2
Physical Properties of Marine Sediments
54
acoustic and elastic properties. Table 2.3 summarizes the cores used for this sediment classification.
2.5.1
Full Waveform Core Logs as
Acoustic Images
That terrigenous, calcareous and biogenic siliceous sediments differ distinctly in their acoustic
properties is shown by four transmission seismogram sections in Figure 2.17. Terrigenous
sediments from the Bengal Fan (40KL, 47KL) are
composed of upward-fining sequences of turbidites characterized by upward decreasing attenuations and P-wave velocities. Coarse-grained
basal sandy layers can easily be located by lowfrequency waveforms and high P-wave velocities.
Calcareous sediments from the Rio Grande Rise
(GeoB2821-1) in the western South Atlantic also
exhibit high- and low-frequency signals which
scarcely differ in their P-wave velocities. In these
sediments high-frequency signals indicate finegrained nannofossil ooze while low-frequency
signals image coarse-grained foraminiferal ooze.
The sediment core from the Meteor Rise
(PS2567-2) in the Antarctic Ocean is composed of
diatomaceous and foraminiferal-nannofossil ooze
deposited during an advance and retreat of the
Polar Frontal Zone in glacial and interglacial
stages. Opal-rich, diatomaceous ooze can be identified from high-frequency signals while foraminiferal-nannofossil ooze causes higher attenuation
and low-frequency signals. P-wave velocities
again only show smooth variations.
Acoustic images of the complete core lithologies present the colour-encoded graphics of the
transmission seismograms, in comparison to the
lithology derived from visual core inspection (Fig.
2.18). Instead of normalized transmission seismograms instantaneous frequencies are displayed
here (Taner et al. 1979). They reflect the dominant
frequency of each transmission seismogram as
time-dependent amplitude, and thus directly
indicate the attenuation. Highly attenuated lowfrequency seismograms appear as warm red to
white colours while parts with low attenuation and
high-frequency seismograms are represented by
cool green to black colours.
In these attenuation images the sandy turbidite bases in the terrigenous cores from the
Bengal Fan (40KL, 47KL) can easily be distinguished. Graded beddings can also be identified
from slightly prograding phases and continuously
decreasing travel times. In contrast, the transition
to 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 increased
attenuation can be observed by slightly warmer
colours at higher transmission times (> 140 µs). In
this part of the core (> 3.2 m) sediments are mainly
composed of coarse-grained foraminiferal ooze,
while farther downcore (3.2 - 5.6 m) fine-grained
nannofossils prevail in the pelagic sediments.
The acoustic image of the calcareous core from
the Rio Grande Rise (GeoB2821-1) shows much
more lithological changes than the visual core
description. Cool colours between 1.5 - 2.5 m
depth indicate unusually fine grain sizes (Breitzke
1997). Alternately yellow/red and blue/black
colours in the lower part of the core (> 6.0 m)
reflect an interlayering of fine-grained nannofossil
and coarse-grained foraminiferal ooze. Dating by
orbital tuning shows that this interlayering coincides with the 41 ky cycle of obliquity (von
Dobeneck and Schmieder 1999) so that finegrained oozes dominate during glacial and coarsegrained oozes during interglacial stages.
The opal-rich diatomaceous sediments in core
PS2567-2 from the Meteor Rise are characterized
by a very low attenuation. Only 2 - 3 calcareous
layers with significantly higher attenuation
(yellow and red colours) can be identified as
prominent lithological changes.
2.5.2
P- and S-Wave Velocity,
Attenuation, Elastic Moduli and
Permeability
As the acoustic properties of water-saturated
sediments are strongly controlled by the amount
and distribution of pore space, cross plots of Pwave velocity and attenuation coefficient versus
porosity clearly indicate the different bulk and
elastic properties of terrigenous and biogenic
sediments and can thus be used for an acoustic
classification of the lithology. Additional S-wave
velocities (and attenuation coefficients) and
elastic moduli estimated by least-square inversion
specify the amount of bulk and shear moduli
which contribute to the P-wave velocity (Breitzke
2000).
The cross plots of the P-wave parameters of
the four cores considered above illustrate that
terrigenous, calcareous and diatomaceous
sediments can uniquely be identified from their
position in both diagrams (Fig. 2.19). In terri-
Physical Properties of Marine Sediments
54
acoustic and elastic properties. Table 2.3 summarizes the cores used for this sediment classification.
2.5.1
Full Waveform Core Logs as
Acoustic Images
That terrigenous, calcareous and biogenic siliceous sediments differ distinctly in their acoustic
properties is shown by four transmission seismogram sections in Figure 2.17. Terrigenous
sediments from the Bengal Fan (40KL, 47KL) are
composed of upward-fining sequences of turbidites characterized by upward decreasing attenuations and P-wave velocities. Coarse-grained
basal sandy layers can easily be located by lowfrequency waveforms and high P-wave velocities.
Calcareous sediments from the Rio Grande Rise
(GeoB2821-1) in the western South Atlantic also
exhibit high- and low-frequency signals which
scarcely differ in their P-wave velocities. In these
sediments high-frequency signals indicate finegrained nannofossil ooze while low-frequency
signals image coarse-grained foraminiferal ooze.
The sediment core from the Meteor Rise
(PS2567-2) in the Antarctic Ocean is composed of
diatomaceous and foraminiferal-nannofossil ooze
deposited during an advance and retreat of the
Polar Frontal Zone in glacial and interglacial
stages. Opal-rich, diatomaceous ooze can be identified from high-frequency signals while foraminiferal-nannofossil ooze causes higher attenuation
and low-frequency signals. P-wave velocities
again only show smooth variations.
Acoustic images of the complete core lithologies present the colour-encoded graphics of the
transmission seismograms, in comparison to the
lithology derived from visual core inspection (Fig.
2.18). Instead of normalized transmission seismograms instantaneous frequencies are displayed
here (Taner et al. 1979). They reflect the dominant
frequency of each transmission seismogram as
time-dependent amplitude, and thus directly
indicate the attenuation. Highly attenuated lowfrequency seismograms appear as warm red to
white colours while parts with low attenuation and
high-frequency seismograms are represented by
cool green to black colours.
In these attenuation images the sandy turbidite bases in the terrigenous cores from the
Bengal Fan (40KL, 47KL) can easily be distinguished. Graded beddings can also be identified
from slightly prograding phases and continuously
decreasing travel times. In contrast, the transition
to 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 increased
attenuation can be observed by slightly warmer
colours at higher transmission times (> 140 µs). In
this part of the core (> 3.2 m) sediments are mainly
composed of coarse-grained foraminiferal ooze,
while farther downcore (3.2 - 5.6 m) fine-grained
nannofossils prevail in the pelagic sediments.
The acoustic image of the calcareous core from
the Rio Grande Rise (GeoB2821-1) shows much
more lithological changes than the visual core
description. Cool colours between 1.5 - 2.5 m
depth indicate unusually fine grain sizes (Breitzke
1997). Alternately yellow/red and blue/black
colours in the lower part of the core (> 6.0 m)
reflect an interlayering of fine-grained nannofossil
and coarse-grained foraminiferal ooze. Dating by
orbital tuning shows that this interlayering coincides with the 41 ky cycle of obliquity (von
Dobeneck and Schmieder 1999) so that finegrained oozes dominate during glacial and coarsegrained oozes during interglacial stages.
The opal-rich diatomaceous sediments in core
PS2567-2 from the Meteor Rise are characterized
by a very low attenuation. Only 2 - 3 calcareous
layers with significantly higher attenuation
(yellow and red colours) can be identified as
prominent lithological changes.
2.5.2
P- and S-Wave Velocity,
Attenuation, Elastic Moduli and
Permeability
As the acoustic properties of water-saturated
sediments are strongly controlled by the amount
and distribution of pore space, cross plots of Pwave velocity and attenuation coefficient versus
porosity clearly indicate the different bulk and
elastic properties of terrigenous and biogenic
sediments and can thus be used for an acoustic
classification of the lithology. Additional S-wave
velocities (and attenuation coefficients) and
elastic moduli estimated by least-square inversion
specify the amount of bulk and shear moduli
which contribute to the P-wave velocity (Breitzke
2000).
The cross plots of the P-wave parameters of
the four cores considered above illustrate that
terrigenous, calcareous and diatomaceous
sediments can uniquely be identified from their
position in both diagrams (Fig. 2.19). In terri-