57
2.6
Sediment Echosounding
genous sediments (40KL, 47KL) P-wave velocities
and attenuation coefficients increase with decreasing porosities. Computed S-wave velocities are
very low (≈ 60 - 65 m s
-1
) and almost independent
of porosity (Fig. 2.20a) whereas computed S-wave
attenuation coefficients (at 400 kHz) vary strongly
from 4·10
3
dB m
-1
in fine-grained sediments to
1.5·10
4
dB m
-1
in coarse-grained sediments (Fig.
2.20b). Accordingly, shear moduli are also low and
do not vary very much (Fig. 2.21b) so that higher
P-wave velocities in terrigenous sediments mainly
result from higher bulk moduli (Fig. 2.21a).
If calcareous, particularly foraminiferal components (FNO) are added to terrigenous sediments
porosities become higher (≈ 70 - 80%, 47KL). Pwave velocities slightly increase from their minimum of 1475 m s
-1
at 70% porosity to 1490 m s
-1
at
80% porosity (Fig. 2.19a) mainly due to an increase
in the shear moduli from about 6.5 to 8.5 MPa
whereas bulk moduli remain almost constant at
about 3000 MPa (Fig. 2.21b). However, a much
more pronounced increase can be observed in the
P-wave attenuation coefficients (Fig. 2.19b). For
porosities of about 80% they reach the same
values (about 200 dB m
-1
) as terrigenous sediments of about 55% porosity, but can easily be
distinguished because of higher P-wave velocities
in terrigenous sediments. S-wave attenuation
coefficients increase as well in these hemipelagic
sediments from about 1·10
5
dB m
-1
at 65% porosity
to about 2.5·10
5
dB m
-1
at 80% porosity (Fig.
2.20b), and are thus even higher than in
terrigenous sediments.
Calcareous foraminiferal and nannofossil oozes
(NFO) show similar trends in both P-wave and Swave parameters as terrigenous sediments, but are
shifted to higher porosities due to their additional
intraporosities (GeoB2821-1; Figs. 2.19, 2.20).
In diatomaceous oozes P- and S-wave wave
velocities increase again though porosities are
very high (>80%, PS2567-2; Figs. 2.19, 2.20). Here,
the diatom shells build a very stiff frame which
causes high shear moduli and S-wave velocities
(Fig. 2.21). It is this increase in the shear moduli
which only accounts for the higher P-wave
velocities while the bulk moduli remain almost
constant and are close to the bulk modulus of sea
water (Fig. 2.21). P-wave attenuation coefficients
are very low in these high-porosity sediments
(Fig. 2.19b) whereas S-wave attenuation coefficients are highest (Fig. 2.20b).
Permeabilities estimated from the least square
inversion mainly reflect the attenuation characteristics of the different sediment types (Fig. 2.22).
They reach lowest values of about 5·10
-14
m
2
in
fine-grained clayey mud and nannofossil ooze.
Highest values of about 5·10
-11
m
2
occur in diatomaceous ooze due to their high porosities.
Nevertheless, it should be kept in mind that these
permeabilities are only estimates based on the
input parameters and assumptions incorporated in
Biot-Stoll’s model. For instance one of these
assumptions is that only mean grain sizes are
used, but the influence of grain size distributions
is neglected. Additionally, the total porosity is
usually used as input parameter for the inversion
scheme without differentiation between inter- and
intraporosities. Comparisons of these estimated
permeabilities with direct measurements unfortunately do not exist up to know.
2.6
Sediment Echosounding
While ultrasonic measurements are used to study
the structure and composition of sediment cores,
sediment echosounders are hull-mounted acoustic
systems which image the upper 10-200 m of sediment
coverage by remote sensing surveys. They operate
with frequencies around 3.5-4.0 kHz. The examples
presented here were digitally recorded with the
narrow-beam Parasound echosounder and
ParaDIGMA recording system (Spieß 1993).
2.6.1
Synthetic Seismograms
Figure 2.23 displays a Parasound seismogram
section recorded across an inactive channel of the
Bengal Fan. The sediments of the terrace were
sampled by a 10 m long piston core (47KL). Its
acoustic and bulk properties can either be directly
compared to the echosounder recordings or by
computations of synthetic seismograms. Such
modeling requires P-wave velocity and wet bulk
density logs as input parameters. From the
product of both parameters acoustic impedances I
= v P ·ρ are calculated. Changes in the acoustic
impedance cause reflections of the normally
incident acoustic waves. The amplitude of such
reflections is determined by the normal incidence
reflection coefficient R = (I 2 -I 1 ) / (I 2 +I 1 ), with (I 1 )
and (I 2 ) being the impedances above and below
the interface. From the series of reflection
coefficients the reflectivity can be computed as
impulse response function, including all internal
2.6
Sediment Echosounding
genous sediments (40KL, 47KL) P-wave velocities
and attenuation coefficients increase with decreasing porosities. Computed S-wave velocities are
very low (≈ 60 - 65 m s
-1
) and almost independent
of porosity (Fig. 2.20a) whereas computed S-wave
attenuation coefficients (at 400 kHz) vary strongly
from 4·10
3
dB m
-1
in fine-grained sediments to
1.5·10
4
dB m
-1
in coarse-grained sediments (Fig.
2.20b). Accordingly, shear moduli are also low and
do not vary very much (Fig. 2.21b) so that higher
P-wave velocities in terrigenous sediments mainly
result from higher bulk moduli (Fig. 2.21a).
If calcareous, particularly foraminiferal components (FNO) are added to terrigenous sediments
porosities become higher (≈ 70 - 80%, 47KL). Pwave velocities slightly increase from their minimum of 1475 m s
-1
at 70% porosity to 1490 m s
-1
at
80% porosity (Fig. 2.19a) mainly due to an increase
in the shear moduli from about 6.5 to 8.5 MPa
whereas bulk moduli remain almost constant at
about 3000 MPa (Fig. 2.21b). However, a much
more pronounced increase can be observed in the
P-wave attenuation coefficients (Fig. 2.19b). For
porosities of about 80% they reach the same
values (about 200 dB m
-1
) as terrigenous sediments of about 55% porosity, but can easily be
distinguished because of higher P-wave velocities
in terrigenous sediments. S-wave attenuation
coefficients increase as well in these hemipelagic
sediments from about 1·10
5
dB m
-1
at 65% porosity
to about 2.5·10
5
dB m
-1
at 80% porosity (Fig.
2.20b), and are thus even higher than in
terrigenous sediments.
Calcareous foraminiferal and nannofossil oozes
(NFO) show similar trends in both P-wave and Swave parameters as terrigenous sediments, but are
shifted to higher porosities due to their additional
intraporosities (GeoB2821-1; Figs. 2.19, 2.20).
In diatomaceous oozes P- and S-wave wave
velocities increase again though porosities are
very high (>80%, PS2567-2; Figs. 2.19, 2.20). Here,
the diatom shells build a very stiff frame which
causes high shear moduli and S-wave velocities
(Fig. 2.21). It is this increase in the shear moduli
which only accounts for the higher P-wave
velocities while the bulk moduli remain almost
constant and are close to the bulk modulus of sea
water (Fig. 2.21). P-wave attenuation coefficients
are very low in these high-porosity sediments
(Fig. 2.19b) whereas S-wave attenuation coefficients are highest (Fig. 2.20b).
Permeabilities estimated from the least square
inversion mainly reflect the attenuation characteristics of the different sediment types (Fig. 2.22).
They reach lowest values of about 5·10
-14
m
2
in
fine-grained clayey mud and nannofossil ooze.
Highest values of about 5·10
-11
m
2
occur in diatomaceous ooze due to their high porosities.
Nevertheless, it should be kept in mind that these
permeabilities are only estimates based on the
input parameters and assumptions incorporated in
Biot-Stoll’s model. For instance one of these
assumptions is that only mean grain sizes are
used, but the influence of grain size distributions
is neglected. Additionally, the total porosity is
usually used as input parameter for the inversion
scheme without differentiation between inter- and
intraporosities. Comparisons of these estimated
permeabilities with direct measurements unfortunately do not exist up to know.
2.6
Sediment Echosounding
While ultrasonic measurements are used to study
the structure and composition of sediment cores,
sediment echosounders are hull-mounted acoustic
systems which image the upper 10-200 m of sediment
coverage by remote sensing surveys. They operate
with frequencies around 3.5-4.0 kHz. The examples
presented here were digitally recorded with the
narrow-beam Parasound echosounder and
ParaDIGMA recording system (Spieß 1993).
2.6.1
Synthetic Seismograms
Figure 2.23 displays a Parasound seismogram
section recorded across an inactive channel of the
Bengal Fan. The sediments of the terrace were
sampled by a 10 m long piston core (47KL). Its
acoustic and bulk properties can either be directly
compared to the echosounder recordings or by
computations of synthetic seismograms. Such
modeling requires P-wave velocity and wet bulk
density logs as input parameters. From the
product of both parameters acoustic impedances I
= v P ·ρ are calculated. Changes in the acoustic
impedance cause reflections of the normally
incident acoustic waves. The amplitude of such
reflections is determined by the normal incidence
reflection coefficient R = (I 2 -I 1 ) / (I 2 +I 1 ), with (I 1 )
and (I 2 ) being the impedances above and below
the interface. From the series of reflection
coefficients the reflectivity can be computed as
impulse response function, including all internal