METHOD FOR SEISMIC DATA ACQUISITION AND PROCESSING

Abstract

Methods are described for separating the unknown contributions of two or more sources from a commonly acquired set of wavefield signals representing a wavefield where the sources are laterally located relatively close to each other and fire relatively close in time, and where the contributions from different sources are separated using different source encoding techniques in different parts of a frequency band of interest.

Claims

1. A wavefield acquisition and/or processing method to separate sources above a certain frequency, below which the data acquisition and/or processing is performed without separating of the sources, the method comprising: encoding at least two different sources relative to each other; obtaining wavefield recordings for the encoded at least two sources; partitioning the obtained wavefield recordings into a first dataset containing frequencies below the certain frequency, and a second dataset containing frequencies above the certain frequency; for the first dataset containing frequencies below the certain frequency, identifying a first contribution of the least two sources to the obtained wavefield recordings as generated by the at least two sources jointly; for the second data set containing frequencies above the certain frequency, separating a second contribution of at least one of the at least two sources to the obtained wavefield recordings as generated by the at least two sources individually in an absence of the other sources; generating subsurface representations of structures or Earth media properties using the separated second contribution of at least one of the at least two sources above the certain frequency and the identified first contribution generated by the at least two sources jointly below the certain frequency; and outputting the generated subsurface representations.

2. The method of claim 1, wherein the encoding step comprises encoding in the time domain.

3. The method of claim 1, wherein the encoding step comprises encoding in the spatial domain.

4. The method of claim 1, wherein the encoding step comprises encoding by changing firing times of the at least two sources relative to each other.

5. The method of claim 4, wherein the step of changing the firing times comprises changing the firing times relative to each other such that the at least two sources constructively interfere at and below the certain frequency.

6. The method of claim 1, wherein a lateral separation between the at least two sources is sufficiently small such that source locations are considered to belong to a same spatial location for wavelengths corresponding to and below the certain frequency.

7. The method of claim 4, wherein the firing times of the at least two sources occur within 80 ms.

8. The method of claim 6, wherein the lateral separation of the at least two sources is less than 150 m.

9. The method of claims 6, wherein the lateral separation of the at least two sources is less than 300 m.

10. The method of claim 1, wherein the certain frequency is lower than 6 Hz.

11. The method of claim 1, wherein a frequency band above the certain frequency is separated using a method of signal apparition.

12. The method of claim 1, wherein a frequency band above the certain frequency is separated using a method of random dithers.

13. The method of claim 11, wherein a frequency band below the certain frequency is processed using the method of signal apparition, but an average or sum of the at least two sources is obtained as opposed to the separation of the at least two sources.

14. The method of claim 13, wherein the frequency band below the certain frequency is not separated but considered to correspond to the average or sum of the at least two sources.

15. The method of claim 1, further comprising de-convolving an effective source signature for the at least two sources, from the frequency band below the certain frequency.

16. The method of claim 1, wherein spatial locations of the data corresponding to the frequency band below the certain frequency is associated with a mean lateral location between the at least two sources.

17. The method of claim 1, wherein data corresponding to the frequency band below the certain frequency are laterally spatially regularized to original shot locations.

18. The method of claim 1, further comprising combining data corresponding to the frequency band below the certain frequency with the separated data for the frequency band above the certain frequency to obtain full frequency response data sets for the at least two sources separately.

19. The method of claim 1, wherein the obtaining step comprises obtaining marine seismic data or seabed seismic data, wherein the at least two sources are towed by one or more vessels.

20. An apparatus to separate sources above a certain frequency, below which the sources are not separated, the apparatus comprising: processing circuitry configured to encode at least two different sources relative to each other; obtain wavefield recordings for the encoded at least two sources; partition the obtained wavefield recordings into a first dataset containing frequencies below the certain frequency, and a second dataset containing frequencies above the certain frequency; for the first dataset containing frequencies below the certain frequency, identify a first contribution of the least two sources to the obtained wavefield recordings as generated by the at least two sources jointly; for the second data set containing frequencies above the certain frequency, separate a second contribution of at least one of the at least two sources to the obtained wavefield recordings as generated by the at least two sources individually in an absence of the other sources; generate subsurface representations of structures or Earth media properties using the separated second contribution of at least one of the at least two sources above the certain frequency and the identified first contribution generated by the at least two sources jointly below the certain frequency; and output the generated subsurface representations.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0081] In the following description reference is made to the attached figures, in which:

[0082] FIG. 1 illustrates how in a conventional marine seismic survey all signal energy of sources typically sits inside a signal cone bounded by the propagation velocity of the recording medium and how this energy can be split in a transform domain by applying a modulation to a second source;

[0083] FIG. 2 shows a synthetic example embodiment of the invention, where two sources are fired simultaneously in a survey (Source A and B) being recorded at a receiver. At location of Source C a reference data set is available to compare with the combined response of Sources A and B, and their separated responses;

[0084] FIG. 3 shows the responses due to Sources A and B if they would have been generated separately (reference solutions after the separation of the simultaneous source experiment);

[0085] FIG. 4 shows the simultaneous source response of Source A and B as well as the single source response at the reference position in between (Source C);

[0086] FIG. 5 shows the 10 Hz high cut filtered reference data for Sources A and B;

[0087] FIG. 6 shows the 10 Hz high cut filtered and source signature corrected simultaneous source response of Source A and B as well as the 10 Hz high cut filtered single source response at the reference position in between (Source C);

[0088] FIG. 7 shows the general practice of marine seismic surveying;

[0089] FIG. 8 shows the general practice of land seismic surveying;

[0090] FIG. 9 summarizes key steps for one embodiment of the methods disclosed herein; and

[0091] FIG. 10 illustrates how the methods herein may be computer-implemented;

DETAILED DESCRIPTION

[0092] The following examples may be better understood using a theoretical overview as presented below.

[0093] It is herein proposed to use the fact that multiple isolated sources shot using either the signal apparition approach (Robertsson, 2016), or indeed any other approach wherein the sources are fired in close temporal and/or physical separation, are treated as a master shot with a sequence of subsidiary shots all fired in close temporal proximity. For example, if each separate source fires in a time sequence approximately 10 milliseconds apart, then even if five such sources are fired as a sequence and subsequently isolated, the total time from first to last firing would be around 40 milliseconds.

[0094] It is advantageous to choose a firing sequence such that all sources fire within half a period of the upper end of the low frequency band that we wish to construct using the present invention. Therefore, if this upper limit is as low as 6 Hz, then the sources should all fire within 80 ms.

[0095] It is also herein proposed to acquire simultaneous source data where all sources are located in vicinity of each other such that the size of the effective source array of the master shot is comparable to the wavelength of interest for the low frequencies to be reconstructed. For example, typical conventional source arrays may have a dimension of 15 m between the outer sub arrays. Such an arrangement is considered to be sufficient to have point source like data up to say 120 Hz. A similar argument would then apply to reconstructing frequencies up to 12 Hz for sources spaced 150 m apart. In other words, the master shot may contain sources spaced as far apart as 150 m if we wish to reconstruct data up to 12 Hz using the present invention. Similarly, the master shot may contain sources spaced as far apart as 300 m if we wish to reconstruct data up to 6 Hz using the present invention.

[0096] Such arrangement of sources can be towed by a single vessel or if one desires my multiple vessels in the vicinity of each other.

[0097] One attribute of signal apparition is that at low frequencies, the signal cones in f-k space largely overlap. This feature can thus be exploited in this invention such that the low frequencies (typically 10 Hz or below) from any of the sources can be treated as energy contributing to all sources. However, for higher frequencies, where the signal cones in f-k space do not overlap, each input source can thus be separated. Since the higher frequencies tend to come primarily from shallower reflectors in the sub-surface, the energy output of a single or dual vibrator or airgun sub-array will in most cases be sufficient to provide the requisite signal-to-noise ratio, whereas the lower frequencies from all source elements can be treated as a composite (and therefore higher energy) output source, suitable for the deeper reflectors (and the lower frequency components of all depths).

[0098] Within the typical spatial separation of the sources (as an example, 50 m lateral separation between sources is quite common, though the approach would work at all achievable separations), as well as the typical temporal separation as noted above, the low frequencies (as an example, up to 10 or 15 Hz) would be largely constructive in impact, whilst their exact contribution would also be known. The effect would thus be to be able to use the constructive interference effect of the low frequency contributions from all of the individual source elements (land or marine vibrators or airgun sub-array sets) in the shot sequence to generate the overall low frequency energy required to ensure return from deep sub-surface reflections (but well within the spatial or temporal positive contribution Fresnel zone) whilst providing the specific isolated energy from the higher frequency energy from each of the contributing land or marine vibrators or sub-array sets to each isolated shot.

[0099] Source characteristics (individual element timing and/or near-field measurements) are often used for producing a far-field source signature. These approaches will ensure that the signature for the source sequence overall (corresponding to the master shot) would be known and could therefore be taken into consideration during processing to yield an excellent low-frequency response where the small timing differences have been deconvolved from the recorded low frequency data.

[0100] We note that in order to provide a good low frequency response it will be beneficial for all source elements to fire within a short time that is small compared to the period of the low frequency energy of interest. For example, if we are considering 10 Hz data or lower, the period of these data is 100 ms or greater. Making sure that all source elements fire at their respective source separation encoded timings within say 30 ms will ensure an overall good source signature for the low frequencies with good signal-to-noise in the recorded data.

[0101] Although the low frequency response will only vary slowly laterally with respect to shot points (due to the longer wavelengths of emitted and recorded data), the effective shot point associated with the low frequency response from the master shot comprising several sources within a larger area, should be associated with the average lateral location. For example, if two sources of similar characteristics are used during simultaneous source acquisition, the low frequency response obtained by the present invention should be allocated to a shot point that lies in between the two source locations. In an additional optional step, it is therefore proposed to regularize the low-frequency responses which are not associated with the same shot points as the intermediate to high frequency separated source data. The regularization should preferably be applied in the 2D horizontal plane so that the low frequency response can be reconstructed to the correct locations also in the cross-line direction (e.g., including many parallel sail lines). This process can be carried out using any known method for spatial regularization and is not expected to be particularly difficult as the low frequencies are spatially well sampled due to their longer wavelengths.

[0102] The regularization algorithm can also involve a modeling step where the averaging process of the generated response due to the simultaneously firing sources is included for instance through a Fourier representation in terms of modelling and regularization. This will further improve the accuracy and quality of the regularization allowing for somewhat greater separation of sail lines.

[0103] Following the deconvolution of the effective source signature and a convolution with a desired source signature consistent with the source signature of the higher frequencies and/or regularization of a low frequency response at all shot locations where the intermediate to high frequencies have been separated by other means (e.g., signal apparition or random dithering), the two data sets (low and intermediate/high frequencies) can now be combined into a full bandwidth data set at all the desired shot point locations.

[0104] The proposed approach would in all cases result in fewer vibrators or sub-arrays being required per source point compared with conventional or time encapsulated techniques, whilst simultaneously increasing the total used energy per shot sequence. This in turn would result in an increase in the total used energy per square kilometre of survey area whilst reducing the instantaneous peak output.

[0105] In another embodiment of the present invention we carry out the simultaneous source separation for the entire bandwidth including the low frequencies. As an example, the method of signal apparition (Robertsson et al., 2016) allows for exact simultaneous source separation given sufficient sampling along the direction of spatial encoding (there is always a lowest frequency below which source separation in theory is exact). It is the only exact method there exists for conventional marine and land seismic sources such as airgun sources and dynamite sources.

[0106] Signal apparition is also a method that is particularly suitable to separate the response from two sources that are close to each other. The effect of signal apparition is to map source contributions into opposite locations of the frequency wavenumber space thus making their subsurface response appear as different from each other as possible even if the two sources are excited at nearby locations.

[0107] A particularly interesting acquisition configuration will therefore include sources that are close to each other (towed by the same vessel for example 25 m or 50 m apart from each other). Clearly, the response from the signal generated by two sources close to each other and recorded on common receiver will be similar but not identical. In addition, we are interested in using small time shifts (for instance 10 ms or 20 ms) and a modulation function with select A=e.sup.iT. For low frequencies the difference in the emitted source signature from shot point to shot point will be very small as the time shift is small compared to the period of the frequency of interest at low frequencies (e.g., below say 5 Hz or 10 Hz if we consider a time shift of say 10 ms or 20 ms).

[0108] Equation (0.2) gives some insight into what happens at the very low frequencies. At low frequencies (i.e., below 10 Hz or 5 Hz), almost all energy will remain within the cone centered at 0 wavenumber (where the average of the signal due to the two sources is mapped) and very little energy will be mapped to the cone centered at the Nyquist wavenumber (where the difference of the signal due to the two sources is mapped). This is a consequence of the fact that the response due to the two source looks very similar at low frequencies for two reasons. First, the sources are close to each other and for low frequencies in particular the response will be very similar. Secondly, the modulation function will not introduce a significant variation from shot point to shot point for low frequencies if the time-shift is small.

[0109] In the general case, Andersson et al. (2016) shows how the data in the two cones will correspond to the average of the sources in the cone centred at zero wavenumber and the difference between the two sources at the Nyquist wavenumber (by setting a.sub.0()=a.sub.1()=1 in their equation 6).

[0110] In this embodiment of the invention where the same source separation is carried out through the entire bandwidth the deconvolution step to correct for source signatures should not be carried out as this is already implicitly carried out in the separation process. However, as described above, the separated quantity is associated with the average of the response of the simultaneous sources for low frequencies and a lateral spatial regularization step to associate the low frequency response exactly with desired shot locations may be carried out. After this optional step the two data sets (low and intermediate/high frequencies) can now be combined into a full bandwidth data set at all the desired shot point locations.

[0111] In another embodiment of the invention, the spatial separation of the sources and the relative activation time of all sources are chosen to ensure a signal-to-noise ratio that will be close to the signal-to-noise ratio of a survey where all sources would have been fired at the same time and the same location. Within this spatial separation and within that relative activation time the separate sources will appear as one single source for practical purposes. The design of a survey along these criteria is obvious from the above by for instance requiring that all sources should be located within half a wavelength or less from each other and activated within half a period or less from each other to ensure constructive interference of the emitted signals below the lowest frequency corresponding to the half wavelength of separation in space and half a period of separation in time. Survey design could begin with choosing the frequency for which the above conditions should be satisfied. The choice of the frequency may depend on a number of different parameters including the desired resolution, the depth to the reservoir, the need for stable inversion, etc.

Example

[0112] A synthetic example was created using an acoustic 3D finite-difference synthetic data set mimicking a seabed seismic acquisition geometry over a complex sub surfaced model. For simplicity the example is limited to the effect for a single simultaneous source shot being recorded on a receiver. A more complete example would have encompassed an entire grid of shots to carry out the simultaneous source separation at the higher part of the frequency band of interest and to enable regularization in a plane for source positions for the low frequency part of the frequency band of interest as described above.

[0113] FIG. 2 shows a schematic view of the example. We consider here a simultaneous source experiment with two sources denoted Source A and Source B (A similar example could also have been created for a larger number of sources firing simultaneously).

[0114] FIG. 2 also shows the location of a virtual reference Source position referred to as Source C which in the case of two simultaneous sources is located right in between Sources A and Source B.

[0115] FIG. 3 shows the reference responses due to Source A and Source B separately as if the separation of the combined sources would have been perfect throughout the entire frequency band. The responses from Sources A and B is different as they illuminate the subsurface differently due to their different source locations. In addition we have included a small timeshift of 20 ms between the firing times to represent the encoding that would have been carried out from shot point to shot point (e.g., due to the modulation sequence in the method of signal apparition as described above).

[0116] FIG. 4 shows the simultaneous source response of Source A and B (i.e. the data that would have been acquired before separation) as well as the response at the reference position Source C. We note that there are quite some differences between the simultaneous source response (Source A+B) and the response at the reference position in between. Again, this is a result of both the different illumination of the subsurface as well as the time shift between the two sources due to the source encoding from shot point to shot point.

[0117] FIG. 5 shows the reference responses due to Source A and Source B separately as if the separation of the combined sources would have been perfect throughout the entire frequency band but now with a 10 Hz high cut filter applied. In contrast to FIG. 3, we can now see that the responses from Sources A and B are very similar as the sources are close to each other compared to the minimum wavelength in the spectrum (150 m at 10 Hz). The small time shift between the two sources (20 ms) is also present but small compared to frequency band in the graphs (10 Hz corresponds to a period of 100 ms).

[0118] Finally, in FIG. 6 we show the simultaneous source response of Source A and B (i.e. the data that would have been acquired before separation) as well as the response at the reference position Source C. In contrast to FIG. 4, we have applied at 10 Hz high cut filter to both graphs. In addition we have deconvolved the combined source signature from Sources A and B and reapplied a source signature without the 20 ms time shift. There is an excellent agreement between the simultaneous source response (Source A+B) and the response at the reference position in between.

[0119] The example illustrates how we can obtain the low frequency response of the seismic survey without an explicit source separation method that relies on encoding shots from shot point to shot point (e.g., using the method of signal apparition or the method of random dithers) for low frequencies. The low frequency response will correspond to the seismic response at the average location of the simultaneously firing sources (provided that the sources are closely located to each other compared to the minimum wavelength in the low frequency part of the frequency spectrum of interest). It will also be desirable to regularize the low frequency response in 2D (i.e. both inline and crossline source line locations) to reconstruct the response at the desired source locations and not just at average simultaneous source locations. This step is carried out using conventional methods for regularization well known to those skilled in the art.

[0120] Finally, the recovered low frequency part of the data illustrated in this example is added to the source separated response (e.g., using signal apparition or random dithers) of the remaining bandwidth of the data to yield the full bandwidth response of the separated sources.

[0121] In FIG. 9, the key steps for one embodiment of the methods disclosed herein are summarized. In a first step, 901, At least two different sources are encoded relative to each other using the methods disclosed herein, enabling the separation of the sources above a certain frequency and below which the data acquisition and/or processing is carried out without performing such separation of the sources. In a second step, 902, wavefield recordings are obtained for the encoded at least two different sources in accordance with the general practice of marine or land seismic acquisition and the methods disclosed herein. In a third step, 903, the obtained wavefield recordings are partitioned into two datasets containing frequencies above and below the certain frequency. In a fourth step, 904, for the partition containing frequencies below the certain frequency, a contribution of the least two sources to the obtained wavefield recordings is identified as generated by the at least two sources jointly. In a fifth step, 905, for the partition containing frequencies above the certain frequency, a contribution of at least one of the at least two sources to the obtained wavefield recordings as generated by the at least two sources individually in the absence of the other sources is separated. In a sixth step, 906, Subsurface representations of structures or Earth media properties are generated using the separated contribution of at least one of the at least two sources above the certain frequency and/or the identified contribution generated by the at least two sources jointly below the certain frequency. In a seventh step, 907, the generated subsurface representations are output.

[0122] The methods described herein may be understood as a series of logical steps and (or grouped with) corresponding numerical calculations acting on suitable digital representations of the acquired seismic recordings, and hence can be implemented as computer programs or software comprising sequences of machine-readable instructions and compiled code, which, when executed on the computer produce the intended output in a suitable digital representation. More specifically, a computer program can comprise machine-readable instructions to perform the following tasks:

[0123] (1) Reading all or part of a suitable digital representation of the obtained wave field quantities into memory from a (local) storage medium (e.g., disk/tape), or from a (remote) network location;

[0124] (2) Repeatedly operating on the all or part of the digital representation of the obtained wave field quantities read into memory using a central processing unit (CPU), a (general purpose) graphical processing unit (GPU), or other suitable processor. As already mentioned, such operations may be of a logical nature or of an arithmetic (i.e., computational) nature. Typically the results of many intermediate operations are temporarily held in memory or, in case of memory intensive computations, stored on disk and used for subsequent operations; and

[0125] (3) Outputting all or part of a suitable digital representation of the results produced when there no further instructions to execute by transferring the results from memory to a (local) storage medium (e.g., disk/tape) or a (remote) network location.

[0126] Computer programs may run with or without user interaction, which takes place using input and output devices such as keyboards or a mouse and display. Users can influence the program execution based on intermediate results shown on the display or by entering suitable values for parameters that are required for the program execution. For example, in one embodiment, the user could be prompted to enter information about e.g., the average inline shot point interval or source spacing. Alternatively, such information could be extracted or computed from metadata that are routinely stored with the seismic data, including for example data stored in the so-called headers of each seismic trace.

[0127] Next, a hardware description of a computer or computers used to perform the functionality of the above-described exemplary embodiments is described with reference to FIG. 10. In FIG. 10, the computer includes a CPU 1000 (an example of processing circuitry) that performs the processes described above. The process data and instructions may be stored in memory 1002. These processes and instructions may also be stored on a storage medium disk such as a hard drive (HDD) or portable storage medium or may be stored remotely. Further, the claimed advancements are not limited by the form of the computer-readable media on which the instructions of the inventive process are stored. For example, the instructions may be stored on CDs, DVDs, in FLASH memory, RAM, ROM, PROM, EPROM, EEPROM, hard disk or any other information processing device with which computer communicates, such as a server or another computer.

[0128] Further, the claimed advancements may be provided as a utility application, background daemon, or component of an operating system, or combination thereof, executing in conjunction with CPU 1000 and an operating system such as Microsoft Windows 10, UNIX, Solaris, LINUX, Apple MAC-OS and other systems known to those skilled in the art.

[0129] The hardware elements in order to achieve the computer can be realized by various circuitry elements, known to those skilled in the art. For example, CPU 1000 can be a Xenon or Core processor from Intel of America or an Opteron processor from AMD of America, or may be other processor types that would be recognized by one of ordinary skill in the art (for example so-called GPUs or GPGPUs). Alternatively, the CPU 1000 can be implemented on an FPGA, ASIC, PLD or using discrete logic circuits, as one of ordinary skill in the art would recognize. Further, CPU 1000 may be implemented as multiple processors cooperatively working in parallel to perform the instructions of the inventive processes described above.

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