## 2 Materials and Methods
### SIP Measurements and Chemical Analysis of Samples in the Laboratory  
Laboratory measurements allow us to investigate the frequency dependence of the graphite-rich rocks, minimizing electromagnetic sources of noise, which cannot be avoided in field SIP imaging data. Rock samples were taken for laboratory measurements from the site near the PSIP measurements and the DAS-1 line P2b (see Figure 1). After the SIP measurements, X-ray fluorescence spectroscopy and LECO analysis were conducted on four rock samples to obtain the chemical composition (see Table 2). The Epsilon 5 X-ray fluorescence analyzer from analytical measured the elemental contents. The method measures the concentration of main oxide elements in %. The silicon content varies within the samples between 35.0% and 37.0%, and the concentration of aluminum oxide between 8.9% and 11.5%. Sample Size d4 contains higher readings of FeO (2.1%) and S (1.1%), indicating a higher pyrite content possibility. The other measured parameters show slight variations. The carbon content of the samples varies between 44.2% and 47.7%. These concentrations were determined using the carbon measurement of the LECO C/S 200 (Inductive combustion of the sample and measurement of the CO\({}_{2}\) concentration within an IR measuring cell). SIP laboratory measurements were conducted on two scales: rock plugs and ground material. For all laboratory measurements, the PSIP instrument was used to collect 61 frequencies in the range between 0.01 and 10,000 Hz using ten measurements per decade. SIP measurements in rock plugs were used to investigate the intrinsic SIP response of the rocks (i.e., without grinding). To this end, we extracted seven core plugs from the rock samples mentioned above, with the plugs drilled in different directions to investigate possible anisotropy (Figure 1b). Plugs 1, 1 cr, 3 and 3 cr were drilled from the same rocks (cr denotes a plug extracted at the same position but perpendicularly), while for a fourth sample, plugs were extracted only in one direction but with different lengths (4l and 4s). The core plugs have a radius of 19 mm and lengths between 40 and 62 mm (Table 3). SIP measurements in the rock plugs were conducted in a sample holder similar to the one used by [PERSON] et al. [46], with a length of 250 mm and a radius of 19.5 mm. Non-polarizing voltage electrodes are located in the middle of the sample holder at a distance of 50 mm \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Elements** & _d = 1-2 mm (\%)_ & _d > 10 mm (\%)_ & _d < 0.45 mm and_ _d = 0.45-1 mm (\%)_ & _d = 0.45-1 mm and_ _d > 10 mm (\%)_ \\ \hline SiO\({}_{2}\) & 35.0 & 37.0 & 36.0 & 36.0 \\ TiO\({}_{2}\) & 0.6 & 0.5 & 0.6 & 0.5 \\ Al\({}_{2}\)O\({}_{3}\) & 9.4 & 8.9 & 11.5 & 9.9 \\ FeO & 1.0 & 2.1 & 0.9 & 1.5 \\ MnO & 0.01 & \textless{}0.01 & 0.01 & 0.01 \\ MgO & \textless{}1 & \textless{}1 & \textless{}1 & \textless{}1 \\ CaO & \textless{}1 & \textless{}1 & \textless{}1 & \textless{}1 \\ Na\({}_{2}\)O & 0.9 & 0.7 & 0.9 & 1.0 \\ K\({}_{2}\)O & 3.8 & 4.2 & 3.7 & 4.1 \\ P\({}_{2}\)O\({}_{5}\) & \textless{}0.5 & \textless{}0.5 & \textless{}0.5 & \textless{}0.5 \\ S & 0.5 & 1.1 & 0.6 & 0.8 \\ H\({}_{2}\)O\({}^{110\text{-}\text{C}}\) & 0.3 & 0.3 & 0.3 & 0.2 \\ H\({}_{2}\)O+ & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 \\ C & 47.7 & 44.2 & 45.1 & 45.4 \\ Summe & 99.2 & 99.0 & 99.5 & 99.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Laboratory analysis (X-ray fluorescence spectroscopy and LECO analysis) of four graphite sediments collected at the site. The grain sizes of the analyzed samples (_d_) are given for comparison. from the plate electrodes used for the current injection, which are made of stainless steel found at both ends of the column. To investigate the SIP response for varying textural properties, we ground the same rock that was used to extract the plugs 4s and 4l.