2015 SEG NSAP conference_Vp in near waterbottom_Copyright

2015 SEG NSAP conference_Vp in near waterbottom_Copyright

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  Variations in compressionwave velocities in near-waterbottom sediments Alan J. Foley*, Svitzer Surveys Summary Geotechnical engineers performing site investigations in the near seabed routinely discard geophysical data as inaccurate, referring to these data as “remote sensing”. The reason for this is that their results of in-situ testing or laboratory tests on cores do not match the results of geophysical surveys. This may be because geophysical measurements in unconsolidated sediments are ambiguous or just plain wrong. In an unconsolidated section, typically an active river delta or bay with a high rate of deposition or large amount of organic material, the start of the normally consolidated section is difficult to determine and may be at a greater depth than experience would indicate. This is highlighted by the lack of definition of the seabed or "mudline" in drilling terminology; definitions are made in terms of acoustic contrast, i.e. p-wave impedance, visibility or navigability of a vessel. There is no absolute definition of the seabed in terms of load-bearing capacity or s-wave impedance, primarily due to the difficulty of measuring s-wave signals or testing the seabed. However the drilling community, with its focus on formation pressures and mudweights, has some insight into the transition from an unconsolidated section into normal compaction. In this highly saturated condition, with saturation values in the 60 - 80% range, the pores of the unconsolidated section are in hydraulic communication with the seabed and pore pressures increase with depth on the hydrostatic curve, any overpressure in this zone will be dissipated through the water column. As depth increases through this zone shear wave velocities through the sediment, having commenced at very low values in the order of tens or low hundreds of meters per second, rise. However the behavior of p-wave velocities is quite different. Commencing at the mudline p- wave velocities are in the range of the overlying fluid, where clays in brine/sediment mixture form sols. As burial depths increase the velocity in the fluid column continues in the 1400 - 1500m/s range until a point above the boundary of normal compaction when the fluid velocity approaches the velocity of the sediment grains of the formation asymptotically; in most cases this will be a p-wave velocity decrease of as much as 30% relative to fluid velocity. As burial depth of the sediment column increases further sediment grain velocities increase until they surpass those of the overlying fluid column and velocity increase follows the normal compaction curve. Introduction In marine near-seabed environments there is a transition of the mode of transmission of seismic signals as the sediment compaction increases and saturation decreases. In the typical marine seismic section acoustic signals are transmitted through the water column to the seabed, thence signals are transmitted to depth by the seabed sediment to reflectors below the seabed. The two modes of transmission must either coexist in a zone where both modes overlap, or transition at a discrete interface where one mode ceases and the other commences. Evidence of the dual mode transmission is rarely observed in field data, but sufficient occurrences of dual mode transmission in unconsolidated, saturated soils are documented to support the tandem modes transition from fluid to sediment grain transmission. The boundary point between the two modes is the start of the “normal” compaction curve. This is the point from which the effective stress on the sediment column increases linearly with depth and the sediment grains and pore spaces transmit p-wave energy in tandem. At this interface pore pressures can increase above the hydrostatic curve as sediment becomes consolidated and allows hydraulic sealing of pores from the seawater column. But where is this point and what happens to acoustic signals above the start of normal compaction? Theory and Method The underlying theory of this effect was described by Biot in 1956. He proposed that p-wave velocities are controlled by: a) The velocity of elastic waves through the sediment matrix b) The velocity of elastic waves through the pore fluid. The implication of Biot’s work, and the subsequent Biot- Gassman equations, is that there are three seismic wave velocities in unconsolidated, fluid saturated sediment: i) The s-wave velocity through the sediment matrix ii) The p-wave velocity through the sediment matrix iii) The p-wave velocity through the fluid within the matrix This p-wave behavior is neither intuitive nor regularly observed in practice. A brief review of the effect of this behavior explains the lack of observation of these variations. In any reflection or acoustic survey the first signal received will be that transmitted by the fluid transmission route through the near-seabed. The p-wave signal transmitted via sediment grains is slower than that of the fluid column, often by a factor of 2 or 3. Typical values of the fluid transmission velocity are 1200 – 1500m/s whilst sediment grain values are in the order of 400m/s. Therefore this lower velocity signal appears later in the reflection record on normal incidence travel paths and occurs rarely on offset receivers. In
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  P-wave variation inthe seabed boreholes and logged wells the highly saturated, unconsolidated section is behind the well conductor and not logged by any conventional sonic methods. Thus, the only instance of seismic velocities measurement in this unconsolidated zone is during site investigations. Usually only s-wave velocities are considered as they are a direct measurement of the stiffness or strength of the formation. Methods used for velocity determination are:  Reflection using a streamer or seabed array  Refraction using a seabed detector array  Transmission using a borehole or seismic cone system  Surface wave analysis  Crosshole tomography Reflection techniques are usually relied on in deepwater. Examples In rare cases both s-wave and p-wave velocities are measured in the same location. I use two examples to highlight the transition of p-wave velocities from those of the fluid column to those of the sediment grain, in the Fraser River Delta of Western Canada and in San Francisco Bay. The Fraser River example illustrates an interesting phenomenon. An asymptotic sonic velocity variation is described by Bowers (2002) in addressing formation overpressure. In figure 2, as the fluid p-wave velocities make their asymptotic approach to those of the sediment grain, the p-wave sediment grain velocity increases asymptotically toward the fluid velocity. In the San Francisco Bay example, the two transmission modes do not converge above the bottom of the sampling hole. Onshore in the Hanford boreholes the behavior of p-waves measured by Redpath(2007) is compared with that of s- waves in environments with differential compaction. The huge velocity contrast of the s-waves is not registered by p- waves. This may be attributed to p-wave transmission below the water table. To determine the reliability of measured velocities in saturated sediment calculate Poisson’s Ratio from the results. If measured Poisson’s Ratio values match the value of the sediment type the measurements are valid. Most lithified sediments have Poisson’s ratios in the 0.20 – 0.35 range whereas plastic mixtures, sols and incompressible liquids approach 0.5. The behavior of mixtures of solids and fluids is complex, especially if the solids are clay sized and immersed in brine, and beyond the scope of this presentation. Figure 1: San Francisco Bay p-wave borehole velocity survey (USGS, 1992)
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  P-wave variation inthe seabed Figure 2: Fraser River Delta, BC near surface velocities from both borehole, surface wave and refaction surveys from Park et al, 2005 Conclusions The measurement of changes in p-wave velocity in near water bottom environments is ambiguous. Care must be taken to use an appropriate technique. Velocity measurements affect many areas of offshore operations including: Jack up site investigation Seabed strength estimation for structural design Pore pressure estimation Well conductor installation Well tophole engineering Navigable waterway survey Shallow water flow analysis Accurate, unambiguous velocity measurement in the near seabed is a critical basis for lateral extrapolation of geotechnical data. The use of transmission velocity techniques can supplement and confirm reflection results. References Biot, M.A., 1956a, Theory of propagation of elastic waves in fluid-saturated porous solid, I: Low-frequency range: Journal of Acoustic Society of America, 28:168-178 Hamilton, E.L. 1976, Shear wave velocity versus depth in marine sediments: a review: Geophysics, 68:985-996 Gibbs, J.F. et al, 1992, USGS OFR 92-287 Seismic Velocities and Geologic Logs in San Francisco Bay Ivanov, J. et al, 2000, Mapping Poisson’s ratio of unconsolidated materials, SAGEEP 2000 Bowers, G. L., 2002, Detecting High Overpressure: The Leading Edge, 21, 174–177 Lee, M.W, 2003, USGS Bulletin 2197 Velocity Ratio and its Application to Predicting Velocities Park et al, 2005, Underwater MASW to evaluate stiffness of water-bottom sediments, The Leading Edge, 24, 724 – 728 Redpath, B.B., 2007, PNNL-16559 Downhole Measurements of Shear and Compression-Wave Velocities in Boreholes C4993, C4996, C4997 and C4998 at the Waste Treatment Plant DOE Hanford Site Acknowledgement I would like to acknowledge the assistance of Bruce B. Redpath in providing data, background and commentary for these observations, Dan Ebrom for several references and Steve Danbom for many helpful comments and suggestions. Joelle Barnes edited this paper. AJF 1/5/2015 ©2015 Society of Exploration Geophysicists, Australian Society of Exploration Geophysicists, Chinese Geophysical Society, Korean Society of Earth and Exploration Geophysicists, and Society of Exploration Geophysicists of Japan Link: http://www.seg.org/footer/segdl-tos