NO337220B1 - Acoustic source system for permanent monitoring of a underground reservoir - Google Patents

Description

BACKGROUND

Field of the invention

[0001] The present invention generally relates to the field of seismic surveys, and in particular to an acoustic source system for permanent reservoir monitoring.

Known and related art

[0002] In the production phase of an oil or gas field, it is desirable to monitor the formation on the production field, ie the formation that contains and surrounds the reservoir that contains the hydrocarbons. For this purpose, a permanent array of seismic receivers can be deployed on a fixed surface at the reservoir. The present invention is useful in onshore and offshore applications. The term "solid surface", as used here, can therefore be considered the interface between the earth's crust and air or a volume of water.

[0003] For the sake of simplicity, the invention is described using an example that includes an offshore reservoir and a seagoing source vessel. It is readily understood that "source vessel" may be substituted for "helicopter" or "truck" in land-based applications. Where appropriate, a distinction is made between onshore and offshore applications.

[0004] In the marine example, the seismic sensors typically comprise hydrophones to record primary or pressure waves (P-waves) and 3C geophones or accelerometers capable of recording secondary or shear waves (S-waves) in three dimensions. Today, a typical commercially available sensor can have a sensitivity of around 40 ng, i.e. in the order of 10"<8>m/s<2>. It is assumed that a permanent recording array is deployed over the formation to be monitored, that the nodes are supplied with electrical power through a cable and that they transmit their recorded data to the surface through a communication line. The power supply is sized to feed the sensors, and the communication preferably uses standard protocols that can handle a large number of addressable nodes using inexpensive standard components, for example ethernet over copper cable. and the communication lines are provided in an ocean bottom cable (OBC - ocean bottom cable) which connects the sensors in the array deployed on the seabed. Seismic receivers and array are, power supplies and suitable options for communication are widely known in the area, and are therefore not discussed further here.

[0005] The present invention relates to seismics with a controlled acoustic source, i.e. applications where the acoustic source is an artificial seismic source capable of producing acoustic energy with enough power to penetrate to a predetermined depth in a subsurface formation and produce detectable echoes in the form of P waves and S waves.

[0006] In today's offshore applications, a specialized source vessel can be used. A source vessel tows an array of air guns which are fired repeatedly to produce a series of graphs (traces) which are recorded by the permanent receiver array. However, the costs of hiring a source vessel form a significant part of the monitoring costs, and therefore limit how often a monitoring survey can be carried out. Furthermore, a source vessel may be unavailable for hire at a given time, for example at the start of an injection procedure which is known to cause displacements in the formation to be monitored.

[0007] Another problem with a source vessel is associated with exact positioning. To monitor changes in the reservoir over time, so-called 4D surveys are required. As is well known in the field, shots in a survey taken some time, for example a week or more, after a previous survey should be fired at locations as close as possible to the shot locations in the previous survey. Maneuvering a large towed source array to the exact position and orientation as in a previous survey is time-consuming and/or imprecise.

[0008] Furthermore, a source vessel has undesirable effects on operations and on the environment. Operationally, the source vessel and its towed equipment interfere with other marine activities. For example, it is usually forbidden to tow and fire an acoustic source in a safety zone around installations in a field to avoid damage to the installation. The survey must also be interrupted when there are fishing vessels in the area. Environmentally, the powerful acoustic energy that travels from the surface to the seabed with each shot disrupts marine life. For example, the survey must be stopped when there are marine mammals nearby, and the acoustic energy released by a gas-filled swim bladder can kill a fish. While killing a significant percentage of fish in a relatively small area is unlikely to affect the overall population of migratory fish, it can set back a site-bound population for several years. In addition to the purely operational and environmental effects, further costs are incurred due to interrupted surveys and potential damage to fisheries.

[0009] To better understand the present invention, a brief description of some known seismic sources follows.

[0010] It is common to divide seismic sources into impulse and vibration sources. In an impulse source, all acoustic energy is released at once. Examples include explosives, canisters

(thumpers) and air cannons. Today, explosives are rarely used because of their negative impact on the environment and problems associated with storing explosives on a vessel at sea.

[0011] A dunker is usually used in land-based seismic, and works by dropping a mass onto a base plate. A typical impulse is achieved by dropping, for example, 5,000 kg 2-3 m, and thus has an energy of around 5,000 kg x 10 m/s<2>x (2 to 3) m = 100 to 150 kJ. This energy is greater than that required to obtain usable results with today's sensors, and can damage equipment or even the formation, if repeated over time. Pulse sources of this type are therefore not suitable in the present application.

[0012] An array of air guns is typically towed behind a specially equipped source vessel, such as the source vessel mentioned above. The air cannons are charged with compressed air from a source vessel to provide repeated shots in accordance with a predetermined shot pattern. The output signal from such an array is shaped into a desired signature. More specifically, the array is towed at a predetermined depth D below the sea surface and the individual air cannons are fired in a sequence so that unwanted bubble pulses interfere destructively and the array as a whole produces the desired source signature. For this, the bubble period is calculated using the Rayleigh-Willis equation, which states that the bubble period T =k Q<1/3>/p<5/6>, where k is a constant that depends on the units used, Q is the supplied energy and p is the ambient pressure in the fluid in which the bubble expands. The pressure is often expressed as water depth, for example (D+10) where D is the water depth in meters and 10 meters of water depth corresponds to 1 atmosphere.

[0013] Operational and environmental effects of air gun arrays are briefly described above, and should be taken into account. Furthermore, it is impractical to produce the required power Q as pressurized gas at the seabed, for example as it would require long and expensive pipes. However, it is noted that individually addressed sources are useful for producing a predetermined source signature.

[0014] Vibrating springs are also known as "vibro ice springs" after the original invention patented by the Continental Oil Company (Conoco) in the 1950s. A vibrating source distributes the acoustic energy over time, specifically during "sweeps" over a predetermined frequency range. Linear, non-linear and randomized sweeps, particularly randomized sweeps designed for an array of receivers, are well known in the art. The detected signals are correlated to produce the equivalent of an impulse. The correlation also improves the signal-to-noise ratio (SNR).

[0015] Traditional vibrating sources are typically driven by hydraulics, although electromechanical equivalents are also well known. Some vibrating sources apply the force perpendicular to the ground to produce P-waves, and some comprise a base plate with elements inserted into the ground and vibrate parallel to the ground to produce S-waves. US 4,143,736 gives an example of a hydraulic S-wave source, and US 3,313,370 (from 1964) gives an example of an electromechanical S-wave source.

[0016] US patent application no. 2010232260 provides a complementary review of prior art, and describes an electromechanical seismic source capable of producing P-waves with a reported ground force of around 300kN in a frequency range from 4 Hz to at least 125 Hz. This electromechanical source can, for example, replace a hydraulically driven seismic source on a commercially available, purpose-built truck commonly used in land-based seismic. However, it is believed to be practicable to adapt such a source for underwater applications.

[0017] Other seismic sources known in the area include in particular plasma sound sources, also commonly known as "kickers", where electrical power is collected in a battery of capacitors, and the collected electrical charge is suddenly released between two electrodes placed close to each other. The arc (corona) forms a plasma and vapor bubble which expands rapidly and produces an initial shock wave. When the bubble pressure drops below the ambient pressure, the bubble collapses and produces a second shock wave. Again, the bubble period T follows from the Rayleigh-Willis equation, and thus depends on the supplied energy Q and the pressure p in the liquid containing the kicker electrodes.

[0018] US patent application with publication number 2009/0154290 (Radtke et al) describes a vibrating seismic source that fires at least two pulse sources separated by a time interval selected to produce an acoustic maximum signal at a selected frequency, and especially at low frequencies. A kicker can be used as an impulse source, and this embodiment can be further improved by producing a reflector so that the reflected signal from the expansion wave arrives at a target at the same time as the wave from the bubble contraction.

[0019] As in the air cannon example above, the device utilizes interference to produce a desired signature, and uses the Rayleigh-Willis equation to calculate the bubble period. Thus, the supplied energy Q and the pressure p in the fluid containing the kicker electrodes can be used to control the output frequency. Thus, the source described in the '290 application can be controlled to produce any frequency in the range used in vibroseis sweeps, and a 2 Hz signal is reported to produce detectable signals about 1.5 km (4,600 ft) from the source with a added energy in the order of 100 J.

[0020] International patent application WO 20110497202A2 (Radtke et al) describes a container for a kicker according to the '290 application above, and discusses several parameters for producing a directed device. Generally useful results include that stainless steel walls capable of withstanding pressure on the seabed (e.g. around 6 mm) are practically transparent to acoustic waves in the frequency range of a randomized vibroseis sweep, and that unwanted leakage of acoustic energy can be prevented by a gas pocket in the container. In addition, the container may contain acoustic reflectors to improve the output signal as briefly described above. Thus, the pressure inside the container and/or the energy supplied to the kickers (on the order of a few hundred joules) can be controlled to produce a directed acoustic output signal with a controllable frequency suitable for a randomized vibroseis sweep.

[0021] Systems comprising both acoustic sources and sensors are known from the publications US3873961, WO2005052303 and US2005004725.

[0022] Pneumatically or hydraulically driven sources can often cause unwanted noise and other problems in the source signal, mainly due to their relatively long response times. Furthermore, equipment such as pumps and pipes to pump gas or liquid to the seabed itself will be expensive, heavy and space-consuming. As these problems quickly become unmanageable with increasing deployment depth, it is assumed that the acoustic source should have an electrical power supply.

[0023] Furthermore, a seismic source for long-term deployment on a production field should preferably be of a vibrating type partly to limit the power requirement (ie the energy per shot) and partly to avoid damage to nearby installations and to reduce the footprint on the environment. Such a seismic source should preferably also be directional for similar reasons.

[0024] A first object of the present invention is thus to produce an improved vibrating acoustic source for deployment on a seabed. A second purpose is to produce such an acoustic source with lower manufacturing and operating costs than previously known in the area.

SUMMARY OF THE INVENTION

[0025] The above objectives are achieved with an acoustic source according to claim 1 and a method according to claim 8. Further features and advantages appear from the independent patent claims.

[0026] More specifically, the invention relates to an acoustic source system for permanent monitoring of a reservoir, comprising an acoustic source adapted for long-term deployment on a fixed surface at the reservoir; a common power source for supplying electrical power to the acoustic source and a sensor array monitoring the reservoir, and a common communication system for the acoustic source and the sensor array, wherein the acoustic source is individually addressable through the common communication system.

[0027] The acoustic source is adapted for long-term deployment on a fixed surface at the reservoir being monitored, i.e. dry land or a seabed above the reservoir. As the sources remain in place, there is no need to consider potentially different source positions in different surveys. The invention thus ensures that differences observed from survey to survey are due to geology, not changes in the data collection. Furthermore, there is no need for a source vessel (or a helicopter or a vibrose vehicle in land-based applications), so surveys can be conducted more frequently. Frequent surveys mean improved monitoring, i.e. a better overview of changes in the underground. Furthermore, the environmental problems are eased, such as regard for marine mammals and fish in the present example. As is well known in the art, permanent acoustic sources can be deployed in a pattern that uses spatial filtering to reduce noise and thus improve the signal to noise ratio (SNR). Reduced rental costs, improved monitoring, reduced impact on the environment and improved spatial filtering are improvements in their own right, and can also reduce operating costs further, for example because a survey can be carried out without regard to marine mammals, fish and fisheries. These and other benefits are described more fully below.

[0028] "Long-term" shall be considered the longest practicable time the acoustic source can remain deployed before requiring maintenance or replacement. That is to say, from an operational perspective, the acoustic source can be considered a permanent installation. Furthermore, the term "adapted long-term deployment" includes acoustic sources that have not yet been deployed and sources that have been brought in after deployment. Once the acoustic source is deployed on the solid surface, a survey can be initiated as needed at any time given by events in production, such as the start or stop of an injection process, before the start-up of a new production well, unexpected pressure events, etc. Repositioning of acoustic sources in a 4D survey is also no problem with permanently deployed acoustic sources. This improves the monitoring of the reservoir as described above.

[0029] Furthermore, the terms "reservoir" and "formation", as used in the following, are not limited to the entire formation, but may be limited to smaller areas of interest. For example, it is known in the field that injecting a fluid into a reservoir to stimulate production often causes displacements in the formation. Consequently, it may be desirable to monitor a single injection well more closely than other parts of the production field for a shorter or longer period.

[0030] A key feature of the invention is that time plays little role, so that the acoustic source can be charged by using low power, and so that the acoustic energy can be distributed over time instead of in a more energetic pulse.

[0031] In general, electrical energy supplied at low power is preferred because it requires cables of smaller cross-section and less electrical insulation than a transmission line designed for the transmission of higher power. In particular, low-power electricity supplied through the infrastructure provided for the sensor array is preferred, e.g. to use as much as possible of existing power sources, cabling, etc. In some cases, most or all of the necessary power supplies and power transfers are already installed with the sensor array, such that little or no upgrading of the infrastructure is required.

[0032] Similarly, the existing communication system can be expanded with the addresses of the acoustic sources, and used to control any acoustic source, including

fire a shot. Shots with relatively little acoustic energy can be combined or correlated with known techniques to produce valuable output. In addition to saved investment costs for infrastructure, a common communication system for the sources and receivers facilitates the operation of the entire system, and can thus improve the system's reliability and security.

[0033] The acoustic sources can thereby be charged by using a common power system which also supplies the sensor array either simultaneously or at different times. Similarly, a common communication system can be used to fire a shot sequence and to transmit seismic data at a later time.

[0034] There is ample time for charging, so the acoustic sources can be fed at low power and accumulate the energy until the accumulated energy is sufficient for a shot. Shots from multiple acoustic sources and detected seismic signals can be controlled using a single control unit and the common communication system so that operation is further simplified.

[0035] In a preferred embodiment, an array of individually addressable acoustic sources is produced. Thus, any desired acoustic signature can be implemented.

[0036] The acoustic source can be an impulse source. The signal from several individual acoustic sources can be added with positive interference and produce a combined, energy-rich output signal.

[0037] Alternatively, the acoustic source can be a vibration source which distributes the acoustic energy over time. The signals obtained can be correlated as briefly described in the introduction.

[0038] In both cases, an array of acoustic sources that distributes the emitted acoustic energy over time reduces the risk of damage to nearby equipment and/or the need for shielding, disconnection in the event of a shot or other protective measures. Perhaps even more importantly, there is less environmental impact, e.g. on marine life, as the high-energy shots that pass through the water from an air cannon are replaced with acoustic energy from a source array on the seabed, and further as the acoustic energy is distributed over a longer time interval.

[0039] The supplied electrical power is preferably collected as energy in a capacitor or a battery. The capacitor can then be discharged directly into a kicker or further transformed into mechanical energy stored in a spring.

[0040] In a second aspect, the invention relates to a method for using the acoustic source described above. The method comprises the steps of: storing energy from the common power source in the acoustic source until the stored energy has reached a predetermined level,

providing a firing signal to the acoustic source using the common communication system, and

to use the firing signal to trigger a shot.

[0041] The operational and environmental advantages of the method are the same as those in the first aspect. In summary, the benefits are reduced environmental impact and reduced costs: Firstly, there are purely operational benefits, as source vessels with towed equipment do not affect other marine activities. Second (and more importantly), marine life is less affected if acoustic energy is distributed over time. Third, the survey can start if necessary at any time depending on events in production, for example the start or stop of an injection process, before the start of operation of a new production well, unexpected pressure events, etc. When using a towed source, the availability of the source is always an issue. Finally, permanently deployed acoustic sources ensure the shot positions and thus repeatable shot signatures. Consequently, differences observed from survey to survey are due to geology, not changes in data acquisition.

[0042] By using one communication system, e.g. Ethernet, for both the sensor array and the acoustic source, it is possible to reuse infrastructure, i.e. to connect the acoustic source to the existing power supply and communication system used for the monitoring sensor array.

[0043] Any arrangement of multiple sources known in the art may be used with the present invention.

[0044] The method may further comprise the step of firing a number of acoustic sources one by one at different times and correlating the resulting signals. This step compensates for a possible lower energy than a shot from an array of air guns. This means that you take advantage of the fact that time is far less important than with a towed source.

[0045] Preferably, the method enables the firing of shots selectively to monitor a single injection well. Regardless of whether a source array is deployed in its entirety around an injection well or in a larger area, the acoustic sources associated with the injection well can be fired selectively to reveal any changes caused by injection. That is, it may be unnecessary to load and fire all sources in an extended source array to detect changes caused by fluid injected through the injection well. Any changes also probably occur when an injection starts and for some time afterwards, so it may be sufficient to monitor the area around the injection well for a limited period of time, in contrast to a permanently increased monitoring around the injection well.

[0046] Other features and advantages of the invention will be apparent from the attached independent claims and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The invention will be explained by means of an example of an embodiment with reference to the attached drawings, where: Fig. 1 illustrates an example of an embodiment of the invention;

Fig. 2 illustrates an acoustic source according to the present invention and

Fig. 3 illustrates an alternative acoustic source according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0048] The drawings are schematic and intended to illustrate the principles of the invention. The elements therein are therefore not necessarily to scale, and numerous details known to those skilled in the art have been omitted for clarity.

[0049] Fig. 1 depicts an array 100 comprising sensor units 120 and acoustic sources 130 deployed on a fixed surface 10, which is a seabed in the present example. The receiver array, i.e. the sensor array 100, monitors an underground formation, e.g. a reservoir for hydrocarbons. The sensor units 120 are shown as rectangles and the acoustic sources 130 are shown as circles for illustration purposes only. Furthermore, the acoustic sources 130 generally form an array that is different from the array of sensor units, and preferably has an acoustic signal with a relatively low power distributed over time. Thus, the acoustic sources 130 appear randomly placed between the sensors 120 in the array of sensor units. Any acoustic source 130 may be placed outside the sensor array 100 .

[0050] A cable 102 supplies electrical power through a hub 105 on the seabed and further through an OCB 110 to the elements 120, 130 in the array 100. The cable 102 and OBC 110 comprise power supply lines, communication lines and other elements known in the field.

[0051] A main concept in the present invention is to supply low power through the existing infrastructure, including through the cable 102, instead of through specially made cables with a larger diameter. Lower power means longer time to supply the energy needed for a shot, so shots can be fired, for example, every 50 minutes instead of every 10 seconds. Even so, the time it takes to perform a survey is short compared to the time required to get a seismic vessel to the field. The costs involved are also greatly reduced compared to the costs of renting a source vessel, as the invention uses existing infrastructure with a relatively small addition as described in more detail below. Unfavorable weather conditions can shut down a source vessel and special considerations must be taken when working near fixed installations.

[0052] In some embodiments, the acoustic sources 130 are connected to a separate cable for power supply and communications on the seabed. Thereby, the acoustic sources can be deployed at positions outside the network of sensors 120 and/or at different times, e.g. after installation of the sensor network 120. The power supply used for the sensors can still be used for the acoustic sources, and a common communication system can be used for the the acoustic sources and sensors. For this, a preferred communication system should be able to contain a large number of units, each of which is provided with a unique address so that it can be controlled independently of the other acoustic sources or sensor units. Ethernet is an example of a suitable communication technology, but any suitable communication technology can be used with the present invention. In a typical embodiment, the hub 105 is thus equipped with several connectors for power and communication, where each connector makes it possible to add a new segment of sensors. The acoustic source according to the present invention will preferably be able to be plugged into such a connector and thus reuse most of the infrastructure. However, no further assumptions regarding the actual shape of the elements or their relative location should be inferred from Figure 1 or the brief discussion above.

[0053] Fig. 2 illustrates an acoustic source 130 in greater detail. The source comprises an outer pressure-tight container 131 which may have any suitable outer shape. The rectangle shown is chosen for convenience, and may represent a cylinder or any other shape. In a real embodiment, the base plate 135 is designed to provide proper acoustic contact with the seabed 10.

[0054] The device in Figure 2 is a kicker. Potential energy in the form of an electrical charge is stored in capacitors inside the unit 141. When a switch is closed, the stored potential energy is discharged through the electrodes 142 to form a vapor and plasma bubble as discussed in the introduction. The reflector 143 reflects the expanding shock wave so that it arrives at a target at the same time as the contractive shock wave. The chamber enclosed by the reflector 143 and the bottom plate 135 is filled with liquid, and the pressure in this liquid can be controlled to produce a controlled frequency, for example in the range 2-100 Hz, as explained in the sections on prior art. The kicker electrodes must be surrounded by a liquid to form the necessary vapor and plasma bubble. However, a container containing the liquid can be deployed on dry land or underwater. Thus, a kick source can be used in applications both on land and offshore.

[0055] As described in the introduction, a bottom plate 135 made of 6-8 mm stainless steel is transparent to the outgoing acoustic waves. An optional air-filled chamber between the reflector 143 and the pressure-tight container 131 prevents the shock waves from propagating in any direction other than through the bottom plate 135.

[0056] For the kicker embodiment in Figure 2, the acoustic source 130 receives supplied electrical power through the cable 110 on the left side of Figure 2. The supplied power, ie the amount of energy per second, is suitable for sensor use, and is therefore relatively low. This means that the time required to collect the necessary energy in the capacitors will be correspondingly long. However, operating costs do not increase with increasing time between shots as they would with a source vessel.

[0057] A control module 132 is used to direct the input power through a power line 133 to the capacitors inside the unit 141. As soon as the charge, i.e. potential energy, has reached a predetermined level, the control module connects the incoming power line directly to the outgoing power line on the right side in Figure 2, so that the next acoustic source downstream can be charged in a similar way. Alternatively, incoming power from a central power source can be distributed evenly between all the acoustic sources. However, such an approach will easily cause the power to fall below a level suitable for charging the capacitors, and impose additional costs on special components that enable charging the capacitors with very low power.

[0058] A communication line 134 carries control signals from control module 132 to unit 141. These control signals are simple instructions to toggle one switch when the capacitors are charged, and another switch to fire the trigger, and do not require ethernet communication or the like. If ethernet connections are used, the addressable MAC will be inserted in a control module 132. Of course, sources downstream should have uninterrupted communication regardless of the signaling, so the communication line 134 also represents the continuous communication line, e.g. a twisted pair cable for the ethernet, which passes through the acoustic source 130 .

[0059] More specifically, signals instructing each individual acoustic source to fire at a specific time are sent over the communication line to the individually addressable control modules 132. If desired, the control modules can include synchronized clocks or other means to compensate for delay in a signal passing through a long cable 110 or a cable containing one or more repeaters.

[0060] Fig. 3 illustrates an alternative acoustic source that stores potential energy in a spring 153. The input and output connections 110, the pressure-tight container 131, control module one 132, the internal power line 134 and the communication line 134 correspond to those in Figure 2, and is described above. The bottom plate 135 should, however, be adapted to receive impulses from a hammer piston 154, and is most probably different from the bottom plate in Figure 2 in a real embodiment.

[0061] The power unit 151 transforms a relatively low electrical input power into mechanical power required to lift the piston 153 in a support structure 152 against the force of spring 153. For example, incoming power can be collected in capacitors until the collected energy is sufficient to lift the piston 154 a notch in a rack and pinion arrangement (not shown). Such an arrangement also illustrates problems that can arise if the input power is evenly distributed over all the acoustic sources mentioned above. If the input power is very low, the capacitors or batteries must have very small leakage currents to avoid a large part of the input power being lost through leakage.

[0062] The required output power for the acoustic wave is in the order of a few hundred joules. When a mass m is dropped from a height h, as is known, an energy mgh is released, where g is the acceleration of gravity, i.e. 9.8 m/s<2>. A significant proportion of this energy can be lost to output energy different from the desired wave energy, for example as heat loss. A falling mass therefore requires a large mass and/or a large height to generate the required output signal. This is the main reason why the energy is stored in a spring 153 instead of as height in the gravitational field.

[0063] A coil wound around the piston 154 can gradually lift the piston in a rack and pinion arrangement as described above. However, when the piston 154 is released and forced back through the coil by the spring 153, such a coil would induce eddy currents in the piston and act as a brake.

[0064] A possible solution is produced by a hydraulic arrangement as shown in Figure 3. This includes only short, local hydraulic lines, so that the long response times associated with lines to the surface are avoided. In Figure 3, a linear piston (not shown) is displaced inside a supply line 155 by induction, i.e. electrical power from the power unit 151 sent through a coil 156. The linear piston moves hydraulic oil from a tank 157 to lift cylinders 158 with a piston area much larger than the diameter of the supply line 155. The hammer piston 154 is gripped by separate hydraulic grippers 159 which release the hammer piston 154 when they receive a signal to do so from the control module 132. A check valve (not shown) prevents the oil from returning to the tank 157 during the lifting process, and a hydraulic return circuit (not shown) returns the oil to tank 157 for another iteration.

[0065] While the invention is described by way of example, the scope of the invention is defined by the appended patent claims.

Claims (10)

1. Acoustic source system for permanent monitoring of a reservoir, where the system comprises a sensor array (100) for monitoring the reservoir, deployed on a fixed surface at the reservoir, where the sensor array (100) is supplied with electrical energy from a common energy source and communicates through a common communication system, characterized by an acoustic source (130) which is adapted to long-term deployment on the fixed surface at the reservoir, is individually addressable through the common communication system, and is arranged to receive and store electrical energy from the common low-power energy source adapted to the infrastructure for the sensor array (100), and to receive a firing signal through the common communication system to release the stored energy as acoustic energy with an output power greater than the input power supplied from the common energy source. 2. Acoustic source system according to claim 1, comprising an array of individually addressable acoustic sources (130). 3. Acoustic source system according to claim 1 or 2, where the acoustic source (130) is an impulse source. 4. Acoustic source system according to one of the preceding claims, where the acoustic source (130) is a vibration source. 5. Acoustic source system according to one of the preceding claims, where the supplied electrical power is collected as energy in a capacitor (141, 151). 6. Acoustic source system according to claim 5, where the acoustic source is a kicker. 7. Acoustic source system according to claim 5, where the supplied electrical energy is collected as energy in a spring (153). 8. Method for using the acoustic source system according to one of the preceding claims, characterized by the steps: to store energy from the common energy source in the acoustic source (130) until the stored energy has reached a predetermined level, to generate a firing signal to the acoustic the source using the common communication system, and using the firing signal to release the stored energy as acoustic energy with an output power greater than the input power supplied from the common energy source. 9. Method according to claim 8, where a group of acoustic sources is triggered to emulate a P-wave from an array of air guns as the P-wave front would reach the seabed. 10. Method according to claim 8, further comprising the step of triggering a number of acoustic sources one by one at different times and correlating the resulting signals.