# Computation of Optimized Electrode Arrays for 3-D Electrical Resistivity Tomography Surveys
## 5 Field Data  
### Panormos (Rethymno)  
The field data were collected for a geotechnical study at the city of Panormos, east of Rethymno (Crete, Greece) (Figure 13). The selected grid was 94 \(\times\) 75 m, where 16 parallel survey lines (5 m apart) were used to collect the geoelectrical data. For each line, 48 electrodes were used with 2 m probe spacing. Each line collected 1216 (dipole-dipole) and 638 (gradient) data, producing an overall complete data set of 19,456 dipole-dipole and 10,208 gradient data in total from the 16 lines. All data were used to produce the comprehensive FULL data set (dd + gr), which consists of 29,664 measurements. Figure 12: Speed comparison between Jacobian and Stummer optimization approach. Figure 13: Map of Crete (Greece) indicating the location of the case studies discussed in the text. The final Optim-2D optimized data set that was created by the 2D optimization procedure selected 15,214 measurements (10,035 dd and 5179 gr). The Optim-3D data set with 13,970 measurements were created from the 3-D optimization procedure which selected 8658 dipole-dipole and 5312 gradient data. The amount of optimum data that was selected from the 2-D and 3-D algorithms from the comprehensive data set is 51% and 47%, respectively. The Res-STUM optimum data set used 14,943 measurements (50%). All data were inverted with the same inversion parameters and are presented with a common resistivity scale from 3.4 to 205 ohm.m. The estimated RMS error for all inversion models is ranged from 3.0 to 3.87% after five or six iterations. The deepest depth layer reaches 15.0 m below the ground (Figures 14-17). Based on the geoelectrical model in the research area, a surface layer of about 2.5 to 3.3 m thick is detected with resistivity values from 20 to 70 ohm.m. This layer coincides with the topsoil and the recent surface deposits. Within this layer, there are areas of increased resistivity values due to local concentrations of rock masses, resulting from the disintegration of the rocky background. From the depth of 3.3 m up to the maximum depth of the geophysical survey (15 m), a formation with high resistivity values over than 70 ohm.m) appears, which is associated with the rocky background of the slates and shale in the wider area. The rocky background entirely covers the western area and extends east to a horizontal distance of 60 m. It also appears that the rocky background is not continuous and exhibits an intrinsic heterogeneity, as can be observed from changes in resistivity within the same formation. This means that the rock is fractured internally, mainly at sections where lower resistivity values are registered. Such an area is located on the northwest side of the area (X = 0-26 m and Y = 35-75 m) where it appears with resistivity values in the range of 30 to 40 ohm.m. The most characteristic geophysical anomaly is the area to the eastern side, which appears with very low resistivity values below 20 ohm.m. This zone is superficial and appears to continue at the deeper slices. This area seems to lean westward and is bound west and east by the rocky background. To the east, this rocky background appears with high resistivity values mainly from the depth of 7 m.  
The above range of low resistivity values can either be connected to a paleo-river due to the conductive signature of the sediments because of the water flowing in them. This area may also have originated from a local fragment of the rocky background so that, again, the water has the ability to move freely in this area and affect the values of resistivity. The inversion results show a comparable accuracy among the resistivity models reconstructed by the inversion of the comprehensive, optim-2D, optim-3D and Res-STUM data sets: the mapping of the bedrock, which was the main interest of the study, is reconstructed at the same position and depth slices of both comprehensive and optimum inversion results. Figure 14: Inversion results from the comprehensive field data set. Figure 15: Inversion results from the Optim-2D field data set.