`
`
`Ex. PGS 1074
`
`
`EX. PGS 1074
`
`
`
`
`
`

`

`GEOPHYSICS,
`
`VOL. 35, NO. 6 (DECEMBER
`
`1910),
`
`I’.
`
`lOSP1072,
`
`14 FIGS.
`
`ELECTROACOUSTIC
`SEISMIC
`STREAMERST
`
`CHARACTERISTICS
`
`OF MAR
`
`I NE
`
`J. W. BEDENBENDEK,
`
`K. C. JOHNSTON,
`
`AND
`
`E.
`
`B
`
`NEITZE
`
`I,*
`
`To specify intelligently an unambiguous tow-
`noise level
`in marine seismic exploration,
`the
`electroacoustic characteristics of the streamer
`must be understood. In this paper, these charac-
`teristics are examined as a basis for an industry-
`wide standard for specifying streamer tow-noise
`level.
`Sources of tow noise-including electrical, am-
`bient, flow, radiated, and mechanically induced-
`are examined and the important parameters that
`control their amplitude spectrum are presented.
`The theoretical bases for various means of re-
`ducing the components of tow noise are analyzed
`and compared with experimental results.
`
`With this background of tow rloise sources and
`noise-reduction schemes, the noise signal
`is
`traced from the hydrophonejsc,awater
`interface
`to the recording system. The h, clrophone array’s
`amplitude response and phase response are deter-
`mined from the transfer characteristics of the
`coupling circuit and the recording system.
`Finally, a method for specifying tow-noise level
`is proposed. The specified tcnv-noise level
`is
`refere?zccd to the hydrophone inlmt terminals and
`is mo&oved at the output of the r-ecording system.
`Therefore, the standard requires that the impor-
`tant electroacoustic characteristics outlined
`in
`this paper be specified.
`
`INTRODUCTION
`In offshore exploration for petroleum, a ship
`pulls 24 or more groups of hydrophones through
`the water at constant velocity. Each group may
`be up to several hundred feet long and, spaced
`throughout its length, there may be five or more
`in electrical
`hydrophones, usually connected
`parallel. These groups are joined together end-
`to-end to form what is called a streamer. The
`streamer is typically 2.5 inches in diameter and
`8000 ft long. Hydrophones are mounted inside a
`thin-walled, flexible, plastic jacket, which is filled
`with a fluid having a low specific gravity, so that
`the streamer is approximately neutrally buoyant
`in seawater. The streamer
`is normally main-
`tained at constant depth by dynamic controllers.
`Each hydrophone group (or data channel) is
`coupled to the recording system aboard ship by
`an electrical network. In addition to the reflected
`signal from the earth’s subsurface, the hydro-
`phones are subjected to a number of acoustic and
`
`mechanical disturbances. The response of the
`hydrophones to these disturbailces is commonly
`called tow noise. Currently, there are a variety of
`methods of specifying this tow noise; this variety
`makes it difficult to compare crew performance
`and data quality on a common Ijasis.
`A common understanding should exist between
`those specifying a maximum
`ljermissible tow-
`noise level for recording marinc seismic data and
`those attempting
`to meet or crcced this speciti-
`cation. No uniform standard of specifying tow
`noise now exists in the exploration industry to
`bridge this concept gap. It is holjcd that the back-
`ground of electroacoustic charat,teristics given in
`this paper will
`lead to an understanding and
`acceptance of a proposed intlustry-wide stan-
`dard for specifying tow noise.
`is presented on
`First, background information
`noise sources; this is followed I)y the means of
`reducing noise generated by various sources.
`Next, the array of hydrophones and the coupling
`
`i manuscriptreceived by the Editor October 13, 1969; revised manuscript received August 3, 1970.
`* Services Group, Texas Instruments Incorporated, Dallas, Texas.
`Copyright @ 1970 by the Society of Exploration Geophysicists. All rights reserved.
`
`1054
`
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`
`

`

`Characteristics
`
`of Seismic Streamers
`
`1055
`
`NOISE
`
`SOURCES
`
`Noise contributed by a multitude of sources
`limits
`the quality of data recorded in marine
`seismic exploration. Each survey contract usually
`includes a clause to the effect that “data will not
`be recorded if the tow-noise level is equal to or
`greater” than some specified value The following
`noise sources encountered in offshore exploration
`will be examined separately in their approximate
`ascending order of importance:
`
`1. Electrical noise,
`2. Ambient sea noise,
`3. Radiated noise,
`4. Flow noise,
`5. Mechanically induced noise.
`
`resistance is negligible compared to other noise
`circuit between the sensors and recording equip-
`sources. These other sources are one order of
`ment are examined. Finally, a standard
`for
`specifying tow noise is outlined and an example magnitude or more above source and input-ampli-
`of its use is given.
`fier electrical noises.
`Crossfeed and fluctuation of wires in stray
`magnetic fields are easily controlled with proper
`design and construction techniques. It is possible
`to achieve all-other-to-one-channel
`crossfeed
`values 100 db below signal level for one channel.
`Of the four electrical noise sources listed, 60 hz
`pickup
`from
`the ship’s power distribution
`is
`dominant. This noise source must be carefully
`controlled by providing high electrical impedance
`to ground and high impedance on the order of
`100 megohms between channels.
`In summary, proper design and precautions can
`easily eliminate electrical sources as a significant
`component of streamer tow noise.
`
`Each source is examined from the standpoint
`of where it originates, how the noise energy is
`propagated from its origin to the streamer hydro-
`phone arrays, and what parameters determine its
`spectral characteristics and limiting values. Typi-
`cal values for the noise level are presented and
`discussed.
`
`Electrical noise
`
`The electrical noise component of tow noise
`may
`be represented by
`input equivalent noise of the amplifier,
`1.
`2.
`thermal noise represented by the equivalent
`resistance of the streamer,
`miscellaneous noises due to crossfeed be-
`tween channels, fluctuations of wires within
`the earth’s magnetic field, changes in line
`capacitance due to wire fluctuation, etc.,
`60 hz pickup from the power distribution of
`the boat.
`
`3.
`
`4.
`
`is
`Input equivalent noise of the amplifiers
`typically 0.1 to 0.2 NV rms over an 8 hz to 300 hz
`bandwidth.
`The source resistance of the streamer, as mea-
`sured from the amplifier input, is typically about
`2000 ohms. This is equivalent to a thermal noise
`voltage, over a 300 hz frequency band at 28’C, of
`only 0.1 pv rms.
`Thus, as will be shown later, the summation of
`electrical noises due to amplifier input and source
`
`Ambient sea noise
`
`is the only thing about ambient sea
`Variability
`noise that is constant. This variability
`is due to
`the presence or absence of dominant noise sources
`such as high waves, shipping traffic, and marine
`life. The purpose here is not to dwell on and ex-
`plain any of these possible noise sources but to
`identify the dominant source in the seismic fre-
`quency passband.
`Figure 1 puts the ambient noise-level range of
`the seismic band in perspective by showing the
`upper and lower limits between one hz and 10
`kbz (Wenz, 1963). Although for each limiting
`condition a single curve is drawn for the spec-
`trum
`level,
`it should be emphasized that
`the
`nominal variability around each curve is f 10
`db. Except for an unusual event such as an earth-
`quake, the ambient environment is bounded by
`the two curves in the figure. The lower curve
`represents thermal noise due to molecular agita-
`tion; the upper curve represents simultaneous
`conditions of high seas, heavy traffic, and other
`sources-all of which would not normally occur
`at the same time These ambient noise levels are
`those which would be measured with a single
`omnidirectional hydrophone.
`The most important source of ambient noise
`in the seismic passband is the condition of the
`sea surface. Knudsen et al (1948) showed ambi-
`ent-noise results which did not extend below 100
`hz, but Marsh (1963) presented a theory on the
`origin of the Knudsen spectrum, ascribing it to
`the generation of sound by surface waves and
`
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`
`

`

`1056
`
`Bedenbender,
`
`Johnston,
`
`and Neitzel
`
`FREQUENCY (Hz)
`
`the Knudsen
`basis,
`extending, on a theoretical
`spectrum down to one hz. Figure 2 shows Marsh’s
`results;
`these curves were calculated
`from
`the
`following equations:
`
`P = 2.90R1.Z/j’.Gi
`
`forj
`
`< 13.5 hz,
`
`[I2 = 94,tjH’.“,/j”
`
`forj
`
`> 13.5 hz,
`
`\rhere
`
`in a one-hz band,
`P= mean pressure, pbar
`ii = crest-to-trough wave height,
`ti,
`_f= frequency, hz.
`
`range associated with each
`jvave-height
`The
`sea state of Figure 2 is given by Urick
`(1967).
`It
`is doubtful whether operations would continue
`in
`sea state 5 or above (8 to 12 ft waves) because of
`crew safety-regardless
`of tow-noise considera-
`ations.
`noise pressure levels may be found
`Randlimited
`the previous equations between
`by
`integrating
`desired frequency
`limits. For a given sea state,
`for example,
`the noise level
`in the one hz to 500
`hz band is 16 db above that
`in the 8 hz to 500 hz
`band. This
`result shows the
`importance
`of in-
`creasing
`the
`low-frequency
`limit
`to reduce
`the
`ambient noise.
`is
`this discussion
`The point
`to be made from
`that ambient noise due to rough sea conditions
`over which
`there
`is no control causes excessive
`tow noise and, at times, requires work stoppage.
`
`Kadiated tzoise
`
`is trans-
`is that noise which
`noise
`Radiated
`mitted
`through
`the water
`frclm an
`identifiable
`source
`to
`the streamer groulbs.
`In marine ex-
`ploration,
`there are several sources: lead-in cable,
`depth controllers,
`tail buoy and
`line, and
`the
`ship. The last is dominant by far. Comprehensive
`measurements of ship’s noise h;l\re been made for
`larger military
`ships but only sketchy
`informa-
`tion is available
`for noise radial ed by geophysical
`vessels.
`sources-ma-
`three major
`Ship’s noise has
`chinery, hydrodynamic,
`and
`l)ropeller-and
`the
`overall spectrum depends primarily
`on the ship’s
`size, speed, and construction.
`.\t a given surve?-
`speed,
`the radiated noise sho11lt1 be similar
`for
`ships used in geophysical exploration.
`vi-
`Machinery
`noise arises
`frolu mechanical
`the
`brations
`transmitted
`through
`the hull
`into
`water and actuated by
`the
`-hip’s engines, air
`compressors, etc. Narrowband
`peaks in the spec-
`trum,
`having
`frequencies
`d(,l)endent
`on
`the
`machinery’s
`speeds of rotation or multiples
`of
`rotation speeds, are expected
`to dominate. This
`is one source of 60 hz energy observed on most
`seismic records. Amplitudes depend on machinery
`characteristics, mounting,
`degree of unbalance,
`etc.
`Hydrodynamic
`motion
`through
`
`the ship’s
`from
`noise result-
`the water;
`it is uot propagated
`to
`
`Downloaded 08/24/14 to 96.241.189.185. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`

`

`1057
`
`FIG. 2. Effect of sea state on sound-pressure spectrum level of amhient sea noise
`
`FREQUENCY (Hz)
`
`great distances (as discussed under flow noise)
`but is strong in the near field, i.e., in the turbulent
`boundary layer of the ship, and may excite hull
`resonances that are subsequently radiated to the
`streamer groups. Noise radiated due to these
`resonances is expected to be peaked, narrowband
`noise.
`Propeller noise is important because the pro-
`pellers act directly on the water; the level of this
`noise can be expected to be high. The peak in the
`spectrum is associated with the blade frequency
`given by j=nV/60,
`where IZ is the number of
`blades and V is the propeller’s rotational speed
`in rpm. At 800 rpm, the frequency of radiated
`propeller noise is 10 hz for a 3-bladed propeller.
`Although the ship’s noise may be a large por-
`tion of the tow noise, especially for the near
`groups, there is still much to learn about this
`noise source. The effect of ship’s noise is cur-
`rently reduced by increasing the offset or dis-
`tance to the hydrophone groups and by arraying
`the hydrophones. Experimental
`results for the
`latter are given in a following section.
`
`Flow noise is caused lay pressure fluctuations in
`the turbulent boundary
`layer and by vortex
`shedding from individual surface-roughness par-
`ticles; it does not include noise due to flow-in-
`
`duced vibrations such as cable strumming (con-
`sidered to be mechanically inducecl and discussed
`in a following section). At present I owing speeds,
`flow noise is not the largest component of tow
`noise but, as speeds increase, it can become
`dominant due toits dependence on higher powers
`of velocity. Thus, an understanding of flow noise
`is necessary. Much of the develolnnent of this
`section
`is based on the
`flow-noise study of
`Skudrzyk and Haddle (1960).
`FTow noise is a near-fieid effect and is com-
`monly called “pseudo sound”
`(Lighthill, 1952)
`because it lacks the property of f)ropagation at
`or near the characteristic speed of sound. This is
`so because turbulent eddies are carried along with
`the flow, but the pressure fluctuations causing
`the pseudo-sound are largely balanced by fluc-
`tuations in acceleration of volume. of fluid in the
`boundary
`layer. Nevertheless, a hydrophone
`suitably placed in or near a turl)ulcnt boundary
`layer will respond to pressure fluctuations, rc-
`sulting in flow noise.
`Flow coise generated by pressam jlztctuations
`in
`I II a turbulent
`the turbulmt
`bounduvy
`layer.-
`boundary
`layer,
`the fluctuating velocity com-
`ponent V’ normal to the bountl;lry can he ap
`proximated by
`
`1;’ = O.O1l’,
`
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`
`

`

`I 058
`
`Bedenbender,
`
`Johnston,
`
`and Neitzel
`
`ti
`>
`
`-
`
`20
`
`V = 10 KNOTS
`
`5
`
`-60 -
`
`-80
`0.1
`
`I
`1
`
`I
`10
`FREQUENCY (Hz)
`
`I
`100
`
`1000
`
`FIG.
`
`3. _kpproximate sound-pressure spectrum of flow noise.
`
`to the
`tangent
`steam velocity
`where V=free
`inches/set. The power spectrum
`boundary
`in
`level P(U) of the pressure due to this fluctuating
`velocity
`(referred to a unit frequency interval)
`can be estimated by
`
`P(W) = 0.75. 10-6~~p4V36*
`
`for w <WO,
`and
`
`P(w) = 1.5. 10-6c&12V6/w36*?
`
`for W>WO,
`where
`
`(Skudrzyk and-
`
`w = frequency, radians/set,
`CL,= the Kraichnan constant
`Haddle, 1960),
`p= density of fluid, lb-sec2/inch4,
`6* = displacement thickness of boundary layer,
`inches,
`wo=2irV/3*.
`
`These relationships give order of magnitude only.
`Clearly, they predict a flat flow-noise spectrum
`up to w=wo and a decrease of 9 db/octave
`for
`W>Wo.
`If it is assumed that the relations for boundary-
`layer thickness for a flat plate apply to a streamer,
`the displacement thickness is approximated by
`
`6*(x) = O.O74X/(V.~/V)“~,
`
`where
`
`x= distance from leading edge,
`v = kinematic viscosity.
`
`The value of x for a streamer is assumed to be an
`average distance between major discontinuities
`(boots, coupling devices, weighl~, fins, etc.); i.e.,
`at these discontinuities, the boundary layer
`is
`detached from the streamer and must reattach
`further downstream.
`Based on these assumptions and equations, the
`flow noise due to pressure flllctuations
`in the
`boundary layer is given as a function of frequency
`for any tow velocity V. Flow noise can be brack-
`eted for V= 2 and V = 10 knots as shown in Fig-
`ure 3. Neglecting the change in boundary-layer
`thickness with velocity, we see that the flow-
`noise dependence on velocity gi\,es an increase of
`9 db/octave of velocity below the break and 18
`db/octave above the break.
`‘l’hese respective
`values are 8.7 and 20 db/octa\e of velocity for
`Figure 3, which includes the change in boundary-
`layer thickness. We again emljhasize that these
`values give order of magnitude only.
`As the hydrophone diameter
`increases, the
`hydrophone becomes less sensitive to the near-
`field effect of flow noise. This Ilappens when the
`hydrophone is large enough in comparison with
`the physical size of “turbulent c-c,lls” to somewhat
`integrate out over its surface the effect of the
`cells.
`The results given in the preceding paragraphs
`are valid only for frequencies i<V/2d,,
`where
`d,=hydrophone
`diameter
`in
`inches; i.e.,
`the
`hydrophone must be smaller than about one-half
`the spatial wavelength V/f ol the turbulence
`that generates the noise. Tllcrefore,
`for
`fre-
`quencies higher than jP = V/2tE,,, the sensitivity
`
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`
`

`

`Characteristics
`
`of Seismic Streamers
`
`1059
`
`of a hydrophone to flow noise is expected to de-
`crease. Values off, for a hydrophone diameter of
`1.25 inches for 2 and 10 knots are 16 and 81 hz,
`respectively. For the flow noise at high fre-
`quencies (well above fp), doubling the hydro-
`phone’s diameter reduces its sensitivity to flow
`noise by approximately 6 to 12 db. Thus, the
`height of the curves in Figure 3 should actually
`decrease somewhat above the frequencies marked
`as fp.
`The preceding discussion and results are for a
`single hydrophone. The results generally will be
`be different when the output is taken from an
`array of hydrophones because the array can
`attenuate flow noise.
`It is difficult to describe an array’s effect on
`flow noise, since the nature of the noise is not
`well understood. If the flow noise had components
`which were coherent along the length of the
`array, the array would attenuate these compo-
`nents according to its inline response; on the
`other hand, if the flow noise were random, it
`would be attenuated by the array by a factor of
`one divided by the square root of the number of
`elements in the array. An array’s total effect on
`flow noise is due to a partial combination of the
`above two situations. The resulting reduction in
`flow noise can be significant.
`Possible coupling of pressure fluctuations with
`natural frequencies of the streamer walls or other
`components has not been included. For conven-
`tional streamer constructions, these natural fre-
`quencies are well above the seismic frequency
`band and resonance is not a problem.
`Flow noise gemrated by surface
`roughness.-
`Noise due to surface roughness is insignificant
`if the roughness is buried in the laminar sublayer
`of the turbulent boundary layer. This is the case
`if the surface is relatively smooth and/or
`the
`relative velocity low. However, as the velocity
`increases, the thickness of laminar sublayer de-
`creases and the individual roughness particles
`pierce the sublayer and shed vortices. The per-
`missible roughness height before roughness be-
`comes significant in producing noise is approxi-
`mately
`
`12 = 0.06/V]<,
`
`where
`
`The streamer skin is quite smooth, and surface
`roughness is not thought to be a significant factor.
`It must be emphasized that this discussion re-
`fers to distributed surface roughness and not to
`abrupt changes or discontinuities in streamer
`cross-sectional shape or area; such discontinuities
`do produce noise in the streamer, as can be ob-
`served when a weight or some other discontinuity
`is placed close to a hydrophone, but the authors
`have not attempted to describe this effect quan-
`titatively. Qualitatively, the flow noise occurring
`is still pseudosound and has been found to be
`attenuated
`rapidly with distance. Hydrody-
`namically unstable objects can also introduce
`motion which can be propagated along the
`streamer; such motions are sources of mechani-
`cally induced noise, which is discussed next.
`
`A+?ecka9zcially GuJuced noise
`
`induced noise is caused by
`Mechanically
`transverse motion or perturbations in the longi-
`tudinal motion of the streamer. This includes
`boat noise propagated along the streamer, tail-
`buoy noise, and cable strumming caused by
`vortex shedding from the lead-in cable or the
`streamer itself. The noise is the response of the
`hydrophones to static-pressure head changes,
`dynamic-pressure head changes, and accelera-
`tions. These inputs will first be discussed, followed
`by consideration of the forcing functions causing
`them.
`If a single hydrophone is forced to vibrate
`sinusoidally in a plant at right angles to the
`streamer, the motion of the hydrophone can be
`expressed as
`
`X = il sin wt,
`
`x = .4w cos wt,
`. .
`X = -
`
`i’lw2 sin wt,
`
`where A is the amplitude in inches. The output
`of the hydrophone due to displacement (static-
`pressure variations) is given by
`
`e,,(f) = K,S.A sin d,
`
`where
`
`S = sensitivit.y of Irydrophone, pvjpbar,
`/r, = 2560 pbars/inch of water head.
`
`h= roughness height, inches,
`Vk = velocity, knots.
`
`.
`
`Considering the dynamic-pressure head to be
`the stagnation pressure &?/2
`coinciding with
`
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`
`

`

`1060
`
`Bedenbender,
`
`Johnston,
`
`and Neitzel
`
`the velocity of motion, the hydrophone output
`due to velocity is
`
`ev(l) = R&p(ilw cos 4’/2,
`
`where
`
`p = fluid density, lb-sec2/inch4,
`kr= 68,950 pbars/psi.
`
`This expression is only approximate because of
`the pressure distribution around the hydrophone,
`but the result is adequate for comparison pur-
`poses.
`the output due to acceleration is
`Similarly,
`approximated by
`
`e,(t) =
`
`-
`
`(k~SmAw2 sin &)/_A,,
`
`where
`
`m = mass of hydrophone, lb-se?/inch,
`A,=projected
`area of crystal normal
`motion, inche?.
`
`to X
`
`Thus, the amplitudes are given by
`
`I?, = 68,950SmAw2/A P.
`
`For A = 10e4 inch, A,= 1.5 inch2, nz= 0.0005 lb-
`sec2/inch, and S= 15.4 pv/pbar, and at a fre-
`quency of 5 hz,
`
`/+I,, = 3.94 /Lv,
`
`/i = 5.03. 10-4 /.Lv,
`
`Ii,, = 35 pv.
`
`Hence, in the seismic band of 5 hz to 100 hz, the
`hydrophone output due
`to acceleration can
`dominate unless some form of acceleration can-
`cellation is utilized.
`The physical sources which cause the above
`motions wilt now be discussed.
`Tail huoq’.-Noise
`is introduced into the tail-
`end of the streamer when the line attached to the
`marker buoy is jerked. This noise component
`is
`primarily due to wave action on the buoy and rc-
`~11s mainly in Iransversc motions.
`Most of the energy in tail-buoy noise is at
`lower frequencies (below 30 hz) and is a strong
`function of sea state; it does not vary with tow-
`ing speed as much as does total streamer tow
`noise over a 4 knot to 8 knot range. Tail-buoy
`
`noise propagates forward along the streamer in
`heavy seas but is rapidly attenuated with dis-
`tance so that it is probably not significant for
`more than a few groups. Vortex shedding and
`consequent strumming of the tail-buoy line have
`not been found to be serious prol)lems.
`Boat fzoise propagated dowel lead-in cabk-
`Sources for this noise include hull and equipment
`vibrations
`forced primarily
`lay rotating ma-
`chinery aboard, i.e., diesels, compressors, gen-
`erators, screws, etc. Kate that many of these same
`sources also generate vibrations producing radi-
`ated ship’s noise (discussed pre\-iously).
`Boat noise is propagated
`(town the lead-in
`cable to the streamer as either transverse or
`longitudinal waves. Motion of the boat-end of
`the lead-in cable as the boat reacts to the sea can
`also be a source of noise througll the introduction
`of higher-frequency harmonics or perhaps even
`the fundamental frequency. Significant boat noise
`can occur as peaks throughout the entire 5 hz to
`100 hz range.
`Noise caused by flow-induced vibrations.--Flow-
`induced vibrations are caused I)y periodic shed-
`ding of vortices from cylindrical or bluff shapes
`in a flow having a component of velocity normal
`to the longitudinal axis of the Ijody. This pheno-
`menon, acting in conjunction with the pitching
`motion of the boat and resultant tugging on the
`lead-in cable, is often the predominant source of
`streamer tow noise in the 5 hz 1 o 20 hz frequency
`range. The ship’s vertical nrotion effectively
`increases the magnitude of the, velocity compo-
`nent normal to the cable and can cause vortex
`shedding and cable strumming I o be in the seismic
`frequency band.
`The
`frequency of vortex shedding for flow
`normal to a cylindrical shape is given by the fami-
`liar Strouhal number relationship
`
`where
`
`N,sl= Strouhat number,
`d= diameter of cylinder,
`I’= velocity of flow,
`J,= frequency of vortex sllc.~ltling.
`
`llrc previous equa-
`For a first approximation,
`tion applies to flow having a tangential velocity
`component also, if V is taken as the normal com-
`ponent only. The Strouhal nulnber is essentially
`
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`
`

`

`
`
`z >
`
`rl
`5
`2,
`E$
`z:‘=
`5-i
`y
`z-
`2
`
`si
`&
`n
`2
`
`;
`VI
`
`i
`
`:
`
`36
`
`24
`
`12
`
`0
`
`-12
`
`-24
`
`-36
`
`-421
`0
`
`Characteristics
`
`of Seismic Streamers
`
`1061
`
`-
`
`-
`
`WITH LEAD-IN
`
`STRUM
`
`-
`
`WITHOUT
`
`LEAD-IN
`
`STRUM
`
`?
`
`y\_____
`
`50
`
`100
`FREQUENCY
`
`(Hz)
`
`150
`
`21
`
`0
`
`l:rc. 4. Sound-pressure spectrum showing effects of leati-in calde slrum.
`
`for Reynolds numbers be-
`constant at iVsl=0.21
`tween 3X lo2 and 105. The expected
`range of
`Reynolds numbers
`for seismic
`lead-in cables
`is
`approximately
`103 to 105.
`the normal
`For steady
`tow
`in low sea states,
`component of velocity
`is quite small (on the order
`of 0.5 to 1.5 ft/sec). The resulting Strouhal
`fre-
`quencies given by ,f,=O.Zl
`V/d
`for a 1.25inch
`diameter cable range
`from 1.0 to 3.0 hz. These
`frequencies are low enough so that
`they may not
`directly cause high noise levels. However, when
`vertical boat motion
`is considered,
`it is clear that
`Strouhal
`frequencies
`can occur
`in
`the seismic
`band. For
`instance, consider
`the motion of the
`boat such that
`the vertical motion of the lead-in
`cable
`is y = B sin WL. The maximum
`velocity
`is
`then 2rjB. For a pitching amplitude of 5 ft and
`a 10 set period,
`the maximum
`velocity
`is 3.14
`ft/sec. When
`this velocity
`combines with
`a
`steady normal velocity component of one ft/sec,
`the resulting
`strumming
`frequency
`j8
`is 8 hz.
`Pitching motion occurring simultaneously with
`wave crests reaching
`the lead-in cable at the air-
`water
`interface
`thus can produce vortex shed-
`ding and cable strum of sizable amplitude
`in the
`5 hz to 20 hz frequency
`range. This cable strum is
`apparent
`to one observing
`lead-in cable behavior;
`it appears
`in bursts of energy at a predominant
`frequency which
`is dependent on the operating
`configuration
`and the sea state.
`
`lead-in cable
`frequencies of tile
`The natural
`vibrating
`transversely
`as a situple string
`are
`approximated
`by
`
`where
`
`T = tension, lb,
`nz, = mass per unit
`L = length,
`inches,
`n = an integer.
`
`length,
`
`lb-bcc”/inch2,
`
`j/z x I) ordinarily
`is
`frequency
`fundamental
`The
`harlnonic multiples
`less than one hz. However,
`can be excited by higher frequency
`forcing func-
`tions. The lead-in cable is cxpectcll
`to resonate at
`a certain higher harmonic wheel that harmonic
`coincides or nearly coincides with
`the Strouhal
`frequency.
`Noise due to cable strum prolulgates along the
`streamer at a speed of 2.500 to 5000 ft/sec, de-
`pending on cable tension. Thus,
`I he propagation
`speed is higher at the front of thr streamer and is
`higher for higher towing speeds.
`Figure 4 shows a comparison of the sound-
`pressure spectrum of one trace
`for a test under
`conditions as nearly
`the same as possible except
`for lead-in cable strum. Clearly,
`the conditions
`were not
`identical, since boat motion and wave
`action were not combining
`to callse cable strum.
`Generally, cable strum causes a narrowband peak
`
`Downloaded 08/24/14 to 96.241.189.185. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`

`

`1062
`
`Bedenbender, Johnston,
`
`and Neitzel
`
`in the 8 hz to 15 hz range. The maximum ampli-
`tude of this peak may be approximately 6 to 24 db
`above a similar case where no cable strum occurs.
`
`MEANS
`
`FOR REDUCING
`
`TOW NOISE
`
`Devices and techniques for reducing tow noise
`of seismic streamers must be based on an under-
`standing of the mechanism of the noise. Since
`there are different sources and propagation paths
`for noise in the streamer system, a single noise-
`reduction device may be effective in reducing only
`one noise mode. Means of reducing radiated noise,
`flow noise, and mechanically induced noise are
`discussed and specific
`reduction
`techniques
`analyzed.
`is an important
`the hydrophones
`Arraying
`factor in reducing tow noise. A properly designed
`array can attenuate
`(at least in discrete fre-
`quency bands) all three noise components pre-
`viously listed. Because of the importance of the
`array, it is discussed separately in a following sec-
`tion.
`
`Radiated noise
`
`Of course, the simplest way to reduce noise
`radiated from the towing vessel is to increase the
`offset distance from the vessel to the hydrophone
`groups. The offset distance should be determined
`by seismic considerations, but sometimes tow
`noise has dictated otherwise.
`The usual expression for approximating trans-
`mission loss of sound propagated through sea-
`water combines spherical spreading loss with an
`absorption loss (Urick, 1967) and is stated as
`
`TL = 20 log (dz/dl) + at(dz - d,),
`
`where
`
`TL= transmission loss, db,
`dz = distance at which TL is to be determined,
`yards,
`dl = reference distance, yards,
`~~l=absorption coefficient, db/lOOO yards.
`
`The absorption coefficient CQ, a function of fre-
`quency and temperature, decreases with
`fre-
`qukncy and
`is approximately 0.0015 db/lOOO
`yards at 100 hz. Thus, absorption loss is negligible
`for distances and frequencies common to seismic
`systems.
`induced noise of most kinds is
`Mechanically
`also attenuated with increasing distance from the
`
`noise source. Flow noise and ambient sea noise are
`not appreciably different at different places along
`the streamer or different for changes in o&et dis-
`tance. Consequently, tow noise versus offset com-
`parisons show changes in both radiated noise and
`mechanically induced noise but not in ambient
`noise and flow noise.
`The preceding transmission-loss equation is not
`applicable to total tow noise but only to the
`radiated component. Attenuation of total tow
`noise with offset distance is found to be less than
`that for the radiated part. Effects of noise at-
`tenuation with distance are sometimes masked by
`local differences along the streamer, e.g., poor
`ballasting of some sections; i.e., a group in the
`center of the streamer can be noisier than the
`group closest to the boat because of improper
`ballasting or other causes.
`If the tow noise of only the group closest to the
`boat is being compared for different offset dis-
`tances (other conditions being equal),
`the at-
`ienuation can be estimated by a similar equation,
`T-Lo = Klog (Cb,'0r),
`
`where
`
`OS, 01= offset distances being compared,
`K = appropriate constant,
`TLO= noise level change in db at 02 relative
`to 0,.
`
`In tow-noise testing performed by the authors,
`the value of K has been found to vary from 4 to 9.
`
`Flow lzoise
`
`The lowest value of flow noise is achieved by
`having the smoothest, most continuous surface
`possible presented to the flow and by controlling
`ballast forces so that a nearly uniform streamer-
`depth protie is maintained. A clean streamer de-
`sign also has obvious advantages in drag reduc-
`tion, and drag reduction usually results in a corre-
`sponding reduction in flow noise.
`If fairly large discontinuities in the streamer
`cross-section cannot be avoided, it is imperative
`that the discontinuities not be placed too close to
`any hydrophone. It has been found that placing a
`discontinuity closer than approximately 10 ft to a
`bydrophone causes an observable increase in tow
`noise at normal towing speeds. The 10 ft separa-
`tion is adequate because most of the flow noise due
`to a discontinuity is near-field flow noise (pseudo-
`
`Downloaded 08/24/14 to 96.241.189.185. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`

`

`Characteristics
`
`of Seismic Streamers
`
`1063
`
`TRACE
`
`20
`
`4 STRETCH SECTIONS
`
`1000 FT OFFSET
`
`FAIRED
`
`LEAD-IN
`
`-48
`
`‘I
`‘I.._
`
`sound) and is attenuated very rapidly I\-ith dis-
`tance.
`-4s mentioned in an earlier section, flow noise is
`not thought to be the largest