CA1128189A - Field fan filter - Google Patents
Field fan filterInfo
- Publication number
- CA1128189A CA1128189A CA306,674A CA306674A CA1128189A CA 1128189 A CA1128189 A CA 1128189A CA 306674 A CA306674 A CA 306674A CA 1128189 A CA1128189 A CA 1128189A
- Authority
- CA
- Canada
- Prior art keywords
- delay lines
- output
- coupled device
- signals
- fan filter
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
- G01V1/28—Processing seismic data, e.g. analysis, for interpretation, for correction
- G01V1/36—Effecting static or dynamic corrections on records, e.g. correcting spread; Correlating seismic signals; Eliminating effects of unwanted energy
- G01V1/364—Seismic filtering
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
- G01V1/16—Receiving elements for seismic signals; Arrangements or adaptations of receiving elements
- G01V1/20—Arrangements of receiving elements, e.g. geophone pattern
Abstract
FIELD FAN FILTER
ABSTRACT
Apparatus for fan filtering seismic data in the field comprising inputs for receiving signals from each seismometer in a group, summing amplifiers for summing the received signals by pairs, charge-coupled device delay lines for providing two different delays to each of the summed pairs, amplifiers for taking the difference of each of the two delayed signals from each summed pair, and means for weighting and summing the outputs of all pairs. The time delays of each of the charge-coupled device delay lines are proportional to and contolled by a single clock, which can be time-variable. The clock may be held at a single frequency during the taking of a given record, but in the preferred embodiment the clock is varied during recording to provide a narrower pass band of apparent velocities for deeper reflections than for the shallow reflections.
ABSTRACT
Apparatus for fan filtering seismic data in the field comprising inputs for receiving signals from each seismometer in a group, summing amplifiers for summing the received signals by pairs, charge-coupled device delay lines for providing two different delays to each of the summed pairs, amplifiers for taking the difference of each of the two delayed signals from each summed pair, and means for weighting and summing the outputs of all pairs. The time delays of each of the charge-coupled device delay lines are proportional to and contolled by a single clock, which can be time-variable. The clock may be held at a single frequency during the taking of a given record, but in the preferred embodiment the clock is varied during recording to provide a narrower pass band of apparent velocities for deeper reflections than for the shallow reflections.
Description
BACKGROI~ OF THE INVENTION
_ This invention relates to the fan filtering of seismic data and more particularly to a portable fan ~ilter suitable fo~ field use which includes means by which the filter characteristics ~ay be varied during the period o~ a seismic reco~d.
References which arta believed to be relevant to the present invention include Wide-band Velocity Filtering--The Pie Slice Process, by Peter Embree, et al., Geophysics, v. 28, No~ 6, December 1963, pp.
948-974; U.S. Patent 3,564,494, issued to Frasier, et al., on ~ebruaxy 16, 1971; and U.S. Patent 3,576,522, issued to Doty, et al., April 27, lg71 .
The Embree article contains a general description of the velocity filtering technique herein referred to as fan Eiltering. The velocity filterin~ technique is intended to improve signal-to-noise ratio by rejecting seismic signals impinging upon a seismic group or spread from directions which differ from the normal by ~ore or less than some preselected angle. Prior to the velocity filtering tech~ique, such signals were filtered by ~requency or tiMe-domain filters connected to the outputs of each seismometer and/or by spatial ~iltering, which amou~ts to placing a group of seismometers of a selected length with selected spacing along the surface of the e~rth. In one sense, the velocity filter combines the two filtering techniques into one with the result that some noise or unwanted signals which would previously be passed by both 8 spatial filter arrangement and a frequency filter arran8ement are removed by the velocity filter. A straig~tforwa~d embodiment of a device according to the teachings of E~bree ~ould include a lsrge nu~ber of ~requency or ti~e domain filters connected to the outputs of individual seismometers so th~t in effect the spatial frequency char~cteri~tics o~ a given spread would vary accordin~ to frequency of received sign~ls.
. ' ~
The Frasier and Doty pa~ents bo-th illustrate improvements to the Embree teaching. These patents each teach specific embodiments of a fan filter in which the large n~mber of frequency or time-domain filters required by Embree is reduced to a single filter. These patents teach that instead of filtering the output of each seismometer with a time-domain filter, the same result can be obtained by feeding each seismometer output into a time-delay element, tapping the delay elemen-t at two points, and subtracting the signal a-t one delay from the signal at a second delay. The dual-delayed and subtracted signals from each seismometer are then weighted and summed and then fed to a single frequency filter. The final result of this operation is a velocity filtering characteristic essentially identical to that -taugh-t by Embree.
It was recognized specifically in the Doty patent that the cut-off velocity of the filter taught by the patent could be adjusted by changing the time delays of the delay elements in unison. FIGURE 10 of the Doty pa-tent illustrates the filter adjustment by showing a variable-speed drive control connected to the transport drive which drives a magnetic recording drum which is the basic time-delay element disclosed by Doty.
Even with the improvements taught ~y the above-referenced patents, no fan filter i9 presently available which is truly useful in the field in the sense of being man-portable. That is, there has been no small, lightweight, low-power fan-filter device produced which would be practical for field operations. Each of these known devices presupposes -that the signal received at each seismometer is of sufficient strength and signal-to-noise ratio that it may be recorded on magnetic tape and played back later to be processed through one of the known fan-filtering or velocity-filtering devices. In many cases, this supposition is not correct and the signals, such as ground roll, which the velocity filter can remove, are of such strength that they totally mask the signal which is desired to be recorded from a single seismometer. ~or example, a large amplitude ground-roll signal may cause the recorders to reduce their input gain, ~hich is required to avoid saturation, to such a point that the desired signals are no longer detectable. It would be very desirable in such cases to remove the ground-roll signal prior to recording of the information.
Even where reasonable signals can be recorded from individual seismometers, such recording becomes complex when it is intended to la-ter velocity-filter the data in the apparatus of the prior art.
Without fan filtering, the prior field groups of, for instance, 12 seismometers were all wired in series or parallel and thus provided only a single electrical output and required only a single recording track.
Since these signals must be separated for velocity filtering, each seismometer must have its own pair of leads back to the recording truck ; and a separate recording track must be provlded for each seismometer, when fan filtering is done after recording. Where 12 seismometers are employed per group, this would required a 12-fold increase in the size of cables and recording capacity to make it possible -to velocity-filter the data later.
While it has been recognized in the prior apparatus using delay lines that the cut-off velocity of a velocity filter can be adjusted by adjusting the delays of all delay lines proportionally, no simple means for continuously adjusting the delay-line time periods have been provided and in particular no means for continuously adjusting the time periods during a given record have been provided.
Accordingly, an object of the present invention is to provide a portable velocity filter suitable for use in field operations.
Another object of the present invention is to provide a velocity filter having means for simply and continuously changing the cut-off velocity of a velocity filter.
These and other ob~ects are achie~ed by providing apparatus having a plurality of inputs for receiving signals from seismometers, summers for combining the received signals by pairs, charge-coupled device delay lines for provlding two differen~ delays to each of the summed signals, sub~ractors for taking the difference between ~he two differently delayed signals, and weishtins and summing means for combining each of the differences. In addition, there is provided a digital master cloc~ which control~ the time-delay of each of the charge-coupled device delay lines, including means for adjusting the clock frequency. The cut-off velocity of the resulting an filter is thus adjustable over a wide range and, in essence, is instantly adjustable by ch~nging the clock frequency.
In one aspect of this invention there is provided a portable fan filter for processing a group of seismic signals in the field comprising:
filter inputs adapted to receive signals from an even number of seismometers arranged in a linear spread;
a plurality of summing ~eans each having a pair of inputs with each pair of inputs conn~cted to a pair of the filter inputs selected to receive signals from a pair of seismometers symmetrically spaced ~bout the center of the seismometer spread and an output for providing the sum of the signals received from said pair o~ seismometers;
a plurality of charge ~oupled device delay lines equal to ~he numb~r 3f said s~mming means each having an input connected to the output o~ one of said summing means and a pair of outputs for providing a reproductlon of the signal received from sald summing me~ns with different time del~ys;
~ plur~l~ty o~ subtra~tor me~ns equal to the number of said delay lin~s each h~ving two inputs coupled to the two ~ 4 -outputs of one of the delay lines and an output or providing the difference between the two delay lines output s~gnals;
a plurality of welghting means equal to the number of said sub~ractor means each having an inpu~ connected to a subtractor output and an output for providing an amplitude adjusted reproduction o~ said subtractor output according to the position of the seismometers in the spread from which the signal originated; and, a final summing means having a plurality of inputs connected to the outputs of said weighting means and an output for providing the sum of said input signals.
In another aspect of his invention there is provided in a method of seismic prospacting in which acoustic energy reflected from subsurface interfaces is received by a plurality of seismometers and output signals from said seismometers are coupled to recording equipm~nt to be recorded for later pro~ess-ing, the improvement comprising coupl.ing the output signals from said seismometer~ to the recording equipment by means o~ a portable ~an filter po~itioned in the field near a group of seismometars, wherein said fan filter incorporates charge csupled device delay lines.
The pre~ent invention may be more completely under-stood by r~ading the follow~ng detailed description of the preferred embodiment with r~erence to the accomp~nylng drawing ~here~n:
~ IGURE 1 is a gen~rali~ed bloek and schematic di~gram of ~ fan filter ac~ordlng to th~ present lnventlon.
FIGURE 2 ls a deta~l~d s~hematic diagram o a representa~lve portion of FI~URE 1, FIGURE 3 is a detailed schematic d~agram of the weighting res:istors and output ~ummer portion of FIGU~E 1.
FIGURE 4 is a detailed schematic diagram of a multlple-frequency clock suitable ~or use wi~h the circuitry shown in FIGURES 1 and 2.
DESCRIPTION OF THE PREFERRED ~MBODIMENT
With reference to FIGURE 1 there is shown in block diagxam form the basic elements of a fan filter aocording to the present invention. The fil~er has twelve inpu~s labeled 1 through 12 in FIGURE 1. These inputs are adapted to receive the outputs of twelve
_ This invention relates to the fan filtering of seismic data and more particularly to a portable fan ~ilter suitable fo~ field use which includes means by which the filter characteristics ~ay be varied during the period o~ a seismic reco~d.
References which arta believed to be relevant to the present invention include Wide-band Velocity Filtering--The Pie Slice Process, by Peter Embree, et al., Geophysics, v. 28, No~ 6, December 1963, pp.
948-974; U.S. Patent 3,564,494, issued to Frasier, et al., on ~ebruaxy 16, 1971; and U.S. Patent 3,576,522, issued to Doty, et al., April 27, lg71 .
The Embree article contains a general description of the velocity filtering technique herein referred to as fan Eiltering. The velocity filterin~ technique is intended to improve signal-to-noise ratio by rejecting seismic signals impinging upon a seismic group or spread from directions which differ from the normal by ~ore or less than some preselected angle. Prior to the velocity filtering tech~ique, such signals were filtered by ~requency or tiMe-domain filters connected to the outputs of each seismometer and/or by spatial ~iltering, which amou~ts to placing a group of seismometers of a selected length with selected spacing along the surface of the e~rth. In one sense, the velocity filter combines the two filtering techniques into one with the result that some noise or unwanted signals which would previously be passed by both 8 spatial filter arrangement and a frequency filter arran8ement are removed by the velocity filter. A straig~tforwa~d embodiment of a device according to the teachings of E~bree ~ould include a lsrge nu~ber of ~requency or ti~e domain filters connected to the outputs of individual seismometers so th~t in effect the spatial frequency char~cteri~tics o~ a given spread would vary accordin~ to frequency of received sign~ls.
. ' ~
The Frasier and Doty pa~ents bo-th illustrate improvements to the Embree teaching. These patents each teach specific embodiments of a fan filter in which the large n~mber of frequency or time-domain filters required by Embree is reduced to a single filter. These patents teach that instead of filtering the output of each seismometer with a time-domain filter, the same result can be obtained by feeding each seismometer output into a time-delay element, tapping the delay elemen-t at two points, and subtracting the signal a-t one delay from the signal at a second delay. The dual-delayed and subtracted signals from each seismometer are then weighted and summed and then fed to a single frequency filter. The final result of this operation is a velocity filtering characteristic essentially identical to that -taugh-t by Embree.
It was recognized specifically in the Doty patent that the cut-off velocity of the filter taught by the patent could be adjusted by changing the time delays of the delay elements in unison. FIGURE 10 of the Doty pa-tent illustrates the filter adjustment by showing a variable-speed drive control connected to the transport drive which drives a magnetic recording drum which is the basic time-delay element disclosed by Doty.
Even with the improvements taught ~y the above-referenced patents, no fan filter i9 presently available which is truly useful in the field in the sense of being man-portable. That is, there has been no small, lightweight, low-power fan-filter device produced which would be practical for field operations. Each of these known devices presupposes -that the signal received at each seismometer is of sufficient strength and signal-to-noise ratio that it may be recorded on magnetic tape and played back later to be processed through one of the known fan-filtering or velocity-filtering devices. In many cases, this supposition is not correct and the signals, such as ground roll, which the velocity filter can remove, are of such strength that they totally mask the signal which is desired to be recorded from a single seismometer. ~or example, a large amplitude ground-roll signal may cause the recorders to reduce their input gain, ~hich is required to avoid saturation, to such a point that the desired signals are no longer detectable. It would be very desirable in such cases to remove the ground-roll signal prior to recording of the information.
Even where reasonable signals can be recorded from individual seismometers, such recording becomes complex when it is intended to la-ter velocity-filter the data in the apparatus of the prior art.
Without fan filtering, the prior field groups of, for instance, 12 seismometers were all wired in series or parallel and thus provided only a single electrical output and required only a single recording track.
Since these signals must be separated for velocity filtering, each seismometer must have its own pair of leads back to the recording truck ; and a separate recording track must be provlded for each seismometer, when fan filtering is done after recording. Where 12 seismometers are employed per group, this would required a 12-fold increase in the size of cables and recording capacity to make it possible -to velocity-filter the data later.
While it has been recognized in the prior apparatus using delay lines that the cut-off velocity of a velocity filter can be adjusted by adjusting the delays of all delay lines proportionally, no simple means for continuously adjusting the delay-line time periods have been provided and in particular no means for continuously adjusting the time periods during a given record have been provided.
Accordingly, an object of the present invention is to provide a portable velocity filter suitable for use in field operations.
Another object of the present invention is to provide a velocity filter having means for simply and continuously changing the cut-off velocity of a velocity filter.
These and other ob~ects are achie~ed by providing apparatus having a plurality of inputs for receiving signals from seismometers, summers for combining the received signals by pairs, charge-coupled device delay lines for provlding two differen~ delays to each of the summed signals, sub~ractors for taking the difference between ~he two differently delayed signals, and weishtins and summing means for combining each of the differences. In addition, there is provided a digital master cloc~ which control~ the time-delay of each of the charge-coupled device delay lines, including means for adjusting the clock frequency. The cut-off velocity of the resulting an filter is thus adjustable over a wide range and, in essence, is instantly adjustable by ch~nging the clock frequency.
In one aspect of this invention there is provided a portable fan filter for processing a group of seismic signals in the field comprising:
filter inputs adapted to receive signals from an even number of seismometers arranged in a linear spread;
a plurality of summing ~eans each having a pair of inputs with each pair of inputs conn~cted to a pair of the filter inputs selected to receive signals from a pair of seismometers symmetrically spaced ~bout the center of the seismometer spread and an output for providing the sum of the signals received from said pair o~ seismometers;
a plurality of charge ~oupled device delay lines equal to ~he numb~r 3f said s~mming means each having an input connected to the output o~ one of said summing means and a pair of outputs for providing a reproductlon of the signal received from sald summing me~ns with different time del~ys;
~ plur~l~ty o~ subtra~tor me~ns equal to the number of said delay lin~s each h~ving two inputs coupled to the two ~ 4 -outputs of one of the delay lines and an output or providing the difference between the two delay lines output s~gnals;
a plurality of welghting means equal to the number of said sub~ractor means each having an inpu~ connected to a subtractor output and an output for providing an amplitude adjusted reproduction o~ said subtractor output according to the position of the seismometers in the spread from which the signal originated; and, a final summing means having a plurality of inputs connected to the outputs of said weighting means and an output for providing the sum of said input signals.
In another aspect of his invention there is provided in a method of seismic prospacting in which acoustic energy reflected from subsurface interfaces is received by a plurality of seismometers and output signals from said seismometers are coupled to recording equipm~nt to be recorded for later pro~ess-ing, the improvement comprising coupl.ing the output signals from said seismometer~ to the recording equipment by means o~ a portable ~an filter po~itioned in the field near a group of seismometars, wherein said fan filter incorporates charge csupled device delay lines.
The pre~ent invention may be more completely under-stood by r~ading the follow~ng detailed description of the preferred embodiment with r~erence to the accomp~nylng drawing ~here~n:
~ IGURE 1 is a gen~rali~ed bloek and schematic di~gram of ~ fan filter ac~ordlng to th~ present lnventlon.
FIGURE 2 ls a deta~l~d s~hematic diagram o a representa~lve portion of FI~URE 1, FIGURE 3 is a detailed schematic d~agram of the weighting res:istors and output ~ummer portion of FIGU~E 1.
FIGURE 4 is a detailed schematic diagram of a multlple-frequency clock suitable ~or use wi~h the circuitry shown in FIGURES 1 and 2.
DESCRIPTION OF THE PREFERRED ~MBODIMENT
With reference to FIGURE 1 there is shown in block diagxam form the basic elements of a fan filter aocording to the present invention. The fil~er has twelve inpu~s labeled 1 through 12 in FIGURE 1. These inputs are adapted to receive the outputs of twelve
2~
seismometers spaced in a linear array. In the preferred embodiment it is assumed that these twelve seismorneters are spaced 50 feet apart in the linear spread. As used herein, the term seismometer means either a single seismic detector or a subgroup or subarray comprising a number of seismic detectors connected in series or parallel to provide a single seismic signal output. Six summers numbered 13 through 18 are provided for summing the signals received on inputs 1 through 12 in pairs. Each pair which :is summed is symmetrically spaced about the center of the array, which is mid-way between seismometers 6 and 7. Thus, summer 13 adds the outputs of seismometers 6 and 7; summer 14 adds -the outputs of seismometers 5 and 8; etc. The outputs of summers 13 through 18 are connected to the inputs of dual-delay lines 19 through 24, respectively.
Each of the dual-delay lines provides two different time delays to the same input signal. The time delays indicated in FIGURE 1 correspond to a typical arrangement in which the seismometers are spaced 50 feet apart. The outputs of each of the dual-delay lines 19 through 24 are connected to positive and negative inputs of summers 25 through 30, respectively. In this way the summers 25 through 30 provide at their outputs the difference between the two differently time-delayed signals.
The ou-tputs of the summers 25 through 30 are connected through resistors 31 through 369 respectively, to the input of a summing amplifier 37.
The resistors 31 through 36 provide weighting of the seismometer output signals in accordance with their distance from the cen-ter of the array as is well-known in the art of fan filtering. Amplifier 37 provides a composite output signal to an output 38, which is adapted for connection to a seismic recorder. The recorder would be typically either a truck-mounted analog or digital system, as is well-known in the art, but may also be a portable field recorder unit. The signal on output 38 is treated in the same manner as the prior art treated the output of a serially or parallel-connected group of 12 seismometers with one e~ception. ~s is known in the velocity filtering art~ the signal on output 38 is a transformed 90@ pnlse. Before this signal is converted to a visual form, it should be corrected by a filter such as the -filter illustrated as element 62 of Fig. 4 oE the above-referenced U. S. Patent
seismometers spaced in a linear array. In the preferred embodiment it is assumed that these twelve seismorneters are spaced 50 feet apart in the linear spread. As used herein, the term seismometer means either a single seismic detector or a subgroup or subarray comprising a number of seismic detectors connected in series or parallel to provide a single seismic signal output. Six summers numbered 13 through 18 are provided for summing the signals received on inputs 1 through 12 in pairs. Each pair which :is summed is symmetrically spaced about the center of the array, which is mid-way between seismometers 6 and 7. Thus, summer 13 adds the outputs of seismometers 6 and 7; summer 14 adds -the outputs of seismometers 5 and 8; etc. The outputs of summers 13 through 18 are connected to the inputs of dual-delay lines 19 through 24, respectively.
Each of the dual-delay lines provides two different time delays to the same input signal. The time delays indicated in FIGURE 1 correspond to a typical arrangement in which the seismometers are spaced 50 feet apart. The outputs of each of the dual-delay lines 19 through 24 are connected to positive and negative inputs of summers 25 through 30, respectively. In this way the summers 25 through 30 provide at their outputs the difference between the two differently time-delayed signals.
The ou-tputs of the summers 25 through 30 are connected through resistors 31 through 369 respectively, to the input of a summing amplifier 37.
The resistors 31 through 36 provide weighting of the seismometer output signals in accordance with their distance from the cen-ter of the array as is well-known in the art of fan filtering. Amplifier 37 provides a composite output signal to an output 38, which is adapted for connection to a seismic recorder. The recorder would be typically either a truck-mounted analog or digital system, as is well-known in the art, but may also be a portable field recorder unit. The signal on output 38 is treated in the same manner as the prior art treated the output of a serially or parallel-connected group of 12 seismometers with one e~ception. ~s is known in the velocity filtering art~ the signal on output 38 is a transformed 90@ pnlse. Before this signal is converted to a visual form, it should be corrected by a filter such as the -filter illustrated as element 62 of Fig. 4 oE the above-referenced U. S. Patent
3,564,494. Alternatively, this filter function can be performed by a general purpose computer at the time of processing the recorded data.
With the 50-foot seismometer spacing and the delay time intervals indicated in FIGU~E I the velocity filter in FIGURE 1 provides a velocity cut-off at 6 dB attenuation of 25,000 ft/sec.
With reference now to FIGURE 2, there is shown a de-tailed schematic diagram for a portion of FIGURE 1 corresponding, for example, -to inputs 1 and 12, summer 18, delay elements 24, and summer 30. While in FIGURE 1 an input such as 1 is shown as having a single lead, each seismometer actually has a pair of wires carrying its signal to the `
input. In the preferred embodiment, summer 18 is a differential input integrated circui-t amplifier 40 having input and feedback resistors arranged to provide the summing of the inputs and a gain of eight. The two leads from seismometer No. 1 may be connec-ted ~o inputs 42, for example, while the two leads from seismometer No. 12 may be connected to inputs 44. Each of the four leads is connected to amplifier 40 by means of 25K ohm resistors. A 200K ohm feedback resistor sets the gain of this amplifier at eight and a 0.001 microfarad feedback capacitor provides a cut-off frequency of approximately 500 Hz. The output of amplifier 40 is coupled to the input of an anti-alias filter comprising a second differential amplifier 46 and associated input and feedback resistors and capacitors. This filter illustrated in FIGURE 2 has a cut-off frequency of 250 Hz with a slope of twelve d~/octave and in addition provides a gain of four. This anti-alias filter is not part of the apparatus illustrated in FIGURE 1 and is not essential to the operation of a velocity filter, but is included in the preferred - ~8~-embodiment for reasons well-known in the seismic prospecting art. An anti-alias filter could be placed in the lead connecting each seismometer to filter inputs 1 through 12 instead of after the summing amplifier, but this would require more filters and is not preferred.
The output of amplifier 46 is connected to two inpu-ts of a dual-delay element 48. In this preferred embodiment, delay element 48 is a dual 512-stage analog delay line sold by Reticon Corporation of 910 Benicia Avenue, Sunnyvale, California, under the Part No. SAD-1024. The pin numbers shown in FIGURE 2 for delay element 48 correspond to this preferred part which is provided in a 16-pin, dual in-line package.
- Both of the 512-bucket shift registers contained in element 48 receive the same input and provide two different delays for this input by having different clock frequencies applied to each of the two shift registers.
Each of the two shift registers requires a two-phase clock input and therefore four clock inputs 50 are provided by a dual flip-flop device 52. The dual Elip-flop used for device 52 in the preferred embodiment is manufactured by RCA Corporation of Somerville, New Jersey, and sold under the Part No. CD4013. Device 52 receives clock signals on inputs 54, which are connected to the circuitry illustrated in FIGURE 4 and described later. The pin numbers indicated for device 52 correspond to the sta~dard 14-pin, dual in-line package provided by RCA.
Each of the delay lines in device 48 has two outputs and both outputs of each are used so that the output is available during an entire clock period. These two pairs o outputs numbered 56 and 58 are coupled through identical resistors to the inpu-ts of a smoothing filter comprising a differential amplifier 60 and associated resistors and capacitors as illustrated in FIGU~E 2. This arran~ement actually provides two separa-te but identical filters, one being for the outputs 56, which are coupled to the negative input of amplifier 60, and the other for outputs 58, which are coupled to the posi-tive input of _ `~g~ _ `~2~
amplifier 60. These filters have a cut-off frequency of 250 Hz and a slope of 12 dB/octave. In addition, the input and feedback arrangement provides a gain of eight in ampliEier 60. The output of amplifier 60 is the difference between the two signals appearing in outputs 56 and 58 of the delay element 48. ~he output of amplifier 60 therefore corresponds to the output of the sum~ing element 30 in FIGURE 1. In addition, a resistor 62 is shown by means of dotted line connections to be bypassing signal from the output of amplifier 46 to the negative input of amplifier 60. ~eference to FIGU~E 1 shows that the delay element 23 includes one delay of zero time. In general, only one of the delay elements in any velocity filter would require an actual zero time delay.
That one element would contain resistor 62 to bypass the delay element 48 and thereby provide zero time delay. With this one exception, that is resistor 62, the circuitry illustrated in FIGURE 2 is used six times to provide -the six different channels il]ustrated in FIGURE 1 With reference now to FIGURE 3, There is illustrated the weighting and final summing portion of the circuitry of FIGURE 1 in greater detail. Resistors 31 through 36 and differential amplifier 37 are the same as illustrated in :FIGURE 1. In addition, there is illustrated the feedback and balancing resistors and capacitors used in the preferred embodiment.
With reference now to FIGURE 4, there is illustrated the circuitry which provides the eleven different clock frequencies necessary to generate the twelve time delays illustrated in FIGURE 1.
FIGURE 4 includes a basic clock oscillator illustrated generally at 64 and down-counting circuitry illustrated at 66. Clock 64 is a digital multi-vibrator, comprising two N0~ gates 68 having inputs of each coupled together so that they act as inverters. A capacitar 70, resistor 72, and n-channel FET 74 set the time period of this multi-vibrator. Capacitor 70 is variable to adjust the basic clock frequency _ ~Q _ ancl the resistance oL F~T 74 is adjusted by controlling the voltage on its gate input 76. For the time periods illustrated in FIGURE 1 these adjustments are set to provide a clock period of 3.906 microseconds at the output 78 of clock 64. In this preferred embodiment, gates 68 are Type CD4001A, also manufactured by RCA Corporation. Output 78 is coupled to the input of another NOR gate 80, which acts as a buffer for driving the inputs to the count-down circuitry 66.
The count-down circuitry 66 is basically a set of digital count-down circuits designed to provide the ten frequencies in addition to the basic clock frequency which are used to provide the eleven different time delays in addition to the zero time delay illustrated for the time-delay units in FIGURF 1. The eleven outputs of the clock and cormt-down circuits are numbered 82 through g2 and are also labeled with the time-delay each output provides. Each of these outputs 82 through 92 is connected to a clock input such as inputs 54, shown in FIGURE 2, in accordance with the desired time delays. It is apparent that -the reason that different clock frequencies are needed is that each of the delay units used in the preferred embodiment have a Eixed number of storage buckets, that is 512, so that various time delays can be achieved by using different clocking frequencies. In practice, this is a simpler procedure than would be providing various length shift registers, all clocked with the same frequency. In the event such shift registers were available, such an arrangement would be quite suitable.
Output 82 is connected directly to the output of gate 80 to provide the basic clock frequency which therefore provides a basic 2-ms time delay. Outputs 83, 84, and 85, which provide frequencies for 4-, 8-, and 16-ms time delays are simply the outputs oi the first three stages of a binary counter device 94, which in the preferred embodiment is a Type CD4024 integrated circuit produced by RCA Corporation. Output 86 provides a clock frequency for generating a 6-ms delay by use of an RC~ Type CD4018 divide by N counter 96 in conjunction with a portion of an RCA Type CD~011 gate package 98 to set the device 96 into a divide-by-three configuration. Output 87 prov:ides a frequency of half of that of output 86 by dividing output 86 by two in a RCA Type 4013 flip-flop unit 100. In essentially the same manner, outputs 88 and ~9 provide clock frequencies for 10- and 20-ms time delays, respectively, by means of circuits 10~ and 104 arranged in a divide~by-five configuration and de~ice 106 dividing the ou-tput 8~ signal by two. Output 90 provides a frequency for a time-delay of 14 ms, again by arrangement of Type 4018 and Type 4011 devices 108 and 110 in a divide-by-seYen configuration.
Again, output 91 provides a frequency for an ]8-ms time delay by means of devices 112 and 114 in a divide by nine configuration. The final divider output 92 provides a frequency for a 22-ms time delay by means of an ~CA Type CD4029 up-down counter used in combination with a NOR
gate 81 (which is a portion of the device including NOR gates 68 and 80) to provide divide-by-eleven function.
While clock 64 and divider circuitry 66 have been shown in terms of specific apparatus, it is apparent that other voltage controlled oscillators which are well-known in the art could bè used in place of -the basic ~lock illustrated here at 64 and other digital circuitry could provide the various digital down-counting functions provided by circuitry 66. As noted above, the dividing circuitry 66 could be totally eliminated if the analog delay lines were each of the appropriate length, instead of each being of the same length. It would also be appropriate to make clock 64 manually or mechanically adjustable by means of, for example, a potentiometer placed in series or parallel with resistor 72 instead of by means of the FFT transistor 7~.
In operation, all of the circuitry illustra-ted in FIG~S 1 through 4 is connected together, as illustrated, in a man-portable battery-powered package. The inputs l through 12 of FIGURE 1 are each --]~12 -connected to ind:ividual seismometers in a group of 12 seismometers placed in a linear spread at, for example, 50-foot spacing. Output 38 of FIGURE I is then co~mected to a master cable lead connected to a recording truck, or in the al-ternative, connected to the input of a field recorder. If a single velocity cut-off is desired, the clock 64 would then be set -to provide a 3.906-ms per:iod so that the exact time delays illustrated in FIGURE 1 would be provided in the velocity filter.
Other clock periods can be selected if other cut-off velocities are desired. A seismic initiation would then be made and the returns sensed by seismometers connected to inputs 1 through 12 would then be processed by the filter of FIGURE 1 and provided on output 38 for recording.
The preferred use of the apparatus illustrated in FIGVRES 1 through 4 is to vary the velocity cut-off of the filter during the recording of the returns from each initiation. This is achieved by synchronizing a voltage ramp to start each -time that an initiation occurs. The voltage ramp signal is applied to input 76 of the clock 64.
This ramp causes the output frequency of clock 64 to sweep in proportion to the voltage at the input. The changing clock frequency causes the time delays illustrated in FIGURE 1 to vary with time. In this way, the velocity cut-off of the velocity filter can ~e varied with time during initiation. It is desirable during the early part of a return to have a lower-velocity cutoff since desirable returns from shallow layers approach the seismometer group at steeper angles than those from deeper layers. Since the returns from shallow layers also occur earlier in time than those from deeper layers, it is a simple matter to vary the clock frequency so that there is a low-velocity cut off in the early portion of the returns and a higher velocity cutoff in the later portion of the returns.
While it is apparent that no particular sweep of velocity cut-offs with time would be optimum for every area of seismic exploration, it is also apparent that the circuitry illustrated herein provides a wide degree of flexibility, since the velocity cut-off is voltage controllable. Thus, for example, in a land cable truck recording sys-tem it would be appropriate to provide a control vol~age line from the truck to the various velocity filters in the field so that the operator of the recording unit could vary the velocity filter cut-off a-t will. In a radio control group recorder type system it would be appropriate to include in the group calling code a selection of cut-off velocities or preprogrammed velocity ramps.
It is apparent that other modifications and changes can be made in the apparatus of -the present invention without depar-ting from the scope of the presen-t invention as defined by the appended claims.
- ?~-
With the 50-foot seismometer spacing and the delay time intervals indicated in FIGU~E I the velocity filter in FIGURE 1 provides a velocity cut-off at 6 dB attenuation of 25,000 ft/sec.
With reference now to FIGURE 2, there is shown a de-tailed schematic diagram for a portion of FIGURE 1 corresponding, for example, -to inputs 1 and 12, summer 18, delay elements 24, and summer 30. While in FIGURE 1 an input such as 1 is shown as having a single lead, each seismometer actually has a pair of wires carrying its signal to the `
input. In the preferred embodiment, summer 18 is a differential input integrated circui-t amplifier 40 having input and feedback resistors arranged to provide the summing of the inputs and a gain of eight. The two leads from seismometer No. 1 may be connec-ted ~o inputs 42, for example, while the two leads from seismometer No. 12 may be connected to inputs 44. Each of the four leads is connected to amplifier 40 by means of 25K ohm resistors. A 200K ohm feedback resistor sets the gain of this amplifier at eight and a 0.001 microfarad feedback capacitor provides a cut-off frequency of approximately 500 Hz. The output of amplifier 40 is coupled to the input of an anti-alias filter comprising a second differential amplifier 46 and associated input and feedback resistors and capacitors. This filter illustrated in FIGURE 2 has a cut-off frequency of 250 Hz with a slope of twelve d~/octave and in addition provides a gain of four. This anti-alias filter is not part of the apparatus illustrated in FIGURE 1 and is not essential to the operation of a velocity filter, but is included in the preferred - ~8~-embodiment for reasons well-known in the seismic prospecting art. An anti-alias filter could be placed in the lead connecting each seismometer to filter inputs 1 through 12 instead of after the summing amplifier, but this would require more filters and is not preferred.
The output of amplifier 46 is connected to two inpu-ts of a dual-delay element 48. In this preferred embodiment, delay element 48 is a dual 512-stage analog delay line sold by Reticon Corporation of 910 Benicia Avenue, Sunnyvale, California, under the Part No. SAD-1024. The pin numbers shown in FIGURE 2 for delay element 48 correspond to this preferred part which is provided in a 16-pin, dual in-line package.
- Both of the 512-bucket shift registers contained in element 48 receive the same input and provide two different delays for this input by having different clock frequencies applied to each of the two shift registers.
Each of the two shift registers requires a two-phase clock input and therefore four clock inputs 50 are provided by a dual flip-flop device 52. The dual Elip-flop used for device 52 in the preferred embodiment is manufactured by RCA Corporation of Somerville, New Jersey, and sold under the Part No. CD4013. Device 52 receives clock signals on inputs 54, which are connected to the circuitry illustrated in FIGURE 4 and described later. The pin numbers indicated for device 52 correspond to the sta~dard 14-pin, dual in-line package provided by RCA.
Each of the delay lines in device 48 has two outputs and both outputs of each are used so that the output is available during an entire clock period. These two pairs o outputs numbered 56 and 58 are coupled through identical resistors to the inpu-ts of a smoothing filter comprising a differential amplifier 60 and associated resistors and capacitors as illustrated in FIGU~E 2. This arran~ement actually provides two separa-te but identical filters, one being for the outputs 56, which are coupled to the negative input of amplifier 60, and the other for outputs 58, which are coupled to the posi-tive input of _ `~g~ _ `~2~
amplifier 60. These filters have a cut-off frequency of 250 Hz and a slope of 12 dB/octave. In addition, the input and feedback arrangement provides a gain of eight in ampliEier 60. The output of amplifier 60 is the difference between the two signals appearing in outputs 56 and 58 of the delay element 48. ~he output of amplifier 60 therefore corresponds to the output of the sum~ing element 30 in FIGURE 1. In addition, a resistor 62 is shown by means of dotted line connections to be bypassing signal from the output of amplifier 46 to the negative input of amplifier 60. ~eference to FIGU~E 1 shows that the delay element 23 includes one delay of zero time. In general, only one of the delay elements in any velocity filter would require an actual zero time delay.
That one element would contain resistor 62 to bypass the delay element 48 and thereby provide zero time delay. With this one exception, that is resistor 62, the circuitry illustrated in FIGURE 2 is used six times to provide -the six different channels il]ustrated in FIGURE 1 With reference now to FIGURE 3, There is illustrated the weighting and final summing portion of the circuitry of FIGURE 1 in greater detail. Resistors 31 through 36 and differential amplifier 37 are the same as illustrated in :FIGURE 1. In addition, there is illustrated the feedback and balancing resistors and capacitors used in the preferred embodiment.
With reference now to FIGURE 4, there is illustrated the circuitry which provides the eleven different clock frequencies necessary to generate the twelve time delays illustrated in FIGURE 1.
FIGURE 4 includes a basic clock oscillator illustrated generally at 64 and down-counting circuitry illustrated at 66. Clock 64 is a digital multi-vibrator, comprising two N0~ gates 68 having inputs of each coupled together so that they act as inverters. A capacitar 70, resistor 72, and n-channel FET 74 set the time period of this multi-vibrator. Capacitor 70 is variable to adjust the basic clock frequency _ ~Q _ ancl the resistance oL F~T 74 is adjusted by controlling the voltage on its gate input 76. For the time periods illustrated in FIGURE 1 these adjustments are set to provide a clock period of 3.906 microseconds at the output 78 of clock 64. In this preferred embodiment, gates 68 are Type CD4001A, also manufactured by RCA Corporation. Output 78 is coupled to the input of another NOR gate 80, which acts as a buffer for driving the inputs to the count-down circuitry 66.
The count-down circuitry 66 is basically a set of digital count-down circuits designed to provide the ten frequencies in addition to the basic clock frequency which are used to provide the eleven different time delays in addition to the zero time delay illustrated for the time-delay units in FIGURF 1. The eleven outputs of the clock and cormt-down circuits are numbered 82 through g2 and are also labeled with the time-delay each output provides. Each of these outputs 82 through 92 is connected to a clock input such as inputs 54, shown in FIGURE 2, in accordance with the desired time delays. It is apparent that -the reason that different clock frequencies are needed is that each of the delay units used in the preferred embodiment have a Eixed number of storage buckets, that is 512, so that various time delays can be achieved by using different clocking frequencies. In practice, this is a simpler procedure than would be providing various length shift registers, all clocked with the same frequency. In the event such shift registers were available, such an arrangement would be quite suitable.
Output 82 is connected directly to the output of gate 80 to provide the basic clock frequency which therefore provides a basic 2-ms time delay. Outputs 83, 84, and 85, which provide frequencies for 4-, 8-, and 16-ms time delays are simply the outputs oi the first three stages of a binary counter device 94, which in the preferred embodiment is a Type CD4024 integrated circuit produced by RCA Corporation. Output 86 provides a clock frequency for generating a 6-ms delay by use of an RC~ Type CD4018 divide by N counter 96 in conjunction with a portion of an RCA Type CD~011 gate package 98 to set the device 96 into a divide-by-three configuration. Output 87 prov:ides a frequency of half of that of output 86 by dividing output 86 by two in a RCA Type 4013 flip-flop unit 100. In essentially the same manner, outputs 88 and ~9 provide clock frequencies for 10- and 20-ms time delays, respectively, by means of circuits 10~ and 104 arranged in a divide~by-five configuration and de~ice 106 dividing the ou-tput 8~ signal by two. Output 90 provides a frequency for a time-delay of 14 ms, again by arrangement of Type 4018 and Type 4011 devices 108 and 110 in a divide-by-seYen configuration.
Again, output 91 provides a frequency for an ]8-ms time delay by means of devices 112 and 114 in a divide by nine configuration. The final divider output 92 provides a frequency for a 22-ms time delay by means of an ~CA Type CD4029 up-down counter used in combination with a NOR
gate 81 (which is a portion of the device including NOR gates 68 and 80) to provide divide-by-eleven function.
While clock 64 and divider circuitry 66 have been shown in terms of specific apparatus, it is apparent that other voltage controlled oscillators which are well-known in the art could bè used in place of -the basic ~lock illustrated here at 64 and other digital circuitry could provide the various digital down-counting functions provided by circuitry 66. As noted above, the dividing circuitry 66 could be totally eliminated if the analog delay lines were each of the appropriate length, instead of each being of the same length. It would also be appropriate to make clock 64 manually or mechanically adjustable by means of, for example, a potentiometer placed in series or parallel with resistor 72 instead of by means of the FFT transistor 7~.
In operation, all of the circuitry illustra-ted in FIG~S 1 through 4 is connected together, as illustrated, in a man-portable battery-powered package. The inputs l through 12 of FIGURE 1 are each --]~12 -connected to ind:ividual seismometers in a group of 12 seismometers placed in a linear spread at, for example, 50-foot spacing. Output 38 of FIGURE I is then co~mected to a master cable lead connected to a recording truck, or in the al-ternative, connected to the input of a field recorder. If a single velocity cut-off is desired, the clock 64 would then be set -to provide a 3.906-ms per:iod so that the exact time delays illustrated in FIGURE 1 would be provided in the velocity filter.
Other clock periods can be selected if other cut-off velocities are desired. A seismic initiation would then be made and the returns sensed by seismometers connected to inputs 1 through 12 would then be processed by the filter of FIGURE 1 and provided on output 38 for recording.
The preferred use of the apparatus illustrated in FIGVRES 1 through 4 is to vary the velocity cut-off of the filter during the recording of the returns from each initiation. This is achieved by synchronizing a voltage ramp to start each -time that an initiation occurs. The voltage ramp signal is applied to input 76 of the clock 64.
This ramp causes the output frequency of clock 64 to sweep in proportion to the voltage at the input. The changing clock frequency causes the time delays illustrated in FIGURE 1 to vary with time. In this way, the velocity cut-off of the velocity filter can ~e varied with time during initiation. It is desirable during the early part of a return to have a lower-velocity cutoff since desirable returns from shallow layers approach the seismometer group at steeper angles than those from deeper layers. Since the returns from shallow layers also occur earlier in time than those from deeper layers, it is a simple matter to vary the clock frequency so that there is a low-velocity cut off in the early portion of the returns and a higher velocity cutoff in the later portion of the returns.
While it is apparent that no particular sweep of velocity cut-offs with time would be optimum for every area of seismic exploration, it is also apparent that the circuitry illustrated herein provides a wide degree of flexibility, since the velocity cut-off is voltage controllable. Thus, for example, in a land cable truck recording sys-tem it would be appropriate to provide a control vol~age line from the truck to the various velocity filters in the field so that the operator of the recording unit could vary the velocity filter cut-off a-t will. In a radio control group recorder type system it would be appropriate to include in the group calling code a selection of cut-off velocities or preprogrammed velocity ramps.
It is apparent that other modifications and changes can be made in the apparatus of -the present invention without depar-ting from the scope of the presen-t invention as defined by the appended claims.
- ?~-
Claims (21)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A portable fan filter for processing a group of seismic signals in the field comprising:
filter inputs adapted to receive signals from an even number of seismometers arranged in a linear spread;
a plurality of summing means each having a pair of inputs with each pair of inputs connected to a pair of the filter inpwts selected to receive signals from a pair of seismometers symmetrically spaced about the center of the seismometer spread and an output for providing the sum of the signals received from said pair of seismometers;
a plurality of charge coupled device delay lines equal to the number of said summing means each having an input connected to the output of one of said summing means and a pair of outputs for providing a reproduction of the signal received from said summing means with different time delays;
a plurality of subtractor means equal to the number of said delay lines each having two inputs coupled to the two outputs of one of the delay lines and an output for providing the difference between the two delay line output signals;
a plurality of weighting means equal to the number of said subtractor means each having an input connected to a subtractor output and an output for providing an amplitude adjusted reproduction of said subtractor output according to the position of the seismometers in the spread from which the signal originated; and a final summing means having a plurality of inputs connected to the outputs of said weighting means and an output for providing the sum of said input signals.
filter inputs adapted to receive signals from an even number of seismometers arranged in a linear spread;
a plurality of summing means each having a pair of inputs with each pair of inputs connected to a pair of the filter inpwts selected to receive signals from a pair of seismometers symmetrically spaced about the center of the seismometer spread and an output for providing the sum of the signals received from said pair of seismometers;
a plurality of charge coupled device delay lines equal to the number of said summing means each having an input connected to the output of one of said summing means and a pair of outputs for providing a reproduction of the signal received from said summing means with different time delays;
a plurality of subtractor means equal to the number of said delay lines each having two inputs coupled to the two outputs of one of the delay lines and an output for providing the difference between the two delay line output signals;
a plurality of weighting means equal to the number of said subtractor means each having an input connected to a subtractor output and an output for providing an amplitude adjusted reproduction of said subtractor output according to the position of the seismometers in the spread from which the signal originated; and a final summing means having a plurality of inputs connected to the outputs of said weighting means and an output for providing the sum of said input signals.
2. A portable fan filter according to Claim 1 wherein each of said delay lines comprises a pair of solid state charge coupled device delay lines.
3. A portable fan filter according to Claim 2 wherein each delay line has the same number of delay elements and further includes clocking circuitry for providing a different clock frequency to each delay line selected to provide the different time delays.
4. A portable fan filter according to Claim 3 wherein said clocking circuitry includes a variable frequency oscillator from which each of the different clock frequencies is derived, whereby said different clock frequencies are synchronized and may all be proportionally varied by varying the oscillator frequency.
5. A portable fan filter according to Claim 4 wherein said variable frequency oscillator is voltage controllable.
6. In a method of seismic prospecting in which acoustic energy reflected from subsurface interfaces is received by a plurality of seismometers and output signals from said seismometers are coupled to recording equipment to be recorded for later processing, the improvement comprising coupling the output signals from said seismometers to the recording equipment by means of a portable fan filter positioned in the field near a group of seismometers, the fan filter circuit being characterized by the incorporation of charge coupled device delay lines for providing different time delays and a master clock means for controlling the time delays of the charge coupled device delay lines, the fan filter circuit cutoff velocity being proportional to the time delays of the charge coupled device delay lines.
7. The improved method according to Claim 6 wherein said portable fan filter comprises:
filter inputs adapted to receive signals from an even number of seismometers;
a plurality of summing means each having a pair of inputs with each pair of inputs connected to a pair of the filter inputs selected to receive signals from a pair of seismometers symmetrically spaced about the center of the seismometer spread and an output for providing the sum of the signals received from said pair of seismometers;
a plurality of charge coupled device delay lines equal to the number of said summing means each having an input connected to the output of one of said summing means and a pair of outputs for providing a reproduction of the signal received from said summing means with different time delays;
a plurality of subtractor means equal to the number of said delay lines each having two inputs coupled to the two outputs of one of the delay lines and an output for providing the difference between the two delay line output signals;
a plurality of weighting means equal to the number of said subtractor means each having an input connected to a subtractor output and an output for providing an amplitude adjusted reproduction of said subtractor output according to the position of the seismometers in the spread from which the signal originated; and a final summing means having a plurality of inputs connected to the outputs of said weighting means and an output for providing the sum of said input signals.
filter inputs adapted to receive signals from an even number of seismometers;
a plurality of summing means each having a pair of inputs with each pair of inputs connected to a pair of the filter inputs selected to receive signals from a pair of seismometers symmetrically spaced about the center of the seismometer spread and an output for providing the sum of the signals received from said pair of seismometers;
a plurality of charge coupled device delay lines equal to the number of said summing means each having an input connected to the output of one of said summing means and a pair of outputs for providing a reproduction of the signal received from said summing means with different time delays;
a plurality of subtractor means equal to the number of said delay lines each having two inputs coupled to the two outputs of one of the delay lines and an output for providing the difference between the two delay line output signals;
a plurality of weighting means equal to the number of said subtractor means each having an input connected to a subtractor output and an output for providing an amplitude adjusted reproduction of said subtractor output according to the position of the seismometers in the spread from which the signal originated; and a final summing means having a plurality of inputs connected to the outputs of said weighting means and an output for providing the sum of said input signals.
8. An improved method according to Claim 6 wherein the fan filter cutoff velocity is proportional to the time delays of said delay lines and the time delays are controlled by a variable frequency clock, further including the step of varying the clock frequency during recording of returns from a single seismic initiation, whereby the cutoff velocity is varied during recording of returns from said single seismic initiation.
9. In a method of seismic prospecting in which acoustic energy reflected from subsurface interfaces in response to a seismic initiation is detected by a plurality of seismometers which produce signals correlated to the detected acoustic energy and the seismometer signals are coupled to a data storage means to be stored for later pro-cessing, the improvement including the step of coupling the seismometer signals through a fan filter circuit to the data storage means, the fan filter circuit being characterized by the incorporation of charge coupled device delay lines for providing different time delays and a master clock means for controlling the time delay of each of the charge coupled device delay lines, the fan filter circuit being man-portable for facilitating deployment of the fan filter circuit in the field together with the seismometers for on-site velocity filtering of the seismometer signals prior to storage by the data storage means, the fan filter circuit cutoff velocity being proportional to the time delays of the charge coupled device delay lines.
10. A method according to Claim 9, further including the step of synchronously and proportionally varying the different time delays of the charge coupled device delay lines under control of the master clock means for adjusting the fan filter circuit cutoff velocity from one seismic initiation to another.
11. A method according to Claim 9, further including the step of synchronously and proportionally varying the different time delays of the charge coupled device delay lines under control of the master clock means during the receipt of the returns from a single seismic initiation, whereby the fan filter circuit cutoff velocity is adjusted during the receipt of the returns from the single seismic initiation.
12. A method according to Claim 11, wherein the different time delays of the charge coupled device delay lines are varied as a ramp function of time.
13. A method according to Claim 11, wherein the different time delays of the charge coupled device delay lines are varied during the receipt of the returns from the single seismic initiation so that there is a low-velocity cutoff in the early portion of the returns and a higher velocity cutoff in the later portion of the returns to provide a narrower pass band of apparent velocities for deep reflections than for shallow reflections.
14. A method according to Claim 9, wherein the master clock means includes an oscillator having a selectively variable frequency, further including the step of varying the oscillator frequency for synchronously and proportionally varying the different time delays of the charge coupled device delay lines, whereby the fan filter circuit cutoff velocity can be adjusted from one seismic initiation to another.
15. A method according to Claim 9, wherein the master clock means includes an oscillator having a selectively variable frequency, further including the step of varying the oscillator frequency for synchronously and proportionally varying the different time delays of the charge coupled device delay lines during the receipt of the returns from a single seismic initiation, whereby the fan filter circuit cutoff velocity is adjusted during the receipt of the returns from the single seismic initiation.
16. A method according to Claim 15, wherein the different time delays of the charge coupled device delay lines are varied as a function of time.
17. A method according to Claim 15, wherein the different time delays of the charge coupled device delay lines are varied during the receipt of the returns from the single seismic initiation so that there is a low-velocity cutoff in the early portion of the returns and a higher velocity cutoff in the later portion of the returns to provide a narrower pass band of apparent velocities for deep reflections than for shallow reflections.
18. A man-portable fan filter circuit suitable for use in the method according to Claim 9 and comprising filter inputs adapted to receive signals from an even number of seismometers arranged in a linear spread; a plurality of summing means each having a pair of inputs connected to a respective pair of filter inputs for receiving signals from a pair of seismometers symmetrically spaced about the center of the seismometer spread and an output for providing the sum of the received pair of seismometer signals; a plurality of charge coupled device delay lines equal to the number of summing means each having an input connected to the output of a respective one of the summing means and a pair of outputs for providing a reproduction of the signal received from the summing means with different time delays controlled by a master clock means; a plurality of subtractor means equal to the number of charge coupled device delay lines each having two inputs connected respectively to the two outputs of a respective one of the charge coupled device delay lines and an output for providing the difference between the received charge coupled device delay line signals; a plurality of weighting means equal to the number of subtractor means each having an input connected to a respective subtractor output for providing an amplitude adjusted reproduction of the signal received from the sub-tractor means correlated to the position of the pair of seismometers in the seismometer spread; and a final summing means having a plurality of inputs connected respectively to the outputs of the weighting means and an output for providing the sum of the received weighting means signals, thereby producing a velocity filtered signal.
19. A man-portable fan filter circuit according to Claim 18, wherein the master clock means provides different clock frequencies to the charge coupled device delay lines for producing different time delays.
20. A man-portable fan filter circuit according to Claim 19, wherein the master clock means includes an oscillator having a selectively variable frequency from which each of the different clock frequencies is derived, whereby the different clock frequencies are synchronized and may all be proportionally varied by varying the oscillator frequency.
21. A man-portable fan filter circuit according to Claim 20, wherein the oscillator is voltage controllable.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US83510077A | 1977-09-21 | 1977-09-21 | |
| US835,100 | 1977-09-21 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1128189A true CA1128189A (en) | 1982-07-20 |
Family
ID=25268577
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA306,674A Expired CA1128189A (en) | 1977-09-21 | 1978-06-30 | Field fan filter |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA1128189A (en) |
| DE (1) | DE2841125A1 (en) |
| FR (1) | FR2404234A1 (en) |
| GB (1) | GB2004648B (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4486865A (en) * | 1980-09-02 | 1984-12-04 | Mobil Oil Corporation | Pressure and velocity detectors for seismic exploration |
| US4935903A (en) * | 1989-05-30 | 1990-06-19 | Halliburton Geophysical Services, Inc. | Reinforcement of surface seismic wavefields |
| US4937793A (en) * | 1989-05-30 | 1990-06-26 | Halliburton Geophysical Services, Inc. | Processing method for marine seismic surveying utilizing dual streamers |
| EG19158A (en) * | 1989-08-25 | 1996-02-29 | Halliburton Geophys Service | System for attenuation of water-column reverberation |
| US5774416A (en) * | 1995-04-07 | 1998-06-30 | Pgs, Tensor, Inc. | Method and device for attenuating water column reverberations using co-located hydrophones and geophones in ocean bottom seismic processing |
| US5621699A (en) * | 1995-07-07 | 1997-04-15 | Pgs Ocean Bottom Seismic, Inc. | Apparatus and method of calibrating vertical particle velocity detector and pressure detector in a sea-floor cable with in-situ passive monitoring |
| US5754492A (en) * | 1996-02-12 | 1998-05-19 | Pgs Tensor, Inc. | Method of reverberation removal from seismic data and removal of dual sensor coupling errors |
| EP2751599A4 (en) * | 2011-11-18 | 2016-06-29 | Geco Technology Bv | Coherent noise attenuation |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3274541A (en) * | 1963-02-15 | 1966-09-20 | Texas Instruments Inc | Seismic velocity filtering |
| US3576522A (en) * | 1966-08-01 | 1971-04-27 | Continental Oil Co | Method and apparatus for analog velocity filtering |
| CA970461A (en) * | 1971-07-29 | 1975-07-01 | Atlantic Richfield Company | Velocity selective array |
| US3877056A (en) * | 1973-01-02 | 1975-04-08 | Texas Instruments Inc | Charge transfer device signal processing system |
| US4035628A (en) * | 1975-10-24 | 1977-07-12 | Westinghouse Electric Corporation | Analog transversal filtering and correlation with progressive summation of analog signals |
-
1978
- 1978-06-30 CA CA306,674A patent/CA1128189A/en not_active Expired
- 1978-08-21 FR FR7824281A patent/FR2404234A1/en active Pending
- 1978-09-07 GB GB7836024A patent/GB2004648B/en not_active Expired
- 1978-09-21 DE DE19782841125 patent/DE2841125A1/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| GB2004648A (en) | 1979-04-04 |
| GB2004648B (en) | 1982-09-22 |
| DE2841125A1 (en) | 1979-03-29 |
| FR2404234A1 (en) | 1979-04-20 |
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