2
Physical Properties of Marine Sediments
34
the striking advantage of the almost continuous
wet bulk density log clearly becomes obvious.
Measured with an increment of 5 mm a lot of finescale variations are defined, a resolution which
could never be reached by the time-consuming
analysis of discrete samples (Gerland 1993;
Gerland and Villinger 1995).
The precision of wet bulk densities can be
slightly improved, if the iterative scheme for the
mass attenuation coefficient is applied (Fig. 2.5a).
For core PS1725-2 wet bulk densities computed
with a constant mass attenuation coefficient are
compared with those derived from the iterative
procedure. Below 1.9 g cm
-3
the iteration produces
slightly smaller densities than are determined with
a constant mass attenuation coefficient and are
thus below the dotted 1:1 line. Above 1.9 g cm
-3
densities based on the iterative procedure are
slightly higher.
If both data sets are plotted versus the wet
bulk densities of the discrete samples the
optimization essentially becomes obvious for high
densities (>2.0 g cm
-3
, Fig. 2.5b). After iteration
densities are slightly closer to the dotted 1:1 line.
While this improvement is usually small and
here only on the order of 1.3% (≈0.02 g cm
-3
), differences between assumed and true grain density
affect the iteration more distinctly (Fig. 2.5c). As
an example wet bulk densities of core PS1725-2
were calculated with constant grain densities of
2.65, 2.75 and 2.10 g cm
-3
, values which are typical
for calcareous, terrigenous and diatomaceous
Fig. 2.5 Influence of an iterative mass attenuation coefficient determination on the precision of wet bulk densities. The gamma ray attenuation log of gravity core
PS1725-2 was used as test data set. (a) Wet bulk densities
calculated with a constant mass attenuation coefficient
('processing porosity' =50%) are displayed versus the data
resulting from the iteration. A pore fluid density of 1.024 g cm -3
and a constant grain density of 2.7 g cm -3 were used, and
the iteration was stopped if densities of two successive
steps differed by less than 0.1‰ (b) Cross plot of wet
bulk densities measured on discrete samples versus wet
bulk densities calculated from gamma ray attenuation
with a constant mass attenuation coefficient ( ) and
with the iterative scheme (+). (c) Influence of grain density on iteration. Three grain densities of 2.65, 2.75 and
2.1 g cm -3 were used to calculate wet bulk densities. Modified after Gerland (1993).
Physical Properties of Marine Sediments
34
the striking advantage of the almost continuous
wet bulk density log clearly becomes obvious.
Measured with an increment of 5 mm a lot of finescale variations are defined, a resolution which
could never be reached by the time-consuming
analysis of discrete samples (Gerland 1993;
Gerland and Villinger 1995).
The precision of wet bulk densities can be
slightly improved, if the iterative scheme for the
mass attenuation coefficient is applied (Fig. 2.5a).
For core PS1725-2 wet bulk densities computed
with a constant mass attenuation coefficient are
compared with those derived from the iterative
procedure. Below 1.9 g cm
-3
the iteration produces
slightly smaller densities than are determined with
a constant mass attenuation coefficient and are
thus below the dotted 1:1 line. Above 1.9 g cm
-3
densities based on the iterative procedure are
slightly higher.
If both data sets are plotted versus the wet
bulk densities of the discrete samples the
optimization essentially becomes obvious for high
densities (>2.0 g cm
-3
, Fig. 2.5b). After iteration
densities are slightly closer to the dotted 1:1 line.
While this improvement is usually small and
here only on the order of 1.3% (≈0.02 g cm
-3
), differences between assumed and true grain density
affect the iteration more distinctly (Fig. 2.5c). As
an example wet bulk densities of core PS1725-2
were calculated with constant grain densities of
2.65, 2.75 and 2.10 g cm
-3
, values which are typical
for calcareous, terrigenous and diatomaceous
Fig. 2.5 Influence of an iterative mass attenuation coefficient determination on the precision of wet bulk densities. The gamma ray attenuation log of gravity core
PS1725-2 was used as test data set. (a) Wet bulk densities
calculated with a constant mass attenuation coefficient
('processing porosity' =50%) are displayed versus the data
resulting from the iteration. A pore fluid density of 1.024 g cm -3
and a constant grain density of 2.7 g cm -3 were used, and
the iteration was stopped if densities of two successive
steps differed by less than 0.1‰ (b) Cross plot of wet
bulk densities measured on discrete samples versus wet
bulk densities calculated from gamma ray attenuation
with a constant mass attenuation coefficient ( ) and
with the iterative scheme (+). (c) Influence of grain density on iteration. Three grain densities of 2.65, 2.75 and
2.1 g cm -3 were used to calculate wet bulk densities. Modified after Gerland (1993).