Seismic streamer locator

Method and means are provided for determining the position of a submerged marine streamer towed behind an exploration vessel. A sonic ring around feedback system is employed to redundantly ascertain the distance to various hydrophones housed in the streamer from an outboard mounted transponder capable of generating high frequency sound pulses of short duration.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention finds principal application within the field of 
marine seismic exploration. More particularly, the invention is concerned 
with a method and means for accurately determining the position of a towed 
marine seismic streamer. 
2. 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, it 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 the precise 
location of the individual hydrophones at the time the pressure waves are 
detected. As the vessel is continually moving and as the streamer may 
extend for thousands of feet behind the vessel, accurate location of 
hydrophones is difficult. 
Various systems have been developed to provide accurate information as to 
the location of the vessel, 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. 
In normal operations, the vessel travels at a speed of approximately 3 
meters per second and sets off original seismic propagations approximately 
every 10 seconds. The use of seismic propagations at a shorter interval is 
limited by the time required for the dissipation of all reflected seismic 
waves. In particular, the use of an air gun at intervals of less than 4 
seconds will not permit sufficient dissipation of the sound waves and will 
result in data that is difficult or impossible to evaluate due to the 
reflected noise. Thus, the use of an air gun, in combination with the 
hydrophones for range estimation, presents problems and does not allow for 
a lack of redundancy in precisely locating the hydrophones. 
SUMMARY OF THE INVENTION 
The present invention relates to apparatus for use in determining the 
location of a marine streamer towed behind an exploration vessel. The 
apparatus includes an acoustic source mounted outboard from the stern of 
the vessel beneath the water surface which is capable of emitting high 
frequency sound pulses of short duration upon an external command. A 
plurality of hydrophones is housed in the streamer capable of receiving 
pulses from the acoustic source and transmitting signals through separate 
channels in the streamer in response thereto. To provide redundant 
measuring capability, a preselected number of hydrophone signals will each 
trigger additional pulses from the acoustic source. Measurement of the 
elapsed time from a first externally initiated pulse to the receipt of the 
last predetermined signal generated by a hydrophone permits an accurate 
determination of the range. 
Preferably, a pair of acoustic sources are mounted apart and outboard from 
the stern of the vessel which are capable of emitting the high frequency 
sound pulses. The pair of sources may be used in different time frames to 
acquire data as to the location of the hydrophones or they may use 
different frequency pulses which are distinguishable by the hydrophones 
and in response to which different signals may be returned to the vessel. 
It is also preferred to use the return signals from the hydrophones to 
adjust the amplitude of the acoustic source pulses to minimize the power 
required for transmission, hence minimizing reverberations. 
The frequency of the sounds pulses are preferably in the range between 3.5 
and 250 kilohertz and the pulse should normally have a duration running 
from the time required for a single cycle to approximately 20 cycles. 
In one preferred embodiment, all of the hydrophone channels are 
simultaneously monitored and the acoustic source is triggered into 
generating additional pulses only after the signal is received responsive 
to the previous pulse by the last or furthest hydrophone in the streamer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a diagrammatic representation of an exploration vessel 10 towing 
a marine seismic streamer as viewed from above. The track of the vessel is 
indicated by dashed line 12 and the streamer 14 arcuately trails to one 
side. A plurality of depth controllers 16 of conventional design maintain 
the bulk of the streamer at a depth beneath the surface of from 
approximately 5 to 10 meters. Tail buoy 18 is affixed to the trailing end 
of streamer 14 and maintains the end of the streamer at the surface. A 
plurality of hydrophones 20 are spaced along the length of the streamer 
for detecting pressure variations and transmitting signals announcing the 
receipt of same along the streamer to recording apparatus aboard the 
vessel. In addition, the streamer 14 will also house a plurality of depth 
sensors 22 and magnetic compasses 24 which may be interrogated for 
information as to the depth and orientation of the streamer at the 
locations of these instruments. 
An air gun 26 is mounted outboard from the stern of vessel 10. In a 
conventional gun, air, compressed to a pressure in the range, 34 to 550 
atmospheres, is suddenly released from a submerged chamber over a period 
of a few milliseconds to generate an acoustic impulse. 
A pair of high frequency acoustic sources, 28 and 30, are mounted outboard 
from the vessel stern and are typically spaced apart from each other at a 
distance of 20 to 40 meters. Acoustic sources 28 and 30 generate high 
frequency pulses of short duration which are received by the hydrophones 
20. Upon receipt of the pulses, the hydrophones emit a signal which is 
transmitted to the vessel along the streamer. The transmitted hydrophone 
signals are used to trigger additional pulses from the sources 28 and 30 
in a controlled oscillation loop. Measurement of the time involved for a 
given number of oscillations allows redundant, accurate calculations of 
the distance to the hydrophones, given the velocity of the pulses in 
water. With the calculated range, location of the hydrophones may be 
determined precisely in conjunction with the depth data obtained by 
interrogation of sensors 22. 
FIG. 2 illustrates in block form functional circuitry which may be used to 
accomplish the range-finding objectives stated above. In accordance with 
FIG. 2, acoustic source 100 is triggered into initiating a pulse, via 
external start 110 and trigger 120, of high frequency and short duration. 
The pulse will preferably be in the range, 2 to 100 kilohertz and more 
preferably in the range, 3 to 10 kilohertz. The pulse length is preferably 
from monocycle to 20 cycles. Longer pulses may be used but serve no useful 
purpose. The acoustic source may be piezoelectric, ferroelectric, or 
electromagnetic in nature. Preferably, the source will comprise a 
piezoelectric or ferroelectric device having a pencil-shaped acoustic beam 
oriented in the general direction of the streamer. Such units having a 
frequency in the range of 2 to 8 kilohertz and capable of generating unit 
cycle pulses are commercially available. 
As mentioned above, the acoustic sources are preferably mounted outboard 
from the stern of the exploration vessel and are separated by a distance 
of 20 to 40 meters for triangulation purposes. 
The pulse from acoustic source 100 travels through the water at a speed of 
approximately 1500 meters per second and contacts the streamer hydrophone 
130. Hydrophones, such as hydrophone 130, are spaced along the length of 
the streamer at distances from 100 to 500 meters, and most preferably, at 
400 meters. As the pulse is detected, the hydrophone responds and 
transmits a signal through the streamer to the vessel. Such signals will 
normally be transmitted along separate electrical conductors extending to 
each hydrophone. Transmitted signals from the hydrophone pass through a 
gate 140 which blocks all signals except those that are expected during 
preselected time intervals. Since the approximate distances from the 
acoustic sources to the individual hydrophones are known from the spacing 
of the hydrophones along the streamer, the approximate time "window" for 
receipt of the signals from the individual hydrophones may be determined. 
Gate 140 thus serves to block spurious signals generated by reflections 
from the water surface and ocean floor. 
Since the purpose of the present invention is to redundantly determine the 
location of each hydrophone along the streamer by a ring-around feedback 
system, the acoustic source must initiate a pulse upon receipt of an 
incoming hydrophone signal. To prevent the generation of confusing 
hydrophone signals, each hydrophone is preferably monitored sequentially 
through individual channels. 
The signals passing through gate 140 are amplified and shaped in unit 150. 
The shaped signals are passed in parallel through a clocked counter 160 
and gain control unit 170. The gain control unit automatically adjusts the 
transmission power of acoustic source 100 in response to the strength of 
the signals from the amplifier 150 to minimize power consumption. Clocked 
counter 160 counts the number of feedback signals emanating from the 
selected hydrophone and times the interval required for a preset number of 
repetitious signals. 
Since the only time lapse of significance is the time required for passage 
of the acoustic pulse through the water, this time may be repetitiously 
measured and the average value determined to accurately ascertain the 
range. 
Circuitry is also provided to automatically retrigger an acoustic pulse 
through trigger 120 in response to the signals passing through counter 
160. After a predetermined number of signals, preferably six, have been 
received, the counter resets to zero to await the beginning of additional 
range-finding operations for successive hydrophones through external start 
110. 
Although FIG. 2 depicts only a single acoustic source, it is preferable to 
use a pair of sources so that independent range determinations may be 
triangulated to pinpoint the hydrophone position with either the knowledge 
of the depth or the appropriate compass headings. 
If two acoustic sources are employed, they should be alternately used to 
prevent confusing cross signals or should use differing frequency outputs 
so that distinguishable signals may be generated by the hydrophones. 
In another preferred embodiment, all of the channels from the hydrophones 
are simultaneously monitored and the receipt of the hydrophone signals are 
individually timed. However, the acoustic source is triggered into 
generating a succeeding pulse only after receipt of the incoming 
hydrophone signal emanated from the last or furthest hydrophone in the 
streamer.