2-ERTH2020_(Induced Polarisation) - Exploration Geophysics at the

Report
ERTH2020
Introduction to Geophysics
The Induced Polarisation (IP) Method
ERTH2020
1
Induced Polarisation
“equivalent circuits”
• Induced Polarisation
• DC Resistivity
-
-
+
I
I
+
completely
described by Ohm’s
law
 =
ERTH2020


+
-
resistance
R
C
“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:
C1
ERTH2020
P1
P2
C2
7
Induced Polarisation
• DC resistivity
→direct electrical connection (electrodes)
→ flow of current
→ electrical potential in the ground
P2
P1
C2
• IP methods C1
→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).
Small IP surveys often use
porous-pot type electrodes
ERTH2020
(Telford, 1990)
9
Induced Polarisation
ERTH2020
10
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
ERTH2020
11
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!
C1
ERTH2020
P1
P2
C2
12
IP Effect
Steady state voltage: Vp
charge time
(primary voltage)
IP effect
Residual voltage:
Vs
(secondary voltage)
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 macrostructure.
ERTH2020
14
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
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
ERTH2020
Reynolds, 2011, p.374
16
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 concn which
may extend 1 mm into the pore fluid
(Equilibrium: No applied electrical field)
ERTH2020
18
IP 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 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
• 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
ERTH2020
(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 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
ERTH2020
21
IP Effect – Sources
Time-domain IP profile using a pole–dipole array
over the Gortdrum copper–silver body in Ireland
• Electrode polarisation depends
strongly on the surface area
Chargeability
Apparent resistivity
• The IP method is more
sensitive to disseminated
conductors than to massive
ones
• This sets the IP method apart
from the DC resistivity and EM
(electromagnetic) methods,
which typically give a weak
response over a disseminated
target
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 are arranged as for a
normal DC resistivity survey
• 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
ERTH2020
23
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
University of British Columbia (UBC-GIF)
ERTH2020
25
Time-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)

K = geometric factor
VP depends on the
 = 
(depends on electrode array)
“charging time”

• The 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)
ERTH2020
26
Time-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
ERTH2020
27
Time-Domain IP – Apparent Chargeability
• The apparent chargeability, ma, is defined by
ma 
1
Vp
tn
 V ( t ) dt
(Units: milliseconds)
t1
• where 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 t
• The apparent chargeability is the area under (part of) the
voltage decay curve, divided by the “primary” voltage Vp
• In 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
ERTH2020
28
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
Charging time 1 minute
Integration 1 minute
ERTH2020
m (ms)
material
Charging time 3 seconds
Integration 0.02 to 1 second
m (ms)
mineral
m (ms)
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
ERTH2020
31
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.
ERTH2020
32
2D electrical imaging surveys
n-spacing
24 msec (n=4)
n-spacing
15 ohm-m
ERTH2020
Rocky’s Reward, WA (NiS), 1986, dipole-dipole
• Dipole-dipole IP data are commonly displayed as separate
pseudosections of apparent resistivity and apparent chargeability
(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 are a convenient means of plotting data acquired
using a variety of current and potential electrode separations in a
single plot
• They do not represent true cross-sections of ra and ma, except in
the sense that the depth of penetration increases as the “nspacing” increases
• 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°
ERTH2020
36
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
survey line
of Leonora, Western Australia
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
observed
data
Mt. Isa in northwest Queensland, Australia,
is hosted
by
relatively flat-lying middle Proterozoic siltstone and shale
units. Mineralization occurs preferentially within black shale
inversion
units as fine-grained sphalerite and galena2Dwith
minor pyrite.
• The recovered model after inversion shows the superposed
geologic section. The inversion nicely delineates
the resistive
observed data
overburden of limestones on the right.
2D inversion
• The resistivity at depth is not correlated with mineralization,
however.
ERTH2020
41
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
42
IP Data Interpretation
Example membrane polarisation
Induced-polarization detection and mapping of contaminant plumes
• 2D time-domain IP & DC Resistivity
mapping of a contaminant plume at
the Massachusetts Military
Reservation.
EDB (10)
EDB (100)
monitoring wells
EDB (0.02)
survey line
• The plume consists of approximately
265 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 plot
ERTH2020
Sogade et al, 2006
43
IP Data Interpretation
Example membrane polarisation
Geological Cross-Section
EDB (0.02)
EDB (10)
EDB (100)
ERTH2020
Sogade et al, 2006
44
IP Data Interpretation
Example membrane polarisation
EDB (100)
Dipole-dipole pseudosection,
electrode separation a = 24.38
Extrapolated plume concentration
data for benzene and EDB based on
ground-monitoring wells
ERTH2020
Sogade et al, 2006
45
IP Data Interpretation
Example membrane polarisation
2D resistivity Section
Conductive zone ~1000  ∙  (Groundwater?)
EDB (100)
Conductive zone ~300  ∙  (Clays?)
Log Resistivity ( ∙ )
2D IP Section
EDB (100)
Chargeability Anomalies
(Contaminants ?)
Chargeability (mV/V)
ERTH2020
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 and chargeability
results
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
 1 −  and 
ERTH2020
D.W. Oldenburg and Y. Li, 1994, "Inversion of induced polarization data", Geophysics, 59, P.1327-1341
50
IP Data Interpretation
DC / IP data are
gathered together
DC
Invert potentials for
conductivity (background)
model
IP
Use -model for forward
mapping of chargeability
Least-Squares
Inversion
Invert for chargeability
models
ERTH2020
51

similar documents