35
sediments. Displayed as cross-plot, calculations
with grain densities of 2.65 and 2.75 g cm
-3
almost
coincide and follow the dotted 1:1 line. However,
computations with a grain density of 2.10 g cm
-3
differ by 1.3 - 3.6% (0.02 - 0.08 g cm
-3
) from those
with a grain density of 2.65 g cm
-3
. This result is
particularly important for cores composed of terrigenous and biogenic material of significantly
different grain densities like the terrigenous and diatomaceous components in core PS1821-6. Here, an
iteration with a constant grain density of 2.65 g cm
-3
would give erroneous wet bulk densities in the
diatomaceous parts. In such cores a depthdependent grain density profile should be applied in
combination with the iterative procedure, or otherwise wet bulk densities would better be calculated
with a constant mass attenuation coefficient.
However, if the grain density is almost constant
downcore the iterative scheme could be applied
without any problems.
2.2.3
Electrical Resistivity
(Galvanic Method)
The electrical resistivity of water-saturated sediments depends on the resistivity of its solid and
fluid constituents. However, as the sediment
grains are poor conductors an electrical current
mainly propagates in the pore fluid. The dominant
transport mechanism is an electrolytic conduction
by ions and molecules with an excess or deficiency of electrons. Hence, current propagation in
water-saturated sediments actually transports
material through the pore space, so that the
resistivity depends on both the conductivity of
the pore water and the microstructure of the
sediment. The conductivity of pore water varies
with its salinity, and mobility and concentration of
dissolved ions and molecules. The microstructure
of the sediment is controlled by the amount and
distribution of pore space and its capillarity and
tortuosity. Thus, the electrical resistivity cannot
be considered as a bulk parameter which strictly
only depends on the relative amount of solid and
fluid components, but as shown below, it can be
used to derive porosity and wet bulk density as
bulk parameters after calibration to a ‘typical’
sediment composition of a local sedimentation
environment.
Several models were developed to describe
current flow in rocks and water-saturated sediments theoretically. They encompass simple plane
layered models (Waxman and Smits 1968) as well
as complex approximations of pore space by self
similar models (Sen et al. 1981), effective medium
theories (Sheng 1991) and fractal geometries
(Ruffet et al. 1991). In practice these models are of
minor importance because often only few of the
required model parameters are known. Here, an
empirical equation is preferred which relates the
resistivity (R s ) of the wet sediment to its fractional
porosity (φ) (Archie 1942)
m
f
s
a
R
R
F
−
⋅
=
=
φ
(2.12)
The ratio of the resistivity (R s ) in sediment to
the resistivity (R f ) in pore water defines the
formation (resistivity) factor (F). (a) and (m) are
constants which characterize the sediment
composition. As Archie (1942) assumed that (m)
indicates the consolidation of the sediment it is
also called cementation exponent (cf. Sect. 3.2.2).
Several authors derived different values for (a)
and (m). For an overview please refer to Schön
(1996). In marine sediments often Boyce’s (1968)
values (a = 1.3, m = 1.45), determined by studies
on diatomaceous, silty to sandy arctic sediments,
are applied. Nevertheless, these values can only
be rough estimates. For absolutely correct porosities both constants must be calibrated by an
additional porosity measurement, either on
discrete samples or by gamma ray attenuation.
Such calibrations are strictly only valid for that
specific data set but, with little loss of accuracy,
can be transferred to regional environments with
similar sediment compositions. Wet bulk densities
can then be calculated using equation 2.3 and
assuming a grain density (cf. also section 3.2.2).
Electrical resistivities can be measured on split
cores by a half-automated logging system (Bergmann 1996). It measures the resistivity (R s ) and
temperature (T) by a small probe which is manually inserted into the upper few millimeters of the
sediment. The resistivity (R f ) of the interstitial
pore water is simultaneously calculated from a
calibration curve which defines the temperatureconductivity relation of standard sea water (35‰
salinity) by a fourth power law (Siedler and Peters
1986).
The accuracy and resolution that can be achieved compared to measurements on discrete
samples were studied on the terrigenous square
barrel kastenlot core PS2178-5 from the Arctic
Ocean. If both data sets are displayed as cross
plots porosities mainly range within the dashed
10% error lines, while densities mainly differ by
2.2
Porositiy and Wet Bulk Density
sediments. Displayed as cross-plot, calculations
with grain densities of 2.65 and 2.75 g cm
-3
almost
coincide and follow the dotted 1:1 line. However,
computations with a grain density of 2.10 g cm
-3
differ by 1.3 - 3.6% (0.02 - 0.08 g cm
-3
) from those
with a grain density of 2.65 g cm
-3
. This result is
particularly important for cores composed of terrigenous and biogenic material of significantly
different grain densities like the terrigenous and diatomaceous components in core PS1821-6. Here, an
iteration with a constant grain density of 2.65 g cm
-3
would give erroneous wet bulk densities in the
diatomaceous parts. In such cores a depthdependent grain density profile should be applied in
combination with the iterative procedure, or otherwise wet bulk densities would better be calculated
with a constant mass attenuation coefficient.
However, if the grain density is almost constant
downcore the iterative scheme could be applied
without any problems.
2.2.3
Electrical Resistivity
(Galvanic Method)
The electrical resistivity of water-saturated sediments depends on the resistivity of its solid and
fluid constituents. However, as the sediment
grains are poor conductors an electrical current
mainly propagates in the pore fluid. The dominant
transport mechanism is an electrolytic conduction
by ions and molecules with an excess or deficiency of electrons. Hence, current propagation in
water-saturated sediments actually transports
material through the pore space, so that the
resistivity depends on both the conductivity of
the pore water and the microstructure of the
sediment. The conductivity of pore water varies
with its salinity, and mobility and concentration of
dissolved ions and molecules. The microstructure
of the sediment is controlled by the amount and
distribution of pore space and its capillarity and
tortuosity. Thus, the electrical resistivity cannot
be considered as a bulk parameter which strictly
only depends on the relative amount of solid and
fluid components, but as shown below, it can be
used to derive porosity and wet bulk density as
bulk parameters after calibration to a ‘typical’
sediment composition of a local sedimentation
environment.
Several models were developed to describe
current flow in rocks and water-saturated sediments theoretically. They encompass simple plane
layered models (Waxman and Smits 1968) as well
as complex approximations of pore space by self
similar models (Sen et al. 1981), effective medium
theories (Sheng 1991) and fractal geometries
(Ruffet et al. 1991). In practice these models are of
minor importance because often only few of the
required model parameters are known. Here, an
empirical equation is preferred which relates the
resistivity (R s ) of the wet sediment to its fractional
porosity (φ) (Archie 1942)
m
f
s
a
R
R
F
−
⋅
=
=
φ
(2.12)
The ratio of the resistivity (R s ) in sediment to
the resistivity (R f ) in pore water defines the
formation (resistivity) factor (F). (a) and (m) are
constants which characterize the sediment
composition. As Archie (1942) assumed that (m)
indicates the consolidation of the sediment it is
also called cementation exponent (cf. Sect. 3.2.2).
Several authors derived different values for (a)
and (m). For an overview please refer to Schön
(1996). In marine sediments often Boyce’s (1968)
values (a = 1.3, m = 1.45), determined by studies
on diatomaceous, silty to sandy arctic sediments,
are applied. Nevertheless, these values can only
be rough estimates. For absolutely correct porosities both constants must be calibrated by an
additional porosity measurement, either on
discrete samples or by gamma ray attenuation.
Such calibrations are strictly only valid for that
specific data set but, with little loss of accuracy,
can be transferred to regional environments with
similar sediment compositions. Wet bulk densities
can then be calculated using equation 2.3 and
assuming a grain density (cf. also section 3.2.2).
Electrical resistivities can be measured on split
cores by a half-automated logging system (Bergmann 1996). It measures the resistivity (R s ) and
temperature (T) by a small probe which is manually inserted into the upper few millimeters of the
sediment. The resistivity (R f ) of the interstitial
pore water is simultaneously calculated from a
calibration curve which defines the temperatureconductivity relation of standard sea water (35‰
salinity) by a fourth power law (Siedler and Peters
1986).
The accuracy and resolution that can be achieved compared to measurements on discrete
samples were studied on the terrigenous square
barrel kastenlot core PS2178-5 from the Arctic
Ocean. If both data sets are displayed as cross
plots porosities mainly range within the dashed
10% error lines, while densities mainly differ by
2.2
Porositiy and Wet Bulk Density