2 Physical Properties of Marine Sediments - Blogs Unpad

2.4 Acoustic and Elastic Propertiesabout one magnitude higher, and velocities aresignificantly lower than for P-waves. The consequenceis that S-waves are very difficult to recorddue to their high attenuation, though they are ofgreat value for identifying fine-scale variations inthe elasticity and microstructure of marine sediments.This is even valid if the low S-wave velocitiesare taken into account and lower frequenciesare used for S-wave measurements than for P-wave recordings.The two gray-shaded areas in Figure 2.12 marktwo frequency bands typical for ultrasonic studieson sediment cores (50 - 500 kHz) and sedimentechosounder surveys (0.5 - 10 kHz). They aredisplayed in order to point to one characteristic ofacoustic measurements. Attenuation coefficientsanalyzed from ultrasonic measurements on sedimentcores cannot directly be transferred tosediment echosounder or seismic surveys. Primarilythey only reflect the microstructure of thesediment. Rough estimates of the attenuation inseismic recordings from ultrasonic core measurementscan be derived if ultrasonic attenuation ismodeled, and attenuation coefficients are extrapolatedto lower frequencies by such model curves.2.4.2 Full Waveform Ultrasonic CoreLoggingTo measure the P-wave velocity and attenuationillustrated by Biot-Stoll’s model an automated, PCcontrolledlogging system was developed whichrecords and stores digital ultrasonic P-waveformstransmitted radially across marine sediment cores(Breitzke and Spieß 1993). These transmissionmeasurements can be done at arbitrary small depthTable 2.2 Physical properties of sediment grains, pore fluid and sediment frame used for the computation ofattenuation and phase velocity curves according to Biot-Stoll’s sediment model (Fig. 2.12).Parameter Sand Silt ClaySediment GrainsBulk Modulus Κ g [10 9 Pa] 38 38 38Density ρ g [g cm -3 ] 2.67 2.67 2.67Pore FluidBulk Modulus Κ f [10 9 Pa] 2.37 2.37 2.37Density ρ f [g cm -3 ] 1.024 1.024 1.024Viscosity η [10 -3 Pa⋅s] 1.07 1.07 1.07Sediment FrameBulk Modulus Κ m [10 6 Pa] 400 150 20Shear Modulus µ m [10 6 Pa] 240 90 12Poisson Ratio σ m0.25 0.25 0.25Pore SpacePorosity φ [%] 50 60 80Mean Grain Size d m [10 -6 m] 70 30 2Permeability κ [m 2 ] 5.4·10 -11 2.3·10 -12 7.1·10 -15Pore Size Parameter a = d m /3φ /(1-φ ) [10 -6 m] 23 15 2.7Ratio κ / a 2 0.1 0.01 0.001Structure Factor a' = 1-r 0 (1-φ -1 ) 1.5 1.3 1.1Constant r 00.5 0.5 0.547