FR2495783A1 - APPARATUS AND METHOD FOR DETERMINING THE POSITION OF A IMMERED MARINE FLUTE AND TRAILERED BY AN EXPLORATION VESSEL - Google Patents

Abstract

THE INVENTION CONCERNS A METHOD AND AN APPARATUS FOR DETERMINING THE POSITION OF A SUBMERSIBLE WATER FLUTE, TOWED BY AN EXPLORATION VESSEL. A NETWORK OF AT LEAST THREE TRANSPONDERS 10, 12, 14, FIXED TO THE SEA BOTTOM 16, RECEIVES AN ACOUSTIC CONTROL SIGNAL FROM A VESSEL 18 WHICH TOWS A FLUTE 20 CARRYING RECEIVERS 34. ON RECEPTION OF THIS SIGNAL, THE TRANSPONDERS 10, 12, 14 RETURN ACOUSTIC PULSES HAVING CLEARLY DIFFERENT FREQUENCIES FROM ONE ANOTHER, DEPENDING ON THE TRANSPONDERS, BOTH TO THE RECEIVERS 34 AND TO THE VESSEL 18. IT IS THUS POSSIBLE TO DETERMINE THE DISTANCE BY TRIANGULATION BETWEEN THE COMTANCE EACH HYDROPHONE 34 AND THE VESSEL 18. FIELD OF APPLICATION: MARINE SEISMIC EXPLORATION.

Description

The invention relates primarily to the field of seismic exploration

  Marine. The invention more particularly relates to a means for accurately determining the

  position of a towed marine seismic flute.

  In marine seismic prospecting, a ship

  explores a seismic flute with several

  pressure-sensitive detectors commonly referred to as

  the hydrophone. A source of seismic energy, for example an air gun or an explosive charge, is used to propagate pressure waves through the water, in the

  underlying seabed. Part of the energy is reflected

  dug by subterranean geological discontinuities and is then detected by hydrophones as pressure variations in the surrounding water. The mechanical energy of these pressure variations is transformed into an electrical signal by the hydrophones and this signal is transmitted, via the flute, to a device

  registration on board the vessel. The data

  picked can then be interpreted by the man of

  art to provide information about the

underwater geological formations.

  For the signals to have a meaning, it is ne-

  to know the location of the hydrophones indicated

  viduals at the moment when the pressure waves are detected.

  Since the ship is moving continuously and

  that the flute can extend over several thousand

  very behind the ship, it is difficult to locate

  accurately the hydrophones of the flute.

  Various devices have been developed to

  to obtain precise information as to the position of the ship.

  In a common application, several sub-transponders

  mariners generate output signals having a single frequency in response to an interrogation signal from the ship. The travel time of the interrogation signal and

  the transponder response signal is measured and the

  distance or distance from each transponder is calculated.

  The position of the ship with respect to the transponders can then

  triangulation if the positions of the trans-

weights are known.

  However, it is rare for the flute to be dragged directly following the direction of the ship's course. Although the flute is connected to the rear of the ship, its mass is submerged below the surface of the water under

  the action of depth control devices extends

  along the flute. Therefore, the velocity of the transverse current at the depth of the flute can be

  different from that of the transverse current affecting the

  turns, so that the flute follows the ship forming a

  certain angle with the road of the latter. Other facets

  that it is useless to enumerate, may also vary the path followed by the flute with respect to the

ship route.

  A method for estimating the position of

  te, described in the prior art, is based on the addition of a radar reflector on a tail buoy placed at the end of the flute. Onboard radar equipment can then be used, when the sea is in optimal conditions, to locate the end of the flute and the position of the individual hydrophones interspersed. Such devices are, however, generally unreliable and leave a doubt

as to the data requested.

  A second method described in the prior art uses a very sensitive and expensive apparatus for measuring the yaw and pitch angles of the end of the

  flute adjacent to the ship. These data, associated with

  their cap taken by magnetic compass along the flute,

  and at the known depth of the flute, allow

  empirically determine the positions of the hydrophones.

  The subject of the invention is another precise means for locating the submerged marine flute, this means being

  designed to eliminate the disadvantages of the prior art.

  The invention relates to an apparatus for deter-

  undermine the position of an immersed marine flute towed behind an exploration vessel. This apparatus comprises means for triggering an acoustic signal of

  order from the ship; at least three transpon-

  spacecraft arranged spatially in known positions on the sea floor, in order to establish distinct acoustic paths to the ship and the flute, each of these

  transponders that can respond to the pro-

  coming from the ship by emitting acoustic pulses of clearly different frequencies; several receivers spaced, carried by the flute and able to receive the

  different acoustic impulses emitted by trans-

  and relay individual signals

  tincts along the flute towards the ship under the control of said acoustic pulses; a ship receiver

  able to receive and distinguish acoustic impulses

  different ticks emitted by transponders; and means for measuring the time interval between the

  triggering of the control signal and the reception of

  the signals relayed by the spaced receivers, enclosed in the

  flute and the time interval between the triggering of the

  control and reception of impulses from trans-

  spawners by the ship's receiver.

  Transponders are preferably placed next

  A non-collinear relationship exists and each receiver of the stream is serviced by an independent channel, housed in the flute, for relaying signals to the ship. Receptors can be active or passive, but they are preferably passive in order to minimize their weight and

  cost. The apparatus may also include means

  able to measure the speed of the ship relative to the transponder network placed on the sea bed. These means for measuring the speed of the ship may comprise a device for measuring frequency variations,

  Doppler effect, impulses generated by trans-

spawners.

  The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting example and in which: FIG. 1 is a schematic view showing a network of transponders with respect to a surface exploration vessel towing a marine flute ; and FIG. 2 is a schematic view showing the effect of vessel displacement on the acoustic paths.

  ticks between the ship and a transponder.

  According to the invention, it is necessary to set up several

  acoustic transponders on the bottom of the sea

  or close to the bottom The transponders are preferably

  on the seabed, in non-cooperative networks

  linear each consisting of at least three transponders.

  Each transponder of a given triplet is preferably placed at a sufficient distance from the other transponders to establish a range suitable for the receivers of the ship and the flute in a given water depth. Although the present invention relates to the location of the ship and

  of the flute in relation to a given network and not by

  port to the actual geographical location, the latter

  relationship can be established once one knows the

  placement of transponders. Well-known methods are described in the prior art for determining the location of transponders and their calibration and is therefore unnecessary.

to come back to it.

  FIG. 1 represents a single network of three acoustic transponders, indicated generally

  10, 12 and 14, these transpon-

  being placed on the sea floor. An exploratory ship 18 is shown on the surface, towing a

marine flute 20.

  Transponders of the requested type are available.

  in commerce and normally include

  than base 22 resting on the seabed, and a cable

  24 fixed between the base plate 22 and the body 26 of the

  transponder. A float 28, connected to the body 26 of the trans-

  by means of a cable 30, maintains this body 26 at a certain height above the seabed, this height being determined by the length of the cable 24. The float 28 also constitutes a means of recovery

in case of breakage of the cable 24.

  The ship 18 is equipped with an acoustic transmitter

  32 which emits acoustic signals of control or interrogation

  transmission, through water, to the transponders and which, in turn, receives response signals. All transponders in the network preferably respond to a signal of a single frequency transmitted by the transceiver of the ship. However, coded signals can be generated to control individual transponders from the ship, if this is

wish.

  The marine flute 20 is immersed below the

  water surface by means of several conventional devices

  depth control units (not shown) and normally encloses hydrophones (not shown) and depth sensors (not shown) which can

  be interrogated from the ship for the purpose of obtaining information

  training. In addition, the flute also contains several

  acoustic receivers 34 spaced along its length. These re-

  receivers 34 are able to detect the signals produced

  transponders and to relay identical answers.

  reliable along the flute towards the ship. Normally,

  the flute has individual channels extending from each

  receiver to the ship to transmit the information

  mation. Although the receivers may be active or

  it is preferable that they be passive.

  To determine the position of the receivers 34 and, therefore, the flute, the acoustic transceiver

  32 of the vessel is triggered in order to emit an acoustic signal

  control. On receipt of the signal, after

  transmission lai in water, each transponder emits an acoustic pulse of a distinct frequency. These pulses are detected by the transceiver 32 and by the acoustic receivers 34 housed in the fl ate. For clarity, only Figure 1 represents the paths

  acoustic signals, in dotted lines, for the transmission

  ship-receiver, transponders and a single receiver of the flute. It is clear, however, that

  analogous routes can be traced for each of the

  receivers housed in the flute. Arrows I, 12 and 13 represent the control pulse which moves along the dotted lines from the ship to the transponders

  the arrows R1, R2 and R represent the repetition pulses

  response transponders to the ship; and the arrows

  R'1, R'2 and R'3 indicate the routes traveled by

  pulses to the receiver housed in the flute. Since the spatial positions of transponders on the sea bed and the speed of sound in water are known, the

  receiver position can be determined by triangulation

  tion, knowing the travel time of each pulse to

from his transponder.

  Appropriate means shall be provided on board the vessel

  to measure the time interval between the transmission of the control signal and the reception of pulses from

  transponders and receivers.

  Figure 2 represents a single ship

  placing on the surface of the water, at a time T0 and then at a time T1. As shown, the transceiver of the ship emits at time T a pulse that moves in line

  right, following the indicated path, to the transponder.

  At the reception of the signal, at time Td, the transponder emits a pulse which is detected by the transceiver of the ship at time T1. It can be determined from this figure that the instant Td is given by the formula

(T - T) -

T- = T + O (1+

  d O -C o v is the speed of the ship relative to the transponder and

  this is the speed of propagation of the acoustic pulses.

  The ratio v can be determined in a number of ways. However, a preferred method relies on the measurement of the Doppler variation of the frequency received from the transponder. It is obvious that to determine the speed, the transponders must

  able to produce pulses at very low frequencies

  the ship's receiver must be able to measure the

Apparent variation of frequency.

  The ratio can also be calculated from the rate of change of the distance in the direction of the transponders and the ship. This rate of variation of the distance can be determined easily knowing the posi-

  the speed and speed of the ship in relation to transpon-

makers.

  The ratio v / c for normal speeds of

  in seismic operations is usually less than 0.002, carv is about 3 meters per second

  and it is about 1500 meters per second. If one

glide the term v / c, we have: (Ti - T0

T = T + 2

d 0 2

  with an error less than 0.2%. An error of this

  may be acceptable in the depths of the sea in which the oil industry

  certain types of seismic operations.

  Knowing the instant Td of issuing

  transponders and the measured time of detection.

  tion of the pulses by the receivers of the flate, as transmitted to the ship, the distance of

  each transponder to each receiver. These distances can

  then be triangulated to give the position of each receiver in a fl ight, in real time, using an on-board computer or from the recorded data, during an analysis performed

return from the mission.

  It goes without saying that many modifications

  can be made to the apparatus and the process de-

  written and represented without departing from the scope of the invention.

Claims (8)

  1. Apparatus for determining the position of a submerged marine flute (20) towed by a vessel (18)   of exploration, characterized in that it comprises a   yen (32) for triggering a control acoustic signal from the ship, at least three transponders   (10, 12, 14) arranged spatially in con-   on the sea floor in order to establish distinct acoustic paths with the ship and the flute, each   transponder capable of responding to the control signal pro-   coming from the ship by emitting acoustic pulses   of a different frequency and different from that of the   transponders, several spaced apart receivers (34), carried by the flute and able to receive the different acoustic pulses emitted by the transponders and which can relay individual signals separately on the   along the flute towards the ship, in response to the said   a receiver (32) on board the ship and   receiving and distinguishing the different acoustic pulses emitted by the transponders and means   (32) intended to measure the time interval between the   triggering of the control signal and the reception of   signals relayed by spaced receivers along the   flute, as well as the time interval between the triggering   control signal and pulse reception by the ship's receiver.   2. Apparatus according to claim 1, characterized   in that the transponders have a non-collinear disposition.   3. Apparatus according to claim 1, characterized   rised in that the receivers are passive.   4. Apparatus according to claim 1, characterized   in that each receiver of the flute is served by an independent channel passing through the flute in order to relay signals to the ship.   5. Apparatus according to claim 1, characterized   in that it further comprises means (32) for   to measure the speed of the ship in relation to trans- spawners.   Apparatus according to claim 5, characterized   in that the means for measuring the speed of the   include an element intended to measure the vari-   Doppler effect of the frequency of the pulses produced by the transponders. 7. A method for determining the position of a submerged marine flute (20), towed by a survey ship (18), characterized in that it consists in producing an acoustic control signal from the ship, to transmit this acoustic signal for controlling at least three transponders (10, 12, 14) arranged spatially in known positions on the sea floor and which, in response to the acoustic control signal, emit acoustic pulses of significantly different frequencies,   to detect acoustic impulses of response from trans-   laying eggs by means of several spaced-apart receivers (34),   carried by the flute (20) and relaying the signals   tincts along this flute towards the ship, in response   said impulses, to receive and distinguish the impulse   acoustic transponders to the ship and to measure   the time interval between the generation of the acoustic signal   control and reception of signals relayed by   the receivers spaced along the flute, as well as the   time interval between generating the command signal   and receiving pulses to the ship.   8. Method according to claim 7, characterized in that it also consists in measuring the variation, by Doppler effect, of the frequency of the pulses emitted by transponders.