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`Seismic 15
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`465
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`discontinuity such as connectors, bulkheads, etc., mode
`transfers occur causing the generation of higher noise har-
`monics. Vibration provides the dominant low-frequency
`noise component whilst turbulent flow contributes signifi-
`cantly towards the higher frequencies.
`A numerical analysis of these types of streamer noise was
`undertaken, providing the basis for a new acoustic design.
`The design and theory were tested in a suite of marine trials
`using a specially instrumented streamer under a wide variety
`of operating conditions. This paper reports on the basic
`principles of the acoustic deGgn and tests these against field
`measurements.
`
`The ever increasing demand for high resolution marine
`seismics is producing significant changes in the design of
`both acoustic sources and streamers. Current proposals for
`digitally multiplexed seismic streamers are generally de-
`signed to attack problems such as spatial aliasing, group
`directivity response, and raypath perturbations. However,
`there is also a fundamental requirement to improve the
`signal-to-noise (S/N) ratio of the seismic section.
`The effective resolving power of a seismic return can be
`gauged in terms of good SIN bandwidth of target and as a
`general principle the two characteristic forms of noise,
`namely shot and streamer noise, are reduced using different
`approaches. Shot noise can usually only be reduced through
`effective processing and by taking advantage of massive
`statistical redundancy in seismic data. As such, shot noise
`must be registered by the complete acquisition system with
`as much fidelity as the primary returns. In contrast, streamer
`noise will be attenuated by both processing and effective
`acoustic streamer design. The latter is considered to be a
`fundamental problem and is the subject of this paper.
`To demonstrate the impact of the problem, Figure I
`indicates the spectra of an air gun and a streamer both
`currently in use in the North Sea. To simulate true signal
`levels at depth, the air gun signal was spectrally modified by
`a Q factor of 100, and representative spreading and transmis-
`sion losses were imposed for a target depth of 2.5 sec.
`The clear disparity between the two spectra signifies the
`magnitude of the problem in attaining a good SIN ratio and is
`clearly a difficulty that is exacerbated at progressively higher
`frequencies. Whilst demonstrating the unsuitability of this
`particular acoustic source the display enforces one further
`point. Within the acquisition system dynamic range, the
`
`Slgnal/nolse
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`Spectra
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`IO
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`L 5
`T 2
`i
`. - 1
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`3
`al
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`L -4
`6l
`;
`g
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`-7
`-10
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`0
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`2.5
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`I
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`5
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`I
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`/
`10
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`7.5
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`Hz x 10
`FIG. 1.
`
`FIG. 6. Predicted harmonic distortion for GS-44D-P by dif-
`ferent procedures under different assumed conditions. NI
`= numerical integration, HT = householder
`transformation.
`
`nonlinearity of sensitivity is represented as THD(G) and the
`same terminology applied to other terms. From those figures
`we note that results obtained by different methods show
`quite good consistency and the typical total harmonic distor-
`tion is ranged from 0.03 to 0. IO percent.
`
`Conclusions
`Methods to predict geophone performance parameters
`have been outlined. It also has been shown that very good
`consistency between the predicted and measured parameters
`is achievable through the developed programs. The informa-
`tion thus obtained will be quite helpful whenever finer
`adjustment of the parameters is required. Incorporated with
`those developed programs, the design processes could be
`facilitated.
`
`References
`Parker, R. J., and Studders. R. J.. 1962, Permanent magnets and
`their application: New York. John Wiley and Sons.
`Jackson, J. D., 1975, Classical electrodynamics: New York. John
`Wiley and Sons.
`Steer, J., and Bulirsch. R.. 1980, Introduction to numerical analysis:
`New York. Springer Verlag.
`Colonias, J. S., 1974, Particle acceleration design: Computer pro-
`grams: New York, Academic Press.
`Lapidus. L., and Seinfeld, J. H., 1971. Numerical solution of
`ordinary differential equations: New York. Academic Press.
`Bierman, G. J., 1977. Factorization methods for discrete sequential
`estimation: New York, Academic Press.
`Huan. S. L., and Murphy, L. P., 1982. Relation of geophone
`distortion between being driven electrically and mechanically:
`Houston, AMF Geo Space Corp.
`Brown, G. G., 1982, Computerized measurement of spring constant
`versus displacement for geophone suspension mechanism: Hous-
`ton, AMF Geo Space Corp.
`
`s15.4
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`Advanced Acoustic Design for a New
`Seismic Streamer
`J. H. Peacock. Britoil; C. G. Sykes, N. W. Crrtwrot~,
`Standard Telephones and Cables; und L. G. Peurdon,
`Britoii, Englmd
`In order to obtain better quality marine seismic data, noise
`induced in the marine streamer must be reduced. A signifi-
`cant component of the total noise field appears as a conse-
`quence of turbulent flow and vibrational excitation. Each
`primary excitation can develop one of three general modes
`of propagation down the streamer. These modes, which are
`constructionally dependant. propagate at velocities which
`are significantly slower than water and at points of acoustic
`
`Downloaded 09/11/14 to 173.226.64.254. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
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`Ex. PGS 1076
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`466
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`Seismic 15
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`signal needs to be matched to the noise spectrum which itself
`should be minimized at higher frequencies.
`Streamer noise can be categorized as follows:-
`
`(a) Ambient. This is dominated by firstly traffic noise in the
`oceans which is progressively increasing in magnitude and
`becoming more directional. The prime factor here is the
`higher speed of ships and the increased speed of reciprocat-
`ing machinery. The second major component consists of
`surface and swell noise. These are sea-state dependent and
`both exhibit a short-term spatial and temporal coherence.
`
`(b) Ship-radiated. Below 50 Hz this noise is periodic and
`dominated by propeller fundamental frequency and associat-
`ed harmonics. In the range of 50 to 150 Hz, noise from the
`vessel machinery is usually observed. At frequencies in
`excess of 150 Hz, hydrodynamic flow over the vessel hull
`and propeller cavitation dominate with increasing effect at
`higher frequencies and at higher ship speeds.
`
`(c) Electrical. This usually exhibits a high coherency
`through electromagnetic pick-up and low-level random char-
`acter.
`
`(d) Towing noise. Tow noise may be either induced by
`turbulent flow over the streamer or vibration-induced
`through direct transfer from the towing/tail assemblies and
`from residual vibration within the streamer. The first two
`categories of noise are characterized by the fact that im-
`provement in SIN ratio can, in the main, only be achieved by
`processing. The third category demands improvement in
`instrumentation and is voiced as an argument in favor of
`telemetry systems. The last category of noise can be at-
`tacked by the acoustic design of the streamer. In all catego-
`ries, fidelity in registration of all forms of noise corruption is
`essential.
`A detailed analysis of towing noise yields the following
`mechanisms of induction.
`
`Turbulence induced noise. (I) Turbulent flow along the
`streamer skin will be directly transferred as a pressure field
`to the hydrophone. (2) Turbulent flow will directly induce
`pressure waves at major discontinuities within streamer such
`as at bulkheads and spacers.
`
`Vibration induced noise. (3) Irregularities in the streamer
`profile, particularly those penetrating the turbulent bound-
`ary layer, will be directly excited into vibration through
`vortex shedding. Notably influential in this category is the
`presence of depressor birds. (4) Excitation of the tow cable
`and tail buoy, though attentuated by the isolators, will be
`transferred through the streamer by the modes described
`below.
`For each of these categories of vibration noise induction
`there are three modes of transfer to the hydrophone: (a)
`breathing of Bulge waves, which propagate as a sectional
`diameter change; (b) extensional waves, which are a longitu-
`dinal mode of propagation within the skin; and (c) longitudi-
`nal extensional waves within the strain member.
`The characteristic velocities of these modes of propaga-
`tion are
`(a) breathing waves:
`
`(b) hose extensional waves,
`
`(c) strain member vibration.
`
`where E,, = Young’s modules of skin, E, = Young’s
`modules of strain member, h = skin thickness, Pf = fluid
`density, n,. = skin density, Pi = strain member density, R =
`internal skin wall radius, and g = Poisson’s ratio.
`The three modes of transfer will couple wherever the skin,
`strain member, and fluid fill are connected and are particu-
`larly influenced by the presence of blocking mechanisms
`within the section such as bulkheads, spacers, etc. An
`examination of characteristic modes indicates that propaga-
`tion velocities are significantly slower than water velocity.
`Transferral to other propagation modes occurs at acoustic
`boundaries and causes the induction of noise at higher
`harmonics.
`For design purposes particular streamer configurations
`have been numerically modeled and yield the following
`summary conclusions. Considerable spatial variance in the
`noise spectrum is predicted being a function of the proximity
`of bulkheads. spacers. connectors, etc. Furthermore, theory
`predicts that the vibrational component of noise will domi-
`nate at the lower frequencies and is relatively independent of
`towing speed. Turbulence induced noise is predicted to have
`a high dependency on towing speed and will progressively
`dominate at high frequencies. Separation of the induction
`mechanisms defines the appropriate design philosophy being
`that the group design can usually only influence the turbulent
`component whilst good acoustic and mechanical design at
`the section or full streamer level is the principal method of
`reducing the vibrational component.
`In response to theoretical model predictions and subse-
`quent new design principles, four 100-m streamer sections
`were constructed and fully instrumented. A suite of trials
`was configured to evaluate the streamer design and to assess
`predicted induction mechanisms and mode transfers. Vibra-
`tion levels within the streamer section were monitored at
`strategic points as was the streamer profile and towing
`dynamics. New techniques of vibration isolator construction
`were also evaluated. For the trials two test sites were chosen
`to reflect the wide ranges of expected operating conditions
`and at each site both the ambient noise field and that
`generated from the towing vessel were closely monitored
`using a sonobuoy array.
`In the final section of this paper. theoretical predictions of
`the modeled system are analyzed against results derived
`from field measurements. A comparison between the trial
`streamer and systems in current use is undertaken to evalu-
`ate the effectiveness of the new acoustic and mechanical
`design.
`
`Ocean Bottom Seismometer: An
`Engineering Perspective
`E. A. Bowden und M. J. Prior, Mobil R & D
`The ocean bottom seismometer (OBS) is an autonomous
`data gathering and recording system consisting of an acous-
`
`515.5
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`Downloaded 09/11/14 to 173.226.64.254. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
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`Ex. PGS 1076
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`

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`This article has been cited by:
`
`1. Thomas Elboth, Fugro Geoteam, Dag HermansenAttenuation of noise in marine seismic data 3312-3316. [Abstract]
`[References] [PDF] [PDF w/Links] [Supplemental Material]
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`Downloaded 09/11/14 to 173.226.64.254. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
`
`Ex. PGS 1076