The present invention relates to a tool for positioning an explosive charge at

5 depth, either below ground level or underneath a sea bed. The tool is particularly useful in seismic surveying as a means for positioning an explosive charge.

Seismic surveying involves the use of an explosive charge buried at depth and of one or more hydrophones located some distance away from the charge. When the charge is detonated, the resulting sound waves travel at different speeds through different l o media. Careful selection of the relative position of a charge and one or more hydrophones can allow the user to calculate, from information provided by the hydrophone, the speed of sound through the particular medium under investigation. The user maybe able to draw conclusions from this information, such as the presence or absence of oil deposits in the area.

15 Planting an explosive charge can be a complex engineering problem. In certain types of media , it is possible simply to make a bore hole for the explosive charge by known drilling methods. These methods involve rotation and or reciprocation, and also require water or air to flush soil from the hole. In softer media, flushing may be employed alone, using a reciprocating motion of a water jet, to carve a hole. However, these methods

20 require quantities of water. The charge may be lowered into the borehole together with a detonator attached to a detonation wire, either through the drilling pipe into the media below, or after withdrawal of the drilling or flushing tool.

However, in certain types of media, such as loose sand and soft mud, this technique is not feasible. The borehole simply closes up after the drilling or flushing

25 operation and before the explosive charge can be inserted. To address the problem of borehole closure in soft or loose media, there are known techniques of loading through the drilling pipe. There has also been developed a technique in which the explosive charge to be planted is mounted inside the body of a boring shaft and secured at its end inside a disposable ground penetration tip thereof. The shaft may be inserted into the ground by

30 a simple pushing action, in which the tool displaces the surrounding medium, or

commonly, by a ramming method whereby the shaft is subjected to short, intermittent bursts of pressure which serve to hammer the shaft into the ground. This ramming action may shake the shaft so vigorously that the surrounding material, for example, loose sand, is shaken clear of the shaft ground penetration tip by the vibrating motion thereof. The ground penetration tip of the shaft is pointed on its outer surface to facilitate its motion through the ground and has a generally cylindrical body which serves as a chamber for receiving and securing an end portion of the explosive charge to be planted. The cylindrical body of the tip is seated in the shaft sufficiently securely such that the tip is not dislodged from the shaft during the ramming process. However, the outer surface of the tip extends from its apex across the end of the shaft. In this way, the tip remains connected to the shaft as the downward bore is made but becomes dislodged from the shaft when the shaft is retracted. The explosive charge becomes dislodged also upon retraction of the shaft.

The methods of the prior art have attendant disadvantages. Notably, the detonation wire, which must be in communication with the detonator connected to the explosive charge and also with a detonation point at the ground surface, must extend along the exterior of the shaft and must therefore come into contact with the ground through which the borehole is being made. Often, the ground material is abrasive and tends to rub against the detonation wire causing breakage thereof. Even in relatively smooth media, such as soft mud, the high pressure reached at depth means that a significant abrasive force is exerted on the detonation wire. Accordingly, these prior art methods always suffer from the possibility that the detonation wire will be broken and the tool must therefore be retracted without detonation of the explosive charge. Not only is this inconvenient and expensive to rectify, it is also environmentally unacceptable to leave undetonated explosive charges buried in the ground. Accordingly, it is common to use back-up detonation wires.

However, these also are subject to breakage and abrasion.

Another problem with prior methods of burial of explosive charges is that the detonation wire on the ground surface is susceptible to lightning strikes. This may be a particularly important consideration in climates where lightning storms are frequent, as is often the case in locations which prove fruitful for oil prospectors.

When the wire is unfolded or unwound there is a real hazard from lightning because of an antenna effect which may develop a potential difference between the ends of the open wire. Normally the ends of the cap wire are shorted exept when in the action of connecting to the blaster via firing line. When folded cap wires are used, further hazard results because the cap wire is hoisted up the inside of the ram pipe, using a steel wire and pulley. Because folded cap wires have to be unfolded in order to feed into the pipe, there is an increased risk with regard to lightnening strikes, compared to using spooled wire, which feeds smoothly into the pipe. According to the present invention there is provided a tool for burying an explosive charge comprising an elongate body for insertion into the ground, the body having attachment means on a proximal part thereof for attachment of the tool to a thrust means, mounting means for a spool for a detonation wire on an intermediate part thereof, and a chamber in a distal part thereof for receipt of an explosive charge and a detonator, the chamber having an opening at the distal end of the body, which opening is closable by a ground penetration tip adapted to be releasably secured to the distal end of the body and to receive and grip a distal portion of an explosive charge received within the chamber, the chamber having an opening in the region of the intermediate part of the tool for receipt of a detonation wire from the spool whereby, upon forcing the tool, with an explosive charge to be buried and a detonator received within the chamber, and a detonation wire coiled on a spool mounted on the mounting means and connected to the detonator, downwardly into the ground by applying downward force to the tool by means of the thrust means, the ground penetration tip penetrates the ground and the tool carries the explosive charge to a desired depth and whereby, upon retracting the tool, the ground penetration tip is disconnected from the body and retains its grip on the explosive charge, thus withdrawing the explosive charge from the chamber as the tool is retracted, and the detonation wire remains connected to the detonator and uncoils on the spool as the tool is retracted.

The tool of the invention has a number of significant advantages over the prior art methods described above. The detonation wire is coiled on the spool within the tool

and is only uncoiled when the explosive charge has been laid and the tool is retracted from the ground. Accordingly, the detonation wire is not subject to the same risks of lightning strikes as is the case with prior art methods in which the detonation wire remains on the ground surface. Lightning strikes represent a very serious risk to workers laying explosive charges of the type used in seismic surveying. Lightning strikes can be particularly prevalent in certain geographical locations which are also fruitful ground for oil explorers.

A further advantage of a tool according to the invention is that the detonation wire connecting the detonator to a detonation point at the ground surface does not come into contact with the ground in which the explosive charge is being buried. Accordingly, the risk of detonation wire breakage or weakening due to abrasion against the ground is much reduced compared to the prior art.

Yet another advantage of the tool of the invention is the ease and speed of deployment using the invention. The spool may be loaded into the chamber directly after its removal from the manufacturer's packing. Only a small amount (for example about 1 metre) of unwinding is needed in order to mount the spool inside the chamber and with the wire connected to the detonator.

In order that the invention may be clearly understood and readily carried into effect, a preferred embodiment thereof will now be described with particular reference to the accompanying drawings in which: Figure 1 is a side elevation of a part of the tool of the invention which shows the ground penetration tip secured to the explosive charge which, in turn, is connected to a detonator and detonation wire coiled on a spool;

Figure 2 shows a side elevation of the tool of the invention; Figure 3 shows a side elevation of the tool of the invention which has been partly cut away to reveal the interior detail thereof and in which the explosive charge is being withdrawn from the tool.

For the avoidance of doubt, it should be understood that the following description is intended to describe particularly a single preferred embodiment of the invention. It will be obvious to those skilled in the art that many of the features described may be altered or adapted to suit the particular requirements of a given terrain or the type

of explosive charge to be buried. Thus, the dimensions of the tool and ancillary features thereof which are not explicitly claimed as part of the invention are included merely to assist reproduction of the invention and are not to be taken as limiting its scope in any way.

Referring to Figure 1 , there is shown an explosive charge 1 which is mounted and gripped by ground penetration tip 2. Explosive charge 1 may comprise any suitable commercially available explosive charge. For example, Powergel 3000, Dynoseis and Pentolite manufactured by Nobel Explosives may be used. Other suitable explosives include To vex and Seismogelit.

In the embodiment shown, explosive charge 1 is formed as a pair of explosive charge packages 3 and 4 which are cylindrical in shape and are screwed together as indicated at join 5

Ground penetration tip 2 is generally conical in shape but has wings 6 which allow tip 2 to be inserted into the ground but hinder any attempted retraction thereof. Therefore, wings 6 serve to anchor tip 2 in the ground once a desired depth has been reached. Ground penetration tip 2 is disposable and is normally manufactured from a hard wearing plastics material such as high molecular weight polyethylene.

Disposable tip 2 has a generally cylindrical body portion 7 which is adapted to grip explosive charge 1. There are many ways in which explosive charge 1 can be secured to body portion 7 of tip 2. For example, a screw threaded portion on tip 2 may receive explosive charge 1 in a screw threaded engagement. Alternatively, retention clips or the like may be used.

Explosive charge 1 is connected to a detonator 9 which is secured to explosive charge 1 by any suitable securing means such as a screw thread.

Detonator 9 is connected to detonation wire 10 which is coiled on a spool 1 1. For convenience, a funnel member 12 is provided between spool 11 and detonator 9 to assist in locating detonation wire 10 in its path between spool 11 and detonator 9. One of the purposes of funnel 12, is to reduce the size of the opening from the spool chamber. This has the important advantage of preventing mud and gravel from being forced up into the spool chamber. At a typical depth of 20 metres, the pressure is sufficient to force any fluid medium, and its attendant hard/sharp objects into the tool through the open distal

opening when the tool is withdrawn from the hole.

Referring to Figure 2, ground penetration tip 2 is shown mounted within the main elongate body 13 of tool 14. Ground penetration tip 2 is secured to body 13 by suitable releasable means such as a retention clip, or simply by a close fit arrangement. Body 13 comprises a chamber 18 for receipt of explosive charge 1 and detonator 9.

Reference numeral 15 indicates a housing for spool 11. Spool 11 has a rotatabie axle 16 mounted in housing 15.

Whilst tool 14 has a main elongate body 13 which is generally formed from a length of tube or pipe, for example, of mild steel, tool 14 also has a shank 17 for connection to a thrust means. The thrust means is not shown in the accompanying drawings but is of known construction and may comprise a ramming or pushing mechanism.

Commonly, such a mechanism comprises a tube or pipe which is perhaps 75 feet in length, one end of which fits over shank 17 and is secured thereto by means of a bolt. This tube or pipe may be provided with a racked portion, which racked portion is driven by a gear. Thrust may be applied as short, intermittent burst of power which cause shank 17, and hence tool 14, to vibrate whilst being urged downwardly into the ground. This vibrating or ramming action is often convenient when burying an explosive charge in ground which is composed largely of loose sand, shale or similar media. The vibrating motion of tool 14 serves to shake material loose from around its ground penetration tip 2 thus clearing a bore for passage of the tool into the ground.

As an alternative to a ramming mechanism the thrust means may simply utilise a pushing action. This may also be achieved using a gear in combination with the racked member on the pipe or tube. In this case, tool 14 is inserted into the ground smoothly with little or no vibrating motion. Power is applied to shank 17 not in short, intermittent bursts, but constantly so as to achieve a prolonged pushing action. This type of thrust mechanism is particularly suitable when tool 14 is to be inserted in soft mud or a similar medium. The thrust means may alternatively comprise the use of rubber wheels to push down a smooth pipe. When thrust is applied to shank 17 by the thrust means, tool 14 is driven into the

ground and explosive charge 1 is carried to a desired depth. Ground penetration tip 2 remains secured to body 13 during the downward motion of tool 14. Explosive charge 1 remains secured in ground penetration tip 2 at all times.

When explosive charge 1 has reached its desired depth, the thrust means is withdrawn upwardly and, being secured to shank 17, carried tool 14 upwardly also. As tool 14 is retracted, wings 6 anchor tip in the ground and prevent its withdrawal therefrom. As tool 14 continues to move upwardly, tip 2 is released therefrom and remains anchored in the ground. Explosive charge 1 remains secured to tip 2 and is therefore withdrawn from body 13 through the distal opening therein and remains buried in the ground at the desired depth while the tool with withdrawn.

When tool 14 has been sufficiently withdrawn from the ground, detonation wire 10 can be released therefrom and attached to a detonation point. It is possible to dismantle spool 11 from housing 15 and then cut detonation wire 10. Alternatively, wire 10 may be cut in the region of the distal end of body 13 as it emerges therefrom. Depending on the hole depth and cap wire length, there may be spare wire on the roll. The usual technique is to cut the wire at the required length, then open the chamber and remove the remaining wire, still on the spool.

Tool 14 may then be re-used.