US3702635A

US3702635A - Seismic energy source using liquid explosive

Info

Publication number: US3702635A
Application number: US95003A
Authority: US
Inventors: John B Farr
Original assignee: BP America Production Co
Current assignee: BP America Production Co
Priority date: 1970-11-10
Filing date: 1970-11-10
Publication date: 1972-11-14
Anticipated expiration: 1989-11-14
Also published as: CA935037A

Abstract

This invention is a method of generating seismic waves, for example for use in seismic geophysical prospecting, which involves forcing liquid explosive below the surface of the earth through a conduit to form a localized accumulation in an unstable formation which may be water, marsh, loose soil, etc. The liquid explosive has sufficient body so that it disperses only slowly through the formation. After this accumulation is placed it is detonated from the earth''s surface to produce seismic waves. Preferably the liquid explosive is formed in place from two or more components, each of which has very little tendency to detonate prior to mixture, and each of which is pumped down a separate conduit, which increases the safety of the procedure. The detonation is accomplished remotely by sudden application of pressure, for example by exploding a thin, substantially continuous column of the liquid explosive between the detonator and the localized accumulation, or by exploding a column of gas between detonator and accumulation. If the unstable earth formation contains a substantial amount of solids, such as in the case of marsh or loose earth, a rod-like solid member is forced down into the formation and then partly withdrawn to form a cavity which is filled with liquid explosive through conduits provided in this member. The region around the local accumulation of liquid explosive can be treated in several ways to enhance transfer of seismic energy to the earth beyond the initial cavity, for example by permeating the formation around the cavity with a nondetonating liquid, e.g., water, up to a distance of several feet or by using a soilconditioning liquid which after a short while tends to solidify and increase the acoustic impedance of the treated region.

Description

United States Patent Farr [ Nov. 14, 1972 [54] SEISMIC ENERGY SOURCE USING LIQUID EXPLOSIVE [72] Inventor: John B. Farr, Tulsa, Okla.

[73] Assignee: Amoco Production Company, Tulso,

Okla.

22 Filed: Nov. 10, 1970 21 Appl.No.: 95,003

[52] US. Cl. ..166/299, 166/300, 175/1 [51] Int. Cl ..E21B 43/76, E21b 47/00 [58] Field of Search.....166/299, 300; 175/1; 102/30, 102/21, 23

[56] References Cited UNITED STATES PATENTS 3,593,793 7/1971 Kelseaux 166/299 X 3,561,532 2/1971 Fletcher ..l66/299 2,708,876 5/1955 Nowak 166/299 3,075,463 1/1963 Eilers et a1. 102/21 X 3,075,464 1/1963 Woodle et a1. ..102/23 3,100,542 8/1963 Stark ..175/1 3,380,551 4/1968 Lang ..166/299 X Primary ExaminerDavid H. Brown Attorney-Paul F. Hawley 5 7 ABSTRACT This invention is a method of generating seismic waves, for example for use in seismic geophysical prospecting, which involves forcing liquid explosive below the surface of the earth through a conduit to form a localized accumulation in an unstable formation which may be water, marsh, loose soil, etc. The liquid explosive has sufficient body so that it disperses only slowly through the formation. After this accumulation is placed it is detonated from the earth's surface to produce seismic waves. Preferably the liquid explosive is formed in place from two or more components, each of which has very little tendency to detonate prior to mixture, and each of which is pumped down a separate conduit, which increases the safety of the procedure. The detonation is accomplished remotely by sudden application of pressure, for example by exploding a thin, substantially continuous column of the liquid explosive between the detonator and the localized accumulation, or by exploding a column of gas between detonator and accumulation. If the unstable earth formation contains a substantial amount of solids, such as in the case of marsh or loose earth, a rod-like solid member is forced down into the formation and then partly withdrawn to form a cavity which is filled with liquid explosive through conduits provided in this member.

The region around the local accumulation of liquid explosive can be treated in several ways to enhance transfer of seismic energy to the earth beyond the initial cavity, for example by permeating the formation around the cavity with a nondetonating liquid, e.g., water, up to a distance of several feet or by using a soil-conditioning liquid which after a short while tends to solidify and increase the acoustic impedance of the treated region.

10 Claims, 8 Drawing Figures SEISMIC ENERGY SOURCE USING LIQUID EXPLOSIVE BACKGROUND OF THE INVENTION 1. Field of the Invention Seismic geophysical exploration as it has been practiced for many years frequently employs the detonation of a relatively small body of explosive, usually dynamite, to generate seismic waves in the earths subsurface which travel through the earth, reflecting from layered beds back to a plurality of geophones or seismometers to produce electric waves which are amplified and processed, ultimately to be recorded in a manner permitting determination of the depth and dip of the seismic reflecting horizons. Processes of this general description have been widely employed to determine the lay of the formations and hence determine possible favorable structural or stratigraphic traps for the accumulation of petroleum.

My invention relates to the initial part of such a system, namely the seismic source. Seismic exploration in soft areas, such as marshes, swamps, and water, or in general any terrain which is unstable, has been hampered by the fact that it is mechanically difficult as well as somewhat dangerous to place a solid explosive, such as dynamite, beneath the surface of the earth. Detonation at the surface is undesirable because there is ineffective resistance to energy flow into the air. Accordingly, it is desirable to place the explosive source at least ten feet or more under the surface, be it liquid or semi-solid. I have found that a liquid explosive may be employed with greater safety than that inherently present when using a solid explosive in a method which permits rapid placement and effective detonation of the liquid accumulation. Generally speaking, a solid probe is forced into the unstable formation, the liquid explosive or preferably the components of such an explosive, none of which have much tendency to detonate, are pumped through conduits in the probe to form a localized accumulation at the bottom of the probe and the accumulation is then detonated remotely from the surface at a time which can be determined with precision, a necessary condition for a source used in seismic prospecting.

2. Description of the Prior Art The Kowastch US. Pat. 1,049,735 teaches that in blasting consolidated formations, where a tamping plug (Le, a packer) can be employed to seal off the blast hole, constituents of the explosives can be introduced through conduits in a tube extending through the packer. By this means Kowastch produces the explosive in a volume in the blast hole isolated by the packer in a manner free from danger and at the place where the explosion will ultimately take place.

The Owen US. Pat. No. 1,627,991 describes a method for loading blast holes by packing off a tube within a drilled hole and injecting through the tube an explosive mixture of oxygen and a hydrocarbon. This mixture is then ignited by a spark. Such a system requires a hole (in which the tube and packer are to be located) with walls of sufiiciently coherent or solid character so that when the packer expands against the walls, they furnish resistance to further expansion of the packer. This is, of course, not possible in case of an unstable earth formation, to which my invention is directed. Furthermore, the energy per unit volume of explosive which can be obtained on detonation of a gas mixture is far too low to produce seismic energy great enough to be employed in most seismic prospecting unless the cavity defined by the hole and packer be of such a large size that it is uneconomic to prepare it.

The only other patent which is considered pertinent is the Merten et al. US. Pat. No. 2,353,484, which is directed to seismic sources for geophysical prospecting. Merten et al. describe their method as involving forcing a plurality of pointed probes into the earth, injecting a gaseous fuel through some and compressed oxygen or air through others, the resultant mixture being detonated to generate a seismic wave over a large horizontal area. As in the Owen patent the explosive is again gaseous, which is quite difficult to confine in an unstable earth formation to which my invention is directed, and also has the disadvantage of low explosive energy per unit volume. In attempting to get around the difficulty of the low specific energy, Merten et al. teach and claim uniformly distributing the explosive charge over a substantial horizontal ground area, of dimensions at least one-tenth of the vertical distance between surface and the nearest underground reflecting layer. For example, it teaches and claims using an area of at least square feet for the gaseous mixture. On the other hand my invention makes use of concentrated small charges of a liquid (including a slurry) type explosive which has sufficient body so that it disperses only slowly through the formation, unlike a gaseous mixture. It requires the use of only a single rod or probe, rather than the plurality of tubing rods used in the Merten et al. invention. The use of a concentrated charge rather than a diffused one results in the presence in the explosion of much higher frequency components than when using the gaseous mixture, which improves subsurface resolution and offsets the deleterious low-frequency effects encountered when shooting gaseous explosives in the weathered layer. Also, normal moveout can be removed from individual recordings, permitting summation in record processing over areas much larger than possible in the Merten et al. scheme. Finally, propagation of detonation in thin sheets of explosive as in the Merten et al. situation, is well-known to be a difficult problem and without extensive precautions, which are not considered by those inventors, results in only a portion of the gaseous explosive remaining undetonated. This, of course, is again contrasted with the effective, complete detonation found when using a liquid explosive which is essentially a lumped charge at the bottom of the probe. This change in system also leads to improvement in highspeed implacement of the concentrated charge without the use of drills, manual loading, and firing, all matters which are not taught by Merten et al.

A series of less pertinent patents have been issued on employing liquid explosives in wells in order to increase the drainage area into the well for recovery of valuable liquid constituents, such as petroleum. Among these is the Chestnut US. Pat. No. 3,982,405, using nitromethane containing a sensitizer (preferably an organic amine), the liquid being pumped into fractures in the formation and ignited there by detonation of the liquid remaining in the bore. The Zandmer US. Pat. No. 2,246,611, employs essentially this same method after acidizing the formation to increase the size of fissures communicating with the bore before placing the liquid explosive. The following US. Pats. generally relate to the type of liquid explosive used rather than dealing with variations in the method beyond that discussed above: 2,746,51 1; 3,075,463; 3,104,706; 3,270,815; 3,239,305; 3,336,981; and 3,336,982. None of these patents are of pertinence in connection with this invention, which is on the method of producing a simple and rapid arrangement for producing an explosive seismic source in an unstable earth formation. In the first place, there is no need to drill a well when using my method. Second, only incidentally do such explosions provide a seismic source, and under circumstances of little importance to geophysical prospecting, since they relate to ordinary oil and gas wells which follow rather than precede seismic prospecting and they also are employed in consolidated formations rather than water, marshes, bogs, loose earth, and the like.

Various publications from the Bureau of Mines and others discuss the problems in using liquid explosives, but not in advantageous methods of placement of liquid explosive in an unstable earth formation so that a" geophysical seismic source can be produced.

It is, of course, known in seismic exploration on water-covered areas to use a gas gun. This is a hollow pipe closed at the top end and carried in a partially emerged, essentially vertical, condition. An explosive gas, for example propane and compressed air, is forced into the upper part of the gun until a suitable sized charge is produced. This charge is then detonated within the body of the gun to produce a low-energy seismic pulse adequate for determining geophysically the floor of the body of water and perhaps the location of reflecting beds below this point of the order of about 1 ,000 feet at most.

SUMMARY OF THE INVENTION The gist of this invention arises in providing a simple, relatively safe, rapid method for placing liquid explosive below the surface in an unstable formation where it can be detonated with timed precision to form a tamped seismic source for use in geophysical prospecting. A rod-like solid probe furnished with conduits (as distinguished from a hose or the like) is forced through the unstable formation. The liquid explosive (or preferably separate non-explosive components thereof) is pumped through the probe into the unstable formation to form a localized accumulation. Preferably this liquid has sufficient body, i.e., enough viscosity, so that it disperses only slowly through the formation. It is also preferred to pump constituents of the liquid explosive through separate conduits, each component having very little tendency to detonate, so that the explosive is actually formed at the zone where it will be subsequently detonated, which enhances the safety of the procedure. The liquid explosive may have solid components; as the term liquid explosive is used hereafter, it includes explosive slurries in which the liquid part is in itself detonatable. The detonation takes place from the surface by sudden application of pressure. In certain liquid explosives this can be accomplished by dropping a weight, but it is preferred to connect the localized accumulation with an electric detonating means, such as a spark plug, arc, or the like, by either a thin, substantially continuous column of a liquid explosive or by a column of explosive gas.

An important variation on this basic technique is the provision for treating the region around the local accumulation of liquid explosive to increase transfer of seismic energy. If the unstable region is dry, it can be wetted for several feet around the accumulation with water or other non-detonating liquid, which will cause a marked increase in transmission of the seismic energy into the earth below. Another usable technique is to provide a soil-conditioning liquid which permeates the zone about the explosive and tends to become plastic or solid a short while after injection.

BRIEF DESCRIPTION OF THE DRAWINGS The attached drawings form a part of this specification. In these drawings the same reference number in a subsequent figure refers to the same or a corresponding part. All of the drawings are diagrammatic representations of a cross section of the earth with one type of probe used to practice the claimed method.

FIG. 1 shows the lower end of the probe ready to penetrate the unstable earth formation.

FIG. 2 shows this probe at its maximum depth in the formation. It is assumed in the figure that this formation contains a considerable volume of solids.

FIG. 3 shows the probe withdrawn somewhat to create a temporary cavity at the lower end.

FIGS. 4 and 5 show the components of the liquid explosive being forced through the probe to form the explosive liquid in the temporary cavity at the bottom.

FIG. 6 shows the arming of an explosive by connecting the localized accumulation at the bottom to a detonating means by an explosive gas.

FIGS. 7 and 8, in turn, show the detonation of the accumulation of liquid explosive and the withdrawal of the probe.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention will first be described in relation to its use in a marsh. In such a region it is difficult and timetaking to force a solid explosive charge down to the order of ten feet or more due to the solids content in the marsh. A gas gun similarly has the difficulty of both weak energy generation and interference with the downgoing seismic pulse due to the solids which accumulate in the barrel of the gun as the pipe is forced down through the marsh.

Referring now to FIGS. 1 and 2, I provide a rod-like solid member 11, sometimes called a probe, which is in this case in the form of a hollow cylinder or pipe, preferably fitted at the lower end with auger-type flukes 12 and a central mixer tip 13 connected to stem 14. This rod 11 is adapted for rotation by a conventional earth auger mechanism (not shown) mounted on a marsh buggy (not shown). The auger mechanism is similar to the arrangement used in drilling fencepost holes and the like. Accordingly, when the rod 11 is rotated about its cylindrical axis with the flukes 12 in contact with the marsh 15, the rod 11 is rapidly drawn into the marsh 15 until the tip 13 is at a desired maximum depth of the order of ten feet or more. During this rotation, and, accordingly, forcing of this rod 11, the tip 13 is maintained in the position shown in FIGS. 1 and 2 by latching stem 14 by any desired mechanical means to the hollow rod 11. Due to the nature of the solids in marshy areas, such solids will simply be pressed around the outside of the rod 1 l.

It may be that at a particular location there is insufficient water in the unstable earth formation to permit rapid forcing of the rod 11 to the desired depth, in which case a source of water (not shown) carried on the marsh buggy is connected to the top of stem 14 so water passes down through the inside of this conduit and through the openings in the tip 13 to flush the region immediately surrounding rod 1 l and lubricate this rod while the implanting process is proceeding. On the other hand, if the solids in the marsh 15 are quite loose, it is frequently possible to force the rod 11 to bottom simply by the pulldown mechanism ordinarily present in the earth auger mechanism and it is not even necessary to rotate this rod.

A possible next step is shown in FIG. 3. The rod 11 and tip 13 (fixed to rod 11 by the clamping of stem 14) are drawn back a suitable distance. This creates a temporary cavity 16 below the tip 13, that is, a space at least not filled with solids. If desired, during this partial withdrawal of rod 11 and tip 13, a further injection of water through stem 14 can be carried out, simply to keep solids from sucking back into the cavity 16. Ordinarily this withdrawal will not be more than the order of a foot or at most two feet. Put another way, it is not necessary to get a very large cavity 16, and when using a rod 11 with an outside diameter of the order of four inches, the withdrawal given above will create a suffrcient cavity. Such withdrawal is not needed when the unstable formation contains considerable liquid. The purpose of this step is to permit easy flow of the liquid explosive or its components into the zone just below the probe. Obviously, if there is little resistance to flow as in the case of open water or marsh areas with little solids there is no need of such withdrawal.

One method of placing liquid explosive (or a liquid explosive containing solid particles, hereinafter also called a liquid explosive) is to pump it down through the conduit or opening in stem 14 and tip 13. The amount of liquid explosive used is not particularly critical. Ordinarily I wish to use of the order of one to two quarts of explosive, which in terms of weight will be about I to 2 pounds. In all cases I prefer to keep the amount of liquid explosive accumulation near the minimum required for the job. In no case do I want it to exceed a maximum dimension of over three feet, both from the fact of excessive explosive and the fact that it may not all detonate substantially simultaneously a very definite requisite of a seismic source for geophysical prospecting in such situations. This can be introduced at the top of the opening in stem 14 and forced down by application of a head of water. This forcing of the liquid explosive down with water is also desirable from the standpoint of flushing the bore or conduit of stem 14 free from clinging particles of liquid explosive. However, a much more preferred way of placing the liquid explosive in the cavity 16 below tip 13 is illustrated diagrammatically in FIG. 4.

The liquid explosive in this case is made up of two or more components, none of which has a substantial tendency by itself to detonate. Each component is then separately introduced through a conduit in rod 11 into the cavity where the components mix simply by the pumping action involved in forcing the components into this cavity. If each component is forced down the same conduit in stem 14, one preferably employs a short column of water to separate components as they are being pumped down. The arrangement shown in FIG. 4 permits more rapid loading of the cavity 16. In this case the hollow rod 11 is provided with a plurality of conduits, such as conduits l7 and 18, each of which is separate from all others but all of which feed into the hollow bore of rod 11. One component is pumped down each conduit. Accordingly, the components of the liquid explosive do not mix (and therefore do not form a detonatable explosive) until they pass into the bore. The components then mix to form a localized accumulation of liquid explosive 19. This sensitized explosive is then forced below the lower end of rod 11 by flushing water introduced through conduit 24 in stem 14.

A particularly advantageous mechanical arrangement of sealing such conduits, except at the time when they are employed to force the components of the liquid explosive into cavity 16, is shown in this FIG. 4. In this

arrangement conduits

17 and 18 terminate above the end of rod 1 1. Until the tip 13 is pulled above this point, these conduits l7 and 18 are shut off from the lower end of rod 11. Only when the tip 13 has been retracted within rod 1 l to a specified point, as shown in FIG. 4, are the

conduits

17 and 18 open for use. This furnishes a kind of valve for the side conduits in the stem 14.

In this embodiment, following the injection of the components of the explosive, the tip 13 may be lowered, as shown in FIG. 5, to the point where it insures that all of the sensitized explosive has been ejected from the inside of the lower part of rod 11 into the cavity 16 to form the localized accumulation 19 as previously mentioned. Frequently this step is unnecessary and the sensitized explosive will simply run out of the bottom of rod 1 1 under the attraction of gravity.

As discussed subsequently, preferably the explosive has sufi'rcient viscosity so that it has enough body so that it will disperse only slowly through the unstable marshy formation 15. Accordingly, it will remain essentially below the probe where placed, until detonation.

There are a number of schemes which may be employed for detonation of the explosive after it has been suitably placed in the unstable formation. In general, these all involve actuation of a device near the earths surface which results in sudden application of pressure to the sensitized liquid explosive. This can be done, for example by removing the stem 14 and dropping a weight through the resulting hollow opening in the probe 11. This is frequently undesirable, both from a safety standpoint and from the fact that there is little weight or body of considerable inertia resting on the liquid explosive and tending to stem or confine the upward explosive pressure upon detonation. The weight dropping technique can also involve some error or uncertainty in timing which is highly undesirable, since one necessity of a seismic source in geophysical prospecting is to know the time of explosion at least to one millisecond. Accordingly, I prefer to connect the localized accumulation of explosive 19 with the detonator (which may be at the earths surface as shown in FIG. 6 or may be placed at the end of the explosive chamber) by a column of fluid explosive material. One method of accomplishing this is shown in FIG. 6.

In FIG. 6 the tip 13 has been pulled back into rod 11,

7 but not to the point where the

conduits

17 and 18 are in fluid communication with the space below. Through the center bore or conduit 24 of stem 14, an explosive mixture of a gaseous fuel, such as propane, acetylene, hydrogen, or the like, and either compressed air or bottled oxygen is introduced from the top for a sufficient period of time so that it flows down through the center conduit 24 and out the tip 13 to form a localized accumulation 22 which at the bottom is in contact with the localized accumulation of liquid explosive 19. This arrangement forms a long, thin, substantially continuous column of explosive gas from the liquid explosive up to a detonating means at the top end of stem 14. This detonator is shown at 23 as closing off the top of the central opening 24 in stem 14, but this is simply a diagrammatic representation; ordinarily such a detonating means producing an electric spark, arc, or the like, will use a spark plug or the like which has been permanently mounted adjacent but communicating with the central bore 24 in stem 14 near its top or alternatively incur the bottom.

There is no particular need to shut off the flow of the explosive gas when the operator believes the liquid explosive mass is armed. If desired, the flow can be continued until the desired actuation time is reached. Excess gas will just flow out around the mass and bubble up to the surface or disperse in the formation. This latter technique is particularly useful if the unstable formation will not retain even a low pressure (of the order of p.s.i.) so that flow must be maintained to in sure continuity of the explosive gas between mass and detonating means.

After the column of explosive gas exists in essentially continuous connection between the explosive accumulation and the detonating means, the spark plug or other detonator is actuated and the explosion of the gas results in application of sudden (essentially simultaneous) pressure causing detonation of the liquid explosive accumulation 19, as shown in FIG. 7. Of course, a confined, substantially continuous column of explosive liquid could be employed instead of the explosive gas. The procedural steps involved are essentially the same, though in this case it would probably be desirable to use more electric energy in the detonator, or employ a light explosive cap.

It is to be noted that the tip 13 and stem 14 form an inertial mass above the liquid explosive accumulation 19, very effectively stemming it at the time of explosion. This, of course, increases the flow of seismic energy downwards into the surrounding formations rather than wasting it up the conduits in the stem.

There are other methods of enhancing the transfer of energy from a certain quantity of liquid explosive in a localized accumulation to the formations below. One is to flush the unstable formation around the explosive with water in the region say, out to two to five feet radially from the accumulation. This can be accomplished simply by following the placement of the sensitized explosive as in FIG. 4 by pumping in the desired volume of water down the conduits l7 and 18 into the space above the explosive, from which the water permeates outward around it. Generally speaking, the detonation of the explosion is considerably more effective in a region saturated with water than with only loose earth. In

a marsh ordinarily there is natural water saturation, and no water need be added, but in an exceptional case such water can be added as described. Usually it is unnecessary to use a volume of water more than sufficient to create an approximate sphere of approximately two to five feet about the localized accumulation of explosive.

Another means of still further enhancing the transfer of seismic energy lies in stiffening the unstable earth formation 15 around the accumulation of liquid explosive prior to its detonation. This is accomplished by pumping a liquid soil conditioner into the area immediately adjacent the localized accumulation, for example to a radial distance of several feet from the liquid explosive accumulation, and allowing it to set until it has hardened what soil there is at this point. There are many types of soil conditioners, for example those taught in U.S. Pat. Nos. 3,268,002 and 2,842,338, which can be pumped down in liquid phase and which rapidly set. Generally speaking I may use any which can be prepared in liquid form and which after placement will rapidly harden unconsolidated sand, earth, and the like. No particular technique is essential here. In fact, the placement is ordinarily carried out like that of the water immediately above. The important point is that the region immediately about the explosive become solid or at least plastic before the detonation of the charge.

After the first charge has been fired, the method may be repeated in the same hole if further records are to be taken. Ultimately the probe 11 is pulled from the ground, using the hydraulic pulldown in reverse, as diagrammatically shown in FIG. 8. Ordinarily there is no need to fill the hole since this will naturally occur in such formation.

If the solids are of sufficient low concentration it is not necessary to employ flukes 12 and rotate the rod 1 1 in order to place it. One can in this case use instead of the rod 11 an ordinary pipe section and simply force it directly into the ground by the pulldown mechanism on the marsh buggy.

On the other hand when applying this method in a region of loose or unconsolidated soil in a region above the water table, a few modifications in technique are desired. In this case use of the earth auger type of probe becomes quite important, as does the initial use of water to flush and lubricate the auger while it is being implaced in the unstable earth formation.

Other techniques are now known to implace a rod or probe in the ground without use of an earth auger, for example, the so-called explosive fencepost system disclosed awhile back by the U.S. Army, and others. Such arrangements are, of course, suitable for forcing the rod-like solid member below the surface of the formation to its desired depth. The retractor drilling system developed by ContinentaI-Emsco and the Roy Cullen research group (see New Concept in Drilling Rigs Designed to Lower Well Costs, pages 1 13-1 17, World Oil, Dec. 1967) has a double continuous-track clamp which can force a cylindrical metal object into the ground. This is another arrangement which can be employed when the amount of solids are too heavy for easy penetration of the probe and the operator does not wish to rotate it.

In using this process on the open water or in a region in which there are very little solids concentration, the method is essentially that already described, except for the fact that ordinarily the probe can be maintained at its desired depth (of the order of ten feet down to the tip, or more) and approximate vertical orientation by simply attaching it to the vessel carrying the seismic source. It will be unnecessary to lift the rod in order to form a cavity, of course, but it is still desirable to lift the tip and force the components of the liquid explosive down through separate conduits in the rod 11, as previously described. Another difference which is compatible with this change in medium is that ordinarily in this case the arrangement for detonation will involve use of a thin column of the liquid explosive accomplished by throttling the flow of components or total liquid explosive at the tailend of deposition of the localized accu mulation, and detonation of the upper end of the remaining thin column of liquid explosive which is rather spun out or extended from the accumulation up to the detonator. One factor enhancing this type of performance is the fact that I prefer to have at least a certain minimum viscosity of explosive, as described below.

I prefer to incorporate a bodying agent into the formula for the liquid explosive such that the explosive in place in the formation will have a minimum effective viscosity of at least centipoises and desirably at least 100 to 200 centipoises or more. In the case of a Newtonian liquid this is relatively unambiguous. I prefer to use a Brookfield viscosimeter if the material is thixotropic. The fluid should show a measurement on a Brookfield viscosimeter equipped with a No. 2 spindle at 60 rpm in the range of viscosity from around 50 to several thousand, and preferably from around 100 to 200 up to several thousand. These readings are, of course, in centipoises.

I do not wish to be limited to any particular formula for the liquid explosive or to any particular bodying agent used therewith. Suitable examples of both have been already given, for example, in the application of Clarence R. Fast, Ser. No. 3,511, filed Jan. 16, 1970. In this case, the thickening agent is added to the nitromethane. According to my preferred arrangement this mixture is pumped down one conduit, such as conduit 17, while the sensitizer (an organic amine) is pumped down another conduit, such as conduit 18. Thus, each component has very little tendency to detonate by itself; whereas, the resultant mixture of the two components at the bottom of the probe can be readily denoted.

Another example of a liquid explosive which can be used in the system described immediately above is also a two-component system, the two components being substantially equal by volume. One component is nitromethane, the other one is made up of about 60 percent liquid ammonium nitrate, and about 40 percent of solid pulverized potassium chlorate, KC 10,. Liquid ammonium nitrate is a commercially available product from The Dow Chemical Company and Commercial Solvents Corporation. It is made by passing ammonia vapor through prilled ammonium nitrate. It is a liquid but contains no water. To this second component is added as thickening agent a material known commercially as Polyox, defined as long-chain polymers of ethylene oxide and having a molecular weight in the range of one million to ten million. Enough Polyox is added to the second component to give a viscosity reading on a viscosimeter, as described above, at least as great as the minimum figures given and preferably about twice the amount of these figures, due to the diluting action of the nitromethane. The two components in this case also have very little tendency to detonate until they are mixed together. The components form upon mixing a localized accumulation with very little tendency to disperse or leak away through the unstable formation in which such accumulation is placed. Even in open water the liquid stays together in a localized accumulation until such time as the means for detonating from the surface is connected to it. An additional advantage to this mix is that in a relatively short time after the two components are mixed of the order of 5 to 10 minutes the explosive is no longer detonatable. As a result, this mass, if not detonated soon after placement, may be safely left in place. In case of rnisfire the same safety feature prevails. This is also true in case of accidental spills on the surface.

Reference was earlier made to incorporation of solids into the liquid explosive. As discussed in the Fast application, Ser. No. 3,51 l, earlier referred to, I contemplate the use, for example, of finely divided aluminum, for example paint grade powdered aluminum. This can be added to any component, since it normally does not add substantially to the detonating ability of the component. I prefer to use a concentration of around 10 percent to 15 percent of such additive with a maximum of the order of 25 percent of the total weight of the other components. The powdered aluminum enhances the energy generated in the explosion upon detonation. It will be understood in the appended claims that when I refer to a liquid explosive, I am referring not only to true liquid but to slurries in which the liquid components are detonatable and are the major part and any divided solid is a minor constituent.

The method taught and claimed hereafter provides a system for high-speed implacement of a liquid explosive as a seismic source for geophysical prospecting without the use of drills and manual loading. As a result, this considerably speeds up and renders less expensive this part of the geophysical prospecting method.

I claim:

1. A method of generating seismic waves in an unstable earth formation comprising a. forcing liquid explosive below the surface of the earth through a conduit to form a localized accumulation in said formation, said liquid explosive having sufficient body to disperse only slowly through said formation, and

b. detonating by sudden application of pressure from the earths surface said localized accumulation, whereby seismic waves are produced in said formation.

2. A method of generating seismic waves in accordance with claim 1 in which said liquid explosive comprises at least two components each of which alone has very little tendency to detonate, said two components being forced through separate conduits to join only in the region of said localized accumulation.

3. A method of generating seismic waves in accordance with claim 2 including the step of connecting said localized accumulation to a detonating means near the surface of the earth by a confined continuous column of an explosive gas prior to detonation.

4. A method of generating seismic waves in accordance with claim 2 including the step of connecting said localized accumulation to a detonating means near the surface of the earth by a substantially continuous column of said liquid explosive of cross section small compared to that of said localized accumulation, said connection being effected prior to detonation.

5. A method of generating seismic waves in an unstable earth formation containing solids comprising a. forcing a rod-like solid member below the surface of said formation at least feet,

b. partly withdrawing said solid member to form a cavity in said formation,

c. filling said cavity with a liquid explosive through conduits in said member,

d. connecting said explosive in said cavity with a detonating means by a confined column of a fluid explosive, and

e. actuating said detonating means.

6. A method of generating seismic waves in accordance with claim 5 in which said liquid explosive comprises at least two components none of which has substantial tendency to detonate, said at least two components being forced through separate conduits in said solid member to join only in the region of said localized accumulation.

7. A method of generating seismic waves in accordancewith claim 6 including the step of flushing said components from said conduits with an inert liquid following filling of said cavity and before connecting said explosive to said detonating means.

8. A method of generating seismic waves in accordance with claim 7 including the step of flushing said formation with water in the region adjacent said solid member prior tofilling said cavity with a liquid explosive.

9. A method of generating seismic waves in accordance with claim 7 including the step of permeating said formation with a non-detonating liquid to a distance of at least several feet in all directions from said cavity prior to detonation, whereby transferance of seismic energy from such detonation to the earth is increased.

10. A method of generating seismic waves in accordance with claim 7 including the step of permeating said formation with a soil-conditioning liquid to a distance of at least several feet in all directions from said cavity prior to detonation, whereby transferance of seismic energy from such detonation to the earth is increased.

Claims

2. A method of generating seismic waves in accordance with claim 1 in which said liquid explosive comprises at least two components each of which alone has very little tendency to detonate, said two components being forced through separate conduits to join only in the region of said localized accumulation.
3. A method of generating seismic waves in accordance with claim 2 including the step of connecting said localized accumulation to a detonating means near the surface of the earth by a confined continuous column of an explosive gas prior to detonation.
4. A method of generating seismic waves in accordance with claim 2 including the step of connecting said localized accumulation to a detonating means near the surface of the earth by a substantially continuous column of said liquid explosive of cross section small compared to that of said localized accumulation, said connection being effected prior to detonation.
5. A method of generating seismic waves in an unstable earth formation containing solids comprising a. forcing a rod-like solid member below the surface of said formation at least 10 feet, b. partly withdrawing said solid member to form a cavity in said formation, c. filling said cavity with a liquid explosive through conduits in said member, d. connecting said explosive in said cavity with a detonating means by a confined column of a fluid explosive, and e. actuating said detonating means.
6. A method of generating seismic waves in accordance with claim 5 in which said liquid explosive comprises at least two components none of which has substantial tendency to detonate, said at least two components being forced through separate conduits in said solid member to join only in the region of said localized accumulation.
7. A method of generating seismic waves in accordance with claim 6 including the step of flushing said components from said conduits with an inert liquid following filling of said cavity and before connecting said explosive to said detonating means.
8. A method of generating seismic waves in accordance with claim 7 including the step of flushing said formation with water in the region adjacent said solid member prior to filling said cavity with a liquid explosive.
9. A method of generating seismic waves in accordance with claim 7 including the step of permeating said formation with a non-detonating liquid to a distance of at least several feet in all directions from said cavity prior to detonation, whereby transferance of seismic energy from such detonation to the earth is increased.
10. A method of generating seismic waves in accordance with claim 7 including the step of permeating said formation with a soil-conditioning liquid to a distance of at least several feet in all directions from said cavity prior to detonation, whereby transferance of seismic energy from such detonation to the earth is increased.