US1943725A - Method of creating artificial seismic waves - Google Patents

Description

Jan. 16, 1934. Q DUDLEY ET AL 1,943,725

METHOD OF CREATING ARTIFICIAL SEISMIC WAVES Filed Jan. 9, 1929 gvweutozy Patented Jan. 16, 1934 UNITED STATES FFEQE Parent METHOD OF CREATING ARTIFICIAL SEKSIVHC WAVES Application January 9,

3 Claims.

This invention has to do primarily with improvements in methods of setting up violent compressional waves, or seismic disturbances, at selected points within the crust of the earth for 5 various purposes. More specifically, it has reference to the use of explosive materials inthe accomplishment of the desired result.

An object of the invention has been to bring about disturbances of desired magnitude by the employment of a minimum quantity of explosives. The invention is particularly applicable to the comparatively recent development of geophysical methods of exploring the sub-surface of the earth. In the practice of one of these recent methods, it is customary to employ large charges of explosives which are set oif at given points to generate series of compressional waves through the earths crust. These waves radiate in all directions and are then detected by suitable instruments, either of mechanical or electrical construction, and operating on the principles of a seismograph or a magnetophone. The vibrations thus detected are usually automatically recorded by the instruments so that the nature of the intervening sections of the earths crust between the points of explosion and the points at which the detectors are located may be studied from the records formed.

It is commonly the practice to transmit these compressional waves over relatively large distances, which may be as much as five, or even ten, miles. Heretofore it has been necessary to employ large charges of dynamite, explosive gelatin or other explosive material, in order to produce disturbances of sufiicient magnitude to be detected at the distant points. Frequently charges of 300 or 400 lbs. have been employed, and sometimes even more. This not only tends to render the exploratory work quite expensive, but it tends to severely disturb the surface of the earth by the creation of large craters. Where the fields of exploration are in more or less settled areas, or on farms which are being worked, the damage done is a considerable item. Heretofore, it has been the practice to place the charge of explosive openly into a relatively shallow hole and to cover it up comparatively lightly.

Now, by the present invention, it is contemplated to confine the charge much more closely. This has been found not only to decrease the amount of charge required, but also to aid materially in reducing the damage done at the surface of the ground. One important factor in the production of artificial seismic disturbances is the formation of a relatively sharp impulse by a com- 1929. Serial No. 331,340

paratively sudden explosion. The confinement of the charge as contemplated by the present invention serves to 'bring about a much sharper impulse than the methods previously employed. This is apparently due to the fact that the effect of the explosive charge is not distributed over so great a period of time but instead its force is accumulated up to the point where the confining means gives way and permits the desired sudden impulse to be sent out.

There are various ways in which the present invention may be carried out. Under a preferred form of application of the invention it is contemplated to place the charge within a metal container or shell to form what may be termed a torpedo. However, it is not altogether essential to provide a special container, since somewhat the same efiect may be produced by boring a relatively deep hole into the ground of, say, from 30 to 150 feet, or more, in depth and burying the charge therein so that the charge may be confined in a. relatively small space at the bottom of this hole. If desired, both methods of confinement may be adopted so that a loaded shell or torpedo may be placed at the bottom of a relatively deep hole and then covered over.

Another feature of the invention, where it is applied to the seismograph method of geophysical exploration, is in the provision of a separate charge at the surface of the ground, or even slightly elevated therefrom, to be set off simultaneously with the buried charge. This is in order to provide for the generation of sufficiently intense sound waves in the air to be transmitted to the detector at the distant receiving point. The simplest method of determining distances between the pointof explosion and the receiving stations is to note the time of transit of the sound wave through the air. Its velocity is known, or may be determined, by the making of suitable corrections for temperature, as well as wind velocity and direction. Where the ground charge has been placed relatively near the surface, it has sometimes been relied upon to create the air wave as well as the ground wave, but in this prac- Figure 1 discloses, partly in section, a shell or tube loaded with the explosive to form a torpedo.

Figure 2 is a detail of a cap for the torpedo.

Figure 3 illustrates diagrammatically the arrangement of the charge, placed at a considerable depth in the ground, and the firing and detecting apparatus.

Referring now to Figure 1, a tube or section of casing 10 may be employed to form the main body of the torpedo. This tube may advantageously be 4 or 5 inches in diameter and of sufficient length to contain the desired quantity of explosive. It should, for example, be about 8 feet long, if it has an inside diameter of about 4%; inches and when it is desired to employ about 100 lbs. of the explosive. The tube may be formed of steel, or any other suitable metal, and should have a wall of sufiicient thickness to momentarily resist the force of the explosion. A wall thickness of between A; and of an inch has been found satisfactory for a tube of the size mentioned. A section of a regular fifteen-pound well casing of 5 inch outside diameter might be employed.

One end of the tube may be closed off by means of a cap 11, which may be provided with internal threads co-operating with external threads on the tubing. Now, a quantity of the desired explosive, in the form of dynamite or sticks of quarry gelatin, may be introduced into the opposite or upper end of the tube and may be tamped by means of a wooden tamping bar to fill the tube to a point within 8 or 9 inches of the top of the latter. In the upper space which remains a priming mixture, composed of, say, 60% nitro-glycerin dynamite, may be introduced. It has been found that the use of this type of primer in the proper quantities serves to bring about the desired high speed of detonation of the gelatin. Two caps 14, for electrically firing the primer sticks of nitroglycerin, may then be inserted into the end of the tube. While only a single firing cap is actually necessary, it has been found desirable to employ two, connected either in parallel or series, so that if one should fail to perform its function properly, the other will, nevertheless, be effective and it is practically certain that the charge will be exploded. A closure or cap 15 may be screwed onto the upper end of the tube to complete the confinement of the charge. A small opening 16 (Fig. 2) should be provided in this cap to permit the passage of the electrical conductors leading to the firing caps 14.

Referring now to Figure 3, a hole 17 may be drilled into the ground by any suitable means, such as a post-hole auger or the like. It is, of course, essential to the effectiveness of the geophysical methods, in which the present invention may be advantageously employed, that the drilling apparatus should be readily portable. The hole 1'7 will preferably be drilled to a depth of at least 30 feet, and preferably to a greater depth, of as much as 150 feet, for example, or even more. The greater depths are particularly desirable where the charge is placed in the hole in a free condition rather than in the form of a torpedo such as illustrated in Figure 1. Where a torpedo of this type is employed, it is lowered into the bottom of the hole and is connected by means of a suitable electricity-conducting cable 18 with a blasting machine 19, or other suitable source of current for discharging the caps 14. The cable 18, which is in the form of a double conductor, is preferably about 1000 feet in length in order that the operator who is stationed at the blasting machine may be at a suitable distance from the explosion. If desired, that portion of the cable which is within the hole 17 may be of less substantial or cheaper construc tion than that along the surface of the ground which may be used many times over again.

In order that the instant of the explosion may be noted at the blasting station, and so that the impulse may be immediately transmitted to suitable recording apparatus or wireless transmitting outfit located at this point, a blasting cap 20 is preferably placed in series with the caps 14. This cap then will be set ofi simultaneously with the two in the torpedo and a signal may be passed by suitable conductors 21 to the recording or transmitting apparatus 22. Any suitable means may be used to cause the transmission of a signal to the apparatus 22 when the blasting cap 20 is exploded and such means may, for instance, include a wire in circuit with the conductors 21 and adapted to be ruptured by the explosion of the cap 20. The transmitting apparatus 22 is preferably provided with a wireless antenna 22a.

In the conduct of the seismograph method of exploration it is frequently desirable that the report of the explosion should be transmitted through the air. This is for the purpose of permitting the actual straight-line distances between the point of explosion and the recording stations to be noted. Computation of this distance may be readily made when the actual time of travel of the sound waves produced is known. It requires merely a knowledge of the air conditions, as to temperature and wind velocity and direction, so that the speed of travel of the sound waves in a particular direction may be accurately determined. Where this method of measuring the distances is adopted, it has been found advantageous to place a small charge of explosive 23 above the surface of the ground, either resting directly on the surface or held slightly above it. A firing cap which is electrically connected, in se ries or parallel, with the caps 14 and 20 may be lodged within this charge to explode it.

In placing the main charge either openly or in the form of the torpedo 10 into the hole 17, it is preferable to fill the hole above the charge with dirt and to pack it by means of a suitable wooden tamping bar so as to form a resistance to the blowing out of the earth in a vertical direction. It will be apparent that where the tube 10 is employed, the detonation or generation of compressional waves will not take place until the explosive force has been built up sufficiently to shatter this tube and therefore a much sharper reaction will be brought about than where the charge is not confined. In much the same way the confinement of the charge directly within a relatively deep hole, which offers substantial resistance in all directions, will serve to concentrate the effectiveness of the explosion, holding it in resistance until it has reached a sufficient magnitude and then transmitting it through the earth, rather than permitting a large part of its force to be dissipated out at the top of the hole with the resulting creation of a large crater.

The compressional waves set up in the earth by means of the explosion will radiate in all directions and will be picked up by one or more detectors 24 of any suitable design but preferably responsive to sound waves passing through the air as well as to the compressional waves passing directly through the earth. At the same time the sound waves set up by the air charge 23 will travel toward the detector but, due to the slower speed of the waves in the air than in the ground these waves will not reach the detector until after the ground waves. Impulses received by the detector may be transmitted in any way, as by the conductors 25, to a recording unit 26 which may, if desired, be in the form of, or include, an oscillograph. Of course if desired, a separate device for detecting sound waves in the air may be placed at or above the surface of the ground and elec-- trically connected to the recording-receiving unit 26. At the instant of the explosion a signal may be transmitted by wireless, as previously explained, from suitable transmitting apparatus 22 and may be received and recorded by means of the unit 26 along with the other impulses. The unit 26 is shown as provided with a suitable wireless antenna 26a. Due to the great velocity of the radio transmission of the signal, its time of travel may be ignored or may be accounted for by a suitable correction factor. The time interval between the reception of the signal and the reception of the air sound waves will provide the basis for computing the distance, as mentioned before.

While certain specific constructions and measurements have been stated in the foregoing discussion, it is to be understood that these are purely for the purpose of illustration and that the invention is capable of many modifications. It is not desired to be limited in the scope of the invention other than by the terms of the claims which follow.

What we claim is:

1. A method of creating artificial compressional earth waves which comprises firing an explosive charge in the ground at a depth of more than thirty feet in a tightly packed hole, said charge being insufficient to disrupt the surface of the earth from said depth, so that substantially the entire force of the explosion will be transmitted to the earth.

2. A method of creating artificial compressional earth waves which comprises drilling a hole of narrow bore and to a depth of more than thirty feet into theground, placing a charge of explosive into the bottom of said hole, packing the hole above said charge with earth, and firing the explosive, the firing of the explosive at a depth of more than thirty feet and the packing of the hole above the charge serving to substantially prevent disrupting the earth at the surface thereof.

3. A method of exploring the sub-surface of the earth which comprises confining a charge of explosive buried in the ground, firing said explosive, simultaneously firing a charge of explosive directly above the first mentioned charge and above the surface of the ground, simultaneously exploding a separate cap above the surface of the ground, and recording at a distance the explosion of said cap and the reception of the compressional waves transmitted by said charges through the ground and through the air.

OSCAR E. DUDLEY. CON LUCID.

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