Part II: Self Potential Method and Induced Polarization (IP)

Size: px

Start display at page:

Download "Part II: Self Potential Method and Induced Polarization (IP)"

Patrick Osborne
5 years ago
Views:

1 Part II: Self Potential Method and Induced Polarization (IP) Self-potential method (passive) Self-potential mechanism Measurement of self potentials and interpretation Induced polarization method (active) Sources of the IP effect IP measurements: TD and FD

2 1. Self-potential method Basic idea: measure natural potential differences in the subsurface (without injecting current) Origin of SP: Spontaneous polarization occurs about dissimilar materials, near varying concentrations of electrolytic solutions (electrochemical effects), and due to the flow of fluids (electrokinetic effect). The controlling factor is underground water, be it simply by flowing through pore space (streamline potential), be it by electrochemical effects (for example, corrosion/weathering) Where do we find (large) SPs: zones containing sulfides, magnetite, graphite ( natural batteries )

3 Advantage: measurement is simple very well suited for detecting metallic bodies in the subsurface Drawback: Qualtitative, low penetration depth (30 m) SP measurements useful for resisitivity measurements to eliminate noise (background voltages) Electrokinetic effects interesting for engineering purposes: - Flow of gasses and fluids in pipes - leakage of a reservoir within the foundation or abutment of a dam - movement of ionic fluids to or within the groundwater - flow of geothermal fluids - movement of water into or through a karst system

4 2. Mechanism of self-potential Four different electrical potentials are recognized: A) Streaming potential caused by flow of a liquid with certain electrical properties under a pressure gradient through a membrane, pipe, capillary, or porous medium (with different electrical properties). 1 E k = ζ P k 4πη ρ E k = resulting electrokinetic potential; E k > 0 if higher potential is on the high pressure side ζ = adsorption potential of double layer between solid and solution) k = solution dielectric constant (= effective permittivitiy ε) P = pressure difference (causing fluid flow through pore space) ρ = electrical resistivity η = viscosity

5 Adsorption potential of and potential drop across solid-liquid double layer Zeta potential (ξ ) is the electric potential that exists at the "slip plane" that is, at the interface between the hydrated particle and the bulk solution

6 Steady state: =>=> + <= = steady voltage (streaming potential) Advective electric current =>=> Counterbalanced by return flow (electromigration) <=

8 Electrokinetic decontamination (remediation) of soils: Electroosmotic potential is proportional to ζ potential but ζ varies with soil type, ph, as well as with type and concentration of ions in the solution => determine ζ potential before treatment

9 Variation of ζ of soils (kaolinite) with surfactant concentration Effect of ph on ζ potential of soils with 10 3 M AlCl 3 in the presence of 10 3 M surfactant concentrations

10 B) Liquid-junction, or diffusion, potential is caused by the displacement of ionic solutions of dissimilar concentrations. E d = RT ( I Fn( I a a + I I c c ) ) ln C C 1 2 R = gas constant, 8.31 (J/K) F = Faraday constant (C/mol) T = absolute temperature in K I a = mobility of anions I c = mobility of cations C = solution concentration Na + Cl - : I a /I c =1.49 (@25 C) => E d [mv] = 11.6 ln C C 1 2

11 C) Nernst, or shale, potential occurs when similar conductors have a solution of differing concentrations about them. E s = RT Fn C ln 1 C 2 n=1 => E s [mv] = 59.1 ln C C 1 2 Combined diffusion and shale potential: Electrochemical self-potential Na + Cl - : E s [mv] ϑ[ C] = ln C C 1 2

12 D) Mineralization, or electrolytic contact, potential is produced at the surface of a conductor with another medium.

15 2. Induced polarization (IP) method 1. Time-domain IP Chargeability M

17 3 sec charging time, then 1 sec integration

20 a) normal distribution of ions in a porous sandstone; b) membrane polarization due to an applied DC voltage

21 electrode polarization due to an applied DC voltage

22 Frequency-domain IP ρ 0.1 Hz ρ 10 Hz PFE = 100 ρ0.1 ρ ρ PFE=Percentage frequency effect PFE = 2π 10 5 ρ0.1 ρ ρ ρ MF= Metal factor

1. Resistivity of rocks

1. Resistivity of rocks RESISTIVITY 1) Resistivity of rocks 2) General principles of resistivity surveying 3) Field procedures, interpretation and examples 4) Summary and conclusions INDUCED POLARIZATION 1) General principles