37
Fig. 2.7 Formation factor versus porosity for six gravity cores retrieved from different sedimentation provinces in
the South Atlantic. Porosities were determined on discrete samples by wet and dry weights and volumes, formation
factors by resistivity measurements. The dashed lines indicate Archie’s law for a = 1 and cementation exponents (m)
between 1 and 5. For a description of the sedimentation provinces, core numbers, coring locations, sediment
compositions, water depths and constants (a) and (m) derived from linear least square fits please refer to Table 2.1.
Unpublished data from M. Richter, University Bremen, Germany.
less than 5% (Fig. 2.6a). The core logs illustrate
that the largest differences occur in the laminated
sandy layers, particularly between 1.5 and 3.5 m
and between 4.3 and 5.1 m depth (Fig. 2.6b). A
drainage of the sandy layers, slight differences in
the depth scale and different volumes considered
by both methods probably cause this scatter.
Nevertheless, in general both data sets agree very
well having used Boyce’s (1968) values for (a) and
(m) and a typical terrigenous grain density of
2.67 g/cm
3
for porosity and wet bulk density
computations. This is valid because the sedimentation environment of core PS2178-5 from the
Arctic Ocean is rather similar to the Bering Sea
studied by Boyce (1968).
The influence of the sediment composition on
the formation factor-porosity relation illustrates
Figure 2.7 for six provinces in the South Atlantic.
For each core porosities and formation factors
were evaluated at the same core depths by wet
and dry weights and volumes of discrete samples
and by electrical resistivities. Displayed as a cross
plot on a log-log scale the data sets of each core
show different trends and thus different cementation exponents. Jackson et al. (1978) tried to
relate such variations in the cementation exponent
to the sphericity of sediment particles. Based on
studies on artificial samples they found higher
cementation exponents if particles become less
spherical. For natural sediments incorporating a
large variety of terrigenous and biogenic particle
sizes and shapes, it is difficult to verify similar
relations. It is only obvious that coarse-grained
calcareous foraminiferal oozes from the Hunter
Gap show the lowest and fine-grained, diatombearing hemipelagic mud from the Congo Fan
upwelling the highest cementation exponent.
Simultaneously, the constant (a) decreases with
increasing cementation exponent (Tab. 2.1). Possibly,
in natural sediments the amount and distribution
of pore space are more important than the particle shape. Generally, cementation exponent (m),
2.2
Porositiy and Wet Bulk Density
Fig. 2.7 Formation factor versus porosity for six gravity cores retrieved from different sedimentation provinces in
the South Atlantic. Porosities were determined on discrete samples by wet and dry weights and volumes, formation
factors by resistivity measurements. The dashed lines indicate Archie’s law for a = 1 and cementation exponents (m)
between 1 and 5. For a description of the sedimentation provinces, core numbers, coring locations, sediment
compositions, water depths and constants (a) and (m) derived from linear least square fits please refer to Table 2.1.
Unpublished data from M. Richter, University Bremen, Germany.
less than 5% (Fig. 2.6a). The core logs illustrate
that the largest differences occur in the laminated
sandy layers, particularly between 1.5 and 3.5 m
and between 4.3 and 5.1 m depth (Fig. 2.6b). A
drainage of the sandy layers, slight differences in
the depth scale and different volumes considered
by both methods probably cause this scatter.
Nevertheless, in general both data sets agree very
well having used Boyce’s (1968) values for (a) and
(m) and a typical terrigenous grain density of
2.67 g/cm
3
for porosity and wet bulk density
computations. This is valid because the sedimentation environment of core PS2178-5 from the
Arctic Ocean is rather similar to the Bering Sea
studied by Boyce (1968).
The influence of the sediment composition on
the formation factor-porosity relation illustrates
Figure 2.7 for six provinces in the South Atlantic.
For each core porosities and formation factors
were evaluated at the same core depths by wet
and dry weights and volumes of discrete samples
and by electrical resistivities. Displayed as a cross
plot on a log-log scale the data sets of each core
show different trends and thus different cementation exponents. Jackson et al. (1978) tried to
relate such variations in the cementation exponent
to the sphericity of sediment particles. Based on
studies on artificial samples they found higher
cementation exponents if particles become less
spherical. For natural sediments incorporating a
large variety of terrigenous and biogenic particle
sizes and shapes, it is difficult to verify similar
relations. It is only obvious that coarse-grained
calcareous foraminiferal oozes from the Hunter
Gap show the lowest and fine-grained, diatombearing hemipelagic mud from the Congo Fan
upwelling the highest cementation exponent.
Simultaneously, the constant (a) decreases with
increasing cementation exponent (Tab. 2.1). Possibly,
in natural sediments the amount and distribution
of pore space are more important than the particle shape. Generally, cementation exponent (m),
2.2
Porositiy and Wet Bulk Density