WO2001075481A2

WO2001075481A2 - A seismic source, a marine seismic surveying arrangement, a method of operating a marine seismic source, and a method of de-ghosting seismic data

Info

Publication number: WO2001075481A2
Application number: PCT/IB2001/000521
Authority: WO
Inventors: Nicolae Moldoveanu
Original assignee: Schlumberger Technology Corporation; Schlumberger Canada Limited; Services Petroliers Schlumberger
Priority date: 2000-04-03
Filing date: 2001-03-29
Publication date: 2001-10-11
Also published as: NO335281B1; NO20024719D0; WO2001075481A3; BR0110058A; GB0222913D0; CA2405068A1; AU3951201A; NO20024719L; GB2376301B; GB2376301A; AU2001239512B2

Abstract

A staggered vertical marine seismic source contains upper and lower arrays (10, 11) of emitters of seismic energy (S11, S12; S21, S22). The upper array (10) is horizontally displaced relative to the lower array (11). The source is used in a marine seismic surveying arrangement that has means for moving the source and at least one seismic receiver. In use, the source is moved through the water in a direction parallel to the direction in which the two arrays are displaced. The arrays (10, 11) are fired sequentially, and the time delay between the firing of the first-fired array and firing of the second-fired array is chosen such that each seismic emitter in one array is fired at the same x-and y-coordinates as the corresponding emitter in the other array. The seismic wavefields generated by firing the two arrays are thus generated at the same x- and y co-ordinates, but at different depths. The seismic data recorded at the receiver(s) as a consequence of firing the first array can be used to de-ghost the seismic data acquired as a result of firing the second array or vice-versa, thereby eliminating or reducing the effect of source-side ghost reflections and reverberations.

Description

A Seismic Source, a Marine Seismic Surveying Arrangement, a Method of operating a Marine Seismic Source, and a Method of De-ghosting Seismic Data

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. In the marine seismic surveying arrangement shown in Figure 1, 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.

One problem associated with conventional marine seismic surveying is that of "ghost reflections". 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)). As is the case for ghost reflections, 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.

A number of schenies^'for minimising the effect of ghost reflections and reverberations on seismic data have been proposed.. For most survey arrangements, the attenuation of ghost reflections and reverberations is equivalent to separating the up-going and down- going seismic wave fields.

F. J. Barr and J. J. Saunders have proposed, in a paper presented at the 59th SEG Meeting (1989), a method of attenuating ghost reflections and reverberations by recording the reflected seismic signal using two different types of seismic receivers, namely using both hydrophones and geophones The up-gomg wave field is recorded by the hydrophone and the geophone with the same polarity, while the down-going wave field is recorded by the hydrophone and the geophone with opposite polarities The difference between the signal recorded by the hydrophone and the signal recorded by the geophone allows the up-gomg wavefield to be separated from the down-going wavefield

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 deπvatives of different signs, so that compaπng 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

These pπor art methods separate the up-gomg and down-going wave fields at the receiver location That is, they attempt to remove the ghost reflections and reverberations that aπse after the seismic energy has been reflected by the target geological structure This is known as receiver-side deghostmg These pπor art methods do not, however, address the problem of the ghost reflections and reverberations that occur before the seismic energy is reflected by the target geological structure

A first aspect of the present invention provides a maπne seismic source comprising a first array of N emitters of seismic energy, where N is an integer greater than 1; and a second array of N emitters of seismic energy, wherein, m use, the first array is disposed at a first depth and the second array is disposed at a second depth greater than the first depth, and the j^th emitter of the first array (j = 1, 2 N) is displaced by a non-zero honzontal distance d_H from the j^th emitter of the second array along a first direction, and the jth emitter of the first array and the jth emitter of the second array both he in a vertical plane parallel to the first direction 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 dπ/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-, z₂), where z- ≠ z₂, 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.

Preferred features of the invention are set out in the dependent claims.

Preferred embodiments of the present invention will now be described by way of illustrative examples with reference to the accompanying figures, in which: 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, and

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.

Consider a hypothetical emitter of seismic energy S having identical emission characteristics to the emitters SI and S2, placed at the mid-point between the upper emitter SI and the lower emitter S2. This emitter S would generate up-going and down- going source wavefields at a reference time t. The total wavefield S(t) emitted by the hypothetical emitter S is the sum of the up-going and down-going source wavefields, that is:

S(t) = u(t) + d(t) (1)

In this equation, 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:

S_\(t) = u(ι-dή + d(ι+dή (2)

S₂(ή = u(t+dή + d(t-dt) (3)

In these equations, Si is the wavefield emitted by the upper emitter S 1 and S₂ 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 S₂ or vice versa. On the assumption that d is small, the terms in 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)

S₂ (t) = u(t) + u'(t)dt + d(t) - d'(t)dt (5)

In equations (4) and (5), u'(t) and d'(t) are the time derivatives of u(t) and d(t)l, respectively.

The sum of the two source wavefields Si and S and the difference between the two source wavefields Si and S₂ can be derived from equations (4) and (5) as follows:

Sum = Sι(t) + S₂(t) = 2u(t) + 2d(t) (6)

Dif= S₂(t) - 5/(

- 2d(i)d (7)

Integrating both sides of equation (7) with respect to time leads to the following result:

Intdif= 2u(t)dt - 2d(t)dt (8)

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):

d(t) = (Sum - Intdif/ dt)/4 (9)

Thus, by using a vertical source array that consists of two emitters of seismic energy that have identical emission characteristics, with one emitter disposed above the other, it is possible to derive the down-going source wavefield d(t) using equation (9) above. This allows the effect of the up-going wavefield u(t) to be eliminated when seismic data acquired using the source is processed. The principle of reciprocity is a fundamental principle of wave propagation, and states that a signal is unaffected by interchanging the location and character of the sources and receivers. For example, if 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, then using 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. (By "corresponds") it is meant that 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.)

One consequence of the principle of reciprocity is that the theory described above with relation to equations (1) to (9) above could be used for wave field separation using two vertically separated receivers. This would provide a method of receiver - side de- ghosting, which would enable the up-going wave field at the receiver, which contains the primary reflection, to be separated from the down-going wavefield caused by reflection or scattering at the sea surface.

The above discussion relates to a vertical source array that contains just two emitters, with one emitter being disposed above the other. However, the same principle can be applied to 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. It is, however, necessary for the first and second arrays of emitters to have substantially identical emission characteristics to one another - that is, 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. Furthermore, 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.

If the upper and lower emitters SI and S2 were fired simultaneously, a receiver would record the combination of the wavefield generated by the upper emitter SI and the wavefield generated by the lower emitter S2. It would therefore not be possible to apply the de-ghosting method outlined above, since the difference between the two wavefields would not be known. To apply the method using the seismic source shown in Figure 3, it would be necessary to maintain the source stationary in the water, and fire the two emitters one after the other. This would generate two distinct wavefields Si, S₂ that could be recorded separately and processed according to equations (1) to (9). However, it would be inconvenient in practice to have to hold the source stationary in the water.

In principle, 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. However, this method would also be inconvenient to carry out.

In a prefened embodiment of the present invention, therefore, a staggered vertical source is used 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. In use, 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. In the case of a source having two anays of emitters, 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. Thus, 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 In this embodiment, 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

In addition to being separated m the vertical direction (z-direction), 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 In Figure 4, 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 As noted above, the two emitter anays of the source shown in Figure 4 have a horizontal displacement, dπ. 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. In addition to the source, 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).

In a particularly prefened embodiment, the horizontal displacement between the two emitter anays is substantially equal to the shot point interval of the marine seismic surveying anangement. Thus, for a seismic surveying anangement that generates a shot point interval of, for example, 25m, the horizontal displacement of the emitter anays of the seismic source is preferably approximately 25m.

In this embodiment, 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.

In Figure 4 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.

The parameters of the survey anangement are as follows:

Number of receiver stations: 204 Receiver interval: 25m Receiver depth: 52m Sweep bandwidth: 5-90Hz Fold: 90.

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.

In the prefened embodiment described above, the seismic source consists of two anays each containing two marine vibrator units. The present invention is not, however, limited to this precise anangement. For example, each of the source anays could contain more than two emitters of seismic energy. Moreover 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.

In the embodiment shown in Figure 4, 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.

Furthermore, 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.

Claims

CLAIMS:

1. A marine seismic source comprising: a first anay of N emitters of seismic energy, where N is an integer greater than one; nd a second anay of N emitters of seismic energy, wherein, in use, the first array is disposed at a first depth and the second anay is disposed at a second depth greater than the first depth, the j^th emitter of the first array (j = I , 2...N) is displaced by a non-zero horizontal distance d_H from the j^th emitter of the second anay along a first direction, and the jth emitter of the first anay and the jth emitter of the second anay both lie in a vertical plane parallel to the first direction.

2. A marine seismic source as claimed in claim 1 wherein the N emitters of the first anay are ananged along the axis of the first anay and the N emitters of the second array are ananged along the axis of the second anay.

3. A marine seismic source as claimed in claim 2 wherein, in use, the first and second anays are disposed such that their axes lie substantially in a common vertical plane.

4. A seismic source as claimed in claim 2 or 3 wherein, in use, the first and second anays are disposed such that the axis of the first anay and the axis of the second anay are each substantially horizontal.

5. A seismic source as claimed in any preceding claim wherein each of the first and second anays of emitters of seismic energy comprises N airguns.

6. A seismic source as claimed in any of claims 1 to 5 wherein each of the first and second anays emitters of emitters of seismic energy comprises N marine vibrator units.

7. A seismic source as claimed in any preceding claim wherein the first and second depths are chosen such that the time taken for seismic energy to travel from the first depth to the second depth is greater than twice the reciprocal of the maximum frequency emitted, in use, by the seismic sources.

8. A marine seismic source substantially as described hereinabove with reference to Figure 4 of the accompanying drawings.

9. A marine seismic surveying anangement comprising: a marine seismic source as defined in any preceding claim; means for moving the seismic source; and an anay of one or more seismic receivers.

10. A marine seismic surveying anangement as claimed in claim 9 and further comprising control means for firing a selected one of the first and second anays of emitters of seismic energy.

11. A marine seismic surveying anangement as claimed in claim 9 or 10 wherein the horizontal displacement dπ between the j¹ emitter of the first anay and the j^lh emitter of the second anay is substantially equal to the shot point interval of the surveying anangement.

12. A marine seismic surveying anangement as claimed in claim 11 wherein the shot point interval of the surveying anangement is approximately 25m.

13. A method of operating a marine seismic source as defined in any of claims 1 to 8, the method comprising the steps of: a) moving the seismic source at a speed v along the first direction; b) ^' firing one of the first and second anays of emitters of seismic energy; and c) firing the other of the first and second anays of emitters of seismic energy at a time d_H/v after step (b).

14. A method as claimed in claim 13 wherein step (b) comprises firing the first anay.

15. A method of processing marine seismic data comprising the steps of: a) 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 anay; b) firing a second emitter of seismic energy at a point in the fluid medium having components (xi, yi, z ), where Zi ≠ z₂, and detecting the resultant second seismic data at the receiver anay; and c) using one of the first second seismic data to reduce the effects of source-side reflection and/or scattering at the sea surface on the other of the first and second seismic data.

16. A method as claimed in claim 15 wherein step (c) comprises calculating

d(t) = (Sum - Intdif/dt) I 4

where Sum is the sum of the first and second seismic data; Intdif is the integral with respect to time of the difference between the first and second seismic data; and Idl is the time for seismic energy to travel from the point (xj, y_{1 ;} zi) to the point (xi, yi, z ).

17. A method as claimed in claim 15 or 16 wherein the first and second emitters of seismic energy are comprised in a seismic source as claimed in any of claims 1 to 8.