ABEM Terrameter SAS 1000 / SAS 4000
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LATERAL LOGGING
Current is injected via cable electrode 5.6 m, and the potential is measured between the ring
electrode at the bottom of the probe and the cable electrode 0.4 m
The SAS 1000 / 4000 will automatically perform the necessary conversions, and display the
resistivity of the surrounding medium, measured with the ”Lateral” configuration.
7.6
FLUID RESISTIVITY AND ESTIMATION OF TDS
The SAS LOG can be used to measure the resistivity of a fluid, thus enabling you to
determine the Total Dissolved Solids (TDS) in the fluid.
Resistivity is a function of water temperature, and a standard temperature (usually 25
C) must
be specified for reporting resistivities or conductivities. For resistivity, the approximate
conversion formulas for common standard temperatures are:
18
=
T
(0.62 + 0.021 T)
20
=
T
(0.58 + 0.021 T)
25
=
T
(0.48 + 0.021 T)
where T is the temperature in
C in the range 5 - 50
C and
T
the measured fluid resistivity at
the temperature T.
Concentrations by weight
Concentrations of the common ions found in ground water are often reported by weight in
parts per million (ppm). One ppm defines one part by weight of the ion to a million parts by
weight of water, and it is numerically equivalent to milligrams per liter. Total dissolved solids
(TDS) is also reported as ppm.
The following diagram illustrates the relationships between resistivity and TDS for various
salt solutions and for an average of natural ground waters.
The TDS estimation for an average of natural ground waters is:
TDS
ppm
= 6400/
25
where
25
is in
m.
Conductance is sometimes preferred for the estimation of TDS (rather than resistance, its
reciprocal) since it increases with salt content. Conductance is measured in Siemen (= S =
mho = (
m)
-1
), but for comparative purposes results are usually reported as electrical
conductivity (EC) which is measured in S/m. Since most ground waters have conductivities of
much less than 1 S/m (and since this method is often used to determine the TDS of ground
water), it is often more convenient to use mS/m.
7.7
PLOTTING OF LOGGING DATA
The most commonly used data format for borehole is LAS (Log ASCII Standard). The
Terrameter SAS 4000 / SAS 1000 Utilities software has an option to convert logging data
from S4K raw data files to LAS format.
