2
Physical Properties of Marine Sediments
30
and wet bulk densities by gamma ray attenuation
and electrical resistivity measurements.
The porosity (φ) characterizes the relative
amount of pore space within a sample volume. It is
defined by the ratio
V
V
volume
sample
total
space
pore
of
volume
f
=
=
φ
(2.1)
Equation 2.1 describes the fractional porosity
which ranges from 0 in case of none pore volume
to 1 in case of a water sample. Multiplication with
100 gives the porosity in percent. Depending on
the sediment type porosity occurs as inter- and
intraporosity. Interporosity specifies the pore
space between the sediment grains and is typical
for terrigenous sediments. Intraporosity includes
the voids within hollow sediment particles like
foraminifera in calcareous ooze. In such sediments
both inter- and intraporosity contribute to the
total porosity.
The wet bulk density (ρ) is defined by the
mass (m) of a water-saturated sample per sample
volume (V)
V
m
volume
sample
total
sample
wet
of
mass
=
=
ρ
(2.2)
Porosity and wet bulk density are closely
related, and often porosity values are derived from
wet bulk density measurements and vice versa.
Basic assumption for this approach is a twocomponent model for the sediment with uniform
grain and pore fluid densities (ρ g ) and (ρ f ). The wet
bulk density can then be calculated using the
porosity as a weighing factor
( ) g
f
ρ
φ
ρ
φ
ρ
⋅
−
+
⋅
=
1
(2.3)
If two or several mineral components with
significantly different grain densities contribute
to the sediment frame their densities are averaged in (ρ g ).
2.2.1
Analysis by Weight and Volume
The traditional way to determine porosity and wet
bulk density is based on weight and volume
measurements of small sediment samples. Usually
they are taken from the centre of a split core by a
syringe which has the end cut off and a definite
volume of e.g. 10 ml. While weighing can be done
very accurately in shore-based laboratories measurements onboard of research vessels require
special balance systems which compensate the
shipboard motions (Childress and Mickel 1980).
Volumes are measured precisely by Helium gas
pycnometers. They mainly consist of a sample cell
and a reference cell and employ the ideal gas law
to determine the sample volume. In detail, the
sediment sample is placed in the sample cell, and
both cells are filled with Helium gas. After a valve
connecting both cells are closed, the sample cell is
pressurized to (P 1 ). When the valve is opened the
pressure drops to (P 2 ) due to the increased cell
volume. The sample volume (V) is calculated from
the pressure ratio (P 1 /P 2 ) and the volumes of the
sample and reference cell, (V s ) and (V ref ) (Blum
1997).
2
1
1
P
P
V
V
V
ref
s
−
+
=
(2.4)
While weights are measured on wet and dry
samples having used an oven or freeze drying,
volumes are preferentially determined on dry
samples. A correction for the mass and volume of
the salt precipitated from the pore water during
drying must additionally be applied (Hamilton
1971; Gealy 1971) so that the total sample volume
(V) consists of
f
salt
dry
V
V
V
V
+
−
=
(2.5)
The volumes (V f ) and (V salt ) of the pore space and
salt result from the masses of the wet and dry
sample, (m) and (m dry ), from the densities of the pore
fluid and salt (ρ f =1.024 g cm
-3
and ρ salt = 2.1 g cm
-3
)
and from the fractional salinity (s)
( ) f
dry
f
f
f
s
m
m
m
V
ρ
ρ
⋅
−
−
=
=
1
(2.6)
with :
s
m
m
m
dry
f
−
−
= 1
(
)
salt
dry
f
salt
salt
salt
m
m
m
m
V
ρ
ρ
−
−
=
=
(2.7)
with :
(
)
dry
f
salt
m
m
m
m
−
−
=
Together with the mass (m) of the wet sample
equations 2.5 to 2.7 allow to compute the wet bulk
density according to equation 2.2.
Wet bulk density computations according to
equation 2.3 require the knowledge of grain
Physical Properties of Marine Sediments
30
and wet bulk densities by gamma ray attenuation
and electrical resistivity measurements.
The porosity (φ) characterizes the relative
amount of pore space within a sample volume. It is
defined by the ratio
V
V
volume
sample
total
space
pore
of
volume
f
=
=
φ
(2.1)
Equation 2.1 describes the fractional porosity
which ranges from 0 in case of none pore volume
to 1 in case of a water sample. Multiplication with
100 gives the porosity in percent. Depending on
the sediment type porosity occurs as inter- and
intraporosity. Interporosity specifies the pore
space between the sediment grains and is typical
for terrigenous sediments. Intraporosity includes
the voids within hollow sediment particles like
foraminifera in calcareous ooze. In such sediments
both inter- and intraporosity contribute to the
total porosity.
The wet bulk density (ρ) is defined by the
mass (m) of a water-saturated sample per sample
volume (V)
V
m
volume
sample
total
sample
wet
of
mass
=
=
ρ
(2.2)
Porosity and wet bulk density are closely
related, and often porosity values are derived from
wet bulk density measurements and vice versa.
Basic assumption for this approach is a twocomponent model for the sediment with uniform
grain and pore fluid densities (ρ g ) and (ρ f ). The wet
bulk density can then be calculated using the
porosity as a weighing factor
( ) g
f
ρ
φ
ρ
φ
ρ
⋅
−
+
⋅
=
1
(2.3)
If two or several mineral components with
significantly different grain densities contribute
to the sediment frame their densities are averaged in (ρ g ).
2.2.1
Analysis by Weight and Volume
The traditional way to determine porosity and wet
bulk density is based on weight and volume
measurements of small sediment samples. Usually
they are taken from the centre of a split core by a
syringe which has the end cut off and a definite
volume of e.g. 10 ml. While weighing can be done
very accurately in shore-based laboratories measurements onboard of research vessels require
special balance systems which compensate the
shipboard motions (Childress and Mickel 1980).
Volumes are measured precisely by Helium gas
pycnometers. They mainly consist of a sample cell
and a reference cell and employ the ideal gas law
to determine the sample volume. In detail, the
sediment sample is placed in the sample cell, and
both cells are filled with Helium gas. After a valve
connecting both cells are closed, the sample cell is
pressurized to (P 1 ). When the valve is opened the
pressure drops to (P 2 ) due to the increased cell
volume. The sample volume (V) is calculated from
the pressure ratio (P 1 /P 2 ) and the volumes of the
sample and reference cell, (V s ) and (V ref ) (Blum
1997).
2
1
1
P
P
V
V
V
ref
s
−
+
=
(2.4)
While weights are measured on wet and dry
samples having used an oven or freeze drying,
volumes are preferentially determined on dry
samples. A correction for the mass and volume of
the salt precipitated from the pore water during
drying must additionally be applied (Hamilton
1971; Gealy 1971) so that the total sample volume
(V) consists of
f
salt
dry
V
V
V
V
+
−
=
(2.5)
The volumes (V f ) and (V salt ) of the pore space and
salt result from the masses of the wet and dry
sample, (m) and (m dry ), from the densities of the pore
fluid and salt (ρ f =1.024 g cm
-3
and ρ salt = 2.1 g cm
-3
)
and from the fractional salinity (s)
( ) f
dry
f
f
f
s
m
m
m
V
ρ
ρ
⋅
−
−
=
=
1
(2.6)
with :
s
m
m
m
dry
f
−
−
= 1
(
)
salt
dry
f
salt
salt
salt
m
m
m
m
V
ρ
ρ
−
−
=
=
(2.7)
with :
(
)
dry
f
salt
m
m
m
m
−
−
=
Together with the mass (m) of the wet sample
equations 2.5 to 2.7 allow to compute the wet bulk
density according to equation 2.2.
Wet bulk density computations according to
equation 2.3 require the knowledge of grain