Note: Descriptions are shown in the official language in which they were submitted.
~s~
01 SUBMERGED MARINE STREAMER LOCATOR
BACKGROUND OF THE INVENTION
Field of the Invention
os The present invention finds principal applica-
tion 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 floorO
Part of the energy will be reflected by subfloor geolog~
ical 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 ~he hydro-
phones 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 neces-
sary to know the placement of the individual hydrophones
at the time the pressure waves are detected. As the
vessel is continuously movlng and as the streamer may
extend for thousands of feet behind the vessel, accurate
location of the streamer hydrophones is difficult~
~ arious systems have been developed to provide
accurate information as to the location of the vessel. In
a common application a plurality of underwater trans-
ponders generate unique output frequency signals in
~ 2
01 response to an interrogation signal from the ship. Thetransit time for the interrogation signal and the trans-
ponder's response signal is measured and the distance or
range from each transponder is calculated. The vessel's
05 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 addi-
tion 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 skreamer end adjacent the vessel.
These data, coupled with magnetic compass headings taken
along the streamer and the known depth o~ the streamer,
permits one to empirically calculate the hydrophone
locations.
--3--
It is an object of this invention to provide an
accurate, alternative means for locating the submerged streamer
which overcomes the c~eEiciencies of the prior art.
SUMMARY OF THE INVENTIO
The present invention provides apparatus for use in
0 1~
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 coustic 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 respond-
ing to a single command signal from said vessel by emitting
acousti.c 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 indïvidually relaying a distinct signal along
2Q the streamer to the vessel responsIve to each of said received
signals; a vessel receiver capable of receiving and distinguish-
ing 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 receip.t
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 receiver, whereby the distance of
each of said streamer receivers from each of said known
3Q positions of said transponders may be calculated.
5~,~æ
-3a-
The present invention also provides a method for
determining the location of a submerged marine seismic streamer
while being towed by an exploration vessel, which comprises:
posit.ioning at least three transponders at known spaced apart
locations on a sea floor; towing a marine seismic from an
exploration vessel in the sea generally above said trans-
ponders; 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
acoustic signal from each of said transducers; 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 signals at said
vessel; and measurin~ the time intervals from generation of
said acoustic command signal to receipt of each of said response
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 transducer signals recorded at said
vessel, 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-
colinear relationship and each streamer receiver is serviced
~ya 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 ~lith respect to the array of transponders situated on
the ocean
æ
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 DRAWING~
05 FTG. 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 illus-
trates the effect of shipment movement on the acoustic
paths between vessel and transponder.
DET~ILED DESCRIPTION OF T~E 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
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 dis~
tance apart to give adequate range to the ship and
streamer receivers in a given water depth. ~hile the
present lnvention is concerned wi~h 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
transpander placement. Well known methods are described
in the art for determination of the transponder placement
and calibration and are therefore not to be considered
here.
Referring to FIG. l of the drawings, there is
sho~m a single array of three acoustic transponders,
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.
rrransponders of the type required are commer-
cially available and normally comprise a base plate 22,
resting on the sea floor, and a cable 24 attached between
~L~95~62
01 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
- ~ OS 28 also provides a means of retrieval if cable ~ is
severed.
Vessel 18 is equipped with an acoustic trans-
ceiver 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 belo~ the
water surface by a plurality of conventional depth
controllers (not shown) and will normally house hydro-
phones (n~t 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 ~coustic receivers 34 spaced along the length
of the streamer. Receivers 34 are capable of de~ecting
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 o~ receivers 34 and
thus the streamer position, the vessel 15 acoustic trans-
ceiver 32 is triggered to send an acoustic command signal.
Upon receipt of the signal, after the delay in trans-
mission time through the water, each transponder transmits
an acoustic pulse of a distin~uishable frequency. These
pulses are detected by transceiver 32 and by the acoustic
01 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
ns should be understood, however, that similar paths could be
drawn for each of the receivers housed in the streamer.
Arrows Il, I2 and I3 represent the command pulse travel-
; ling alon~ the dashed lines from the ship to the trans-
ponders, arrows R1, R2, and R3 represent the responsive
pulses from the transponders to the vessel and arrows R'l,
R'2 and R'3 indicate the pulse lines of travel to the
receiver housed in the streamer. Since the spatial posi-
tions of the transponders on the sea floor and the speed
o~ sound through the water are known, the receiver posi~
tion 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 th`e com-
mand signal and the receipt of the pulses from the trans-
ponders and the receivers.
In FIG. 2 of the drawings, there is illustrateda single vessel moving along the water's surface at time
To and at a subsequent time Tl. As shown therein, the
vessel's transceiver initiates a pulse at time To which
travels in a straight line along the indicated path to the
transponder. Upon receipt of the signal at time Td the
transponder transmits a pulse which is detected by the
vessel transceiver at time Tl. From the figure it may be
derived that the time, Td~ is given by the formula-
Td = To + (Tl - To) (1 v)
2 c
01 wherein v is the vessel's velocity with rèspect to the
transponder and c is the propagation speed of the
acoustic pulses.
The v ratio may be determined in a number of
05 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
lS 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
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 = To + (Tl To)
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 trans-
mitted to the vessel permits the calculation of the dis-
tance from each transponder to each receiver. These dis-
tances may then be triangulated to give the location of
~s~7~æ
01 each receiver in a streamer in real time by a shipboard
computer or from the recorded data in post missionanalysis.
05
