as: DE1548450B2; DE1548450A1; GB1097420A; NL6610849A

Description

March 17, 1970 s. PATERSON 3,500,949

MARINE SEISMQGRAPHIC PROSPECTING Filed Dec. 18, 19s? 3 Sheets-Sheet 1 554% gig,

March 17, 1970 s. PATERSON 3,

MARINE SEISMOGRAPHIC PROSPECTING Filed Dec. 18, 1967 3 Sheets-Sheet 2 arch 17, 1970 Filed Dec. 18, 1967 S. PATERSON MARINE SEISMOGRAPHIC PROSPECTING FIG. 6

3 Sheets-Sheet 5 US. Cl. 181.5 6 Claims ABSTRACT OF THE DISCLOSURE Underwater seismic prospecting is carried out by detonating an immersed bubble-forming explosive having a length-to-diameter ratio exceeding :1 thereby suppressing bubble pulsation which would normally occur unless the explosive is located close to the water surface.

This invention relates to a method of marine seismographic prospecting, and to explosive charges for use therein.

In marine seismographic prospecting pressure waves are generated by firing explosive charges immersed in water, and the arrival at a ship of the pressure waves reflected from underwater rock strata are recorded on seismographs on the ship. When an explosive charge is fired at a great depth, about half the explosive energy is usefully transformed into a spherically expanding shock wave. After this has been radiated, the remaining half of the energy still remains largely in the product gases, whose pressure greatly exceeds the ambient hydrostatic pressure. In consequence, the bubble of gas expands, setting the water into outward motion. Because of the inertia of the water, this process does not terminate when the bubble pressure has fallen to the ambient pressure, but instead an overshoot takes place. In due course, the water comes to rest, with the bubble now greatly enlarged and at a very low pressure. Gravitational force now reverses the motion, and the bubble is rapidly contracted again, overshooting once more takes place and the bubble is compressed to a high pressure and a relatively small volume. As the motion is again reversed, a second pressure pulse is radiated outwards. This entire cycle is repeated several times with decreasing amplitude, and a series of pulses is radiated. The peak pressure in the first pulse, although small compared with that in the primary shock, is of much longer duration and, in consequence, the momentum and energy transferred are not negligible when compared with those of the primary shock. This first bubble pulse is received by the recording equipment during the time interval in which the reflected signals are arriving and the record can become confused. In order to suppress bubble pulsation, one can work with charges quite close to the surface, and this is what is done at present. Release through the surface then suppresses bubble oscillations, but it also greatly reduces the intensity of the outgoing shock wave. In typical cases, instead of radiating one half the explosive energy in the shock, only oneeighth may be thus radiated. Elimination of the bubble action would permit firing at greater depths and could, therefore, lead to as much as a four-fold increase in efliciency.

In accordance with the present invention, a method of seismographic prospecting for producing a record of underwater rock strata comprises generating a pressure wave by the firing underwater of an explosive charge having a length to diameter ratio exceeding 5:1 and recording the resulting seismic waves after reflection or refraction from underwater rock layer interfaces. Preferably the length to diameter ratio is between l and 100:1 since such charges may contain suflicient charge per unit length to ates ac give a strong pressure wave whilst still being convenient to handle. Heavy charges having a length to diameter ratio exceeding :1 are cumbersome to work with in prospecting conditions. The charge distribution preferred is from 0.2 to 5 lb. per foot.

The invention also includes waterproof charges of detonatable explosive for use in underwater seismographic prospecting having a length to diameter ratio exceeding 5:1. It will be obvious that the explosive charges need not be cylindrical and that diameter in this context is used in a wider sense as being the maximum cross-sectional dimension. The explosive charge may be a unitary charge comprising explosive in a rigid container such as, for example, a tinplate canister or a tubular cardboard container, or the explosive charge may be contained in a flexible tube of, for example, synthetic thermoplastic which may be supplied in coiled form in bulk. Alternatively, the explosive charge may be an assembly of spaced charges connected for substantially instantaneous initiation and assembled so that the overall length to diameter ratio of the assembled charge exceeds 5:1. It will be apparent that the smaller charges in the assembly may or may not have a length to diameter ratio exceeding 5:1. The charges may, for example, be connected together for initiation by a line of detonating fuse-cord.

The invention further includes an apparatus assembly for seismic prospecting, com-prising an explosive charge of the invention located underwater and means for recording a seismic wave reflected or refracted from an underwater rock layer interface, consequently on detonation of the explosive charge.

When marine seismographic prospecting is carried out in accordance with the method of the invention, using an elongated charge, the bubble pulse is much diminished in comparison to the bubble pulse obtained using the charges of the same weight and of the squat shape normally used, whereas the primary shock wave is not substantially different. The bubble pulse suppression effect is more marked when the charges are orientated horizontally on firing.

The invention is further illustrated by the following description of examples of explosive charges designed for use in putting the invention into practice. In the description reference is made to the accompanying drawings in which:

FIG. 1 shows a diagrammatical longitudinal section of a canister primer cartridge,

FIG. 2 shows a diagrammatical longitudinal section of a canister main-charge cartridge,

FIG. 3 shows a diagrammatical longitudinal section of an elongated flexible cartridge assembly,

FIG. 4 shows a bundled assembly of the cartridges shown in FIG. 3,

FIG. 5 is a sectional view on the line VV of FIG. 4,

FIG. 6 shows a spaced assembly of the cartridges shown in FIG. 3.

EXAMPLE 1 The canister primer cartridge shown in FIG. 1 has an elongated tubular tinplate body 11, sealed at one end with a recessed tinplate cap 12 which is formed with a pocket 13 to accommodate a detonator and at the other end with a tinplate cap 14 formed with an internal screw-threaded portion 15 adapted for attachment by screw engagement to a screw-ended canister cartridge of a less sensitive main explosive charge. The cartridge is filled with a charge of powdered explosive consisting (by weight) of 88.5% of ammonium nitrate, 3.5% of anthracite and 8% of trinitro toluene.

The canister cartridge of main explosive charge shown in FIG. 2 has a tubular tinplate body 16 sealed with

tinplate end caps

17, 18 formed respectively with matching male and female threads. The cartridge is filled with a charge of powdered explosive consisting (by weight) of 92% of ammonium nitrate and 8% of trinitrotoluene.

The bodies of both cartridges are inches long and 2 inches in diameter. In use, the primer cartridge and optionally one or more cartridges of main charge attached thereto, are immersed in the water over the area to be surveyed and fired by an electric detonator disposed in the detonator pocket 13. The length to diameter ratio of the assembled charge may be varied at will by variation in the number of cartridges.

EXAMPLE 2 The explosive cartridge shown in FIG. 3 has an elongated

tubular polyethylene body

19, 25 inches long and having an internal diameter of 0.625 in., and a wall thickness of 0.1 in. A polyethylene sleeve 20 is closely fitted over one end of the cartridge and this sleeve is used to attach a detonator or primer or to attach a further similar tubular charge to vary the length of the charge. The cartridge is filled with an explosive charge which may, for example, have the composition of the primer or main charge given in Example 1.

EXAMPLE 3 In the assembly shown in FIG. 4, four of the cartridges 21 as shown in FIG. 3 are bundled together and held by strap 22. The explosive loading is thus readily varied by varying the number of cartridges in the bundle.

EXAMPLE 4 In the charge assembly shown in FIG. 6 a number of cartridges 23, as shown in FIG. 2, are spaced apart along a rope 24- and attached thereto by tapes 25. Detonators 26 are attached one to each cartridge by sleeves 27.

The cartridges shown in FIG. 1 may also be used in a spaced assembly wherein each cartridge is provided with a detonator.

What I claim is:

1. In the method of seismographic prospecting for producing a record of underwater rock strata, said method being of the type which includes firing an explosive immersed in the water thereby forming a bubble of combustion gases and recording the resulting seismic waves after reflection or refraction from the underwater rock layer interfaces, the improvement comprising predetermining parameters for a suitable firing depth for an elongated condensed explosive charge having a length to maximum cross sectional dimension ratio exceeding 5:1, said predetermining step including selecting a depth at which a generalized shock wave will be utilized to provide energy from the shock pulse for directing the wave toward the rock strata and to control the bubble travel in a manner to prevent bubble breakthrough at the water surface, thereby providing substantially greater energy from the shock pulse than is provided under conditions where bubble breakthrough does occur; submerging the charge to at least said predetermined depth; and firing the charge, whereby undesired multiple pressure pulses of the type usually associated with explosion bubbles which do not break through the water surface are suppressed.

2. A method as claimed in claim 1 wherein the length to diameter ratio of the explosive charge is between 10:1 and :1.

3. A method as claimed in claim 1 wherein the explosive charge distribution is from 0.2 to 5 lb./sq. in.

4. A method as claimed in claim 1 wherein the explosive charge is contained in a container the walls of which are constructed of a material selected from the group consisting of tinplate, cardboard and synthetic thermoplastic.

5. A method as claimed in claim 1 wherein the explosive charge comprises an assembly of spaced charges connected for substantially instantaneous initiation.

6. A method as claimed in claim 5 wherein the spaced charges are connected together for initiation by detonating fuse-cord.

References Cited UNITED STATES PATENTS 2,675,882 4/1954 Bazzoni et al l81.5 2,842,056 7/1958 Klotz 18l.5 3,009,526 11/1961 Andrews et a1. l81.5 3,256,501 6/1966 Smith 34012 X 3,326,126 6/1967 Berthmann et al. l81.5 2,619,186 11/1952 Carlisle 181.5

BENJAMIN A. BORCHELT, Primary Examiner T. H. WEBB, Assistant Examiner U.S. Cl. X.R. 340-5, 12