2 Physical Properties of Marine Sediments - Blogs Unpad

2Physical Properties of Marine Sedimentsacoustic and elastic properties. Table 2.3 summarizesthe cores used for this sediment classification.2.5.1 Full Waveform Core Logs asAcoustic ImagesThat terrigenous, calcareous and biogenic siliceoussediments differ distinctly in their acousticproperties is shown by four transmission seismogramsections in Figure 2.17. Terrigenoussediments from the Bengal Fan (40KL, 47KL) arecomposed of upward-fining sequences of turbiditescharacterized by upward decreasing attenuationsand P-wave velocities. Coarse-grainedbasal sandy layers can easily be located by lowfrequencywaveforms and high P-wave velocities.Calcareous sediments from the Rio Grande Rise(GeoB2821-1) in the western South Atlantic alsoexhibit high- and low-frequency signals whichscarcely differ in their P-wave velocities. In thesesediments high-frequency signals indicate finegrainednannofossil ooze while low-frequencysignals image coarse-grained foraminiferal ooze.The sediment core from the Meteor Rise(PS2567-2) in the Antarctic Ocean is composed ofdiatomaceous and foraminiferal-nannofossil oozedeposited during an advance and retreat of thePolar Frontal Zone in glacial and interglacialstages. Opal-rich, diatomaceous ooze can be identifiedfrom high-frequency signals while foraminiferal-nannofossilooze causes higher attenuationand low-frequency signals. P-wave velocitiesagain only show smooth variations.Acoustic images of the complete core lithologiespresent the colour-encoded graphics of thetransmission seismograms, in comparison to thelithology derived from visual core inspection (Fig.2.18). Instead of normalized transmission seismogramsinstantaneous frequencies are displayedhere (Taner et al. 1979). They reflect the dominantfrequency of each transmission seismogram astime-dependent amplitude, and thus directlyindicate the attenuation. Highly attenuated lowfrequencyseismograms appear as warm red towhite colours while parts with low attenuation andhigh-frequency seismograms are represented bycool green to black colours.In these attenuation images the sandy turbiditebases in the terrigenous cores from theBengal Fan (40KL, 47KL) can easily be distinguished.Graded beddings can also be identifiedfrom slightly prograding phases and continuouslydecreasing travel times. In contrast, the transitionto calcareous, pelagic sediments in the upper part(> 5.6 m) of core 47KL is rather difficult to detect.Only above 3.2 m depth a slightly increasedattenuation can be observed by slightly warmercolours at higher transmission times (> 140 µs). Inthis part of the core (> 3.2 m) sediments are mainlycomposed of coarse-grained foraminiferal ooze,while farther downcore (3.2 - 5.6 m) fine-grainednannofossils prevail in the pelagic sediments.The acoustic image of the calcareous core fromthe Rio Grande Rise (GeoB2821-1) shows muchmore lithological changes than the visual coredescription. Cool colours between 1.5 - 2.5 mdepth indicate unusually fine grain sizes (Breitzke1997). Alternately yellow/red and blue/blackcolours in the lower part of the core (> 6.0 m)reflect an interlayering of fine-grained nannofossiland coarse-grained foraminiferal ooze. Dating byorbital tuning shows that this interlayering coincideswith the 41 ky cycle of obliquity (vonDobeneck and Schmieder 1999) so that finegrainedoozes dominate during glacial and coarsegrainedoozes during interglacial stages.The opal-rich diatomaceous sediments in corePS2567-2 from the Meteor Rise are characterizedby a very low attenuation. Only 2 - 3 calcareouslayers with significantly higher attenuation(yellow and red colours) can be identified asprominent lithological changes.2.5.2 P- and S-Wave Velocity,Attenuation, Elastic Moduli andPermeabilityAs the acoustic properties of water-saturatedsediments are strongly controlled by the amountand distribution of pore space, cross plots of P-wave velocity and attenuation coefficient versusporosity clearly indicate the different bulk andelastic properties of terrigenous and biogenicsediments and can thus be used for an acousticclassification of the lithology. Additional S-wavevelocities (and attenuation coefficients) andelastic moduli estimated by least-square inversionspecify the amount of bulk and shear moduliwhich contribute to the P-wave velocity (Breitzke2000).The cross plots of the P-wave parameters ofthe four cores considered above illustrate thatterrigenous, calcareous and diatomaceoussediments can uniquely be identified from theirposition in both diagrams (Fig. 2.19). In terri-54