Method and apparatus for directional designature

Abstract

Methods and apparatuses for directional designature in shot domain are provided. Azimuth and take-off angles are calculated for each record in the seismic data. Directional designature is then applied to the seismic data using a source signature dependent on the azimuth and take-off angles.

Claims

1. A method for seismic exploration of a formation under the seafloor, the method comprising: receiving marine seismic data recorded by receivers towed above the formation following shots of a source; for each of the shots, calculating azimuth angles and take-off angles relative to the source for records in the marine seismic data, acquired by different among the receivers, obtaining a source signature related to the shot and dependent on the azimuth angles and the take-off angles, and applying a directional designature to the records using the source signature; and generating an image of the formation using data obtained by applying shot-by-shot the directional designature, to assess whether oil and/or gas reservoirs are present in the formation.

2. The method of claim 1, wherein at least one of the take-off angles is calculated using a source-side ghost delay time obtained using a bootstrap approach.

3. The method of claim 2, wherein the at least one of the take-off angles is calculated using the source-side ghost delay, t, a depth of the source z and a signal propagation velocity in water, v.sub.w.

4. The method of claim 3, wherein the at least one of the take-off angles is calculated using the formula
cos =0.5 t v.sub.w/z.

5. The method of claim 2, wherein mirror data for a mirror source located symmetric to the source relative to water surface is generated, and the source-side ghost delay is obtained by jointly inverting the mirror data and the marine seismic data.

6. The method of claim 1, wherein the source signature is obtained using near-field sensor measurements.

7. The method of claim 1, wherein the marine seismic data includes second records acquired by the receivers following another shot of another source, and the method further comprises: calculating other azimuth angles and other take-off angles for the second records; obtaining a second source signature for the other shot and dependent on the other azimuth angles and the other take-off angles; and applying directional designature to the second records using the second source signature.

8. The method of claim 1, wherein the marine seismic data is acquired as a continuous time recording, and, the marine seismic data is pre-processed to generate separate records related to each of the shots, before performing calculating, obtaining and applying steps shot-by-shot.

9. An apparatus configured for seismic exploration of a formation under the seafloor, the apparatus comprising: an interface configured to receive marine seismic data recorded by receivers following shots of a source; and a data processing unit configured, for each of the shots, to calculate azimuth angles and take-off angles relative to the source for records in the marine seismic data acquired by different among the receivers, to obtain a source signature related to the shot and dependent on the azimuth angles and the take-off angles, and to apply a directional designature to the records using the source signature, wherein the data processing unit is further configured to generate an image of the formation using data obtained by applying shot-by-shot the directional designature, for assessing whether oil and/or gas reservoirs are present in the formation.

10. The apparatus of claim 9, the data processing unit calculates at least one of the take-off angles using a source-side ghost delay time obtained using a bootstrap approach.

11. The apparatus of claim 10, wherein the data processing unit calculates the at least one of the other take-off angles using the source-side ghost delay, t, a depth of the source z and a signal propagation velocity in water, v.sub.w using the formula
cos =0.5 t v.sub.w/ z.

12. The apparatus of claim 9, wherein the data processing unit is configured to calculate at least one of the take-off angles using a source-side ghost delay time, the ghost time delay being obtained by generating mirror data for a mirror source located symmetric to the source relative to the water surface, and then jointly inverting the mirror data and the marine seismic data.

13. The apparatus of claim 9, wherein the data processing unit is configured to obtain the source signature using near-field sensor measurements.

14. The apparatus of claim 9, wherein the marine seismic data includes second records acquired by the receivers following another shot of another source, and the data processing unit is further configured: to calculate other azimuth angles and other take-off angles related to the other source for the second records, to obtain a second source signature dependent on the other azimuth angles and the other take-off angles, and to apply directional designature to the second records using the second source signature.

15. The apparatus of claim 9, wherein the marine seismic data is acquired as a continuous time recording, and, the data processing unit is further configured to pre-process the marine seismic data to generate separate records related to each shot, before performing calculating, obtaining and applying steps shot-by-shot.

16. A computer readable recording medium non-transitorily storing executable codes, which, when executed by a computer, make the computer perform a method for seismic exploration of a formation under the seafloor using directional designature, the method comprising: receiving marine seismic data recorded by receivers towed above the formation following shots of a source; for each of the shots, calculating azimuth angles and take-off angles relative to the source for records in the marine seismic data, the records having been acquired by different among the receivers following the shot, obtaining a source signature related to the shot and dependent on the azimuth angles and the take-off angles, and applying a directional designature to the records using the source signature; and generating an image of the formation using data obtained by applying shot-by-shot the directional designature, to assess whether oil and/or gas reservoirs are present in the formation.

17. The computer readable recording medium of claim 16, wherein at least one of the take-off angles is calculated using a source-side ghost delay time.

18. The computer readable recording medium of claim 16, wherein the marine seismic data includes second records acquired by the receivers following another shot of another source, and the method further comprises: calculating other azimuth angles and other take-off angles for the second records; obtaining a second source signature for the other shot, the second signature being dependent on the other azimuth angles and the other take-off angles; and applying directional designature to the second records using the second source signature.

19. The computer readable recording medium of claim 17, wherein mirror data for a mirror source located symmetric to the source relative to water surface is generated, and the source-side ghost delay is obtained by jointly inverting the mirror data and the marine seismic data.

20. The computer readable recording medium of claim 16, wherein the source signature is obtained using near-field sensor measurements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:

(2) FIG. 1 illustrates a conventional narrow azimuth data acquisition system;

(3) FIG. 2 illustrates definition of the take-off angle;

(4) FIG. 3 is a graph of spectral content of a source signature's dependence on the take-off angle for a 0 azimuth angle;

(5) FIG. 4 is a graph of spectral content of a source signature's dependence on the take-off angle for a 90 azimuth angle;

(6) FIG. 5 is an image including two graphs (a first one being amplitude versus time, and a second one in the upper right corner of the first one being amplitude versus frequency);

(7) FIG. 6 illustrates a method for extracting a take-off angle;

(8) FIG. 7A represents the layout of a data acquisition system acquiring azimuth data whose azimuth coverage is illustrated in FIG. 7B;

(9) FIG. 8 is a shot/receiver map of one super shot (i.e., data is acquired with the system in FIG. 7A);

(10) FIG. 9 represents raw data belonging to the super shot in FIG. 8;

(11) FIG. 10 represents data in FIG. 9 after applying conventional designature;

(12) FIG. 11 represents data in FIG. 9 after applying directional designature according to an embodiment;

(13) FIG. 12 illustrates traces of a common middle point gather;

(14) FIG. 13 illustrates the traces in FIG. 12 after applying conventional designature;

(15) FIG. 14 illustrates the traces in FIG. 12 after applying directional designature according to an embodiment;

(16) FIG. 15 represents raw data in a common channel domain;

(17) FIG. 16 illustrates the data in FIG. 15 after applying conventional designature;

(18) FIG. 17 illustrates the data in FIG. 15 after applying directional designature according to an embodiment;

(19) FIG. 18 is a flowchart of a method for directional designature according to an embodiment; and

(20) FIG. 19 is a data processing apparatus according to an embodiment.

DETAILED DESCRIPTION

(21) The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to marine data acquisition and processing. However, similar embodiments and methods may be used for a land data acquisition system and for surveys using electromagnetic waves.

(22) Reference throughout the specification to one embodiment or an embodiment means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases in one embodiment or in an embodiment in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

(23) Some of the following embodiments perform designature of marine seismic data recorded by receivers. An azimuth angle (defined based on the in-line and cross-line distance between the source and the receiver generating a record following the shot) and a take-off angle are then associated with each record. A record is a series of amplitude versus time pairs of values recorded by a receiver following a shot of a source. The source may be (but it is not limited to) an air-gun source. A source signature dependent on the azimuth and the take-off angles is obtained (i.e., estimated or received). The source signature may be obtained shot-by-shot or when considered necessary (e.g., when changes of the source and/or data acquisition configuration occurs). A directional designature is the applied using this source signature. The data obtained after applying the directional designature is further processed to yield an image of an explored formation under the seafloor.

(24) Near-field sensor measurements (which are often acquired using hydrophones, but other sensors may also be employed) may be used to obtain the source signature. The near-field measurements may be received separately from the marine seismic, or incorporated, as appropriately time-wise, in the marine seismic data (at the time of the survey or later when data is assembled and/or pre-processed). Near-field hydrophones are often installed 1-2 m above each air-gun of a source to monitor the air-gun's operation (e.g., to detect if the air-gun misfired) and/or the air-gun's firing moment. As described in the article, The signature of an air gun array: Computation from near-field measurements including interactionsPractical considerations by Ziolkowski et al., published in Geophysics, 48(2), 1984, pp 105-111, near-field hydrophone data may be used to derive source signatures. The isotropic point source signatures of each air-gun in the source, known as notional signatures, can be derived from the near-field hydrophone measurements. If the directional information (i.e., azimuth angles and take-off angles) is provided, source signatures can be obtained by beam-forming using the notional signatures. For example, FIG. 5 illustrates two different source signatures (in time domain and in frequency domain) without free surface ghost. Lines 510 and 530 correspond to the vertical (take-off angle=0) source signature, and lines 520 and 540 correspond to the source signature (take-off angle=50 and azimuth=0). The difference of bubble energy (proportional with the square of the amplitude) and spectrum (i.e., frequency versus amplitude profile) are noticeable between these two source signatures.

(25) Knowledge of the take-off angles and azimuth angles associated with each record in the data is necessary to apply directional designature (i.e., using an angle-dependent source signature). As illustrated in FIG. 6, the take-off angle can be obtained if the source side ghost-delay time t and air-gun's depth z are known. The delay time t between the primary and ghost can be expressed as:
t=2z cos /v.sub.w (2)
where v.sub.w is the water velocity. The take-off angle can then be calculated as:
cos =0.5tv.sub.w/z (3)

(26) The source side ghost-delay time t can be extracted from the data using a bootstrap method as, for example, described in the article, Premigration deghosting for marine towed streamer data using a bootstrap approach, by Wang et al., published in 82.sup.nd Annual International Meeting, SEG, 2012, Expanded Abstracts, pp. 1-5 (the contents of which is incorporated herewith by reference in its entirety). This bootstrap method, which self-determines parameters for shot deghosting, may be used for deriving take-off angles. The recorded data in shot domain is first used to create mirror data (see source 610, mirror source 620 and water surface 630 in FIG. 6). Then, both the recorded and mirror data are jointly inverted to obtain t, which is then used in equation 3 to calculate a take-off angle. This take-off angle paired with the azimuth angle estimated from shot and receiver locations (at the time when the source is fired and when the signal is detected, respectively) enables estimating the source signature by beam-forming using the notional signatures.

(27) FIGS. 9-17 represent data recorded using a wide azimuth data acquisition system whose layout is illustrated in FIG. 7A. Five air-gun sources 710-750 were towed at staggered positions to provide data with ultra-long offsets (up to 18 km) and full azimuth coverage within a 9 km offset. A super shot is a term associated with a sequence of shots from all sources to cover many (most) azimuth angles. In-line distance from sources 710 and 750 to spreads 760 and 770, respectively, was about 300 m. In-line interval between adjacent sources (e.g., 710 to 720, 720 to 730, etc.) was about 2.1 km, and cross-line distances between adjacent sources were about 1.2 km. The spreads included about 10 variable-depth streamers with 120 m there-between. This layout is merely exemplary and it is not intended to be limiting.

(28) FIG. 7B is a circular graph illustrating azimuth coverage achieved with this data acquisition system towed back and forth. The circles correspond to distance from the source in increments of 1 km, and the azimuth is divided in 10 sectors, the shade of gray for each sector corresponding to the amount of records therein.

(29) FIG. 8 is a shot/receiver map of one super shot in the system illustrated in FIG. 7A. FIGS. 9-11 represent data corresponding to line 810 of azimuth angle 0 (i.e., along the towing direction) and to line 820 of azimuth angle 90.

(30) FIG. 9 illustrates the raw data before designature as time (T, the vertical axis) versus inline or cross-line position (X, axes other than vertical). In the right-side of the time axis in FIG. 9, the position axis corresponds to line 810, and in the left-side of the time axis in FIG. 9, the position axis corresponds to line 820. FIG. 10 represents the same data as in FIG. 9 in frequency (F, vertical) versus position (X) domain after the conventional designature method (not considering the source directivity) has been applied. The gray shades correspond to different amplitudes.

(31) FIG. 11 is the same data and in the same format as FIG. 10, after applying directional designature in shot domain according to an embodiment.

(32) A comparison of FIGS. 10 and 11 (see 1010 versus 1120) reveals that the resulting data is richer in information in high frequency (50-80 Hz range) when using directional designature than the conventional designature method. It is also noticeable that directional designature has a bigger impact for the 90 azimuth angle (left-side, 1010 vs. 1110) than for the 0 azimuth angle (right-side, 1020 vs. 1120), which is consistent with the source directivity spectra illustrated in FIG. 5.

(33) To further illustrate the improvements provided by using methods according to the above-described embodiments instead of the conventional method, FIGS. 12-14 illustrate traces of a common middle point (CMP) gather. Time flows from the upper to the lower part of the traces, and the departure from vertical of each trace corresponds to the detected signal amplitude. The first signal for each trace is the water bottom (W.B.) reflection. The horizontally running line above the traces represents azimuth angles (varying between 0 and 90) corresponding to each of the traces. FIG. 12 illustrates the raw data, FIG. 13 represents data in FIG. 12 after applying the conventional designature method, and FIG. 14 represents the same data after applying the directional designature according to an embodiment.

(34) In FIG. 12, one can observe (see 1210 and 1220 areas) that the traces are inconsistent among different azimuths. FIG. 13 indicates (see 1310 and 1320 areas) that the conventional designature (which uses a single signature) is unable to properly correct this inconsistency. In contrast, as observed in FIG. 14 (see 1410 and 1420 areas), the directional designature provides more consistency among different azimuths. Thus, the directional signatures represent the source signatures more accurately. This improvement may be useful to better predict a multiple model and thereby reduce residual multiples in the processed data.

(35) FIGS. 15-17 indicate that directional designature methods also properly remove the bubble energy because the shot-by-shot directional far-field signatures better represent the source signatures in the recorded seismic data. FIG. 15 represents raw data in a common channel domain, i.e., time vertically versus shot number horizontally (offset 7 km), with the shades of gray illustrating different amplitudes. In this representation, the bubble energy yields the echoes in the lower part of the graph. FIG. 16 represents the same data as in FIG. 15 after applying the conventional designature method, and FIG. 17 also represents the same data as in FIG. 15 after applying the designature method according to an embodiment. Comparing features in the lower parts of FIGS. 15-17 reveals that the directional designature attenuates the bubble energy much better than the conventional designature.

(36) In view of FIGS. 12-17, an advantage of directional designature is that data becomes more cohesive along all azimuths and take-off angles.

(37) FIG. 18 is a flow diagram of a method 1800 for directional designature according to an embodiment. Method 1800 includes receiving marine seismic data recorded by receivers following a shot of a source (may be an air-gun source or other type of source), at 1810. Method 1800 further includes calculating azimuth angles and take-off angles relative to the source for records in the marine seismic data acquired by different receivers, at 1820.

(38) Method 1800 also includes obtaining a source signature dependent on the azimuth angles and the take-off angles at 1830. In one embodiment, the source signature is obtained using near-field measurements, which may be acquired by hydrophones placed in water, above (e.g., 1-2 m) each air-gun of the source. In another embodiment, the source signature is derived using modelled notional sources. Method 1800 finally includes applying directional designature to the records using the source signature, at 1840. Directional designature means that for each record, it is used a record source signature inferred from the source signature based on the azimuth and the take-off angle associated with the record.

(39) In one embodiment, one or more of the take-off angles is/are calculated using a source-side ghost delay time obtained using a bootstrap approach. The take-off angles may also be calculated using the source-side ghost delay, t, a depth of the air-gun source z and a signal propagation velocity in water, v.sub.w (e.g., formula 3). The source-side ghost delay may be obtained by jointly inverting mirror data (generated for a source located symmetric to the air-gun source relative to water surface) and the marine seismic data.

(40) If the marine seismic data correspond to plural shots of the air-gun, then steps 1820-1840 are performed shot-by-shot. If the marine seismic data includes second records related to another shot of another source, method 1800 may further include performing steps 1820-1840 for the second records.

(41) If the marine seismic data is acquired as a continuous time recording, and, the marine seismic data may be pre-processed to separate records related to each shot, before performing steps 1820-1840. Note that if the data is blended data (i.e., with time-overlapping records), then the pre-processing includes de-blending.

(42) FIG. 19 illustrates a block diagram of a data processing apparatus 1900 usable for directional designature according to an embodiment. Hardware, firmware, software or a combination thereof may be used to perform the various steps and operations. Apparatus 1900 includes one or more data processors 1910, coupled to an interface 1920 and a memory 1930.

(43) Interface 1920 may include one or more user interface mechanisms such as a mouse, keyboard, microphone, touch pad, touch screen, voice-recognition system, etc. Interface 1920 may be used to receive marine seismic data recorded by receivers, and near-field sensor measurements.

(44) Data processor(s) 1910 may be controlled by a user via interface 1920 (i.e., data processing parameters may be input by the user via the interface). Data processor(s) 1910 is/are configured to calculate azimuth angles and take-off angles relative to the source for records in the marine seismic data, the records having been acquired by different receivers following the shot. Data processor(s) 1910 is/are further configured to obtain a source signature dependent on the azimuth angles and the take-off angles. Data processor(s) 1910 is/are also configured to apply directional designature to the records using the source signature.

(45) Data processor(s) 1910 may be configured to calculate one or more of the take-off angles using a source-side ghost delay time obtained using a bootstrap approach. In one embodiment, data processor(s) 1910 calculate the at least one of the other take-off angles using the source-side ghost delay, t, a depth z of the source and a signal propagation velocity v.sub.w in water (e.g., formula 3).

(46) Data processor(s) 1910 may be configured to generate mirror data for a source located symmetric to the source relative to the water surface, and to obtain the source-side ghost delay by jointly inverting the mirror data and the marine seismic data.

(47) If the marine seismic data corresponds to more than one shot, data processor(s) 1910 may be configured to perform calculating, obtaining and applying steps of method 1800, on a shot-by-shot basis. If the marine seismic data may include second records related to shots of another source, data processor(s) 1910 may be configured to calculate other azimuth angles and other take-off angles for the second records, to obtain a second source signature dependent on the other azimuth angles and the other take-off angles, and to apply the directional designature to the second records using the second source signature.

(48) If the marine seismic data is blended data acquired as a continuous time recording, and, the data processing unit may be further configured to deblend the marine seismic data (i.e., generate separate records for each shot) before performing calculating, obtaining and applying steps.

(49) Interface 1920 may also include a display, which may be any type of known display or presentation screen, such as LCD, plasma displays, cathode ray tubes (CRT), etc. Data processor(s) 1910 may control the display to show images of the raw data and/or data after applying the directional designature.

(50) Memory 1930 may include a random access memory (RAM), a read-only memory (ROM), CD-ROM, removable media and any other forms of media capable of storing data. Memory 1930 may store the received marine seismic data, and/or data resulting after applying the directional designature. Memory 1930 may also store executable codes which, when executed on a processor, make the processor perform seismic data processing methods according to various embodiments described in this section.

(51) The disclosed exemplary embodiments provide methods and data processing apparatus performing directional designature. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

(52) Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

(53) This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.