Lecture N. 6: Induced Polarisation (IP) H. SAIBI November 6 , 2014

Outline Principles; Areas of application; Measurement; Equipment and layout; Interpretation; Case histories.

Generic Subsurface Charge Distribution

Induced Potentials After current is switched off (or turned on), the voltage between potential electrodes takes 1s -1 min to decay (or build up) The ground acts somewhat like a capacitor. Overvoltage decay times and rise times are measured and are diagnostic of the nature of the subsurface. Applications: Metallic deposits with low EM anomalies and high resistivity; Disseminated Cu, Pb-Zn ores, Au; Pyrite, chalcopyrite, magnetite, clay, graphite. Fig. The overvoltage effect produced by induced polarisation after an applied current is switched off.

IP Techniques Time domain (pulse transient); Frequency domain (using harmonic signals): Traditional variable-frequency IP (using two or more frequencies of < 10 Hz); Phase domain (measure phase delays between current and voltage); Spectral IP (measure phases and amplitudes at frequencies 10-3 to 4·103 Hz). Using conventional resistivity arrays Most commonly double-dipole configuration; Schlumberger arrays for broad reconnaissance surveys.

Origin of IP Macroscopic IP s sensitive to dielectric rater than conductivity characteristics. Disseminated (poorly conductive) ore body is polarized (develops surface charges) by the imposed current; When the current is switched off, the charges cause transient current through the conductive overburden. These currents flow in the same direction and cause the overvoltage effect. “Equivalent” electrical circuit Fig. Macroscopic effect of grain polarisation over a disseminated ore body.

Origin of IP Microscopic Grain (and electrode) polarization: Electrolytic (membrane) polarization: Fig. Development of membrane polarisation associated with (A) a constriction within a channel between mineral grains, and (B) negatively charged clay particles and fibrous elements along the sides of a channel (Fraser et al., 1964)

The electrical double layer at the clay-mineral-electrolyte interface (Revil et al., 2012)

Time-domain IP Measuring apparent chargeability (M) Apparent chargeability (Ma) increases with increasing duration of the pulses (3-5 s); Graphite has Ma=11.2 ms, magnetite - 2.2 ms at 1 s integration. Polarisation voltage Chargeability: Overvoltage Observed voltage Overvoltage Apparent chargeability: Fig. (A) Application of a pulsed current with alternate polarity, and the consequent measured voltage showing the effect of the overvoltage (Vp) and the rise-time on the leading edge of the voltage pulse. (B) To forms of measurement of the overvoltage at discrete time intervals V(t1), etc., and by the area beneath the overvoltage curve (A).

Variable-frequency IP Using the same array as in DC resistivity measurements but driving AC current at several frequencies. Measuring a (frequency): a decreases with frequency; This decrease is measured as the Frequency Effect (FE): FE can also be expressed as the Metal Factor (variation of apparent conductivity): Apparent resistivities at low and higher frequencies Impedence of the capacitor decreases with frequency: Z=1/iC; hence the total resistance decreases. Apparent conductivities at low and higher frequencies

Spectral (complex resistivity) IP Using AC current at a range of frequencies from 30 to 4000 Hz. Measuring complex impedance: The Cole-Cole model for complex resistivity: Geometric factor of the array IP chargeability DC resistivity Angular frequency Time constant (Relaxation time) Typical values: M: 0-1 (depending on mineral content) : 10-4-104 (depending on grain size) c: 0.2-0.6 (depending on grain size distribution) Fig. A typical IP spectral response (Pelton et al., 1983)

Cole-Cole relaxation spectra For varying frequencies, complex resistivity describes a semicircle in (ReZ, ImZ) plane: The critical frequency at which the maximum phase shift is measured is indicative of : Fig. Cole-Cole relaxation spectra for Debye and Cole-Cole dispersions for =1/2 and c=0.5 (Pelton et al., 1983) =1/2 and c=0.5 Independent of resistivity

Cole-Cole complex electrical resistivity spectra

Equivalent electrical circuit for the Cole-Cole model

Chargeability of various materials Chargeability (ms) Groundwater Alluvium Gravels Precambrian volcanics Precambrian gneisses Schists Sandstones Argilites Quartzites 1-4 3-9 8-20 6-30 5-20 3-12 3-10 5-12 Material Metal factor (mhos/cm) Massive sulfides Fracture-filling sulfides Massive magnetite Porphyry copper Dissem. Sulfides Shale-sulfides Clays Sandstone- 1-2% sulfides Finely dissem. Sulfides Tuffs Graphitic sandstone and limestone Gravels Alluvium Precambrian gneisses Granites, monzonites, diorites Various volcanics Schists Basic rocks (barren) Granites (barren) Groundwater 10000 1000-10000 3-3000 30-1500 100-1000 3-300 1-300 2-200 10-100 1-100 4-60 0-200 0-60 0-80 10-60 1-10 1

Displays of IP data Profiles and maps of apparent chargeability (time-domain IP); Pseudo-sections (combined with a ) Fig. How to plot a pseudo-section. For a dipole-dipole array with current and potential electrodes at 1-2 and 3-4 respectively (n=1), the measuring point is plotted at A; for dipoles at 4-5 and 8-9 (n=3), the data value is plotted at B.

Environmental Applications Freshwater lens is well detected by chargeability For saline water lens, both a and Ma Show only broad anomalies Ratio of overvoltages at 0.5 and 5 minutes is a good indicator  a Fig. Scale model experimental results of apparent resistivity, chargeability, and ratio () of overvoltage measured after 0.5 minutes and 5 minutes after current switch obtained across a buried hemispherical lens of (A) freshwater and (B) saltwater. After Ogilvy and Kuzmina (1972) Ma

IP Case History Identification of a contamination with cyanide complexes (slags from plating works; Cahyna et al., 1990); Resistivity survey failed to detect the contamination; IP chargeability identified both the known and unknown slag deposits. Fig. Chargeability map over a site contaminated with cyanide complexes. The location of a known outcrop of slag is indicated at A. Contours are in % chargeability. Shaded areas indicate the interpreted extent of contaminated land. From Cahyna et al., 1990.

IP response of a bezene contaminant plume, USA, along with contours of benzene concentration (Sogade et al., 2006)

Imaginary conductivity as a function of interfacial geometric factor Sp for different geomaterials (Kruschwitz et al., 2010)

Apparent intrinsec chargeability map and vertical cross-section in support of a paleometallurgical investigation (Florsch et al., 2011)