CA1274298A

CA1274298A - Apparatus and method for locating towed siesmic floats

Info

Publication number: CA1274298A
Application number: CA000496906A
Authority: CA
Inventors: Joseph Philip Lloyd
Original assignee: Shell Canada Ltd
Current assignee: Shell Canada Ltd
Priority date: 1984-12-31
Filing date: 1985-12-05
Publication date: 1990-09-18
Anticipated expiration: 2007-09-18
Also published as: USH388H

Abstract

A B S T R A C T

APPARATUS AND METHOD FOR LOCATING
TOWED SEISMIC FLOATS

A method and apparatus for locating a towed float relative to the towing vessel. The method determines the range from at least two known spaced apart locations on the towing vessel to the float and determines the position of the float from the intersection of the at least two ranges. The apparatus includes a microwave transceiver selectedly interconnectable with a plurality of microwave antennas at known spaced apart locations on a towing vessel, a microwave transponder on each float whose locations is to be determined, and suitable controller and/or computer means to calculate ranges and intersections of ranges.

Description

-l- 63293-2610 APPARATUS AND METHOD FOR LOCATING
TO~ED SEISMIC FLOATS
The ;nventlon relates to marine seismic exploration, and more particularlyl relates to de~ermining the position of a towed marine seismic source.
In marine seismic exploration, impulsive sources, for example air guns, are suspended at some preselected depth beneath a float. The Eloat is towed by an exploration vessel ànd there may be a plurali~y of such floats towed behind the exploration vessel. The exploration vessel may also tow a streamer cable to detect energy propagating upwardly from subsurface strata lying beneath the body of water in which the vessel operates.
- The exploration vessel may determine its location in the body of water through the use of conventional navigation systems.
Such systems determine the vessel's location but do not determine the location of any float(s), having impulsive sources attached thereto, that may be towed at varying positions and distances by the vessel. Early attempts to locate floats with respect to the vessel by employing the radar of the vessel and mounting radar reflectors on the float have not proven to be sufEiciently accurate nor reliable. Similarly, attempts to use acoustic location devices have had the same or similar shortcomings.
These and other limitations and disadvantages are overcome by the present inventionl The invention provides an apparatus for determining the position of a towed seismic float relative to a towlng exploration vessel during marine seismic exploration, comprising: means Eor determining a Eirst range to said towed seismic Eloa~ Erom a Eirst , ,~r~

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-2- 63293-2610 preselected location on said seismic vessel, means for determining a second range to said towed seismic float Erom a second preselected location on said seismic vessel, and means eor determining the point of intersection of sa:id -first and second ranges, wherein: a microwave transponder means is disposed on said seismic float responsive to a preselected coded signal; master microwave transceiver means are disposed on said seismic vessel for transmitting preselected coded si~nals functionally related to a preselected set of first commandsi a plurality of spaced apart microwave antennas are disposed at known locations on said seismic vessel; antenna switching means are present for operatively interconnecting said master microwave transceiver with one of said plurality of antennas in functional response to a second set of preselected commands; and controller means are present for generating said first and second sets of preselected commands to said master transceiver and said antenna switching means, respectively.
The invention will now be described by way of example in more detail with reference to the accompanying drawings, in which Figure 1 is a plan view of a vessel towing several floats;
Figure 2 is a plan view of a portion of the apparatus of the present invention; and 4~

fig. 3 is a block diagram of the apparatus of the present invention.
Referring now to Eig. 1, there may be seen a simplifled plan view of a vessel 5 towing a plurality of floats 21-28. More particularly, the vessel 5 has mounted thereon a master micro-wave transceiver 10 which may be selectively intercoDnected with microwave antennas 11, 12, 13 or 14. These mlcrowave antennas 11-14 are located at spaced apart and known locations relative to the centre of the vessel 5. Although four antennas are shown ]o in fig. I (and fig. 3) 9 this number of antennas is by way of illustration only and is not intended as any limitation on the scope of the present invention.
The vessel 5 also has appropriate towing gear 18 and 19 for towing floats 21-28 with cables 20. Each of the floats 21-28 have a microwa~e transponder 31-38, respectively, suitably located on the float, also at a known position of the float.
Although eight floats are depicted in fig. 1, this number of -float~ is for illustration only and is not intended as any limitation on the scope of the present invention.
Referring now to fig. 3, there may be seen a block diagram of the apparatus of the present invention. More specifically, the master transceiver 10 is shown connected to an antenna switch 15 for interconnecting the transceiver 10 with either antenna 11, antenna 12, antenna 13, or antenna 14. Advantageous embodiments of the present invention employ four such antennas.
However, more than four and fewer than four antennas are considered within the scope of the present invention.
Continuing to refer to fig. 3, there are also depicted in shadow or outline form the illustrative number of eight floats 3Q 21-28, as depicted in fig. 1. AgainJ the number of eight floats is by way of illustration and not by way of limitation. Each float 21-28 has a microwave transponder 31-38 located thereon at a known location of the float, as described hereinbefore.

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Also depicted in fig. 3 is a controller 16 that interfaces the transceiver and antenna system with a local computer 17. The local computer 17 has a keyboard 18 for inputting into the compute~ 17 the desired frequency for determinlng the location of a float and for what floats and in what sequence, if any. The computer 17 may also have associated therewith a local display l9a and a remote display 19b; there may be more than one of the local and/or remote displays. The computer 17 also receives data on the course, speed, pitch, roll, etc. of the vessel through an external vessel status interface 40.
The local computer 17 also provides the float location, vessel heading and time to a host computer SO in the appropriate format for use by the host computer 50. The host computer 50 may store this information or use it for on-vessel processing or preprocessing of seismic data, as is known in the art.
Referring now to fig. 2, there may be seen a simplified plan view of a portion of the apparatus of the presen~
invention, which may be e~ployed to describe t~e operation of the present invention. Thls description is offered by way of 2~ illustration only and not by way of any limitation on the scope of the present invention. More particularly , the antennas 11 and 13 may be seen as well as the transponder 31, which is associated with the float 21 (not shown). Also depicted is an x-y coordinate system centred at the centre of the vessel 5, and distances A4 and A2 from the antennas 11 and 13 to the transponder 31, respectively; the x axis represents the longitudinal (fore and aft~ axis of the vessel 5.
Continuing to refer to fig. 2, the x and y coordinates of the antennas 11 and 13 are known because of their fixed and known location on vessel 5. What is sought is the x and y coordinates of the transponder 31, or equivalently the position of the float 21 (or any other preselected float) relative to the vessel 5. This is accomplished as described hereinbelow.

Referring now to fig. 2 and 3, the master transceiver 10 is connected by the antenna switch 15 to the antenna 11, in response to directions from the controller 16. The controller 16 also directs the master transceiver 10 to l.ocate the float 21.
In advantageous embodiments of the present invention, the master transceiver 10 broadcasts a coded pulse sequence that selects only the float transponder 31 as the responder over the antenna 11. All the float transponders 31-38 will receive the broadcast, but all will disregard the broadcast except for the transponder 31. Each float transponder may have its own unique coded pulse train to which it automatically responds.
The transponder 31 will recognize its coded pulse sequence and as quickly as possible transmit a response. Alternatively, each float transponder ~ay have a different transmission frequency band althovgh this is not necessary for the present invention when each transponder has its own unique pulse sequence. For embodi~ents of the present invention which employ . transponders having different transmission frequencies, it is possible to detect the location of all the floats substantially 2Q simultaneously by employing a plurality of receiver channels tunes to these separate frequencies in the master transceiver 1~. The controller 16 measures the time from transmission until it first detects a transmission back from the float transponder 31. This length of tims includes the time of transmission over and back, or twice the distance A4, as well as fixed equipment ; times, such as the reception-to-transmission turnaround time forthe transponder 31. These fixed equipment delays are subtracted out to find the ~'flight time" and thereby the distance A4, as is described hereinbelow. This distance A4 is also corrected for 3Q any roll and pitch of the vessel 5 during this flight tiMe.
In a similar manner, the distance A2 is determined when the master transceiver 10 is connected to the antenna 13. The intersection of the distances A2 and A4 determines the position of the transponder 31, as determined by the controller 16.

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The fixed equipment delays may be diferent for each float transponder and should accordingly be determined for each transponder. The controller 16 will contain these delays for each transponder, once determined. ~hese de!lays may be deter-mined by spacing the transponder a known calibrated distancefrom one or more antenna, or alternatively by using delay lines havlng known delays to compare against a transponder signal receipt.
Thus, the method of the present invention determines a IQ plurality of ranges to a preselected location on a towed float from a pluraliey of spaced apart antennas having known locations on a vessel, and determines the float's location from the intersection of these ranges from the known locations on the vessel. This method requires a minimum of two such ranges, but lS may employ any number than two. If more than two ranges are employed, each pair of ranges will determine a float position;
these float positions may then be averaged or otherwise manipulated mathematically to arrive at one position to be employed as the location of the float. Two antennas are used in an advantageous embodiment to determine the location of each float, Further, as depicted in fig. 1, the antennas 11 and 13 would be employed to measure floats on the port side of the vessel 5 ~i.e. the floats 21-24), and the antennas 12 and 14 to measure floats on the starboard side of the vessel 5 (i.e. the floats 25-28).
As depicted in fig. 2, the position of each float is determined rela~ive to an x y coordinate system aligned along the longitudinal axis of the vessel and centred at the centre of the vessel. Alternatlvely, other coordinate systems may be used 3Q and they may have their origins at locativns other than the centre of the vessel.
Many other variations and modifications may be made in the apparatus and techniq~es hereinbefore described, by those having experience in this technology, without departing from the concept of the present invention. Accordingly, it should be ~.~ 74~ 38 clearly understoocl that the apparatus and methods depicted in the accompanying drawings and referred to in the foregoing description are illustrative only and are not intended as limitations on the scope of the invention.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for determining the position of a towed seismic float relative to a towing exploration vessel during marine seismic exploration, comprising:
means for determining a first range to said towed seismic float from a first preselected location on said seismic vessel, means for determining a second range to said towed seismic float from a second preselected location on said seismic vessel, and means for determining the point of intersection of said first and second ranges, wherein:
a microwave transponder means is disposed on said seismic float responsive to a preselected coded signal;
master microwave transceiver means are disposed on said seismic vessel for transmitting preselected coded signals functionally related to a preselected set of first commands;
a plurality of spaced apart microwave antennas are disposed at known locations on said seismic vessel;
antenna switching means are present for operatively interconnecting said master microwave transceiver with one of said plurality of antennas in functional response to a second set of preselected commands; and controller means are present for generating said first and second sets of preselected commands to said master transceiver and said antenna switching means, respectively.