Materials Nominal Resistivity (Ω-m) Quartz 3 x 102 - 106 Granite 3 x 102 – 106 Granite (weathered) 30 – 500 Consolidated shale 20 – 2 x 103 Sandstones 200 – 5000 Sandstone (weathered) 50 – 200 Clays 1 - 102 Boulder clay 15 – 35 Clay (very dry) 50 - 150 Gravel (dry) 1400 Gravel saturated 100 Lateristic soil 120 -750 Dry sandy soil 80 - 1050 Sand clay/clayed sand 30 -215 Sand and gravel (saturated) 30 – 225 Mudstone 20 – 120 Siltstone 20 -150

Table 2: Resistivity Values for Some Common Geological Materials (Anthony et al. 2006).