2
Physical Properties of Marine Sediments
68
In order to correct for slight temperature variations in the laboratory and to transfer laboratory
measurements to in situ conditions, usually
temperature and in situ corrections are applied.
Temperature variations mainly affect the pore
fluid. Corrections to in situ conditions should
consider both the influence of reduced temperature and increased hydrostatic pressure at the
sea floor.
Porosity and Wet Bulk Density
In standard sea water of 35‰ salinity density
increases by maximum 0.3·10
-3
g cm
-3
per °C
(Siedler and Peters 1986), i.e. by less than 0.1%
per °C. Hence, temperature corrections can usually be neglected.
Differences between laboratory and in-situ
porosities are less than 0.001% (Hamilton 1971)
and can thus be disregarded, too. Sea floor wet
bulk densities are slightly higher than the corresponding laboratory values due to the hydrostatic pressure and the resulting higher density of
the pore water. For Central Pacific sediments of
75 - 85% porosity Hamilton (1971) estimated a
density increase of maximum 0.01 g cm
-3
for water
depths between about 500 - 3000 m and of
maximum 0.02 g cm
-3
for water depths between
about 3000 - 6000 m. Thus, corrections to sea floor
conditions are usually of minor importance, too.
However, for cores of several hundred meter length
the effect of an increasing lithostatic pressure has
to be taken into account.
Electrical Resistivity
If porosities and wet bulk densities are determined
by galvanic resistivity measurements (Sect. 2.2.3)
varying sediment temperatures are considered by
computation of the formation factor (F) (see Eq.
2.12). While the resistivity (R s ) of the sediment is
determined by the small hand-held probe (cf. Sect.
2.2.3) the resistivity (R f ) of the pore fluid is derived from a calibration curve which describes the
temperature (T) - conductivity (c) relation by a
fourth power law (Siedler and Peters 1986)
4
4
3
3
2
2
1
1
T
c
T
c
T
c
T
c
c
R
o
f
+
+
+
+
=
−
(2.22)
The coefficients (c 0 ) to (c 4 ) depend on the
geometry of the probe and are determined by a
least square fit to the calibration measurements in
standard sea water.
P-wave velocity and attenuation
Bell and Shirley (1980) demonstrated that the Pwave velocity of marine sediments increases
almost linearly by about 3 m s
-1
per °C while the
attenuation is independent of sediment temperature, similar to the temperature dependence of
sound velocity and attenuation in sea water.
Hence, to correct laboratory P-wave velocity measurements to a reference temperature of 20°C
Schultheiss and McPhail’s (1989) equation
(
)
T
v
v
T
−
⋅
+
=
20
3
20
(2.23)
can be applied, with (v 20 ) = P-wave velocity at
20°C (in m s
-1
), (T) = sediment temperature (in °C)
and (v T ) = P-wave velocity measured at temperature (T) (in m s
-1
).
To correct laboratory P-wave velocity measurements to in situ conditions a modified time-average equation (Wyllie et al. 1956) can be used
(Shipboard Scientific Party 1995)
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
⋅
+
=
lab
situ
in
lab
situ
in
c
c
v
v
1
1
1
1
φ
(2.24)
(v lab ) and (v in-situ ) are the measured laboratory
(20°C, 1 at) and the corrected in situ P-wave
velocities, (c lab ) and (c in-situ ) the sound velocity in
sea water at laboratory and in situ conditions and
(φ) the porosity. If laboratory and sea floor
pressure, temperature and salinity are known from
tables or CTD measurements (c lab ) and (c in-situ ) can
be computed according to Wilson’s (1960)
equation
STP
P
T
c
c
c
c
c
S
+
+
+
+
=
14
.
1449
(2.25)
(c T ), (c P ), (c S ) are higher order polynomials which
describe the influence of temperature (T), pressure
(P) and salinity (S) on sound velocity. (c STP )
depends on all three parameters. Complete
expressions for (c T ), (c P ), (c S ), (c STP ) can be found
in Wilson (1960). For T = 20°C, P = 1 at and S =
0.035 Wilson’s equation results in a sound velocity of 1521 m s
-1
.
Physical Properties of Marine Sediments
68
In order to correct for slight temperature variations in the laboratory and to transfer laboratory
measurements to in situ conditions, usually
temperature and in situ corrections are applied.
Temperature variations mainly affect the pore
fluid. Corrections to in situ conditions should
consider both the influence of reduced temperature and increased hydrostatic pressure at the
sea floor.
Porosity and Wet Bulk Density
In standard sea water of 35‰ salinity density
increases by maximum 0.3·10
-3
g cm
-3
per °C
(Siedler and Peters 1986), i.e. by less than 0.1%
per °C. Hence, temperature corrections can usually be neglected.
Differences between laboratory and in-situ
porosities are less than 0.001% (Hamilton 1971)
and can thus be disregarded, too. Sea floor wet
bulk densities are slightly higher than the corresponding laboratory values due to the hydrostatic pressure and the resulting higher density of
the pore water. For Central Pacific sediments of
75 - 85% porosity Hamilton (1971) estimated a
density increase of maximum 0.01 g cm
-3
for water
depths between about 500 - 3000 m and of
maximum 0.02 g cm
-3
for water depths between
about 3000 - 6000 m. Thus, corrections to sea floor
conditions are usually of minor importance, too.
However, for cores of several hundred meter length
the effect of an increasing lithostatic pressure has
to be taken into account.
Electrical Resistivity
If porosities and wet bulk densities are determined
by galvanic resistivity measurements (Sect. 2.2.3)
varying sediment temperatures are considered by
computation of the formation factor (F) (see Eq.
2.12). While the resistivity (R s ) of the sediment is
determined by the small hand-held probe (cf. Sect.
2.2.3) the resistivity (R f ) of the pore fluid is derived from a calibration curve which describes the
temperature (T) - conductivity (c) relation by a
fourth power law (Siedler and Peters 1986)
4
4
3
3
2
2
1
1
T
c
T
c
T
c
T
c
c
R
o
f
+
+
+
+
=
−
(2.22)
The coefficients (c 0 ) to (c 4 ) depend on the
geometry of the probe and are determined by a
least square fit to the calibration measurements in
standard sea water.
P-wave velocity and attenuation
Bell and Shirley (1980) demonstrated that the Pwave velocity of marine sediments increases
almost linearly by about 3 m s
-1
per °C while the
attenuation is independent of sediment temperature, similar to the temperature dependence of
sound velocity and attenuation in sea water.
Hence, to correct laboratory P-wave velocity measurements to a reference temperature of 20°C
Schultheiss and McPhail’s (1989) equation
(
)
T
v
v
T
−
⋅
+
=
20
3
20
(2.23)
can be applied, with (v 20 ) = P-wave velocity at
20°C (in m s
-1
), (T) = sediment temperature (in °C)
and (v T ) = P-wave velocity measured at temperature (T) (in m s
-1
).
To correct laboratory P-wave velocity measurements to in situ conditions a modified time-average equation (Wyllie et al. 1956) can be used
(Shipboard Scientific Party 1995)
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
⋅
+
=
lab
situ
in
lab
situ
in
c
c
v
v
1
1
1
1
φ
(2.24)
(v lab ) and (v in-situ ) are the measured laboratory
(20°C, 1 at) and the corrected in situ P-wave
velocities, (c lab ) and (c in-situ ) the sound velocity in
sea water at laboratory and in situ conditions and
(φ) the porosity. If laboratory and sea floor
pressure, temperature and salinity are known from
tables or CTD measurements (c lab ) and (c in-situ ) can
be computed according to Wilson’s (1960)
equation
STP
P
T
c
c
c
c
c
S
+
+
+
+
=
14
.
1449
(2.25)
(c T ), (c P ), (c S ) are higher order polynomials which
describe the influence of temperature (T), pressure
(P) and salinity (S) on sound velocity. (c STP )
depends on all three parameters. Complete
expressions for (c T ), (c P ), (c S ), (c STP ) can be found
in Wilson (1960). For T = 20°C, P = 1 at and S =
0.035 Wilson’s equation results in a sound velocity of 1521 m s
-1
.