fig1 shows the present invention as it is typically used in combination with an oil well perforating gun assembly . an oil well 14 is drilled through earth formations 12 until a desired formation 13 , which may contain oil and gas , is penetrated . the well 14 is then &# 34 ; completed &# 34 ; by inserting therein a steel casing 16 and cementing it in place to hydraulically isolate the desired formation 13 from other portions of the earth formations 12 . a perforating gun assembly 10 is lowered into the well 14 , typically by means of an armored electrical cable 18 comprising at least one insulated electrical conductor ( not shown ). the cable 18 is lowered into the well 14 by means of a winch 34 or other device known in the art until the gun assembly 10 is positioned at the depth of the desired formation 13 . the gun assembly 10 includes the firing circuit 24 , an ebw detonator and booster 22 , and explosive shaped charges disposed in a carrier assembly 20 . the gun assembly 10 is attached to the cable 18 by a cable head 25 , which makes both electrical and mechanical connections from the assembly 10 to the cable 18 . when so controlled by the system operator , a surface control unit 30 applies an electrical voltage to the cable 18 to initiate the firing circuit 24 , thereby detonating the ebw detonator and booster 22 , which detonates the gun assembly 10 . fig2 shows the perforating gun assembly 10 as a functional block diagram . the surface control unit 30 includes a high voltage ac power supply 40 which , by control of the equipment operator , applies power at a preselected frequency to the cable 18 for activating the firing circuit 24 . the functional components of the firing circuit 24 , shown in fig2 include a bandpass filter 35 , a voltage multiplier 37 , and a discharge circuit 39 . the response characteristics of the bandpass filter 35 will be further explained . in the present embodiment of the invention , the circuits 35 , 37 , 39 can be combined in a single assembly ( shown as the firing circuit , number 24 ) which can be sealed in a plastic potting compound . the potting compound can be a room temperature , chemically reactively setting material such as polymer resin . fig3 shows the functional components of the firing circuit 24 in more detail as a circuit diagram . the bandpass filter 35 comprises a first surge voltage protector ( svp ) 50 which shunts out voltage exceeding a preselected voltage . the first svp 50 protects against accidental energizing of the detonator 22 by unintentional application of voltages to the cable 18 which may be caused by such electrical sources as lightning strikes . resistors 52 and 58 , capacitors 54 and 56 , and inductors 51 and 53 can form the active components of the present embodiment of the bandpass filter 35 . the component values selected for the resistors 52 , 58 , capacitors , 54 , 56 and inductors 51 , 53 shown in fig3 provide a passband frequency of about 1 khz and a &# 34 ; cut - off &# 34 ; response of about - 30 db at 500 hz and 2 khz . alternating current ( ac ) at a frequency near the passband frequency will readily pass through the filter 35 to the voltage multiplier circuit 37 , but ac having frequencies higher or lower than the - 30 db &# 34 ; cut - off &# 34 ; frequencies will be substantially blocked by the filter 35 . the passband and cut - off response of the filter 35 is preferably narrow enough to enable activation of the firing circuit 24 by application of substantially monochromatic ac at the passband frequency to reduce the possibility of unintended activation of the firing circuit 24 . the response characteristics of the filter 35 in the present embodiment are selected to provide sufficiently narrow passband response but also enable the filter 35 to be constructed from resistors 52 , 58 , capacitors 54 , 56 and inductors small enough in size to fit in the gun carrier 10 . other passband frequencies and cut - off response characteristics can be selected for the filter 35 by selecting different values of resistors , capacitors and inductors . methods of selecting values of capacitors , resistors and inductors to obtain the desired filter response are known in the art . the passband frequency and the cut - off characteristics of the filter 35 preferably are selected to exclude passage of ac having frequencies of sources of electrical power already present at the wellbore location . for example , the passband frequency of the bandpass filter 35 can be chosen to exclude the frequency of a general utility electric power source , typically 50 or 60 hz . exclusion of the utility power source frequency can reduce the possibility of accidental activation of the firing circuit by unintended application of utility line voltage to the cable 18 . the passband frequency of the filter 35 should also be selected to fall within the passband response of the cable 18 , which as is known in the art typically has a - 30 db response cut - off at frequencies above about 80 to 100 khz . ac at a frequency which can pass through the filter 35 is rectified and multiplied in the voltage multiplier 37 . diodes 62 , 66 , 70 , 74 , 78 , 82 , 86 half - wave rectify the ac and apply the rectified voltage to capacitors 60 , 64 , 68 , 72 , 76 , 80 , 84 which are connected in series . the multiplied voltage output from the multiplier 37 , which is about 1600 volts with a voltage input to the multiplier 37 of 200 volts , is conducted to the discharge circuit 39 through a current - limiting resistor 92 . another resistor 90 bleeds off any charge remaining in the system when no power is applied to the filter 35 . the discharge circuit 39 comprises a second svp 96 which conducts at about 1500 volts , and a high - voltage capacitor 94 which stores the applied voltage from the multiplier 37 until the conduction threshold voltage of the second svp 96 is exceeded . when the voltage on the high - voltage capacitor 94 exceeds the conduction threshold of the second svp 96 , the current stored in the high - voltage capacitor 94 is discharged through the ebw ( not shown in fig3 ), causing detonation . a test resistor 98 can be connected in parallel to the second svp 96 so the voltage in the discharge circuit 39 can be monitored for testing purposes without exposing the system operator to potentially hazardous high voltages . the bandpass filter 35 can also be a digital filter comprising a microprocessor ( not shown ) which samples the applied electrical power at spaced apart time intervals and compares the rate of change of the voltage to a programmed amount of rate of change of voltage . if a match is found , the applied power is passed through the filter . this type of digital filter is known in the art . the digital embodiment of the bandpass filter 35 is typically includes programmable cut - off characteristics . the digital bandpass filter can be programmed to have cut - off characteristics similar to the analog filter in the first embodiment of the invention , for example , a 1 khz passband filter can have - 30 db cut - off at 500 hz and - 30 db cutoff at 2 khz . it is to be understood that the digital filter can also be programmed to have extremely &# 34 ; sharp &# 34 ; cut - off characteristics , for example , a 1 khz passband filter could have - 30 db cut - offs of 900 and 1 , 100 hz . a particular advantage of the digital filter is that sharp cut - off response is possible while maintaining component sizes compatible with insertion of the filter 35 in the gun assembly ( 10 in fig1 ). fig4 shows an embodiment of the invention including a plurality of gun carriers 20 and a plurality of firing circuits 24 , each having a different passband frequency filter 35 , forming part of the same gun assembly 10 . the gun assembly in fig4 can be used for perforating a plurality of desired formations ( shown as 13 in fig1 ). detonation of a selected gun carrier 20 is performed by charging the cable 18 with ac having a frequency which substantially matches the passband frequency of the bandpass filter 35 in the gun carrier 20 which is desired to be detonated .