Nov. 15, 1960
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`A. 1.. PARRACK
`APPARATUS RELATED TO SEISMIC EXPLORATION
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`2,960,176
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`Filed Oct. 16, 1957
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`2 Sheets-Sheet l
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`PGS Exhibit 1015
`PGS v. WG
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`

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`Nov. 15, 1960
`
`A. |_. PARRACK
`APPARATUS RELATED To SEISMIC EXPLORATION
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`2,960,1 76
`
`Filed Oct. 16, 1957
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`2 Sheets-Sheet 2
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`i?
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`PGS Exhibit 1015
`PGS v. WG
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`

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`tent
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`are
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`. ICC
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`2,960,176
`Patented Nov. 15, 1960
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`United 5
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`2,960,176
`APPARATUS RELATED TO SEISMIC
`EXPLORATION
`Alvin L. Parrack, Bellaire, Tex., assignor to
`Texaco Inc., a corporation of Delaware
`Filed Oct. 16, 1957, Ser. No. 690,562
`1 Claim. (c1. 181-5)
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`This invention is concerned with seismic exploration
`in general, and more speci?cally with apparatus for elimi
`nating the effects of a “following,” or “ghost” wave.
`In re?ection type seismic exploration that is carried
`out while employing a high-explosive'charge to generate
`the seismic energy, it is most common to detonate such
`a charge ‘beneath the surface of the earth at a distance
`su?icient that the detonation takes place within the sub
`‘surface formations of the earth, i.e., where the density
`is such that the propagation of seismic energy takes place
`at a relatively high velocity and with good propagation.
`The seismic energy that is generated by such a detona
`tion travels outwardly from the point of detonation in all
`directions.‘ For this reason there is created, in addition
`to a desired downward traveling wave, a secondary, or
`following wave which is the product of the energy that
`traveled upward from the detonation and then was re
`?ected back down again from the surface (or the bottom
`of ‘the weathered layer). Such following wave is harm
`ful' with respect to the interpretation of the results in
`determining the presence of re?ecting sub-surface layers.
`This is so because the “following” wave adds an addi
`tional seismic signal that tends to confuse the signals as ‘
`they are received by a transducer at the surface of the
`earth. ‘Furthermore, the “following” wave may exist in
`van overlapping manner with the direct wave that has
`traveled only from the detonated charge downward to a
`re?ecting surface and back up to the transducer.
`Consequently, it is an object of this invention to over
`come the di?iculties indicated above in connection with
`the following wave that exists in conjunction with re
`?ection seismic prospecting.
`Another object of this invention is to disclose appa
`ratus for substantially eliminating the effects of the fol
`lowing wave ‘as it exists upon detection at the surface
`of the earth.
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`_
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`Brie?y, the invention is concerned‘ with seismic ex
`ploration, and relates to apparatus for eliminating the
`effects of a following wave coming later in time than a
`direct wave emanating from the same source. The in
`vention comprises ?ltering and attenuating said direct
`wave to create a third wave having substantially the same
`shape and amplitude as said following wave. The inven
`_ tion also comprises delaying said third wave for a time
`duration equal to the time between the commencement
`of .saiddirect and following waves, and applying said
`third wave in opposition to said following wave to can—
`'cel out the effects thereof, leaving said direct wave unob
`scured.
`The above and other objects and bene?ts of the inven
`tion will appear more fully below, and in connection
`with the speci?cation which is illustrated in the drawings,
`‘in which:
`i' 'Fig. 1 is a schematic diagram taken as a cross-section
`of a bit of the earth’s surface, and illustrating the paths
`of travel of seismic waves;
`Fig. 2 is an idealized diagram showing representative
`traces illustrating theiseismic waves after transformation ’
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`2
`by the transducer and at various stages throughout the
`process; and
`Fig. 3 is a block diagram illustrating one arrangement‘
`of elements which might be employed for carrying out
`the invention.
`Referring to Fig. 1, it is pointed out that this invention
`is concerned with a method useful in connection with re
`?ection seismic exploration, wherein there is employed the
`detonation of a high-explosive charge C which would
`be located beneath the surface 11 of the earth, as indi
`cated in the diagram of Fig. 1. Such charge C is located
`some distance beneath an upper re?ecting horizon, such
`as the weathered layer indicated in Fig. 1 by the dashed
`line W1—W1.
`Of major importance are seismic wave indications that
`have been re?ected from sub-surface layers, e.g., a hori
`zon indicated in Fig. 1 as R1—R1. The presence of this
`energy is determined at or near the surface 11 of the
`earth by means of a seismic transducer or detector D.
`Of course more than one such detector will usually be
`employed, but re?ection seismic techniques are generally
`well-known so that no further reference to the details
`thereof need be given here. It will be clear to anyone
`skilled in the art that this invention is described in terms
`of a single channel of seismic information only.
`It is to be noted that whenever the high explosive charge
`is detonated, there are two paths of travel for the seismic
`energy that is propagated, or emanates from the location
`of the charge C, that are of interest in this disclosure.
`These two paths are indicated in Fig. l by the solid and
`dotted lines respectively. The solid line path C, R, D
`is the path of travel for a direct re?ected wave. While
`the dotted line C, W, R, D is the path of travel for a
`“following” or “ghost” wave that is created by reason of
`the energy which travels upward from the charge and is
`re?ected back down from the surface of the weathered
`layer.
`It is to be noted that the coefficient of re?ection of the
`surface (or weathered layer) is negative, so that the “fol
`lowing” wave becomes reversed in phase .relative to the
`direct wave, just described above. This “ghost” or “fol
`lowing” wave is undesirable since it adds confusion to the
`waves being received at the detector. Furthermore, the
`following wave may overlap the direct wave, and the
`more complete the overlap the more the direct wave
`will be obscured. Also the following wave may at times
`overlap the direct wave of another re?ector which may
`exist close to and below the re?ector shown in Figure 1
`thereby creating confusion when one attempts to inter
`pret the direct wave from the deeper re?ector, even though
`the following wave should not overlap its own direct
`wave.
`Apparatus for eliminating the effects of the following
`wave (in re?ection seismic operations or other similar
`?elds) in accordance with this invention, is best described
`in connection with the illustration of Fig. 2. In Fig. 2,
`there is shown a trace A which represents, in an idealized
`manner, the seismic energy waves that are received by
`the detector D illustrated in Fig. 1 (after automatic gain
`control has been applied).
`'
`It is pointed out that the direct wave and its following
`wave (as described above) are shown in Fig. 2 as spaced
`apart considerably. This is done merely for clarity of
`the description, and it will be understood that the princi
`ples are the same whatever the spacing involved.
`It will be noted that there is a wave 12, marked pulse
`1 in Fig. 2, which represents the seismic wave that ?rst
`reaches the transducer or detector D. This is the wave
`commonly known as the P-wave. Then after an interval
`of time, the next wave 13 that is illustrated, is indicated
`as being pulse 2. This pulse, or wave 13, represents the
`direct re?ected wave which has reached detector D over
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`a direct path from the charge to the re?ecting surface
`and back, e.g., path C, R, D of Fig. 1. Finally, there is
`illustrated a third wave 14, marked pulse 3, which is
`somewhat lesser in amplitude and has a more stretched
`out shape by reason of the further travel through the
`earth. This wave 14 represents the following wave which
`has arrived at the detector D traveling over a path in
`cluding the distance up to the surface re?ecting layer and
`back in addition to the direct path, e.g., the path indicated
`by the dotted line C, W, R, D of Fig. 1.
`It will be noted that wave 14 is reversed in phase rela
`tive to wave 13, as well as being somewhat different in
`shape. This is because of the negative coefficient of re
`?ection at the surface, as mentioned above, and on ac
`count of the higher frequency components having been
`additionally attenuated by the greater distance of travel
`through the earth.
`Trace B illustrates waves 18 and 19 that have been
`derived from Waves 13 and 14 respectively. It Will be
`noted that in Fig. 2, trace B is shown vertically aligned
`beneath trace A, such that wave 18 is in line with wave
`14. Furthermore, it is to be noted that (as indicated by
`the oblique dashed line) trace B is derived from trace A,
`beginning with a point between wave 12 and Wave 13.
`Now it is pointed out that by adding trace A with trace
`B, in an algebraic manner, the resulting trace A+B will
`contain a wave 121 (pulse 1) and a wave 131 (pulse 2),
`but will have no following wave, e.g., wave 14, although
`a wave 191 will now be included.
`-
`With the above diagrammatic explanation, it will be
`appreciated that the apparatus of this invention as so
`far described, includes means to carry out the following
`steps: beginning with a direct re?ected wave and its fol
`lowing wave, (1) ?ltering this direct wave to change its
`shape so as to match that of its following wave, (2) at
`tenuating the amplitude of the direct wave so as to make
`it substantially equal the amplitude of the following Wave,
`and (3) delaying it in time for a period equal to the time
`between the commencement of the direct wave and the
`commencement of its following wave. Finally, (4) ap
`plying the resulting wave in opposition to the following
`wave to cancel out the latter, e.g., by algebraically adding
`them together. When these steps have been carried out,
`the waves illustrated on trace A+B will result.
`After the above-described steps have been carried out,
`there still remains a'wave which has not been canceled,
`i.e., the wave 191 found on trace A+B. However, by
`repeating the same steps again, but beginning with the
`waves 18 and 19, a new wave 20 followed by wave 28,
`may be formed so as to match the shape of the wave 191
`in amplitude and frequency components. Then by de
`laying additionally the occurrence of waves 20 and 28 rela
`tive to waves 18 and 19, the resulting wave 20 Will be
`applied in opposition to Wave 191 to cancel out the latter,
`e.g., by adding them algebraically. Thus a combined sig
`nal such as that illustrated by trace A+B+C will result.
`It is pointed out that by thus making two complete
`combining procedures, the ?rst “following” wave and in
`addition the newly created lesser following wave, will be
`both eliminated, and the only undesired wave remaining
`will be a relatively low amplitude and low frequency
`pulse as illustrated by wave 28' in Fig. 2. It will be ap
`preciated also, that the method could be carried out for
`additional complete combining procedures as desired, in
`order to cancel wave '28’ and other remaining waves (not
`shown) and leave only a still lesser wave (not shown),
`which would be of little consequence. The determina
`tion as to how far to carry the process of eliminating un
`desired remaining waves, will depend upon the economic
`considerations as well as the amplitudes which are dis
`turbing to the ?nal results.
`It is pointed out that in the description set forth above,
`the “following” wave has a phase opposite to that of its
`direct wave that precedes it, on account of the fact that
`most surface re?ection layers will have a negative co
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`efficient of re?ection. However, the invention could be
`practiced equally well if the coef?cient were positive;
`by merely inverting the direct wave, along with the
`?ltering, attenuating and delaying thereof. Consequently
`such a step will be understood in the event of a positive
`coe?icient.
`Referring to Fig. 3, it is pointed out that there is here
`illustrated one arrangement of apparatus which might be
`employed to carry out the steps of this invention. Many
`other and different arrangements and combinations of
`elements for carrying out the invention will suggest them
`selves to anyone skilled in the art.
`There is shown a transducer or detector 15 which is
`the source of electric waves corresponding to the seismic
`wave energy, e.g., as picked up at location D in the Fig.
`1 illustration. The signals generated by detector 15 will
`be transferred into an automatic gain control circuit 16.
`At the same time these signals are fed to a time delay
`switch control circuit 17.
`Automatic gain control circuit 16 may take various
`forms, and preferably is one of the types well known in
`the art of re?ection seismic exploration, wherein the
`variation in amplitude of the received seismic energy is
`held relatively constant in order to facilitate the making
`of a record thereof. The purpose for the time delay
`circuit 17 will appear more fully below. However, the
`details of this circuit form no part, per se, of the inven
`tion and any feasible type of elements and circuit ar
`rangement may be employed.
`Output signals from the automatic gain control cir
`cuit 16 are carried simultaneously over two paths, as in
`dicated. One is a direct path 21 that carried the signals
`comparable to trace A, as indicated by the letter (A)
`on the drawing. The other path is indicated as a path
`22 which leads to a switch 23 that is normally open
`when not actuated. From the other side of the switch
`23, the path continues to a ?lter 24. The path then con~
`tinues to an attenuator 25 and from there to a delay line
`26.
`The ?lter 24, attenuator 25 and delay line 26 are
`conventional electronic elements which might take vari
`ous different forms for accomplishing the desired re
`sults. The circuits for these elements are not part of
`this invention per se, and thus will not be illustrated.
`It will be noted that the time delay circuit 17 ac
`tuatesv switch 23, as indicated by a dashed line 27.
`The output signals from the delay line 26 are carried
`over two paths simultaneously. One path is represented
`by a line 31 that illustrates the fact that these signals are
`added directly with the signals that are being transmitted
`over the path 21. Thus a new path 32 carried the
`algebraically combined signals, e.g., the idealized trace
`which is pictorially represented in Fig. 2 by the trace
`A+B.v
`At the same time, the signals from delay line 26
`are carried‘ over a path 33 to another ?lter 34 and then
`from the output thereof to an attenuator 35, and after
`that to' a delay line 36. The output signals from delay
`line 36 are carried over a path 37 to join with the sig
`nals existing on path 32 so that, after an algebraic com
`bination of these signals, the combined total will be
`carried over a path 38 that carried the combined sig
`nals; e.g., which are illustrated as having the idealized
`form as represented by the trace A+B+C of Fig. 2.
`The combined signals may then be carried to a recorder
`39-for transforming them into ‘an observable form, such
`as by employing an oscillograph in the usual manner as
`employed in seismic exploration operations.
`It will be noted that the block diagram of Fig. 3 is
`not a circuit showing in any sense. For example the
`output of delay line 26 will have to be separated, of
`course, when the electrical circuits are considered; be
`cause‘the input of ?lter 34 must not include‘ any of the
`‘signals being carried on path 21 (illustrated by trace A).
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`PGS Exhibit 1015
`PGS v. WG
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`

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`2,960,176
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`block diagram shown in Fig. 3 might take widely dif
`It will be observed that with the above-described
`ferent forms. It is contemplated that the delay lines
`schematically illustrated apparatus, the process according
`26 and 36 may be in the form of magnetic delay ele
`to the illustrations of Fig. 2 may be carried out. How
`ments, since the period of delay involved would not be
`ever, if it should be desired to employ one or more ad
`expected to be too great. The delay periods will vary,
`ditional operations of delaying and canceling the remain
`of course, depending upon the velocity of seismic wave
`ing signals, e.g. wave 281, it would involve the use of
`In addition, the
`‘ propagation for a given detonation.
`additional ,sets of elements such as the ?lter, attenuator
`time ‘would be dependent upon whether or not the meth
`and delay line elements 41, 42 and 43 indicated in Fig. 3
`‘ od is carried out directly, as the signals are created by
`in dashed lines.
`the'detector 15 (from the detonation of a charge in the
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`The various steps of a method that may be carried
`earth), or whether the signals to be operated upon, are
`out with apparatus according to this invention may be
`those taken from a record previously made. For exam
`followed with reference to Fig. 3 in the following manner:
`ple, the signals might be those obtained by means of
`Signals generated by detector 15 are transmitted via
`magnetic recording of the signals which were received
`AGC circuit 16 to a direct transmission path 21 and
`when a charge was detonated. In the latter case the
`15
`simultaneously to a path 22 that includes the switch 23.‘
`vtime delay involved must be varied depending upon the
`At the same time, the same output signals from detector
`speed of play back used in reconstituting the signals as
`15. are carried to a control element in the time delay
`they exist on the magnetic record.
`switch 17, so that upon the arrival of the ?rst energy
`Likewise, time delay switch element 17 may take vari
`or P-wave (illustrated as wave 12 in Fig. 2), the in- V
`ous forms among which are commercially available units.
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`ternal controls of the time delay circuit 17 will be set
`In addition the switch 23 might be any feasible type of
`in action for causing the switch 23 to be closed, after
`electric circuit breaking structure; or it might take the
`a predetermined period. Thus, some time after the
`form of an electronic switch wherein no mechanical mov
`P-wave signal has occurred, switch 23 will be closed
`ing parts are involved.
`and the signals from detector 15 will be transmitted to
`While a particular embodiment of the invention has
`the ?lter 24 and attenuator 25 at the same time, as they
`been shown and described in considerable detail in ac
`are being transmitted over the direct path 21. Thus, the
`cordance with the applicable statutes, this is not to be
`shape of the direct wave, e.g., Wave 13 of Fig. 2, will
`taken as in any way limiting the invention but merely
`be ?ltered and attenuated as desired to form an al
`as being descriptive thereof.
`tered wave, e.g., wave 18, and then will be delivered to
`the input of the delay line 26.
`It is claimed:
`.
`‘In seismic exploration, a device for elimination of
`By having delay line 26 adjustable, the delay period
`“ghost” waves comprising a transducer and a recorder,
`may be set to correspond to the time between-the com
`a ?rst circuit means directly interconnecting said trans
`mencement of the direct wave and its following wave,
`ducer and recorder, a second circuit means in shunt
`e.g., waves 13 and 14, so that the ?ltered and attenuated
`with said ?rst circuit means having ?lter, attenuation
`wave (18) will be delivered to the point where the
`and delay means therein, a switch means in said sec
`direct signals are combined with the delayed signals,
`ond circuit, and a time delay means operatively con
`at the proper time so as to cancel the direct wave (as
`nected to said transducer and to said switch, whereby
`indicated by the above description related to Fig. 2).
`a signal from said transducer passes directly to said
`Then similarly in order to cancel the remaining wave,
`recorder and at a later time passes indirectly in modi
`which is illustrated as wave 19 in Fig. 2; the output
`?ed form to said recorder to eliminate “ghost” waves.
`signals from delay line 26 are carried to the ?lter 34
`and attenuator 35 to be delivered to the delay line 36.
`Here again the wave is shaped by the ?lter and atten
`uator to match the remaining wave that is to be can
`celed; and then delayed prior to mixing, as before, so
`that the newly created wave, e.g., wave 20, will combine
`in a canceling manner with wave 191 (which was pres
`ent on the path 32). .
`Thus the ?nal result will be signals such as those il
`lustrated'by the trace A+B+C of Fig. 2. This will ex
`ist on the path 38, and will be delivered to the recorder
`39 for display. '
`‘
`‘It will be observed that the various elements of the
`
`References Cited in the ?le of this patent
`UNITED STATES PATENTS
`Rieber ____'_ _________ __ Nov. 9, 1954
`Silverman ____________ ..._ Ian. 29, 1957
`Dobrin _____________ __ Apr. 21, 1959
`OTHER REFERENCES
`Van Melle et al.: “Ghost Re?ections Caused by En
`ergy Initially Re?ected Above the Level of the Shot,”
`Geophysics Magazine, October 1953, vol. 18, pages 793
`804.
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`PGS Exhibit 1015
`PGS v. WG