DK161266B - METHOD AND APPLICATION FOR POSITIONING OF A MARINE HYDROPHONE CABLE - Google Patents

Description

DK 161266 B

The invention relates mainly to marine seismic surveys, and more particularly to a method for determining the position of a submerged marine hydrophone cable towed by a survey vessel at which at least three transponders are placed at known, mutually spaced locations on the seabed. marine seismic hydrophone cable is towed by a survey vessel in the lake mainly over said transponders, producing an acoustic command signal from the vessel on a periodic basis, where the acoustic command signal is received by each of the at least three transponders and where in response to the acoustic command signal a clearly distinguishable acoustic signal is generated from each of the transponders, each of the transponder's acoustic response signals being detected in the vessel receiver, each of the transponders acoustic response signals being detected in a plurality of receivers spaced apart along the hydrophone cable transmitting a clear signal along the hydrophone cable 20 in response to each of the detected signals for recording it on the vessel, and where a separate recording of each of the transponder acoustic signals is made on the vessel, and measuring the time intervals from -25 view of the command signal for receiving each of the transponder acoustic signals in at least one receiver on the vessel.

In marine seismic exploration, a survey vessel tows a seismic hydrophone cable with a number of pressure-sensitive detectors, commonly referred to as hydrophones. A seismic energy source, such as an air cannon or explosive charge, is used to generate pressure waves through the water and into the underlying seabed. Part of the energy will be reflected by 35 geological discontinuities below the bottom and then detected by the hydrophones as pressure variations in the surrounding area.

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giving water. The mechanical energy of these pressure variations is converted into an electrical signal by the hydrophones and transmitted through the hydrophone cable to a recording device on board the vessel. The data collected can then be interpreted by those skilled in the art to provide information on geological formations underwater.

In order for the signals to make sense, it is necessary to know the location of the individual hydrophones at the time the pressure waves are detected.

10 Since the vessel is moving continuously and since the hydrophone cable can extend thousands of feet after the vessel, accurate positioning of the hydrophones is difficult.

Various systems have been developed to provide accurate information on the vessel's position.

In a commonly used system, a number of underwater transponders produce signals with a particular output frequency in response to a ship's interrogation signal. The transport time of the question signal and the transponder response signal is measured and the distance or area from each transponder is calculated. The position of the vessel relative to the transponders can then be triangulated, if the position of the transponders is known.

However, it is rare for hydrophone cable 25 to be routed directly along the vessel's path. Although the hydrophone cable is attached to the stern of the vessel, most of the hydrophone cable is submerged under the water surface under the influence of depth regulators along the length of the hydrophone cable. As a result, the transverse current velocity at the hydrophone cable depth may differ from the transverse current affecting the vessel, thereby causing the hydrophone cable to extend at an angle from the vessel's course. Other factors that need not be noted may also create a variation in the hydrophone cable path relative to the vessel's course.

A known method for estimating the position of the hydrophone cable is due to the release of a tail-bending radar reflector located at the end of the hydrophone cable. On-board radar systems can then be used to find the end of the hydrophone cable under optimal sea conditions and the position of the individual hydrophones interpolated. However, such systems are generally unreliable and make the necessary data questionable.

Another known method uses a very sensitive and expensive apparatus to measure the yaw and push angles of the end of the hydrophone cable up to the vessel. These data, together with magnetic compass readings taken along the hydrophone cable and the known depth of the hydrophone cable, make it possible to calculate the hydrophone positions empirically.

US Patent No. 4,229,809 discloses an apparatus for determining the position of a submerged unit tethered to a surface vessel relative thereto. This is accomplished by a cold receiver unit on the surface vessel emitting an acoustic pulse that produces response pulses with discernible carrier waves from two bottom responders and from a transponder receiver on the tethered unit. Response pulses are received partly from all the transponders of the vessel's acoustic receiver and partly from the bottom transponders of the tethered unit's receiver. Then, the submerged 25 unit position is calculated from a series of measurements based on the time difference between the pulses from: (1) the vessel to the bottom transponder, (2) the vessel to the submerged unit, (3) the bottom transponder to the submerged unit and (4) to all of these signals. return.

The present invention has for its object to provide a method and apparatus for accurately locating a submerged hydrophone cable pulled by a survey vessel in a simple and economical manner and with a fast reaction time.

This is accomplished by a method of the present invention which is according to the invention

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peculiar in that the distance between each of the hydrophone cable receivers and each of the known positions of the transponders is calculated by determining the speed of said at least one vessel receiver relative to each of the 5 transponders in calculating the distance change rate from an acoustic response of the transponders. and that the time intervals from generating the acoustic response signals of the transponders to receive each of said response signals forwarded from the separated receivers along the hydrophone cable are measured.

In the method of the invention, a simple and accurate position determination is obtained by calling from acoustic source a set of at least three fixed transponders. These each respond to a signal 15 of its own frequency which is intercepted by a receiver on the hydrophone cable. Only one pulse is sent to and only one pulse is received from each of the bottom transponders, as opposed to the system of U.S. Pat.

No. 4,229,809 using a complex series of signal transmissions.

Furthermore, the method according to the invention calculates the speed and position change between the acoustic source and the transponders on the seabed. In the system according to the invention, a seismic survey vessel 'which sails at a rate of 4-5 knots can be used to pull the hydrophone cable. At this speed the position difference of the cable is more pronounced and it must be taken into account to provide very accurate measurements. In the prior art, for various practical reasons, the speed or position change of the hydrophone cable is not calculated. Eg. For example, the towing vessel moves at lower speeds, which means that the correction factor is less significant and the measurement accuracy is not so necessary in the type of work, ie. reconnaissance inspections, not location of potential oil drilling sites or detailed structure 5

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high resolution mappings. Furthermore, the towing vessel is subjected to a more random and less straight-line maneuver, so that a calculation of speed and position change would not be feasible.

In addition, the known system involves a more complex measurement method, but not a calculation of the speed and position changes of a hydrophone cable, which is essential for the present invention.

This also relates to an apparatus for use in determining the position of a submerged marine hydrophone cable towed through the sea by a survey vessel, relative to a number of known geographical locations on the seabed, comprising means for initiating an acoustic command signal from the vessel as it travels through the lake, at least three transponders which are spatially located at known positions on the seabed to provide clear acoustic trajectories for the vessel and the hydrophone cable towed by the vessel, each of the transponders being able to respond to a single 20 command signal from the vessel upon transmission of acoustic signals at a frequency distinctly different from each of the other transponders, a number of mutually spaced receivers carried by the hydrophone cable and each capable of receiving the 25 different frequencies for acoustic signals emitted by each transponder and each individually known transmits a clear signal along the hydrophone cable to the vessel representative of each of the received signals, at least one vessel receiver capable of receiving and distinguishing the various frequencies emitted by the transponders, means for recording the time interval between the initiation of the command signal for receiving a signal from each of the transponders of the vessel receiver, which apparatus 35 for enabling the aforementioned technical advantages is simply characterized by the existence of a 6

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means for determining the speed of the vessel receiver relative to each of the transponders so that the distance between each of the hydrophone cable receivers and each of the known positions of the transponders can be calculated.

The invention is described in more detail with reference to the accompanying drawings, in which 1 shows a transponder row together with a survey vessel tugging a marine tow wire; and FIG. 2 the effect of vessel movements on the 10 acoustic paths between vessel and transponder.

The present invention requires the placement of a number of acoustic transponders on or near the seabed. The transponders will preferably be arranged on the seabed in non-linear patterns with at least three transponders per second. pattern. Each transponder in a given triplet is preferably spaced a sufficient distance from the culverts to obtain a suitable distance to the ship and tow receivers at a given water depth. The present invention relates to position determination of the vessel and tow wire in relation to a given pattern and not in relation to the actual geographical position, the latter being established from knowledge of the transponder position. Well known methods are described in the literature for determining the transponder position and calibration and are therefore not mentioned here.

X fig. 1 of the drawings, a single pattern of three acoustic transponders, designated 10, 12 and 14, is shown on the seabed 16. An examination ship 18 is shown on the surface towing a tow 20.

Transponders of the type required are commercially available and normally comprise bottom plate 22 resting on the seabed and a cable 24 attached between the bottom plate 22 and a transponder housing 26.

A float 28 is connected to the transponder housing 26 by means of a cable 30 and holds the transponder housing at a height above the seabed, which is determined by the length of the cable 24. The float 28 also allows for salvage if the cable 24 is broken.

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The vessel 18 is equipped with an acoustic transceiver 32 for transmitting acoustic call or question signals through the water to the transponders and receiving response signals therefrom. The transponders of the pattern 5 will preferably respond to a single frequency signal transmitted by the vessel transceiver. However, if desired, encoded signals can be generated from the vessel to activate the individual transponders.

The marine tow 20 is submerged beneath the sea surface by a number of conventional depth regulators (not shown) and will usually contain hydrophones (not shown) and depth gauges (not shown) that can be called from the vessel for information.

In addition, the tow wire will also contain a plurality of 15 acoustic receivers 34 spaced along the length of the tow wire. The receivers 34 are capable of detecting the signals generated by the transponders and transmitting identifiable responses along with the towing vessel to the vessel. Normally, the tow wire will have separate, 20 equal channels leading from each receiver to the vessel to transmit the information. However, although the receivers may be active or powered, the receivers are preferred to be passive.

To determine the position of the receivers 34 and thus the tow position, the acoustic transceiver 32 of the vessel is caused to transmit an acoustic call signal. Upon receiving the signal after the delay in transmission time through the water, each transponder transmits an acoustic pulse at a discernible frequency. These 30 pulses are detected by the transceiver 32 and by the acoustic receivers 34 of the tow wire. For the sake of clarity, the acoustic passageways in FIG. 1 is a dotted line for the vessel transceiver, transponders and a single receiver in the tow wire. It is clear that similar paths could be drawn for each of the recipients in the tow wire. Arrows 1 ^, I2 and I3 indicate the cold pulse moving along the dashed lines from the sky 8

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bet to the transponders. Arrows R 1, R 2 and show the response pulses from the transponders to the vessel, and arrows R * R 2 and R 2 show the paths of the pulses to the receiver in the tow wire. Since the spatial positions of the transponders on the seabed and the velocity of sound through the water are known, the receiver position can be triangulated based on knowledge of the passage time of each pulse from their respective transponders.

On board the vessel, there are suitable means for measuring the time interval between the transmission of the call signal and the reception of the pulses from the transponders and the receivers.

In FIG. 2 shows a single vessel moving on the water surface to a point Tg and a subsequent time T1. As shown, the vessel transceiver sends a pulse to time Tg, which pulse moves in a straight line along the indicated path to the transponder.

Upon receiving the signal at the time, the transponder transmits a pulse detected by the vessel transceiver at the time. From the figure it can be deduced that the time T ^ is given by the formula:

Td = Tg + - Tg) (1 - v) -2- c wherein Ir is the speed of the vessel relative to the transponder, 25 and c are the propagation velocity of the acoustic pulses.

The ratio ^ can be determined in a number of ways. However, a preferred method depends on measuring the Doppler change in the received frequency from the transponder. Of course, in order to determine the speed in this way, the transponders must be able to produce pulses of very stable frequencies and the vessel receiver must be able to measure the approximate frequency shift.

The ratio can also be calculated from the speed at which the distance changes in the direction between the transponders and the vessel. This distance velocity can be easily determined from knowledge of the vessel's position and speed relative to the transponders.

Claims (8)

The ratio ^ for normal ship speeds during seismic operations will generally be less than 0/002 since $ is approx. 3 m per second, and c is approx. 1,500 m per second. If ^ omitted, is obtained A method for determining the location of a submerged marine seismic hydrophone cable towed by a survey vessel at which at least three transponders are placed at known, mutually spaced locations on the seabed, where a marine seismic hydrophone cable is towed by a survey vessel in the sea 30 mainly over said transponders, where an acoustic command signal is generated from the vessel on a periodic basis, wherein the acoustic command signal is received by each of the at least three transponders, and in response to the acoustic command signal, a clearly distinguishable acoustic signal is produced. from each of the transponders, where each of the transponders acoustic response signals is detected in the vessel's receiver. In which each of the transponder acoustic response signals is detected in a plurality of receivers spaced apart along the hydrophone cable, a clear signal is transmitted along the hydrophone cable in response to each of the detected signals for recording it on the vessel, and wherein a separate recording of each of the transponders acoustic signals is made on the vessel, and measuring the time intervals from generating command signal 10 to receiving each of the transponders acoustic signals in at least one receiver on the vessel, characterized in that the distance between each of the hydrophone cable receivers and each of the known positions of the transponders is calculated by determining the velocity of said at least one vessel receiver relative to each of the transponders by calculating the distance change rate from an acoustic response of the transponders and the time intervals from the generation of transponde acoustic response signals to receive 20 each of said response signals transmitted from the separate receivers along the hydrophone cable are measured. Method according to claim 1, characterized in that the mean distance change rate of said at least one vessel receiver relative to each of the transponders is successively measured from changes in the measured time of the signal migration from each of the transponders to the receiver in response to each acoustic command signal during a number of successive generations of the acoustic command signals by inclusion of the last 30 measured distance value of a given number and looping of the earliest measured value. Method according to claim 1, characterized in that the position of the vessel relative to each transponder on the bottom is determined by utilizing the time T ^ which is determined in accordance with the formula DK T = T + 1 u / 1 V , x0 - (1 c} using Tjj to calculate the distance from the transponders to the receivers, which then makes it possible for distances to be triangulated to give the location of each of the receivers and where is the time for a response from a transponder on a command pulse, where Tg is the time of a command pulse introduction, where Tj is the time when the transponder pulse is received, v is the speed of the vessel relative to the transponder and where c is the propagation speed of the acoustic pulses and where the ratio vc 15 is determined from a number of successive vessel positions. Method according to claim 3, characterized in that the initial value of the speed of the vessel v with respect to the transponder is determined. An apparatus for use in determining the position of a submerged marine hydrophone cable being dragged through the sea by a survey vessel, relative to a number of known geographical locations on the seabed, comprising means for initiating an acoustic command signal from the vessel while moving through the lake, at least 25 three transponders, which are spatially located at known positions on the seabed to provide clear acoustic trajectories for the vessel and the hydrophone cable towed by the vessel, each of the transponders being able to respond to a single command signal from The vessel by transmitting acoustic signals at a frequency distinctly different from each of the other transponders, a number of mutually spaced receivers carried by the hydrophone cable and each capable of receiving the different frequencies of acoustic signals emitted by each transponder and each individually transmitted / DK 161 266 B 12 a clear signal along the hydrophone cable to the vessel representative of each of the received signals, at least one vessel receiver capable of receiving and distinguishing the various frequencies emitted by the transponders, means for recording the time interval between the initiation of the command signal to receive a signal from each of the transponders in the vessel receiver, characterized in that a means is provided for determining the speed of the vessel receiver 10 relative to each of the transponders, such that the distance between each of the hydrophone cable receivers and each of the known positions of the transponders can be calculated. 5 C Td = t0 + (Τχ - V 2 with an error of less than 0.2%. An error of this magnitude may be acceptable for the sea depths encountered in the oil industry by some types of seismic operations. for transmitting the pulses from the transponders and the measured time for pulse detection of the receivers in the tow as forwarded to the vessel allows calculation of the distance from each transponder to each receiver.These distances can then be triangulated to obtain the real time position of each receiver in a tow wire using a computer on board the ship or from the recorded data by analysis on land. Apparatus according to claim 5, characterized in that the speed determining means comprises means 15 for forming the mean of a number of the recorded time intervals detected in the vessel receiver from each of the transponders. Apparatus according to claim 5, characterized in that the rate-determining means comprises means 20 for generating the mean of a number of recorded time intervals detected by each of a number of said receivers from each of the transponders. Device according to claim 5, characterized in that the transponders are located in a non-cholinear relationship.