chapter2 - 1 2. Electrical resistivity methods ● The...

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Unformatted text preview: 1 2. Electrical resistivity methods ● The resistivity method is used in the study of horizontal and vertical discontinuities in the electrical properties of the ground. ● It utilizes direct currents or low frequency alternating currents to investigate the electrical properties (resistivity) of the subsurface. ● A resistivity contrast between the target and the background geology must exist. 2 Possible applications of resistivity surveying Fig. 1: Groundwater exploration 3 Possible applications of resistivity surveying Fig. 2: Mineral exploration, detection of cavities 4 Possible applications of resistivity surveying Fig. 3: Waste site exploration 5 Possible applications of resistivity surveying Fig. 4: Oil exploration 6 2.1 Resistivity Fig. 5: Ohm' s law A direct current with strength I [A] flows through a conductor of a limited size. I = qV l current strength [A] Voltage [V] cross section [m²] length resistivity conductivity I : V : q : l : : = 1 / : m (2.1) 7 Resistivity This can be written alternatively in terms of field strength (E [V/m]) and current density (j [A/m²]). 1.6 × 10 − 8 m 10 16 m = E / j [ m ] Resistivity is one of the most variable physical properties. native silver pure sulfur 8 Rock types and resistivity Igneous rocks → highest resistivities Sedimentary rocks → tend to be the most conductive due to their high fluid content Metamorphic rocks → have intermediate but overlapping resistivities Age of the rock is also important for the resistivity. For example: Young volcanic rock (Quaternary) Old volcanic rock (Precambrian) ≈ 10 − 200 m ≈ 100 − 2000 9 Rock types and resistivity Most rock-forming minerals are insulators: 10 8 − 10 16 m However, measurement in-situ : sedimentary rocks: metamorphic/crystalline rocks: 5 − 1000 m 100 − 10 5 m Reason : Rocks are usually porous and pores are filled with fluids, mainly water. As the result, rocks are electrolytic conductors. Electrical current is carried through a rock mainly by the passage of ions in pore waters. ➔ Most rocks conduct electricity by electrolytic rather than ohmic processes. 10 Law of Archie n ≈ 2 0.5 a 2.5 1.3 m 2.5 = a − m S − n w ≈ 1 / 2 Observation: : porosity where Empirical law of Archie S w : fractional pore volume (porosity) : fraction of the pores containing water : resistivity of water (2.2) 11 Example for the application of Archie' s law / w = 6 for = 0.5 / w = 17 × 10 4 for = 0.3 / w = 150 for = 0.1 / w = 1.5 × 10 4 for = 0.01 S = 1 a = 1.5 m = 2 12 Schematic current flow in soil sample...
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chapter2 - 1 2. Electrical resistivity methods ● The...

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