Patent Application
20120180678
1. Field of the Invention
The present invention relates to an explosive system for seismic charges which is safe from detonation by radio frequency (RF) signals and electrostatic discharge (ESD).
2. Description of the Prior Art
The current art of explosive seismic exploration relies on fast-acting electric detonators that typically function in less than 1 millisecond, and up to 1,000 detonators may be fired essentially simultaneously by a relatively low-voltage capacitor discharge. Because of their fast action, seismic electric detonators rely on a very sensitive primary explosive like lead styphnate, lead azide and diazodinitrophenol (DDNP). These seismic electric detonators can never be considered totally safe because they may be actuated by electrostatic discharge or stray voltage and are also susceptible to actuation by stray current and distant lightning strikes.
Accordingly, for safety reasons, seismic charges are currently shipped from the factory without detonators, and the detonators and charges are assembled and combined in the field. Of course, assembly of the charges and detonators in the field presents safety problems since RF signals and electrostatic discharge can cause detonation as the charges are being field-assembled.
An example of an exploding bridge wire (EBW) detonator is illustrated in U.S. Pat. No. 4,777,878. An EBW detonator may, for example, employ a two electrode arrangement in the detonator and have an exploding bridge conductor between the two electrodes. The exploding bridge is located at one end of a column of explosive material used in the device. Within the column of explosive and spaced a distance away from the exploding bridge portion of the detonator is a shock reflector element on an inert but relatively dense material having a high shockwave impedance. The cooperative action of the exploding bridge and the shock reflector intensifies the shockwave propagated through the explosive and causes a detonation because of this intensification.
Exploding foil initiator (EFI) detonators have also been available, and one such detonator is illustrated in U.S. Pat. No. 6,752,083, which is owned by the assignee of the present application. An EFI detonator includes an electrically conductive metal foil which is connected to a source of current. The metal foil includes a narrow neck section that explodes or vaporizes when a high current is discharged quickly through the neck section. The exploding neck section of the foil shears a small flier from a disk that is disposed in contact with the foil. The flier travels or flies through a barrel to impact a secondary explosive, e.g., dynamite, to initiate a detonation.
Because EBW and EFI detonators contain only secondary explosives (e.g. HNS, Nona, and RDX), and require very high power to function, they are known to offer safety against electrostatic discharge, stray current and even lightening strike hazards. These detonators also have extremely short function times that meet or surpass the standard seismic requirement of less than 1 millisecond. The disadvantage of this technology is the requirement of very high voltages, e.g., over one thousand volts, and extremely high currents, usually over one thousand amps to activate these devices. The required voltage and current need only be applied for a very short period of time, e.g., 1-2 microseconds and is typically accomplished by the discharge of a high-voltage capacitor into a low-inductance firing circuit.
A new technology which can operate at lower voltages and currents but still provide very good safety is to use semiconductor bridges (SCB) in place of the metal foil bridge of the EFI. SCB's can be used in two ways. They can be placed in direct contact with sensitive pyrotechnic and primary explosives in which case their use offers only a slight improvement over typical hot-wire low-voltage detonators because they are still susceptible to stray voltages, currents and ESD. However, the SCB can also be used to replace the metal foil bridge in a slapper type detonator. When used this way with only secondary explosives the resulting detonator is ESD safe. When an SCB slapper detonator is also coupled to an addressable switch, then the whole assembly becomes safe from stray voltages and currents and ESD.
Addressable switch technology has been commercially available in the mining and blasting industry for several years. Each of these systems incorporate an addressable switch to isolate the firing circuit of the detonator from the lead wire input until the detonator has been properly addressed and then armed. All systems capable of firing multiple detonators also have built in firing circuit diagnostic capability allowing the identification of detonators that are not properly attached to the firing circuit.
The U.S. Department of Defense and the U.S. Department of Transportation consider it unsafe to transport or store explosive charges which have been assembled with initiation systems without additional safety precautions. Initiation systems that rely on primary explosives must have a shutter that physically isolates the primary explosive from the rest of the explosive train so that even if the primary explosive accidentally detonates it will not initiate the main charge. It is also required that these shutter devices require two independent signals or actions to arm, i.e. to couple the primary explosive component to the initiation circuit. If a an explosive device containing primary explosive in its initiation chain has such a shutter requiring two independent signals to arm and another independent signal to fire, then such a device is considered safe to transport and store with its initiation system installed. For initiation systems that do not contain primary explosives it is not required to have a physical barrier, shutter, that interrupts the initiation chain. Rather in these systems it is acceptable to require just two independent signals to arm the device and a third signal to fire the device. These signals can be mechanical or electrical. A further restriction on such non-primary systems is that at signal of at least 500 volts is required to fire the device.
In accordance with the present invention, a seismic explosive package is provided which comprises a seismic charge, and an addressable switch for use in selecting that seismic charge for detonation. A seismic explosive package according to the present invention further comprises a fireset which is interposed between the addressable switch and the seismic charge. The fireset is for receiving a firing voltage via the addressable switch and for using the firing voltage to produce an actuation voltage. In one embodiment, the actuation voltage may be formed by increasing the magnitude of the firing voltage, and the fireset may, for example, comprise a voltage multiplier for increasing the magnitude of the firing voltage. A seismic explosive package according to the present invention further comprises a Detonating Device, which comprises a secondary explosive. The Detonating Device may, for example, may be either an EBW detonator, an EFI detonator or a Semiconductor Bridge (SCB) Slapper Detonator, which is interposed between the fireset and the seismic charge. The actuation voltage from the fireset is sufficient to cause the Detonating Device to detonate, which in turn detonates the seismic charge.
In accordance with the present invention, a system for detonating seismic explosives is provided which comprises a plurality of explosive devices, as described above. The plurality of explosive devices may be deployed in desired patterns at spaced intervals at or near the earth's surface, and a system according to the present invention may further comprise a base unit having a computer and a power supply for providing selection, firing and trigger signals to the plurality of explosive devices. The base unit selects an explosive device for detonation by providing a selection signal to the addressable switch associated with that explosive device. The base unit also provides the firing signal via the addressable switch to the fireset in the selected explosive device. When a trigger signal is received from the base unit by the fireset, the actuation voltage is presented to the Detonating Device. This actuation voltage causes detonation of the Detonating Device which in turn causes detonation of the seismic charge.
In the accompanying drawings:
It will be appreciated that the present invention may take many forms and embodiments. In the following description, some embodiments of the invention are described and numerous details are set forth to provide an understanding of the present invention. Those skilled in the art will appreciate, however, that the present invention may be practiced without those details and that numerous variations and modifications from the described embodiments may be possible. The following description is thus intended to illustrate and not to limit the present invention.
In this specification and the appended claims: (a) the term “Detonating Device” means a device which contains only secondary explosives and which when detonated causes a seismic charge to detonate. Examples of a Detonating Device include EBWs, EFIs and SBC slapper detonators; and (b) two items are “operative coupled” if they are directly connected or connected through an intermediate device.
With reference first to
Referring now to
In operation, explosive package 101(i) is coupled to base unit 102 by appropriate cabling 103, and base unit 102 provides selection, firing and trigger signals to explosive package 101(i) via cabling 103. A selection signal is first provided which selects the addressable switch associated with seismic charge 204 in explosive package 101(i) for detonation. Thereafter, a firing signal is provided by base unit 102 and this firing signal may, for example, be a voltage between 300 and 500 volts. Addressable switch 201, which has been selected, allows the firing voltage to be presented to fireset 202 which uses the firing voltage to charge a capacitor to produce an actuation voltage. The fireset may, for example, also comprise circuitry for increasing the magnitude of the firing voltage from base unit 102 to produce the actuation voltage. This increase in firing voltage may, for example, be necessary when the Detonating Device is an EBW or EEI detonator and may be accomplished by using a voltage multiplier circuit in fireset 202. Such voltage multiplier circuitry is well known to those skilled in the art. The output of fireset 202 is operatively coupled to the input of Detonating Device 203, and when base unit 102 provides a trigger signal to the selected explosive device, the actuation voltage, which is present at the output of fireset 202, is presented to Detonating Device 203. Detonating Device is thus detonated which in turn detonates seismic charge 204.
An explosive package 101(i) which is made in accordance with the present invention has an advantage over the prior art in that all explosive devices for use in a particular seismic operation may be assembled in a factory as opposed to being assembled in the field. Factory assembly of explosive package 101(i) should not only be cheaper, but also safer than the field assembly of seismic charges which is currently practiced.
An explosive package in accordance with the present invention may also be safely transported and stored while assembled with its own circuitry for initiation; which is of great advantage to the seismic exploration industry. Such devices have not previously been used or available to the seismic exploration community and are possible only by coupling several dissimilar technologies together to form a new invention. This invention couples direct initiation of secondary explosive via EBW, EFI or SCB Slapper technology with the use of addressable switch technology and a seismic charge to create a seismic explosive system with an initiation train requiring two independent arming signals, and an independent firing signal, and an initiator (detonator) that requires more than 500 volts to function. Such a system is safe from accidental initiation due to commonly encountered stray voltages, currents, electrostatic discharge and simple human errors.
It will be appreciated by those skilled in the art that the explosive devices 101(1), 101(2), . . . 101(n) may be arranged in any pattern which the user deems appropriate for the seismic exploration task at hand. A plurality of the explosive devices may, for example, be arranged in series with one another and the series connection of explosive devices may be arranged in parallel with one another. It will also be appreciated by those skilled in the art that the present inventive concept may be used in a detonator package for downhole operation, e.g, for detonating a perforating gun, jet cutter, propellant or other downhole device.
