NZ199066A - Marine seismic streamer location - Google Patents

Description

199066 Fnoi'tt/ Complete Specification Filed: Class: L AUG 1985' Publication Date: M P.O. Journal, No: 1?*?}$,....

NEW ZEALAND PATENTS ACT, 1953 No.: Date: COMPLETE SPECIFICATION "SUBMERGED MARINE STREAMER LOCATOR" P?We. CHEVRON RESEARCH COMPANY, a corporation of the State of Delaware, U.S.A., of 100 West Tenth Street, Wilmington, Delaware, U.S.A. hereby declare the invention for which F/ we pray that a patent may be granted to riffit/us, and the method by which it is to be performed, to be particularly described in and by the following statement: - - 1 - ^ (followed by page la) 199066 SUBMERGED MARINE STREAMER LOCATOR BACKGROUND OF THE INVENTION Field of the Invention The present invention finds principal application within the field of marine seismic exploration. More particularly, the invention is concerned with means for accurately determining the position of a towed marine seismic streamer.

Prior Art In marine seismic prospecting, an exploration vessel tows a seismic streamer having a plurality of pressure sensitive detectors, commonly referred to as hydrophones. A source of seismic energy, such as an air gun or an explosive charge, is used to propagate pressure waves through the water into the underlying sea floor.

Part of the energy will be reflected by subfloor geological discontinuities and subsequently detected by the 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 transmitted through the streamer to recording apparatus aboard the vessel. The collected data may then be interpreted by those skilled in the art to reveal information about the subsea geological formations.

For the signals to be meaningful, it is necessary to know ttfe placement of the individual hydrophones at the time the pressure waves are detected. As the vessel is continuously moving and as the streamer may extend for thousands of feet behind the vessel, accurate location of the streamer hydrophones is difficult.

Various systems have been developed to provide accurate information as to the location of the vessel. In a common application a plurality of underwater transponders generate unique output frequency signals in I99066 response to an interrogation signal from the ship. The transit time for the interrogation signal and the transponder's response signal is measured and the distance or range from each transponder is calculated. The vessel's position with respect to the transponders may then be triangulated if the location of the transponders are known.

However, it is rare for the streamer to trail directly along the path of the vessel. While the streamer is attached to the stern of the vessel, the bulk of the streamer is submerged below the water surface through the action of depth controllers along the length of the streamer. As a result, the cross-track current velocity at the streamer depth may differ from the cross-track current affecting the vessel, thereby causing the streamer to trail at an angle to the vessel's course. Other factors, which are not necessary to enumerate, may also create a variance in the path of the streamer when compared to the vessel track.

One method of estimating the location of the streamer disclosed in the prior art relies upon the addition of a tail buoy radar reflector located at the end of the streamer. On-board radar systems may then be used under optimal sea conditions to find the end of the streamer and the location of the individual hydrophones interpolated. Such systems are generally unreliable however, and render the required data suspect.

A second method taught by the art relies upon very sensitive and expensive apparatus to measure the yaw and pitch angles of the streamer end adjacent the vessel. These data, coupled with magnetic compass headings taken along the streamer and the known depth of the streamer, permits one to empirically calculate the hydrophone locations. 199066 It is an object of this invention to provide an accurate, alternative means for locating the submerged streamer which overcomes the deficiencies of the prior art.

SUMMARY OF THE INVENTION The present invention can be broadly said to consist in an apparatus for use In determining relative to a known geographic location on a sea floor of a submerged marine streamer being towed through the sea by an exploration vessel, which comprises: means for initiating an acoustic command signal from the vessel while moving through the sea; at least three transponders spatially located In known positions on the sea floor so as to provide distinct acoustic paths to said vessel and to a streamer towed by said vessel, each of said transponders capable of responding to a single command signal from said vessel by emitting acoustic signals of a distinctly different frequency from each of other said transponders; a plurality of spaced apart receivers carried by said streamer capable of receiving said different frequencies of acoustic signals emitted by each transponder and individually relaying a distinct signal along the streamer to the vessel responsive to each of said received signals; a vessel receiver capable of receiving and distinguishing said distinctly different acoustic frequencies emitted from said transponders; and means for recording the time Interval from Initiation of a command signal from said vessel to receipt of each signal relayed from said spaced receivers along said streamer and recording the time Interval from Initiation of said command signal to receipt of a signal from each of said transponders by said vessel receivers, whereby the distance of each of said streamer receivers from each of said known positions of said transponders may be calculated.

Preferably, the transponders are placed In a non-col!near relationship and each streamer receiver Is serviced by a separate channel housed In the streamer for relaying signals to the vessel. The receivers may be either active or passive, but are preferably passive to minimize weight and expense. The apparatus may further comprise means for measuring the vessel's velocity with respect to the array of transponders situated on the ocean 199066 01 floor. Said means for measuring the vessel's velocity may include apparatus for measuring the Doppler shift in the frequency of the pulses generated by the transponders.

BRIEF DESCRIPTION OF THE DRAWINGS 05 FIG. 1 of the drawings illustrates a transponder array shown in relationship to a surface exploration vessel towing a marine streamer.

FIG. 2 of the drawings diagrammatically illustrates the effect of shipment movement on the acoustic 10 paths between vessel and transponder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention requires the placement of a plurality of acoustic transponders on, or adjacent, the ocean bottom. Preferably, the transponders will be 15 positioned on the sea floor in non-colinear arrays of at least three transponders per array. Each transponder in a given triplet is preferably placed at a sufficient distance apart to give adequate range to the ship and streamer receivers in a given water depth. While the 20 present invention is concerned with location of the vessel and streamer with respect to a given array and not with respect to the actual geographical location, the latter relationship may be established from knowledge of the transponder placement. Well known methods are described 25 in the art for determination of the transponder placement and calibration and are therefore not to be considered here.

Referring to FIG. 1 of the drawings, there is shown 3 single array of three acoustic transponders, 30 indicated generally by reference "numerals 10, 12 and 14, positioned on the sea floor 16. An exploration vessel 18 is shown on the surface towing a streamer 20.

Transponders of the type required are commercially available and normally comprise a base plate 22, 35 resting on the sea floor, and a cable 24 attached between 199066 the base plate 22 and the transponder body 26. A float 28 connected to the transponder body 26 by means of a cable 30 maintains the transponder body 26 at an attitude above the sea floor determined by the length of cable 24. Float 28 also provides a means of retrieval if cable 30 is severed.

Vessel 18 is equipped with an acoustic transceiver 32 for sending command or interrogation acoustic signals through the water to the transponders and, in turn, receiving responsive signals therefrom. Preferably, all transponders in the array will respond to a single frequency signal emitted by the vessel's transceiver, however, coded signals may be generated to actuate the individual transponders from the vessel, if desired.

The marine streamer 20 is submerged below the water surface by a plurality of conventional depth controllers (not shown) and will normally house hydrophones (not shown), and depth sensors (not shown) which may be interrogated from the vessel for information.

In addition, the streamer will also house a plurality of acoustic receivers 34 spaced along the length of the streamer. Receivers 34 are capable of detecting the signals generated by the transponders and relaying identifiable responses along the streamer to the vessel. Normally the streamer will have individual channels leading from each receiver to the vessel for transmitting the information. Although the receivers may be active, or powered, it is preferred that the receivers be passive.

To determine the location of receivers 34 and thus the streamer position, the vessel's acoustic transceiver 32 is triggered to send an acoustic command signal. Upon receipt of the signal, after the delay in transmission time through the water, each transponder transmits an acoustic pulse of a distinguishable frequency. These pulses are detected by transceiver 32 and by the acoustic I 99066 receivers 34 housed in the streamer. For' the sake of clarity, acoustic travel paths are only shown in FIG. 1 of the drawing as dashed lines for the vessel transceiver, transponders, and a single receiver in the streamer. It should be understood, however, that similar paths could be drawn for each of the receivers housed in the streamer. Arrows 1^, I2 and 13 represent the command pulse travelling along the dashed lines from the ship to the transponders, arrows R^, R2, and R3 represent the responsive pulses from the transponders to the vessel and arrows R'^, R12 and R'3 indicate the pulse lines of travel to the receiver housed in the streamer. Since the spatial positions of the transponders on the sea floor and the speed of sound through the water are known, the receiver position may be triangulated from knowledge of the travel time for each pulse from their respective transponders.

Suitable means aboard the vessel are provided to measure the time interval between the sending of the command signal and the receipt of the pulses from the transponders and the receivers.

In FIG. 2. of the drawings, there is illustrated a single vessel moving along the water's surface at time Tq and at a subsequent time . As shown therein, the vessel's transceiver initiates a pulse at time Tq which travels in a straight line along the indicated path to the transponder. Upon receipt of the signal at time T^ the transponder transmits a pulse which is detected by the vessel transceiver at time T^. From the figure it may be derived that the time, T^, is given by the formula: 199066 ~7~ wherein v is the vessel's velocity with respect to the transponder and c is the propagation speed of the acoustic pulses.

The v_ ratio may be determined in a number of c ways. A preferred method, however, relies upon the measurement of the Doppler shift in the received frequency from the transponder. Naturally, in order to determine the velocity in this manner, the transponders must be capable of generating pulses of very stable frequencies and the vessel receiver must be capable of measuring the apparent change in the frequency.

The ratio may also be calculated from the rate of change of range in the direction of the transponders and the vessel. This range rate may be determined readily from knowledge of the vessel's position and speed with respect to the transponders.

The ratio _v for normal ship speeds during c seismic operations will usually be less than .002, since v is about 3 meters per second and c is about 1,500 meters per second. If the _v term is dropped then: c Td = T0 + (Tx - T0) 2 with an error of less 0.2%. An error of this magnitude may be acceptable for the ocean depths encountered in oil industry for some types of seismic operations.

Knowledge of the time, Td, for the initiation of the pulses from the transponders and the measured time of pulse detection by the receivers in the streamer as transmitted to the vessel permits the calculation of the distance from each transponder to each receiver. These distances may then be triangulated to give the location of J 99066 each receiver in a streamer in real time by a shipboard computer or from the recorded data in post mission analysis. 05 199066

Claims (7)

WHAT WE CLAIM IS:

1. Apparatus for use in determining relative to a known geographic ■Hie (oeailfiiA location on a sea floor^of a submerged marine streamer being towed through the sea by an exploration vessel, which comprises: means for initiating an acoustic command signal from the vessel while moving through the sea; at least three transponders spatially located in known positions on the sea floor so as to provide distinct acoustic paths to said vessel and to a streamer towed by said vessel, each of said transponders capable of responding to a single command signal from said vessel by emitting acoustic signals of a distinctly different frequency from each of other said transponders; a plurality of spaced apart receivers carried by said streamer capable of receiving said different frequencies of acoustic signals emitted by each transponder and individually relaying a distinct signal along the streamer to the vessel responsive to each of said received si gnai s; at least one vessel receiver capable of receiving and distinguishing said distinctly different acoustic frequencies emitted from said transponders; and means for recording the time interval from initiation of a command signal from said vessel to receipt of each distinct signal relayed from said spaced receivers along said streamer and recording the time interval from 199066 - 10 - initiation of said command signal to receipt of a signal from each of said transponders by said vessel receivers, whereby the distance of each of said streamer receivers from each of said known positions of said transponders can be calculated.

2. Apparatus for use In determining the location of a marine streamer as recited In Claim 1, wherein said transponders are In a non-colinear relationship.

3. Apparatus for use In determining the location of a marine streamer as recited In Claim 1, wherein said receivers are passive.

4. Apparatus for use In determining the location of a marine streamer as recited In Claim 1, wherein each streamer receiver Is serviced.by a separate channel In the streamer for relaying signals to the vessel.

5. Apparatus for use In determining the location of a marine streamer as recited In Claim 1, further comprising: means for measuring the vessel's velocity with respect to said transponders.

6. Apparatus for use In determlng the location of a marine streamer as recited In Claim 5, wherein said means for measuring the vessel's velocity Includes means for measuring the Doppler shift In the frequency of the signals generated by the transponders. - 11 - 199066 I

7. A method for determining the location of a submerged marine seismic streamer while being towed by an exploration vessel, which comprises: positioning at least three transponders at known spaced apart locations on a sea floor; towing a marine seismic streamer from an exploration vessel in the sea substantially above said transponders; generating an acoustic command signal from said vessel on a periodic basis; receiving said acoustic command signal by said at least three transponders and in response to said acoustic command signal emitting a distinctly different frequency acoustic response signal from each of said transponders; detecting each of the transponder acoustic response signals at a plurality of receivers spaced along said streamer; relaying a distinct signal along said streamer in response to each of said detected signals for recording thereof at said vessel and recording each of said transponder acoustic response signals at said vessel; and measuring the time intervals from generation of said acoustic command signal to receipt of each of said distinct signals relayed from said spaced receivers along said streamer and the time intervals from generation of said command signal to receipt of each of said transponder acoustic response signals recorded at said vessel, whereby the distance of each of said streamer receivers from each of said known positions of said transponders can be calculated. By+ttS7Thsir Authorised Agents, ' A. J. PARK & SON