Electrical properties of rocks by Atharv Singh Department of Geology BBAU

Presentation of Geophysics

1. Geophysics presentation NAME: ATHARV SINGH CLASS : B. Sc(HONS) GEOLOGY SEMESTER: 5 ROLL NO:191409 TOPIC: ELECTRICAL PROPERTIES OF ROCKS TEACHER: MR. SAUMENDRA KUMAR

2. Contents Piezoelectric and pyroelectric TERMINOLOGY RELATED TO ELECTRIC PROPERTIES OF ROCKS FACTORS AFFECTING ELECTRICAL PROPERTIES OF ROCKS APPLICATIONS OF ELECTRICAL PROPERTIES CONCLUSION

3. Pyroelectric and peizoelectric properties ELECTRICAL CONDUCTION OCCURS WHEN A MINERAL‘S ELECTRONS CAN MOVE THROUGHOUT IT’S STRUCTURE. MOSTLY IN MINERALS CONTAINING METALLIC BONDS. PIEZOELECTIC MINERALS ARE THOSE MINERALS WHICH ARE CHARGED BY PREASSURE CHANGE PYROELECYRIC MINERALS ARE THOSE MINERALS WHICH ARE CHARGED BY TEMPERATURE CHANGE

4. Resistance (R) is defined as being one ohm when a potential difference (voltage; V) across a specimen of one volt magnitude produces a current (i) of one ampere; that is, V = Ri. The electrical resistivity (ρ) is an intrinsic property of the material. In other words, it is inherent and not dependent on sample size or current path. It is related to resistance by R = ρL/A where L is the length of specimen, A is the cross-sectional area of specimen, and units of ρ are ohm-centimetre; 1 ohm-centimetre equals 0.01 ohm-metre. The conductivity (σ) is equal to 1/ρ ohm -1 · centimetre-1 (or termed mhos/cm). In SI units, it is given in mhos/metre, or siemens/metre. Basic terms related to electricity

5. FaCTORS RESPONSIBLE FOR CONDUCTION Electrical conduction occurs in rocks by (1) fluid conduction—i.e., electrolytic conduction by ionic transfer in briny pore water—and (2) metallic and semiconductor (e.g., some sulfide ores) electron conduction. If the rock has any porosity and contained fluid, the fluid typically dominates the conductivity response. The rock conductivity depends on the conductivity of the fluid (and its chemical composition), degree of fluid saturation, porosity and permeability, and temperature. If rocks lose water, as with compaction of clastic sedimentary rocks at depth, their resistivity typically increases.

6. Range of conductors Materials that are generally considered as “good” conductors have a resistivity of 10-5–10 ohm-centimetre (10-7–10-1 ohm-metre) and a conductivity of 10–107 mhos/metre. Those that are classified as intermediate conductors have a resistivity of 100–109 ohm-centimetre (1–107 ohm-metre) and a conductivity of 10-7–1 mhos/metre. “Poor” conductors, also known as insulators, have a resistivity of 1010–1017 ohm-centimetre (108–1015 ohm-metre) and a conductivity of 10-15–10-8. Seawater is a much better conductor (i.e., it has lower resistivity) than fresh water owing to its higher content of dissolved salts; dry rock is very resistive. In the subsurface, pores are typically filled to some degree by fluids.

7. Tables showing Resestivity( ohm- cm) Rocks in situ
sedimentary clay, soft shale 100–5(103)
hard shale 7–50(103)
sand 5–40(103)
sandstone (104)–(105)
glacial moraine 1–500(103)
porous limestone 1–30(104)
dense limestone >(106)

8. Rock salt (108)–(109)
igneous 5(104)–(108)
metamorphic 5(104)–5(109)
rocks in laboratory
dry granite 1012
minerals
copper (18 °C) 1.7(10−6)
graphite 5–500(10−4)
pyrrhotite 0.1–0.6
magnetite crystals 0.6–0.8
pyrite ore 1–(105)
magnetite ore (102)–5(105)
chromite ore >106
quartz (18 °C) (1014)–(1016)

9. Graphical representation

10. Electrical properties of rocks

11. Most rocks contain pore-spaces which are at least partially saturated with ionic fluids. These fluids include: fresh water, brackish water, ocean water and brine. Because pore fluids have a higher conductivity than most rock-forming minerals, electrical current generally prefers to flow through the pore-space whenever possible. As a result, the bulk conductivity of the rock depends significantly upon its porosity, fluid saturation and the type of fluid contained within the pore-space.For rocks which are unsaturated, the pore space is occupied solely by air. Because air is very resistive, it forces the current to flow through the minerals comprising the rock. As a result, unsaturated rocks are poorly conductive. When a sufficient percentage of the pore-space is saturated, the pore fluid is able to offer a more efficient pathway for the current. Thus, the bulk conductivity of rocks generally increases as fluid saturation increases. Factors affecting rock conductivity

12. Factors effecting conductivity of rock Current flows through a rock’s pore-fluid via ionic conduction. As a result, the conductivity of the pore-fluid depends on the concentration of dissolved ions. Pore-fluid conductivity increases as the concentration of dissolved ions increases. This implies that rocks containing more brackish pore fluid are more conductive than rocks containing fresh-water.

13. Tortusity Tortuosity defines the connectivity and complexity of a rock’s pore-space network. For rocks with low tortuosities, the current’s path through the pore space is simple; resulting in efficient conduction of electrical charges. For rocks with high tortuosities, the path the current must take to get through the rock is very indirect. As a result, conduction is inefficient, and the rock is more resistive.

14. Mineralization Electrical current within a rock will choose not to flow through the pore-space if the rock forming minerals are more conductive. This occurs frequently in ore-bearing rocks due to the presence of metal-oxides (magnetite, illmenite, specular hematite), metal-sulphides (pyrite, pyrrhotite, galena) and native metals (gold, silver, copper). One exception is graphite, which despite being entirely comprised of carbon, is very conductive. As expected, the conductivity increases as the concentration of conductive minerals within the rock increases

15. ChargeAbility Chargeability is a physical property related to conductivity. As we learned previously, ionic charges within a rock’s pore water begin to move under the influence of an electric field, resulting in electrical current. However, some of the pore ions do not move uninhibited through the rock and begin to accumulate at impermeable boundaries. This build-up of ionic charges is commonly referred to as induced polarization (IP), as it is responsible for generating electric dipole moments within the rock. We use chargeability to characterize the formation and strength of the induced polarization within a rock, under the influence of an electric field.

16. Factors Impacting Chargeability Sulphide Mineralization:

As we discussed earlier, electrode polarization occurs when the pore path is blocked by metallic particles. A major source of these metallic particles is sulphide mineralization. As the abundance of sulphide minerals within a rock increases, so does the electrode polarization. Therefore, highly mineralized rock tend to be very chargeable.

17. Factors affecting chargeability Clays have a tendancy to partially block the path which ions take through the rock’s pore water. Upon application of an electric potential, positive charge carriers pass easily, while negative carriers accumulate. This results in an “ion-selective” membrane polarization. Clays represent a dominant source of induced polarization in unmineralized sedimentary rocks.

A surplus of both cations and anions occurs at one end of the membrane, while a deficiency occurs at the other end. The reduction of mobility is most obvious at frequencies slower than the diffusion time of ions between adjacent membrane zones; i.e. Slower than around 0.1 Hz. Conductivity increases at higher frequencies.

18. Pore-Water Salinity:

The induced polarization within a rock depends on having a mechanism for accumulating ionic charges. It also depends on the salinity of the pore water; i.e. The concentration of ions within the pore water. As the pore-water salinity increases, so does the capacity of the rock to support a build-up of ionic charges. This results in an increases chargeability for the rock.

19. Chargeability of rocks Material type Chargeability (msec) ground water alluvium 1-4 gravels 3-9 precambrian volcanics 8-20 precambrian gneisses 6-30 schists 5-20 sandstones 3-12 argilites 3-10 quartzites 5-12

20. Bibliography FUNDAMENTAL OF GEOPHYSICS BY WILLIAM LOWRIE RESEARCH GATE BRITANNICA.COM Science direct MINEROLOGY BY DEXTER PERKINS Gpg.geoscience.xyz

21. Thanks to everyone