Note: Descriptions are shown in the official language in which they were submitted.
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
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
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
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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.
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