## 3 Results and Discussion
### The Frequency-Dependence in SIP Measurements Conducted In-Situ and in the Lab  
Figure 4 shows the complex conductivity spectra measured in the field and rock samples measured with the PSIP unit. The in-situ measurements reveal differences in the \(\sigma^{*}\) values measured between the graphite-rich area (indicated by the readings Z3 and Z5 in Figure 4a,b) and the area dominated by gneiss (indicated by the reading Z1 in Figure 4a,b). Although the values are different, the spectra are relatively similar. The difference in the values measured between the gneiss (Z1) and the graphite-rich area is practically two orders of magnitude (for both the conductivity and the polarization) and it is due to the high conductivity and polarization in good electrical conductors [25, 27, 28]. The measurements reveal a slight increase in the polarization values with increasing frequency. However, the lack of a dominant peak in the spectral response indicates well sorted materials, with no dominant grain size [20, 23, 27, 28, 29, 30, 48]. Moreover, the similar spectra resolved in the three locations indicate that, close to the surface, the materials are similarly well-sorted, independent of the graphite content. Figure 4: Measurements conducted (**a,b**) with the PSIP at the site, (**c,d**) laboratory complex conductivity spectra of the drilled core samples, (**e–h**) sediments collected at the quarry and (**i–j**) commercial graphite sediments in terms of (**a,c**,**e,g,i**) in-phase conductivity and (**b,d,f,h,j**) quadrature conductivity. The solid lines represent the Cole-Cole fit of the data. For visibility, the panels (**e–j**) range differs from the panels (**a–d**). Laboratory measurements minimize the sources of data error and are not affected by modeling and inversion uncertainties affecting imaging field data. Hence, they allow us to understand the intrinsic properties of the material and support our interpretation. Figure 4c,d presents the spectra of the plugs, which represent the undisturbed samples. All spectra in the plugs reveal a clear frequency dependence, with a peak in the quadrature conductivity spectra at the low frequencies (<10 Hz). Although the spectra are similar, the polarization values differ for all spectra, with maximum values (i.e., at the critical frequencies) varying in the range between 1 mS/m and 1000 mS/m (Figure 4c,d). While plugs 1 and 2 reveal the highest polarization values (>500 mS/m), plug 4 reveals the lowest. The changes of the polarization response are related to the content of the electrical conductor, independent on the size of the sample, considering that plug 2 is larger than plug 4 (see Table 3). The spectra in Figure 4c,d reveal the significant heterogeneity in the polarization response of the study area. Moreover, it shows that in-situ spectra may provide only a limited insight into the subsurface properties, as they are limited in the depth and volume of investigation. In-situ spectra with large electrode spacing resulted only in noisy readings, as described for the imaging data sets, due to low voltages and high EM coupling in the data. We extracted plugs with perpendicular directions (plug 1, 1 cr, 3, and 3 cr) from the same rocks to investigate anisotropy. Figure 4c,d shows that plugs 1 and 1 cr do not exhibit significant anisotropy, as the measurements reveal consistent values and spectra. However, measurements of plugs 3 and 3 cr demonstrate high differences in the critical frequency and its associated polarization values (100 mS/m and 300 mS/m for plugs 3 and 3 cr, respectively), indicating that anisotropic effects in the electrical properties can not be neglected in the materials in the area where the rock was taken. Figure 4 shows similar spectra for the measurements in plugs 1, 1 cr, 2 and 3 cr; thus suggesting consistent textural properties for the samples, which is not the case for the plugs 3, 4l and 4s.