2“equivalent circuits” Induced Polarisation“equivalent circuits”C+-IRInduced Polarisationresistance“capacitance ”(charge / voltage)I𝑹= 𝑼 𝑰+-DC Resistivitycompletely described by Ohm’s lawIn the previous lectures we saw that the subsurface reacts as a giant resistor when energised by a steady state-current and the bulk resistance could be described with Ohm’s law.However when a current is injected into the ground it also acts as a capacitor (or actually a battery), which stores electrical charge. The two phenomena can be depicted in an equivalent circuit as shown here on the right, where R is the earth resistance and C describes the capacitance. Upon turning off the (polarising) DC-current, the ground gradually discharged and returns to equilibrium which is the observed IP effect.Note that this simple RC circuit is an approximation only and does not fully describe the electrochemical phenomena, but it is useful as a “mental picture”.
3Induced Polarisation Three main causes Electrochemical processes at the interface of metallic minerals / pore fluid: presence of ore deposits.Exchange reactions in clay and shaly sands: hydrogeological applications.Reactions involving organic materials: hydrocarbon exploration.IP - Main Applications:disseminated metallic oresporphyry coppers,bedded lead/zincsulphide-related gold depositsenvironmental related studiesgeothermal explorationVeeken et al., 2009 ; Reynolds, 2011
4Induced PolarisationThe 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.Seigel et al, 2007
5Induced Polarisation Seigel et al, 2007 San Manuel porphyry copper deposit, under 330 m of Gila conglomerate.One of the first IP surveys in the US by Seigel during his PhD study (1948).Seigel et al, 2007
6Induced PolarisationDC & IP over polymetallic deposit in the Altai region (USSR) in the late 1960s(Schlumberger array with AB = 1200 m and MN = 20 m ).The results of dual time-domain and frequency-domain IP and resistivity traverses over a known polymetallic deposit in the Altai region (USSR) in the late 1960s. The deposit lies in volcanics and comes within about 30 m of the ground surface. These traverses employed the Schlumberger array with AB = 1200 m and MN = 20 m. For each measurement, charging current flowed for two minutes and the residual transient after 0.5 s was measured. Dual time-domain profiles of apparent polarizability (nk) are shown, for different polarities of the charging current flow. The small differences in nk for the different current polarities are attributed to nonlinear effects. Also shown are the results of the corresponding frequency-domain profile. The IP parameter measured was the phase shift of the measured voltage, using a frequency of 2.44 Hz. 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 (after Komarov, 1980).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.Seigel et al, 2007
7Principally with the same equipment as Resistivity Measurements: Induced PolarisationPrincipally with the same equipment as Resistivity Measurements:C1C2P1P2IP measurement principally use the same equipment as the DC resistivity method.What is observed in IP is that the voltage, which is measured at the potential electrodes, does not immediately drops to zero when the current is switched off.This means that there is some effect in the ground that stored the energy and then releases that energy after the current is switched off, i.e. there is some kind of capacitor.In DC resistivity measurements we inject the current and while the current is flowing it builds up a potential field which is measured as a voltage.In IP the measurement takes place after the current is switched off and we are interested in the decay of the electrical potential, measured as a time-varying (decaying) voltage.
8Induced Polarisation DC resistivity direct electrical connection (electrodes)flow of currentelectrical potential in the groundC1C2P1P2IP methodsdirect electrical connection (electrodes)flow of current switched offdecay of electrical potentialIP measurement principally use the same equipment as the DC resistivity method.What is observed in IP is that the voltage, which is measured at the potential electrodes, does not immediately drops to zero when the current is switched off.This means that there is some effect in the ground that stored the energy and then releases that energy after the current is switched off, i.e. there is some kind of capacitor.In DC resistivity measurements we inject the current and while the current is flowing it builds up a potential field which is measured as a voltage.In IP the measurement takes place after the current is switched off and we are interested in the decay of the electrical potential, measured as a time-varying (decaying) voltage.
9Small IP surveys often use porous-pot type electrodes Induced PolarisationReconnaissance 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).Small IP surveys often use porous-pot type electrodesTraditionally, special kinds of electrodes were used, so-called porous pot electrodes.If one were to use metal stakes as SP electrodes, electrochemical contact-potentials would build up at the ground contacts which results in high noise levels.Therefore non-polarising electrodes are used. These consist of a metal immersed in a saturated solution of its own salt (e.g. Cu in CuSO4) which is contained in a porous pot that allows the solution to leak slowly and make contact with the ground.(Telford, 1990)
10Induced PolarisationWe see here some equipment that is used for DC resistivity and IP measurements.As you can see the receivers and layout resemble those used in seismic.This kind of equipment is relatively new – measurements are computer controlled and the right pairs of electrodes are automatically selected over the course of measurements.IP measurements also commonly employ a separate generator as a large current is required to observe the IP effect of buried structures.
11Induced PolarisationIP surveys usually use a separate transmitter and receiverPower requirements are higher than for DC res. surveysCables and electrodes must be watched. If a passer-by or animal touched the current electrodes during data acquisition, this could be fatalWe see here some equipment that is used for DC resistivity and IP measurements.As you can see the receivers and layout resemble those used in seismic.This kind of equipment is relatively new – measurements are computer controlled and the right pairs of electrodes are automatically selected over the course of measurements.IP measurements also commonly employ a separate generator as a large current is required to observe the IP effect of buried structures.
12IP 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!C1C2P1P2IP measurement principally use the same equipment as the DC resistivity method.What is observed in IP is that the voltage, which is measured at the potential electrodes, does not immediately drops to zero when the current is switched off.This means that there is some effect in the ground that stored the energy and then releases that energy after the current is switched off, i.e. there is some kind of capacitor.In DC resistivity measurements we inject the current and while the current is flowing it builds up a potential field which is measured as a voltage.In IP the measurement takes place after the current is switched off and we are interested in the decay of the electrical potential, measured as a time-varying (decaying) voltage.
13IP Effect Vp Vs IP effect Steady state voltage: Residual voltage: charge timeVpSteady state voltage:(primary voltage)IP effectVsResidual voltage:(secondary voltage)The observed IP effect is shown in this figure – a voltage is generated upon injecting a DC current in the subsurface.The voltage reaches a maximum, Vp, after a certain time, the charge time.When the current is turned off it can be observed that the voltage decays over a period of time to zero, the residual voltage, Vs, is due to the IP effect.
14IP EffectVoltage shows a large initial decrease, then decays slowly over a timescale of seconds (minutes). This is the IP effectThe rate of decay depends on the electrical properties of the ground and the presence of metallic mineralsThe decay voltage is the result of storage of energy by the ground during the period when the DC current is onThe effect cannot be explained in terms of the atomic or molecular structure of the material, but depend on the macro- structure.The observed IP effect is shown in this figure – a voltage is generated upon injecting a DC current in the subsurface.The voltage reaches a maximum, Vp, after a certain time, the charge time.When the current is turned off it can be observed that the voltage decays over a period of time to zero, the residual voltage, Vs, is due to the IP effect.
15IP Effect – SourcesChemical 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 rockLarger than the normal IP effectRequires presence of metallic minerals (or graphite)Membrane polarisation (electrolytic) (~below 1Hz)Due to variations in the mobility of ions contained within pore fluidsCalled the “normal” IP effectMay occur in rocks which contain no metallic minerals
16IP Effect – Electrode Polarisation Electrode (or grain) polarisationsame process as self-potential.Metal electrode in an ionic solution:No voltage applied:charges with different polarities separatepotential difference between electrode and solution.With voltage applied:currents start flowingchange in potential differenceVoltage turned off:ions diffuse back to equilibriumThe total magnitude of the potential is the Nernst potential and the adsorbed layer gives rise to the Zeta potentialReynolds, 2011, p.374
17IP Effect – Electrode Polarisation Electrolytic conduction only (no IP)Electrolytic and electronic conductionElectrode polarisation occurs when electricity is conducted partly electrolytically and partly electronicallyWhen 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 surfacesIons accumulate at grain surfaces and the grains become PolarisedWhen the current flow is interrupted, ions return to their equilibrium positions voltage decayElectrode polarisation occurs when electricity is conducted partly electrolytically and partly electronicallyThe upper figure shows electrolytic conduction only – there is no IP effect.The lower figure shows a combination of electrolytic and electronic conduction when a metallic mineral grain blocks the pore space.In this case a chemical reaction takes place at the interface between mineral and solution – an electron exchange takes place between the metal and the solution ions at the interface (overvoltage)Electrode polarisation takes place in almost all sulphides and also some oxides such as magnetite and – unfortunately – also graphite.
18IP 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 repelledBuild-up of a layer (“cationic cloud”) of +ve ion concn which may extend 1 mm into the pore fluid(Equilibrium: No applied electrical field)
19IP Effect – Membrane Polarisation ( Applied electrical field)Zone of +ve ion concn 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 otherWhen the voltage is removed, the ions return to their equilibrium positions voltage decayMembrane 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
20IP Effect – Electrode Polarisation IP effect depends on grain sizeLarge sulphide grain → large amount of current through it, but small surface/volume ratioIP however is a function of the amount of grain surface exposed to the electrolytic solutionTherefor, as the grain size is reduced, the IP effect increasesHowever for very small grain sizes, the surface resistance is too largegreatest IP effect for intermediate values of sulphide grain sizesIP observed in mixtures of pyrite and quartz sand for various pyrite grain sizes(Keller and Frischknecht, 1966)
21IP Effect – SourcesIn practice, it is not possible to distinguish between membrane and electrode polarisations on the basis of geophysical IP measurementsMembrane polarisation may give rise to a “background” IP effect equivalent to 0.1% - 10% conductive minerals (typically 1% - 2%)IP is a bulk effect: it does not depend on atomic-scale rock or mineral properties
22IP Effect – Sources(P. Kearey et al., 2007)ChargeabilityApparent resistivityTime-domain IP profile using a pole–dipole array over the Gortdrum copper–silver body in IrelandElectrode polarisation depends strongly on the surface areaThe IP method is more sensitive to disseminated conductors than to massive onesThis sets the IP method apart from the DC resistivity and EM (electromagnetic) methods, which typically give a weak response over a disseminated targetSo electrode polarisation is the dominant effect and it depends strongly on the total surface area.That means that IP is more sensitive to disseminated material containing mineralisation than to massive ones.This is a very attractive feature of the method.The figure shows an example of the resistivity and IP measurement over the Gortdrum copper-silver deposit in Ireland.Although the deposit is of low grade, containing less than 2% conducting minerals, the chargeability anomaly is well defined and centred over the ore body.In contrast, the corresponding apparent resistivity profile reflects the large resistivity contrast between the Old Red Sandstone and dolomitic limestone but gives no indication of the presence of the mineralization.
23Time-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 are arranged as for a normal DC resistivity surveyIn 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
24Time-Domain IP Measurements Typical transmitted and received waveforms in time-domainChargingtimeOff-timeIn time-domain IP a square waveform is usually employed for injection of a DC-current.It consists of a fixed sequence of current-on and –off times and the polarity of the current is changed upon each sequence.The measured voltages resembles the input current waveform. However the potential field in the subsurface is not setup instantaneously so that there is a small delay in the received signal before the voltage reaches a maximum. This is when DC resistivity is measured.Upon turning off the transmitter current, the voltages don’t decay to zero immediately which is due to the IP effect.
25Time-Domain IP Measurements Effect of chargeable groundUniversity of British Columbia (UBC-GIF)In time-domain IP a square waveform is usually employed for injection of a DC-current.It consists of a fixed sequence of current-on and –off times and the polarity of the current is changed upon each sequence.The measured voltages resembles the input current waveform. However the potential field in the subsurface is not setup instantaneously so that there is a small delay in the received signal before the voltage reaches a maximum. This is when DC resistivity is measured.Upon turning off the transmitter current, the voltages don’t decay to zero immediately which is due to the IP effect.
26Time-Domain IP Measurements In time-domain IP, the main parameters used to present and interpret data are apparent resistivity (ra) 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 Vp) 𝜌 𝑎 =𝐾 𝑉 𝑃 𝐼The measured Vp for a short charging time will be less than that measured for a long charging time - this means that ra calculated for a high frequency Tx waveform will be less than that for a low frequency Tx waveform (the frequency-domain IP effect)K = geometric factor(depends on electrode array)VP depends on the “charging time”
27Time-Domain IP – Chargeability The ratio Vs/Vp is called the chargeability (Units: millivolts per volt)In practice it is impossible to measure Vs (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
28Time-Domain IP – Apparent Chargeability The apparent chargeability, ma, is defined bywhere tn is the time corresponding to the last voltage measurement (on the previous slide, n = 4) and V(t) is the decay voltage at time tThe apparent chargeability is the area under (part of) the voltage decay curve, divided by the “primary” voltage VpIn practice, the units are milliseconds (ms)The apparent chargeability depends on the actual values of t1 and tn, and may be different for different field instruments(Units: milliseconds)
29Time-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 chargeabilityBecause 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.
301% Volume concentration Chargeabilities of rocksmineralm (ms)material1% Volume concentrationCharging time 3 secondsIntegration 1 secondCharging time 1 minuteIntegration 1 minuteCharging time 3 secondsIntegration 0.02 to 1 second(from Telford et al., 1990)
31Electrode 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 electrodesPlan ViewA,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
32Electrode 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 setupn = 1, 2, 3, etc.
33Rocky’s Reward, WA (NiS), 1986, dipole-dipole 2D electrical imaging surveysDipole-dipole IP data are commonly displayed as separate pseudosections of apparent resistivity and apparent chargeability15 ohm-m24 msec (n=4)n-spacingRocky’s Reward, WA (NiS), 1986, dipole-dipole(Mutton and Williams, 1994)
342D electrical imaging surveys Combine vertical (sounding) and lateral (profiling) survey methodThis provides a 2D geoelectrical model of the subsurface:vertical and horizontal changes in electrical propertiesassumption: no changes perpendicular to survey lineTypical 1D sounding surveys involve 10 – 20 readingsTypical 2D imaging surveys involve 100 – 1000 readingsIn comparison, 3D would involve several 1000’s of readings
352D electrical imaging surveys Pseudosections are a convenient means of plotting data acquired using a variety of current and potential electrode separations in a single plotThey do not represent true cross-sections of ra and ma, except in the sense that the depth of penetration increases as the “n- spacing” increasesAs a rough rule-of-thumb, the depth of investigation is ( na / 2 ) for the dipole-dipole arrayAlthough 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
36Dipole-Dipole – combine Sounding and Profiling 2D electrical imaging surveysDipole-Dipole – combine Sounding and Profiling45°Data acquisition proceeds by combining Depth Sounding and Lateral Profiling to obtain data from a 2D section of the subsurface.The horizontal location of a data point is at the mid-point of a set of electrodes used.The vertical position (pseudo-depth) of a data point is at a depth proportional to the electrode spacings.
372D electrical imaging surveys Horizontal location of data point at mid-point of set of electrodes usedVertical position (pseudo-depth) of data point at a depth proportional to electrode spacingsThe measured parameter is plotted at the intersection of 45° lines extending from the mid-points of the transmitter and receiver pairsNote that this is a convention only and does not constitute the depth of investigation
38Centenary gold deposit, WA (disc. 1996) 2D electrical imaging surveysThe Centenary gold deposit is a concealed ore body located 110 km north of Leonora, Western AustraliaThe ore body is associated with sulphides and is hosted in the magnetic portion of the Mount Pickering Dolerite.Due to its sulphidic nature, both gravity and induced polarization (IP) were trialled soon after discovery.A dipole–dipole IP and resistivity survey detected a significant chargeability anomaly over Centenary.Centenary gold deposit, WA (disc. 1996)survey lineore bodydrill holeshigh chargeability (ore body)conductive overburdenlow resistivity (ore body)Pittard and Bourne, Exploration Geophysics, 2007, 38, 200–207
40IP Data Interpretation The most common method of interpretation of IP data is via automatic two-dimensional inversionInversion of IP data results in cross-sections of resistivity and chargeability vs depth, which are similar to geological cross sectionsInversion of the data is performed in “real time” by some instruments, and inverted sections are now a standard product delivered by geophysical contractorsNB 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)
41IP Data Interpretation Example electrode polarisationThe Century deposit, approximately 250 km north-northwest of Mt. Isa in northwest Queensland, Australia, is hosted by relatively flat-lying middle Proterozoic siltstone and shale units. Mineralization occurs preferentially within black shale units as fine-grained sphalerite and galena with minor pyrite.The recovered model after inversion shows the superposed geologic section. The inversion nicely delineates the resistive overburden of limestones on the right.The resistivity at depth is not correlated with mineralization, however.2D inversionobserved data
42IP Data Interpretation Example electrode polarisation2D inversionobserved data
43IP Data Interpretation Example membrane polarisationInduced-polarization detection and mapping of contaminant plumesmonitoring wellssurvey lineEDB (0.02)EDB (10)EDB (100)2D time-domain IP & DC Resistivity mapping of a contaminant plume at the Massachusetts Military Reservation.The plume consists of approximately m3 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 plotSogade et al, 2006
44Geological Cross-Section IP Data InterpretationExample membrane polarisationEDB (0.02)EDB (10)EDB (100)Geological Cross-SectionSogade et al, 2006
45Dipole-dipole pseudosection, electrode separation a = 24.38 IP Data InterpretationExample membrane polarisationEDB (100)Dipole-dipole pseudosection, electrode separation a = 24.38Extrapolated plume concentration data for benzene and EDB based on ground-monitoring wellsSogade et al, 2006
46Chargeability Anomalies (Contaminants ?) IP Data InterpretationExample membrane polarisationConductive zone ~1000 𝛺∙𝑚 (Groundwater?)Conductive zone ~300 𝛺∙𝑚 (Clays?)Log Resistivity (𝛺∙𝑚)EDB (100)2D resistivity SectionChargeability Anomalies (Contaminants ?)Chargeability (mV/V)EDB (100)2D IP SectionSogade et al, 2006
47SummaryThe 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 waveformsDetermination of apparent chargeabilityDisplay of IP data, pseudosections and depth of investigationInversion and interpretation of resistivity and chargeability results
48ReferencesVeeken 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–B59Telford, W.M, Geldart, L.P., Sheriff, R.E.: “Applied Geophysics”, 1991, Cambridge University PressReynolds, J.M., "An Introduction to Applied and Environmental Geophysics", 2011, John Wiley & SonsSeigel H., Nabighian M., Parasnis D., Vozoff K., “The early history of the induced polarization method”, March 2007, The Leading Edge, pp. 312Sogade, J.A, Scira-Scappuzzo F., Vichabian Y., “Induced-polarization detection and mapping of contaminant plumes”, 2006, Geophysics, V71, p. B75–B84
50IP 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 algorithmmeasured potential in the absence of chargeability effectsthe apparent chargeability can be computed by carrying out two DC resistivity forward modellings with conductivities 𝜎 1−𝜂 and 𝜎potential including chargeability effectsD.W. Oldenburg and Y. Li, 1994, "Inversion of induced polarization data", Geophysics, 59, P
51IP Data Interpretation DC / IP data are gathered togetherInvert potentials for conductivity (background) modelUse 𝜎-model for forward mapping of chargeabilityInvert for chargeability modelsDCIPLeast-Squares Inversion