Terrameter LS (ABEM, Sweden) is a modern instrument for ERT. However, it has several unusual features which need to be considered.
Maximum number of electrodes: 64Terrameter LS is 64-electrode instrument with two 32-pole connector for electrode cables (2x32). However the basic recommended set supplied with the equipment is the set of 4 cables with 21 electrodes each (4х21). Since the last and the first electrode take-outs shall overlap at the cable ends, the equipment allows to measure with 81 electrodes for the system layout (4x21 - 3). It provides increase the depth of ERT investigation up to 25%.
4x21 modeHere is the challenge: to measure 81-electrode layout by using 64-electrode instrument. To achieve that, only 11 of the 21 electrodes are being used on the two farthest cables (1st and 4th cables). As a result we have 2x21 + 2x11 - 3 = 61 electrodes.
Electrode protocolsABEM instruments use the following idea to create ERT protocol:
Protocol consists of two types of measurements. The first type - shallow measurements (small electrode spacings). The second type - measurements for deeper parts of the geoelectrical section (large electrode spacings). For example, having cables with 5m spacing between electrode takeouts, LONG32 type (41 electrode with 10m unit electrode spacing) and SHORT32 type (41 electrode with 5m unit electrode spacing) were applied. Two protocols with constant spacing between electrodes each.
x2ipi toolbox provide preparing ABEM protocols for 61-electrode measuring scheme.
Since two different protocols (short&long) has been created they are merged into one general protocol ("Menu - Save - Merge SEQ - Lund protocol ABEM"). Then protocol is optimized to provide multichannel mode and saved in .org data file ("Menu - Save SEQ - Lund protocol").
Terrameter LS Toolbox supplied with the equipment converts .org file into .xml. User needs just to specify (in the .org file) the name of the file describing the cable (spread) (.xml, .adr). Using 4-electrode array (without remote electrode) text in second line "LONG32/SHORT32" has to be changed to "4X21". For measurements with remote electrode "B" it has to be "4X21REMOTEC2".
XML protocols files can be opened and viewed in x2ipi.
Examples of protocols (4x21)All following protocols has been optimized for measuring in multichannel mode and tested with Terrameter LS in real field work.
- Schlumberger array (reciprocal)
- Pole-Dipole array
- Dipole-Dipole array
- Combined protocol- Schlumberger + Dipole-dipole