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

Self-potential method (passive)

Self-potential mechanismMeasurement of self potentials and interpretation

Induced polarization method (active)

Sources of the IP effectIP measurements: TD and FD

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 (electrochemicaleffects), 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“)

Advantage: measurement is simplevery 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

2. Mechanism of self-potential

Four different electrical potentials are recognized:

A) Streaming potential

caused by flow of a liquid with certain electrical propertiesunder a pressure gradient through a membrane, pipe, capillary,or porous medium (with different electrical properties).

ρζπη

⋅⋅∆⋅⋅−= kPEk 41

Ek = resulting electrokinetic potential; Ek > 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 resistivityη = viscosity

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

Counterbalanced by return flow (electromigration) <=

Advective electric current =>=>

Steady state: =>=> + <= = steady voltage (streaming potential)

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

Variation of ζ of soils (kaolinite) with surfactant concentration

Effect of pH on ζ potential of soils with10−3 M AlCl3 in the presence of 10−3 M surfactant concentrations

B) Liquid-junction, or diffusion, potential is caused by the displacement of ionic solutions of dissimilar concentrations.

⎟⎟⎠

⎞⎜⎜⎝

⎛+−

−=2

1ln)()(

CC

IIFnIIRTE

ca

cad

R = gas constant , 8.31 (J/K)F = Faraday constant 9.65 ·104 (C/mol)T = absolute temperature in KIa = mobility of anionsIc = mobility of cationsC = solution concentration

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

2

1ln6.11]mV[CCEdNa+Cl-: Ia/Ic=1.49 (@25°C) =>

C) Nernst, or shale, potential occurs when similar conductors have a solution of differing concentrations about them.

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

2

1lnCC

FnRTEs ⎟⎟

⎠

⎞⎜⎜⎝

⎛⋅−=

2

1ln1.59]mV[CCEsn=1 =>

Combined diffusion and shale potential:Electrochemical self-potential

⎟⎟⎠

⎞⎜⎜⎝

⎛⋅

+°−=

2

1ln273

273][7.70]mV[CCCEs

ϑNa+Cl-:

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

2. Induced polarization (IP) method

1. Time-domain IP

Chargeability M

3 sec charging time, then 1 sec integration

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

„electrode“ polarization due to an applied DC voltage

Frequency-domain IP

ρ0.1 Hz

ρ10 Hz

10

101.0100ρ

ρρ −=PFE PFE=Percentage frequency effect

101.0

101.05102ρρρρπ −

⋅=PFE MF= Metal factor