Ch6 Electrical Natural Field Based

7/28/2019 Ch6 Electrical Natural Field Based

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ELECTRICAL METHODS

NATURAL SOURCE

BASED METHODS

ARTIFICIAL SOURCE

BASED METHODS

SELF

POTENTIALMETHOD

MAGNETO

TELLURICMETHOD

TELLURIC

METHOD

RESISTIVITY

METHOD

INDUCED

POLARISATION

METHOD

ELECTRO-

MAGNETICMETHOD



CAUSES BEHIND NATURAL OR SPONTANEOUS

POTENTIALS OCCURING IN THE SUBSURFACE

Weathering of sulphide mineral bodies

Variation in rock properties (mineral content) at

geologic contacts

Bioelectric activity of organic material

Corrosion

Thermal and pressure gradients in

underground fluids etc.

The controlling factor in allcases is underground water

SP Method



Mechanism

Chemical Mechanical

Electro kineticPotential

Or,StreamingPotential

DiffusionOr,

LiquidJunctionPotential

ShaleOr,

NernstPotential

Mineralisation

Potential

Electrochemical or,

Static Self Potential

Background Potentials Main interestduringprospecting

SP Method



A Few Things

About theMineralizationPotential

Most common mineralisation

potential anomalies occur overPyrite, Chalcopyrite, Pyrrhotite,Sphalerite, Galena & Graphite Amplitudes range from a few

millivolts to one volt e.g. 200mVcan be considered a good SPanomaly

Potentials are almost always

‘-‘ve near the upper end of the body and are quite stable intime

SP Method



A FewThings

About theBackgroundPotential

Amplitudes vary greatly but

generally are <100 mill volts

Characteristic regionalbackground potentials

Created by fluid streaming,bioelectric activity in

vegetation, varying electrolyticconcentrations in ground waterand other geochemical action Say, gradient of the order of 10mV/1000ft over severalmiles and may be ‘+’ve or’-‘ve.Also, regional gradientassociated with topography.‘-‘ve while going uphill.

SP Method



SP Method

Negative Center

Oxidizing Zone

(Lower pH

value- pH=2-4,Highly acidic)/

Cathode Half

Cell

Reducing Zone

( Higher pH value-

pH=7-9, Highly

basic/ Anode Half

Cell

Figure: Shows Self-potential mechanism in pyrite (Sato and Mooney, 1960)



SP Method

Key points about the SP Phenomenon

Two electrochemical half-cell reactions of opposite sign, one

cathodic above the water table, the other anodic at depth.

In the cathodic half- cell there is chemical reduction of the

substance in solution-that is, they gain electrons

In the anodic half-cell an oxidation reaction takes place and

electrons are lost.

The mineral zone itself functions only to transport electrons from

anode to cathode.The magnitude of the overall SP effect is determined by the

difference in oxidation potential (Eh) between the solutions at the two

half-cells.



SP MethodField Equipments

Figure: A walking –stick SP electrode

What is a walking –stick electrode ?

A walking stick electrode is a type of

non-polarizing electrode that consists

of a metal immersed in a saturated

solution of its own salt, such as Cu

in CuSO4, Zn in ZnSO4 etc., and

contained in a porous pot which

allows the solution to leak slowly and

make contact with the ground.

Why a non polarizing electrode used?

A simple metal rod used as an electrode results

into electrochemical reactions at the ground

contact creating thereby spurious potentials

magnitude of which is comparable to that of the

SP anticipated from the causative body.

Moreover, the magnitude of this spurious potential

varies from place to place. So, a constant

correction factor can not be introduced. Non-

polarizing electrodes diffuse the effect due to the

spurious potentials.



SP MethodField Equipments

A Potentiometer

The main requirement of the

milli-voltmeter is that its inputimpedance should be large

enough that negligible current will

be drawn from the ground during

the measurement



SP Method



SP Method



SP Method



SP Method



SP Method



SP Method



SP Method



Telluric Method

What is telluric current?

Telluric current is an alternating

electric current with a variable

frequency of I Hz or smaller flowing

horizontally in the upper part of the

earth’s crust, the cause of their

generation being induction effect dueto the electric currents in the

ionosphere produced due to the

motion of particles ionized by solar

radiation.



Telluric Method

RLμvT

f R

2

1h

ΔZZZOS

00

Z

ΔZ

μ

Δμ

With the linear assumption,

0

00S

μ

ΔμZZZ

Some Working

Formula

Here



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Ch6 Electrical Natural Field Based