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Introduction to Geophysics

The Induced Polarisation (IP) Method

1

Induced Polarisation “equivalent circuits”

•

DC Resistivity

-

I

+ •

Induced Polarisation

-

I

+ completely described by Ohm’s law +

-

C

ERTH2020 𝑹 = 𝑼 𝑰 resistance

R

“capacitance ” (charge / voltage) 2

Induced Polarisation

Three main causes

1) Electrochemical processes at the interface of metallic minerals / pore fluid:  presence of ore deposits.

2) Exchange reactions in clay and shaly sands:  hydrogeological applications.

3) Reactions involving organic materials:  hydrocarbon exploration.

IP - Main Applications:

 disseminated metallic ores ⁻ ⁻ ⁻ porphyry coppers, bedded lead/zinc sulphide-related gold deposits  environmental related studies  geothermal exploration ERTH2020 Veeken et al., 2009 ; Reynolds, 2011 3

Induced Polarisation

• The earliest observation ( ~ 1913) of the induced polarization phenomenon associated with sulphide mineralization is attributed to Conrad Schlumberger who observed that if he passed a DC current through rocks containing metallic sulphides and interrupted the current abruptly, the resultant voltages in the Earth decayed slowly rather than instantly.

• Today IP is the primary tool used to explore for several important types of mineral deposits—especially porphyry coppers, bedded lead/zinc and sulphide-related gold deposits. • IP is unique among the controlled-source geophysical methods employed in mineral exploration in that it is based on an interface electrochemical phenomenon, rather than on a purely physical property of rocks or minerals.

ERTH2020 Seigel et al, 2007 4

Induced Polarisation

ERTH2020 Seigel et al, 2007 5

Induced Polarisation

DC & IP over polymetallic deposit in the Altai region (USSR) in the late 1960s (Schlumberger array with AB = 1200 m and MN = 20 m ).

Charging current was 2 minutes and the integration time was 0.5 s. [...] The deposit does show as a minor resistivity depression, but is much more clearly indicated by its IP response, both in the time and frequency domains.

ERTH2020 Seigel et al, 2007 6

Induced Polarisation

Principally with the same equipment as Resistivity Measurements:

C 1 P 1 P 2 C 2

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Induced Polarisation

• DC resistivity →direct electrical connection (electrodes) → flow of current → electrical potential in the ground • IP methods

C 1 P 1 P 2 C 2

→direct electrical connection (electrodes) → flow of current

switched off

→

decay

of electrical potential ERTH2020 8

Induced Polarisation

• Reconnaissance or deep IP surveys often use large current electrodes buried in deep, saline-filled holes (Hence the benefit of electrode arrays where the current electrodes do not need to be moved for each reading).

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Small IP surveys often use porous-pot type electrodes

(Telford, 1990) 9

Induced Polarisation

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Induced Polarisation

• IP surveys usually use a separate transmitter and receiver • Power requirements are higher than for DC res. surveys • Cables and electrodes must be watched. If a passer-by or

animal touched the current electrodes during data acquisition, this could be fatal

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IP Effect

IP-Effect:

(below 1kHz or greater than 1 ms) • If a DC current injected into the ground is

abruptly switched off , the voltage measured at the potential electrodes does not immediately drop to zero!

C 1 P 1 P 2 C 2

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IP Effect

Steady state voltage: Vp (primary voltage) Residual voltage: (secondary voltage) Vs charge time IP effect ERTH2020 13

IP Effect

• Voltage shows a large initial decrease, then decays slowly over a timescale of seconds (minutes). This is the IP effect • The rate of decay depends on the electrical properties of the ground and the presence of metallic minerals • The decay voltage is the result of storage of energy by the ground during the period when the DC current is on • The effect cannot be explained in terms of the atomic or

molecular structure of the material, but depend on the macro structure.

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IP Effect – Sources

• Chemical energy is the main source of the IP effect which is

stored by subsurface structures in two main ways: Electrode polarisation

(overvoltage) (~below 1kHz) – Related to the transition between electrolytic and electronic conduction at the interfaces between pore fluids and metallic minerals in the rock – Larger than the normal IP effect – Requires presence of metallic minerals (or graphite)

Membrane polarisation

(electrolytic) (~below 1Hz) – Due to variations in the mobility of ions contained within pore fluids – Called the “normal” IP effect – May occur in rocks which contain no metallic minerals ERTH2020 15

IP Effect – Electrode Polarisation

ERTH2020 • Electrode (or grain) polarisation same process as self-potential.

Metal electrode in an ionic solution: • •

No voltage applied:

charges with different polarities separate potential difference between electrode and solution.

• •

With voltage applied:

currents start flowing change in potential difference •

Voltage turned off:

ions diffuse back to equilibrium The total magnitude of the potential is the Nernst potential and the adsorbed layer gives rise to the Zeta potential 16 Reynolds, 2011, p.374

IP Effect – Electrode Polarisation

Electrolytic conduction only (no IP) Electrode polarisation occurs when electricity is conducted partly

electrolytically

and partly

electronically

Electrolytic and electronic conduction    When metallic mineral grains block the pore spaces in a rock, an electrochemical barrier must be overcome in order for current to flow across the grain surfaces Ions accumulate at grain surfaces and the grains become Polarised When the current flow is interrupted, ions return to their equilibrium positions  voltage decay ERTH2020 17

IP Effect – Membrane Polarisation

• Many minerals (e.g. clays) have a net -ve charge at the interface between mineral surface and pore fluid • +ve ions are attracted to the surface and -ve ions repelled • Build-up of a layer (“cationic cloud”) of +ve ion conc n may extend 1 mm into the pore fluid which

(Equilibrium: No applied electrical field)

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IP Effect – Membrane Polarisation

( Applied electrical field)

• Zone of +ve ion conc n may extend 1 mm into pore fluid: if the pore has diameter < 1 mm, then, when a voltage is applied , -ve ions will accumulate on one side of the pore and leave the other • When the voltage is removed, the ions return to their equilibrium positions  voltage decay • Membrane polarisation is largest when a rock contains clay materials scattered through the matrix in small (~10%) concentrations and in which the electrolyte has some salinity ERTH2020 19

IP Effect – Electrode Polarisation

IP observed in mixtures of pyrite and quartz sand for various pyrite grain sizes

ERTH2020 • IP effect depends on grain size • Large sulphide grain → large amount of current through it, but small surface/volume ratio • IP however is a function of the amount of grain surface exposed to the electrolytic solution • Therefor, as the grain size is reduced, the IP effect increases • However for very small grain sizes, the surface resistance is too large → greatest IP effect for intermediate values of sulphide grain sizes (Keller and Frischknecht, 1966) 20

IP Effect – Sources

• In practice, it is not possible to distinguish between membrane and electrode polarisations on the basis of geophysical IP measurements • Membrane polarisation 1% - 2%) may give rise to a “ background ” IP effect equivalent to 0.1% - 10% conductive minerals (typically • IP is a bulk effect : it does not depend on atomic-scale rock or mineral properties ERTH2020 21

IP Effect – Sources

• Electrode polarisation depends strongly on the surface area • The IP method is more sensitive to disseminated conductors ones than to massive • This sets the IP method apart from the DC resistivity and EM (electromagnetic) methods, which typically give a weak response over a disseminated target

Time-domain IP profile using a pole–dipole array over the Gortdrum copper–silver body in Ireland

Chargeability Apparent resistivity ERTH2020 (P. Kearey et al., 2007) 22

Time-Domain IP Measurements

• IP measurements can be made in either the time-domain or

frequency-domain

(frequency-domain IP won’t be covered today) – An advantage of time-domain systems is that measurements can be

made over several transmitter cycles and then averaged (or stacked). This process reduces the effect of random noise.

• Current and potential electrodes

normal DC resistivity survey are arranged as for a

• In time-domain (TD) systems, the transmitter current is

abruptly switched off, and the decaying voltage due to the IP effect is measured at a series of delay times

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Time-Domain IP Measurements

Typical transmitted and received waveforms in time-domain Charging time Off-time ERTH2020 24

Time-Domain IP Measurements

• Effect of chargeable ground ERTH2020 University of British Columbia (UBC-GIF) 25

Time-Domain IP Measurements

• In time-domain IP, the main parameters used to present and interpret data are apparent resistivity ( r a ) and chargeability (

m

) • m is a macroscopic physical parameter which represents all of the microscopic phenomena.

• The apparent resistivity is calculated as for DC resistivity using the voltage measured before the transmitter is switched off (denoted V p )

V P

depends on the “charging time” 𝜌 𝑎 = 𝐾 𝑉 𝑃 𝐼 K = geometric factor (depends on electrode array) • The measured V p for a short charging time will be less than that measured for a long charging time this means that r a calculated for a high frequency Tx waveform will be less than that for a low frequency Tx waveform (the frequency-domain IP effect) ERTH2020 26

Time-Domain IP – Chargeability

• The ratio V s /V p is called the chargeability (Units: millivolts per volt) • In practice it is impossible to measure V s (the voltage at current switch-off) • Instead, after an initial delay (  500 msec), the decay voltage is measured at a series of (typically four) delay times. • Measured voltages are then used to approximate the area under the decay curve ERTH2020 27

Time-Domain IP – Apparent Chargeability

• The apparent chargeability , m a , is defined by

m a

 1

V p t

1

t n



V

(

t

)

dt

(Units: milliseconds) • where t n is the time corresponding to the last voltage measurement (on the previous slide, n = 4) and V(t) is the decay voltage at time t • The apparent chargeability is the area under (part of) the voltage decay curve, divided by the “primary” voltage V p • In practice, the units are milliseconds (ms) • The apparent chargeability depends on the actual values of t 1 and t n , and

may be different for different field instruments

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Time-Domain IP – Apparent Chargeability

• Apparent chargeability also depends on the charging time (long charging times give larger decay voltages) • A highly polarisable earth will give rise to a longer IP decay and hence a large chargeability • Because of the practical considerations outlined, the apparent chargeability isn’t equal to the actual chargeability of the ground, even in the case of a uniformly polarisable earth. • Note that the DC resistivity measurement made in the course of an IP survey is useful data. Chargeability is usually interpreted together with the resistivity data. ERTH2020 29

Chargeabilities of rocks

material m (ms) material m (ms) mineral m (ms)

Charging time 1 minute Integration 1 minute ERTH2020 Charging time 3 seconds Integration 0.02 to 1 second 1% Volume concentration Charging time 3 seconds Integration 1 second (from Telford et al., 1990) 30

Electrode arrays: Gradient

• Any of the common DC resistivity electrode arrays may be used for IP surveys - the two most commonly used are the dipole-dipole and gradient arrays .

• For mineral exploration, the gradient array is similar to the Schlumberger array, except that the potential electrodes do not have to be kept in-line with the current electrodes

Plan View A,B current electrodes (fixed) M,N potential electrodes (roving) Because the current electrodes are not moved, the gradient array is useful for reconnaissance surveying of relatively large areas

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Electrode arrays: Dipole-dipole

• With the dipole-dipole array, measurements of apparent resistivity and apparent chargeability are made at several “n-spacings” for each current electrode setup

n = 1, 2, 3, etc.

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2D electrical imaging surveys

• Dipole-dipole IP data are commonly displayed as separate

pseudosections

of apparent resistivity and apparent chargeability 24 msec (n=4) 15 ohm-m ERTH2020 (Mutton and Williams, 1994) 33

2D electrical imaging surveys

• Combine vertical

(sounding) and lateral (profiling) survey method

– This provides a 2D geoelectrical model of the subsurface: • vertical and horizontal changes in electrical properties • assumption: no changes perpendicular to survey line – Typical 1D sounding surveys involve 10 – 20 readings – Typical 2D imaging surveys involve 100 – 1000 readings – In comparison, 3D would involve several 1000’s of readings ERTH2020 34

2D electrical imaging surveys

• Pseudosections using a variety of current and potential electrode separations in a single plot are a convenient means of plotting data acquired • They do not represent true cross-sections spacing” increases of r a and m a , except in the sense that the depth of penetration increases as the “n • As a rough rule-of-thumb, the depth of investigation is ( na / 2 ) for the dipole-dipole array • Although pseudosections are useful for displaying data and for assessing data quality, the resistivity and chargeability

pseudosections do not provide a realistic portrayal of the true subsurface distributions

of these parameters ERTH2020 35

2D electrical imaging surveys

Dipole-Dipole – combine Sounding and Profiling

45°

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2D electrical imaging surveys

• Horizontal location of data point at mid-point of set of electrodes used • Vertical position (pseudo-depth) of data point at a depth proportional to electrode spacings • The measured parameter is plotted at the intersection of 45° lines extending from the mid-points of the transmitter and receiver pairs • Note that this is a convention only and does not constitute the depth of investigation ERTH2020 37

2D electrical imaging surveys

Centenary gold deposit, WA

(disc. 1996) • The Centenary gold deposit is a concealed ore body located 110 km north of Leonora, Western Australia

survey line ore body

• The ore body is associated with sulphides and is hosted in the magnetic

drill holes

portion of the Mount Pickering Dolerite. • Due to its sulphidic nature, both gravity and induced polarization (IP) high chargeability (ore body) were trialled soon after discovery. • conductive overburden A dipole–dipole IP and resistivity survey detected a significant chargeability anomaly over Centenary.

low resistivity (ore body) ERTH2020 Pittard and Bourne, Exploration Geophysics, 2007, 38, 200–207 38

2D electrical imaging surveys

Centenary gold deposit, WA

(disc. 1996)

Example electrode polarisation survey line ore body drill holes

high chargeability (ore body) conductive overburden low resistivity (ore body) ERTH2020 Pittard and Bourne, Exploration Geophysics, 2007, 38, 200–207 39

IP Data Interpretation

• The most common method of interpretation of IP data is via automatic two-dimensional inversion • Inversion of IP data results in cross-sections of resistivity and chargeability vs depth, which are similar to geological cross sections • Inversion of the data is performed in “real time” by some instruments, and inverted sections are now a standard product delivered by geophysical contractors • NB Remember resolution , suppression of features and model equivalence apply to any best-fit geophysical model, so be prepared to supply the relevant modelling information (or ask your contractor to do so) ERTH2020 40

IP Data Interpretation

Example electrode polarisation

• The Century deposit, approximately 250 km north-northwest of relatively flat-lying middle Proterozoic siltstone and shale units. Mineralization occurs preferentially within black shale • The recovered model after inversion shows the superposed overburden of limestones on the right. •

2D inversion

The resistivity at depth is not correlated with mineralization, however.

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IP Data Interpretation

Example electrode polarisation observed data 2D inversion observed data 2D inversion

ERTH2020 http://www.eos.ubc.ca/ubcgif/iag/casehist/century/intro.html

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IP Data Interpretation

Example membrane polarisation

Induced-polarization detection and mapping of contaminant plumes

EDB (0.02) EDB (10) EDB (100)

• 2D time-domain IP & DC Resistivity mapping of a contaminant plume at the Massachusetts Military Reservation.

monitoring wells survey line

• The plume consists of approximately 265 m 3 of fuel that erupted from a broken underground pipeline in the early 1970s. • Benzene and ethylene dibromide (EDB) are the primary contaminants exceeding the allowed maximum concentration levels.

Plan view of the plume site, indicating existing wells; geologic section line CC', IP survey line, as well as the ethylene dibromide (EDB) concentration plot

ERTH2020 Sogade et al, 2006 43

IP Data Interpretation

Example membrane polarisation Geological Cross-Section EDB (0.02) EDB (100) EDB (10)

ERTH2020 Sogade et al, 2006 44

IP Data Interpretation

Example membrane polarisation EDB (100)

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Dipole-dipole pseudosection, electrode separation a = 24.38

Extrapolated plume concentration data for benzene and EDB based on ground-monitoring wells

Sogade et al, 2006 45

IP Data Interpretation

Example membrane polarisation

2D resistivity Section

Conductive zone ~1000

𝛺 ∙ 𝑚

EDB (100) (Groundwater?) Conductive zone ~300

𝛺 ∙ 𝑚

(Clays?) Log Resistivity (

𝛺 ∙ 𝑚

)

2D IP Section

EDB (100) Chargeability Anomalies (Contaminants ?)

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Chargeability (mV/V)

Sogade et al, 2006 46

Summary

• The IP effect , voltage decay after switching off a DC voltage and membrane polarisation and electrode polarisation mechanisms a sources of this effect.

• Time-domain IP measurements, Tx and Rx waveforms • Determination of apparent chargeability • Display of IP data, pseudosections and depth of investigation • Inversion and interpretation of resistivity results and chargeability ERTH2020 47

References

Veeken P.C.H., Legeydo P.J., Davidenko Y.A, Kudryavceva E.O, Ivanov S.A., Chuvaev A.:

“Case History: Benefits of the induced polarization geoelectric method to

hydrocarbon exploration”, 2009, Geophysics, V74, p. B47–B59 Telford, W.M, Geldart, L.P., Sheriff, R.E.: “Applied Geophysics”, 1991, Cambridge University Press Reynolds, J.M., "An Introduction to Applied and Environmental Geophysics", 2011, John Wiley & Sons Seigel H., Nabighian M., Parasnis D., Vozoff K., “The early history of the induced polarization method”, March 2007, The Leading Edge, pp. 312 Sogade, J.A, Scira-Scappuzzo F., Vichabian Y., “Induced-polarization detection and mapping of contaminant plumes”, 2006, Geophysics, V71, p. B75–B84 ERTH2020 48

• Supplementary slides ERTH2020 49

IP Data Interpretation

• Assumption: the ultimate effect of chargeability is to alter the effective conductivity (resistivity) when current is applied (Seigel, 1959).

• This assumption permits the IP responses to be numerically modelled by carrying out two forward modellings using a DC resistivity algorithm measured potential in the absence of chargeability effects potential including chargeability effects the apparent chargeability can be computed by carrying out two DC resistivity forward modellings with conductivities 𝜎 1 − 𝜂 and 𝜎 ERTH2020 D.W. Oldenburg and Y. Li, 1994, "Inversion of induced polarization data", Geophysics, 59, P.1327-1341 50

IP Data Interpretation

IP

DC / IP data are gathered together

DC

Use 𝜎 -model for forward mapping of chargeability Invert potentials for conductivity (background) model Least-Squares Inversion Invert for chargeability models ERTH2020 51