2
Physical Properties of Marine Sediments
52
Table 2.3 Geographical coordinates, water depth, core length, region and composition of the sediment cores
considered for the sediment classification in Section 2.5.
Biot-Stoll’s theory allows us to model P-wave
velocities and attenuation coefficients analyzed from
transmission seismograms. As an example Figure 2.16
displays six data sets for the turbidite layer of core
GeoB1510-2. While attenuation coefficients were
analyzed as described above frequency dependent Pwave velocities were determined from successive
bandpass filtered transmission seismograms (Courtney and Mayer 1993; Breitzke 1997). Porosities and
mean grain sizes enter the modeling computations via
the pore size parameter (a) and structure factor (a’).
Physical properties of the pore fluid and sediment
grains are the same as given in Table 2.2. A bulk and
shear modulus of 10 and 6 MPa account for the
elasticity of the frame. As the permeability is the
parameter which is usually unknown but has the
strongest influence on attenuation and velocity
dispersion, model curves were computed for three
constant ratios κ/a
2
= 0.030, 0.010 and 0.003 of
permeability (κ) and pore size parameter (a). The
resulting permeabilities are given in each diagram.
These theoretical curves show that the attenuation
and velocity data between 170 and 182 cm depth can
consistently be modeled by an appropriate set of
input parameters. Viscous losses due to a global pore
water flow through the sediment are sufficient to explain the attenuation in these sediments. Only if the
turbidite base is approached (188 - 210 cm depth) the
attenuation and velocity dispersion data successively deviate from the model curves probably due
to an increasing amount of coarse-grained foraminifera. An additional damping mechanism which
might either be scattering or resonance within the
hollow foraminifera must be considered.
Based on this modeling computations an
inversion scheme was developed which automatically iterates the permeability and minimizes
the difference between measured and modeled
attenuation and velocity data in a least square
sense (Courtney and Mayer 1993; Breitzke 1997).
As a result S-wave velocities and attenuation
coefficients, permeabilities and elastic moduli of
water-saturated sediments can be estimated. They
are strictly only valid if attenuation and velocity
dispersion can be explained by viscous losses. In
coarse-grained parts deviations must be taken
into account for the estimated parameters, too.
Applied to the data of core GeoB1510-2 in 170, 176
and 182 cm depth S-wave velocities of 67, 68 and
74 m/s and permeabilities of 5·10
-13
, 1·10
-12
and
3·10
-12
m
2
result from this inversion scheme.
2.5
Sediment Classification
Full waveform ultrasonic core logging was applied
to terrigenous and biogenic sediment cores to
analyze P-wave velocities and attenuation coefficients typical for the different settings. Together
with the bulk parameters and the physical properties estimated by the inversion scheme they form
the data base for a sediment classification which
identifies different sediment types from their
Core
Coordinates Water
Core
Region
Sediment
Depth
Length
Composition
40KL
07°33.1'N
3814 m 8.46 m
Bengal
terrigenous clay, silt, sand
85°29.7'E
Fan
47KL
11°10.9'N
3293 m 10.00 m
Bengal
terrigenous clay, silt, sand;
88°24.9'E
Fan
foram. and nannofossil ooze
GeoB
30°27.1'S
3941 m 8.19 m Rio Grande foram. and nannofossil ooze
2821-1
38°48.9'W
Rise
PS
46°56.1'S
4102 m 17.65 m
Meteor
diatomaceous mud/ooze; few
2567-2
06°15.4'E
Rise
foram. and nannofossil layers
Physical Properties of Marine Sediments
52
Table 2.3 Geographical coordinates, water depth, core length, region and composition of the sediment cores
considered for the sediment classification in Section 2.5.
Biot-Stoll’s theory allows us to model P-wave
velocities and attenuation coefficients analyzed from
transmission seismograms. As an example Figure 2.16
displays six data sets for the turbidite layer of core
GeoB1510-2. While attenuation coefficients were
analyzed as described above frequency dependent Pwave velocities were determined from successive
bandpass filtered transmission seismograms (Courtney and Mayer 1993; Breitzke 1997). Porosities and
mean grain sizes enter the modeling computations via
the pore size parameter (a) and structure factor (a’).
Physical properties of the pore fluid and sediment
grains are the same as given in Table 2.2. A bulk and
shear modulus of 10 and 6 MPa account for the
elasticity of the frame. As the permeability is the
parameter which is usually unknown but has the
strongest influence on attenuation and velocity
dispersion, model curves were computed for three
constant ratios κ/a
2
= 0.030, 0.010 and 0.003 of
permeability (κ) and pore size parameter (a). The
resulting permeabilities are given in each diagram.
These theoretical curves show that the attenuation
and velocity data between 170 and 182 cm depth can
consistently be modeled by an appropriate set of
input parameters. Viscous losses due to a global pore
water flow through the sediment are sufficient to explain the attenuation in these sediments. Only if the
turbidite base is approached (188 - 210 cm depth) the
attenuation and velocity dispersion data successively deviate from the model curves probably due
to an increasing amount of coarse-grained foraminifera. An additional damping mechanism which
might either be scattering or resonance within the
hollow foraminifera must be considered.
Based on this modeling computations an
inversion scheme was developed which automatically iterates the permeability and minimizes
the difference between measured and modeled
attenuation and velocity data in a least square
sense (Courtney and Mayer 1993; Breitzke 1997).
As a result S-wave velocities and attenuation
coefficients, permeabilities and elastic moduli of
water-saturated sediments can be estimated. They
are strictly only valid if attenuation and velocity
dispersion can be explained by viscous losses. In
coarse-grained parts deviations must be taken
into account for the estimated parameters, too.
Applied to the data of core GeoB1510-2 in 170, 176
and 182 cm depth S-wave velocities of 67, 68 and
74 m/s and permeabilities of 5·10
-13
, 1·10
-12
and
3·10
-12
m
2
result from this inversion scheme.
2.5
Sediment Classification
Full waveform ultrasonic core logging was applied
to terrigenous and biogenic sediment cores to
analyze P-wave velocities and attenuation coefficients typical for the different settings. Together
with the bulk parameters and the physical properties estimated by the inversion scheme they form
the data base for a sediment classification which
identifies different sediment types from their
Core
Coordinates Water
Core
Region
Sediment
Depth
Length
Composition
40KL
07°33.1'N
3814 m 8.46 m
Bengal
terrigenous clay, silt, sand
85°29.7'E
Fan
47KL
11°10.9'N
3293 m 10.00 m
Bengal
terrigenous clay, silt, sand;
88°24.9'E
Fan
foram. and nannofossil ooze
GeoB
30°27.1'S
3941 m 8.19 m Rio Grande foram. and nannofossil ooze
2821-1
38°48.9'W
Rise
PS
46°56.1'S
4102 m 17.65 m
Meteor
diatomaceous mud/ooze; few
2567-2
06°15.4'E
Rise
foram. and nannofossil layers