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29. THE COMPRESSIONAL-WAVE VELOCITY OF AMAZON FAN ...Roger D. Flood, Carlos Pirmez, and Hezhu YinABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCESFIGURESFigure 1. Cross-plot of porosity and velocity data ...Figure 2. Cross-plot of calculated compressional- ...Figure 3. Downhole plots of porosity (measured on ...Figure 4. Downhole plots of porosity (measured on ...Figure 5. Downhole plots of porosity (measured on ...Figure 6. Downhole plots of porosity (measured on ...Figure 7. Downhole plots of porosity (measured on ...Figure 8. Downhole plots of porosity (measured on ...Figure 9. Downhole plots of porosity (measured on ...Figure 10. Time-depth plot for all Leg 155 sites....Figure 11. Theoretical relationships between ...Figure 12. Relationships between velocity, porosity, ...TABLESTable 1. Source of information used in the ...Table 3. Summary of K0 values used in calculating ...Table 4. Depth at each site as a function of traveltime.Flood, R.D., Piper, D.J.W., Klaus, A., and Peterson, L.C. (Eds.), 1997Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 15547729. THE COMPRESSIONAL-WAVE VELOCITY OF AMAZON FAN SEDIMENTS:CALCULATION FROM INDEX PROPERTIES AND VARIATION WITH CLAY CONTENT1Roger D. Flood,2 Carlos Pirmez,3 and Hezhu Yin4ABSTRACTAcoustic properties of unconsolidated marine sediments contain important information about sedimentary materials, andvelocity structure is important for relating time on seismic profiles to depth in the sediments. We have analyzed the velocitystructure of the Leg 155 Amazon Fan sediments from two independent perspectives. In situ compressional-wave velocitieswere calculated following the Biot model for sediment acoustic properties from shipboard measurements of porosity and graindensity. Some of the parameters for this calculation were derived from the comparison of calculated velocities with those deter-mined from wireline logs and applied to other sites. This approach suggests that surficial sediment deposits are normally con-solidated, but that debris-flow deposits and some buried levees are overconsolidated. We also identified some buried leveeswhere underconsolidated sediments directly underlie the debris-flow deposits. The velocity-depth information was used todetermine a time-depth curve at each site. The curves are similar above 200 meters below seafloor, but diverge at greaterdepths. Sediment depths for deeper layers determined aboard Leg 155 are 10–20 m less than initially determined. An analysisof the relationships between compressional-wave velocity and clay volume and between porosity and clay volume suggests thatclay minerals and framework grains bear the overburden stress in surficial levees and sand units (a matrix-supported structureor isostress condition). Framework grains bear the overburden stress in deeper levees and debris-flow (a grain-supported struc-ture or isostrain condition). These observations are all consistent with the upper ~400 m of the Amazon Fan being uncementedand unlithified, except for some restricted zones where iron sulfides (e.g., hydrotroilite) or gas hydrates may affect the sedimentacoustic properties.INTRODUCTIONCompressional-wave velocity is a primary sediment characteristicthat provides important information about the mechanical propertiesof those sediments that may be difficult to determine otherwise. Thecorrelation of cored sequences to seismic profiles also requires aknowledge of the vertical compressional-wave velocity structure.Seismic velocity information for continental margin settings tradi-tionally comes from the analysis of sonobuoy data and multichannelseismic data. When holes are drilled in a region, additional informa-tion on sediment velocity can come from logging or other downholeexperiments, from velocity measurement on recovered sediment, orthrough velocity calculation from index properties (e.g., density).Log information can also be used to generate synthetic seismogramsfor more detailed core-seismic correlation. Acoustic properties of un-consolidated sediments are sensitive to the mechanical properties ofthe sediments as well as to sediment lithology, thus a detailed under-standing of the acoustic properties of fan sediments may provide im-portant insights into the structure and processes active on the fan.During Ocean Drilling Program (ODP) Leg 155, 17 sites weredrilled on the Amazon Fan to study fan growth patterns, fan deposi-tional processes, and equatorial paleoclimate. Much of our under-standing of the structure of the Amazon Fan comes from interpreta-tion of seismic profiles, thus correlating seismic records to drill sitesis a high priority. Prior to Leg 155, compressional-wave velocity in-formation on the Amazon Fan came from regional sonobuoy studies(Houtz, 1977; Shipboard Scientific Party, 1995a). During Leg 155,wireline logs measured in situ compressional-wave velocity on por-tions of seven of the sites occupied. For sites and depth intervals thatwere not logged, an alternate approach is needed to estimate in situcompressional-wave velocity from routine laboratory measurements.Although velocity can often be measured in the laboratory on coredsediments, most of the sediment recovered from the Amazon Fanduring Leg 155 was gassy, and few laboratory velocity measure-ments could be made. As a result, we need to use an approach thatwill estimate in situ compressional-wave velocity from sediment in-dex properties that were measured aboard ship to make more accuratetime-depth calculations.Two methods have been used to estimate sediment compression-al-wave velocity from measured sediment properties. Hamilton(1980 and references therein) developed an approach based on exist-ing velocity measurements to predict acoustic properties in regionswhere few measurements are available. Stoll (1989) describes theBiot theory applied to sediment acoustic properties and an approachfor calculating acoustic properties from sediment properties such asporosity, grain size, permeability, and effective stress; however, thisapproach also requires that a number of other sediment properties berealistically estimated. This study reports an initial attempt to use thepredictive approach described by Stoll (1989) to determine sedimentcompressional-wave velocity from measured index properties. Thisinformation is then used to determine the time-depth relationship atsites drilled on the Amazon Fan and to provide independent evidenceon the mechanical


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