induced polarization (ip) · induced polarization ... z)=(.001, 1.0, 1.0) recovered chargeability...

Induced Polarization (IP)

Basic principles

Data Acquistion

Pseudosection

Inversion

Case Histories

Induced Polarization

Current injected into ground and the voltage continues

to increase.

Recognized in 1950’s: it was termed Over-voltage.

Understand the effect in terms of charge accumulation.

The phenomenon is called induced polarization.

I source V potential Not chargeable Chargeable

Source

(Amps)

Potential

(Volts)

Chargeability is a microscopic phenomenon

Thoroughly understanding what is happening at the microscopic level

is scientifically challenging. In practice we work with the concept of

“chargeability”

Chargeability

pyrite

chalcocite

copper

graphite

chalcopyrite

bornite

galena

magnetite

malachite

hematite

13.4 ms

13.3 ms

12.3 ms

11.2 ms

9.4 ms

6.3 ms

3.7 ms

2.2 ms

0.2 ms

0.0 ms

Minerals at 1% Concentration in Samples

Chargeability: rocks and minerals

Earth materials are “chargeable”

Initial situation

Neutrality

Apply an electric field

Build up of charges

Net effect

Charge Polarization

Electric dipole

Induced Polarization: Over-voltage

Not chargeable Chargeable

Source

(Amps)

Potential

(Volts)

Chargeability Data: Time domain IP

Intrinsic chargeability

0<n<1 (dimensionless)

Integrate over the decay

Sample a channel

(msec) mV/V

IP data: frequency domain

Percent frequency effect:

Phase:

low freq. f2 high freq. f1

Source

current

Measured

potential

V1 V2

I I

1

12100a

aaPFE

Source

current

Measured

potential

Phase (mrad)

phase (mrad)

Data acquisition

Data are acquired along with DC resistivity data (just

sample a different part of the waveform)

Data are plotted as pseudosections (exactly the

same as DC resistivity)

For IP the data plotted in the pseudosections will

have units (mV/V, msec, mrad, PFE).

Earth

Energy Source In Measured signals

Out = “Data”

Plotting plane

~ v

Plotting plane

~ v

Plotting plane

~ v

Plotting plane

~ v

IG

Va

2Each data point is an apparent resistivity:

~ v ~

v

(Click for animation)

DC resistivity and IP data

Example IP pseudosection

2) A chargeable block.

2) A chargeable block and geologic noise.


3) The “UBC-GIF model”


Pseudosections … conclusions

Except for very simple structures, geologic

interpretations can not be clearly made directly from

pseudosections.

Interpretation is even more difficult in 3D

Given:

- Field observations

- Error estimates

- Ability to forward model

- Prior knowledge

Choose a suitable

misfit criterion

Design model

objective function

Discretize the Earth

Perform inversion

Evaluate results Iterate

Interpret preferred model(s)

Summary: what is needed to invert a data set?

dxmmsm

2

0

dxmm

dx

dx

2

0 )(

dzmm

dz

dz

2

0 )(

Summary of IP data types:

Time domain: Theoretical chargeability (dimensionless). Integrated decay time (msec).

Frequency domain:

PFE (dimensionless) Phase (mrad)

For all data types, J = d .

where J is a sensitivity matrix that requires that the

electrical conductivity σ is known. We find σ by inverting the DC resisitivity data.

DC / IP data

gathered together

Use model for

forward mapping of

chargeability

IP

Data

Invert potentials

for conductivity

model

Potential (i.e. voltage) data

Conductivity model

Invert for

chargeability models

Chargeability model

IP Inversion

Inversion of IP data

Step 1: Invert Vm to obtain .

Step 2: Generate sensitivities

Step 3: Invert the IP data (any form) by solving:

j

i

ijJ

ln

ln

obsdJ subject to > 0.

Example 1: buried prism.

Chargeability model

Data with 5% Gaussian noise

• Pole-dipole; n=1,8; a=10m; N=316; (s, x, z)=(.001, 1.0, 1.0)

Recovered chargeability

Predicted data

Example 2: prism with geologic noise.

Chargeability model

Data with 5% Gaussian noise


Recovered chargeability

Predicted data

Example 3: UBC-GIF model.

Chargeability model Recovered chargeability

Data with 5% Gaussian noise Predicted data


Field Case History

Cluny deposit, Australia

10 lines of DCIP data acquired

Inversion carried out in 3D

Data set #1:

Apparent resistivity,

dipole - pole.

Cluny: 3D resistivity

Eight survey lines

Two survey configurations.

Easting (m) Easting (m)

mS/m

10500 11500 12500

13000

14000

15000

16000

400

450

500

Easting (m)

No

rth

ing

(m

)

Surface topography:

Elevation

Meters

10 lines surveyed

Easting (m) Easting (m)

mS/m

Data set #2:

Apparent resistivity,

pole - dipole.

Conductivity model from 3D inversion of DC

Apparent chargeability,

dipole - pole.

10500 11500 12500

13000

14000

15000

16000

400

450

500

Easting (m)

No

rth

ing

(m

)

Surface topography:

Elevation

Meters

10 lines surveyed

3D Induced polarization (IP)

Click image to see the AVI movie

Chargeability model from 3D inversion of IP

Volume rendered resistivity model Volume rendered chargeability model

3D conductivity and chargeability models

Coming Up

Friday Nov 26: TBL DC resistivity and IP

Monday Nov 29: Quiz

Wednesday/Friday: Review

induced polarization (ip) · induced polarization ... z)=(.001, 1.0, 1.0) recovered chargeability...

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