WO2017108707A1

WO2017108707A1 - System and method for acquisition of seismic data

Info

Publication number: WO2017108707A1
Application number: PCT/EP2016/081774
Authority: WO
Inventors: Guido Jozef Maria Baeten; Ying Ji; Zijian Tang
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2015-12-22
Filing date: 2016-12-19
Publication date: 2017-06-29

Abstract

At least two groups of at least two concerted sources within a first cluster of concerted sources are involved in a system and method for acquisition of seismic data. The groups of concerted sources each comprise a first concerted source, which is induced to start a frequency sweep at a certain time. Every remaining concerted source within the same group is induced to start its respective frequency sweep at the same time as the first one, or within a preselected maximum period later than the first one. The first concerted source in one of the groups is induced to start its frequency sweep upon expiry of a certain delay (slip time) following the preselected maximum period of the other of the groups. The slip time comprises a predetermined fixed minimum amount of slip time supplemented with a relatively small amount of variable slip time, which represents a slip time dither.

Description

SYSTEM AND METHOD FOR ACQUISITION OF SEISMIC DATA

Field of the invention

The present invention relates to a system and method for acquisition of seismic data by recording seismic signals induced by a plurality of sources with a common seismic receiver .

Background of the invention

Seismic acquisition has been of vital importance for exploration of hydrocarbons, such as oil and/or natural gas, from subsurface earth formations, and it is becoming

increasingly used in the context of monitoring the subsurface earth formations during production of these hydrocarbons as well .

The principle of seismic acquisition is that a seismic source is employed to induce seismic waves that propagate downwardly through the subsurface earth formation. The downwardly-propagating seismic waves are reflected by one or more geological structures within the subsurface earth formation, which act as partial reflectors for the seismic waves. It is possible to obtain information about the geological structure of the subsurface earth formation from seismic waves that undergo reflection within the subsurface earth formation and is subsequently acquired by one or more seismic sensors (generally referred to as seismic

"receivers") . Reflected seismic waves are typically recorded during a so-called listening time. Longer listening times allow recording of multiple reflection events and/or

reflection events that occurred deeper under the earth surface .

Various specific methods for seismic acquisition have been described in the literature. One such method, disclosed in pre-grant patent application publication US 2003/0210609 Al, makes use of a so-called slip-sweep acquisition technique first proposed by Justus Rozemond at the 66^TH annual SEG meeting, 1996, in Denver (AQC 3.2 pp. 64-67), which allows for a reduction in the cycle time by efficiently timing multiple sources. The multiple sources are vibrators, employed to transmit seismic energy into the ground. When actuated, the vibrators start emitting the seismic energy in the earth formation at a frequency that changes over time during a period of time. This is referred to as a frequency sweep, and the period of time is called sweep time. In the method of US patent 7,050,356, the vibrators are grouped in vibrator groups, such that at least one of a first and a second vibrator group comprises at least two vibrators. The vibrators are actuated in a so-called simultaneous slip-sweep method. In this method, the or each vibrator in the first vibrator group is actuated at time TQ, and the or each vibrator in the second vibrator group is actuated at time Tl, whereby TQ<T]_<TQ+S+L wherein S is the sweep time and L is the listening time of the first vibrator group.

Summary of the invention

In accordance with a first aspect of the present

invention, there is provided a system for acquisition of seismic data by recording seismic signals induced by a plurality of sources with a common seismic receiver,

comprising:

- a first cluster of sources comprising a plurality of concerted sources grouped in N concerted groups, whereby N is a natural number greater than or equal to two (N > 2), said N concerted groups comprising at least an n^tn group and an (n-l)th group, wherein n is a positive integer number whereby 2 < n < N, and wherein each of the n^h group and an (n-l)th group of concerted sources comprises at least two concurrent sources ; - a common seismic receiver configured to measure seismic signals induced by said plurality of concerted sources from at least the first cluster;

- an n^tn g_roUp actuator system arranged to actuate each concerted source in the nth group of the N concerted groups, wherein each concerted source in said n^tn group is induced to perform a frequency sweep lasting for a predetermined first sweep period SP ( 1 ) . wherein a first concerted source of the _nth g_roUp is induced to start its frequency sweep at time T_n and wherein every remaining concerted source in the nth group of the N concerted groups is induced to start its respective frequency sweep on T_n or within a preselected maximum period

^ATn,max later than T_n;

- an (n-l)th g_rQup actuator system arranged to actuate each concerted source in the (n-l)th g_rQup of the N concerted groups, wherein each concerted source within said (n-l)th group is induced to perform a frequency sweep lasting for the predetermined first sweep period SP( 1 ) , wherein a first concerted source of the (n-l)th g_rQup is induced to start its frequency sweep at time T_(n__]_₎ and wherein every remaining concerted source in the (n-l)th g_rQup of the N concerted groups is induced to start its respective frequency sweep on ^T ₍n- 1 ) ^or within a preselected maximum period ΔΤ _(n_i _{) f max} later than ^'ΐ (_n-i) ;

- a first cluster controller configured in communication with at least the nth _ancj (n-l)th g_rQup actuator systems, and configured to impose T_n is later than the respective starting times of every concerted source in the (n-l)th g_rQup by at least a first slip time ST ( 1 ) , such that T_N = T ( _N_ ]_ ) +

AT _{( N}_ _{1 ) F MAX} + ST⁽D ; wherein ST*¹⁾ = ST⁽¹⁾ _min + AST*¹⁾, wherein ST⁽¹⁾ _min is a predetermined fixed minimum first amount of slip time which is the same for each sweep in the first cluster whereby L (1) < ST⁽¹⁾ _mj__n < L⁽¹⁾+SP⁽¹⁾, wherein L*¹) is a predetermined first listening period and wherein AST^⁾ represents a first slip time dither in a range of from 0 to 0.10 X ST (1 ) _mj__n .

In accordance with a second aspect of the invention, there is provided a method for acquisition of seismic data by recording seismic signals induced by a plurality of sources with a common seismic receiver, comprising:

- operating a first cluster of sources comprising a plurality of concerted sources grouped in N concerted groups, whereby N is a natural number greater than or equal to two (N > 2), said N concerted groups comprising at least an n^tn group and an (n-l)th group, wherein n is a positive integer number whereby 2 < n < N, and wherein each of the nth g_rQup and an (n-l)th group of concerted sources comprises at least two concurrent sources;

- recording seismic signals induced by said plurality of concerted sources from at least the first cluster measured in a common receiver;

- actuating each concerted source in the nth group of the N concerted groups by means of an nth group actuator system, wherein each concerted source in said nth group is induced to perform a frequency sweep lasting for a predetermined first sweep period SP(1), wherein a first concerted source of the nth group is induced to start its frequency sweep at time T_n and wherein every remaining concerted source in the nth group of the N concerted groups is induced to start its respective frequency sweep on T_n or within a preselected maximum period ^ATn,max later than T_n;

- actuating each concerted source in the (n-l)th g_rQup of the N concerted groups by means of an (n-l)th g_rQup actuator system, wherein each concerted source within said (n-l)th group is induced to perform a frequency sweep lasting for the predetermined first sweep period SP ( 1 ) , wherein a first concerted source of the (n-l)th g_rQup is induced to start its frequency sweep at time T_(n__]_₎ and wherein every remaining concerted source in the (n-l)th g_rQup of the N concerted groups is induced to start its respective frequency sweep on ^T ₍n- 1 ) ^or within a preselected maximum period ΔΤ _(n_]__{) iinax} later than T_(n__]j;

- controlling the n^h and (n-l)th g_rQup actuator systems by means of a first cluster controller, whereby imposing T_n is later than the respective starting times of every concerted source in the (n-l)th g_rQup by at least a first slip time S T⁽D, such that T_n = T₍n-l₎ + ΔΤ _max + ST⁽D ;

wherein ST⁽!⁾ = ST⁽¹⁾ _m:L_n + AST⁽!⁾, wherein ST⁽¹⁾ _m:L_n is a predetermined fixed minimum first amount of slip time which is the same for each sweep in the first cluster whereby < ST⁽¹⁾ _mj__n < L⁽¹⁾+SP⁽¹⁾, wherein is a predetermined first listening period and wherein AST^⁾ represents a first slip time dither in a range of from 0 to 0 . 1 0 X ST ( 1 ⁾ _mj__n .

The invention will be further illustrated hereinafter by way of example only, and with reference to the non-limiting drawing .

Brief description of the drawing

Fig. 1 shows a schematic example of a seismic acquisition system and method; and Fig. 2 shows a schematic graph of concerted source groups with dithered slip time.

These figures are not to scale. Identical reference numbers used in different figures refer to similar

components. The person skilled in the art will readily understand that, while the invention is illustrated making reference to one or more a specific combinations of features and measures, many of those features and measures are functionally independent from other features and measures such that they can be equally or similarly applied

independently in other embodiments or combinations.

Detailed description of the invention

The present disclosure involves a first cluster of sources comprising a plurality of concerted sources grouped in N concerted groups. N is a natural number greater than or equal to two (N > 2) . Thus, there are at least two groups of concerted sources within the first cluster, whereby each of the at least two groups comprises at least two concerted sources. Each of the groups of concerted sources comprises a first concerted source which is induced to start its

frequency sweep at a certain time, and every remaining concerted source within the same group is induced to start its respective frequency sweep at the same time as the first one, or within a preselected maximum period later than the first one. A common receiver may be employed to measure seismic signals induced by said plurality of concerted sources from at least the first cluster.

The term "common" in "common receiver" in the context of this disclosure means "shared". The signals from multiple sources are measured by the shared receiver, and recorded. Recording can be done in the receiver, or at a central location for a plurality of common receivers . Similar to the method of US patent 7,050,356 the first concerted source that is induced to start its frequency sweep in one of the at least two groups is later than the

respective starting times of every concerted source within the other of the at least two groups, by at least a first slip time after the preselected maximum period of the other of the at least two groups. However, it is presently proposed that the first slip time comprises a predetermined fixed minimum first amount of slip time supplemented with a relatively small amount of variable slip time which

represents a first slip time dither.

The first slip time dither is suitably a relatively small amount of additional slip time compared to predetermined fixed minimum first amount of slip time. The predetermined fixed minimum first amount of slip time is suitably selected longer than the listening time and shorter than the listening time plus the sweep period.

The relatively small amount of variable slip time may for instance be in a range of from 0 to 0.10 times the

predetermined fixed minimum first amount of slip time.

In the context of this disclosure, the term "dither" refers to an intentionally applied temporal noise which helps to subsequently deblend seismic responses and isolate seismic events arising from seismic waves induced by individual sources from the total seismic signal.

The term "listening time" or "listening period" is used to characterize a user-defined amount of time which

determines the maximum record length that will be available for the seismic traces, after data processing. A desired (maximum) record length may be determined by the amount of time after completing of a frequency sweep that useful seismic information is measurable by the seismic receiver. The desired maximum record length may already be taken into account during the acquisition of the seismic data, by ensuring that the slip time imposed between concerted sources is never shorter than the listening time.

By applying the dithered slip time in accordance with the present disclosure, it may be possible to concurrently measure and record signal from one or more additional clusters of concerted sources using the common receiver, in addition to the first cluster of concerted sources described above, without a need to concerted any of the sources within the one or more additional clusters with any of the concerted sources within the first cluster. An advantage of this proposal is that no inter-cluster concerting is needed, in other words there is no need for any source synchronization between clusters. Hence the number of clusters is not limited and they can be relatively far away from each other without the need for any concerting between the clusters .

In the context of the present disclosure, the term

"concurrent" is used when at least two seismic sources are induced to start close enough in time that is overlap in respective listening periods in time-frequency domain, regardless of whether these at least two sources are together concerted or independent from each other. The sources can be but do not have to be synchronous. Thus, in the present disclosure, concerted sources within a single group in a single cluster are concurrent with each other. Sources belonging to different clusters can also be concurrent, while there is no concerted relationship between these sources.

So, for example a second cluster of concerted sources wherein a plurality of groups of concerted sources are mutually timed using a second slip time dither, can be used concurrently with the first cluster of concerted sources.

All concerted sources within a single cluster may be programmed to perform the identical frequency sweep parameters, including sweeping for the same length of time, with the same sweep waveform, at the same sweep rate and over the same frequency range, and with the same phase.

Comparing one cluster to another cluster, on the other hand, the frequency sweep parameters may be identical or different. An example of employing clusters having mutually different sweep parameters is where the respective ranges of the frequency sweeps employed in first and second clusters may be different. Particularly, one may be covering lower frequencies than the other.

Suitably, the concerted sources within a single concerted group are arranged in a coherent shooting relationship relative to each other. In a coherent shooting relationship, pilot signals for each of the coherent concerted sources may be identical to each other. A predetermined constant time difference between the starting times of the concerted sources within a single concerted group whereby accumulation of all these time differences within the n^tn group added up is smaller than or equal to AT_n^_max, and whereby accumulation of all these time differences within the (n-l)th g_rQup added up is smaller than or equal to ΔΤ _(n_i _{) f max} .

Figure 1 schematically illustrates a system and method for acquisition of seismic data. The system is set up to record seismic signals induced by a plurality of sources (111,112,121,122) with a common seismic receiver 10. The sources are operated in clusters, each comprising concerted groups of concerted sources. Concerted, in this context, means that the sources are actuated in dependence of each other. Figure shows a first cluster 100 and an optional second cluster 200. The sources may be vibrators, which are actuated to perform a frequency sweep over a sweep period. The frequency sweep traditionally starts at a lower frequency and ends at an upper frequency. For typical vibrators, the lower frequency may for instance be 5 Hz and the upper frequency may for instance be 80 Hz.

The concerted sources 111,112,121,122 in the first cluster 100 are grouped in a plurality of concerted groups . A first concerted group 110 and a second concerted group 120, each having two concerted sources, are shown in Figure 1 as an example. However, any plurality of concerted sources grouped in N concerted groups, whereby N is a natural number greater than or equal to two (N > 2), may be employed. The N concerted groups comprise at least an n^tn group (here depicted as the second group 120) and an (n-l)th g_rQup

(depicted as the first group 110), wherein n is a positive integer number whereby 2 < n < N. However, for the remainder of this description it will be assumed n = 2, and the (n-l)th group will be indicated by the first group 110 and the n^h group by the second group 120.

Each concerted group has a group center-of-gravity spot which indicates the center of gravity location averaged over all the concerted sources within the concerted group. For instance, the first and second group center-of-gravity spots of the concerted groups in the first cluster 100 are

schematically represented by X symbols 118 and 128. Each cluster has a cluster center-of-gravity spot, which indicates which indicates the center of gravity location of all the concerted groups within the cluster, averaged over all the concerted groups within the cluster. The cluster center-of- gravity spot of the first cluster 100 is schematically represented by ® symbol 108.

The sources within any single concerted group are concurrent sources. The consequence of concurrency of the sources is that the wave fields generated in the earth are overlapping with each other in time-frequency domain. Each of - li the concerted groups 110,120 has a group actuator system arranged to actuate each concerted source within the actuated group to which the group actuator belongs. Thus the first concerted group 110 has first group actuator 115 and the second concerted group 120 has second group actuator 125. Generally, the n^h group actuator system is configured to actuate each concerted source within the n^h group and the the (n-l)th g_rQup actuator system is configured to actuate each concerted source within the (n-l)th g_rQup. A first cluster controller 105 is configured in communication with the group actuator systems within the first cluster 100.

Each concerted source in the first cluster 100 is induced to perform a frequency sweep lasting for a predetermined first sweep period SP ⁽1⁾ . during which time the frequency is ramped up from f ^^_mi_n to f ⁽ ^ ' _max . Fig. 2 shows a plot of frequency against time. The ramping up of frequency is illustrated linear in time, but other ramp patterns can be used if desired. Fig. 2 further shows a listening time is maintained for each frequency in the frequency sweep.

The first concerted source 111 of the first group 110 is induced to start its frequency sweep at time T]_. Every remaining concerted source in the first group 110 is induced to start its respective frequency sweep on T]_ (as illustrated in Fig. 2), or within a preselected maximum period AT]_^_max later than T]_. The first concerted source 121 of the second group 120 is induced to start its frequency sweep at time T2.

Every remaining concerted source in the second group 120 is induced to start its respective frequency sweep on T2, or within a preselected maximum period ^2 , max later than T2. The first cluster controller 105 is configured to impose T2 is later than the respective starting times of every concerted source in the first group 110, by at least a first slip time STi¹) . Thus ^2 = ^τ1 ^{+ AT}l,max ⁺ STi¹), or, generally for any 2 < n < N: T_n = T_(n_₁₎ + AT_(n__{1) finax} +

ST(1) . (In the special case illustrated in Fig. 2, all concerted sourced within the first group 110 are actuated simultaneously, which effectively means AT_]_^_max = 0, so that

T₂ = T_]_ + ST (!) . )

The slip time ST^) is composed of a predetermined fixed minimum first amount of slip time ST(l)_mj__n that is the same for each sweep in the first cluster, and a variable amount of slip time AST^), which introduces a first slip time dither.

In equation format: ST*¹) = ST(¹)_mj__n + AST ( 1 ) . The minimum amount of slip time is selected longer than the listening period, to allow signal processing with slip-sweep methods (for instance involving cross correlation with the pilot sweep signal) . The maximum amount of slip time is selected as the sweep time plus the listening period. Longer slip times are not productive compared to standard flip-flop acquisition. In equation format: < ST(l)_mj__n <

L ( 1 ) +SP ( 1 ) , wherein is the predetermined first listening period. The first slip time dither, AST^), is suitably selected in a range of from 0 to 0.10 X ST (1) _m-j__n .

Preferably, the first slip time dither may be selected in a smaller range, for instance in a range of from 0 to 0.020 X ST (1 ) _mj__n, preferably in a range of from a range of from 0 to

0.010 X ST (1 ) _mj__n . A smaller range provides a higher

acquisition productivity, at the cost of signal loss during post acquisition source separation.

Additional clusters may be employed. Figure 1 shows, as an example, a second cluster 200, which can be operated fully independently from the first cluster. This means that no concerted inter-cluster source actuation is necessary between sources in the first cluster and sources in the second cluster. This is useful, as clusters can be operated at large distances from each other. The second cluster 200 may be identical to the first cluster 100, although different numbers of concerted groups and different numbers of

concerted sources may be provided. Generally stated, the second cluster 200 comprises a second plurality of concerted sources (211,212,221,222) grouped in M concerted groups (210,220), whereby M is a natural number greater than or equal to two (M > 2) . The M concerted groups comprise at least an m^h group 220 and an (m-l)th g_rQup 210, wherein m is a positive integer number 2 < m < M. Each of the m^h group 220 and an (m-l)th g_rQup 210 of concerted sources comprises at least two sources 211,212, and 221,222 respectively. None of the concerted sources within the second cluster 200 coincides with any of the concerted sources within the first cluster 100, and none of the concerted sources within the second cluster 200 is concerted with any of the concerted sources within the first cluster 100.

An mth group actuator system 225 is arranged to actuate each concerted source (221,222) in the m^h group 220, wherein each concerted source in said m^h group is induced to perform a frequency sweep lasting for a predetermined second sweep period SP ⁽2⁾ . A first concerted source 221 of the m^h group is induced to start its frequency sweep at time T_m, and every remaining concerted source in the m^h group 220 is induced to start its respective frequency sweep on T_m, or within a preselected maximum period AT_m^_max later than T_m. An (m-l)th group actuator system 215 is arranged to actuate each concerted source (211,212) in the (m-l)th g_rQup 210 of the M concerted groups. Each concerted source (211,212) within the (m-l)th group 210 is induced to perform a frequency sweep lasting for the predetermined second sweep period SP(2) . The first concerted source 211 of the (m-l)th g_rQup 210 is induced to start its frequency sweep at time ^'ΐ (_m- ) , and every remaining concerted source (212) in the (m-l)th g_rQup 210 is induced to start its respective frequency sweep on T_(m-i₎, or within a preselected maximum period ΔΤ _(m-i _{) r max} later than T_(m__]j . A second cluster controller 205 is configured in communication with at least the m^h group actuator system 225 and (m-l)th group actuator system 215. It is configured to impose T_m is later than the respective starting times of every concerted source in the (m-l)th group by at least a second slip time ST (2), such that T_m = T_(m__]_₎ + ^AT ₍m-l₎,max ⁺ ST<²⁾. In this equation, ST<²⁾ = ST⁽²⁾ _mi_n + AST⁽²⁾, wherein ST⁽²⁾ _mj__n j__{s a} predetermined fixed minimum second amount of slip time which is the same for each sweep in the second cluster and AST'²⁾ represents a second slip time dither in a range of from 0 to 0.10 X ST⁽²⁾ _mj__n. The predetermined fixed minimum second amount of slip time in the second cluster 200 full fills the condition L<²⁾ < ST⁽²⁾ _mj__n <

L⁽²⁾+SP⁽²⁾, wherein L⁽²⁾ is a predetermined second listening period. All concerted source within one group are laterally separated from each other.

The second frequency sweep may cover a frequency range that is different from the first frequency sweep. This may be done to obtain supplementary data. Some overlap may exist.

For instance, the second frequency range between f ⁽²⁾ _min and f ⁽2⁾ _max may be selected as follows: f ^min ^{< f} ^min < _f(l) _{d f}(2) > _f(l)

^L max ^{dIlu L} max ^ ^L max-

Suitably, the first slip time dither AST^⁾ and the second slip time dither AST ⁽2⁾ _are unpredictable, or quasi unpredictable. The less predictable, the more incoherent the contribution in signals from other concerted source groups . The group controller for each cluster may comprise a random generator or quasi-random generator, configured to determine the respective slip time dithers.

In addition to the slip time dither, the group actuator systems may each impose a source dither with each concerted group for every concerted source that is actuated after the first concerted source of the group that is controlled by the group actuator system. Such source dither is suitably between zero and 5% of the sweep time.

However, because there is a dithered slip time between concerted groups in a selected cluster, it is also possible to maintain coherent firing sequences within concerted groups. Furthermore it allows for separating the signals from the cluster from other signals that may be induced in the common receiver 10 from other clusters that may operate contemporaneously with the selected cluster. In such a case, each group actuator system imposes predetermined constant time differences between the starting times of the concerted sources within a selected concerted group. Accumulation of all these time differences within a selected concerted group added up is smaller than or equal to a preselected maximum period. In a special case, the preselected maximum period is zero, in which case each concerted source within a single selected concerted group starts its frequency sweep exactly simultaneously with the first concerted source within that selected concerted group. Particularly in case of these coherent firing sequences within concerted groups, it is advantageous if the sources within one concerted group are relatively close to each other. This facilitates the subsequent separation of the sources from the blended signals. Relatively close in this context means that every concerted source of any single concerted group within one selected cluster has another concerted source belonging to the same single concerted group located within a lateral vicinity radius of half of a minimum apparent seismic wavelength induced by the frequency sweep. In equation format, the lateral vicinity radius equals VQ /

2f '^'max' wherein VQ represents a minimum apparent seismic velocity of a wave in the ground induced by the concerted sources and C represents a cluster identification number.

In a practical example, all respective group center-of- gravity spots (118,128) of the N concerted groups (110,120) within the first cluster 100 are within a first cluster vicinity radius of 100 m from a first cluster center-of- gravity spot 108 for the first cluster 100. Likewise, all respective group center-of-gravity spots (218,228) of the M concerted groups (210,220) within the second cluster 200 are within a second cluster vicinity radius of 100 m from a second cluster center-of-gravity spot 208 for the second cluster 200. The first cluster center-of-gravity spot 108 and the second cluster center-of-gravity spot 208 are preferably more than 500 m, more preferably more than 1 km, most preferably more than 5 km, removed from each other. In other words, the concerted groups within each cluster are

preferably relatively close to each other compared to the distance between respective clusters.

The system described herein may be deployed on land or in a marine environment off-shore. Suitably, the sources are vibrators. A seismic vibrator source for use on land consists generally of a baseplate configured in contact with the ground. The baseplate is usually supported on a truck. A seismic wave is induced in the subsurface earth formation by applying a vibratory force to the plate, and this is typically done by applying a control waveform known as a "pilot sweep" in the vibrator actuator system. The pilot sweep is generally a constant amplitude swept frequency signal, although the amplitude of the vibration may in practice be ramped up and down at the start and,

respectively, finish of the sweep, for instance to manage inertia of the vibrator mass. Marine vibrators are also available, as evidenced by for instance an article from Western Geco: "Marine Vibrators and the Doppler Effect", by Dragoset, which appeared in Geophysics, Nov. 1988, pp. 1388- 1398, vol. 53, No. 11. More recently, Geokinetics has introduced its AquaVib(TM) marine vibrator. Other examples exist .

Table 1 contains a legend of mathematical symbols used herein .

Table 1

N Number of concerted groups of concerted sources within the first cluster.

M Number of concerted groups of concerted sources within the second cluster.

n A number designating the nth concerted group of concerted sources out of the N groups within the first cluster.

m A number designating the m^h concerted group of concerted sources out of the M groups within the second cluster.

T_n Starting time of a frequency sweep of the first concerted source within the n^h concerted group of concerted sources . Starting time of a frequency sweep of the first concerted source within the m^h concerted group of concerted sources .

^Δτη, max A preselected maximum period later than T_n before which every remaining concerted source in the n^h group of the N concerted groups is induced to start its respective frequency sweep.

^Δ¾, max A preselected maximum period later than T_m before which every remaining concerted source in the m^h group of the M concerted groups is induced to start its respective frequency sweep.

sp ( i ) First sweep period, the sweep period employed in frequency sweeps within the first cluster.

SP⁽2⁾ Second sweep period, the sweep period employed in frequency sweeps within the second cluster.

^Lf (1) mm■ First lower frequency at which each frequency

sweep for every concerted source in the first cluster starts.

f ^{L (}i⁾ max First upper frequency at which each frequency

sweep for every concerted source in the first cluster ends .

f ^L (2) mm. Second lower frequency at which each frequency sweep for every concerted source in the first cluster starts.

f (2) Second upper frequency at which each frequency

^L max

sweep for every concerted source in the first cluster ends .

L⁽D Predetermined first listening period determining the record length of seismic responses selected for frequency sweeps within the first cluster. L⁽2⁾ Predetermined second listening period determining the record length of seismic responses selected for frequency sweeps within the second cluster.

ST⁽D First slip time, which determines the starting time of the first concerted source within the groups of concerted sources within the first cluster .

ST⁽2⁾ Second slip time, which determines the starting time of the first concerted source within the groups of concerted sources within the second cluster .

ΰ CT¹(1) mm. Predetermined fixed minimum first amount of slip time used within the first cluster.

AST ⁽!⁾ A first slip time dither time used within the first cluster, which together with the predetermined fixed minimum first amount of slip time determines the first slip time.

ΰ ST¹(2) mm■ Predetermined fixed minimum second amount of slip time used within the second cluster.

AST ⁽2⁾ A second slip time dither time used within the second cluster, which together with the predetermined fixed minimum second amount of slip time determines the second slip time.

The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims.

Claims

SP 1023 - 20 - C L A I M S

1. A system for acquisition of seismic data by recording seismic signals induced by a plurality of sources with a 5 common seismic receiver, comprising:

- a first cluster of sources comprising a plurality of concerted sources grouped in N concerted groups, whereby N is a natural number greater than or equal to two (N > 2), said N concerted groups comprising at least an nth g_rQup and an0 (n-l)th group, wherein n is a positive integer number whereby 2 < n < N, and wherein each of the nth group and an (n-l)th group of concerted sources comprises at least two concurrent sources ;

- a common seismic receiver configured to measure seismic5 signals induced by said plurality of concerted sources from at least the first cluster;

- an n^tn g_roUp actuator system arranged to actuate each concerted source in the nth g_roUp _Qf the N concerted groups, wherein each concerted source in said nth g_roUp ±_s induced to0 perform a frequency sweep lasting for a predetermined first sweep period SP (1) . wherein a first concerted source of the nth g_roUp is induced to start its frequency sweep at time T_n and wherein every remaining concerted source in the nth group of the N concerted groups is induced to start its respective5 frequency sweep on T_n or within a preselected maximum period

^ATn,max later than T_n;

- an (n-l)th group actuator system arranged to actuate each concerted source in the (n-l)th group of the N concerted groups, wherein each concerted source within said (n-l)th0 group is induced to perform a frequency sweep lasting for the predetermined first sweep period SP(1), wherein a first concerted source of the (n-l)th g_rQup is induced to start its frequency sweep at time T_(n__]_₎ and wherein every remaining concerted source in the (n-l)th g_rQup of the N concerted groups is induced to start its respective frequency sweep on T_(n_]_₎ or within a preselected maximum period ΔΤ _(n_i _{) f max} later than T_(n__]j;

- a first cluster controller configured in communication with at least the n^h and (n-l)th g_rQup actuator systems, and configured to impose T_n is later than the respective starting times of every concerted source in the (n-l)th g_rQup by at least a first slip time S T⁽1⁾, such that T_n = T_(n_]_₎ + T(n-l),max + S T⁽D ;

wherein ST*¹⁾ = ST⁽¹⁾ _m:L_n + AST*¹⁾, wherein ST⁽¹⁾ _m:L_n is a predetermined fixed minimum first amount of slip time which is the same for each sweep in the first cluster whereby < ST⁽¹⁾ _mj__n < L⁽¹⁾+SP⁽¹⁾, wherein is a predetermined first listening period and wherein AST^⁾ represents a first slip time dither in a range of from 0 to 0.10 X ST ⁽1 ⁾ _mj__n .

2. The system of claim 1, wherein the first slip time dither AST ⁽ 1 ⁾ is in a range of from 0 to 0.020 X STi¹^-^,

preferably in a range of from a range of from 0 to 0.010 X ^ΰT^{1 (}D mm^■ .

3. The system of claim 1 or 2, wherein the first slip time dither AST^⁾ is unpredictable or quasi unpredictable.

4. The system of any one of claims 1 to 3, wherein the first cluster controller comprises a random generator or quasi- random generator, configured to determine AST^⁾ for each sweep .

5. The system of any one of claims 1 to 4, wherein the n^h group actuator system and the (n-l)th g_rQup actuator system impose a source dither to each concerted source of the nth group and each concerted source of the (n-l)th g_rQup, respectively.

6. The system of claim 5, wherein the source dither is in a range of from 0 to 0.05 X SP*¹⁾.

7. The system of any one of claims 1 to 4, wherein the n^h group actuator system imposes predetermined constant time differences between the starting times of the concerted sources of the n^h group, whereby accumulation of all these time differences within the n^h group added up is smaller than or equal to AT_n^_max, and wherein the (n-l)th g_rQup actuator system imposes predetermined constant time

differences between the starting times of the concerted sources of the (n-l)th g_rQup, whereby accumulation of all these time differences within the (n-l)th g_rQup added up is smaller than or equal to ΔΤ _(n_i _{) f max} .

8. The system of any one of claims 1 to 4, wherein

ATn,max ⁼ 0 ^anc^ each concerted source within a single group starts its respective frequency sweep simultaneously with the first concerted source within that single group.

9. The system of any one of claims 1 to 8, wherein each concerted source in the n^h group is laterally separated from each other concerted source in the n^h group, and wherein each concerted source in the (n-l)th g_rQup is laterally separated from each other concerted source in the (n-l)th group .

10. The system of any one of claims 1 to 9, wherein the frequency sweep for every concerted source in the first cluster starts at a first lower frequency f ^min ^anc^ ends at a first upper frequency f (l)_max, wherein f^min < ^f ^ max ·

11. The system of claim 9 combined with claim 10, wherein every concerted source of any single concerted group within the first cluster has another concerted source belonging to the same single concerted group that is located within a lateral vicinity radius of half of a minimum apparent seismic wavelength induced by the frequency sweep, which equates to

VQ / 2f ⁽l⁾ _max wherein VQ represents a minimum apparent seismic velocity of a wave in the ground induced by the concerted sources .

12. The system of any one of claims 1 to 11, further

comprising a second cluster of sources comprising a second plurality of concerted sources grouped in M concerted groups, whereby M is a natural number greater than or equal to two

(M > 2), said M concerted groups comprising at least an m^h group and an (m-l)th group, wherein m is a positive integer number 2 < m < M, and wherein each of the m^h group and an (m-l)th group of concerted sources comprises at least two sources, and wherein none of the concerted sources within the second cluster coincides with any of the concerted sources within the first cluster, and wherein none of the concerted sources within the second cluster is concerted with any of the concerted sources within the first cluster.

13. The system of claim 12, further comprising:

- an mth group actuator system arranged to actuate each concerted source in the m^h group of the M concerted groups, wherein each concerted source in said m^h group is induced to perform a frequency sweep lasting for a predetermined second sweep period SP (2) . wherein a first concerted source of the _mth group is induced to start its frequency sweep at time T_m and wherein every remaining concerted source in the m^h group of the M concerted groups is induced to start its respective frequency sweep on T_m or within a preselected maximum period

^ATm,max later than T_m;

- an (m-l)th g_rQup actuator system arranged to actuate each concerted source in the (m-l)th g_rQup of the M concerted groups, wherein each concerted source within said (m-l)th group is induced to perform a frequency sweep lasting for the predetermined second sweep period SP (2) . wherein a first concerted source of the (m-l)th g_rQup is induced to start its frequency sweep at time T_(m__]j and wherein every remaining concerted source in the (m-l)th g_rQup of the M concerted groups is induced to start its respective frequency sweep on ^T ₍m-1) ^or within a preselected maximum period ΔΤ _(m_i _{) f max} later than ^'ΐ (_m- ) ;

- a second cluster controller configured in communication with at least the m^h and (m-l)th g_rQup actuator systems, and configured to impose T_m is later than the respective starting times of every concerted source in the (m-l)th g_rQup by at least a second slip time ST (2) ₍ such that T_M = T(_M__]_) + ΔΤ _(m_ 1) ,max + ST (2) ;

wherein ST (2) = ST(2)_MIN + AST (2), wherein ST(2)_MIN is a predetermined fixed minimum second amount of slip time which is the same for each sweep in the second cluster whereby L(2) < ST(2)_MIN < L(2)+SP(2), wherein L<²) is a predetermined second listening period and wherein AST (2) represents a second slip time dither in a range of from 0 to 0.10 X ^ΰςτ¹ (2) mm■ · and wherein each concerted source in the m^tn group is laterally separated from each other concerted source in the _mth group, and wherein each concerted source in the (m+l)th group is laterally separated from each other concerted source in the (m+l)th g_rQup .

14. The system of claim 12 or 13, wherein all respective group center-of-gravity spots of the N concerted groups within the first cluster are within a first cluster vicinity radius of 100 m from a first cluster center-of-gravity spot for the first cluster, and wherein all respective group center-of-gravity spots of the M concerted groups within the second cluster are within a second cluster vicinity radius of 100 m from a second cluster center-of-gravity spot for the second cluster, and wherein the first cluster center-of- gravity spot and the second cluster center-of-gravity spot are more than 500 m removed from each other.

15. A method for acquisition of seismic data by recording seismic signals induced by a plurality of sources with a common seismic receiver, comprising:

- operating a first cluster of sources comprising a plurality of concerted sources grouped in N concerted groups, whereby N is a natural number greater than or equal to two (N > 2), said N concerted groups comprising at least an n^tn group and an (n-l)th group, wherein n is a positive integer number whereby 2 < n < N, and wherein each of the nth group and an (n-l)th group of concerted sources comprises at least two concurrent sources;

- recording seismic signals induced by said plurality of concerted sources from at least the first cluster and measured in a common receiver;

- actuating each concerted source in the nth group of the N concerted groups by means of an nth group actuator system, wherein each concerted source in said nth group is induced to perform a frequency sweep lasting for a predetermined first sweep period SP⁽1⁾, wherein a first concerted source of the _nth group is induced to start its frequency sweep at time T_n and wherein every remaining concerted source in the nth group of the N concerted groups is induced to start its respective frequency sweep on T_n or within a preselected maximum period

^ATn,max later than T_n;

- actuating each concerted source in the (n-l)th group of the N concerted groups by means of an (n-l)th group actuator system, wherein each concerted source within said (n-l)th group is induced to perform a frequency sweep lasting for the predetermined first sweep period SP⁽1⁾, wherein a first concerted source of the (n-l)th group is induced to start its frequency sweep at time ^ (n-1) and wherein every remaining concerted source in the (n-l)th group of the N concerted groups is induced to start its respective frequency sweep on ^T ₍n-1₎ ^or within a preselected maximum period ΔΤ _(n_i _{) f max} later than ^'ΐ (_n-i) ;

- controlling the nth _ancj (n-l)th group actuator systems by means of a first cluster controller, whereby imposing T_n is later than the respective starting times of every concerted source in the (n-l)th group by at least a first slip time ST⁽D, such that T_N = T₍n-l₎ + ΔΤ _MAX + ST⁽D;

wherein ST*¹⁾ = ST⁽¹⁾ _m:L_n + AST*¹⁾, wherein ST⁽¹⁾ _m:L_n is a predetermined fixed minimum first amount of slip time which is the same for each sweep in the first cluster whereby < ST⁽¹⁾ _mj__n < L⁽¹⁾+SP⁽¹⁾, wherein L*¹⁾ is a predetermined first listening period and wherein AST^⁾ represents a first slip time dither in a range of from 0 to 0.10 X ST⁽l⁾ _mj__n.