Induced Potential

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InducedPotential

Group 12LiyanaAhmedElyse


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Introduction

Induced Potential/ Polarisation (IP) is a technique of measuring aninduced potential field in the ground in order to map the geologicalsubsurface.

From measurements of the induced potential field the chargeabilityand resistivity of the subsurface can be calculated.


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Physical Principles

Anode and the cathode of an electrolytic cell become polarizedupon the passage of an electrical current through the cell.

This effect provides a means of detecting metallic minerals in

the earth either in the form of solid ore bodies or asdisseminated particles and forms the basis for the inducedpolarization.

It Consists of first inducing polarization on a metallic mass and

subsequently detecting that polarization.

The presence of the mass will distort the current flow in amanner governed by the relative resistivity of the metal and thesurrounding rock.


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Physical Principles

The surface along which the current filaments enter the masscorresponds to the cathode of an equivalent electrolytic cell,and the surface where they leave to the anode

A direct current will polarize the mass , positive on the sidewhere the current enters and negative where it leaves.

If the current is interrupted, the dipole on the mass willdissipate itself by sending a current through the surrounding

medium.

The recorder will indicate the ohmic drop caused by thispolarization current.


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Electrical dipole


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Measurement Techniques

Time domain IP ( Pulse transient)

Frequency domain (Using harmonic signals)

Traditional variable-frequency IP, using two or more frequencies of < 10Hz

Phase domain-measure phase delays between current and voltage

Spectral IP-measure phases and amplitudes at frequencies 10-3 to

4103 Hz


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Time Domain IP

Time domain IP involves transmitting current into the ground and thenswitching it off.

The current flow induces a potential field which the receivers measureduring the on and off time.


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The voltage peaks during the on-time and begins to decay as soon as thecurrent is switched off.

From the on-time peak voltage measurement the apparent resistivity canbe calculated.

Chargeability is calculated from the off-time measurements of thetransient voltage decay.

Time Domain IP


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Frequency Domain IP

In frequency domain IP an alternating current is transmitted into theground at a low frequency which induces a potential field in thesubsurface.

The receivers measure the phase shift in the signal and from this thechargeability and resistivity can be calculated.


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Implementation

Apparatus:Electrodes, data recorder, current transmitter, power source for

transmitter, receiver

Procedure:1. Four electrodes are used; 2 to transmit current, 2 to receive

current.2. Current is transmitted to ground, and is switched on and off

rapidly.3. Current flow induces potential field, which the receivers measure

during the on and off time.4. Data obtained by receivers are recorded.


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Electrode Arrangements

Factors that affect choice of

electrode array (in order of relativeimportance):

1. Signal-to-noise ratio2. EM coupling rejection

3. Survey speed and economy4. Resolution of subsurface bodies5. Array symmetry6. Other matters such as safety,topographic effect, communication

and ease of interpretation.Dipole-Dipoleelectrode arrangementis most commonly used in measuring

Induced Potential.Row 1 Row 2 Row 3 Row 4

0

2

4

6

8

10

12

Column 1

Column 2

Column 3


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Chargeability Measurement of IP effect (ratio of Vp, over-voltage to Vo, primary

voltage)

(units : mV/V or %)

In reality, apparent chargeability (Ma) is measured:

Area (A) beneath the voltage-time decay curve over a timeinterval (t1 to t2) and normalized by the supposed steady-state primary voltage, Vo

(units of mVs/V)


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Frequency Effect Frequency Effect in Frequency-Domain is equivalent to

Chargeability in Time-Domain.


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Pseudosection

- IP data & resistivitydata recorded, plotted insection form calledpseudosection.

Horizontal dimension:

Distance

Vertical dimension:Electrodespacings used to take the

measurements

* In this profile the elevation ofground surface has also beenincorporated.


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Applications

Induced potential is used identify:

Minerals (sulphide)Metal bearing zones(iron)

Polarizable materials(clay) Surface materials(ores)Ground water contamination Fracture zones


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Applications

It measures resistivity

Measures permeability

Maybe used to locate pipe or cable

Detects up normality of electrical properties

Geothermal exploration


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Advantages

Deep depth investigation

High signal to noise-ratio

Data can be collected with otheractivities (e.g. resistivity survey)

Improves the resolution of theanalysis of resistivity data


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Limitations

Induced potential requires: ExperienceMore power Relatively large areas

Relatively more crew members Electrodes to be inserted in the ground

CostlyData cannot be interpreted easilyNot suitable in hard and dry ground


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Induced Potential

Used in mappingGeological

Subsurface

Different

ElectrodeArrangements

& Distances

Techniques:Time-Domain &

Frequency Domain

Limitations ofTechnique

Wide Range ofApplications

Chargeability,Frequency Effect &

Resistivityused to produce

Pseudosection

PhysicalProperties


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Thank You

Induced Potential

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