Patent Description:
This section provides background information to facilitate a better understanding of the various aspects of the disclosure. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.

In order to discover and/or evaluate subsea formations for the purpose of hydrocarbon extraction, marine seismic surveys can be used. One form of marine seismic survey is called a towed steamer survey. In that, boats tow long streamers that have sensors located thereon, a source creates an impulsive wave that travels down through the water and into the formation thus reflecting and reverberating, and the reflections and reverberations travel back through the water and are detected by the sensors on the streamers. The data generated from the detected signals at the streamers can be used to evaluate features of the formation and to present a visual representation of the formation that can be used to determine the presence of various features including hydrocarbon deposits.

Another form of survey is known as an ocean bottom seismic (OBS) survey. Similar to the towed streamer survey an impulsive source is used, but instead of streamers being towed by a boat, sensors are placed directly onto the seafloor. The sensors on the seafloor can be nodal (cabled or independent), or can be in the form of seabed sensor cable (similar to streamers). The sensors detect the reflections and reverberations thus generating data that can be analyzed and presented to show various features of the formation.

Seabed surveys are generally accepted as beneficial with respect to quality of data in comparison to towed streamer surveys. This is especially the case in deep water. However, for various reasons, OBS surveys are quite (often magnitudes) more expensive and time consuming than towed steamer surveys. As a result, commercially there is a strong bias toward towed streamer surveys outside of special circumstances where OBS survey data is needed. According to various combinations of embodied features herein some of these associated issues are addressed, including efficiency in cost and operation.

<CIT> disclose a method of seismic acquisition that utilizes an arrangement of marine sources where each source is positioned at a water depth shallow enough for the surface ghost notch to fall at a frequency greater than or equal to the maximum radiated frequency of interest. If the marine seismic source has a ratio of signal bandwidth to maximum frequency that is less than one half, then it is possible to deploy it at a greater depth at which ghost notches fall below and above its frequency band but not in it. Further, by placing two or more sources at different depths for the same frequency, any undesired nulls in the radiation pattern caused by the deeper tow can be filled in.

According to an aspect of the present invention, there is provided a marine seismic survey method, comprising activating a vibrator array of two or more marine vibrators to emit a plurality of radiation patterns. The relative phases of two or more marine vibrators of the array located at the same depth are controlled so as to produce complimentary source gradient wavefields, wherein a notch corresponds to a polarity change from positive to negative; and a notch in a source gradient wavefield corresponds to a peak in the complementary source gradient wavefield. The complimentary source gradient wavefields comprises at least a first radiation pattern that has a first notch at a take-off angle that is not close to vertical, and less than a maximum take-off angle of interest, and at least a second radiation pattern that does not have a notch at a take-off angle close to that first notch.

In embodiments, the first radiation pattern has a first notch at a take-off angle greater than about <NUM> degrees and less than <NUM> degrees, wherein the maximum takeoff angle of interest is <NUM> degrees.

In embodiments, the plurality of radiation patterns are emitted at the same spatial location.

In embodiments, the plurality of radiation patterns are emitted at different spatial locations.

The method may comprise performing crossline reconstruction of sources during processing of survey data acquired in response to the source gradient wavefields.

The method may further comprise that the groups of vibrators comprise vibrator pairs with a separation distance of equal to or greater than <NUM>/<NUM> of the minimum wavelength of the source gradient wavefield produced by the vibrator pair.

The disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of various features may be arbitrarily increased or reduced for clarity of discussion.

It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. Specific examples of components and arrangements are described below to simplify the disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the disclosure may repeat reference numerals and/or letters in the various examples.

Survey equipment including one or more seismic sources and seismic receivers can be used to perform surveying of a target structure. In some examples, the target structure can be a subsurface structure beneath an earth surface. Surveying such a subsurface structure can be performed for various purposes, such as for identifying a subsurface element of interest, including a hydrocarbon-bearing reservoir, a fresh water aquifer, a gas injection zone, or other subsurface elements of interest.

Although reference is made to performing surveying of a subsurface structure, techniques or mechanisms according to some implementations can also be applied to perform surveys of other structures, such as human tissue, plant tissue, animal tissue, a mechanical structure, a solid volume, a liquid volume, a gas volume, a plasma volume, and so forth.

Different types of seismic sources are employed in seismic surveys. For example, a seismic source can include an air gun, which when activated releases compressed air to produce a pulse of acoustic energy. Another type of seismic source is a seismic vibrator, which produces acoustic energy based on oscillating movement of a vibrating element that impacts a structure in the seismic vibrator. The oscillating movement of the vibrating element can be controlled by an activation signal, which can be a sinusoidal wave signal or other type of signal that causes oscillating movement of the vibrating element.

The phase of the activation signal can be controlled for various purposes, such as to perform noise reduction or for other purposes. Generally, a seismic vibrator refers to any seismic source that produces a wavefield in response to an activation signal whose phase can be adjusted independently at each frequency. In particular, the vibrator may be a volumetric seismic source, i.e. it generates a wavefield through changing its volume.

Traditionally, a seismic source (or a collection of seismic sources) is activated such that the seismic source(s) produce(s) an approximately monopolar or omnidirectional source wavefield. A monopolar or omnidirectional wavefield is a wavefield that radiates energy generally equally in all directions. In practice, this directionality is modified by the aperture effect of the source array (since the source array typically includes sources in different horizontal and/or vertical locations), and if the source is located adjacent to the sea surface, the directionality is also modified by the interference effect of the sea surface. To produce an approximately omnidirectional wavefield using a seismic vibrator array that includes multiple seismic vibrators, the seismic vibrators are controlled to be in-phase with respect to each other. For example, if all vibrators are at the same depth, then no two vibrators within the array have a phase difference whose cosine is less than zero. A seismic vibrator array can refer to any arrangement of multiple seismic vibrators.

In addition to being able to produce an approximately omnidirectional source wavefield, a seismic vibrator array can also be controlled to produce a source gradient wavefield. A source gradient wavefield is a wavefield that has a substantially different radiation pattern to that of the omnidirectional source wavefield. Whereas the omnidirectional source wavefield radiates energy equally in all directions, the gradient source radiates energy with different polarity in different directions. For example, if the gradient source is oriented in the y-direction, then the wavefield will have a positive polarity in the positive y- direction, and a negative polarity in the negative y-direction. The source then has zero-output in at least one direction where the changeover from positive to negative polarity occurs. If the time domain wavefield due to a source at position yl is defined as S(yl,t), then the gradient of this wavefield in the direction is given as dS(yl,t)/dy. While it may not be possible to generate a wavefield corresponding exactly to this derivative term, this can be approximated as the difference of two omnidirectional sources at the same depth: <MAT>.

In equation <NUM>, <NUM> y is the separation between the two omnidirectional sources. Therefore, the source gradient can be generated by locating two or more sources together, and having two or more sources sweeping with opposite polarity (corresponding to the difference in Eq. <NUM>). In this case, the output signals produced by at least two seismic vibrators are <NUM>° out-of-phase, in which case the at least two seismic vibrators are considered to be in anti-phase. In other examples, the at least two seismic vibrators may not be exactly in anti-phase, but the property that the source has different polarity in different directions may still be obtained. For example, this would be the case when the sources are at the same depth, and any two are out-of-phase by an angle whose cosine is less than zero. The source gradient wavefield produced by the omnidirectional sources according to the foregoing configurations is not an idealized mathematical source gradient wavefield. To achieve production of a mathematical source gradient wavefield, the omnidirectional sources would have to be <NUM>° out-of-phase, their separation, <NUM> y, would have to approach nil, and their amplitudes would have to approach infinity. In practice the output level of the omnidirectional sources cannot approach infinity, so there is a practical trade-off between "close enough" to approximate the idealized mathematical gradient and "far enough" apart to generate useable output level. The "source gradient wavefield" produced by a source array according to some implementations is thus an approximate source gradient wavefield.

The seismic vibrator array can also be controlled such that the seismic vibrator array is an omnidirectional source that produces an omnidirectional source wavefield. To produce the omnidirectional source wavefield, the seismic vibrators of the seismic vibrator array are controlled such that they are in-phase (with some of the seismic vibrators slightly out-of-phase to account for different positions of the seismic vibrators, e.g. different depths of the seismic vibrators in a body of water, assuming the seismic vibrator array is part of a marine survey arrangement).

Using the seismic vibrator array according to some implementations, greater flexibility is offered since the seismic vibrator array is selectively controllable to be an omnidirectional source or a gradient source. During a survey operation, the seismic vibrator array can be controlled to be an omnidirectional source for some shots, and can be controlled to be a gradient source for other shots, such that a target shot pattern can be developed. A "shot" can refer to an activation of the seismic vibrator array.

<FIG> is a schematic diagram of an example marine seismic survey <NUM> arrangement that includes a marine vessel <NUM> that tows a seismic vibrator array <NUM> according to some implementations through a body of water <NUM>. The seismic vibrator array <NUM> includes seismic vibrators <NUM> that can be activated in response to activation signals produced by a controller <NUM> and provided by the controller <NUM> over a link <NUM> to the seismic vibrator array <NUM>. In the example of <FIG>, a series <NUM> of seismic receivers <NUM> (sensors) are deployed on the water bottom <NUM>. The receivers <NUM> may be deployed in a cable or nodal form.

The seismic receivers <NUM> are configured to detect wavefields reflected from a subsurface structure <NUM> that is underneath an earth surface (which in <FIG> is the water bottom <NUM>, such as the sea floor or sea bottom). The subsurface structure <NUM> can include one or multiple subsurface elements of interest <NUM>. Source wavefields propagated by the seismic sources <NUM> are propagated into the subsurface structure <NUM>. The subsurface structure <NUM> reflects a part of the source wavefields, where the reflected wavefields are detected by the seismic receivers <NUM>. Measured data acquired by the seismic receivers <NUM> can be communicated to the controller <NUM> for storage or for processing.

The seismic vibrators <NUM> in the seismic vibrator array <NUM> can be controlled to either be in-phase or out-of-phase to cause production of an omnidirectional source wavefield or a source gradient wavefield, respectively. The controller <NUM> can send activation signals to the seismic vibrator array <NUM> to control the seismic vibrator array <NUM> to produce an omnidirectional source wavefield in a first shot (i.e., first activation of the seismic vibrator array <NUM>) and to produce a source gradient wavefield in a second shot.

In some examples, activation of the seismic vibrator array <NUM> can be controlled such that a pattern of omnidirectional source wavefields and source gradient wavefields are produced in successive shots. This pattern can be an alternating pattern, where the seismic vibrator array <NUM> alternates between producing an omnidirectional source wavefield and a source gradient wavefield in successive shots. In other examples, other activation patterns can be produced. Non-limiting examples of wavefield generation using a seismic vibratory array are described in published patent application No. <CIT>.

<FIG> is a schematic diagram of a rear view of the example survey arrangement of <FIG>. As depicted in the example of <FIG>, the survey vibrator array <NUM> includes seismic vibrators at various different depths, Dl, D2, and D3. Although seismic vibrators are shown at three different depths in the illustrated example, it is noted that in other examples, the seismic vibrators can be included at less than three depths or at more than three depths. The seismic vibrators at different depths are configured to be activated with activation signals in different frequency ranges. For example, seismic vibrators <NUM>-<NUM> at depth D3 can be configured to be activated using activation signals that sweep from <NUM> to <NUM> hertz (Hz). Sweeping an activation signal from a first frequency to a second frequency refers to controlling the activation signal such that the frequency of the activation signal is changed from the first frequency to the second frequency.

Seismic vibrators <NUM>-<NUM> at depth D2 can be configured to be activated using activation signals that sweep from <NUM> to <NUM>. Seismic vibrators <NUM>-<NUM> at depth Dl can be configured to be activated by activation signals that sweep from <NUM> to <NUM>. In other examples, the activation signals for the seismic vibrators at different depths can be swept in different frequency ranges.

More generally, a shallower set of one or more seismic vibrators is swept in a higher frequency range, and a deeper set of one or more seismic vibrators is swept in a lower frequency range.

The seismic vibrators are separated by a separation distance L. The vibrators may be connected to one another by a rigid spacer device. The rigid spacer device may be permanently spaced or may be moveable by way of a contracting and extending device such as a telescoping member or folding member. In some examples, the separation distance L can be <NUM>/<NUM> of the shortest wavelength of interest, and in some implementations, no larger than <NUM>/<NUM> of the shortest wavelength of interest. The shortest wavelength of interest is dependent on the maximum frequency output by the two or more seismic vibrators, and can therefore vary for different seismic vibrators, such as when deployed at different depth levels as described above. One way to define the shortest wavelength of interest can be to define the maximum take-off angle of interest, θ, which then allows the shortest wavelength of interest to be defined as, <MAT>.

Here, λmin is the shortest wavelength of interested, fmax is the maximum output frequency (e.g. for the current depth level), and c is the velocity of sound in water. The separation distance can therefore change for the vibrators deployed at different depth levels, provided the vibrators at the different levels emit different frequency bands as described. Thus, seismic vibrators <NUM>-<NUM> may be separated by a distance LI, and seismic vibrators <NUM>-<NUM> may be separated by a distance L2.

Each separation distance LI and L2 is sufficiently large such that a useable output level for the source gradient wavefield is produced, while sufficiently small to retain the characteristics of the idealized mathematical gradient. As noted above, the separation distance can be generally <NUM>/<NUM> of the minimum wavelength of the source gradient wavefield produced by the respective seismic vibrators. In other examples, the separation distance can be greater than <NUM>/<NUM> the wavelength of this minimum wavelength, as long as the separation distance allow for production of a source gradient wavefield.

In the example of <FIG>, the seismic vibrators <NUM>-<NUM> at depth D3 are driven in-phase. That is to say the relative phase has a cosine that is greater than zero. As a result, the seismic vibrators <NUM>-<NUM> do not produce a source gradient wavefield. Instead, the pair of seismic vibrators <NUM>-<NUM> is configured to produce just an omnidirectional source wavefield.

Although two pairs (Pair <NUM> and Pair <NUM>) of seismic vibrators <NUM>-<NUM> are shown at depth D2 in <FIG>, it is noted that in other examples, just two seismic vibrators <NUM>-<NUM> can be provided at depth D2, where these two seismic vibrators are separated by distance L2. Similarly, just one seismic vibrator <NUM>-<NUM> can be provided at depth D3.

To produce an omnidirectional source wavefield using the seismic vibrator array <NUM> depicted in <FIG>, the seismic vibrators <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> are driven to be in-phase. The seismic vibrators <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> are considered to be in-phase even though the activation signals for the seismic vibrators <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> may be slightly out-of-phase, with phase delays provided between the respective activation signals to account for depth differences of the seismic vibrators <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM>. The net effect of the activation signals that are slightly out-of-phase is that the seismic vibrators <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> at different depths produce wavefields as if they were driven in-phase.

On the other hand, to produce a source gradient wavefield, the left and right seismic vibrators <NUM>-<NUM> at depth Dl are driven to be out-of-phase (more specifically in anti-phase), and the left pair of seismic vibrators <NUM>-<NUM> and right pair of seismic vibrators <NUM>-<NUM> at depth D2 are also driven to be out-of-phase (more specifically driven anti-phase).

Causing the seismic vibrators to be out-of-phase can be accomplished by sweeping the seismic vibrators to be in anti-phase (or close to anti-phase, for example, to preserve energy output, or to account for depth differences). Sweeping seismic vibrators in anti-phase refers to activating a first of the seismic vibrators using an activation signal that is in anti-phase with respect to an activation signal used to activate another of the seismic vibrators. The seismic vibrators sweeping in anti-phase are separated by a suitable distance (such as further described above) to generate a source gradient signal. As noted above, the separation distance is frequency dependent, with an example of the separation distance being <NUM>/<NUM> of the minimum wavelength of the source gradient wavefield.

In the discussion above the gradient wavefield is generated using wavefields that were approximately in anti-phase and where the separation between the source elements met a criterion that ensured the wavefield had the necessary gradient-type properties. In accordance to embodiments of this disclosure, the directivity patterns are controlled using combinations of phases that vary from shot-to-shot and frequency-to-frequency in order to produce complimentary wavefields. The directivity can also be controlled by varying the separation of the source elements.

These directive wavefields may have a number of characteristics, such as, the emitted wavefield has at least one notch (close to zero output energy) at one or more take-off angles other than close to the vertical, but less than the maximum take-off angle of interest. For example, greater than <NUM> degrees take-off angle and less than <NUM> degrees take-off angle, where for a particular survey, <NUM> degrees is the maximum take-off angle of interest; and each emitted wavefield has one or more complementary wavefields, where the complimentary wavefield does not have a notch that coincides with a notch in the other wavefields.

Using an array of marine vibrators, the directivity of the output wavefield can be controlled by varying the relative phase of the vibrators within the array, and also by varying the distance between the vibrator elements within the array. Consider for example the case of an array consisting of two vibrators, for example vibrators <NUM>-<NUM> at depth D1 in <FIG>. If the vibrators are separated by a distance L1 of <NUM> in the crossline direction the radiation pattern for an output frequency of <NUM>, observed in the crossline direction, is shown in <FIG>. At this frequency, <NUM> corresponds to half of the longest wavelength in the data. The amplitude response of the omnidirectional source (as a function of crossline take-off angle) is shown in <FIG>, and the response of the source gradient is shown in <FIG>. The positive polarities are indicated by reference number <NUM>, and negative polarities are indicated with reference number <NUM>. The gradient emits energy with positive polarity in one direction (positive take-off angles) and with negative polarity in another direction (negative take-off angles). There is a crossover from positive to negative at zero take-off angle. This source gradient is interleaved with the omnidirectional source, which has only positive polarity. The notch at the cross-over point in <FIG> is indicated by the black dotted line <NUM> in <FIG>.

Now, consider a case where neither an omnidirectional source wavefield nor a gradient source wavefield is emitted, but rather wavefields are emitted from each source of the pair of sources that are neither in-phase nor in anti-phase. This produces crossline amplitude responses as shown in <FIG> shows the radiation patters in dB as a function of takeoff angle for a pair of vibrators within an array that is out of phase by -<NUM> degrees. <FIG> shows the response for a pair of vibrators within an array that is out of phase by <NUM> degrees. The positive polarity are indicated by the numeral <NUM>, the negative polarity by the numeral <NUM>, and the dotted line <NUM> indicates the position of the notch on the alternate plot. Note that both responses have a peak where the other has a notch (indicated by the dotted black line <NUM> on the alternate figure). The notches occur at the point where the polarity changes from positive to negative. Neither of the output curves correspond to an omnidirectional source, and neither correspond to a gradient source, yet they do have similar characteristics, namely variations in the polarity of the emitted wavefield, and notches where the polarity changes from positive to negative. Key characteristics of these two outputs are that they have been designed such that the notches of one output correspond to peaks of the other output, and that the notches occur at the point where the output changes polarity. The notches occur at take-off angles of <NUM> degrees and -<NUM> degrees. These takeoff angles are not close to vertical, and for a particular seismic survey, less than the greatest takeoff angle of interest. Such a pair of complimentary outputs can be used for similar applications to omnidirectional and gradient wavefields.

<FIG> illustrate another example of two outputs that have the similar characteristics, but with the corresponding notch values and peaks occurring at different take-off angles. <FIG> illustrates a radiation pattern as a function of take-off angle for a pair of vibrators within an array that is out of phase by negative (-)<NUM> degrees and <FIG> illustrates a radiation pattern as a function of take-off angle for a pair of vibrators within an array that is out of phase by <NUM> degrees. Again, these two outputs correspond neither to an omnidirectional source nor to a source gradient. In this case the notches occur at approximately negative (-)<NUM> degrees and <NUM> degrees respectively. Both of these take-off angles correspond to peaks in the alternate radiation pattern (black dotted lines <NUM>). While one of these notches occurs at a take-off angle that may be larger than the largest angle of interest in a marine seismic survey, the other notch does occur within a range that is not close to the vertical, but less than the take-off angle of interest.

It is also possible to generate output wavefields with multiple notches and peaks. For example, <FIG> shows the crossline amplitude response for the case where the separation of the two elements is now <NUM> (L1 in <FIG>), and one source pair are <NUM> degrees out of phase (<FIG>) the other are -<NUM> degrees out of phase (<FIG>). This configuration has a separation equal to the longest wavelength. As such, this configuration would not be suitable to generate certain gradient wavefields. However, it is suitable to generate a pair of complimentary directive wavefields as disclosed herein. These responses are more complex than those in the previous figures, but they still exhibit similar characteristics, each has peaks where others have notches, and there is a change from positive to negative polarity at those notches.

The examples above consider a pair of sources, with two complimentary directivity patterns. <FIG> illustrate examples where the array of sources consists of more than two marine vibrators, and we can also devise examples where there are more than two complimentary patterns. The figures illustrated an example where the same pair of sources are designed to produce a repeating pattern of four sources. In this particular case, the source pair are out of phase by negative (-)<NUM> degrees (<FIG>), <NUM> degrees (<FIG>), <NUM> degrees (<FIG>) and <NUM> degrees (<FIG>). This particular pattern contains an omnidirectional source (<NUM> degrees phase difference) and a source gradient (<NUM> degrees phase difference). The notches are at -<NUM> degrees, <NUM> degrees, and <NUM> degrees take-off angle. The distinction between this set of four sources, and an omnidirectional plus gradient configuration is that this set of four complimentary patterns contains at least one source radiation pattern that has a notch at a take-off angle that is not close to the vertical, and less than a defined maximum take-off angle of interest.

The radiation patterns above are idealized versions. In practice it may also be necessary to consider the effect of the sea-surface reflection on these responses. The impact of this will be to introduce a further notch close to maximum/minimum take-off angles. These notches will be present for all configurations, and thus it is not possible to fill these notches with information from a complimentary directivity pattern. It should also be noted that the idealized patterns will represent target outputs which may not be precisely reproduced in practice due to experimental perturbations. The phase differences between elements in the array may also change as a function of frequency, as the location of the notche(s) is(are) a function of the phase difference, the separation of the elements, and the output frequency. The above describes radiation patterns in the crossline direction. It should be understood that source arrays can be configured to emit directionality patterns that can vary in any direction, and also in multiple dimensions.

<FIG> shows an example shot pattern that can be produced using the seismic vibrator array <NUM> (<FIG>) as towed by the marine vessel <NUM>, according to some examples. The tow path of the marine vessel <NUM> is indicated by <NUM> (i.e., shot line). As shown in <FIG>, stars <NUM> and arrows <NUM> represent respective shots (i.e., shot points) of the seismic vibrator array <NUM>. A star <NUM> represents a respective activation of the seismic vibrator array <NUM> that produces an omnidirectional source wavefield. An arrow <NUM> represents an activation of the seismic vibrator array <NUM> that produces a source gradient wavefield. In <FIG>, the first two stars along the path <NUM> are referred to as <NUM>-<NUM> and <NUM>-<NUM>, respectively, and the first arrow along path <NUM> is referred to as <NUM>-<NUM>. A general reference to stars <NUM> includes a reference to <NUM>-<NUM> and <NUM>-<NUM>, and a general reference to arrows <NUM> includes a reference to <NUM>-<NUM>.

In the example of <FIG>, an alternating pattern of omnidirectional source activations and source gradient activations is depicted, where successive shots alternate between an omnidirectional source activation (activation of the seismic vibrator array <NUM> that produces an omnidirectional source wavefield) and a source gradient activation (activation of the seismic vibrator array <NUM> that produces a source gradient wavefield).

The phase of the seismic vibrator array <NUM> can be controlled from shot-to-shot such that a residual shot noise (RSN) from one shot can be mitigated in the next shot. For a given shot, residual shot noise can result from a previous shot or from previous shots. If a shot pattern of the seismic vibrator array <NUM> is an alternating pattern that alternates between omnidirectional source activations and source gradient activations in successive shots, then residual shot noise from the omnidirectional shot activation can have a relatively strong effect on a subsequent source gradient activation.

By controlling the phase of the successive shots to reduce residual shot noise, the shot interval (the distance or time) between the successive shots can be reduced to increase in-line sampling using the omnidirectional source activations and source gradient activations, without compromising survey data quality. In-line sampling refers to acquiring survey data in response to respective shots of the seismic vibrator array <NUM>. Increasing in-line sampling refers to acquiring a greater amount of survey data, since a larger number of shots are provided.

Increasing in-line sampling can improve results of acquiring survey data. For example, increasing in-line sampling can improve results of performing crossline wavefield reconstruction using survey data acquired in response to source gradient wavefields. Crossline wavefield reconstruction is discussed further below.

In some examples, residual noise removal or reduction can be accomplished by varying the phase of the omnidirectional source activation by <NUM>° from omnidirectional source to omnidirectional source, while keeping the phase of the gradient source constant. For example, in <FIG>, the phase of the omnidirectional source (represented by star <NUM>-<NUM>) can be set at +<NUM>°, while the phase of the next successive omnidirectional source (represented by star <NUM>-<NUM>) can be set at -<NUM>°. Thus, the phases of successive omnidirectional sources are varied. The omnidirectional source <NUM>-<NUM> and omnidirectional source <NUM>-<NUM> thus have a phase difference of <NUM>° from one another. The phases used for the source gradient sources (represented by arrows <NUM>) do not have to be modified. Other combinations of phases can be used to achieve residual shot noise reduction.

Control of the seismic vibrators of the seismic vibrator array <NUM> can also be split based on frequency, for example, such that the separation between the seismic vibrators is optimized to produce a gradient for different bandwidths. In some cases, the seismic source array <NUM> is controlled to produce just higher-frequency source gradients. In other words, the seismic vibrators of the seismic source array <NUM> that are configured to generate higher frequency wavefields are controlled to produce source gradient wavefields for at least certain shots. At lower frequencies, however, the respective seismic vibrators of the seismic vibrator array <NUM> are controlled to be swept in-phase, and thus would produce just omnidirectional source wavefields, and not source gradient wavefields.

For example, in the arrangement of <FIG>, the seismic vibrators <NUM>-<NUM> and <NUM>-<NUM> (that produce wavefields at higher frequencies) can be controlled to alternate between in-phase and antiphase, such that omnidirectional source wavefields and source gradient wavefields are alternately produced from shot-to-shot. However, the seismic vibrators <NUM>-<NUM> (that produce wavefields at a lower frequency) are controlled to be in-phase (so that the seismic vibrators <NUM>-<NUM> do not produce source gradient wavefields).

As noted above, survey data acquired in response to source gradient wavefields (such survey data is referred to as "source gradient data") can be used to perform crossline reconstruction of sources. Reconstruction of a source refers to estimating a source based on actual sources.

<FIG> illustrates a shot pattern produced by the seismic vibrator array <NUM> traversing along shot lines paths <NUM>, <NUM>, and <NUM>. In <FIG>, the darker stars represent actual shot points, while the lighter (dashed or dotted) stars represent reconstructed shot points. A direction of the arrow <NUM>, <NUM>, or <NUM> is the in-line direction (or direction of travel of the seismic vibrator array <NUM>). The crossline direction is the direction represented by dual arrow <NUM>, which is generally perpendicular to the in-line direction. Crossline reconstruction refers to reconstruction of sources (shot data) between actual sources in the crossline direction <NUM>. Crossline reconstruction can be accomplished by performing interpolation between the actual sources. In <FIG>, the reconstructed shot data provided by the crossline reconstruction include reconstructed sources <NUM> and <NUM>. The reconstructed sources <NUM> are between paths <NUM> and <NUM>, while the reconstructed sources <NUM> are between paths <NUM> and <NUM>.

In the use of source gradient data (survey data acquired in response to a source gradient wavefield) for crossline reconstruction, the use of a dedicated low-frequency seismic vibrator(s) can obviate having to employ a low-frequency source gradient wavefield, as crossline reconstruction may not have to be performed at low frequencies. This has the added benefit of increasing low-frequency output, since the source gradient wavefield may result in reduced output energy. Varying the frequency outputs of different seismic vibrators can also allow the seismic vibrators to repeat sweeps at different time intervals, to allow in-line sampling to be varied for different frequencies. In some cases, this may allow for an omnidirectional source wavefield and source gradient wavefield to be acquired without aliasing.

Crossline reconstruction can include beyond Nyquist source side reconstruction. An example of beyond Nyquist source side reconstruction is described in Massimiliano Vassallo et al. , "Crossline Wavefield Reconstruction for Multi- Components Streamer Data: Part <NUM> -MultiChannel Interpolation by Matching Pursuit (MFMAP) Using Pressure and Its Crossline Gradient," SOCIETY OF EXPLORATION GEOPHYSICISTS (<NUM>). Whereas the method of Vassallo et al. performs reconstruction of the receiver side wavefield, it is noted that methods that use measurements of pressure and its crossline gradient can be adapted for application on the source-side (e.g. for source wavefield reconstruction), as the source wavefield and corresponding gradient wavefield have similar properties to the pressure wavefield and its gradient.

In addition to performing crossline reconstruction, <FIG> also depicts in-line reconstruction to reconstruct omnidirectional sources between actual omnidirectional sources in an in-line direction. For example, along path <NUM>, omnidirectional sources represented by lighter stars <NUM> are reconstructed omnidirectional sources provided by in-line reconstruction.

In a seismic ocean bottom survey arrangement as illustrated in <FIG>, survey receiver locations can be fixed. In a traditional water bottom survey, a marine vessel towing a seismic source would repeat source lines at close spacings. However, if source gradients are available using techniques or mechanisms according to some implementations, the source line spacing (spacing between arrows <NUM>, <NUM>, <NUM> in <FIG>, for example), can be increased, such that survey time can be reduced (since fewer shots have to be performed). In the common-receiver domain, the combination of the alternating omnidirectional-gradient source array with a multi- component beyond-Nyquist reconstruction technique can allow a smaller crossline sampling to be recovered from the wider crossline sampling depicted in <FIG>.

To further increase survey efficiency, one alternating omnidirectional-gradient source array <NUM> may be used simultaneously with another alternating omnidirectional-gradient source array <NUM>. For example, the sources may use a simultaneous source technique based on time or phase dithering, phase sequencing, or a frequency-sparse technique. An example of time dithering is described in <NPL>. An example of phase sequencing is described in U. Patent Publication No. <CIT>, which claims priority to Provisional Application No. <CIT>. An example of a frequency-sparse technique is described in <CIT>, which claims priority to Provisional Application No. <CIT>.

By controlling the directionality of the source array it is possible to acquire seismic data that can be used to de-alias the seismic sources using beyond Nyquist reconstruction techniques. By controlling the phase of multiple seismic source arrays is it possible to acquire simultaneous source data that can be more easily separated, for example, where sequences of phases are used to move the energy from one shot into the empty part of the frequency-wavenumber space of another shot.

By using the phase control method, data acquired from simultaneous sources can be made to appear almost identical to aliased data acquired from a single source, by observing the two datasets in the frequency-wavenumber domain. A synthetic data example of this is illustrated in <FIG>.

<FIG> is a plot showing data from a single line of sources sampled at <NUM>. The Nyquist sampling of this data is <NUM>, thus in the frequency wavenumber domain there are six copies of the data. There is one true version of the data (centered on <NUM> Wavenumber) and <NUM> aliased replicas of the data (centered at -<NUM>, -<NUM>, -<NUM>, <NUM>, and <NUM><NUM>/m wavenumber).

<FIG> is a plot showing data from six adjacent lines of sources, with each line being sampled at <NUM>. The phase for each source along the first line remains constant from shot-to-shot, along the second line it varies by <NUM> degrees from shot-to-shot, along the third line by <NUM> degrees, along the fourth line <NUM> degrees, the fifth line <NUM> degrees, and the sixth line <NUM> degrees. This has the effect of shifting the origin of the signal cones in frequency-wavenumber space by <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM><NUM>/m, respectively. Since the wavenumber axis is cyclical, this is equivalent to shifts of <NUM>, <NUM>, <NUM>, -<NUM>, -<NUM>, and -<NUM>. These shifts are the same as the positions at which the six copies of the data occur in the aliased data in the <FIG> plot. Thus, the <FIG> plot representing the six simultaneous sources, appears very similar to the <FIG> plot, which contains only one source.

The differences are much clearer in the time-offset domain. A portion of the data used to generate the frequency-wavenumber plots are shown in <FIG>. These are plotted with a trace spacing of <NUM>, so the <FIG> plot showing the aliased data (<FIG>) has every <NUM> out of <NUM> traces empty, whereas the simultaneous source data (<FIG>) plotted in <FIG> has data on every trace (in fact, each trace contains the data from <NUM> sources).

Problems such as those in <FIG> can be solved using any number of antialiasing methods. In particular, they can be solved by using beyond Nyquist reconstruction methods. This is done by introducing different types of complimentary data. For seismic sources, these types of complimentary data may be due to source arrays emitting omnidirectional and source gradient wavefields, or sources emitting wavefields with other types of directivity pattern. Thus, by using directive sources it is possible to solve aliasing problems like that shown in <FIG>. Such problems are much more difficult to solve using a seismic source that emits only a single directivity pattern, for example, the conventional tuned air gun array.

Likewise, if the simultaneous source data in <FIG> had been acquired with a single directivity pattern, then there is not enough information for the wavefields to be separated. However, since directive sources can be used to solve the beyond-Nyquist aliasing problem represented by <FIG>, this means that such directive sources can also solve the simultaneous source problem in <FIG>.

Thus, the various combinations of embodied features herein relate to the acquisition of simultaneous source seismic data using directive sources and phase control from shot-to-shot to enable a new type of simultaneous source separation. This is done by acquiring the data using source arrays emitting specific directivity patterns, and with the phase for each simultaneous source changing from shot-to-shot in a prescribed way.

Using an array of marine vibrators, the directivity of the output wavefield can be controlled by varying the relative phase of the vibrators within the array, and also by varying the distance between the vibrator elements within the array. For the advanced processing methods required the directivity patterns must be chosen such that they are complimentary to one another.

The choice of phase pattern is more complicated, as a number of factors needs to be taken into account, including the number of simultaneous sources, how those sources are sampled along each source line, and how the sources emitting different directionality patterns are distributed.

In the example in <FIG>, there were six sources that were each sampled within the Nyquist sampling criteria. The required phase pattern then involves defining six regularly sampled points along the wavenumber axis (from minus Nyquist wavenumber to positive Nyquist wavenumber) and defining the phase shift required from shot-to-shot in order to place the origin of each shot at one of those six regularly sampled points. In this case, the following patterns of repeating phase shifts are required: Source <NUM> : <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> degrees; Source <NUM>: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> degrees; Source <NUM> : <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> degrees; Source <NUM>: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> degrees; Source <NUM>: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> degrees; and Source <NUM>: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> degrees.

However, it may be the case that the shots along each line are not sampled within the Nyquist criteria. If instead those shots are aliased, and sampled at an interval of <NUM>, there will then be one true version of the data in frequency-wavenumber space, and one aliased replica. The aliased replica will be centered on the Nyquist wavenumber. Thus, to give a problem of the same complexity, only another <NUM> simultaneous sources can be introduced. The following patterns of repeating phase shifts can be used in this case: Source <NUM> : <NUM>, <NUM>, <NUM> degrees; Source <NUM>: <NUM>, <NUM>, <NUM> degrees; and Source <NUM> : <NUM>, <NUM>, <NUM> degrees.

Effectively, this is a trade-off between the requirements on source sampling, and the number of simultaneous sources that can be used. Of course, the same number of sources could be used with the increased sampling, and this would just make the problem more complex (and would require a different set of repeating phase shifts). Note, that because the same type of directive sources can be used for source separation that can be used for beyond Nyquist reconstruction, the type of data acquired using this method can be used for both simultaneous source separation and wavefield reconstruction.

The fact that the sources may emit different directionality patterns have not been accounted for in the sequences above. In the case where each source emits only a single directionality pattern, then the sequences of phases above can be applied directly. One beneficial way to use directionality patterns may be to have the same source emitting different directionality patterns along each source line. For example, a single source array alternates between emitting the omnidirectional radiation pattern, and the source gradient radiation pattern. In this case, it may be possible to use a single sequence of phases along the line, but it may also be desirable to treat each radiation pattern as a different source. One particular benefit of this, is that it can separate the residual shot noise from one shot to the next, allowing for shorter listening times to be used.

Considering the six source case above, if there were two alternating directivity patterns, then Source <NUM> and Source <NUM> would correspond to the alternating directivity patterns emitted along the first source line, Source <NUM> and <NUM> to the patterns emitted along the second source line, and Source <NUM> and <NUM> to the patterns emitted along the third source line. By acquiring the data in this way, the residual shot noise (the energy remaining from the previous shot) can be separated from the data for the current shot. Note the pairs were chosen such that the difference between the sequences alternates between <NUM> and <NUM> degrees in this example.

<FIG> illustrates a geometry used to demonstrate the separation of simultaneous source data acquired according to aspects of this disclosure. The synthetic data are generated using a modified version of the SEAM model (SEg Advanced Modelling). The triangles <NUM> in <FIG> indicate receiver positions, these may represent hydrophones and/or accelerometers on a seismic streamer cable, or they may represent receivers deployed on the sea bed. The two lines of stars and arrows indicate two source lines <NUM>, <NUM> that are acquired simultaneously. The star <NUM> indicates a source with an omnidirectional radiation pattern (similar to that shown in <FIG>) and the arrow <NUM> indicates a source with a gradient directivity pattern (similar to that shown in <FIG>). Data for each of these source lines is generated and is summed together to represent data that might have been acquired in a simultaneous source experiment.

The sampling between each omnidirectional source (star <NUM>) is <NUM>, and the sampling between each gradient source (arrow <NUM>) is also <NUM>. For this dataset, the Nyquist sampling is <NUM>. Thus each source type is aliased by a factor of four, meaning that in the corresponding frequency-wavenumber plot there will be one true copy of the data and three aliased replicas.

The simultaneous source data can be acquired using phase-shifts from shot-to-shot along each source line that places the simultaneous source data between the aliased replicas introduced by the spatial sampling along that line. In this case, the following sequences can be used: Sequence <NUM> (Source <NUM>, directivity pattern <NUM>): <NUM>, <NUM>, <NUM>, <NUM> degrees; Sequence <NUM> (Source <NUM>, directivity pattern <NUM>): <NUM>, <NUM>, <NUM>, <NUM> degrees; Sequence <NUM> (Source <NUM>, directivity pattern <NUM>): <NUM>, <NUM>, <NUM>, <NUM> degrees; and Sequence <NUM> (Source <NUM>, directivity pattern <NUM>): <NUM>, <NUM>, <NUM>, <NUM> degrees.

These example phase shifts could have been determined by dividing <NUM> by the number of required sequences, and multiplying the result by the sequence number minus <NUM>, i.e., <MAT> where ψs is the required phase shift from shot-to-shot for sequence s, and n is the number of sequences required. Other sequences may be used that are defined by similar expressions.

An example of the data generated from this geometry (see, <FIG>) is shown in <FIG>, wherein each panel shows the data corresponding to <NUM> receivers for each of the six sources. <FIG> show the data in turn, for the simultaneous source lines measured along the line of receivers aligned to the phase of source <NUM> with directivity pattern <NUM>, and source <NUM> with directivity pattern <NUM>, respectively. <FIG> and <FIG> show the desired non-simultaneous data for sources <NUM> and <NUM>, respectively, and <FIG> show the results of using a simultaneous source version of the extended-Generalized Matching Pursuit (E-GMP) algorithm. Extended generalized matching pursuit is described for example in published patent application No. <CIT>. <FIG> show the difference between the desired non-simultaneous data and the separated data (with the gain increased by a factor of <NUM>). It is clear that by using the acquisition scheme described in this disclosure that the simultaneous source data can be separated with a high level of accuracy.

The above describes the combinations of directive sources with phase sequencing to enable simultaneous source separation. An alternative, that is also novel, could be to use a combination of directive sources with other phase based simultaneous source encoding methods, for example, it may be desirable to use pseudo-random phase (or time delays), or to allow a small amount of pseudo-random variation to the sequence of phases from shot-to-shot. As well as using phase based encoding, a spatial encoding scheme such as pseudo-random spatial sampling may also be combined with directive sources.

Various mathematical methods of interpolation and reconstruction can be used with respect to source signals and receiver signals. Various embodiments of vibrators can be used. According to various embodiments, a method where the vibrator physically produces a known and controllable source signal gradient that is in turn used at the receiver to account for a source signal gradient and to apply reconstruction to produce seismic images with a finer spatial sampling is disclosed, and is functionally equivalent to having physically produced source signals from different locations than was actually used. Through reconstruction, resulting data is generated is as if there were sources from source locations where no source was actually present.

Utilizing aspects disclosed herein marine seismic surveys can be planned and implemented to improve efficiencies while obtaining quality data as described for example with reference to <FIG>. In view of the improved efficiencies, quality data may be acquired utilizing seismic ocean bottom surveys as opposed to towed receivers. <FIG> illustrates a seismic survey utilizing omnidirectional source technology where the source vessel <NUM> travels along predefined paths <NUM> (in this case in a back/forth pattern), for example with <NUM> spacing between sail lines (shot lines), while activating the source in a predefined manner. On the outer portion of the illustrated vessel path are a series <NUM> of seismic receivers <NUM> that are located on the seafloor. In <FIG> there are <NUM> source lines <NUM> that provide the source input for the survey, and the survey can take thirty-six hours as a result.

<FIG> illustrates a seismic ocean bottom survey <NUM> in accordance to one or more aspects. The vibrator array <NUM> can be driven so as to know the source gradients or adequate approximations thereof so that fewer source (shot lines) <NUM> can be used relative to the survey in <FIG>, by reconstructing shot lines <NUM> (dashed lines). As opposed to <FIG> the actual source or shot lines <NUM> are spaced about <NUM> meters apart. According to various combinations of embodied features herein with respect to the directed source (source gradients) that are provided by using at least two vibrators as detailed herein activating in anti-phase to produce source gradients, when detected by an omnidirectional receiver <NUM> such allows for reconstruction of shot point data as if from points where no actual shot point was present or occurred, this results in an improvement in efficiency. <FIG> illustrate improvements in the marine ocean bottom survey that formerly took thirty-six hours and thirteen source lines <NUM> in <FIG> took only five shot lines <NUM> and about twelve hours in <FIG> with reconstructed shot lines <NUM> to produce usable data.

With respect to the time and cost comparison of the surveys in <FIG>, it can be estimated that the operational cost of the survey <NUM> (<FIG>) using source gradient technology can be in the magnitude of one half that of a survey that does not use source gradient technology. It is also possible to achieve a situation where the survey using the source gradient technology is at least one third the cost of a survey that does not use the source gradient technology.

A survey can be planned using and being based on directional vibratory sources (dipole sources) producing source gradients, in a number of ways. One way is to first establish the survey area and the resolution (e.g. actual shots versus reconstructed shots) for the survey data. Once the resolution is determined, the desired source lines and/or shot points (actual or reconstructed) can be determined. The frequency(s) can also be determined. Once the desired source lines, shot points and/or frequencies are determined, with respect to the source gradient vibrator, it is then determined which source points (or source lines or combinations thereof) will actually be performed, and which will reconstructed.

Another way of survey design includes determining the survey area and the resolution at which the survey data will occur. Once the resolution is determined, and based on the understanding and attributes of the marine vibrator source gradient technology, the shot lines/points that are needed for use of the marine vibrator source gradient in order to provide a source that can provide the desired resolution of survey data can be determined.

Once the actual shooting path of the source vessel is known, the cost and time of the survey can be determined based at least in part on the speed and tack of the vessel and the time the survey will take, as well as other accountable costs such as fuel, man hours etc. Based on at least some of those variables, a price estimate for a survey using the source gradient technology can be determined.

By way of the source gradient technology it is possible to determine an improvement in efficiency between a survey performed with a monopole source vibrator and a source gradient source vibrator. This can be done by determining the costs associated with a survey that will meet each shot line/point that would be needed with a monopole source, and compare such with that required with a source gradient source vibrator, as shown e.g. in <FIG>. This can be done by a computing device that has a human input device and a data display device.

Software programs can be used for survey planning with source gradient vibrator surveys. The computer programs can take into account various aspects of the survey such as survey area, water depth, desired image/data output, desired resolution, source power, depth of survey into the earth formation, type of earth formation, vessel speed, vessel cost, and other factors.

It is possible for a survey to be designed to fit a certain cost versus quality parameter. It may be the case that a client desires a survey of lesser data accuracy, but that meets a lower cost structure. In that case, the resolution can be lower, and it is possible that the number of reconstructed shot points can be increased at the expense of some data quality. Also, the number of actual shot points can be reduced. Conversely, to the extent a client desires more accuracy and can accept a higher cost, fewer reconstructed shot points can be used and/or more actual shot points can be used. These calculations can be performed by a computer that is programmed to take in various parameters of a survey, and can produce (in a visual manner via a display device) a survey design that will meet those criteria.

A marine seismic survey method according to an aspect of the disclosure includes activating a vibrator array of two or more marine vibrators to emit a plurality of radiation patterns with at least a first radiation pattern that has a first notch at a take-off angle that is not close to vertical, and less than a maximum take-off angle of interest, and at least a second radiation pattern that does not have a notch at a take-off angle close to that first notch. In an example, the first radiation pattern has a take-off angle greater than about <NUM> degrees and less than <NUM> degrees, wherein the maximum take-off angle of interest is <NUM> degrees. The plurality of radiation patterns may be emitted at the same spatial location or at different spatial locations.

<FIG> is a block diagram of a computer system <NUM>, which can be part of the controller <NUM> shown in <FIG>. The computer system <NUM> includes a seismic vibrator control module <NUM>, which is executable on one or multiple processors <NUM> to control seismic vibrators of the seismic vibrator array <NUM>. The computer system <NUM> can also include a processing module <NUM>, which is executable on the processor(s) <NUM> to perform any of the tasks discussed above, such as crossline reconstruction, in-line reconstruction, up-down source side wavefield reconstruction, and/or multi-component imaging, in some examples. Note that the processing module <NUM> can be provided in a computer system that is separate from a computer system including the seismic vibrator control module <NUM>. The processor(s) <NUM> can be coupled to a network interface <NUM> (to allow the computer system <NUM> to communicate over a network) and a storage medium (or storage media) <NUM>, to store data and machine-executable instructions.

Claim 1:
A marine seismic survey method, comprising activating a vibrator array (<NUM>) of two or more marine vibrators (<NUM>) to emit a plurality of radiation patterns, characterized in that the relative phases of two or more marine vibrators (<NUM>) of the array (<NUM>) located at the same depth are controlled so as to produce complimentary source gradient wavefields,
wherein
- a notch corresponds to a polarity change from positive to negative; and
- a notch in a source gradient wavefield corresponds to a peak in the complementary source gradient wavefield, and wherein the complimentary source gradient wavefields comprise at least a first radiation pattern that has a first notch at a take-off angle that is not close to vertical, and less than a maximum take-off angle of interest, and at least a second radiation pattern that does not have a notch at a take-off angle close to that first notch.