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Transcript
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.
Fig. Macroscopic effect of grain polarisation
over a disseminated ore body.
“Equivalent”
electrical circuit
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-mineralelectrolyte 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):
Apparent resistivities at low
and higher frequencies
Impedence of the
capacitor decreases
with frequency:
Z=1/iC; hence the
total resistance
decreases.
• FE can also be expressed as the Metal Factor (variation of apparent
conductivity):
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:
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
•
The critical frequency at which the maximum phase shift is measured is indicative of :
Independent of
resistivity
Cole-Cole complex electrical resistivity
spectra
Equivalent electrical circuit for the
Cole-Cole model
Chargeability of various materials
Material
Chargeability (ms)
Groundwater
Alluvium
Gravels
Precambrian volcanics
Precambrian gneisses
Schists
Sandstones
Argilites
Quartzites
0
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
0-200
10-100
0-60
0-80
10-60
1-10
1
0
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

Ma
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)
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 crosssection in support of a paleometallurgical investigation
(Florsch et al., 2011)