NO342820B1 - A data acquisition unit - Google Patents

Abstract

The invention relates to a data acquisition unit (1) for lowering to a sea floor (2), com prising a radio antenna (13) movable between a retracted position and an extended position. The antenna (13) and a gas-filled chamber(17) with variable volume interact in such a way that a compression of the chamber(17) corresponds to a movement (22) of the antenna (13) towards the retracted position, while an expansion of the chamber (17) corresponds to a movement (23) of the antenna (13) towards the extended position. The Chamber (17) is provided with an initial pressure (pce) above atmospheric pressure (patm), adapted to place the antenna (13) in the extended position when ambient pressure (på) is atmospheric, and place the antenna (13) in the retracted position when the ambient pressure (pa) equals the pressure at or above the sea floor (2).The invention relates to a data acquisition unit (1) for lowering to a sea floor (2), com prizing a radio antenna (13) movable between a retracted position and an extended position. The antenna (13) and a gas-filled chamber(17) with variable volume interact in such a way that a compression of the chamber(17) corresponds to a movement (22) of the antenna (13) towards the retracted position, while an expansion of the chamber (17) corresponds to a movement (23) of the antenna (13) towards the extended position. The Chamber (17) is provided with an initial pressure (pce) above atmospheric pressure (patm), adapted to place the antenna (13) in the extended position when ambient pressure (på) is atmospheric, and place the antenna (13) in the retracted position when the ambient pressure (pa) equals the pressure at or above the sea floor (2).

Description

Description

The invention relates to a data acquisition unit for lowering to a sea floor, comprising a buoyancy-increasing means for raising the data acquisition unit from the sea floor to a sea surface, and a radio communication system. The invention also relates to a method for deploying the data acquisition unit at the sea floor, for acquiring data, and retrieving the data acquisition unit from the sea floor.

A data acquisition unit comprises at least one sensor for measuring at least one physical quantity. One kind of data acquisition unit is a seismic node which before data acquisition is lowered to a sea floor. The seismic node comprises one or more geophones that sense analogue seismic signals which are either natural or produced by a seismic energy source, typical an air gun, and reflected by subterranean strata. The seismic signals are converted to digital data by software, time stamped by a clock and stored in a memory. The seismic node is then raised to the sea surface by increasing its buoyancy, e.g. by releasing a weight. Then the seismic node is picked up from the sea and brought aboard a vessel, and the seismic data is retrieved from the memory for processing and interpretation. Other kinds of data acquisition units for lowering to a sea floor are used for sensing environmental data such as atmospheric pressure and seawater temperature for weather forecasting or science, or for military purposes.

Data acquisition units may be provided with pick-up buoys attached by ropes. The data acquisition units can then be picked up from the sea by throwing out a heaving line with a hook from the vessel, and hook the pick-up buoys and haul them in together with the data acquisition units. The heaving line is typically thrown by a pneumatic heaving line thrower. The pick-up buoys may also be picked up by a long boat hook.

Buoys with radio communication systems with antennas are known from prior art. A known problem is that the antennas may be hit or snagged by surrounding objects, and thereby destroyed or torn off the buoy. A possible problem with data acquisition units with antennas that are lowered to the sea floor is that the antennas may be hit or snagged by the sea floor or objects on the sea floor.

WO 2013169115 A1 concerns an apparatus for deployment and retrieval of a measurement system on an ocean bottom, comprising devices for temporary storing the measurement system, movable control faces, software and devices for manoeuvring and at least one ballast tank for ballast. The ballast comprises a slurry consisting of free soluble salt and a saturated solution of the salt in water, or a solid body or insoluble coarse or fine grained material. The apparatus comprises a wireless modem on the apparatus on the surface and at the ocean bottom, respectively. This modem contains GPS positioning equipment, equipment for acoustic communication and equipment for radio communication.

US 6625083 B2 relates to a method and device for seismic exploration of an underwater subsurface zone using seismic receivers coupled with the bottom of the water mass. Acquisition units comprising a spire where seismic receivers are arranged, a measuring compartment for acquisition and storage of the data received by the receivers, as well as removable and selective floating devices are used. The acquisition units are launched and, under the effect of gravity, they attach to the bottom and couple the receivers with the underlying formations. The respective positions thereof are first detected, seismic acquisition operations are carried out using an underwater source so as to collect seismic data on the formation, then the floating devices are actuated so as to bring all or part of acquisition units back up to the surface or a data collection device (vehicle or passive collection unit) is lowered to the neighbourhood of the various units. In one embodiment inflatable buoys associated with storage modules of the acquisition units are inflated and disconnected from the acquisition units. The storage modules thus come back up to the surface where they are recovered.

US 4692906 A discloses an ocean bottom seismometer with a data collection unit that will stand on the seabed while collecting data. The unit has an antenna and ballast which may be ejected to provide positive buoyancy for retrieval.

US 3961589 A and US 7165504 B1 discloses a subsea device with an antenna movable in and out of a device. The antenna is moved due to pressure differences between a chamber in the device and the water outside the device.

The purpose of the invention is to provide a data acquisition unit for lowering to a sea floor, comprising a buoyancy-increasing means for raising the data acquisition unit from the sea floor to a sea surface, and a method for deploying the data acquisition unit at the sea floor, for acquiring data, and retrieving the data acquisition unit from the sea floor. A further purpose is that the data acquisition unit shall have a radio communication system with an antenna that is not easily hit or snagged by surrounding objects or the sea floor when the data acquisition unit is under water. At least the invention shall provide an alternative to prior art. Further purposes and advantages of the invention and how they are achieved will appear from the description, the drawings and the claims.

The invention thus relates to a data acquisition unit for lowering to a sea floor, comprising a buoyancy-increasing means for raising the data acquisition unit from the sea floor to a sea surface, and a radio communication system including an antenna. According to the invention the antenna is movable between a retracted position and an extended position; the data acquisition unit comprises a gas-filled chamber with variable volume; the antenna and chamber interact in such a way that a compression of the chamber corresponds to a movement of the antenna towards the retracted position, while an expansion of the chamber corresponds to a movement of the antenna towards the extended position; a portion of the antenna or chamber is exposed to ambient pressure, an increase of the ambient pressure forces the antenna towards the retracted position and compresses the chamber, a compression of the chamber causes an increase of chamber pressure, which counteracts the movement of the antenna towards the retracted position and forces the antenna towards the extended position; wherein the chamber is provided with an initial pressure above atmospheric pressure, adapted to place the antenna in the extended position when the ambient pressure is atmospheric, and place the antenna in the retracted position when the ambient pressure equals the pressure at or above the sea floor.

Preferably the chamber has a connection for changing the pressure of the chamber. This allows providing the chamber with the initial pressure in a convenient way.

Further the antenna preferably also forms a gripping portion for a catching means, for picking up the data acquisition unit when it floats in the sea. The catching means may be a heaving line with a hook which is thrown out from a vessel, which hooks the antenna and haul the data acquisition unit aboard the vessel by the antenna. The heaving line is typically thrown by a pneumatic heaving line thrower. The catching means may also be a long boat hook. The gripping portion of the antenna allows catching the data acquisition unit in the sea in a convenient way. The antenna is preferably provided with a lug that both serves as a stopper for the movement of the antenna towards the retracted position, and serves as a stopper for preventing the catching means, e.g. the hook, from sliding off the antenna.

In one embodiment the chamber comprises a cylinder and the antenna is connected with a piston movable in the cylinder, and the piston forms a movable wall of the chamber. An inward movement of the piston compresses the chamber and pulls the antenna into the cylinder, and an expansion of the chamber pushes the piston and antenna outwards. The ambient pressure increases during a lowering of the data acquisition unit in the sea, and pushes the piston and the antenna to the retracted position. This in turn compresses the chamber, causing the chamber pressure to increase. When the ambient pressure falls during rising of the data acquisition unit in the sea, the chamber pressure pushes the piston and antenna to the extended position.

I an alternative of the above embodiment the antenna is telescopic. This makes it possible to use a cylinder that is shorter than the extended antenna.

In another embodiment the chamber comprises a helical, flexible and springy tube, the antenna is within or integral with the tube. In a variant of this embodiment the chamber comprises a flexible tube, the antenna is helical and springy and within or integral with the tube. In these embodiments an increase of the ambient pressure compresses the tube and forces the tube towards a coiled, retracted position, a decrease of the ambient pressure allows the tube to expand and straighten to the extended position.

The invention also relates to a method for deploying the data acquisition unit of the invention at the sea floor, for acquiring data, and retrieving the data acquisition unit from the sea floor, comprising:

- bringing the data acquisition unit in a state of negative buoyancy;

- providing the chamber of the data acquisition unit with an initial pressure above atmospheric pressure, which causes the antenna to move to the extended position; - lowering the data acquisition unit from the sea surface to the sea floor, the ambient pressure is initially atmospheric and the antenna is initially in the extended position, the ambient pressure increases during the lowering of the data acquisition unit, which causes the antenna to move to the retracted position;

- acquiring data;

- actuating the buoyancy-increasing means of the data acquisition unit, which causes the data acquisition unit to get positive buoyancy and rise from the sea floor to the sea surface, the ambient pressure decreases during the rising of the data acquisition unit, which causes the antenna to move to the extended position; and - catching the data acquisition unit in the sea.

The buoyancy-increasing means of the data acquisition unit may be actuated by sending a remote signal to the data acquisition unit by an underwater acoustic communication system. This signal can be sent from a nearby vessel. Alternatively, the buoyancy-increasing means of the data acquisition unit may be actuated by a clock. This clock can be included in the data acquisition unit and set to actuate the buoyancy increasing means at a predetermined time when the data acquisition is planned to be finished.

Preferably the radio communication system is activated to send a localization signal from the antenna when the data acquisition unit has returned to the sea surface. This activation can be done by the underwater acoustic communication system.

Alternatively, the activation is carried out by a sensor or switch that senses the ambient pressure, and that activates the radio communication system when the pressure is atmospheric or slightly above, i.e. at a distance below the sea surface.

As discussed in the introductory part of the description, data acquisition units for lowering to a sea floor exist in various kinds. The invention is not limited to data acquisition units for lowering to a sea floor of any specific kind.

The invention will now be described with reference to the accompanying drawings, in which:

Fig. 1a illustrates a data acquisition unit according to the invention at a sea surface, before lowering to a sea floor;

Fig. 1b illustrates the data acquisition unit of fig. 1a during lowering to a sea floor;

Fig. 1c illustrates the data acquisition unit of fig. 1a at the sea floor;

Fig. 1d illustrates the data acquisition unit of fig. 1a rising from the sea floor;

Fig. 1e illustrates the data acquisition unit of fig. 1a returned to the sea surface;

Fig. 2 is an enlargement of a portion of fig. 1a;

Fig. 3a illustrates another embodiment of the data acquisition unit according to the invention at the sea surface; and

Fig. 3b illustrates the data acquisition unit of fig. 3a during lowering to the sea floor.

Fig. 1a illustrates a data acquisition unit 1 according to the invention, floating at a sea surface 6, for lowering to a sea floor for acquiring seismic data.

The data acquisition unit 1 has a front section 33, a mid-section 32 and a tail section 4. A measurement equipment package 8 is located partly in the front section 33, partly in the mid-section 32, and includes geophones for sensing seismic signals. The measurement equipment package 8 also includes a battery, a processor, a clock and software for converting analogue electric signals produced by the geophones to digital seismic data. Further the measurement equipment package 8 includes a memory for storing the seismic data.

The data acquisition unit 1 further includes ballast tanks 11. When the ballast tanks 11 are filled with sea water, the data acquisition unit 1 has a positive buoyancy. The ballast tanks 11 may be filled with a heavy slurry of salt and brine, and the data acquisition unit 1 then gets a negative buoyancy.

The data acquisition unit 1 includes a not illustrated underwater acoustic communication system for communication between the data acquisition unit and a boat when the data acquisition unit is under water.

The data acquisition unit 1 also includes a radio communication system with a radio antenna 13 for communication between the data acquisition unit 1 and the boat, and for localizing the data acquisition unit 1 when it floats in the sea. The antenna 13 is connected with a piston 21 which is movable in a cylinder 20 which is located partly in the tail section 4, partly in the mid-section 32 of the data acquisition unit 1. The inner end of the cylinder 20 has an inner cap 41 which forms a gas tight closure of the cylinder, and the opposite outer end of the cylinder 20 has an outer cap 34 which forms a mechanical closure, but not a gas tight closure of the cylinder 20.

The piston 21 and antenna 13 are movable in an inward direction 22 and an outward direction 23. Reference is made to fig. 2, which illustrates the outer portion of the tail section 4 as illustrated in fig. 1a in more detail. The outer cap 34 has a central hole 47 which guides the antenna 13 during movement of the piston 21 in the cylinder 20. The outer cap 34 also has a vent hole 40 which allows ingress of water during movement of the piston 21 away from the outer cap 34, and egress of water during movement of the piston 21 towards the outer cap 34. The piston 21 is provided with seals 38 which seal between the piston and the cylinder. The cylinder 20 is provided with a flange 37 with threaded bolt holes 36 for fastening the outer cap 34 by bolts 35. This allows removing and replacing the outer cap 34 during insertion and withdrawal of the piston 21.

The inner side of the outer cap 34 limits outward movement of the piston 21 in direction 23, as illustrated in fig. 1a and 2. The antenna 13 and piston 21 are then in an extended position. The antenna 13 has a lug 19 serving as a stopper which abuts the outer side of the outer cap 34 during inward movement of the antenna 13 and piston 21 in direction 22, as illustrated in fig. 1b. The antenna 13 and piston 21 are then in a retracted position.

The piston 21, the cylinder 20 and the inner cap 41 forms a chamber 17, in which the piston 21 forms a movable wall. The chamber 17 thus has variable volume. An inward movement of the antenna 13 and piston 21 in direction 22 compresses the chamber 17 and its content, and an outward movement of the antenna 13 and piston 21 in direction 23 expands the chamber and its content.

Ambient pressure pa exerts a force on the antenna 13, and propagates through the vent hole 40 of the outer cap 34 and exerts a force on the outer side of the piston 21. These forces try to move the antenna 13 and the piston 21 inwards in direction 22. The force exerted on the antenna 13 equals the ambient pressure pa times the crosssectional area of the antenna 13. The force exerted on the piston 21 equals the ambient pressure pa times the area of the piston 21 minus the ambient pressure pa times the cross-sectional area of the antenna 13. Thus, the total force exerted by the ambient pressure pa on the antenna 13 and the piston 21 equals the ambient pressure pa times the area of the piston 21. In the sea the ambient pressure pa is the atmospheric pressure plus the hydrostatic pressure, which is proportional to the depth of the sea, and thus, the deeper the data acquisition unit 1 is in the sea, the greater is the force exerted by the ambient pressure pa on the antenna 13 and the outer side of the piston 21.

The chamber 17 is filled with air. The chamber pressure pc exerts a force on the inner side of the piston 21, and this force tries to move the piston 21 and the antenna outwards in direction 23. This force equals the chamber pressure pc times the area of the piston 21. According to Boyle's Law the chamber pressure pc increases when the chamber volume is reduced. This means that the force of the chamber pressure pc increases when the chamber volume is reduced when the piston 21 is moved inwards in direction 22.

The deploying of the data acquisition unit 1 at the sea floor, the acquiring of data, and the retrieving of the data acquisition unit 1 will now be explained with reference to fig.

1a-e.

Fig. 1a illustrates the data acquisition unit 1 before deployment. The data acquisition unit 1 is then at a boat at the sea surface, where it is brought in a state of negative buoyancy by filling the ballast tanks 11 with a heavy slurry of salt and brine through not illustrated valves.

Since the data acquisition unit is at a boat at the sea surface, the ambient pressure is atmospheric. The chamber 17 has a connection for filling or removing air, formed by a tube 18 and a valve 31, for changing the chamber pressure pc. By means of this connection, the chamber pressure pc is set to an initial pressure pce above atmospheric pressure before the data acquisition unit 1 is lowered from the sea surface. Since the initial chamber pressure pce is greater than the ambient pressure pa, the piston 21 is moved outwards in direction 23 until the piston abuts the outer cap 34, and the antenna 13 is in the extended position, see fig. 2.

Fig. 1b illustrates the data acquisition unit 1 during lowering from the sea surface to the sea floor. This can be done by throwing the data acquisition unit 1 overboard from the boat, or by using some kind of launcher, e.g. a chute. Since the buoyancy is negative, the data acquisition unit 1 sinks. The data acquisition unit is steered into a predetermined trajectory by control faces 7 of the tail section 4.

As the data acquisition unit 1 sinks in the sea the ambient pressure pa increases, and the force of the ambient pressure increases correspondingly, as discussed above. At a certain depth, which depend on the initial chamber pressure pce, the ambient pressure pa exceeds the initial chamber pressure pce. The force of the ambient pressure pa then exceeds the force of the initial chamber pressure pce, and the piston 21 is moved inwards in direction 22, away from the outer cap 34. The movement of the piston 21 reduces the volume of the chamber 17, which compresses the air in the chamber, which increases the chamber pressure pc, and increases the force of the chamber pressure pc. The piston moves until the force of the chamber pressure pc equals the force of the ambient pressure pa, i.e. the piston moves until the chamber pressure and the ambient pressure are equal. However, since the data acquisition unit 1 continuously sinks, the force of the ambient pressure continuously increases, and the piston 21 continuously moves inwards in direction 22. The piston 21 therefore continues to move inwards in direction 22 until the lug 19 of the antenna 13 abuts the outer cap 34, and the antenna 13 and the piston 21 is in the retracted position, see fig. 1b. The chamber pressure pc is then pcr. Just as the antenna 13 and piston 21 reaches the retracted position, also the ambient pressure pa=pcr. The ambient pressure pa increases during a further sinking of the data acquisition unit 1, but this will not change the position of the piston 21 and the antenna 13, since the lug 19 of the antenna prevents a further inwards movement of the piston 21.

Fig. 1c illustrates the data acquisition unit 1 landed at the sea floor 2. Before landing the control faces 7 are oriented transversely as brakes in order to slow the descent.

The front section 33 is divided in portions that can unfold to form support legs 9 with feet 10. The support legs 9 and feet 10 are folded out some time before landing, which further slows the descent. After the landing the measurement-equipment package 8 is lowered to the sea floor 2 (not illustrated) by a not illustrated mechanism. The acquisition of data is then carried out. The acquisition of data consists of firing distant air guns, which produces seismic waves, and sense the seismic waves and their reflections by the geophones of the measurement-equipment package 8. The geophones produce analogue signals which by means of the software are converted to digital seismic data which are stored in the memory. The clock time stamps the data for later processing and interpretation.

After some time, the data acquisition unit 1 is given a command to finish data acquisition and return to the sea surface. This command is transmitted through the underwater communication system. The measurement equipment-package 8 is then lifted back into the data acquisition unit 1, and the support legs 9 with the feet 10 are folded up into the front section 33. Not illustrated valves of the ballast tanks 11 are opened, which causes the heavy slurry of salt and brine forming the ballast to flow out of the ballast tanks and lighter sea water to flow into the ballast tanks. This causes the data acquisition unit 1 to get a positive buoyancy and rise from the sea floor.

Fig. 1d illustrates the data acquisition unit 1 rising from the sea floor. The ballast tanks 11 are filled with sea water, and the control faces 7 are oriented in the longitudinal direction of the data acquisition unit 1, to minimize their braking effect. The ambient pressure pa is greater than the chamber pressure pcr, and the antenna 13 is held in its retracted position by the ambient pressure pa.

The ambient pressure pa is continuously reduced during the rising of the data acquisition unit 1, and at a certain depth the ambient pressure pa falls below the chamber pressure pcr of the retracted position. The chamber pressure pcr then forces the piston 21 outwards in direction 23. The chamber 17 then expands, and the chamber pressure pc falls. The data acquisition unit 1 continues to rise, the ambient pressure pa therefore continues to fall, and the piston 21 is pushed further outwards in direction 23 by the chamber pressure pc. This continues until the piston 21 reaches the outer cap 34, and the antenna 13 is in the extended position, see fig. 1e. The chamber pressure pc and the ambient pressure pa is then equal to the initial chamber pressure pce. The data acquisition unit 1 is then at the same depth as when the piston 21 and the antenna 13 started to move inwards in direction 22 during descent of the data acquisition unit, as discussed with reference to fig. 1b.

Fig. 1e illustrates the data acquisition unit 1 when it has risen to the sea surface 6. The data acquisition unit 1 floats at the sea surface 6 with the antenna 13 in the extended position. A not illustrated pressure switch detects that the ambient pressure has fallen to atmospheric, and turns on the radio communication system. The radio communication system sends out a localization signal from the antenna. To save the battery, the localization signal is intermittent. The localization signal might be too weak to be detected by the boat, and therefore, after a predetermined time the intensity of the localization signal is increased. To save the battery, the intervals of the intermittent signal are then longer.

To visually localize the data acquisition unit, the antenna 13 may be provided with an electric light. The electric light is supplied with electricity from a battery which is located in an internal space 39 of the piston 21. The internal space 39 of the piston also comprises electronics of the radio communication system.

To facilitate catching the data acquisition unit in the sea, the antenna 13 also forms a gripping portion for a catching means, e.g. a heaving line with a hook which is thrown out from the boat, and which hooks the antenna 13. The lug 19 of the antenna forms a stopper which prevents the hook from sliding off the antenna. The data acquisition unit is then brought aboard the boat, and the seismic data is retrieved from the memory.

The following example further explains the invention:

Cylinder length: Lc = 0.5 m

Length of antenna excluding lug, including piston: La = 0.45 m

Length of chamber in retracted position: Lr = Lc-La=0.5 m -0.45 m = 0.05 m Cylinder and piston diameter: Dp = 0.06 m

Piston area: Ap= πDp<2>/4 = π*0.06m<2>/4 = 2.8 * 10<-3>m<2>

Atmospheric pressure: patm = 1 bar

Definition bar: 1 bar = 10<5>N/m<2>

Hydrostatic pressure ph is phs=0 at the sea surface, and increases with 1 bar for every 10 m depth.

Ambient pressure pa is at any depth the sum of atmospheric pressure and hydrostatic pressure, i.e.

pa=patm+ph (I)

At the sea surface:

pas=patm+phs=1 bar+0=1 bar

Chamber pressure pc is pce in extended position of antenna and pcr in retracted position of antenna.

In extended position chamber is filled to 2 bar above atmospheric pressure, i.e. pce=2 bar patm = 3 bar

Force acting on the piston at any depth: Fp = (pc-pa)*Ap (II)

Force acting on the piston at the sea surface:

Fps = (pce-pas)*Ap = (3-1)*10<5>N/m<2>*2.8*10<-3>m<2>= 560 N

This force is assumed to be acceptable for forcing the piston and antenna to the extended position.

At any depth during the piston's movement between extended position and retracted position the sum of forces acting on the piston is 0, i.e. Fp=0

Equation (II) gives:

0= (pc-pa)*Ap, which means that when the piston is between extended position and retracted position:

pa=pc (III)

Let phe be hydrostatic pressure at the depth where the piston leaves the extended position during descent and reaches the extended position during ascent.

Ambient pressure from equation (III), at this depth:

pa=pc=pce=3 bar

Equation (I), rearranged:

ph=pa-patm

At this depth:

phe= pce-patm=3 bar- 1 bar = 2 bar

Since hydrostatic pressure ph = 0 at the sea surface and increases with 1 bar for every 10 m depth, the piston leaves/reaches the extended position at a depth of 20 m.

Chamber pressure pcr in retracted position:

According to Boyle's Law (assumes constant temperature, ideal gas) the product of pressure and volume is constant.

Thus:

pcr*Lr*Ap = pce*Lc*Ap

Rearranged: pcr=pce*Lc/Lr=3 bar*0.5 m/0.05 = 30 bar

Let phr be the hydrostatic pressure at the depth where the piston reaches the retracted position during descent and leaves the retracted position during ascent. Ambient pressure from equation (III), at this depth:

pa=pc=pcr=30 bar

Equation (I), rearranged:

ph=pa-patm

At this depth:

phr=pa-patm=30 bar- 1 bar = 29 bar

Since hydrostatic pressure ph = 0 at the sea surface and increases with 1 bar for every 10 m depth, the piston leaves/reaches the retracted position at a depth of 290 m. The sea floor then should be minimum 290 m deep. But the sea might be much deeper, e.g. 1000 m, which means the antenna reaches the retracted position above the sea floor.

Fig. 3a and 3b illustrate another embodiment of the data acquisition unit according to the invention. In this embodiment the data acquisition unit comprises a flexible tube 25 which forms the air-filled chamber 17. The antenna 13 is placed within the tube 25. Alternatively, the antenna 13 is integral with the tube 25, e.g. moulded into the tube. A lug 19 is located in the outer end of the tube/antenna. Fig. 3a illustrates the data acquisition unit 1 at the sea surface 6 with the tube 25 and the antenna 13 in the extended position, extending from a tube storage room 45 in which the tube is fastened and the antenna is coupled to electronics of the radio communication system. Fig. 3b illustrates the data acquisition unit 1 during lowering to the sea floor with the tube 25 and the antenna 13 in a coiled, retracted position, stored in the tube storage room 45. Except for the tube 25 and the antenna 13, the data acquisition unit 1 of fig.

3a-b is the same as in fig. 1a-e, and the description of this will not be repeated.

In one alternative the antenna 13 of fig. 3a and 3b is helical and springy, made e.g. from steel, which when not subjected to any forces will assume a coiled shape. The tube may be made from a flexible, relatively soft plastic material, which allows the tube to be compressed and expanded.

In another alternative the tube 25 itself is helical and springy, and when not subjected to any forces, it will assume a coiled shape. The tube is at the same time flexible, made e.g. from a plastic material, which allow the tube to be compressed and expanded. In this alternative the antenna 13 may be flexible, e.g. a steel wire which assumes the shape of the tube.

Both alternative will function the same way:

When the chamber pressure pc is higher than the ambient pressure pa, the chamber 17 will expand. Put another way: if the pressure pc inside the tube 25 is higher than the pressure pa outside the tube, the tube will be inflated. Forces of the internal pressure pc cause the tube to straighten out and assume the extended position. This is the situation illustrated in fig. 3a.

When the external ambient pressure pa is higher than the internal chamber pressure pc, the tube will be compressed. Forces of the internal pressure pc that cause the tube 25 to straighten out are cancelled out by forces of the external pressure pa, and the tube will return to its coiled shape. Since the tube 25 and antenna 13 are fastened in the tube storage room 45, the coiled tube will place itself in the tube storage room. This is the situation illustrated in fig. 3b.

The tube storage room 45 has a conical entering portion 46. The lug 19 has an upper closing portion 42 and a lower conical portion 44. The lower conical portion 44 fits into the entering portion 46, and guides the closing portion 42 in place when the tube coils up in the storage room 45.

Similar considerations related to depth and pressure as for the embodiment of fig. 1ae apply.

Claims (15)

Claims

1. A data acquisition unit (1) for lowering to a sea floor (2), comprising a buoyancyincreasing means (11) for raising the data acquisition unit (1) from the sea floor (2) to a sea surface (6), and a radio communication system including an antenna (13), the data acquisition unit is c h a r a c t e r i z e d b y :

- the antenna (13) is movable between a retracted position and an extended position;

- a gas-filled chamber (17) with variable volume;

- the antenna (13) and chamber (17) interact in such a way that a compression of the chamber (17) corresponds to a movement (22) of the antenna (13) towards the retracted position, while an expansion of the chamber (17) corresponds to a movement (23) of the antenna (13) towards the extended position;

- a portion of the antenna (13) or chamber (17) is exposed to ambient pressure (pa), an increase of the ambient pressure (pa) forces the antenna (13) towards the retracted position and compresses the chamber (17), a compression of the chamber (17) causes an increase of chamber pressure (pc), which counteracts the movement of the antenna (13) towards the retracted position and forces the antenna (13) towards the extended position;

wherein

the chamber (17) is provided with an initial pressure (pce) above atmospheric pressure (patm), adapted to place the antenna (13) in the extended position when the ambient pressure (pa) is atmospheric, and place the antenna (13) in the retracted position when the ambient pressure (pa) equals the pressure at or above the sea floor (2).

2. The data acquisition unit (1) of claim 1, wherein the chamber (17) has a connection (18, 31) for changing the chamber pressure (pc).

3. The data acquisition unit (1) of claim 1 or 2, wherein the antenna (13) also forms a gripping portion for a catching means.

4. The data acquisition unit (1) of claim 3, wherein the antenna (13) is provided with a lug (19) that both serves as a stopper for the movement of the antenna (13) towards the retracted position, and serves as a stopper for preventing the catching means from sliding off the antenna (13).

5. The data acquisition unit (1) of any of the preceding claims, wherein the antenna (13) is provided with an electric light.

6. The data acquisition unit (1) of any of the preceding claims, wherein the chamber (17) comprises a cylinder (20) and the antenna (13) is connected with a piston (21) movable in the cylinder (20), the piston (21) forms a movable wall of the chamber (17), an inward (22) movement of the piston (21) compresses the chamber (17) and pulls the antenna (13) into the cylinder (20), an expansion of the chamber (17) pushes the piston (21) and antenna (13) outwards (23).

7. The data acquisition unit (1) of claim 6 depending on claim 5, wherein the piston (21) comprises a battery for the electric light.

8. The data acquisition unit (1) of claim 6 or 7, wherein the piston (21) comprises electronics of the radio communication system.

9. The data acquisition unit (1) of any of the claims 1-5, wherein the chamber (17) comprises a helical, flexible and springy tube (25), the antenna (13) is within or integral with the tube (25), an increase of the ambient pressure (pa) compresses the tube (25) and forces the tube (25) towards a coiled, retracted position, a decrease of the ambient pressure (pa) allows the tube (25) to expand and straighten to the extended position.

10. The data acquisition unit (1) of any of the claims 1-5, wherein the chamber (17) comprises a flexible tube (25), the antenna (13) is helical and springy and within or integral with the tube (25), an increase of the ambient pressure (pa) compresses the tube (25) and forces the tube (25) towards a coiled, retracted position, a decrease of the ambient pressure (pa) allows the tube (25) to expand and straighten to the extended position.

11. A method for deploying the data acquisition unit (1) of any of the preceding claims at the sea floor (2), for acquiring data, and retrieving the data acquisition unit (1) from the sea floor (2), comprising:

- bringing the data acquisition unit (1) in a state of negative buoyancy;

- providing the chamber (17) of the data acquisition unit (1) with an initial pressure (pce) above atmospheric pressure (patm), which causes the antenna (13) to move to the extended position;

- lowering the data acquisition unit (1) from the sea surface (6) to the sea floor (2), the ambient pressure (pa) is initially atmospheric and the antenna (13) is initially in the extended position, the ambient pressure (pa) increases during the lowering of the data acquisition unit (1), which causes the antenna (13) to move to the retracted position;

- acquiring data;

- actuating the buoyancy-increasing means (11) of the data acquisition unit (1), which causes the data acquisition unit (1) to get positive buoyancy and rise from the sea floor (2) to the sea surface (6), the ambient pressure (pa) decreases during the rising of the data acquisition unit (1), which causes the antenna (13) to move to the extended position; and

- catching the data acquisition unit (1) in the sea.

12. The method of claim 11, wherein actuating the buoyancy-increasing means (11) of the data acquisition unit (1) comprises sending a remote signal to the data acquisition unit (1) by an underwater acoustic communication system.

13. The method of claim 11 or 12, wherein actuating the buoyancy-increasing means (11) of the data acquisition unit (1) is done by a clock.

14. The method of any of the claims 11-13, further comprising activating the radio communication system to send a localization signal from the antenna (13), the activating is carried out by a sensor or switch sensing the ambient pressure (pa).

15. The method of any of the claims 11-14, further comprising activating the radio communication system to send a localization signal from the antenna (13), wherein the localization signal is intermittent, and after a predetermined time, the localization signal is sent with longer intervals and increased intensity.