This article describes some of the noise generation mechanisms that is believed to affect towed seismic streamers. By combining fluid mechanical insight and analysis of seismic noise records it is clear that a significant amount of noise observed is generated in the turbulent boundary layer surrounding seismic streamers. This noise primarily originates from two different sources. First there is the low frequency hydrostatic pressure variation, and secondly there is the dynamical
flow noise. The dynamical flow noise appears to pass a threshold when the angle between the flow direction and the streamer exceeds around6−15◦. During operations it should therefore be a goal to keep the tow angle below this critical value.
Previously, elevated noise levels have been explained by bulge-waves, tugging, strumming, vibrations and electrical interference. Engineering improvements in modern equipment have greatly reduced the influence of these sources of noise. The work towards these improvements has not addressed the turbulent streamer boundary layer which inevitably will create unwanted noise. As a direct consequence we see that in order to reduce the recorded noise levels further, changes to the physical design of seismic streamers alongside with improved signal processing algorithms need to be addressed. Engineering optimizations to improve the transfer of signal between the streamer hose, the fill materials and the hydrophones can probably improve the S/N-ratio on future streamers. However, more radical design changes might also have to be considered to really tackle the influence of flow noise.
Chapter 4
Investigation of flow and flow noise around a seismic streamer cable
Thomas Elboth<thomae@math.uio.no>
Fugro Geoteam AS, Hoffsveien 1c, P.O.Box 490 Skøyen N-0213 Oslo, Norway Mechanics Division, Department of Mathematics, University of Oslo, Norway Difrik Lilja<d.lilja@fugro.no>
Fugro Geoteam AS, Hoffsveien 1c, P.O.Box 490 Skøyen N-0213 Oslo, Norway Bjørn Anders Pettersson Reif<Bjorn.Reif@ffi.no>
Norwegian Defense Research Establishment (FFI), P.O.Box 25 2027 Kjeller, Norway Mechanics Division, Department of Mathematics, University of Oslo, Norway Øyvind Andreassen<Oyvind.Andreassen@ffi.no>
Norwegian Defense Research Establishment (FFI), P.O.Box 25 2027 Kjeller, Norway Article originally published as:
T. Elboth, D.Lilja, B. A. Pettersson Reif and Ø. Andreassen: Investigation of flow and flow noise around a seismic streamer cable, Geophysics XXX, in press (2010)
Abstract
In marine seismic explorations, flow noise from the turbulent boundary layer that forms around a streamer cable due to its relative motion through water significantly affects the quality of col-lected data. Understanding this noise generation mechanism is valuable for the development of future seismic streamer cables.
In this study, we qualitatively characterize the area of turbulent flow surrounding a seismic streamer cable, and relate this characteristic to the statistics of the measured noise signal. The main finding is that the boundary layer thickness around a seismic streamer appears to be around 25 cmin an ocean environment. This is significantly larger than the2.5to5 cmthat has been reported in the literatures from laboratory experiments. We attribute this discrepancy to the un-steadiness of the ocean environment. Estimations of the spatial extent of the recorded boundary layer noise indicate that the “optimal” hydrophone separation needs to be about0.5 min or-der for the noise to be uncorrelated. The SNR (signal-to-noise ratio) on streamer cables would
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therefore be improved if hydrophones were placed more densely than the current industry
α = Streamer incidence angle (degree) U0 = Sow speed or free-stream velocity (m/s)
y = Distance from the cable surface (m) L = Streamer length (m)
Λ = Integral length scale (m) ρ = Fluid density (kg/m3)
ν = Kinematic viscosity of water (1.01·10−6m2/s) Re = Reynolds number;ReL=UL/ν;Reδ =Uδ/ν Cxy = Correlation coefficient of timeseriesxandy
4.2 Introduction
The word seismology is often associated with earthquakes. However, the tightly related term
“seismic” comprises a valuable technology used extensively by the oil and gas industry in its exploration, development, and reservoir management operations. Marine seismic exploration is normally done by towing long flexible streamer cables in the ocean. These cables are pop-ulated with pressure sensors (hydrophones), to acquire information about the subsurface geol-ogy. Pressure recordings are made from subsurface reflections of energy arising from a pressure source (air guns).
Various factors add complexity to streamer cable operations. Examples are wave motion from surface waves and currents that cause pressure fluctuations and rattling of the streamer. Other factors are tugging as swells abruptly force the towing vessel to different towing speeds, the presence of seismic equipment such as module cans and depth controllers along the streamer, as well as biological related flow phenomena, e.g., with barnacle growth. We also mention the wake of the towing vessel and ambient turbulence that often is present in the ocean.
All of these uncontrolled conditions of the ocean operating environment will result in elevated levels of noise and make it difficult to acquire data with sufficient quality. Different types of ocean ambient noise such as seismic interference, noise from oceanic traffic, and noise from marine creatures also affect seismic data. These types of noise will not be addressed in any detail in this paper. However, it should be recognized that they often contribute significantly to the total noise level.
As the ratio of inertial to viscous fluid forces, known as the Reynolds number, becomes large, a statistically axisymmetric turbulent boundary layer (TBL) is formed around a streamer cable. In fluid mechanics the TBL refers to a thickness beyond which the velocity is
essen-tially the free-stream velocityU01. This is customarily defined as the distance from a surface or wall to the point where the time-averaged velocity isu(y= 0.99U0). This is often denoted δ99. Note that in an ocean environment the free-streamU0might contain ambient turbulence, characterized by low intensities and large scales. It differs from the more intense small-scale TBL formed by the relative motion between the cable surface and the surrounding water.
Apart from the recent work byCipolla and Keith(2008) little or no previous work has dealt with the turbulent flow around sea-towed streamer cables. Industrial flows are significantly more complex than what is normally experienced in a laboratory environment, or what is sim-ulated on computers. In the present work we seek insight into the TBL that surrounds the streamer cables in an ocean environment. Visualizations of the boundary layer based on a dye release experiment,along with noise recordings are presented and analyzed. In addition, we will consider the placement and density of hydrophones inside the streamer cable in an effort to determine the optimal configuration in order to obtain the best possible signal-to-noise ratio (SNR).
4.2.1 Previous works
The flow past a circular cylinder is a frequently studied problem in fluid mechanics. Unfortu-nately, most of the work performed has been on steady flow normal to the cylinder axis. Towed streamers are typically aligned in an angle|α| ≤15◦with the flow, and the available literature for such alignments is more limited. The flow on axially symmetric cylinders havingα= 0◦, was studied byWillmarth and Yang(1970),Denli and Landweber(1979), andWillmarth et al.
(1976). They investigated wall pressure fluctuations in conjunction with the mean velocity profiles. Furthermore,Willmarth and Sharma(1984) andSnarski and Lueptow(1995) also per-formed similar studies in whuch turbulent flow properties were investigated. In the latter study, two different groups of fluctuations in the TBL surrounding a cylinder were identified. The first was low-frequency high-energy fluctuations originating from the outer parts of the boundary layer. The second was in the form of small-scale high-frequency disturbances related to the so-called TBL burst-sweep cycle. Based on measurements,Lueptow et al.(1985) andFurey (2005) provided statistics of the distribution of the Reynolds stresses within the TBL atα= 0◦. InCipolla and Keith(2003) andKeith et al.(2005) details about how the boundary layer thick-ness scale with the streamer length were presented. Recent work byKeith et al.(2008) also provided measurements on the boundary layer thickness on a long cylinder in a water-tank. In this study, they report a thicker boundary layer compared to boundary layers developed in wind tunnels, water tunnels, or pipe flows. Examples of such measurements and simulations are the work by Willmarth et al.(1976) andTutty(2008a), who indicate a TBL thickness for a seismic cable in the range of2.5to5.0 cm.
Neves and Moin(1994a,b) conducted numerical simulations of axial flow and presented de-tailed turbulence statistics.
Lueptow et al.(1985),Heenan and Morrison(2002a), and a number of other researchers, have shown that even small misalignments of the axis of the cylinder relative to the mean tow-ing direction will cause asymmetry of the boundary layer. As a result the fluctuattow-ing wall pressure levels around the cylinder are significantly modified.
1U0is here measured using the cable as the reference system.
Based on water-tank experiments,Bull and Dekkers(1993) observed that vortex shedding might occur for a limited range of Reynolds number for long cylinders (with the length to diameter ratioL/d≈3000) at inclination angles as low asα= 1◦. They also suggested that the vortex shedding might be relevant to turbulence-generation (and thus also to noise-generation) in thick axisymmetric TBLs. On the other hand, in wind tunnel experiments performed byHeenan and Morrison (2002a,b), no vortex shedding was observed for inclination between0-6◦. Their hypothesis was that low-frequency noise was caused by streamer cable oscillation or buckling. However, their experiment was performed on a rather short cylinder (smallL/dratio), which seems to suggest that the boundary layer, might not have had time to develop sufficiently for shedding to occur.
It should be noted, that cross-flow induced vortices at small angles could remain attached to the surface and remain within the boundary layer. This phenomenon is sometimes referred to as
’trailing vortices’, see e.g.,Ramberg(1983) andThomson and Morrison(1971). The spectral characteristics of flow over a cylinderL/d = 23at different inclination angles were investi-gated bySnarski(2004). He reported significant Reynolds number effects for small inclination angles, and suggested that the energy spectra for high Reynolds number flow might be built up of contributions from both low-frequency large-scale shed vorticity and higher frequency small-scale boundary layer turbulence.
From a geophysical standpoint, investigation to identify sources of noise on streamer cables was reported bySchoenberger and Mifsud(1974), and byFulton(1985). Since then, a signif-icant amount of effort within the seismic industry has focused on improving streamer system technology to reduce the effects of many of the identified sources of noise. BothPeacock et al.
(1983),Bjelland(1993), andDowling(1998) contributed to the understanding of noise genera-tion mechanisms related to fluid-filled streamer cables. The primary mechanism was identified to be longitudinal bulge-wave propagation inside the cable that created low-frequency noise.
Brink and Spackman(2004) andDowle(2006) showed that modern foam-filled streamer cables are less sensitive to such internal bulge-waves. Important recent contribution to the understand-ing of flow-noise on streamer cables are the works byKnight (1996) andCipolla and Keith (2008).
As a result of design improvements, it can be concluded that the relative importance of flow-noise compared to other types of flow-noise has increased, c.f.Elboth et al.(2009b). To further reduce the level of recorded noise during seismic acquisition, the influence of flow-noise needs to be addressed.