FR3065293A1 - METHOD AND SYSTEM FOR ACQUIRING MARINE SEISMIC DATA - Google Patents

Abstract

The invention relates to a method for acquiring marine seismic data in an observation zone (23), in which: A fleet (21) of independent seismic sources, deployed under water and generating seismic waves, is developed, according to a predefined trajectory (T) between an initial position and an end position, above a plurality of groups of autonomous underwater vehicles each equipped with at least one seismic wave sensor, each group being pre-positioned below and at the right of an initial portion of the trajectory (T) and held in position in a pattern (P1, P2 ... P6) of deployment; The seismic wave sensors of the same group are monitored for measuring the seismic waves when said fleet (21) is moving above said group; and, as long as the final position is not reached, at least one group is moved over which the fleet (21) has finished moving to a new position below and to the right of a portion of the trajectory (T) not yet traveled by the flotilla (21) of seismic sources.

Description

TECHNICAL AREA

The present invention relates generally to marine seismic exploration, and more specifically to a method and system for acquiring marine seismic data in an observation area from a fleet of independent seismic sources and groups of vehicles. autonomous submarines evolving in a coordinated fashion (in formation or in packs) and carrying seismic wave sensors.

CONTEXT REVIEW

The acquisition and processing of marine seismic data generates a profile (an image) of a geophysical structure under the ocean floor. Although this profile does not provide a precise location for oil and gas deposits, it does suggest to those skilled in the art the presence or absence of these deposits. Thus, providing a high resolution image of geophysical structures under the ocean floor is an ongoing process.

Reflective seismology is a process of geophysical exploration to determine the properties of the subsurface of the earth, which is particularly useful for the determination of the deposits indicated above. Reflective marine seismology is based on the use of a controlled source of energy that sends a wave to the seabed. By measuring the time necessary for reflections and / or refractions to return to several seismic wave sensors, it is possible to assess the depth of the characteristics causing these reflections. These characteristics may be associated with submarine hydrocarbon deposits.

A conventional system for generating seismic waves and recording their reflections outside the geological structures present in the subsurface is illustrated in FIG. 1. A ship 1 tows a network of sensors 2 of seismic waves, for example hydrophones, provided on flutes seismic 3 (“streamers” in Anglo-Saxon terminology). The seismic streamers 3 can be arranged horizontally, that is to say, be at a constant depth relative to the surface 4 of the ocean, or they can have other spatial arrangements. The vessel 1 also tows one or more seismic sources 5 configured to generate a seismic wave 6. The seismic wave 6 propagates downward towards the ocean floors 7 and penetrates them until a reflection structure 8 (reflector ) finally reflect the seismic wave. The reflected seismic wave 9 propagates upwards until it is detected by the seismic wave sensors 2 provided on the seismic streamers 3. On the basis of the data collected by the seismic wave sensors 2 then as the ship 1 is moving on a trajectory 10 covering the observation area 11 diagrammed here by a cube, an image of the subsurface is generated.

Figure 2 schematically illustrates a perspective of a system similar to the system of Figure 1, in a conventional configuration. The system comprises a vessel 12 towing, for example at a speed of approximately 4.5 knots, one or more seismic sources 13i and 13 ₂ as well as a network 14 composed of a plurality of seismic streamers 15i to 15i4- In this nonlimiting example, fourteen seismic streamers are shown, but their number varies from one system to another, typically between six and fourteen seismic streamers. When they are towed by the ship 12 which is moving in the direction indicated by the arrow 16, the seismic streamers 15i at 15m form lines generally parallel to the direction of movement. The length of the seismic streamers typically varies according to the systems between 6 and 10 kilometers. In the transverse direction indicated by arrow 17, the lines formed by the seismic streamers 15i to 15i4 are generally regularly spaced, typically by a distance of the order of 100 meters. Each seismic streamer 15i to 15i4 carries a plurality of seismic wave sensors. In the example of Figure 2, only 18i to 18i sensors ₄ located closer to the vessel 12 and 13i sources, ₂ and 13 to 19 _i4 19i sensors located farther from the vessel 12 and 13i sources 13 ₂ have been shown in FIG. 2, for the sake of clarity.

The positions of the sensors 18i to 18 _i4 are determined to obtain a predefined minimum distance (“near offset” in English terminology) between the sources and these sensors. Similarly, the positions of the sensors 19i to 19 _i4 are determined to obtain a maximum distance ( "offset maximum" or "far offset" in English terminology) between the sources and the sensors. Typically, the maximum offset is generally equal to 8 kilometers.

Thus, a system composed of the fourteen seismic streamers 15i to 15i ₄ of length equal to 8 kilometers and regularly spaced 100 meters apart will cover an observation area with a sliding window of 8 kilometers by 1.3 kilometers corresponding to the seismic spread ( "Seismic spread" in Anglo-Saxon terminology) of the system.

However, taking into account the reflection of waves, and more precisely the existence, between a source and a sensor, of a mid-point ("mid-point" in English terminology) vertical to which a wave emitted by the source is reflected to be received by the sensor, the corresponding imaged surface will in fact cover only half of the sliding window approximately. For the nonlimiting example given, the imaged surface, represented schematically in dotted lines under the reference 20, therefore extends over 0.7 km in the direction of arrow 17.

Consequently, to obtain acquisition data corresponding to the entire observation area without coverage holes, the ship must follow a trajectory which will make it possible to pass the seismic streamers at least partially over areas where the ship is already passed. The trajectory followed by the ship generally comprises a plurality of parallel sections partially overlapping two by two, and a plurality of curved sections each connecting two parallel sections and corresponding to a maneuver of half-turn of the ship. The trajectory can be defined so that two parallel sections connected by the same curved section are two partially overlapping sections. Alternatively, two parallel sections connected by the same curved section may be provided distant and without overlap.

In all cases, the number of parallel sections and half-turns necessary for the system to acquire data may become significant depending on the extent of the observation area.

Typically, for an observation zone delimited by a rectangular surface 40 kilometers wide by 60 kilometers long (observation zone or polygon considered as standard in the context of marine seismic studies), the vessel 12 of the system of the figure 2 must plan to cover 86 parallel sections (60 / 0.7) of 40 kilometers each, and 85 curved sections for U-turns.

Assuming that the ship maintains a continuous progression at 4.5 knots, it takes 413 hours for the course of the parallel sections, to which should be added of the order of 4 hours per U-turn, for a total of 753 hours.

In view of the above, there is a need to find acquisition methods and systems which make it possible to significantly reduce the total time necessary for acquiring marine seismic data in a given observation area.

ABSTRACT

The subject of the present invention is a method of acquiring marine seismic data in an observation area, method in which:

-We evolve a fleet of independent seismic sources, deployed underwater to at least a given depth and generating seismic waves, according to a predefined trajectory between an initial position and a final position of said observation area, above a plurality of groups of autonomous underwater vehicles each equipped with at least one seismic wave sensor, each group of autonomous underwater vehicles being pre-positioned below and in line with an initial portion of said predefined trajectory to form an elementary zone for receiving seismic waves in which the autonomous underwater vehicles of the same group are held in position according to a deployment pattern, the elementary zones of reception formed simultaneously by the plurality of groups of vehicles autonomous submarines being successive in the direction of the predefined trajectory;

-We control the seismic wave sensors of autonomous underwater vehicles of the same group so that they receive and record seismic waves during a measurement period corresponding at least to the period during which the fleet of seismic sources evolves at - above the same group; and

-as long as the final position is not reached, a coordinated movement of the autonomous underwater vehicles of at least one group is controlled, above which the flotilla of seismic sources has finished evolving towards a new position to form a new elementary area for receiving seismic waves below and in line with a portion of the predefined trajectory not yet traveled by said fleet of seismic sources.

In addition to the main characteristics which have just been mentioned in the preceding paragraph, the method according to the invention may have one or more additional characteristics among the following:

each group of autonomous submarine vehicles is preferably pre-positioned so that the elementary reception zones formed simultaneously by the plurality of groups of autonomous submarine vehicles are substantially contiguous in the direction of the trajectory;

the seismic source fleet occupies an area of predefined width in a direction transverse to the trajectory, and the deployment pattern of each group of autonomous underwater vehicles extends, in said transverse direction, over a corresponding width preferably substantially at said predefined width;

the deployment pattern of each group of autonomous underwater vehicles can advantageously extend, in a direction parallel to the trajectory, over a length corresponding to a predefined maximum offset;

- groups of autonomous underwater vehicles can be prepositioned or moved to a new position so that there is no overlap between the elementary reception areas formed;

- The trajectory can advantageously be predefined so that the zones to be traversed by the fleet of seismic sources are without overlap;

- The trajectory is for example predefined in a continuous serpentine shape with a plurality of parallel sections two by two and without overlap, and a plurality of curved sections each connecting two parallel and adjacent sections, preferably contiguous;

- In one possible embodiment, the number of groups of underwater motorized vehicles is chosen to correspond, during prepositioning, to at least one parallel section of the trajectory;

each new position for a group of autonomous underwater vehicles is preferably optimized to allow minimal movement of the group between two successive positions.

The invention also relates to a system for acquiring marine seismic data in an observation area, comprising:

a fleet of independent seismic sources capable of being deployed underwater to at least a given depth, and of being controlled to move in a coordinated fashion according to a predefined trajectory to cover said observation area between an initial position and a position final while generating seismic waves; and

a plurality of groups of underwater motorized vehicles, each underwater motorized vehicle being equipped with at least one seismic wave sensor, the underwater motorized vehicles of the same group being capable of evolving in a coordinated manner;

system in which:

each group of autonomous underwater vehicles is able to be pre-positioned below and in line with an initial portion of said trajectory to form a basic area for receiving seismic waves in which the autonomous underwater vehicles of a same group are maintained in position according to a deployment pattern, the elementary reception zones formed simultaneously by the plurality of groups of autonomous underwater vehicles being successive in the direction of the trajectory;

- the seismic wave sensors of autonomous underwater vehicles of the same group are configured to receive and record seismic waves during a measurement period corresponding at least to the period during which the flotilla of seismic sources evolves above said same group; and

-as long as the final position has not been reached, the autonomous underwater vehicles of at least one group above which the flotilla of seismic sources has finished evolving are capable of moving in a coordinated manner to a new position to form a new elementary area for receiving seismic waves below and in line with a portion of the predefined path not yet traveled by said fleet of seismic sources.

The system according to the invention may have one or more additional characteristics among the following:

- the fleet of seismic sources may include high frequency seismic sources and low frequency seismic sources;

- The fleet of seismic sources comprises for example a plurality of autonomous surface equipment, able to follow said trajectory in a coordinated manner, each surface equipment being connected to at least one seismic source of said fleet by means of an umbilical cable;

- At least one autonomous surface equipment can then be configured to deploy underwater, by means of said umbilical cable, the seismic source corresponding to said at least a given depth;

- autonomous underwater vehicles of the same group are able to communicate with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the description which follows, made with reference to the appended figures, in which:

- Figure 1, already described above, is a diagram of a conventional seismic study system using a ship towing seismic sources and streamers;

- Figure 2, already described above, is a schematic perspective view of a conventional configuration of a seismic study system similar to the system of Figure 1;

- Figure 3 schematically shows, in top view, an example of a fleet of seismic sources and a group of autonomous submarine vehicles of a marine seismic data acquisition system according to the invention;

- Figures 4a to 4d illustrate, in schematic top view and for an example of a marine seismic data acquisition system according to the invention, the principle of pre-positioning and movement of groups of autonomous underwater vehicles of the system as the system source fleet moves along its path;

- Figures 5a and 5b schematically illustrate in perspective the situations corresponding to Figures 4a and 4b;

- Figures 6a to 6c illustrate elements of a seismic wave generation system that can be used as a fleet of sources according to the invention;

- Figure 7 illustrates a side view and a front view of an example of an autonomous underwater vehicle capable of being used in a group of an acquisition system according to the invention;

- Figure 8 is a flow diagram of a method of acquiring marine seismic data according to the invention.

DETAILED DESCRIPTION

The invention is based on the combined use on the one hand, of a fleet of independent seismic sources capable of being deployed and operating underwater at least at a given depth, and on the other hand, of a plurality groups of autonomous underwater vehicles (AUV or “Autonomous Underwater Vehicle” in English terminology), each autonomous submarine vehicle carrying at least one seismic wave sensor, the AUVs of the same group being able to move in training towards a desired target position in the ocean.

One advantage of using autonomous underwater vehicles lies in particular in the ease of deployment and recovery of this type of equipment at different locations. These vehicles can indeed be deployed and / or recovered either from a boat or a seaside.

Such AUVs, as well as various possibilities as to their use for seismic studies, are described in numerous patent applications published in the name of the applicant, such as FR documents. 3,000,225, US 2013/0083623, US 2013/0083624 and US 2013/0081564

Here, the present invention proposes to develop the seismic sources of the flotilla according to a predefined trajectory in an observation area, above a plurality of groups of autonomous underwater vehicles, each group being positioned beforehand below and to the right of an initial portion of the trajectory so as to form an elementary reception zone according to a reason for deployment of the autonomous submarine vehicles in the group.

The elementary reception zones formed simultaneously by the plurality of groups of autonomous underwater vehicles can advantageously be successive in the direction of the predefined trajectory, and preferably substantially contiguous.

The seismic wave sensors of autonomous underwater vehicles of the same group are controlled so as to receive and record the seismic waves reflected during a measurement period corresponding at least to the period during which the flotilla of seismic sources evolves above said same group. Preferably, to optimize the autonomy of autonomous underwater vehicles, the measurement period during which the seismic wave sensors of vehicles of the same group are activated does not exceed the period during which the source flotilla evolves above this group.

As soon as the source fleet has finished evolving over a given group of autonomous underwater vehicles, that group moves to a new target position to form a new elementary area for receiving seismic waves below and in line with a portion of the trajectory not yet traveled by said fleet of seismic sources. Each group of autonomous submarine vehicles is thus capable of being used several times at different target positions, as long as the fleet of sources evolves on its trajectory.

In one possible embodiment, the autonomous underwater vehicles of one or more groups are able to move themselves to their new positions.

In another possible embodiment, to save the autonomy time of the autonomous underwater vehicles, it is also possible to provide for using at least one command and control boat to transport the underwater vehicles of at least one group closest to the position it should occupy. The autonomous underwater vehicles of said at least one group are then deployed underwater and reach their positions. After the flotilla has finished evolving above the autonomous underwater vehicles of this group, these vehicles can be recovered by said command and control boat (or even by another command and control boat), in view provide maintenance including recharging the batteries of autonomous underwater vehicles. The command and control boat which has recovered the autonomous underwater vehicles can then transport them as close as possible to their next positions, before deploying them again underwater.

As a variant or in combination, provision may be made for having at least one additional group of underwater vehicles in addition to the groups of autonomous underwater vehicles which have been pre-positioned. This additional group is kept in reserve on a command and control boat and deployed as needed.

FIG. 3 schematically represents, in top view, an example of a flotilla 21 of seismic sources according to the invention. The flotilla 21 here comprises N independent seismic sources 21i to 21n capable of operating underwater. An individual seismic source can be an air gun, a vibration source, or other known seismic sources. The flotilla 21 moves here to the right of FIG. 3, and is illustrated at an instant preceding its passage over a group comprising M autonomous underwater vehicles 22i at 22m, group which has been pre-positioned under the water, as indicated above, to form an elementary reception zone with a deployment pattern Pi of the autonomous underwater vehicles inside the group. To simplify, the group is thus assimilated here and in the following description to the deployment pattern Pi and to the associated elementary reception area.

In a preferred but nonlimiting embodiment, the flotilla 21 of seismic sources occupying an area of predefined width Ls, for example 2.5 kilometers, in a direction transverse to the trajectory, the deployment reason for each group of vehicles under -autonomous sailors extends, in said transverse direction, over a width corresponding substantially to the predefined width Ls, as visible in FIG. 3 with the deployment pattern Pi. Consequently, the imaginary surface (area for acquiring seismic data) extends transversely over the width Ls, unlike a system of seismic sources and streamers towed by a ship for which, as indicated in the introduction, the imaged surface is reduced approximately by half compared to the seismic spreading of the system.

As a result, the trajectory followed by the flotilla 21 of seismic sources can be determined so that the areas to be traversed by the flotilla 21 of seismic sources are without overlap, which considerably reduces the time necessary for acquiring the data.

In addition, the deployment pattern of each group of autonomous submarine vehicles extends, in a direction parallel to the trajectory, over a length Ig, the value of which preferably corresponds to a predefined maximum offset. In FIG. 3, the length Ig of the deployment pattern Pi is fixed at 8 kilometers, which corresponds to a standard maximum offset value.

An example of a seismic study, using the fleet 21 of independent seismic sources of FIG. 3 and a plurality of groups of autonomous underwater vehicles having deployment patterns similar to the pattern Pi of FIG. 3, will now be explained in reference to FIGS. 4a to 4d which illustrate, in diagrammatic view from above, the principle of pre-positioning and displacement of the groups of autonomous submarine vehicles as and when the fleet 21 of sources moves on its trajectory. Reference will also be made to FIGS. 5a and 5b which schematically give a perspective representation corresponding to FIGS. 4a and 4b.

To fully understand the advantages of a system according to the invention compared to the conventional system using a vessel towing seismic streamers described in the introduction, it is considered in this nonlimiting example that the seismic study must allow the acquisition of seismic data in an observation zone 23 delimited by the standard rectangular surface 40 kilometers wide by 60 kilometers long. In this example, it is also assumed that the source fleet 21 moves faster than the groups of autonomous underwater vehicles. For example, the speed of movement of the flotilla 21 is approximately 3 knots while that of the groups is approximately 1 knot.

Figures 4a and 5a show the initial configuration of the acquisition system, respectively in top view and in perspective.

The trajectory T followed by the flotilla 21 of sources has been defined beforehand to optimally cover the observation zone 23 between an initial position (position of the flotilla 21 of sources at the top left of FIG. 4a) and a final position (bottom left of Figure 4a).

The path T here has the shape of a continuous coil, with a plurality of parallel sections two by two, and a plurality of curved sections each connecting two parallel sections and corresponding to a half-turn maneuver of the fleet 21 of sources. The path T is preferably defined so that two parallel sections connected by the same curved section are two sections immediately adjacent and without overlap, preferably contiguous.

The system, in this exemplary implementation of the invention, here comprises six groups (Gi to Gô in FIG. 5a) of autonomous underwater vehicles which have been pre-positioned, in accordance with the invention, below (for example separated by 60 meters from the depth at which the sources of the flotilla will evolve) and in line with an initial portion of the trajectory T, as shown by the six corresponding deployment patterns Pi to Pô shown in FIG. 4a. More precisely, five groups Gi to G5 (assimilated to the patterns Pi to P5 of deployment in FIG. 4a) were prepositioned to the right of the very first rectilinear section of the trajectory T to form five elementary reception zones, while the sixth group Gô (assimilated to the Pô deployment pattern) has been pre-positioned to the right of the start (in the direction of travel of the path T) of the second rectilinear section. It can be seen in FIGS. 4a and 5a that there is advantageously no overlap between the elementary reception zones thus formed, and that the elementary reception zones formed by the six groups in position are substantially contiguous not only in the direction of the trajectory T, but also preferably in the transverse direction (see the patterns P5 and Pô in FIG. 4a, and the groups G5 and Gô corresponding in FIG. 5a).

FIG. 4b represents a particular instant of the study corresponding to a period in which the fleet 21 of sources evolves for the first time above the group of deployment pattern P3. FIG. 4b also represents the new positions towards which the first two groups, above which the flotilla 21 has already passed once, have been moved to form new patterns Pi and P2 corresponding to it. FIG. 5b represents this same situation seen in perspective.

Similarly, FIG. 4c represents a particular instant of the study corresponding to a period when the flotilla 21 is evolving for the first time above the group of deployment pattern P5, and also represents the new positions towards which the third and fourth groups, above which the flotilla 21 has already passed once, have been moved to form new patterns P3 and corresponding P4. At this time, and even when the flotilla 21 has not yet started its first U-turn to cover the second straight section of the path T, the first four groups and the sixth group are therefore already advantageously in place at the right of this second section of trajectory.

Each new position for a group of autonomous underwater vehicles has also been optimized to allow minimal movement of the group between two successive positions.

As can be seen in FIG. 4d, the fifth group (corresponding to the pattern P5 of deployment) here has a particular displacement, due to its positioning at the end of the first section of line. Indeed, the latter must be moved as indicated by the arrow to be positioned just below the deployment pattern Pi in Figure 4d.

As indicated previously, the autonomous underwater vehicles of one or more groups among the groups Gi to Gô may be able to move by themselves to their new positions.

However, to save the autonomy time of autonomous underwater vehicles, it is also possible to use at least one command and control boat to transport underwater vehicles from at least one group closest to the position that 'he must occupy. The autonomous underwater vehicles of said at least one group are then deployed underwater and reach their positions. After the flotilla has finished evolving above the autonomous submarine vehicles of this group, these vehicles can be recovered by said command and control boat (or even by another command and control boat), with a view to provide maintenance including recharging the batteries of autonomous underwater vehicles. The command and control boat which has recovered the autonomous underwater vehicles can then transport them as close as possible to their next positions, before deploying them again underwater. In particular, in the nonlimiting case of the acquisition represented in FIGS. 4a to 4d, taking into account the greatest distance which separates the two successive positions of the group G5 from the deployment pattern P5, it is advantageous to provide that the group G5 is recovered by a command and control boat, brought as close as possible to its new position, then deployed again.

In other words, the movement of groups of AUVs can be carried out directly underwater, or indirectly using one or more control boats from which the AUVs can be recovered, recharged and then redeployed.

As a variant or in combination, provision may be made for having at least one additional group of underwater vehicles in addition to the six groups of autonomous underwater vehicles which have been pre-positioned. This additional group is for example kept in reserve on a command and control boat, and brought to be deployed underwater as and when required. Thus, in the nonlimiting case of the acquisition represented in FIGS. 4a to 4d, instead of moving the group G5 of the deployment pattern P5 as indicated by the arrow in FIG. 4d, it is possible to plan to use an additional group (not shown) which would replace the G5 group for the new position, in order to avoid a displacement of this G5 group.

The movement sequence (direct or indirect) of the groups as the source fleet 21 travels, as shown in FIGS. 4a to 4d, is repeated cyclically as long as the fleet 21 has not reached the position end of the trajectory.

In this example, the groups are pre-positioned and moved so that there is no overlap between the elementary reception areas formed.

The combination according to the invention of a fleet of independent sources evolving in line with groups of autonomous underwater vehicles carrying the seismic wave sensors makes it possible to optimize the trajectory to be followed in the observation area, and by means of Consequently, the duration of acquisition of seismic data. In particular, for the system of FIGS. 4a to 4b, the trajectory T followed by the flotilla 21 comprises only 24 parallel sections (i.e. 60 / Ls with Ls here equal to 2.5 kilometers) of 40 kilometers each, and 23 curved sections for the U-turns. The U-turn maneuvers are also much faster than in the case of streamers towed by a ship. Assuming that the flotilla 21 maintains a continuous progression at 3 knots, it therefore only needs 173 hours for the course of the 24 parallel sections, to which it is advisable to add the order of 0.5 hour per U-turn, a total of 185 hours, much lower than the 753 hours required for a towing vessel to acquire data in the same observation area.

Furthermore, such an acquisition methodology provides several geophysical advantages not equaled by a conventional system. With this invention, the predefined minimum distance is reduced to zero, compared to 100-300 meters minimum with a conventional system. In addition, this invention produces double azimuth offsets, both positive and negative, while a conventional system is limited to a single azimuth. Finally, this invention offers the possibility of placing seismic sensors in different and variable water depths, ranging from very shallow (less than 5 meters) to significantly deep (of the order of 100 meters) while a system classic is constrained by minimum water depths (typically 20 meters) for the safety of the vessel and its equipment and maximum water depths (typically 50 meters) so as not to damage the flutes.

It will be understood at this stage that the system described with reference to FIGS. 3 and 4a to 4d that the implementation of the system has been optimized both on the dimension chosen for Ls and on the number of groups of vehicles used to allow data acquisition to lower costs, and on the assumption that groups of vehicles move (directly or indirectly) more slowly than the fleet of sources. Of course, other system configurations can be envisaged without departing from the scope of the invention, by using other dimensioning parameters, and other hypotheses linked to the displacement capacities of the fleet of sources and groups of vehicles. autonomous submarines.

FIGS. 6a to 6c illustrate, by way of nonlimiting example, elements of a system for generating seismic waves which can be used to produce a fleet 30 of sources according to the invention:

The flotilla 30 comprises a plurality of autonomous surface equipment (USV or "Unmanned Surface Vessel" in English terminology), such as for example the surface equipment referenced 31i and 31 ₂ in FIGS. 6a to 6c. Each surface equipment is also connected to at least one seismic source by means of an umbilical cable. Thus, for example, the surface equipment 31i is connected to the seismic source 32i and / or to its support 33i by means of an umbilical cable 34i, and the surface equipment 312 is connected to the seismic source 322 and / or to its support 332 by means of an umbilical cable 342. An umbilical cable is understood here as comprising a communication cable (coaxial, electrical conductor or optical fiber). An umbilical cable may further include power supply connections. The seismic sources can advantageously be vibration sources. However, one can also consider using air cannons. Seismic sources can transmit at different frequencies. For example, the seismic source 32i carried by the seismic source support 33i is a low frequency source (preferably in a frequency band of the order of 4 to 32 Hertz), while the seismic source 32 ₂ carried by the support 33 ₂ of seismic source is a high frequency source (preferably in a frequency band of the order of 25 to 125 Hertz). In order to reduce interference with waves reflecting from the sea surface, the length of an umbilical cable is preferably dependent on the frequency of the acoustic signal emitted by the corresponding seismic source, so that the sources can evolve underwater at different depths, for example a depth H1 of approximately 25 meters for the low frequency source 32i (FIG. 6b) and a depth H2 of approximately 5 meters for the high frequency source 32 ₂ (FIG. 6c).

At least one autonomous surface equipment 31i, respectively 312, can be configured to deploy underwater, by means of said umbilical cable 34i, respectively 342, the seismic source 32i, respectively 322 corresponding to the given depth. At the end of the acquisition, this same surface equipment can be configured to recover, for example by a command for winding the umbilical cable, the support and the associated seismic source.

The surface equipment, and consequently the supports and the associated seismic sources, are controlled so as to evolve in formation, thus forming the fleet of independent sources within the meaning of the present invention, for example from a non-control vessel. represented with whom they are in communication. As a variant, the supports 33i and 332 are pieces of equipment having guidance and self-propulsion capacities, and are capable of evolving in formation to form the fleet of independent sources within the meaning of the invention. The different sources of the flotilla are controlled to emit acoustic waves while the flotilla is moving along the acquisition path.

A schematic representation (side view and front view) of an autonomous underwater vehicle 40 capable of being used in a group of an acquisition system according to the invention is given in FIG. 7:

The autonomous underwater vehicle 40 comprises a body 41 and a propulsion system 42. The body 40 contains various elements (not shown) such as the operating electronics, a power supply module, memories to store in particular the mission parameters and the measurements carried out. The body also carries, preferably on its upper part, a module 43 for transmitting and / or receiving acoustic signals, and a communication module 44 for exchanging data with acoustic or optical positioning systems. The vehicle 40 is finally provided with at least one seismic sensor 45, for example a hydrophone. For further details concerning possible AUV structures, see the aforementioned documents FR. 3,000,225, US 2013/0083623, US 2013/0083624 and US 2013/0081564

Several autonomous underwater equipment such as that shown schematically in FIG. 7 are configured or controlled to operate in a coordinated fashion within the same group which can, as explained above, be pre-positioned or repositioned according to the deployment reasons such that the patterns Pi to P6 described with reference to Figures 3, and 4a to 4d. By “coordinated” evolution is meant a movement of the various underwater vehicles in the acquisition zone according to a pattern of trajectories, with for example trajectories substantially parallel to one another (displacement in formation), each vehicle occupying positions different, in depth and / or in a plane parallel to the sea surface. For its movement, autonomous underwater equipment can rely on its own propulsion and / or on sea currents.

A hierarchy can be defined within the same group by using one of the vehicles as the master vehicle, the other vehicles then being considered as slave vehicles receiving instructions from the master vehicle. The group defines in this case a pack of vehicles and it is necessary to provide on the various vehicles means of communication between vehicles (including for example the module 43 for the emission and / or the reception of acoustic signals), or at any the least between the master vehicle and each slave vehicle. In this case, the positioning information can be transmitted only to the master vehicle, for example by means of dedicated surface equipment (not shown) serving as a reference point for making it possible to know the position of the vehicles, each slave vehicle being able to follow its trajectory either by calculating its own position in relation to the information received from the master vehicle, or by obeying instructions transmitted by the master vehicle.

As a variant, the vehicles of the same group evolve in swarms, that is to say without any particular hierarchy. In this case, the positioning information, for example coming from the aforementioned dedicated surface equipment, is preferably transmitted to each of the vehicles of the group. All or part of the vehicles can also exchange information allowing them to know at any time their relative position with respect to each other.

The measurements carried out by each of the vehicles in the group can be aggregated and processed after the recovery of the vehicles in the group from the water, by recovering the data stored in each of the vehicles. Alternatively, the measurements can be transmitted to the dedicated surface equipment during the acquisition mission, in real time or periodically, then relayed for further processing to a remote control station (not shown) using any wireless communication system (not shown). Depending on the distance separating the surface equipment and the control station, the most suitable communication system will be chosen, which may in particular be any short or long range radio frequency system, or even a cellular communication system or a communication system. by satellites. In another variant, the measurements can be transmitted to the dedicated surface equipment during the mission, in real time or periodically, for processing during the mission.

An example of steps that can be implemented for a method of acquiring marine seismic data in accordance with the invention is now described with reference to FIG. 8 for a system comprising a fleet of sources capable of evolving in formation of extent Ls in width, and Q groups of AUVs. Note that certain steps, in particular related to the deployment underwater of the various elements of the acquisition system are shown as being carried out successively in a certain order. It will however be understood that some of these steps can be reversed in the order of succession, or even carried out simultaneously. The deployment and recovery processes for autonomous underwater vehicles will not be described here:

An initial step So comprises the determination of the trajectory T which will have to be followed in a coordinated manner by a fleet of independent seismic sources capable of being deployed underwater at at least a given depth, to cover the acquisition zone between a position initial and a final position. The trajectory is preferably optimized as a function in particular of the extent Ls of the fleet, the number Q of groups used, the speed of movement of the various items of equipment (sources and AUV), and possibly other external parameters such as parameters environmental (currents in the sea ...) or climatic. The extent Ls and the number Q of groups used are chosen mainly according to economic and geophysical criteria related to imaging needs and equipment costs, and according to the relative speeds of movement of the source fleet and AUVs .

The Q groups of AUVs are then deployed (steps Si) then prepositioned (step S2) below and in line with an initial portion of the trajectory T to form elementary zones for receiving successive seismic waves in the direction of the trajectory T. The AUVs of the same group are then kept in position while awaiting the passage of the flotilla of seismic sources. The seismic sources are also deployed underwater (step S3) then controlled to generate seismic waves while moving along the path T. The seismic wave sensors of the AUVs of a group are controlled to measure the reflected seismic waves , if possible during a measurement period not exceeding the duration during which the seismic source flotilla evolves above this group. When the source fleet has finished evolving above a given group of AUVs, and as long as the final position of the fleet on the trajectory T has not been reached (step Sô), the AUVs of this group move (directly or indirectly) in a coordinated fashion to a new position to form a new elementary area for receiving seismic waves below and in line with a portion of the trajectory T not yet traveled by the fleet of seismic sources ( step S7).

Claims (15)

1. Method for acquiring marine seismic data in an observation zone (23), method in which: -We evolve (S4) a fleet (21; 30) of seismic sources (21i to 21n; 32i, 322) independent, deployed underwater to at least a given depth (Hl, H2) and generating seismic waves, along a predefined trajectory (T) between an initial position and a final position of said observation zone (23), above a plurality of groups (Gi, G2 ... G6) of autonomous underwater vehicles equipped each of at least one seismic wave sensor, each group of autonomous underwater vehicles being pre-positioned (S2) below and in line with an initial portion of said predefined trajectory (T) to form an elementary reception area of seismic waves in which the autonomous submarine vehicles of the same group are held in position according to a pattern (Pi, P2 ... P6) of deployment, the elementary reception zones formed simultaneously by the plurality of groups of vehicles autonomous submarines being successive in the direction of the predefined path (T); -We control (S5) the seismic wave sensors of autonomous submarine vehicles of the same group so that they receive and record seismic waves during a measurement period corresponding at least to the period during which the flotilla (21; 30 ) of seismic sources evolves above the same group; and -as long as the final position is not reached, a coordinated movement (S7) of the autonomous underwater vehicles of at least one group is controlled above which the flotilla (21; 30) of seismic sources has finished evolve towards a new position to form a new elementary area for receiving seismic waves below and in line with a portion of the predefined trajectory (T) not yet traveled by said flotilla (21; 30) of seismic sources. 2. Method according to claim 1, characterized in that each group of autonomous underwater vehicles is pre-positioned so that the elementary reception zones formed simultaneously by the plurality of groups of autonomous underwater vehicles are substantially contiguous in the direction of the trajectory (T). 3. Method according to any one of the preceding claims, characterized in that the fleet (21; 30) of seismic sources occupies an area of predefined width (Ls) in a direction transverse to the trajectory (T), and in that the pattern (Pi, P2 ... P6) of deployment of each group of autonomous underwater vehicles extends, in said transverse direction, over a width corresponding substantially to said predefined width (Ls). 4. Method according to any one of the preceding claims, characterized in that the pattern (Pi, P2 ... P6) of deployment of each group of autonomous underwater vehicles extends, in a direction parallel to the trajectory ( T), over a length (Ig) corresponding to a predefined maximum offset. 5. Method according to any one of the preceding claims, characterized in that the groups (Gi, G2 ... G6) of autonomous underwater vehicles are pre-positioned or moved to a new position so that it does not there is no overlap between the elementary reception zones formed. 6. Method according to any one of the preceding claims, characterized in that the trajectory (T) is predefined so that the areas to be traversed by the fleet (21, 30) of seismic sources are without overlap. 7. Method according to any one of the preceding claims, characterized in that the trajectory (T) is predefined in a continuous serpentine shape with a plurality of parallel sections two by two and without overlap, and a plurality of curved sections connecting each two parallel and adjacent sections, preferably contiguous. 8. Method according to claim 7, characterized in that the number of groups of underwater motorized vehicles is chosen to correspond, during pre-positioning, to at least one parallel section of the trajectory (T). 9. Method according to any one of the preceding claims, characterized in that each new position for a group of autonomous underwater vehicles is optimized to allow minimal movement of the group between two successive positions. 10. System for acquiring marine seismic data in an observation zone (23), comprising: - a flotilla (21; 30) of seismic sources (21i to 21n; 32i, 32 ₂ ) independent capable of being deployed underwater to at least a given depth (Hl, H2), and to be controlled to move from coordinated manner along a predefined trajectory (T) to cover said observation zone (23) between an initial position and a final position while generating seismic waves; and a plurality of groups (Gi, G2 ... G6) of underwater motorized vehicles (22i to 22m; 40), each underwater motorized vehicle being equipped with at least one sensor (45) of seismic waves, underwater motorized vehicles of the same group being able to evolve in a coordinated manner; system in which: each group of autonomous underwater vehicles is able to be pre-positioned below and in line with an initial portion of said trajectory (T) to form an elementary zone for receiving seismic waves in which the autonomous underwater vehicles of the same group are held in position according to a deployment pattern (Pi to Po), the elementary reception zones formed simultaneously by the plurality of groups of autonomous underwater vehicles being successive in the direction of the trajectory (T); - the seismic wave sensors (45) of autonomous underwater vehicles of the same group are configured to receive and record seismic waves during a measurement period corresponding at least to the period during which the flotilla (21; 30) seismic sources evolves above the same group; and -as long as the final position is not reached, the autonomous underwater vehicles of at least one group above which the flotilla (21; 30) of seismic sources has finished evolving are able to move so coordinated to a new position to form a new elementary zone for receiving seismic waves below and in line with a portion of the predefined trajectory (T) not yet traveled by said flotilla (21; 30) of seismic sources. 11. Acquisition system according to claim 10, characterized in that the fleet (30) of seismic sources comprises high frequency seismic sources (322) and low frequency seismic sources (32i). 12. Acquisition system according to any one of claims 10 to 11, characterized in that the flotilla (30) of seismic sources comprises a plurality of autonomous surface equipment (31i, 3I2), capable of following said trajectory. in a coordinated manner, each surface equipment (31i, 3I2) being connected to at least one seismic source (32i, 322) of said fleet by means of an umbilical cable (34i, 342). 13. Acquisition system according to claim 12, characterized in that at least one autonomous surface equipment (31ι, 3I2) is configured to deploy underwater, by means of said umbilical cable (34i, 342), the source seismic (32i, 322) corresponding to said at least one depth (Hl, H2) given. 14. System according to any one of claims 10 to 13, characterized in that the vehicles (22i to 22m; 40) autonomous submarines of the same group are hierarchized into a master vehicle and slave vehicles. 15. System according to any one of claims 10 to 14, characterized in that the vehicles (22i to 22m; 40) autonomous submarines of the same group are able to communicate with each other. 2/5 E LO CM II ω i k S) 0 'bSd œo êobo C 0 BO BO BO KO KO => it is BO BO BO BO BO v 0 <\ il> II> II> BO BO BO BOBO BO: BOBO: : r \ ^ Pi I _g = 8 km