ABEM Terrameter SAS 1000 / SAS 4000
- 81 -
na
a
Pole-dipole
C1
P1 P2
na
a
na
Schlumberger
C1
C2
P1 P2
Gradient array
(s+2)a
C1
C2
(1)
1
P
(1)
2
P
(2)
1
P
(2)
2
P
(n)
1
P
(n)
2
P
+
+
10.2.2
Pseudosection Plotting
Plotting the measured apparent resistivities as pseudosection is a valuable tool for fast
assessment of the data quality. Systematically varying apparent resistivity in a pseudosection
usually shows that the data quality is acceptable, where a modestly experienced field crew
easily picks out outliers indicating measurement technical problems. A pseudosection can also
give a rough hint about the resistivity structure at the site, but must be used with great caution
as the sensitivity function of different electrode arrays lead to images that are strongly
distorted compared to the geometry of the imaged structures.
For the gradient protocols many data points tend to end up in the same location in the
pseudosection using the formulas above, hence it is suggested that pseudosections based on
several sub-sets of the data are plotted separately for data quality assessment. Furthermore, it
it may be useful to make separate plots for each electrode spacing
a
used in order to get a
clear image of the data quality.
10.3
REFERENCES
Dahlin, T. and Zhou, B. (2002) Gradient and mid-point-referred measurements for multi-
channel 2D resistivity imaging,
Procs. 8th Meeting Environmental and Engineering
Geophysics, Aveiro, Portugal, 8-12 September 2002
, 157-160.
Parasnis, D.S. (1997)
Principles of Applied Geophysics
, 5:th ed, ISBN 0 412 64080 5,
Chapman and Hall, London, 429p.
Figure 27. Schematic representations for a) gradient array, b) pole-dipole,
and c) Schlumberger measurement. The background shows the
sensitivity pattern of the configuration, in the gradient case for the
first potential electrode pair (n-factor = 1).
