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2Physical Properties of Marine SedimentsFig. 2.18 Ultrasonic images of the transmission measurements on cores 40KL, 47KL, GeoB2821-1 and PS2567-2 retrieved from different terrigenous and biogenicsedimentation environments. Displayed are the colour-encoded instantaneous frequencies of the transmission seismograms and the lithology derived from visual coreinspections.56
2.6 Sediment Echosoundinggenous sediments (40KL, 47KL) P-wave velocitiesand attenuation coefficients increase with decreasingporosities. Computed S-wave velocities arevery low (≈ 60 - 65 m s -1 ) and almost independentof porosity (Fig. 2.20a) whereas computed S-waveattenuation coefficients (at 400 kHz) vary stronglyfrom 4·10 3 dB m -1 in fine-grained sediments to1.5·10 4 dB m -1 in coarse-grained sediments (Fig.2.20b). Accordingly, shear moduli are also low anddo not vary very much (Fig. 2.21b) so that higherP-wave velocities in terrigenous sediments mainlyresult from higher bulk moduli (Fig. 2.21a).If calcareous, particularly foraminiferal components(FNO) are added to terrigenous sedimentsporosities become higher (≈ 70 - 80%, 47KL). P-wave velocities slightly increase from their minimumof 1475 m s -1 at 70% porosity to 1490 m s -1 at80% porosity (Fig. 2.19a) mainly due to an increasein the shear moduli from about 6.5 to 8.5 MPawhereas bulk moduli remain almost constant atabout 3000 MPa (Fig. 2.21b). However, a muchmore pronounced increase can be observed in theP-wave attenuation coefficients (Fig. 2.19b). Forporosities of about 80% they reach the samevalues (about 200 dB m -1 ) as terrigenous sedimentsof about 55% porosity, but can easily bedistinguished because of higher P-wave velocitiesin terrigenous sediments. S-wave attenuationcoefficients increase as well in these hemipelagicsediments from about 1·10 5 dB m -1 at 65% porosityto about 2.5·10 5 dB m -1 at 80% porosity (Fig.2.20b), and are thus even higher than interrigenous sediments.Calcareous foraminiferal and nannofossil oozes(NFO) show similar trends in both P-wave and S-wave parameters as terrigenous sediments, but areshifted to higher porosities due to their additionalintraporosities (GeoB2821-1; Figs. 2.19, 2.20).In diatomaceous oozes P- and S-wave wavevelocities increase again though porosities arevery high (>80%, PS2567-2; Figs. 2.19, 2.20). Here,the diatom shells build a very stiff frame whichcauses high shear moduli and S-wave velocities(Fig. 2.21). It is this increase in the shear moduliwhich only accounts for the higher P-wavevelocities while the bulk moduli remain almostconstant and are close to the bulk modulus of seawater (Fig. 2.21). P-wave attenuation coefficientsare very low in these high-porosity sediments(Fig. 2.19b) whereas S-wave attenuation coefficientsare highest (Fig. 2.20b).Permeabilities estimated from the least squareinversion mainly reflect the attenuation characteristicsof the different sediment types (Fig. 2.22).They reach lowest values of about 5·10 -14 m 2 infine-grained clayey mud and nannofossil ooze.Highest values of about 5·10 -11 m 2 occur in diatomaceousooze due to their high porosities.Nevertheless, it should be kept in mind that thesepermeabilities are only estimates based on theinput parameters and assumptions incorporated inBiot-Stoll’s model. For instance one of theseassumptions is that only mean grain sizes areused, but the influence of grain size distributionsis neglected. Additionally, the total porosity isusually used as input parameter for the inversionscheme without differentiation between inter- andintraporosities. Comparisons of these estimatedpermeabilities with direct measurements unfortunatelydo not exist up to know.2.6 Sediment EchosoundingWhile ultrasonic measurements are used to studythe structure and composition of sediment cores,sediment echosounders are hull-mounted acousticsystems which image the upper 10-200 m of sedimentcoverage by remote sensing surveys. They operatewith frequencies around 3.5-4.0 kHz. The examplespresented here were digitally recorded with thenarrow-beam Parasound echosounder andParaDIGMA recording system (Spieß 1993).2.6.1 Synthetic SeismogramsFigure 2.23 displays a Parasound seismogramsection recorded across an inactive channel of theBengal Fan. The sediments of the terrace weresampled by a 10 m long piston core (47KL). Itsacoustic and bulk properties can either be directlycompared to the echosounder recordings or bycomputations of synthetic seismograms. Suchmodeling requires P-wave velocity and wet bulkdensity logs as input parameters. From theproduct of both parameters acoustic impedances I= v P·ρ are calculated. Changes in the acousticimpedance cause reflections of the normallyincident acoustic waves. The amplitude of suchreflections is determined by the normal incidencereflection coefficient R = (I 2-I 1) / (I 2+I 1), with (I 1)and (I 2) being the impedances above and belowthe interface. From the series of reflectioncoefficients the reflectivity can be computed asimpulse response function, including all internal57
Page 3 and 4: 2.2 Porositiy and Wet Bulk Densityi
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Page 17 and 18: 2.4 Acoustic and Elastic Properties
Page 27 and 28: 2.5 Sediment ClassificationFig. 2.1
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Page 39 and 40: AppendixFig. 2.25 a, b Parasound se
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Page 43 and 44: References2.7 ProblemsProblem 1What
Page 45: Referencespressional wave in a poro

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