Abstract:
Detonator apparatus, such as a blasting machine detonator, is provided with a miniature transformer having multi-turn primary and secondary coils. The transformer feeds a bridge wire detonator element, and has sufficient impedance to permit impedance matching with a carrier that may be as long as 7500 m. The impedance of the detonator is such that the detonator resists firing when subject to stray currents or commonly present power or communications signals. A blasting machine is provided that is specifically designed to provide a signal having a frequency in the range in which the detonator is sensitive. The blasting machine relies upon semi-conductor switching and timing circuits to control the discharge from a pair of capacitors, rather than upon an output transformer.

Description:
RELATED CASES 
     The present patent application is a divisional of and claims the benefit of patent application Ser. No. 08/992,412 filed on Dec. 17, 1997, now abandoned, the subject matter of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to an apparatus and method for remotely activating blasting devices. Such an apparatus and method may be used, for example, in oil and gas well production in other industries in which remote initiation of explosive devices occurs. 
     BACKGROUND OF THE INVENTION 
     In the production of oil and gas from underground wells, it is known to convey a perforating gun on a wireline down a bore hole of a well to a position where an oil or gas bearing stratum is located, and then to detonate shaped charges in the perforating gun. The shaped charges penetrate the formation, facilitating the entry of oil or gas into the well. 
     Safe and reliable initiation of perforating guns or other firing devices in the well-bore, far removed from the surface, has been a continuing source of design challenges. The explosive train in the perforating gun normally comprises a detonator for setting off a detonating cord. The cord in turn detonates a series of connected shaped charges. The detonator is the first element in the explosive train and is normally the most sensitive to external stimulation. Generally speaking, the safety level of the perforating gun is primarily determined by the safety level of the detonator used. Bridge wire electric detonators have been, and are widely used. When an electric current of sufficient strength is applied to its lead wires the bridge wire is heated and ignites the pyrotechnic material surrounding it. This in turns sets off the primary and secondary explosive charges in the detonator. 
     An inherent problem with bridge wire detonators is the risk of unintentional detonation which may arise from stray currents. A bridge wire detonator does not possess the ability to distinguish between firing current and hazardous electric energy that reaches its lead wires. Typical sources of electrical interference which may cause unintentional detonation are communications equipment, whether cellular telephones or radio, standard 220V, 50 Hz or 110V, 60 Hz line current, electrostatic discharges and lightning. At present when bridge wire detonators are used for perforating jobs, typical safety measures include shutting down electric sources in the well rig environment and turning off communication facilities. It would be advantageous to provide the oil industry a method of initiating perforating guns and a detonator which reduces or eliminates the need to suspend the use of without suspending the electric devices and communication radio in the well rig environment. An additional problem concerns unauthorized use of the detonators. Lost, stolen or mishandled detonators that can be set off by commonly available power sources, whether deliberately or accidentally used, may pose a significant danger. It would be advantageous to have a detonator which will resist detonation except when initiated by an authorized person using a specially designed blasting machine. 
     A known approach to the problem of unintentional detonation is to add extra resistance in series with the bridge wire, making a “resistorized detonator”. A higher voltage than would otherwise be required is used to fire a resistorized detonator, making it more difficult to set off. However, the magnitude of the electric current needed to initiate the detonator remains the same as non-resistorized detonators. 
     Another approach is to increase both the voltage and electric current needed to fire the detonator, so that they substantially exceed the upper limit of routine well rig electrical signals like the exploding bridge wire detonator or exploding foil detonator. This kind of exploding bridge wire or exploding foil detonator is disclosed in U.S. Pat. No. 4,777,878 of Johnson et al. issued Oct. 18, 1988 and U.S. Pat. No. 5,505,134 of Brooks et al., issued Apr. 9, 1996. Another approach, as shown in U.S. Pat. No. 4,708,060 of to Bickes et al., issued Nov. 24, 1987 and U.S. Pat. No. 5,503,077 of Motley issued Apr. 2, 1996, employs a semi-conductor bridge wire to achieve improved safety. 
     Still another method is to isolate the bridge wire, by employing a small transformer in the detonator. The load, generally the bridge wire of the detonator, is connected to the secondary winding of the transformer to form a loop and is electrically isolated from the primary winding of the transformer. The core material of the transformer is chosen to attenuate, or eliminate, spurious electrical power and radiofrequency signals and to respond to firing currents falling within a predetermined range of magnitude and frequency. A blasting machine provides electric current in the predetermined range needed to fire these inductive detonators. 
     A number of embodiments of transformer based detonators are shown in U.S. Pat. No. 4,273,051 of Stratton, issued Jun. 16, 1981. All of those embodiments employ some form of auxiliary energy dissipation means, whether a series or other leakage inductance, a fusible link, or a resistor in parallel with the primary winding. 
     Another example of a ferrite core, broad band attenuator is shown in U.S. Pat. No. 4,378,738 of Proctor et al., issued Apr. 5, 1983. U.S. Pat. No. 4,441,427 of Barrett, issued Apr. 10, 1984 discloses an oil well detonator assembly that uses ferrite materials to protect against radio frequency energy. 
     U.S. Pat. No. 4,544,035 of Voss, issued Oct. 1, 1985 discloses the use of two coils to initiate a detonator in a perforating gun without the coupling of magnetic materials. U.S. Pat. No. 4,806,928 of Veneruso, issued Feb. 21, 1989 discloses the use of coil assemblies arranged on ferrite cores for data transmission between well bore apparatus and the surface and which may also be used to fire perforating guns. 
     U.S. Pat. No. 3,762,331 to Vlahos, issued Oct. 2, 1973 discloses a firing circuit for detonators that uses a step down transformer having a voltage reduction of roughly 100:1 and a secondary coil having only 1 or 2 turns. It operates at a voltage between 60V and 240V and at a signal frequency of the order of 10 KHz. It is powered by a battery in parallel with a storage capacitor, which discharge through an inverter circuit which includes a solid state oscillator and a transformer for stepping up the resulting a.c. voltage to the desired level. This patent also discloses the use of shunt and series capacitance connected to the primary winding of the detonator, and a large step down at the detonator transformer. U.S. Pat. No. 4,145,968 to Klein, issued Mar. 27, 1979 describes primary and secondary windings and a fixed magnetic screen designed to be saturated in the presence of the magnetic flux generated by the primary winding. U.S. Pat. No. 4,297,947 to Jones et al., issued Nov. 3, 1981 discloses the use of a toroid or a magnetic core with removable parts as transformer cores to couple a relatively short (100 m) firing cable and a number of detonators. 
     U.S. Pat. No. 4,304,184 to Jones issued Dec. 8, 1981 discloses a transformer circuit whose primary and secondary windings are not completely isolated. Instead, they are coupled not only magnetically but also electrically. While this configuration may provide protection against hazardous electrical currents at low values and low frequencies, the safety features would be more satisfactory if the two windings were completely isolated electrically. None of the transformer-based detonators noted above appear to be suitable for oil well use. 
     A detonator that can be used in the oil industry at great depth poses special requirements for the coupling transformer. The electric energy supplied from the surface is transmitted along the wireline cable down oil wells as deep as 7,500 m. The cable used for well logging and casing perforation may not be designed for high frequency transmission. The distributed shunt capacitance along the cable is in the order of 0.15 uF/Km. The attenuation for high frequency electrical energy is as high as 3 db/Km (at 20 KHz). Consequently, for effective power transmission along the wireline, a relatively low frequency is preferred. However, electric currents having a frequency lower than 1 KHz will be attenuated by the ferrite core transformer and may not yield a suitable output for energizing the bridge wire in the secondary winding. Therefore, frequency significantly higher than 1 KHz is preferable and the blasting machine must be powerful enough to allow energy dissipation along the wire-line and still secure reliable initiation of the detonator. For optimum power transmission, the inductance of the transformer used in the detonator must be in a certain range at a certain firing current frequency. The inductance of a transformer of some typical known designs may fall in the range of 1-50 μH. Inasmuch as the characteristic impedance of a typical monocable used in well logging is about 30-50Ω, usable for oil well wirelines. 
     By contrast, a transformer having a relatively high primary inductance in the order of 40 mH, would be unsuitable even at the lowest usable frequencies. Also, where the step down is too large, the relatively high voltage needed to fire the detonator makes it impractical for oil well use because of the rapid attenuation of the high frequency voltage signal along the cable. In the view of the inventors of the present invention, the preferred frequency range for effective power transmission is between 3 and 20 KHz, and the primary inductance of the transformer should be in the range of 200 μH and 3 mH. 
     A number of the transformers noted above use magnetic cores which provide a closed magnetic circuit. Some of them may have removable parts to accommodate the firing cable and detonator legwires, as disclosed by U.S. Pat. Nos. 4,297,947 or 4,601,243. When the primary inductance needed is small and a relatively big transformer core (for example, a toroid having outer diameter of 20 mm, placed outside the detonator body) is used, a few turns of winding may be sufficient. However, for a higher impedance the number of winding turns is relatively large, normally in the range 15-80 for the primary winding, depending on the actual size and material properties of the transformer core. Generally the core size of the transformer should be comparable to that of the outside diameter of the detonator. For an oil well detonator this dimension is commonly about 6-7 mm. In the view of the present inventors, as a practical matter, it is difficult efficiently to wind such a large number of turns on a small transformer core, such as a toroid. 
     In the view of the inventors, some of these difficulties may be addressed by using a transformer constructed with a simple core in the form of a column having the desired number of primary and secondary windings on it. A column represents an open magnetic circuit. To achieve efficiency in manufacturing, especially in mass production, it would be advantageous to form the primary and secondary windings by winding separate coils, and then be assembling those coils onto the column shaped core. Alternatively, the primary and secondary windings could be wound on a simple machine sequentially, with the primary winding be embedded, or nested, within the secondary winding, or vice versa. Different shapes of the column can be used, such as a square column, a plate, a tube, a U-shaped core, or other suitable form. 
     In an open magnetic circuit, there is energy loss associated with the high magnetic resistance. It would be advantageous to reduce this loss by using another piece of magnetically permeable material to form a closed magnetic circuit transformer core. Examples of such materials are nickel-iron alloys or permalloys and silicon steel, which have a high magnetic permeability, high curie temperature and are small in volume, low in cost and flexible to form different shapes as required. 
     The oil well use of a transformer-based detonator presents technical challenges. In addition to the extremely long transmission distance (up to 7,500 m long) discussed previously, the high temperature environment also tends to present design challenges. Firstly, magnetic permeability of the core changes with increases in temperature, and drops to near zero above the Curie temperature. Magnetic materials lose their magnetism and the ability to transmit signals beyond the Curie temperature. Advantageously, magnetic materials chosen for transformer cores should have a Curie temperature higher than the highest anticipated temperature in the well, typically 180° C. or higher. Secondly, the ability of most magnetic materials to transmit energy decreases substantially with the increase in temperature due to the decrease in saturation flux density. For example, for a typical maganese-zinc ferrite material, the saturation flux density at room temperature is 4500 Gauss. This decreases to 1750 Gauss when the ambient temperature is 200° C. It is advantageous for the transformer detonator to be able to transmit the required amount of initiation energy at reduced saturation flux density. Thirdly, for ferrite materials there is generally an optimum temperature point at which the core loss at a minimum. Deviation in temperature from that point would result in increased core loss. Even though the detonation location well temperature may vary, it is advantageous to choose a ferrite material which has an optimum core loss temperature close to the expected well temperature. 
     A blasting machine is an electronic device which sends a high frequency electric signal through the wireline to fire the detonator. It is advantageous to provide a blasting machine whose output characteristics match the preferred frequency range of the detonator. 
     U.S. Pat. No. 4,422,378 discloses an ignition circuit for firing detonators having a toroid transformer. It uses a power oscillator having a transistor to provide a firing signal at the resonant frequency of a network of detonators, the transistor being controlled by a current feedback signal. This self-adjusting resonance matching is possible when the inductance and capacitance of the detonators connected in a net are detectable. In some applications, such as those in which diodes are placed in series with the wireline, the inductance of the line can not be obtained and it is difficult automatically to generate the resonant frequency. 
     U.S. Pat. No. 4,422,379 discloses another ignition circuit for firing detonators with a toroid transformer. The oscillator of the circuit is a typical push-pull power amplifier with the use of an output transformer. U.S. Pat. No. 4,848,232 also uses a firing circuit in the form of a push-pull power amplifier with an output transformer. 
     In U.S. Pat. No. 4,601,243, the electrical charge stored by a capacitor is discharged to detonators through a high frequency converting unit which oscillates at a frequency between 50 KHz and 1 MHz. 
     The above referenced U.S. patents commonly have an output transformer. It would be advantageous to eliminate the use of such an output transformer in the blasting machine. First, power output tends to be limited by the size of the transformer. Long transmission distances or initiation of many detonators in one round tends to require a relatively big transformer. This weight and size disadvantage tends to be more pronounced at relatively lower frequencies such as the 3-20 KHz range noted above. When a large, heavy transformer is used the manufacturing cost also tends to increase. 
     It would be advantageous to have an electrically activated detonator operable at great distances, from an electrical signal source, such as may be desired for perforation of an oil well thousands of meters from the surface. 
     It would be advantageous to have a simplified, electrically activated detonator that is relatively insensitive to signals from common electrical sources such as radios, telephones, 50 and 60 Hz supply signals, and other stray or static signals. 
     It would be advantageous to have a blasting machine for activating remote detonators that does not require the use of a large, heavy, and expensive iron core output transformer. 
     SUMMARY OF THE INVENTION 
     The present invention provides, in a first aspect, a detonator for igniting explosive material comprising a multi-turn primary coil for connection to a detonation signal source; a multi-turn secondary coil connected to an explosive igniting element; and a core magnetically linking the coils. The core has a mandrel upon which at least one of the coils is mounted. 
     In a second aspect of the invention there is a detonator for use in a well perforating gun comprising a transformer having a pair of multi-turn coils linked by a magnetically permeable core. The core has a mandrel. One of the coils is a pre-formed coil mounted upon the mandrel. One of the coils is connectible to a detonation signal source and the other coil is connected to an explosive igniting element with which it forms a closed circuit. Explosive material is in contact with the explosive igniting element. 
     The invention may also have a magnetically permeable closure member fit to the mandrel to form a closed loop magnetic circuit. Each of the coils may be a pre-formed coil. Each of the coils may be mounted on a mandrel of the core. The detonator may have closure member fit to each mandrel to form a closed loop magnetic path. 
     In a still further aspect of the invention there is an assembly for causing an explosive charge to explode comprising a blasting machine for generating a detonation signal; a detonator for receiving a detonation signal; and a carrier for carrying a detonation signal from the blasting machine to the detonator; the detonator having a transformer having a pair of multi-turn coils linked by a magnetically permeable core, one of the coils being connectible to the signal carrier; an explosive igniting element connected to the other coil to form a closed circuit; explosive material in contact with said explosive igniting element; and the core having at least one mandrel, and at least one of the coils being a pre-formed coil mounted on the mandrel. 
     In a further aspect of that invention, the blasting machine of the explosive assembly further comprises an energy storage system; a discharge system for releasing energy from the storage system; a switching system operable to control the discharge system to release the detonation signal from the energy storage system for communication of the signal to the detonator along the carrier. 
     In an even further aspect of the invention there is a blasting machine for producing a specific signal for setting off a signal selective detonator, comprising a charge storage system; an output port for connection to the signal selective detonator; a switching system connected between the charge storage system and the output port; a pre-set discharge control system operable to vary flow of charge through the switching system to produce the specific signal. 
     In further aspect of that even further aspect of the invention, the blasting machine further comprising a charging system selectively connectible to the charge storage system when the discharge control system is inoperative. 
     In another further aspect of that even further aspect of the invention the charging system includes a transformer connectible to draw power from a standard line source, and a rectifier connected to the transformer for converting the power to a form storable in the charge storage system. 
     In yet another aspect of the invention there is a detonator for igniting explosive material comprising a primary winding for connection to a detonation signal source; a secondary winding and an explosive igniting element connected thereto; and a core magnetically linking the primary and secondary windings. The core has a first portion made from a first magnetically permeable material for attenuating signals in a first frequency range, and a second portion made from a second magnetically permeable material for attenuating signals in a second frequency range. 
     In a still further aspect of the invention a detonator for igniting explosive material comprises a multi-turn primary coil for connection to a detonation signal source and a multi-turn secondary coil and an explosive igniting element connected thereto. The coils are co-axially mounted and magnetically coupled by a core of low magnetic permeability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made by way of example to the accompanying drawings, which show an apparatus according to the preferred embodiment of the present invention and in which: 
     FIG. 1 is a general schematic drawing indicating the general relationship of a blasting machine, a detonator and a perforating gun in the context of the present invention. 
     FIG. 2 is an electrical schematic of the detonator of FIG.  1 . 
     FIG. 3 a  shows a cross section of the detonator of FIG. 1 with a transformer core having a closed magnetic circuit core. 
     FIG. 3 b  shows a cross section of an alternative detonator to the detonator of FIG. 1 with a transformer core not having a closed magnetic circuit transformer core. 
     FIG. 4 a  shows a general view of the transformer of FIG. 3 a.    
     FIG. 4 b  shows an alternative closed loop transformer for the detonator of FIG.  1 . 
     FIG. 4 c  shows an alternative transformer geometry for the detonator of FIG.  1 . 
     FIG. 4 d  shows a further alternative geometry for an open loop transformer for the detonator of FIG.  1 . 
     FIG. 5 is an electrical schematic of a half bridge inverter for the blasting machine of FIG.  1 . 
     FIG. 6 is an electrical schematic for a self-oscillating driver for the blasting machine of FIG.  1 . 
     FIG. 7 is an electrical schematic for a charging system for the blasting machine of FIG.  1 . 
     FIG. 8 is an electrical schematic an alternative half bridge inverter for the blasting machine of FIG. 1 
     FIG. 9 is an alternative electrical schematic for a full bridge inverter for the blasting machine of FIG.  1 . 
     FIG. 10 is a timer circuit schematic for the full bridge inverter of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The description of the invention is best understood with reference to the figures, in which some proportions have been exaggerated, or shown in schematic form for the purposes of conceptual illustration. 
     The blasting machine of the preferred embodiment is useful, for example, in the oil industry for oil well casing perforation. As such, with reference to FIG. 1, a well bore, such as may be made for an oil, gas or other well, is shown as  20 . It has an inner steel casing  22 , with a more or less annular concrete filling  24  between casing  22  and bore  20 . A formation, or stratum of oil bearing rock is indicated as  26 . A perforating gun assembly  28  has been conveyed down bore  20  on the end of a wireline  30  by which it is physically located in the well. The distance down the well may be 1000 m, or more, up to 7,500 m beneath the ground surface. Wireline  30  also electrically connects assembly  28  with a high frequency blasting machine  32  located on the surface. 
     Perforating gun assembly  28  has at its upper end a collar locator  34  to which wireline  30  is attached. Depending therefrom, perforating gun assembly  28  includes a tube  36  containing a series of shaped charges  38  connected to a detonating cord  40 . Cord  40  terminates at a detonator  42  by which cord  40 , and then charges  38  are ignited. In use, an electrical signal originating at blasting machine  32  is delivered along wireline  30  to detonator  42 . When detonator  42  is set off, it in turn sets off cord  40  which detonates charges  38 . The jets formed by charges  38  penetrate steel casing  22 , concrete filling  24  and oil bearing stratum  26  to establish communication between the well and the rock formation. 
     Referring to the electrical representation of FIG.  2  and the physical presentation of FIG. 3 a  in greater detail, detonator  42  has a detonator casing shell  44  with an internal, closed ended, roughly cylindrical chamber  46 . Explosive material  48  is packed into the end of chamber  46 , and is covered by a partition  50  and pyrotechnic igniter material  52 . The igniter material,  52 , surrounds an embedded filament in the nature of a bridge wire  54 , suspended between the extended ends of two lead wire legs  56  and  58 . Legs  56  and  58  are joined in a closed circuit loop by a multiple turn, secondary winding  60  wound about a magnetically permeable, U-shaped Mn—Zn ferrite core  62 . A keeper, or closure element  64 , again of magnetically permeable material extends between the open legs  66  and  68  of U-shaped core  62  to closed the magnetic circuit of U-shaped core  62 . 
     Referring again to FIGS. 2 and 3 a,  wireline  30  is shielded by a grounded sheathing  70  until it reaches perforating gun assembly  28 , and is grounded through collar locator  34 . Collar locator  34  has a coil  72  for generating an electromagnetic signal when perforating gun assembly  28  passes junctions in casing  22 , so that the exact location of perforating gun assembly  28  in bore  20  can be determined relative to stratum  26 . Collar locator coil  72  has an inductance of 11 H, a typical value for such devices. Wireline  30  extends beyond collar locator  34  to a pair of reversed diodes  74  and  76  on parallel paths. One lead wire  78  of a multi-turn primary winding  80  is connected to diodes  74  and  76 . Winding  80  is wound about U-shaped core  62 , and its remaining lead wire,  82  is grounded. Diodes  74  and  76  are used to permit communication of the firing signal from blasting machine  32  to detonator  42  and to provide high impedance to the small signal generated by collar locator coil  72 . 
     Bridge wire  54  is the part of detonator  42  most sensitive to external stimulation. It forms a closed loop with secondary winding  60 . It is physically protected by a cast-in-place plastic plug  84  which serves also to capture and immobilize legs  56  and  58 , and bridge wire  54  in igniter material  52 . Plug  84  additionally holds diodes  74  and  76 ; the transformer formed by primary winding  80 , secondary winding  60 , U-shaped core  62 , and closure element  64  in place. Bridge wire  54  is also physically and electro-magnetically protected by shell  44  which is, typically, made of a highly conductive metal such as copper or aluminum. Consequently the loop which includes bridge wire  54  remains electrically neutral as it is electrically shielded by shell  44 . 
     Primary winding  80  and secondary winding  60  are pre-formed and then assembled on legs  66  and  68  of U-shaped core  62 , before being locked in place by nickel alloy closure element  64 . This method encourages relatively easy and economical assembly, and contrasts with the method of assembly of threaded-core detonators. Primary and secondary windings  80  and  60  are mounted parallel to each other in an arrangement which reduces the magnetic flux coupled by air and shared by both windings. The number of turns may vary. It is typically in the range of 15 to 80 for primary winding  80  and for secondary winding  60 . In the preferred embodiment the number of turns on primary winding  80  is 24, and the number of turns on secondary winding  60  is 12. In the preferred embodiment, the height of core  62  is 8 mm, its thickness is 1.5 mm, and its width is 6 mm. 
     The number of turns of windings  60  and  80 , the permeability of core  62 , and the geometry chosen affect the range of frequencies to which detonator  42  is most responsive. Core  62  is chosen so that it responds efficiently to electric currents delivered by specifically designed blasting machine  32 , reducing or eliminating electrical hazards. In use, stray DC signals and low frequency AC sources carried on wireline  30  will have little or no effect on bridgewire  54 . Since electrical frequencies in a typical well rig environment have frequencies either below 1 kHz (e.g., DC, 50 or 60 Hz AC) or well above 1 Mhz (radio frequency energy in GHz), the probability of unintentional detonation tends to be reduced. When an appropriate firing current is delivered to wireline  30  the current running through primary winding  80  induces a current in the closed circuit loop formed by secondary winding  60 , legs  56  and  58 , and bridge wire  54 . Bridge wire  54  is then heated to incandescence and ignites pyrotechnic igniter material  52 . The ignited material  52  initiates detonation of detonator  42 , which thereafter sets off the explosive train of cord  40  and shaped charges  38  in perforating gun assembly  28 . 
     The preferred material for core  62  is either a Mn—Zn or an Ni—Zn ferrite chosen to discourage energy transmission at frequencies falling outside the chosen frequency range of blasting machine  32 . In the preferred embodiment, the ferrite has an operating range between 3 and 20 kHz, which is too high for general power transmission interference, and too low for interference by radio or communications signals. As noted above, the ferrite chosen must have a Curie temperature higher than the temperature in bore  20  at the level of oil bearing rock stratum  26 . Typically a Curie temperature of 200° C. or higher is preferred. In the preferred embodiment the ferrite core chosen has an initial permeability of 2500, a Curie temperature of 230° C., a saturation flux density of 5000 Gauss at room temperature and a field strength of 15 Oersted. 
     The preferred material for closure element  64  is a super permalloy (T.M.), an alloy of 80% nickel and 20% iron having an A.C. impedance permeability in the order of 100,000 a Curie temperature of roughly 400° C., and a saturation flux density of 8 Gauss. Due to its high permeability, the thickness of closure member  64  is 0.36 mm. The material cost is low, and the alloy can be formed to the shape desired. Closure member  64  in other embodiments, can also be made of a suitable ferrite for a given frequency range or from other magnetic materials, such as silicon steels. 
     The combination of the material properties of core  62  and closure member  64  provide relatively efficient, and desirable, frequency discrimination. The Mn—Zn ferrite material responds relatively poorly to DC and low frequency AC stimulation, but can operate satisfactorily at higher frequencies as high as a few MHz. By contrast, the magnetic alloy of closure member  64  responds satisfactorily to low frequency AC and DC signals, but tends to attenuate high frequency signals as its permeability decreases with increasing frequency. Also, core losses are approximately proportional to the square of the frequency. Consequently low frequency (&lt;1 KHz) signals are retarded by core  62  and high frequency signals (&gt;1 MHz) are attenuated by closure member  64 . When a firing current is delivered by blasting machine  32  to lead wires  78  and  82  both core  62  and closure member  64  are energized, bridge wire  54  is heated to incandescence, and pyrotechnic material  52  is ignited. Thus the combined effect core  62  and closure member  64  is that of a frequency sensitive filter. 
     Blasting machine  32 , located at the far end of wireline  30  from detonator  42 , is illustrated in electrical schematic form in FIGS. 5,  6 ,  7 , and  8 . It supplies electric current to fire detonator  42  as described at length above. Blasting machine  32  will be described in detail in order of a timing driver, controlling circuit  86 , which provides an oscillating signal; a firing circuit indicated generally as  88 , in the nature of an inverting circuit which receives the oscillating signal; and a charging circuit indicated generally as  90 , which charges energy storage elements of firing circuit  88  to a desired voltage level. 
     The time varying signal generator, or driver, controlling circuit  86 , shown in FIG. 6, has as its principle element a commercially available IR2151 self-oscillating MOSFET and IGBT driver chip  92  having V cc , R t , C t , Com, V b , H o , V s , and L o  ports. A DC source in the nature of a 15V dry cell  94  has a negative terminal connected to the Com port, and a positive terminal connected, through a switch  96 , to V cc . A timing resistor  98  is connected across the R t  and C t  ports, and a timing capacitor  100  connected to between the C t  port and an output terminal ‘D’. A voltage stabilising capacitor  102  is connected from Com to V b . A diode  104  and capacitor  106  are used to provide high side power supply, high side power supply capacitor  106  being connected across V b  and V s . V s  is connected directly to an output terminal ‘B’. H o  and L o  are similarly connected to output terminals ‘A’ and ‘C’ respectively. Closure of switch  96  will cause chip  92  to produce a high side, low power square wave output  108  between terminals ‘A’ and ‘B’, and an opposite, half period phase shifted low side square wave output  110  between terminals ‘B’ and ‘C’, as indicated in FIG.  5 . Chip  92  is capable of generating controlling signals over a wide frequency range. In the preferred embodiment, a controlling signal at 12.75 KHz is produced when resistor  98  has a value of 56 Ω, capacitor  100  has a value of 1000 pF and capacitor  106  has a value of 0.47 μF. 
     Firing circuit  88  is shown in FIG. 5, as a half bridge converter with input ports ‘A’, ‘B’, ‘C’, and ‘D’ corresponding to output ports ‘A’, ‘B’, ‘C’, and ‘D’ of driver  86 . Back to back energy storage capacitors  112  and  114 , whose charging will be described below, are joined in series at a central grounded node  116  and act as power sources for high and low side MOSFETs  118  and  120  respectively, defining a high voltage side  122 , and a low voltage side  124 . In a preferred embodiment the storage of capacitors  112  and  114  are 470 μF capacitors. MOSFETs  118  and  120  are of the high speed switching type with voltage and current ratings of 1000V and 14 A. A voltage limiting Zener diode  126  and an LED  128  are connected in series between high and low voltage sides  122  and  124  as well. The source of MOSFET  118  and the drain of MOSFET  120  are connected at a common node corresponding to input ‘B’, with the drain of MOSFET  118  connected to high voltage side  122  and the source of MOSFET  120  connected to low voltage side  124 . 
     The gate of MOSFET  118  is connected to input ‘A’ across a resistor  130  which, in a preferred embodiment, has a value in the range of 10 to 500Ω. Resistor  130  is used to reduce the quality factor of the input circuit, thereby discouraging parasitic oscillations. Similarly the gate of MOSFET  120  is connected to input port ‘C’ across a resistor  132  or the same magnitude, for the same purpose. 
     A gate to source resistor  134  having a value of 1 MΩ is used to reduce resistance from the gate to the source of MOSFET  118 . A similar resistor  136  is used with MOSFET  120  for the same purpose. A pair of opposed Zener diodes  138 ,  140  having a voltage rating of 18V and a power rating of 1 W each are used to protect the gates and sources of MOSFETs  118  and  120 . Further Zener diodes  142  and  144  connected between the drains and sources, respectively of MOSFETs  118  and  120  provide protection against voltage surges. Higher voltage protection could be obtained by connecting more than one such Zener diode in series. 
     Finally, a 30Ω current limiting resistor  146  extends from input port ‘B’ to a first load terminal  148 , while a second load terminal  150  is connected directly to central grounded node  116 . The current initiating resistor is 30Ω in the preferred embodiment. 
     A third component of blasting machine  32  is charging circuit  90 . As shown in FIG. 7, it has a small step up transformer  152  has a primary coil  154 . Primary coil  154  has one lead connected to a standard, single phase, 115V, 60 Hz AC plug  156 , and has another lead, connected through a current limiting resistor  158  and through a switch  160  to connect with the other side of plug  166 . A current limiting resister  162  and LED  164  in series are connected in parallel with primary coil  154  to indicate the working conditions of the transformer. 
     Secondary coil  166  of transformer  152  has leads  168 ,  170  connected to opposite sides of a full wave bridge rectifier  172 . The positive output of rectifier  172  is connected to high voltage side  122  and the negative side of rectifier  172  to low voltage side  124  of blasting machine  32 . One of leads  168  or  170  is connected by a jumper  174  to grounded node  116 , for the purpose of doubling the voltage level of main capacitors  112  and  114 . 
     In operation, assuming that power storage capacitors  112  and  114  are initially uncharged, charging circuit  90  is plugged in to a suitable source, wireline  30  is disconnected from output load terminals  148  and  150 , and timing circuit switch  96  is open. Charging circuit switch  160  is then closed to charge capacitors  112  and  114 . Once capacitors  112  and  114  have been charged to 300V, switch  160  may be opened or the power source may be disconnected. 
     After perforating gun assembly  28  has been conveyed along bore  20  to an appropriate position amidst oil bearing rock stratum  26 , wireline  30 , and hence, ultimately primary winding  80 , is connected to load terminal  148 . Load terminal  150  is grounded through node  116  and primary winding  80  being connected to ground  82 . When timing signal switch  96  is closed, square wave signals  108  and  110  will be sensed at the respective gates of MOSFETs  118  and  120 , turning them on and off alternatively and giving a peak output current in the range of 1 to 12 A. When a positive voltage of 10 to 15V is applied to terminals A and B (that is, gate to source), MOSFET  118  conducts, capacitor  112  discharges through it and a current runs through current limiting resistor  146  to the load, that is, wireline  30  and the components of detonator  42 , forming a first half cycle of electric current shown as I 1  as shown in FIG.  5 . In the second half of a cycle, MOSFET  120  conducts and MOSFET  118  is switched off. Capacitor  114  discharges to the load, R L , that is, through detonator  42  and current limiting resistor  146  such that an electric current indicated as I 2  in the lower side. In this manner the two (2) MOSFETs  118  and  120  will conduct alternately, yielding an alternating current in load R L  until both capacitors  112  and  114  are discharged. The alternating current produced in this manner is carried along wireline  30  to primary winding  80  to induce a current in secondary winding  60 , and bridgewire  54 , which in turn heats to incandescence and sets off igniter material  52 . In the preferred embodiment, blasting machine  32  constructed using the circuitry described herein has a maximum peak to peak current output of 16A or a maximum peak to peak voltage output of 900V, assuming capacitors  112  and  114  have been charged to 450V each, and resistor  146  has a value of 55Ω. In use the embodiment described yields a signal having relatively high voltage, relatively large current, relatively high momentary power output, and relatively short duration. 
     The apparatus described has been found to discourage unintentional firing due to stray currents from common AC or DC sources, radio frequency energy, lightning and other electrostatic discharges. The inventors have found that it discourages firing, even when commonly used electric sources are applied directly to leadwires  78  and  82  (with the DC firing current of the material 52 of 0.8 A). The inventors have found that detonators made according to the above description have resisted firing when exposed to 115V, 60 Hz AC; 220V, 50 Hz AC; 380V, 50 Hz AC; and when connected to a 705 μF capacitor charged to 600V. 
     Having described the preferred embodiment of the invention, it should be noted that a number of alternatives are possible without departing from the principles or spirit of the invention. The detonator of the present invention can be manufactured in different forms to facilitate its use. For example, a block detonator is a design that provides some space between the detonator and detonating cord by using a block, allowing fluid desensitization. A top fire detonator is designed to start a top-down detonation of the explosive train in the gun. A detonator in capsule version is directly exposed to the high pressure in the well. The present detonator may be manufactured in any of these forms. 
     Four alternative versions of detonator geometry are shown in FIGS. 3 b,    4   b,    4   c  and  4   d.  FIGS. 3 b  shows a transversely mounted detonator transformer  180  having a circular cylindrical ferrite core  182  of a diameter of a 5 mm and a length of 6 mm. A primary winding  184  having 60 turns and a secondary winding  186  of 30 turns are wound in a nested, co-axial fashion about core  182 , that is to say, one winding is embedded within the other. Ferrite core  182  is a simple ferrite bar, and, as shown, is an open magnetic circuit. The magnetic properties of the bar are the same as those of U-shaped core  62  of the preferred embodiment of FIG. 3 a.    
     FIG. 4 b  shows a detonator transformer  200  having a circular cylindrical ferrite core  202 , 6 mm long and 5 mm in diameter, about which a primary winding  204  having 60 turns, and a secondary winding  206  having 30 turns are coaxially wound in a nested fashion. Core  202  is then held about its ends by a U-shaped, or half rectangle shaped, magnetic alloy closure member  208  having a back  210  and legs  212  and  214 . Closure member  208  could also be in the form of a full closed rectangle, or a circle or other shape making a closed loop for capturing core  202  about its ends. 
     Alternatively, the transformer core could be in the form of a bobbin or spindle having at one end a radially extending flanged base or shoulder, with a closure member in the shape of a cap, or thimble, at least partially covering the spindle with continuous magnetically permeable structure extending from one end of the spindle to the other. The foregoing alternatives are only examples of cores that could be used in the present invention, other configurations such as a plate, a square column, or a square or round tube, and other configurations also being possible. 
     FIG. 4 c  shows a detonator transformer  220  having a circular cylindrical ferrite core  222  of a diameter of a 5 mm and a length of 6 mm. A primary winding  224  having 60 turns is wound about one portion of core  222 . A secondary winding  226  of 30 turns is wound about another portion of core  222 . There is no highly magnetically permeable closure member, rather the magnetic circuit of ferrite core  222  is left open. 
     FIG. 4 d  shows a detonator transformer  240  having a core  242  in the form of a C-shaped half cylinder section, much like a half toroid but with a rectangular cross section, having toes  244   246 . A primary winding  248  of 60 turns is wound about toe  244  and a secondary winding  250  of 30 turns is wound about toe  244 . As before, there is no highly magnetically permeable closure member spaning the gap between toes  244  and  246  to form a closed loop path. 
     It will be appreciated that the geometry of the transformer core may vary, and it may be in the form of an open core, or a core having a closure member and a closed loop magnetic path. The core may be solid, or it may be a hollow tube, whether of circular, square, or other section. 
     The relative position of the primary and secondary windings has an effect on output performance. When one winding is embedded within the other, or the two windings are coaxial and close together or abutting, it is possible for a low or non-magnetically permeable material, whether air, a ceramic, paper or plastic core, to couple sufficient magnetic flux between the two windings to permit detonation. For example, a sudden fluctuation in a 150 A current can be enough to trigger detonation. If the axes of the windings are parallel and spaced apart an axial distance, similar to the axial distance shown in FIG. 3 a,  the magnetic flux coupled by air between the two cores is reduced, or minimized. 
     In all cases, the detonator transformer windings present a significant level of impedance to the firing current supplied by blasting machine  32  and coupled by wireline  30 . This is done by using a relatively large number of turns on both the primary and secondary windings, rather more than merely one or two turns. The minimum number of turns has not been determined, but is thought to be at least five. Hand threading multi-turn cores is generally impractical, more so in oil well detonators since a typical inside diameter for a casing, like shell  40 , is 6 mm, implying very small core and winding sizes. It is more economical to form these multi-turn windings by machine, and this is facilitated if, at the time of manufacture, the core presents an open ended spindle, or mandrel, upon which a winding can be wound, or upon which a pre-formed winding can be slipped. The winding, or windings, can then be retained in place either by the mechanical tightness of the winding, an adhesive, or by mechanical means such as a fastener, a bent over flange, or, as in FIGS. 4 a  and  4   b,  by a magnetically permeable closure member. The spindle, or mandrel, portion, or portions of the core, may be circular in section, as in the case of FIGS. 4 b  and  4   c,  or rectangular, as in the case or FIGS. 4 a  and  4   d,  or some other shape or shape as may be found convenient. 
     The alternative blasting machine  260  of FIG. 8 shows a half bridge structure of an inverting circuit using pairs of two IGBTs  262 ,  264  or  266 ,  268  in parallel in place of MOSFETs  118  or  120 , for the purpose of increasing the maximum current out put of the blasting machine. 
     The full bridge  270  of FIG. 9 is for use when a higher voltage output is required than can be produced with the similar half bridge of FIG.  5 . Those elements that are unchanged from FIG. 5 are indicated by the same item numbers as above. A circuit as shown in FIG. 10 can be used to drive full bridge  270  of FIG.  9 . It uses a 555 timer  272  as a square wave signal generator. The circuit is powered by battery  274  controlled by a switch  276 . The frequency of the signal is determined by the values of the capacitors  278  and  280 . The output signal of timer  272  is amplified using transistor  282  whose collector is connected to the primary winding  284  of a small transformer  286 . The transformer is coupled with a ferrite core  290  and has four identical secondary windings  292 ,  294 ,  296  and  298  with the polarity shown, at which output signals identified as GS 1 , GS 2 , GS 3 , and GS 4  are sensed. 
     Referring again to FIG. 9, full bridge  270  has four MOSFETs  300 ,  302 ,  304 , and  306  arranged to work in diagonal pairs to produce a doubled-voltage push-pull effect. MOSFETs  300  and  306  are driven by signals GS 1  and GS 2 , of the same polarity, MOSFETs  302  and  304  are driven by signals GS 3  and GS 4 , of the opposite polarity. MOSFETs  300  and  306  conduct simultaneously as a pair, and alternate with the other pair formed by MOSFETs  302  and  304 . The net result, as before, is to drive an alternating current through a current limiting resistor  310  and the load, R L . In the embodiment shown, each MOSFET  300 ,  302 ,  304  and  306  has a voltage and current rating of 1000V and 14 amperes, respectively. As before, capacitors  112  and  114  have a capacitance of 470 micro F each, in the preferred embodiment. They are connected in series and charged to 800V. The value of current limiting resistor  310  is 80 ohms. 
     Full bridge  270  of FIG. 9 is driven by 555 timer  272  of FIG. 10 at a frequency of 20 KHz. Consequently, a blasting machine constructed using this circuitry has a maximum peak to peak current of 20 A, or a maximum peak to peak voltage of 1600V. Controlling circuit of FIG. 10 is one example of a controlling circuit suitable for use with a full wave inverter. Other electronic circuits could be used as well. 
     Although only two types of power transistors, i.e., MOSFETs and IGBTs, are used in the description of the present invention, other types of power transistors can also be used. Bipolar transistors, Giant Darlington power transistors, and gate turn-off silicon-controlled rectifiers can be used in place of MOSFETs and IGBTs with corresponding changes in the driving circuits according to their driving requirements. 
     In the description of the present invention, the circuits are shown in discrete elements. However, it is understood that the half bridge or full bridge converter can be integrated into a single chip along with its driving circuit, making it more compact and less expensive. 
     In each embodiment described, the blasting machine does not require an output transformer. However, it does not exclude the use of a transformer for other purposes, such as for isolation of electronic circuits, or for impedance matching between the blasting machine and the load. In such uses, the transformer is not involved in the conversion of the DC currents to high frequency AC currents. The transformer is not a necessary part of the converter. 
     In addition to charging circuit  90  shown in FIG. 7, capacitors  112  and  114  can be charged using dry batteries, an oscillating circuit, a step up blasting machine or other suitable circuit. The use of commercially available single phase 11V, 60 Hz AC, as shown in FIG. 7, corresponds to a source commonly available from truck mounted generators at well sites. 
     Although developed mainly for oil well casing perforation, the apparatus of the present invention can also be used in other oil field applications such as exploration, pipe cutting, severing, and so on. Furthermore, the apparatus of the present invention can also be used to replace conventional bridge wire detonators in mining, construction and other engineering projects where the initiation of explosives is involved. 
     This description is made with reference to the preferred embodiment of the invention. However, it is possible to make other embodiments that employ the principles of the invention and that fall within its spirit and scope as defined by the following claims.