# GLE 594: An introduction to applied geophysics ## GLE 594: An introduction to applied geophysics

### Electrical Resistivity Methods

Fall 2004

Earth Properties and Basic Theory

Today : 207-218

Next Lecture : 218-228

1

Introduction

• Link resistivity (ability of the earth to conduct an electric current) to subsurface structure.

• Useful because resistivity of earth materials varies by around 10 orders of magnitude.

(France) and Frank Wenner (United

States) in early 20th century.

• Uses: Archeology, Environmental, Mineral exploration

Electricity Basics

Voltage V - Electrical potential energy per unit charge [volts]

Current i - amount of charge per unit time [amperes] i =

1

R

V

Resistivity R is just a proportionality constant [ohms]

R relates current I to voltage I.

However, no units of length in this form of Ohm’s law.

2

Resistivity

Resistance includes length and area

We want resisitivity

ρ [ ohm/m

] because

- property of the material alone.

- no geometry included

R

= ρ

L

A

[ ohm

] ↑ length

↑ area

↑ resistance

↓ resistance

Conductivity

σ

[siemens/m] or [mhos/m]:

σ =

1

ρ

[ mhos m

]

It is the ability of the electrical charge to move through the material

Electrical Conductivity of Geomaterials

• Non-conductive minerals

Electric field Electric field

Hydrated ion mobility

Electrolyte

Soil (Archie’s law)

Soil (clays)

Equation

σ el

[ ] m

=

0

Double layer surface conduction

.

15 TDS

[ ]

L

σ soil

= a

σ el

S c r n m

σ soil

= n

σ el

+

(

1

− n

)

Θ ρ g

S s

TDS: total dissolved salts a

1; m~1-2.4; c~4-5

Θ ≈

10 -9 S (for Kaolinite)

3

• Non-conductive Minerals

4

### Analogous to Heat and Fluid Flow

Any solutions you know for one of these flows works for the others with the analogous boundary and initial conditions.

(Wang and Anderson 1982)

### Earth as a Circuit

Soils and rocks can be conceptually modeled as a circuit made of a resistor, capacitor, inductor and battery:

Electrodes

R

C

L

B

Resistor R: dissipator of applied energy as heat

Capacitor C: storage of energy as separation of charges

Inductor L:

Battery B: self voltage associated to electromagnetic methods electrokinetics and self-potentials

5

Current Source on Surface

Electric potential at distance away from current source on surface given as V(r)=

ρ

I/2

π r. How?

Boundary conditions:

1)As r =>

, V => 0.

2) V is continuous across any boundary

3) Tangential E continuous across any boundary

4) Normal I continuous across any boundary.

5) Above leads to no vertical current crossing earth-air interface.

Current Flow in a Homogeneous and

Isotropic Medium

Point Current Source: dV

= iR shell

= i

ρ l

A

= i

ρ dr

2

π r

2

Voltage decreases as the inverse of the distance from the current source.

Shape of constant voltages are hemispheres for a single point source.

V

D

= i

ρ

2

π

= ∫

D dV

= i

ρ

2

π

D dr r

2

1

D

=

= i

ρ

2

π

( ) 1 r

D i

ρ

2

π

D

6

Two Current Electrodes: Source and Sink

• Why run an electrode to infinity when we can use it?

source sink r source

P r sink

Total Voltage at P:

V source

= i ρ

2 π r source

V sin k

= i

ρ

2

π r sink

V p

=

V source

V sin k

= i

ρ

2

π

⎜⎜

1 r source

1 r s ink

⎟⎟

Measurement Practicalities

Can’t measure potential at single point unless the other end of our volt meter is at infinity. This is inconvenient. It is easier to measure potential difference (

V) . This lead to use of four electrode array for each measurement.

ρ

Resulting measurement given as

V=V

P1

-V

P2

=

ρ

I/(2

π

)*(1/r

1

-1/r

2

-1/r

3

+1/r

4

).

Can be rewritten

V=

ρ

I*G/(2

π

) where G/2

π is the Geometrical Factor of the array.

7

Current density and equipotential lines for a current dipole d i fraction total current f

=

2

π tan − 1

⎛ 2 z d ⎠ i f

=0.5 at z = d

2 i f

=0.7 at z = d

Wider spacing

Deeper currents

Apparent Resistivity

Previous expression can be rearranged in terms of resistivity:

ρ=(∆

V/I) (2

π

/G).

This can be done even when medium is inhomogeneous. Result is then referred to as Apparent

Resistivity.

ρ

2

ρ

Definition:Resistivity of a fictitious homogenous subsurface that would yield the same voltages as the earth over which measurements were actually made.

1

8

Geometrical Factors

Array

Wenner

Schlumberger

Dipole-Dipole

1. Easy to calculate

ρ a field in the

2. Less demand on instrument sensivity

1. Fewer electrodes to move each sounding

2. Needs shorter potential cables

1. Cables can be shorter for deep soundings

1. All electrodes moved each sounding

2. Sensitive to local shallow variations

3. Long cables for large depths

1. Can be confusing in the field

2. Requires more sensitive equipment

3. Long Current cables

1. Requires large current

2. Requires sensitive instruments

9