Patent Publication Number: US-10787235-B2

Title: Methods and underwater bases for using autonomous underwater vehicles for marine seismic surveys

Description:
PRIORITY 
     The present application is a continuation of U.S. application Ser. No. 14/777,395, filed on Sep. 15, 2015, which is a national stage entry of application PCT/EP2014/055576, filed on Mar. 20, 2014, which claims priority to U.S. provisional patent application No. 61/803,617, filed on Mar. 20, 2013. The entire contents of each of the above documents is incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for performing a marine seismic survey using autonomous underwater vehicles (AUVs) that carry appropriate seismic sensors. 
     Discussion of the Background 
     Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure beneath the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process. 
     Reflection seismology is a method of geophysical exploration to determine the properties of earth&#39;s subsurface, which is especially helpful in the oil and gas industry. Marine reflection seismology uses a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits. 
     A traditional system for generating seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in  FIG. 1 . A vessel  100  tows an array of seismic receivers  110  provided on streamers  112 . Streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface  114 . The streamers may have spatial arrangements other than horizontal. Vessel  100  also tows a seismic source array  116  configured to generate a seismic wave  118 . Seismic wave  118  propagates downward and penetrates the seafloor  120  until a reflecting structure  122  (reflector) eventually reflects the seismic wave. Reflected seismic wave  124  propagates upward until it is detected by the receiver(s)  110  on streamer(s)  112 . Based on the data collected by receiver(s)  110 , an image of the subsurface is generated by further analyses of the collected data. Seismic source array  116  includes plural individual source elements, which may be distributed in various patterns, e.g., circular, linear, at various depths in the water. 
     However, this traditional configuration is expensive because the cost of streamers is high. New technologies deploy plural seismic sensors on the bottom of the ocean (ocean bottom stations) to improve the coupling. Even so, positioning seismic sensors remains a challenge. 
     Newer technologies use autonomous underwater vehicles (AUVs) that have a propulsion system and are programmed to move to desired positions and record seismic data. After recording the seismic data, the AUVs are instructed to return to a vessel or underwater base to recharge their batteries and/or transfer the seismic data. Various methods have been proposed for deploying and collecting the AUVs. However, none of the existing methods fully address the needs of a seismic survey that uses AUVs which land on the ocean bottom to collect the seismic data. 
     Accordingly, it would be desirable to provide systems and methods that provide an inexpensive and efficient method for deploying AUVs on the ocean bottom, to record seismic waves, and resurface after recording the data. 
     SUMMARY 
     According to one embodiment, there is a method for cycling autonomous underwater vehicles that record seismic signals during a marine seismic survey. The method includes deploying plural current AUVs on the ocean bottom; recording the seismic signals during the marine seismic survey with plural current AUVs; releasing from an underwater base a new AUV to replace a corresponding current AUV from the plural current AUVs; recovering the current AUV; and continuing to record the seismic signals with the new AUV. 
     According to another embodiment, there is a method for cycling autonomous underwater vehicles that record seismic signals during a marine seismic survey. The method includes recording the seismic signals during the marine seismic survey with plural current AUVs deployed on the ocean bottom; replacing during the seismic survey a current AUV from the plural current AUVs with a new AUV; and continuing to record the seismic signals with the new AUV. 
     According to yet another embodiment, there is a method of rolling autonomous underwater vehicles that record seismic signals during a marine seismic survey. The method includes recording the seismic signals during the marine seismic survey with plural AUVs deployed on the ocean bottom; instructing an AUV from the plural AUVs, after recording the seismic signals, to move to a new location to be surveyed; and continuing to record the seismic signals with the AUV at the new location during the same marine seismic survey. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: 
         FIG. 1  is a schematic diagram of a conventional seismic survey system; 
         FIG. 2  is a schematic diagram of an AUV; 
         FIG. 3  is a high-level schematic diagram of an AUV; 
         FIG. 4  is a schematic diagram of a rolling scheme for deploying AUVs during a seismic survey system; 
         FIG. 5  is a schematic diagram of a cycling scheme for deploying AUVs during a seismic survey system according to an embodiment; 
         FIG. 6  is a schematic diagram of a seismic survey having streamers and AUVs according to an embodiment; 
         FIG. 7  is a schematic diagram of a deploying base and a recovery base connected to a support vessel according to an embodiment; 
         FIG. 8  is a schematic diagram of a deployment base according to an embodiment; 
         FIG. 9  is a schematic diagram of a control part of a deployment base according to an embodiment; 
         FIG. 10  is a schematic diagram of a seismic system that uses underwater bases for handling AUVs according to an embodiment; 
         FIG. 11  is a flowchart of a method for deploying AUVs from a deployment base according to an embodiment; 
         FIG. 12  is a flowchart of a method for guiding AUVs from a deployment base to a desired target position according to an embodiment; 
         FIGS. 13A and 13B  are schematic diagrams of a recovery base according to an embodiment; 
         FIG. 14  is a schematic diagram of a control part of a recovery base according to an embodiment; 
         FIG. 15  is a schematic diagram of an inlet part of a recovery base according to an embodiment; 
         FIG. 16  is another schematic diagram of the inlet part of the recovery base according to an embodiment; 
         FIG. 17  is a flowchart of a method for recovering AUVs according to an embodiment; and 
         FIG. 18  is a flowchart of a method for deploying a recovery base and recovering AUVs according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of an AUV with seismic sensors for recording seismic waves. Note that an AUV in the following description is considered to encompass an autonomous self-propelled node that has one or more sensors capable of detecting seismic waves. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     Emerging technologies in marine seismic surveys need an inexpensive system for deploying and recovering seismic receivers that are configured to operate underwater. According to an exemplary embodiment, such a seismic system includes plural AUVs, each having one or more seismic sensors. The seismic sensors may include a hydrophone, geophone, accelerometer, electromagnetic sensor, depth sensor, or a combination thereof. 
     The AUV may be inexpensively and efficiently designed, e.g., using internal components available off the shelf. A deployment vessel or underwater base stores the AUVs and launches them as necessary for the seismic survey. The underwater base may be a buoy, a structure deployed on the ocean bottom that has means for communicating with the vessel, a structure floating in water, etc. The AUVs find their desired positions using, for example, an inertial navigation system. However, in another application, the AUVs find their desired positions using a combination of acoustic guidance, waypoint navigation and information from various navigation sensors such as an inertial measurement unit, echo sounder, pressure gauge, etc. Other systems or methods may be used for finding their desired positions. Thus, the AUVs may be preprogrammed or partially programmed to find their desired positions. If the AUVs are partially programmed, the final details for finding the desired position may be received, acoustically, from the vessel or the underwater base when the AUV is launched from the vessel. 
     As the deployment vessel or underwater base is launching the AUVs, a shooting vessel for generating seismic waves may be used to generate seismic waves. The shooting vessel may tow one or more seismic source arrays, each one including plural source elements. A source element may be an impulsive element, e.g., a gun, or a vibratory element. The shooting vessel or another vessel, e.g., the recovering vessel, the deployment vessel, or the underwater base, may then instruct selected AUVs to return to the underwater base or to resurface so they can be collected. In one embodiment, the deployment vessel also tows and shoots source arrays as it deploys the AUVs. In still another exemplary embodiment, only the deployment vessel is configured to retrieve the AUVs. However, it is possible that only the shooting vessel is configured to retrieve the AUVs. Alternatively, a dedicated recovery vessel may wake up the AUVs and instruct them to return to the surface for recovery. 
     In one exemplary embodiment, the number of AUVs is in the thousands. Thus, the deployment vessel is configured to hold all of them at the beginning of the seismic survey and then to launch them as the survey advances. Alternatively, a set of underwater bases is used to handle all the AUVs. 
     In an embodiment, the seismic survey is performed with a combination of seismic sensors on the AUVs and seismic sensors on streamers towed by the deployment vessel, the shooting vessel, or both of them. 
     In still another embodiment, when selected AUVs are instructed to leave their recording locations, they may be programmed to go to a desired rendezvous point where they will be collected by the shooting vessel, the deployment vessel, the recovery vessel, or the underwater base. The selected AUVs may be chosen from a given row or column if that type of arrangement is used. The shooting, deployment, recovery vessel, or the underwater base may be configured to send acoustic signals to the returning AUVs to guide them to the desired position. 
     Once on the vessel or the underwater base, the AUVs are checked for problems, their batteries may be recharged, and the stored seismic data may be transferred to the vessel for processing. After this maintenance phase, the AUVs are again deployed as the seismic survey continues. Thus, in one exemplary embodiment, the AUVs are continuously deployed and retrieved. 
     The above-noted embodiments are now discussed in more detail with regard to the figures.  FIG. 2  illustrates an AUV  200  having a body  202  to which one or more propellers  204  are attached. A motor  206  inside the body  202  activates propeller  204 . Other propulsion systems may be used, e.g., jets, thrusters, pumps, etc. Motor  206  may be controlled by a processor  208 . Processor  208  may also be connected to a seismic sensor  210 . Seismic sensor  210  may be shaped so that when the AUV lands on the seabed, the seismic sensor achieves a good coupling (e.g., direct) with the seabed sediments. The seismic sensor may include one or more of a hydrophone, geophone, accelerometer, etc. For example, if a  4 C (four component) survey is desired, the seismic sensor  210  includes three accelerometers and a hydrophone, i.e., a total of four sensors. Alternatively, the seismic sensor may include three geophones and a hydrophone. Of course, other sensor combinations are possible. 
     A memory unit  212  may be connected to processor  208  and/or seismic sensor  210  for storing a seismic sensor&#39;s  210  recorded data. A battery  214  may be used to power all these components. Battery  214  may be allowed to change its position along a track  216  to alter the AUV&#39;s center of gravity. 
     The AUV may also include an inertial navigation system (INS)  218  configured to guide the AUV to a desired location. An inertial navigation system includes at least one module containing accelerometers, gyroscopes, magnetometers or other motion-sensing devices. The INS is initially provided with the position and velocity of the AUV from another source, for example, a human operator, a global positioning system (GPS) satellite receiver, another INS from the vessel, etc., and thereafter, the INS computes its own updated position and velocity by integrating (and optionally filtrating) information received from its motion sensors. The advantage of an INS is that it requires no external references in order to determine its position, orientation, or velocity once it has been initialized. As noted above, alternative systems may be used, as, for example, acoustic positioning. 
     Besides, or instead of, the INS  218 , the AUV  200  may include a compass  220  and other sensors  222  such as, for example, an altimeter for measuring its altitude, a pressure gauge, an interrogator module, etc. The AUV may optionally include an obstacle avoidance system  224  and a communication device  226  (e.g., Wi-Fi device, a device that uses an acoustic link) or another data transfer device capable of wirelessly transferring data. One or more of these elements may be linked to processor  208 . The AUV further includes an antenna  228  (which may be flush with the body of the AUV) and a corresponding acoustic system  230  for communicating with the deploying, shooting, or recovery vessel or the underwater base. Stabilizing fins and/or wings  232  for guiding the AUV to the desired position may be used together with propeller  204  for steering the AUV. However, such fins may be omitted. The AUV may include a buoyancy system  234  for controlling the AUV&#39;s depth and keeping the AUV steady after landing. 
     Acoustic system  230  may be an Ultra-short baseline (USBL) system, sometimes known as a Super Short Base Line (SSBL). This system uses a method of underwater acoustic positioning. A complete USBL system includes a transceiver, which is mounted on a pole under a vessel, and a transponder/responder on the AUV. The processor is used to calculate a position from the ranges and bearings measured by the transceiver. For example, the transceiver transmits an acoustic pulse that is detected by the subsea transponder, which replies with its own acoustic pulse. This return pulse is detected by the transceiver on the vessel. The time from transmission of the initial acoustic pulse until the reply is detected is measured by the USBL system and is converted into a range. To calculate a subsea position, the USBL calculates both a range and an angle from the transceiver to the subsea AUV. Angles are measured by the transceiver, which contains an array of transducers. The transceiver head normally contains three or more transducers separated by a baseline of, e.g., 10 cm or less. Alternatively, an SBL (short base line) system or an inverted short baseline (iSBL) system may be used. 
     With regard to the AUV&#39;s internal configuration,  FIG. 3  schematically shows a possible arrangement for the internal components of an AUV  300 . AUV  300  has a CPU  302   a  that is connected to INS  304  (or compass or altitude sensor and acoustic transmitter for receiving acoustic guidance from the mother vessel), wireless interface  306 , pressure gauge  308 , and transponder  310 . CPU  302   a  may be located in a high-level control block  312 . The INS is advantageous when the AUV&#39;s trajectory has been changed, for example, because of an encounter with an unexpected object, e.g., fish, debris, etc., because the INS is capable of taking the AUV to the desired final position as it does for currents, wave motion, etc. Also, the INS may have high precision. For example, it is expected that for a target having a depth of 300 m, the INS and/or the acoustic guidance is capable of steering the AUV within +/−5 m of the desired target location. However, the INS may be configured to receive data from the vessel to increase its accuracy. 
     An optional CPU  302   b , in addition to the CPU  302   a , is part of a low-level control module  314  configured to control attitude actuators  316  and propulsion system  318 . The high-level control block  312  may communicate via a link with the low-level control module  314  as shown in the figure. One or more batteries  320  may be located in the AUV  300 . A seismic payload  322  is located inside the AUV for recording the seismic signals. Those skilled in the art would appreciate that more modules may be added to the AUV. For example, if a seismic sensor is outside the AUV&#39;s body, a skirt may be provided around or next to the sensor. A water pump may pump water from the skirt to create a suction effect, achieving a good coupling between the sensor and the seabed. However, there are embodiments in which no coupling with the seabed is desired. For those embodiments, no skirt is used. 
       FIG. 4  illustrates an embodiment in which a seismic survey system  400  includes plural AUVs  402   a - c  distributed across the ocean bottom  404 . AUVs  402  are in direct contact with the ocean bottom  404  to achieve a better coupling between sensors of an AUV and the ocean bottom. AUVs  402   a  are inside an area  406 , which constitutes the active recording area, while AUVs  402   b  and  402   c  are outside active recording area  406 . AUVs  402   b  are inside an already recorded area  408 , while AUVs  402   c  are distributed in an area  409  where they are going to become active and record seismic data. Seismic waves are generated by a seismic source  410  that is towed by a vessel  412 . A seismic source may be towed by another device or it may be an autonomous source. 
     As an example, which is not intended to limit the applicability of the claims, it is possible that area  420 , which is to be surveyed, has a length L of about 50 km and an width W of about 10 km while the active recording area  406  may have a length l of about 4 km and a width of about 2 km. Other numbers are possible depending on the conditions of the seismic survey. 
     As area  420  to be surveyed is too large to be simultaneously covered with AUVs, one approach for recording seismic data over the entire area, without having to fully cover it with AUVs, is to continuously roll a set of AUVs ahead of the seismic source. More specifically, consider that instead of simultaneously distributing AUVs over the entire area  420 , AUVs are distributed only over areas  406 ,  408 , and  409 , which together may represent a percentage of the entire area  420 . For example, it is possible that a total surface of areas  406 ,  408  and  409  constitutes 20% or less of the surface of area  420 . 
     Thus, according to this embodiment, AUVs  402   a  are active inside the recording area  406 , AUVs  402   c  are ready to record seismic data inside the future recording area  409  and AUVs  402   b  just finished recording the seismic data inside the recorded area  408 . In order to have further AUVs ready for recording seismic data as vessel  412  advances along direction X, AUVs  402   b  of recorded area  408  are instructed to roll at new position  411  as indicated by arrow  413 . Thus, the AUVs are rolled from one area to another area while the seismic waves are generated so that a limited number of AUVs can be used to cover the entire survey area  420 . The details about moving the AUVs from one area to another area are discussed later. 
     According to another embodiment illustrated in  FIG. 5 , instead of or in addition to using the rolling procedure discussed with regard to  FIG. 4 , it is possible to use a cycling procedure as now discussed. In other words, the cycling procedure may be an alternative or a complement to the rolling procedure. As illustrated in  FIG. 5 , a seismic survey system  500  includes a set of AUVs  502   a  distributed within a recording area  506 , a set of AUVs  502   b  within an already recorded area  508  and a set of AUVs  502   c  to become active. All the AUVs are located on the ocean bottom and they may be actually attached to the ocean bottom with an appropriate device to obtain a better coupling. Vessel  512  tows seismic source  510  while another vessel  530  stores plural AUVs  532 . If there is a need to replace AUV  502   a  before, during, or after the seismic source  510  has passed the actively recording area  506 , AUV  532  may be deployed to replace AUV  502   a . In one embodiment, a set of AUVs  532  is deployed at the same time to replace an entire set of AUVs  502   a . This cycling of AUVs may happen while the set of AUVs  502   a  is actively recording seismic data. The set of AUVs  532  has recharged batteries and their memories may be empty while the set of AUVs  502   a  to be replaced may have drained batteries and full memories. 
     One reason for taking such an approach is now discussed. Consider, as illustrated in  FIG. 6 , that a traditional seismic survey  600  includes a vessel  642  that tows seismic source  644  and one or more streamers  646 . Streamers  646  include seismic sensors  648  that record seismic waves generated by seismic source  644 . However, while following its path  640 , vessel  642  may encounter an obstacle  650 , for example, a rig or platform, which needs to be avoided, or shallow waters where the vessel cannot go, etc. Thus, vessel  642  follows a modified path  652  that affects the seismic data to be recorded around obstacle  650 . For this reason, AUVs  602   a  are deployed around obstacle  650  to produce in-fill seismic data that is missing from the seismic data recorded with streamers  646 . In one application, AUVs  602   a  are deployed prior to vessel  642  arriving at obstacle  650 . The time prior to this event may be in the order of hours if not days. 
     Thus, it is possible that by the time vessel  642  arrives near obstacle  650 , one or more AUVs  602   a  will need to be replaced with new AUVs  632  that have a freshly charged battery and/or empty memory. Alternatively, it is possible that vessel  642  has passed AUVs  602   a  a first time, and when the same vessel passes AUVs  602   a  a second time, after the vessel has completed its line  640  and is coming back along an adjacent line, some AUVs  602   s  have already stayed for hours or days on the ocean bottom and are ready to be replaced by new AUVs. Thus, a predetermined condition for changing one AUV with another AUV may be related to an amount of power available in the current AUV and/or an amount of available memory in the current AUV. Further, the predetermined condition may be related to enough seismic signals being recorded for processing purposes, and/or enough seismic signals being recorded for quality checking seismic data, and/or a weather forecast (i.e., if one week of bad weather is forecasted, considering a one week time delay in replacing the AUVs). Thus, new AUVs  632  (new in the sense that their batteries are charged and/or memory emptied of previous data) are deployed from support vessel  630  or an underwater base to replace one or more of AUVs  602   a  that need such replacement. As noted above, this replacement or cycling can take place when vessel  642  is away from the AUVs or while the AUVs are actively recording seismic data originated by vessel  642 . 
     Another scenario for which the cycling procedure discussed above is appropriate is when an in-fill mission is performed which lasts for days, e.g., about 10 days. For this embodiment, suppose that a conventional streamer survey is performed first and it lasts 10 days. After this, the AUVs are used to collect seismic data for in-fill reasons, i.e., to fill in the missing data from the seismic data recorded with the streamers. Such a mission may take a couple of days for cross-line orientated shooting lines and another couple of days for in-line orientated shooting lines. Thus, such an in-fill mission may last one or more weeks; during this time, some or all the AUVs deployed on the ocean bottom would eventually need to be replaced with new ones. However, note that a survey may take about 40 days and the AUVs may be cycled about three times during this time interval, resulting in an average of 10 days underwater deployment for each AUV. 
     Next, various possible deployment methods of the above noted AUVs are discussed.  FIG. 7  shows a seismic survey system  700  that includes a vessel  703  that, with the help of a crane  705 , deploys an underwater base (e.g., a cage)  706  underwater and maintains the cage at a given underwater position described by coordinates (x,y,z). To achieve this condition, crane  705  may have a controller  710  that coordinates a heave mechanism  712  for maintaining given position (x,y,z) despite the normal movement of vessel  703 . Note that given position (x,y,z) may be on or above the ocean bottom. In one application, controller  710  is part of a global controller  714  associated with the vessel&#39;s navigation system. 
     Underwater base  706  accommodates one or more AUVs  732  that are deployed when necessary to replace existing AUVs  702   a  already located on the ocean bottom  704 . According to this embodiment, one or more AUVs  702   a  need to be replaced by AUVs  732 , which have charged batteries. For this situation, the fully charged AUVs  732  are deployed from underwater base  706  after being instructed to land next to a corresponding AUV  702   a  needing a replacement. AUVs  702   a &#39;s positions are known because either vessel  703  has used its detection system  707  (e.g., USBL) to determine those positions, or underwater base  706  has used a similar detection system  709 , or AUVs  702   a  have calculated (e.g., using an INS system) their landing positions and have transmitted this information, e.g., using an acoustic modem, to underwater base  706  or vessel  703 . Alternatively, the AUVs positions are known prior to deploying them because they have been pre-plotted. 
     Thus, AUVs  732  know where to land on the ocean bottom  704  after being launched from underwater node  706 . After new AUVs  732  have landed on the ocean bottom  704  next to the AUVs  702   a  that need to be replaced, existing AUVs  702   a  detach from the ocean bottom and navigate toward underwater base  706  to be retrieved on the deck of vessel  703 . If this cycling procedure is taking place during active seismic recording, there is no substantial gap in the recorded data, as the transfer from existing AUVs  702   a  to new AUVs  732  is achieved while recording the seismic data. However, a disadvantage of this procedure might be the noise introduced by those AUVs traveling toward the recording AUVs and/or the potential collisions between the existing AUVs and the new AUVs. Once the underwater base is full with old AUVs, crane  705  retrieves the base and the AUVs on the vessel&#39;s deck and a maintenance phase and/or data transfer phase occurs. 
     The embodiment discussed with regard to  FIG. 7  may be modified to include two underwater bases, a launching base  706  and a recovery base  706 ′. With this configuration, launching base  706  launches new AUVs  732 , and the old AUVs  702   a  do not return to launching base  706 , but rather to recovery base  706 ′. Thus, it is possible to have the following three different scenarios:
         (i) first launch new AUVs  732  and after they land on the ocean bottom, then recover existing AUVs  702   a;      (ii) simultaneously launch AUVs  732  from base  706  and instruct AUVs  702   a  to go to recovery base  706 ′; this scenario is efficient but introduces gaps into the recorded data and it is prone to AUVs collision;   (iii) first instruct existing AUVs  702   a  to go to recovery base  706 ′ and then instruct new AUVs  732  to land on the ocean bottom at the positions previously occupied by the existing AUVs  702   a ; under this scenario it may be possible to land the new AUVs very close to the previous positions of the old AUVs but may also introduce gaps into the recorded data.       

     Those skilled in the art would recognize that the above-discussed embodiments may be varied to achieve the same or similar results. For example, instead of a vessel holding both the deployment and recovery bases, two vessels may be used, each one holding one of the two bases. Alternatively, more than two bases may be used at the same time. Further, it is possible to land the bases on the ocean bottom or to leave them floating from a buoy. Furthermore, the two cranes illustrated in  FIG. 7  may be placed at any location along the deck of the vessel, for example, both cranes at the back of the vessel. In one application, cages  706  and/or  706 ′ are deployed on the ocean bottom and are not suspended in the water. 
     One possible configuration of the deployment base is now discussed. In one embodiment, as illustrated in  FIG. 8 , deployment base  800  may have a control part  810 , a storing part  820  for storing the AUVs, and a support part  830 . The three parts are attached to each other and serve the following purposes: control part  810  may include a control system  812  that coordinates the launching of the AUVs, and also provides beacon signals to the AUVs while traveling to their final destinations. Control part  810  may also include an acoustic device  814  for generating and transmitting the beacon signals as discussed later. Control part  810  may be attached above storing part  820  as illustrated in the figure. Other positions for the control part may be used. Storing part  820  is configured to hold a certain number of AUVs, for example, 20 or 40. Other numbers are possible as would be appreciated by those skilled in the art. For example, it is envisioned that storing part  820  may have dimensions in the order of meters, e.g., 3×3×5 m. Storing part  820  is attached above support part  830  and is configured to be flooded. Storing part  820  has a mechanism to lock the AUVs during operation of the crane, and is configured to provide a communication interface with the AUVs and the control part. 
     Support part  830  may be a strong structure designed to support the weight of the control part and the storing part. Also, the support part is designed in such a way that avoids the burial of the deployment base into the ocean bottom. However, the support part is also designed to partially burry into the ocean bottom to stabilize the storing part as this part needs to be immobile to achieve the desired acoustic guidance performance. 
     A schematic representation of the functional units of a deployment base  900  is illustrated in  FIG. 9 . Deployment base  900  includes, as discussed above with reference to  FIG. 8 , a control system  912  and an acoustic system  914 . Control system  912  is functionally connected to the storing part  920 , as now discussed. Control system  912  may include a variety of elements, some of them illustrated in  FIG. 9 . A clock  940  (which may be a high-precision clock) is connected to a navigation device  942 , and both the clock  940  and the navigation device  942  are connected to a power source  944  (e.g., a battery). Various other electronic components  946  may also be provided in the control system, for example, to interface with storing part  920  and acoustic system  914 . 
     Navigation device  942  may include an inertial navigation system (see, e.g., INS  218 ), an attitude and heading reference system (AHRS), or another similar device. Navigation device  942  is used for determining an accurate position and orientation of the underwater base. For example, when crane  705 , illustrated in  FIG. 7 , deploys underwater base  706 / 900  on the ocean bottom, the underwater base freely rotates while being moved from the vessel to the ocean bottom and also can change its X and Y coordinates (if the X and Y coordinates describe the ocean surface and the Z coordinate describes the depth). However, for guiding the new AUVs to their new positions and for being able to recover the existing AUVs, the underwater base needs to know, as accurately as possible, its own absolute position. By knowing the original coordinates (i.e., when the underwater base is released into the water, the vessel&#39;s GPS is used to determine this position) of the underwater node and its trajectory (using the INS or AHRS) while traveling underwater toward the ocean bottom, the underwater base is capable of calculating its final x,y,z position on the ocean bottom, and also its orientation, e.g., an angle between (i) a longitudinal axis  924  of a launching tube  922  that is used by the storing part to launch the new AUVs and (ii) a reference axis or system of axes (e.g., x). Launching tube  922  may include a locking mechanism  926  for locking a corresponding AUV during a transition of the underwater base from the vessel to the ocean bottom. 
     The x,y,z position and its orientation may also be determined by an acoustic device installed on the vessel, for example, USBL, and this information may be transmitted to the underwater base via an acoustic modem. For the purpose of exchanging this and other information (e.g., status of deployment/recovery, etc.) with the vessel while underwater, the underwater node also has a modem port  950 . A power port  952  is provided for charging the power unit  944  when the underwater node is on the vessel&#39;s deck, or for connecting to an underwater device that has the capability to provide power. Control system  912  may also have a port  954  for synchronizing, when on the vessel&#39;s deck, clock  940 , downloading mission parameters, uploading data acquired during launch and recovery, etc. Alternatively or in addition, a physical connection (cable) may be provided between the underwater base and the vessel. 
     Deployment base  900  also includes an acoustic system  914  for providing guidance to departing and/or arriving AUVs. Acoustic system  914  may include three or more acoustic beacons  970   a - d  (although  FIG. 9  shows four acoustic beacons, an underwater base having only three acoustic beacons is also possible) located, for example, on control part  810  illustrated in  FIG. 8 . Thus, these acoustic beacons may form a short base line (SBL) system  970 . Other locations of the acoustic beacons on the underwater base are possible. Having more acoustic beacons is desirable so that during a seismic survey, each AUV has a “direct view” of at least three acoustic beacons for positioning itself. In one application, at least two of the acoustic beacons are positioned within a base of a pyramid, while at least one of the acoustic beacons is positioned at the top of the pyramid. In this arrangement, each AUV has the capability to position itself in not only a horizontal, but also in a vertical plane relative to the ocean bottom. 
     An acoustic beacon may include a ceramic element  972  that emits the acoustic signal and corresponding electronics  974  that interacts with the control system  912  and also controls the ceramic element.  FIG. 9  illustrates an electric link  980  between the control system  912  and acoustic beacons  970   a - d , and also an electric link  982  between battery unit  944  and acoustic beacons  970   a - d .  FIG. 9  also illustrates a link  984  between control system  912  and launching tubes  922  of storing part  920 . This link  984  may be a wired or wireless link. 
     In one application, a distance between two acoustic beacons may be in the order of meters, for example, 2.5 m. With such a configuration, it is expected that an AUV could detect its position from 1 km away, with a good precision, e.g., 1 m. As the technology improves, it is expected that these numbers will become even better. Control system  912  is programmed to select appropriate frequency channels for the acoustic beacons, to adjust the channels if necessary, to synchronize the acoustic beacons, and to exchange information with the acoustic beacons, e.g., to send commands to interrogate the AUVs. In one application, control system  912  is capable of interrogating the AUVs about their position and their status, instructing them to return to the underwater base, etc. Thus, acoustic system  914  may provide not only AUVs guidance functionality, but also AUVs communication, wired or wireless. 
     An entire sequence for deploying the underwater base and launching the corresponding AUVs is now discussed with reference to  FIGS. 10 and 11 .  FIG. 10  illustrates a seismic survey system  1000  including a vessel  1002  and at least one underwater base  900 . Note that vessel  1002  may carry any number of underwater bases  900 . A heave compensated crane  1005 , similar to that described in the embodiment illustrated in  FIG. 7 , may handle underwater base  900 . From its initial position on the deck (which position may be calculated by the GPS  1007  of the vessel), as illustrated in step  1100  of  FIG. 11 , underwater base  900  computes its final position on the ocean bottom  1004 . The computing step relies not only on the base&#39;s initial position when launched in water, but also on the AHRS or INS system&#39;s output for determining the entire trajectory of the underwater base, from the vessel until it lands on the ocean bottom. The result of the computation step is an accurate final position (x,y,z) on the ocean bottom and an orientation of the base relative to, for example, longitudinal axis  924  of the launching tubes (see  FIG. 9 ). This position may also be calculated by the USBL of the vessel and then transmitted through an acoustic modem or a wire to the underwater base. In step  1102 , the underwater base calculates the absolute position of each beacon, based on the known geometry of the SBL (i.e., the locations of the acoustic beacons) and the final position of the underwater base calculated in step  1100 . The beacons&#39; positions are transmitted in step  1104  to the AUVs  932  that are stored in the storing part of the underwater base. This communication may be wireless or wired. 
     In step  1106 , the control system instructs the locking mechanism to release the corresponding AUV and in step  1108  the AUV is instructed, by the control system, to start its mission. At the same time, control system coordinates in step  1110  the acoustic beacons to send the correct acoustic signals so that the launched AUV can determine its position relative to the underwater base and/or ocean floor. This position determination happens in step  1112 , while the AUV  932  travels from underwater base  900  to target position  980 . The position determination involves the AUV&#39;s processor in calculating distances to at least three acoustic beacons and, based, for example, on triangulation, determining its absolute position relative to target position  980 . This step may be repeated until the AUV reaches its target position. Once at the target position, AUV lands on the ocean bottom in step  1114  and, optionally, may use a drilling device to attach (connect) to the ocean floor. Then, AUV starts recording seismic data. The recording step may be triggered by the underwater base, the vessel, or an internal mechanism of the AUV. 
     The underwater base may use its iSBL or USBL system to compute the final position of AUVs. This data may be stored for later use or transmitted to the vessel. If this embodiment uses a deployment base and a recovery base, after the last AUV has been launched from the deployment base, the deployment base is retrieved in step  1116  back on vessel  1002 , to be prepared for another mission. 
     A method for deploying AUVs underwater at desired target positions is now discussed with regard to  FIG. 12 .  FIG. 12  includes a step  1200  of deploying an underwater base to the ocean bottom. The underwater base includes plural AUVs that need to be deployed at the desired positions. In step  1202  the underwater base calculates its final position and orientation and transmits these results to the AUVs stored in the underwater base. The AUVs also store their target positions. In step  1204  the AUVs are launched, sequentially or simultaneously, and in step  1206  acoustic signals emitted by acoustic beacons of the underwater base are sent to the AUVs for determining their absolute positions. The AUVs know the exact locations of the acoustic beacons (this information was received by each AUV prior to departing the underwater base) and by triangulating the acoustic beacons, the AUVs are able to determine their absolute positions, relative to the ocean bottom. As described in previous embodiments, the AUVs are capable of adjusting their trajectories, based on the calculated absolute positions, to arrive at the target positions. 
     Before, while, or after the newly released AUVs have traveled to their final destination, the existing AUVs are instructed, in one embodiment, to return to a recovery base, e.g., recovery base  706 ′, as discussed with regard to  FIG. 7 . The recovery base is now discussed in more detail with regard to  FIGS. 13A and 13B . 
     Recovery base  1300  may include a control part  1310 , an inlet part  1320 , a storing part  1330 , and a support part  1340  configured to support the control part, the inlet part, and the storing part, and also to prevent a burial of the recovery base. However, support part  1340  may be also designed to partially bury into the ocean bottom to make the rest of the base immobile. Similar to the deployment base, the recovery base may handle the AUVs simultaneously or sequentially. The recovery base may be attached to a heave compensated crane as in  FIG. 7 , or it may be released on the ocean bottom and then be recovered by using, for example, a floating buoy. Control part  1310  may include a control system  1312  and an acoustic system  1314 . While the control system and acoustic system of recovery base may be similar to those described in  FIG. 9 , the inlet part  1320  and storing part  1330  are different. 
     With regard to the inlet part  1320 , the functionality includes detecting that an AUV has entered the recovery base and also instructing AUVs to switch off their propulsion systems. In this way, after an AUV enters through the inlet part  1320  (which is the gate to the storing part  1330 ), the AUV simply falls into the storing part  1330  as its propulsion system is shut down. This is advantageous for conserving the energy left in its battery and also for preventing the AUV from escaping the storing part. For these purposes, as illustrated in  FIGS. 13B and 14 , the inlet part  1320  may have an AUV interface  1322  that is configured to detect the entrance of an AUV and also to identify the AUV. In one application, each AUV has a unique identification ID which may be detected by the AUV interface  1322 . For example, AUV interface  1322  includes an acoustic modem that interrogates the AUV about its ID. After the ID is checked against, for example, a table stored in the memory of the control system  1312 , AUV interface  1322  instructs the AUV to shut down its propulsion system. In one embodiment, the instruction to shut down the propulsion system is sent after a predetermined amount of time to make sure that the AUV has entered the storing part. The storing part  1330  may be simply a chamber for receiving the recovered AUV. In one application, inlet part  1320  has an inclined surface  1324 , as illustrated in  FIG. 15 , for deviating AUVs  1332  into the storing part. 
     Acoustic system  1314  may be different than the one shown in  FIG. 9 . Thus, for this embodiment, acoustic system  1314  may include, as shown in  FIG. 15 , a transducer (e.g., pinger)  1350  located in the center of the inlet part and configured to emit a signal. The AUV  1332  then uses its USBL system  1333  to detect the emitted signal and to approach the inlet part  1320 . Acoustic system  1314  may also include one or more acoustic beacons with a configuration that allows the AUV to find a height of the inlet part. The acoustic beacons may be those shown in  FIG. 14  or others. Control system  1312  is configured to control the transducer and one or more acoustic beacons to coordinate the recovery of the AUVs. With this configuration, once deployed on the ocean bottom, the recovery base sends an acoustic wake-up message to the AUVs that need to be recovered. The recovery base then activates its acoustic system to allow the AUVs to position themselves, regarding the inlet part and find the inlet part. The control system ensures that the beacons emit the required signals at the required instants. Knowing the direction of the inlet part, the AUVs navigate to enter the recovery base. Upon entering the inlet part, each AUV triggers the AUV interface and stops its mission. 
     According to another embodiment illustrated in  FIG. 16 , acoustic system  1614  includes at least two transducers T 1  and T 2 . Transducer T 1  may be used to send a (unique) signal to wake-up AUV  1632  or a set of AUVs that form a group. This unique signal is recognized by a single AUV and not by the others or by the group and not by other groups. AUV  1632  takes off from its location on the ocean bottom and enters a homing phase in step  1700  as illustrated in  FIG. 17 . AUV  1632 , while navigating in the volume of water, it is attracted to the center of the inlet part  1620  by the acoustic system  1614 . This system may operate in a frequency range between 20 and 30 kHz. 
     The two transducers Ti and T 2  are synchronized to transmit, for example, a 10 ms pulse every second (with a transmission from transducer T 2  shifted from 100 ms in time from a transmission from transducer T 1 ) and they are located on a center pole  1662  of the mechanical frame of the recovery base. The two transducers T 1  and T 2  are, in one embodiment, equidistantly located (e.g., 3 m) from the “recovery navigation plane”  1664  that AUV  1632  follows during the homing phase. Central pole  1662  may extend throughout the storing part. In one application, the central pole extends outside the recovery base and ends up with a hook  1663  that connects to a crane. In still another application, the central pole does not enter the storing part, but the second transducer T 2  is placed inside the storing part, symmetrically located from transducer T 1  relative to the recovery navigation plane. The recovery navigation plane  1664  is designed to extend, for example, substantially perpendicular on the center pole  1662 . In one application, the recovery navigation plane intersects inlet part  1620  as illustrated in  FIG. 16 . In still another application, the recovery navigation plane is designed to extend between inlet part  1620  and storing part  1630 . In still another application, inlet part  1320 / 1620  has an actuation mechanism  1690 , controlled for example by the control system  1312 , and configured to lower and raise the inlet part. For example, while the recovery base is traveling from the vessel to the ocean bottom, or the other way around, actuation mechanism  1690  closes the gap between inlet part  1620  and storing part  1630  so that no AUV escapes. However, when the recovery underwater base lands on the ocean bottom, actuation mechanism  1690  raises the inlet part  1620  to provide enough clearance C for the AUV to enter the storing part. In one embodiment, clearance C is equal to or larger than a height of the AUV. 
     The phased receiving array  1660  located on AUV  1632 , e.g., on its nose, may include at least three hydrophones that are configured to capture the signals emitted by transducers T 1  and T 2 . Processing capabilities of the AUV, e.g., its processor and accompanying software, are configured to calculate the direction and/or distance to the center pole  1662  and the navigation attitude, relative to the recovery navigation plane  1664 . 
     Following the recovery navigation plane  1664 , AUV  1632  eventually hits the AUV sensitive deflector  1322  and falls into storing part  1630 . AUV  1632  may be programmed to switch in step  1702  from the homing phase to the impact-detecting mode. To achieve this, the AUV&#39;s processor may be configured to compare an estimated distance to the center pole  1662  or another reference point with a predetermined distance, e.g., 5 m, and when the estimated distance is smaller than the predetermined distance, to automatically switch from the homing phase to the impact detection mode. During the impact detection mode, the AUV may be configured to reduce its speed to a certain percentage of the normal speed, allowing it more time to react and change its course, if necessary, and also to hit the AUV interface  1322  with less force. 
     If located on the upper part of the AUV, the phased receiving array  1660  will directly hit the AUV interface  1322  and the impact detection mode will make the AUV&#39;s processor detect the impact shock, which is characterized by high energy and a larger frequency bandwidth. In one application, any part of the AUV may hit the AUV interface  1322  and make the AUV&#39;s processor detect the impact shock. When the impact shock is detected in step  1702 , the AUV&#39;s processor instructs the thrusters and/or jet pumps to stop in step  1704 , resulting in a slow dive of the AUV down into the storing part, as the AUV is negatively buoyant. 
     The sensitive AUV interface  1322  may be configured to also detect the impact, because, in one embodiment, the AUV interface is made of one or several quadrants of piezoelectric fabric material, such as piezoelectric poly-vinylidene fluoride (PVDF), all of them connect to the acoustic system  1614 . Thus, the acoustic system may condition and process the PVDF generated impact signals and inform the control system of the recovery base accordingly. In one application, the control system may communicate this info to the respective AUV to offer a redundancy mechanism for making sure that the AUV enters the immobilization phase in step  1704 . In step  1706 , the AUVs are stacked in the storing part and then the entire recovery base is brought back on the vessel. 
     The operational model discussed with regard to  FIG. 17  is designed such that not all AUVs arrive at the same time at the inlet part. In other words, it is expected that a single AUV hits the AUV interface at any instant. Thus, the control system knows the number of recovered AUVs and can send a status report to the support vessel using its embedded modem. 
     In one application, recovery base may have one or more cameras  1680  so that visual information of the stack of AUVs captured inside the storing part can also be transmitted via modem to the support vessel. Once the desired AUVs have been recovered, the recovery base is lifted back to the support vessel. 
     A method for recovering AUVs from the ocean bottom is now discussed with regard to  FIG. 18 . In step  1800 , an empty recovery base is deployed on the ocean bottom. Once on the ocean bottom, the acoustic system is activated to wake up the desired AUVs in step  1802  and to broadcast guidance signals for those AUVs in step  1804 . Those AUVs that were woken up start to navigate towards the recovery base and adjust their trajectories in step  1806  based on received signals that are indicative of a recovery navigation plane. The recovery navigation plane is defined by the physical arrangement of the sensors forming the acoustic system. In step  1808 , the AUVs interact with an AUV interface of the recovery base, and as a result of this interaction, the AUVs are instructed to switch off their propulsion system and fall into a storing part of the base. Thus, the AUVs are stacked in step  1820  in the storing part. In one application, the recovery base identifies the AUVs entering the storing part. In step  1812 , the recovery base is brought back on the vessel for recovering the seismic data from the AUVs and for replacing or recharging their batteries. The above noted steps may be performed in the order illustrated in  FIG. 18  or in another order that is consistent with the description of the other embodiments. Also, these steps may be modified, reduced or enlarged based on the discussed embodiments as will be appreciated by those skilled in the art. 
     One or more of the exemplary embodiments discussed above disclose a deployment base, a recovery base, and methods for deploying, recovering, and cycling or rolling AUVs during or after a seismic survey. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. 
     Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments, or in various combinations with or without other features and elements disclosed herein. 
     This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.