CONTROLLED IMPLOSIVE DOWNHOLE SEISMIC SOURCE AND CARRIER MEANS FOR A PLURALITY OF SUCH SOURCES

Abstract: A controlled implosive downhole seismic source (10) having a substantially hollow closed vessel body (18) adapted to be lowered in a borehole and means (22) for causing at least a portion (20) of the vessel body to implode under pressure thereby producing seismic waves. An elongated carrier (62) is adapted to carry in longitudinally spaced relationship a plurality of controlled implosive downhole seismic sources (10).

Description: The present invention relates to a controlled implosive downhole seismic source. In the field of seismology, a wide variety of approaches have been used to create and detect seismic waves. It is known, of course, that seismic sources and seismic detecting means may be placed downhole or on the surface, or any combination thereof. Thus, among the various configurations, it is known to place the seismic source downhole with the detecting means on the surface, or vice versa, or with both on the surface, or with both downhole, or with the detecting means both on the surface and downhole. It is also known that placing the source downhole contains several distinct advantages over placing the source on the surface. First, a downhole source is usually closer to the critical layers of strata which the operator is interested in, particularly in oil and gas exploration. Thus, less energy from the seismic source is lost in traveling to the desired strata layers. Second, most of the earth's surface is covered by a weathered layer of strata which consists of topsoil and fragmented bedrock. That weathered layer of strata often creates problems in sending or receiving seismic waves because of that layer's porosity. Surface sources typically put as much as two-thirds of their energy into unwanted surface or Rayleigh waves. Third, the remaining third of the energy which contains the desired P-waves and S-waves may be largely lost due to the porous nature of the weathered surface layers. See; Hardee, Downhole Periodic Seismic Sources, Geophysical Prospecting 31, 57-71 (1983). Fourth, a surface seismic source must also compete with other surface noises caused by man-made vibration of the earth's surface. Particularly if the testing site is near a concentrated man-made activity, such as construction site or a highway, waves generated by a surface source will mix with those background noises and the readability of the seismic data is seriously impaired. For those reasons, the field of seismology has long sought a downhole seismic source capable of operating under adverse conditions and yet generating acceptable seismic waves. A variety of downhole seismic sources have been suggested as attempts to achieve those goals and overcome those longstanding problems in the field. The most common type of downhole source is an explosive source. However, the explosive source has several disadvantages. First, the explosive source can damage or destroy the borehole. That limits the use of an explosive source in the actual well bore, because of the cost of potentially re-drilling the well and the potential for damage to the formation. The inability to use the source in the well bore is a significant operating limitation when seismic work is being used to map out the boundaries of a developed oil and gas field. In those cases, it is often desirable to dispose the seismic source in the well bore to find the boundaries of such structures as salt domes and subsurface inclines. Because those structures extend generally in a vertical direction, it is often necessary to determine the precise horizontal distances from an existing well at which the structures are found to avoid missing the field altogether on the next well. A second drawback to the explosive downhole seismic source is the lack of control over the quality of the seismic wave generated. Explosions, by nature, are violent reactions not prone to manipulation. It has also be recognized in the art that explosive downhole sources propagate a larger percentage of high frequency waves than is desired, but low frequency waves are usually the waves that reveal the most valuable data. See; Hardee, Downhole Periodic Seismic Sources, Geophysical Prospecting 31, 58 (1983). In an effort to overcome the disadvantages of an explosive downhole source, a variety of prior art devices have attempted to create downhole sources which do not damage the borehole and which give the operator more control over the frequency of the waves propagated. Examples of such devices are disclosed in PCT Application No. F183/00068, U.S. Patent Nos. 4,038,631; 3,909,776; 3,718,205; 3,587,775; and 3,221,833. Except for U.S. Patent No. 3,587,775, all of those devices create a seismic source downhole by mechanical operation, such as air guns, mechanical hammers, and similar devices. U.S. Patent No. 3,587,775 discloses a controlled explosive source in which water is disassociated into hydrogen and oxygen and then ignited to provide a seismic source. However, the devices disclosed in those references suffer from several operating limitations, including: power, depth and hydrostatic pressure. Power is an operational limitation faced by any mechanical device. If the seismic source is powered from the surface of the borehole, additional equipment must be run into the hole along with the logging cable, because a normal logging cable can only pass a maximum of one horsepower down a well. Explosive seismic sources are not faced with that limitation; the power supplied to the source is self contained and not dependent on any connection to the surface, except for the small amount of power needed to ignite the explosive charge. However, unless an independent source of power is included in the device lowered downhole, problems with the second operational limitation (depth) are encountered. The mechanical devices shown in the references mentioned above are connected to the surface by hoses or other means to provide air, fluid, or other materials required to drive those mechanical devices. If the seismic source is needed at depths over a few hundred feet, those additional cables, hoses, and other means, prove too heavy and cumbersome for practical operation. Thus, a second need is defined by finding a downhole source capable of going to significant depths, which necessitates that the source have a self contained power source. The third operational limitation, hydrostatic pressure, is also an operating limitation for mechanical devices and explosive devices. As depth increases in the well, the hydrostatic pressure of drilling fluid, water and other materials in the borehole also increases the pressure on any seismic source placed in the well. For explosive devices, that means much of the explosive power is absorbed by the hydrostatic pressure. For mechanical devices, the device must be insulated or sealed from the surrounding fluid such that the fluid will not interfere with any moving parts, air outlets, or other similar features of the device. What the devices known in the prior art have failed to realize is that hydrostatic pressure can serve as a power source itself and does not require the addition of any hoses, or other cumbersome equipment to power the seismic source. The present invention takes advantage of that natural power source and frees the field of seismic sources from most operational limitations of depth or power. That is achieved through the present invention by an implosive device which is ruptured on command and uses the power applied by the hydrostatic pressure of the fluid surrounding the device to create a seismic source. To the best of Applicant's knowledge, a controlled implosive seismic source has never been utilized in a borehole, although there have been limited experiments with implosive devices used as sources of underwater sound at shallow depths; See; Orr, M. and Schoenberg, M., "Acoustic Signatures From Deep Water Implosions of Spherical Cavities", Journal of Acoustical Society of America, Vol. 59, No. 5, pg. 1155 (May 1976), Reader, W.T. and Chertock, G. "Transient Sounds Due to Implosions of Simple Structures Under Hydrostatic Pressure", presented at 82nd meeting of Acoustical Society of America (October 19, 1971) Urick, R.J., "Implosions as Sources of Underwater Sound", Journal of Acoustical Society of America, pg. 2026 (1964). The differences between those shallow underwater experiments and a usable downhole seismic source are several. The devices disclosed in those publications were crude experimental underwater devices and not the type of device that would be useful in well bore. Further, those devices could not be imploded at predetermined depths, or at predetermined times, and were imploded only as a function of pressure. That makes the timing of recording equipment used to detect the source very cumbersome. Those crude devices could not be initiated remotely, and when coupled with the other factors showing a lack of control over the implosion, it is clear that those devices were not practical seismic sources because, among other factors, those devices could not be lowered into the fluid medium at more than one device at a time. As explained, it is desired to be able to fire a sequence of shots when generating seismic sources. Therefore, it is the object of the present invention to provide a novel apparatus for a controlled implosive downhole seismic source, furthermore to gain the advantages set out above without being limited as a practical matter to the depth at which the source may operate, and furthermore to provide a device having sufficient power to produce high quality seismic waves, without damaging the borehole, and able to operate under high hydrostatic pressure. An implosive source derives its energy from the high pressure encountered downhole; the higher the pressure, the stronger the source output. Such devices involve the sudden collapse of a cavity - the collapse being driven by the hydrostatic pressure of the surrounding fluid - and the abrupt reversal of the momentum of the collapsing wall. A typical device might be a thin-walled sphere or cylinder made of ceramic or glass which is ruptured upon command. The water would rush in from all sides, causing an intense pressure wave to be generated when the walls of water collide. The device embodying the invention described herein is a controlled implosive device. The implosion is selective, i.e. on command. According to the invention, a downhole seismic source apparatus is set out in claim 1. The preamble of claim 1 is based on the teaching of US-A-4 300 654. Further ebodiments are set out in the dependent claims. Other advantages of the invention will become evident in light of the following description considered in conjunction with the referenced drawings in which : Fig. 1 is a cross-sectional view illustrating a first embodiment of a seismic source pursuant to the present invention; Fig. 2 is a cross-sectional view illustrating a second embodiment of a seismic source pursuant to the present invention; Fig. 3 is a cross-sectional view illustrating a third embodiment of a seismic source pursuant to the present invention; Fig. 4 is a side view illustrating a first embodiment of a downhole carrier adapted to carry a plurality of seismic sources pursuant to the present invention; Fig. 5 is a cross-sectional view illustrating a fourth embodiment of a seismic source pursuant to the present invention; Fig. 6 is a diagram of one of the possible selective firing systems. Fig. 7, 8 and 9 are respectively a front view and two cross-sectional views illustrating a second embodiment of a downhole carrier adapted to carry a plurality of seismic sources pursuant to the present invention. DESCRIPTION OF PREFERRED EMBODIMENT The structure of the downhole seismic source pursuant to the present invention may vary according to the conditions under which the seismic source is being used. Variations in the structure and assembly of the seismic sources may be necessitated to alter the types and qualities of the seismic waves generated and in response to varying economic considerations. Such changes will be discussed below. Referring now to Fig. 1, a first illustrative embodiment of a controlled implosive seismic source 10 pursuant to the present invention is shown. Although the present invention contemplates both disposable and reusable seismic sources, the invention and the present preferred embodiment will be described in terms of a reusable seismic source. The controlled implosive seismic source 10 pursuant to the present invention comprises a vessel body, generally designated 18, which is adapted to be lowered into a borehole and which forms a closed vessel chamber 19. In the embodiment shown in Fig. 1, vessel body 18 is formed by a cylindrical body 24 and two end caps, 26 and 20. Cylindrical body 24 forms two openings. A first end cap 26 is secured to and closes one of the two openings in cylindrical body 24. It is contemplated that the cylindrical body walls 24 and first end cap 26 of vessel chamber 18 would be manufactured using a material of sufficient strength and durability to withstand pressures up to 1400 bar and the force of the resulting implosion. First end cap 26 and cylindrical body 24 should be joined in a fluidtight and airtight relationship. A destructible second end cap 20 is secured over the other end of cylindrical body 24. Second end cap 20 should likewise be made of a sufficiently strong material that would allow it to withstand pressures up to 1400 bar. However, second end cap 20 should also be capable of being ruptured by a reasonably small force to allow the vessel body 18 to be preserved and reused. In the present embodiment, second end cap 20 is made of a ceramic material. Ceramic materials are capable of withstanding high compression pressures, but they are also very brittle and can be shattered easily. It is desired that second end cap 20 be capable of shattering into fairly small fragments, so the well bore is not left cluttered with abrasive or other foreign ratter after the testing is done. A rupturing means, generally designated 22, is provided which can take a variety of forms. In the first embodiment shown in Fig. 1, rupturing means 22 is an explosive type rupturing means with connectors 28 communicating through cylindrical body 24 for connection to a firing mechanism (not shown). Leads 30 connect connectors 28 to an explosive material 32 shaped and sized to rupture end cap 20 without damaging the remainder of vessel body 18. Alternative rupturing means 22 are contemplated which would selectively rupture end cap 20 or other configurations of vessel bodies 18. For example, a rupturing means 22 could be provided in the form of an explosive projectile or bullet. Again, considerations such as economics of production, the quality of the source desired to be produced, and other considerations will dictate what type of vessel body design and rupturing means will be employed. In the present invention, including the embodiment illustrated in Fig. 1, the assembly process of seismic source 10 will depend upon the conditions under which it is to be used. For example, for tests at depths where suitable high hydrostatic pressure results, source 10 need merely be assembled and run downhole. However, at shallower depths where seismic source 10 would not have adequate hydrostatic pressure to generate sufficiently powerful seismic waves, an improved seismic wave is generated if a vacuum is maintained inside vessel chamber 19. A vacuum will aid seismic source 10 of the present invention in producing a more powerful wave because of the larger differential in pressure between the exterior of the seismic source 10 and the vessel chamber 19. A second embodiment of a seismic source 10 pursuant to the present invention is illustrated in Fig. 2. That embodiment is anticipated as being a more economical version of the embodiment shown in Fig. 1 due to the simpler design of the vessel body 18' and rupturing means 22'. As in the embodiment of Fig. 1, vessel body 18' forms a closed vessel chamber 19'. Vessel body 18' includes a portion adapted to rupture, in this case a destructible end cap 20'. The rupturing means 22' in this embodiment is a small blasting cap 22' embedded within end cap 20'. End cap 20' contains an explosive material such as a charge of powder 23 for rupturing end cap 20' upon detonation of blasting cap 22'. The cap 20' shown in Fig. 2 may be provided with threads for threadably engaging cooperating threads formed on vessel body 18'. Alternatively, end cap 20' may be secured by any suitable means to vessel body 18' to provide an airtight, fluidtight vessel chamber 19'. Seals 25 placed at the junction of end cap 20' and vessel chamber 18' may be used to effect that airtight, fluidtight function, particularly if end cap 20' is threadably engaged to vessel chamber 18'. The seal should be constructed to withstand pressures up to 1400 bar. In both the embodiments of Fig. 1 and Fig. 2, the openings defined by the walls of vessel body 18 or 18' and covered by destructible caps 20 and 20' should be of sufficient size to provide for rapid fluid communication with the interior of the vessel chamber 19. The opening of the vessel body sealed by cap 20' is selectively sized (and the shape of the vessel body and vessel chamber varied) to create seismic waves having various characteristics. If a seismic wave having a higher percentage of low frequency propagating waves is desired, the surface area on which the implosion impacts should be increased. A third embodiment shown in Fig. 3 is particularly well suited for a carrier adapted to carry a plurality of seismic sources described thereafter in detail with reference to Fig. 7 to 9. The vessel body comprises an end cap forming a base plate 9 adapted to receive the rupturing means 22'' and a destructible jar 20'' sealingly applied against the base plate 9. The seal 25'' should withstand pressures up to 1400 bar. Manufacturing the jars out of glass is particularly economical compared to a ceramic material, although it requires a jar with a thicker wall. Further, experiments have shown that glass is a superior material for imploding downhole in the sense that upon implosion, the jar is completely pulverized while a ceramic material tends rather to break into larger fragments. The base plate 9 comprises a hole forming a barrel for lodging a projectile or bullet 29. One end of an igniter needle 31 sealingly engaging into the base plate 9 penetrates a hollow projectile 29. The other end of the igniter 32 is electrically connected to an ignition circuit (not shown). The arrangement of the above described rupturing means 22'' presents several advantages. At first, placing the rupturing means inside the vessel chamber 19'' protects them from the surrounding fluid at hydrostatic pressure and improves safety. Secondly, rupturing the jar 20'' from the inside requires less powerful rupturing means 22'' since the inner surface of the jar is under tension and the glass tensile strength is roughly only one tenth of its compressive strength. Fig. 5 illustrates another embodiment of a controlled implosive seismic source 10 pursuant to the present invention. In some applications, it is desirable to maintain strict control over the seismic wave propagated. The implosions resulting from the devices shown in the other embodiments can be controlled by varying the size and shape of the vessel body 18 and the vessel chamber 19 to alter the pattern of waves reflected by the vessel chamber 19 when the fluid crashes into the interior of the vessel body 18 defining that chamber 19. The embodiment shown in Fig. 5 is designed to allow more control over the wave propagated. A free sliding piston 34 is disposed inside vessel chamber 19''' near the top of the vessel chamber 19''' adjacent end cap 20'''. A piston stop 36 is disposed in the opposite end of chamber 19'''. Piston 34 should closely and slidingly engage the interior of the cylindrical body 24'''. As the cap 20''' is ruptured, hydrostatic pressure drives piston 34 downward in vessel chamber 19'''. Piston 34 strikes a piston stop 36 disposed at the first end cap 26''' of the vessel chamber 19'''. A seismic wave is propagated as piston 34 strikes piston stop 36. That embodiment provides a highly controllable seismic source. The frequency spectrum of the signal may be controlled by changing the rigidity of the piston stop 36. It is noted that other methods of starting the piston 34 travel could be employed other than rupturing the end cap 20'''. For example, the piston 34 may be held by a release mechanism (not shown) which would release the piston 34 upon command and allow the hydrostatic pressure to drive it toward the piston stop 36. The seismic source illustrated in Fig. 5 operates as follows: the ceramic cap 20''' is broken by a small explosive device like a blasting cap 22' (Fig. 2), allowing a wall of water to rush in; the water drives piston 34 with a force of PA, where P is the hydrostatic pressure and A is the area of piston 34; piston 34 serves to regulate the inrushing water such that the collapse is along a flat surface; piston 34 picks up speed as it gains more and more kinetic energy , the kinetic energy being approximately equal to PA times the distance the piston travels down cylindrical body 24''' within vessel chamber 19'''; piston 34 hits piston stop 36; piston stop 36 acts as a plastic spring, rapidly decelerating the piston and thereby generating a potent acoustical source; the deceleration, and therefore, the shape of the output signal is governed by the stiffness and shape of piston stop 36. The seismic source can be recovered and reused upon replacing piston stop 36 and cap 20'''. The available energy for a device of the embodiment of Fig. 5 with an inside diameter of 102 mm (4 inches) and a nominal piston-travel distance of 203 mm (8 inches) : By shaping the deceleration, the amount of energy going into low-frequency propagation of sound can be controlled. To understand how the shape of the output can be controlled, it must be recognized that the radiated pressure results primarily from the volume acceleration of the cavity. It is monopole-like and is given by the approximation where P rad is the pressure at a distance r from the source, through a medium whose density is ρ. The volume acceleration of the cavity as a function of time is given as (t) and is directly proportional to the acceleration of the piston 34. The shape of the output signal is controlled by shaping the deceleration of piston 34, which is done by choosing an appropriate piston stop 36. For example, if piston stop 36 functions as a plastic spring in the shape of a truncated cone (cone angle 60°) made of, for example, annealed aluminum, the spring of that piston stop is nearly linear, with a constant equal to K. The volume deceleration is then governed by the kinetic energy of piston 34 (including the entrained fluid), the spring constant K and the system mass M (piston mass plus fluid mass), i.e., A = piston cross sectional area KE = kinetic energy where the system's resonance frequency is given by and the pulse width is approximately and KE is the kinetic energy of the flying piston (approximately equal to hydrostatic pressure x piston cross sectional area x the distance the piston traveled). The controlled implosive seismic source pursuant to the present invention may be carried downhole by a tool similar to a core gun manufactured by Schlumberger Well Services. As shown in Fig. 4, an array of seismic sources pursuant to the present invention, labeled generally as 10, are attached to a tool 12 by a carrier 14. A series of the seismic sources may be placed along carrier 14 separated by resilient standoffs 16. Resilient standoffs 16 are used to absorb a portion of the shock from one source implosion such that other sources 10 are not jarred loose or prematurely detonated. Other elements of the carrier 14 are conventional and are not shown. For example, firing mechanisms used to ignite explosive material in seismic sources 10 and selectively implode those sources from the surface could be any manual firing mechanism or automatic firing mechanism well known in the art. One selective firing system, illustrated in Fig. 6, uses diodes and blocking rectifiers in conjunction with the standard seven conductor logging cable. That system could selectively detonate upon to 28 selective implosions. Fig. 6 illustrates the connection for one conductor 38 of the seven standard conductors in a logging cable. Conductor 38 is connected to a first ignitor or firing mechanism 42 through diode 40, and a second ignitor or firing mechanism 44 through an avalanche or similar break-down device, for example zener diode 50 (which, for example, may be a 100V zener diode). Equivalent or similar switching or voltage level circuits can be used, the result being that ignitor 42 and ignitor 44 may be selectively fired by changing the level of the applied voltage. Similarly, ignitors 46 and 48 are electrically connected to conductor 38 through diode 52 and zener diode 54 with the polarities reversed. Thus, the same voltage level change used to effect the selective firing of ignitors 42 and 44 may be used to selectively fire ignitors 46 and 48 by reversing the polarity of the applied voltage. The result is that with form voltages, for example, -200 VDC, -100 VDC, and + 200 VDC, form selective firings can result on each of the seven cable conductors. A second exemplary firing system (not shown) would use downhole standard digital switching to connecting a selected implosion source to a cable wire such that initiation current would be sent to the selected chamber by a surface power source. Systems such as this are known in the art. A third exemplary firing system (not shown) would also use downhole digital switching, but would initiate firing by a downhole capacitor, which would then be recharged from the surface between firings. A fourth exemplary firing system (not shown) such as is used in the Schlumberger Coregun could be used in which the firing of one implosive source would mechanically pull an arming wire for firing the next implosive source. The firing mechanism chosen is, of course, connected to the several seismic 10 sources by conventional electrical connection devices such as multiple pin connectors encased within a high pressure housing (not shown), except in the case of an external firing mechanism, in which no connection is required. The connector and housing should be able to withstand hydrostatic pressures up to 1400 bar. Also standard switching electronics (not shown) and the actual wiring connecting the firing mechanism to seismic sources 10 should be contained within that high pressure housing. It is contemplated that fluid to air connectors (not shown) would be the preferred devices for connecting the switching electronics to the seismic sources 10. The number of seismic sources 10 placed on the carrier 14, may vary according to the test results desired. For example, up to 40 or 50 "shots" may be desired for periodic downhole tests. Periodic downhole tests are found desirable when the seismic detecting means are placed on the surface. Firing those shots at regular intervals will enable the operator reading the results to distinguish the pulses generated by the shots from background noise. That result is obtained because the shots will form pulses at regular intervals, whereas background noise forms pulses at random intervals. Fig. 7 illustrates another embodiment of a carrier adapted to receive a plurality of controlled implosive downhole seismic sources, Fig. 8 and 9 being two corresponding cross-sectional views. Two complementary longitudinal shells 60 and 61 form a hollow substantially cylindrical carrier 62 presenting several longitudinal apertures 63 to the outside of the carrier. The shells are held together by means of a plurality of transverse base plates 109, each base plate being secured to each one of shells 60 and 61 by means of a set of screws respectively 65 and 66. The upper face of each base plate 109 sealingly supports a jar 120, thus forming a closed vessel chamber 119 similar to the one described with reference to Fig. 3. Each base plate is adapted to receive the rupturing means 122 of the type described with respect to Fig. 3 and comprising an ignitor needle 131 and a projectile 129 whose line of firing is slightly tilted with respect to the longitudinal axis of the carrier 62 and directed towards one of tha apertures 63. Obviously this is intended to prevent the projectile from damaging the adjacent base plate located above. The lower face of each base plate 109 is equipped with attaching means for sealingly applying and maintaining the adjacent jar located below against its corresponding base plate 9. A simple and appropriate attaching means 65 consists in a press screw fitted with a rubber plug. More sophisticated attaching means may consist in creating a sufficient vacuum inside the vessel chamber via a valve (not shown) located within the base plate body. The base plates 109 act as shields to prevent the wilful implosion of a jar 120 from accidentally causing the destruction of the neighbouring jars. The base plate is made out of steel and thus mainly reflects the shock waves. While the arrangement has been described with the jar 120 attached on the upper face of the base plate 109, it is of course possible to reverse the base plates 109 and attach the jar 120 on the lower face of the base plate. The surface area covered by the apertures 63 opposite the jar 120 should be sufficient to allow a good implosion (typically around 50% of the outer area of the carrier) while maintaining the shells 60 and 61 of sufficient strength in order to withstand a large number of implosions. Carrier 62 typically presents an outside diameter of 115 mm and an inside diameter of 95 mm , while jar 120 typically presents an outside diameter of 80 mm and a height of 150 mm. The described arrangement advantageously protects the glass jar 120 from accidentally imploding under the action of an external shock. Further, the inside wall of shell 60 is U shaped in order to allow easy withdrawal and re-entry of the assembly composed of shell 61 and the plurality of base plates 109. This renders very practical the mounting of the jars 120 onto the corresponding base plate 109. The same advantage can obviously be obtained when using two symetrical half-shells. Simpler and more economical carriers (not shown) also adapted to prevent the jars from prematurely imploding under the action of an external shock can be used such as a set of longitudinal rods joining the base plates and extending beyond the volume allocated for each individual seismic source. In order to facilitate handling of the carrier 60 and allow a wide choice of the number of seismic sources 10 to be lowered into the borehole, it is contemplated to connect in line several carriers 62 end to end, each carrier being equipped with a limited standard number of seismic sources 10, a typical number being for instance 4. The electrical wiring 66 runs through a protecting jacket 67. The wires are connected at their upper end to a selective firing system of the type described with reference to Fig. 6 and located above the carrier 62. Each ignitor needle is provided with an electrical wire. Also provision is made for additional wires to extend to the lower end of carrier 60 , thus providing the necessary electrical connections in case additional carriers are desired. Sensors (not shown) such as accelerometers for detecting the instant of implosion and its intensity may possibly be added within the cartridge which receives the selective firing system. One of the principal advantages of using hydrostatic pressure for powering a seismic source pursuant to the present invention is that the available power increases as the depth in the well increases. With a fixed power potential, such as is found in an explosive source, increases in depth usually result in more energy being lost, particularly when the seismic detecting means are placed on the surface. However, with the implosive seismic source pursuant to the present invention, that increase in hydrostatic pressure increases the power available to the seismic source 10 and compensates for any loss in power. Further, applicants have found that implosive seismic sources pursuant to the present invention avoid problems with the "bubble effect". The "bubble effect" is created when a mechanical device, such as an airgun, sets up resonance. That resonance is caused by the release of high pressure gas or air into the surrounding medium. As that gas expands, it tends to overexpand and then contract and expand again. That resonance causes a small "bubble wave" generated in addition to the primary signal wave generated by the mechanical action. The "bubble wave" further obscures valuable data and renders it more difficult to distinguish reflected waves from background noise. In testing embodiments of the present invention, applicant has been able to achieve a radius of up 3,100 feet (945 m) using a downhole to surface configuration. In a downhole to downhole configuration, much further distances have been achieved. The high quality of the waves produced by controlled implosive downhole seismic sources pursuant to the present invention serve as ideal means for mapping out highly dipping subsurface structures such as salt domes and inclines. Further, the downhole to surface configuration generates a high percentage of low frequency waves, which better penetrate the weathered layer and are more easily distinguishable from background noise. In the downhole to downhole configuration, applicant has found that the controlled implosive seismic source of the present invention can generate higher than normal frequency waves (up to 300 Hz). Waves in that frequency range are desired because they generate higher resolution data regarding subsurface highly dipping structures such as salt domes and inclines. That frequency band is generally not obtainable using traditional surface seismic sources. By varying the depth of the implosive source and the seismic detecting means in the downhole to downhole configuration, the location and slope of a salt dome can be accurately mapped.

Claims: A seismic source apparatus comprising a plurality of spaced apart implosive seismic sources (10) adapted to be ruptured to cause an implosion under hydrostatic pressure and means for cable controlled, selective rupturing of said seismic sources (10), thereby producing seismic waves, characterized in that: each seismic source includes a wall portion (9,24,26,24"',26"') of a material of sufficient strength to withstand the force of the implosion, said wall portion (9,24,26,24"',26"') being attached to an elongated carrier (14,62) attached to a cable to be lowered into a borehole, and a cover portion (20,20',20",20"') adapted to be ruptured and mounted on said wall portion (9,24,26,24"',26"') in a fluidtight relationship to form, together with said wall portion, a closed vessel chamber (19,19',19",19"'); and said rupturing means includes explosive means (22,22',22") disposed within said vessel chamber of each seismic source and is selectively controlled through said cable for rupturing said cover portion of said seismic source. A seismic source apparatus according to claim 1, characterized in that said explosive means (22,22') comprises: explosive material (32,23) disposed in said vessel chamber (19,19') so as to rupture said cover portion (20,20') upon detonation; and means for detonating said explosive material. A seismic source apparatus according to claim 2 characterized in that said rupturing means further comprises circuit means (Fig.6) electrically connecting said detonating means to a source of electrical potential. A seismic source apparatus according to claim 1, characterized in that said explosive means (22") comprises: projectile means (29) mounted in said wall portion (9) within said vessel chamber (19") so as to rupture said cover portion (20") upon actuation; and means (31) for actuating said projectile means (29) so as to cause it to strike said cover portion (20") of said seismic source. A seismic source apparatus according to claim 4, characterized in that said projectile means (29) is a bullet placed within a barrel and wherein said actuating means (31) includes an igniter needle associated with an electrical power supply. A seismic source apparatus according to any one of claims 1 to 5, characterized in that said wall portions (9) are base plates (109) of sufficient thickness and strength to substantially absorb and/or reflect implosive shock waves propagated to a neighbouring seismic source. A seismic source apparatus according to any one of claims 1 to 6, characterized in that said elongated carrier (62) comprises two longitudinal shells (60,61) having complementary "U" shape cross-sections and adapted to form, when assembled together, a substantially cylindrical tube surrounding said seismic sources, said shells (60,61) having apertures (63) through the wall thereof.

Inventors: BROOKS, JAMES E., JOHNSON, DOUGLAS M., JOHNSTON, LUCIAN K., TAYLOR, DARRELL T., VANNIER, DANIEL F.
Applicants: SCHLUMBERGER LIMITED, SOCIETE DE PROSPECTION ELECTRIQUE  SCHLUMBERGER
Application year: 1986
Main classification: G01V1
IPC codes: G01V1