Water velocity variations in 3D seismic processing

Wombell, R.

doi:10.1190/1.1826447

Cited by 9 publications

(7 citation statements)

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“…When processing 2D, 3D and 4D seismic surveys of regions of high oceanic variability, such as Rockall Trough, it may be necessary to employ techniques that can solve for and then remove the effect of significant fluctuations in water layer sound speed over time periods as short as a few hours and distances as short as a kilometre. The current industry standard solution is known as 'cold-water statics' (Wombell 1997;Xu & Pham 2003). This approach corrects for variation in the average sound speed in the water layer using a layer replacement method borrowed from land-based seismic processing.…”

Section: Implications For Seismic Surveyingmentioning

confidence: 99%

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Seismic imaging of variable water layer sound speed in Rockall Trough, NE Atlantic and implications for seismic surveying in deep water

Jones

Sutton

Hardy

et al. 2010

PGC

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Natural variation in sound speed within ocean water can degrade sub-seabed images from 2D, 3D and 4D seismic reflection datasets. Degrading effects include vertical offset of reflections between adjacent or intersecting sail lines and difficulties in suppressing multiples from water layer reverberations. Here we investigate water layer variability in Rockall Trough, offshore Ireland, in water depths ranging from 200 m to 3.5 km. A compilation of vertical sound speed profiles, calculated from temperature and salinity profiles obtained by probes lowered from ships, shows that the mean sound speed in the water layer mostly varies between 1490 and 1500 m s 21 . Vertical offsets of up to about 15 ms two-way travel time are predicted at seismic line intersections. A significant amount of the total observed sound speed variability can occur along a single seismic sail line. These effects result mainly from spatial and temporal fluctuations in the thicknesses of, and vertical sound speed gradients within, an upper layer of North Atlantic Central Water and a mid-depth layer of Mediterranean Outflow Water. Seismic sections across Rockall Trough commonly show lateral variability in reflectivity within these same two water layers. Some reflective packages contain lens-shaped structures consisting of reflective rims and transparent cores and with diameters between 10 and 50 km. Other reflective packages have abrupt, almost vertical boundaries and no distinct transparent core. We infer that the lateral boundaries of the reflective packages are likely to be associated with significant variations in average water layer sound speed. When processing 2D, 3D and 4D seismic surveys of regions of high oceanic variability, such as Rockall Trough, it is necessary to employ techniques that can solve for and then remove the effect of significant fluctuations in water layer sound speed over time periods as short as a few hours and distances as short as a kilometre.

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Section: Implications For Seismic Surveyingmentioning

confidence: 99%

“…Variation in sound speed within ocean water can cause problems in sub-seabed seismic reflection imaging (Wombell 1997;Barley 1999;Bertrand & Macbeth 2003;Mackay et al 2003;Xu & Pham 2003;Lacombe et al 2006Lacombe et al , 2009. Water layer sound speed variations arise from natural variations in temperature and salinity associated with oceanic currents.…”

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confidence: 99%

Seismic imaging of variable water layer sound speed in Rockall Trough, NE Atlantic and implications for seismic surveying in deep water

Jones

Sutton

Hardy

et al. 2010

PGC

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“…Other authors have suggested techniques to correct for seawater velocity variations with varying degrees of success. Wombell (1997) presented a static correction which gives a first‐order solution to the problem. MacKay et al .…”

Section: Correction Techniquementioning

confidence: 99%

Repeatability enhancement in deep‐water permanent seismic installations: a dynamic correction for seawater velocity variations

Bertrand

MacBeth

2005

Geophysical Prospecting

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A B S T R A C TWith the increasing use of permanently installed seismic installations, many of the issues in time-lapse seismic caused by the lack of repeatability can be reduced. However, a number of parameters still influence the degree of reliability of 4D seismic data. In this paper, the specific impact of seawater velocity variations on time-lapse repeatability is investigated in a synthetic study. A zero-lag time-lapse seabed experiment with no change in the subsurface but with velocity changes in the water column is simulated. The velocity model in the water column is constant for the baseline survey while the model for the repeat survey is heterogeneous, designed from sea salinity and temperature measurements in the West of Shetlands. The difference section shows up to 80% of residual amplitude, which highlights the poor repeatability. A new dynamic correction which removes the effect of seawater velocity variations specifically for permanent installations is developed. When applied to the synthetic data, it reduces the difference residual amplitude to about 3%. This technique shows substantial improvement in repeatability beyond conventional time-lapse cross-equalization. I N T R O D U C T I O NOne of the main issues encountered during 4D seismic experiments is the degree of repeatability between successive surveys. It often determines the degree of confidence in the interpretation of the time-lapse signature. Metrics have been developed to quantify that degree of confidence. The most used are the NRMS (which measures the normalized root mean square difference residual amplitude) and the predictability (which measures the correlation between two traces). Their equations are given in Appendix A. Over the past years, factors affecting repeatability have been studied extensively and solutions or guidelines have been laid out in order to minimize their effects. From the acquisition point of view, dedicated 4D surveys have proved to be more repeatable than the use of legacy data sets. Swanston et al. (2003) compared the repeatabilities obtained for legacy and 4D dedicated sur- * Now at: veys. In their examples, dedicated 4D experiments tended to achieve an NRMS value of around 30% to 40% while it is more likely to exceed 60% for 4D experiments using data not purposely acquired. Such experiments include the Alba Field (towed streamers and seabed receivers) where the NRMS is 89% (Hanson et al. 2003). Similarly, dedicated 4D processing helps to reduce the discrepancies between data sets. Smith et al. (2001) showed that careful 4D reprocessing reduced the difference residual amplitude from 74.6% to 41.3%. New 4D specific processing techniques have also been developed to remove non-production-related difference signals. Cross-equalization (Ross, Cunningham and Weber 1996), warping (Rickett and Lumley 2001), singular-value decomposition (Reid et al. 2005) and geostatistics (Lecerf and Coleou 2002) are some of the methods that are now used to improve the quality of the 4D signature. However, some parameters, often ...

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“…At the same time, water properties can vary relatively rapidly, both temporally and spatially, as a result of several oceanic phenomena, such as the interaction between warm/cold water fronts, eddies, internal waves, plumes, and currents, to name a few. Particularly in deep water areas, if variations of water properties are not properly accounted during the seismic processing, lateral discontinuities (or jitter) on crossline sections can lead to stack deterioration and migration artifacts (WOMBELL, 1996;BERTRAND et al, 2004;LACOMBE et al, 2009;GUERRA et al, 2015;AYOBAMI et al, 2020).…”

Section: Introductionmentioning

confidence: 99%

“…For streamer data, a number of methods have been already formulated, ranging from straightforward static corrections (WOMBELL, 1996) and NMO adjustments of traveltimes (LACOMBE et al, 2009), to more detailed characterizations obtained from reflections occurring in the water column (BRIGHT et al, 2015). These reflection events are related to impedance contrasts produced by termohaline variations of the water properties, which in turn allows detailed visualization of some oceanic processes (HOLBROOK et al, 2003;SONG et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

The structure of water-layer sensitivity kernels in OBN-type acquisition geometries

2021

Proceeding of the 17th International Congress of the Brazilian Geophysical

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The water column is usually taken as an homogeneous velocity layer in OBN-type marine seismic surveys. Heterogeneities caused by salinity and temperature variations, however, may introduce significant artifacts resulting in inaccurate interpretation of sub-sea floor geologic features, especially accounting the long time spans of seismic acquisitions that take place in ultra-deep ocean environments. Here we investigate the underlying structure of sensitivity kernels in this horizontally distributed acquisition geometry, mostly focused on the development of improved water layer velocity reconstructions from bottom node data. Two possible formulations are here considered, relating first arrivals with velocity perturbations, where a reference oceanic state and some overall shape/pattern of the velocity perturbations could be known a priori. The results are expected to provide guidelines for adapted inversion schemes, directed to more detailed water layer velocity reconstruction.

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Water velocity variations in 3D seismic processing

Cited by 9 publications

References 0 publications

Seismic imaging of variable water layer sound speed in Rockall Trough, NE Atlantic and implications for seismic surveying in deep water

Seismic imaging of variable water layer sound speed in Rockall Trough, NE Atlantic and implications for seismic surveying in deep water

Repeatability enhancement in deep‐water permanent seismic installations: a dynamic correction for seawater velocity variations

The structure of water-layer sensitivity kernels in OBN-type acquisition geometries

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