ABEM Terrameter SAS 1000 / SAS 4000
- 42 -
(down), but the cables must always be rolled out with increasing takeout numbers in parallel
with increasing coordinate numbers.
6.2.8
Data Cover
The data cover for resistivity surveying with cable systems in general is dependent on several
factors such as: the total number of electrode take-outs, the number of sections the cable array
is divided into, the measurement array type used (Wenner, pole-pole, dipole-dipole etc.), and
whether there are any restrictions on how the electrodes can be combined. This section shows
the data cover of standard Wenner CVES, while “Appendix E. Address and Protocol Files”
(page 90) summarizes the characteristics of protocol files supplied with Lund Imaging
System.
For the standard Wenner CVES the data cover is enhanced by measuring in two sets, linking
in electrodes from the outer electrode cables. This is demonstrated by the protocol files
WENNER_L and WENNER_S. With roll-along measuring the data cover becomes
continuous, for all but the longest electrode separations where there are gaps in the cover
between the station points (Figure 14). Since there is a substantial over-lap in data cover
between consecutive station points, the .UP and .DWN protocol files are reduced accordingly,
which means measuring is much faster. In practice, this means that the measuring for these
points normally take about half as long time as for the first midpoint. WENNER_S is
designed only to supplement WENNER_L for the shortest electrode spacing, and is generally
not suitable for use alone if only the two central cables are used. If measuring with two cables
only use a protocol dedicated for this (see “Appendix E. Address and Protocol Files”).
Any cable configuration and electrode array could in principle be used for measuring, as long
as the geometry and measuring sequence is specified in the address and protocol files used.
Details on the format of these files are given in section 8.1.5 “Format of Lund Imaging
Address, Protocol and Location Files” (page 60).
The SAS 1000 / 4000 displays measured resistance or apparent resistivity values on the screen
during data acquisition, as well as the smallest inter-electrode distance and the midpoint
coordinate.
Figure 14. Data coverage of standard Wenner CVES, using the measurement protocols
WENNER_L and WENNER_S, for roll-along with three full measurement
stations, plus one station with a cable excluded at each end of the line. A unit
electrode spacing of 1 metre was used.