The electrical sediment conductivity σs (and reciprocal resistivity Rs) was determined using an inductive GEOTEK non-contact resistivity (NCR) sensor. The system applies high-frequency magnetic fields by a transmitter coil inducing electrical eddy currents in the sediment which are proportional to conductivity. Their secondary field is recorded and yields raw and calibrated values for conductivity and resistivity. The resistivity sensor averages over approximately 12 cm core length. A platinum thermometer inserted into a segment continuously measures sediment temperature for temperature compensation. Absolute sensor calibrations using a series of saline standards are performed daily. For subsequent drift and segment end correction, 29.5 cm long insulating spacers were placed between segments during logging. Thus, the characteristic decay of the eddy currents nearby the end-caps was separately recorded for each segment and corrected on basis of a model curve. This method provides a continuous composite record, however the first 2-3 data points from each intersection were often discarded due to some under- or overshooting.
Porosity was calculated according to the empirical Archie's equation
Rs/Rw = k ∙ φ^(-m)
where the ratio of sediment resistivity Rs and pore water resistivity Rw can be approximated by a power function of porosity φ. Following a recommendation by Boyce (1968), suitable for sea water saturated clay-rich sediments, values of 1.30 and 1.45 were used for the constants k and m, respectively. The calculated porosity φ is subsequently converted to wet bulk density ρwet using the equation (BOYCE, 1976)
ρwet = φ ∙ ρf + (1 - φ) ∙ ρm
with a pore water density ρf of 1030 kg/m³ and a matrix density ρm of 2670 kg/m³. For a uniform treatment of all cores, these empirical coefficients were not adapted to individual sediment lithologies. Yet, relative porosity and density changes should be well documented.
