Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/003—Seismic data acquisition in general, e.g. survey design
  • G01V1/006—Seismic data acquisition in general, e.g. survey design generating single signals by using more than one generator, e.g. beam steering or focusing arrays
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/38—Seismology; Seismic or acoustic prospecting or detecting specially adapted for water-covered areas
  • G01V1/3861—Seismology; Seismic or acoustic prospecting or detecting specially adapted for water-covered areas control of source arrays, e.g. for far field control

Definitions

• the present invention relates to a seismic source, in particular to a source for use in marine seismic surveying.
• the present invention also relates to a marine seismic surveying arrangement including a source, to a method of operating the source and to a method of de-ghosting marine seismic data.
• the principle of marine seismic surveying is shown schematically in Figure 1. Seismic energy emitted in a generally downwards direction from a source of seismic energy 1 is reflected by the sea bed 2 and by the earth strata or geological structures beneath the sea bed, and is received by an array of seismic receivers 3 such as hydrophones. Analysis of the energy received at the receiving array 3 can provide information about the earth strata or geological structures beneath the seabed.
• the source of seismic energy 1 is suspended from a survey vessel 4 and the array of seismic receivers 3 is towed by the survey vessel 3.
• Ghost reflections occur when upwardly travelling seismic energy is reflected or scattered downwards at the sea surface.
• a related problem in marine seismic surveying is that of "reverberations”. Reverberations occur when seismic energy is reflected between the sea surface and the sea-bed. The problems of ghost reflections and reverberations are explained in Figures 2(a) to 2(d).
• Figure 2(a) shows a "primary reflection". Seismic energy is emitted downwards by the source 1 , is reflected by a geological feature below the sea bed, and the reflected signal is detected at the receiver 3. An analysis of the seismic signal generated by the primary reflection provides information about the geological feature responsible for reflecting the seismic energy. (In practice, refraction may occur at the sea-bed, but this has been omitted from Figures 2(a) to 2(d) for clarity.)
• Figure 2(b) shows a ghost reflection. Seismic energy that has been emitted upwards by the source is reflected or scattered downwards by the sea surface. The seismic energy that is reflected or scattered downwards may then be incident on the target geological feature, undergo reflection, and be reflected to the receiver.
• Seismic energy that follows the path shown in Figure 2(b) will have a different travel time from the source to the receiver than will energy that follows the primary path of Figure 2(a).
• ghost reflections are an undesirable source of contamination of seismic data since they tend to obscure the interpretation of data produced by the primary reflection.
• Figures 2(c) and 2(d) show reverberations, in which seismic energy undergoes reflections between the sea-bed and the sea-surface.
• Reverberations can occur in the case of seismic energy emitted in an up-going direction by the source ( Figure 2(c)) and also in the case of seismic energy emitted in a down-going direction by the source ( Figure 2(d)).
• reverberations are an undesirable source of contamination of seismic data, since they obscure the interpretation of the primary reflection from the earth's interior.
• Figures 2(b), 2(c) and 2(d) show source-side ghost reflections and reverberations - that is, ghost reflections and reverberations that occur before the seismic energy is reflected by the target geological structure. (Indeed it will be noted that the path of seismic energy shown in Figure 2(d) does not involve a reflection by the target geological structure.) Ghost reflections and reverberations can also occur after the seismic energy has been reflected from the target geological structure, and these are known as receiver- side ghost reflections or reverberations.
• An alternative method for attenuating ghost reflections and reverberations is to use two receivers located at different depths This method is based on the principle that waves travelling in different directions will have spatial de ⁇ vatives of different signs, so that compa ⁇ ng the signal obtained at one receiver with the signal obtained by the other receiver will allow the up-gomg wavefield to be separated from the down-going wavefield
• the use of two arrays of emitters of seismic energy at different depths allows the up- going and down-going seismic wavefields to be separated from one another, as will be described below.
• the effect of source-side ghost reflections and reverberations on the seismic data can be reduced or eliminated.
• a second aspect of the present invention provides a marine seismic surveying arrangement comprising a marine seismic receiver; and a seismic source as defined above; means for moving the seismic source; and one or more seismic receivers.
• a third aspect of the present invention provides a method of operating a marine seismic source as defined above, the method comprising the steps of: moving the seismic source at a speed v along the first direction; firing one of the first and second arrays of emitters of seismic energy; and firing the other of the first and second arrays of emitters of seismic energy after a time d ⁇ /v.
• the time delay of di/v between the firings of the two arrays of seismic sources ensures that each emitter of one array is fired at the same point in the x- and y-directions as the corresponding emitter of the other array, but at different depths. This allows the seismic data generated by one of the arrays to be used to de- ghost the seismic data generated by the other of the arrays.
• a fourth aspect of the present invention provides a method of processing marine seismic data comprising the steps of: firing a first emitter of seismic energy at a point in a fluid medium having components (xi, yi, Zi), and detecting the resultant first seismic data at a receiver array; firing a second emitter of seismic energy at a point in the fluid medium having components (x-, y-, z 2 ), where z- ⁇ z 2 , and detecting the resultant second seismic data at the receiver array; and us ' ing the second' seismic data to reduce the effects of source-side reflections and/or scattering at. the sea surface on the first seismic data.
• Figure 1 is a schematic view of a typical manne seismic surveying arrangement
• Figures 2(a) to 2(d) are schematic illustrations of the problems of ghost reflections and reverberations
• Figure 3 is a schematic view of a vertical source array illustrating the principles of the de-ghostmg method of the present invention
• Figure 4 is a schematic illustration of a vertical source array according to an embodiment of the present invention.
• Figure 5 shows a typical seismic signal recorded by a receiver in a manne seismic surveying arrangement that contains a seismic source according to an embodiment of the present invention
• Figure 6 shows the signal of Figure 5 after processing to attenuate source-side ghost reflections and reverberations
• Figure 7 illustrates the average amplitude spectrum of the signal of Figure 5
• Figure 8 illustrates the average amplitude of the signal of Figure 6
• Figure 3 illustrates the general pnnciple of the de-ghostmg method of the present invention
• Figure 3 shows a vertical source array that consists of two emitters of seismic energy SI and S2 that have identical emission characteristics to one another The emitters are disposed m the water at two different depths
• the upper emitter SI is disposed substantially vertically above the lower emitter S2
• the source array generates a seismic wavefield that has both up-gomg and down-gomg components
• the wavefield travelling upwards generates source-side ghost reflections and up-going reverberations in the water layer.
• the wavefield travelling downwards from the source array generates the primary reflection and also generates down-going reverberations.
• u(t) is the up-going source wavefield and d(f) is the down-going source wavefield emitted by the hypothetical emitter S.
• the emitters SI and S2 generate up-going and down-going wavefields. These wavefields can be described,- relative to time t, by the following equations:
• Si is the wavefield emitted by the upper emitter S 1 and S 2 is the wavefield emitted by the lower emitter S2.
• the time dt is the time that seismic energy would take to travel from the upper or lower emitter SI or S2 to the position of the hypothetical emitter S. Since the hypothetical emitter S is at the mid-point between the upper emitter SI and the lower emitter S2, the time dt is equal to half the time taken for seismic energy to travel between the upper emitter S] and the lower emitter S 2 or vice versa.
• equations (2) and (3) can be expanded using a first-order Taylor expansion, as follows:
• Si (t) u(t) - u'(t)dt + d(t) + d'(t)dt (4)
• u'(t) and d'(t) are the time derivatives of u(t) and d(t)l, respectively.
• Equations (6) and (8) may now be combined, to eliminate u(t). This leads to the following expression for the down-going source wavefield d(t):
• a surveying arrangement with an anay of seismic sources at point A and a receiver at point B gives a certain signal at the receiver
• a receiver array at point A and a single source at point B would lead to the same signal, provided that the source anay conesponds to the receiver array.
• the source array contains the same number of sources as the receiver array has receivers, and that the sources in the source array are arranged in the same locations relative to one another as the receivers in the receiver array.
• the above discussion relates to a vertical source array that contains just two emitters, with one emitter being disposed above the other.
• a source that comprises a first array of two or more emitters of seismic energy disposed above a second array of two or more emitters of seismic energy.
• each emitter array must contain the same number of emitters, and each emitter in one anay must have identical emission characteristics to the conesponding emitter in the other anay.
• the relative anangement and separation of the emitters in one anay must be the same as the relative anangement and separation of the emitters in the other anay.
• the two separate wavefields required for the de-ghosting method could also be obtained by using firing a single emitter at one depth, altering the depth of the emitter, and firing the emitter again.
• this method would also be inconvenient to carry out.
• a staggered vertical source consisting of two emitters or of two emitter anays, with, in use, one emitter or emitter anay being disposed at one depth and the other being disposed at a different depth.
• the two emitters, or two emitter anays are displaced horizontally with respect to one another.
• the source is moved through the water in the direction along which the emitters, or emitter anays, are displaced. There is a time delay between the firing of one of the emitters or emitter anays and the firing of the other emitter or emitter anay.
• the time delay between the firings and the speed of movement of the source are chosen such that, in the case of a source having just two emitters, the point at which the upper emitter is fired has the same x- and y-co-ordinates as the point at which the lower emitter is fired.
• the time delay between firing one anay and firing the other anay is chosen so that the point at which an emitter in one anay is fired has the same x- and y-co-ordinates as the point at which the conesponding emitter in the other anay is -fired, for all emitters in the array.
• the invention makes it straightforward to generate identical seismic wavefields at different depths but at the same x- and y-co-ordinates.
• the seismic data generated by one wavefield can then be used to de-ghost the seismic data generated by the other wavefield, using equation (9) above.
• Figure 4 shows an embodiment of the invention in which the source includes two arrays 10, 11 each having two emitters of seismic energy SI 1, SI 2, S21, S22.
• the four emitters SI 1, SI 2, S21, S22 have substantially identical emission charactenstics to one another
• the separation between the two emitters SI 1, S 12 of the first anay 10 is substantially equal to the separation between the two emitters S21- S22 of the second anay
• one anay 10 is disposed at a depth of four metres, whereas the other anay 1 1 is disposed at a depth of 10 metres
• the axis of each emitter anay is preferably honzontal, so that each emitter SI 1, S12 of the first anay 10 is at a depth of 4 metres and each emitter S21, S22 of the second anay 11 is at a depth of 10m
• the source is intended to be moved through the water at a speed v, and this is most conveniently done by to wing the source from a survey vessel, as shown in Figure 1
• the two anays are also displaced m a horizontal direction
• the direction of displacement of the two anays is the direction m which the source is towed in use
• the anays are displaced by a honzontal distance d H
• the direction m which the anays are displaced, and in which the source is moved use is chosen to be the x-direction for convenience of description
• the two anays are not displaced in the direction perpendicular to the direction of movement of the source (in Figure 4 this is the y-direction and extends out of the plane of the paper)
• An emitter of one anay and the conesponding emitter of the other anay are both disposed in a common vertical plane, that is parallel to the direction of movement of the source
• the difference m depth between the first and second emitter anays should be chosen such that 11 dt ⁇ f max , where f max is the maximum frequency the seismic data.
• the time dt is determined by the depth difference between the two emitter anays and by the velocity of seismic energy in water, which is a known quantity
• the embodiment of Figure 4 is intended for use with a maximum frequency f max ⁇ 90Hz, and a depth difference of 6m has been found to be acceptable in this case
• the two emitter anays of the source shown in Figure 4 have a horizontal displacement, d ⁇ . The horizontal displacement is measured between an emitter of the anay nearer the towing vessel and the conesponding emitter of the array further from the towing vessel.
• the marine seismic source shown in Figure 4 can be used in a marine seismic surveying anangement.
• the anangement would also comprise one or more seismic receivers, and means, such as a towing vessel, for moving the source through the water.
• the marine seismic surveying anangement would also comprises control means for firing the emitters, and recording means for recording seismic data acquired by the receiver(s).
• the horizontal displacement between the two emitter anays is substantially equal to the shot point interval of the marine seismic surveying anangement.
• the horizontal displacement of the emitter anays of the seismic source is preferably approximately 25m.
• the emitter anays are fired in a "flip-flop" sequence at equal intervals of, in this example, 25m. That is to say, the emitters on the anay nearer the towing vessel are fired initially and they may be fired consecutively, or simultaneously. After a time delay that is equal to the time required for the towing vessel to travel 25m, the emitters of the anay further from the boat are fired. This results in two shot records generated at points having the same x-co-ordinate and the same y-co-ordinate, but at different depths.
• the arra at the shallower depth is shown as the anay nearer the towing vessel.
• the invention is not limited to this, however, and the anay at the shallower depth could be the anay further from the towing vessel.
• the signals generated at the receiver or receiver anay as the result of firing the first emitter anay and subsequently firing the second emitter anay are recorded in any conventional manner. Since, as explained above, the signals were emitted by the two emitter anays at the same x- and y- co-ordinates but at different z-co-ordinates, the results can be analysed using the theory outlined above with regard to equations (1) to (9). In particular, by calculating the sum of the two signals and the integral with respect to time of the difference between the two signals, it is possible to compute the down- going source wavefield using equation (9). Thus, the present invention enables the effects of the up-going source wavefield to be removed from the processed seismic data. The effect of source-side ghost reflections and reverberations is thus eliminated, or at least significantly reduced.
• Results obtained using a seismic source according to the present invention and the de- ghosting method of the present invention are illustrated in Figures 5-8. These figures relate to a survey carried out using a source having two emitter anays, each anay having two marine vibrator anays as the seismic emitters.
• the source was towed with the anays at depths of 4m and 10m respectively, with a 25m in line displacement (by "in-line displacement' 1 is meant displacement along the towing direction) between the two anays.
• the average water depth was 52m.
• An ocean bottom cable (OBC) dual sensor cable, 10km in length, disposed on the sea bed was used as the receiver.
• the two anays of marine vibrators were fired in a flip-flop mode as described above.
• Receiver interval 25m Receiver depth: 52m Sweep bandwidth: 5-90Hz Fold: 90.
• Figure 5 The data recorded in the OBC sensors as a result of firing the emitter anay at a depth of 10m is shown in Figure 5. This shows the data after preliminary processing operations.
• the emitter anay at a depth of 4m generated another record (not shown) at the same x, y location.
• Figure 6 illustrates the data of Figure 5 after processing, using equation (9) and the data recorded using the emitter anay at a depth of 4m, to remove the up-going wavefield. That is, Figure 6 shows the data of Figure 5 after de-ghosting to remove the effect of source-side ghost events and reverberations.
• Figures 7 and 8 show the average amplitude spectra for the seismic data of Figures 5 and 6 respectively. It will be seen that the resolution and the signal-to-noise ratio have both been improved by de-ghosting process.
• the seismic source consists of two anays each containing two marine vibrator units.
• the present invention is not, however, limited to this precise anangement.
• each of the source anays could contain more than two emitters of seismic energy.
• the de-ghosting method of the present invention could in principle be applied if seismic data acquired using a single seismic emitter at one depth and seismic data acquired using an emitter having identical emission characteristics at a different depth (but at the same x- and y-co- ordinates) is available.
• each receiver anay is an in-line emitter anay - that is, the emitters of each anay are ananged along the axis of the anay.
• the axis of each anay is coincident with the towing direction when the source is in use.
• the invention is not, however, limited to use with in-line emitter anays.
• the seismic source of the invention is not limited to a source that contains marine vibrator units.
• the source could also consist of anays of other emitters of seismic energy such as, for example, air guns.

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• 2001
  • 2001-03-29 CA CA002405068A patent/CA2405068A1/en not_active Abandoned
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  • 2001-03-29 BR BR0110058-0A patent/BR0110058A/pt not_active Application Discontinuation
  • 2001-03-29 AU AU2001239512A patent/AU2001239512B2/en not_active Ceased
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