electrical resistivity methods 13

Electrical Methods

0Principles & Applications

0 Graeme Taylor0 [email protected]

Info On Applied Geophysics

Field Geophysics By John Milsom, Published by John Wiley and Sons, 2003 (or any of the previous versions)

Reynolds, J. M. 1997. An Introduction to Applied and Environmental Geophysics, John Wiley and Sons Ltd, Chichester,796 pp. (includes examples from Sourton !)

An Intro. To Geophysical Exploration Kearey, Brooks & Hill Blackwell 2002.

Material that contains explicit info about the Sourton Area

*****BEER, K.E. and FENNING, P.J. 1976. Geophysical anomalies and mineralisation at Sourton Tors, Okehampton, Devon. Institute of Geological Sciences, Report 76/1.*****

Taylor, G.K., Hake, D.M., King, I.R., & Bowers R. (2001). The Sourton Tors geophysical anomaly revisited. Geoscience in south-west England, 10, 166-171.

http://www.dartmoor-npa.gov.uk/sheet_1c-b.pdf

Which also contains all the references for the regional geology

http://galitzin.mines.edu/INTROGP/index.jspUseful Online Course

MaterialAnd if you have access to it >>>

http://www.geologyuk.com/geophysics/textbook.htm

http://www.dartmoor-npa.gov.uk/sheet_1c-b.pdf

http://galitzin.mines.edu/INTROGP/index.jsp

Electrical Methods

0Resistivity Methods

0Self Potential SP0Electromagnetic Methods EM0Induced Polarisation IP0Ground Probing Radar GPR

Electrical Methods Applications0 Depth to bedrock0 Lithology/lithological boundaries0 Depth to the water table0 Groundwater contamination0 Buried targets e.g.

0 mineralised targets0 archaeological artifacts0 UXB and other metallic objects

Resistivity ?0What is it ?0How can we exploit variations in ground resistivity

to show geology, fluids or other targets? (Some intro ideas on how current flows in the ground)

0How do we apply such ideas in the field ?(Survey types, basic interpretation)

0Fieldwork, Equipment & Applications/Case Histories?

Resistance An analogy

Measuring Resistance- Ohm’s Law

Ohm found that the current, I, was

proportional to the voltage,V, for a broad

class of materials that we now refer to as

ohmic materials. The constant of

proportionality is called the resistance of

the material and has the units of voltage

(volts) over current (amperes), or ohms.

Ohm’s Law

0V = I *Ror

0R= V/ I

BUT will this work in the Earth ?

V - is Voltage in Volts

I is Current in Amperes

R is the resistance in Ohms

It's Resistivity, NOT Resistance

Resistance depends not only on the material but also the geometry of the wire. i.e. length and cross sectional area

We want to define a property that describes a material's ability to transmit electrical current that is independent of the geometrical factors. RESISTIVITY

Earth Materials as Conductors

0Conductors0 native metals, some sulphide minerals e.g. Chalcopyrite,

Pyrrohotite, Pyrite + graphite

0Semi Conductors0 Rock salt

0Insulators0 silicate minerals

This means that the vast majority of rocks will not be good conductors in their own right

Poor conductors

Good conductors

Good insulators

Poor insulators

Resistivity Conductivity

How then does electricity flow in rocks?

0 In metals by the flow of electrons which can be stripped from the outer atomic shell of a metallic atom

0 this is known as Ohmic or electronic conduction

0 In a liquid this is not possible instead we have

0Ionic or electrolytic conduction - this is the flow of current by the movement of +ve and -ve ions

Ionic Conduction in Rocks and sediments

0 Pore Space - to contain the Fluid

0 Water saturation - you have to have fluid or the ions cannot flow

0 Salinity - no Salts = no Ions

0 Temperature - affects a salts ability to break down into ions and ability to move

0 Permeability - the pores have to be connected

Archie’s Equation?

The Controlling Factors0Porosity - depends upon

0 Grain size0 Sorting/Packing0 Diagenesis/cementation0 Normally expressed as a fraction or perecentage0Total Void / Total Rock Volume

0Water SaturationSw = Volume filled with fluid / Total Porosity

0SalinityThe equivalent concentration of common salt (NaCl) that would

give rise to the same resistivity of fluid as is actually filling the pore space.

Implications of Electrolytic Conduction

0Resistivity is the most variable property of rocks because0 it depends upon three important factors which all vary

themselves i.e. Porosity, Sw and Salinity

0It is not possible to say what the resistivity of the rock might be even if you know 2 of the 3 e.g. A sandstone with high porosity and salinity will still have a very high resistance if it is dry !!!

Summary I0 Ionic (Electrolytic) conduction is much more common in

rocks than Electronic0 Electrolytic conduction depends on Porosity, Sw and

salinity0 We will need to measure the grounds resistivity (not

resistance)

Current Flow in a uniform medium

N.B. Potential = Voltage

The voltage change from a single current electrode to any point in the half space representing the earth is given by the expression above. In this expression, V is voltage, I is current, (rho) is resistivity, and r is the distance between the current electrode and the point the voltage is measured. Notice that this expression is nothing more than Ohm's law with the resistance, R equal to over 2r

An expression for the apparent resistivity

rIV2

Constant Resistivity Experiment

Current flow from

2 electrodes

Path % Current

1 17

2 32

3 43

4 49

5 51

6 57

Actually Measuring Resistivity

Potential distribution in a uniform World

The potential computed along the surface of the earth is shown in the graph. The voltage we would observe with our voltmeter is the difference in potential at the two voltage electrodes, V.

C CP P

+ -

Measuring Resistivity - What happens if we change the distance

between current electrodes?

What happens if the Earth is not uniform ? Current Flow in a Layered

Earth

Electrode Spacing and Apparent Resistivity Plots

Variation in Apparent Resistivity: Layered Versus Homogeneous Media

Current Density

Current Flow in Layered Media v Current Electrode Spacing

A 2nd

Exampleof current Flow in layered media

Summary II0The measurements made are V (potential difference

between 2 electrodes) and the applied current050% of the current or more will penetrate no deeper than

the current electrode separation 0We calculate the measured apparent resistivity using a

modified form of Ohm’s Law that allows for the geometry of the electrode array

0Current will preferentially flow in the low material0Current penetrates deeper into the ground with expansion

of the electrode distance

Electrode Arrays

WENNER

SCHLUMBERGER

Asymmetric

Overview of surveys

0Vertical Electrical Sounding V.E.S.0 Electrical Sounding, Drilling

0Constant Separation Traversing0 Profiling, Trenching

0Tomography or 2D Surveying0 which is a combination of both the

above methods

Expanding Arrays for V.E.S. 1WennerC PP C

aa aaa a

e.g. 0.25, 0.5, 1, 2, 4, 6, 8, 12, 16, 20, 24, 32, 48, 64….. metres

Expanding Arrays for V.E.S. 2

Sclumberger

A BM N

VES-1a=1ma=2ma=3ma=4ma=5m

VES-1AB/2=1.5, MN/2=0.5

AB/2=2, MN/2=0.5

AB/2=3, MN/2=0.5

AB/2=4, MN/2=0.5

AB/2=5, MN/2=0.5

AB/2=5, MN/2=1

a,m R ρa1234

AB/2 R ρa1.5234

ρa ρa

a,m AB/2

Wenner Sounding Schlumberger Sounding

Data Table

Field Curve

Data Table

Field Curve

Multilayered Earth Models I

Relatively thin middle layer Varying half-space resistivity

Multilayered Earth Models II

Varying the thickness of the second layer

VES Data Plotting Convention

• Plot apparent resistivity as a function of the log of some measure of electrode separation.• Wenner – a spacing• Schlumberger – AB/2• Dipole-Dipole – n spacing• Asymptotes:• Short spacings << h1, ρa=ρ1.• Long spacings >> total thickness of overlying layers, ρa=ρn• To get ρa=ρtrue for intermediate layers, layer must be thick relative to depth.

Multilayer Curves Summary

0 There is always at least one more layer than there are ‘turning points’ on the sounding graph

0 If three layers and 1 > 2 > 3 the it may well appear as if you only have 1 > 2 Equally the same thing happens for 1 < 2 < 3

0 Also very thin layers or layers with resistivities similar to those above or below may disappear

Resistivity Profiling/CST

• Maps• Profiles

• Locate Boundaries

ConstantSeparationTraversing

Constant Arrays for C.S.T. I

WennerC PP C

aa aC PP C

aa aC PP C

aa a

1 2 3 4 5 6 n

Wenner array

Profiling: a-spacing is fixed, move the whole array

Constant Arrays for C.S.T. II

Schlumberger

C PP C

1 2 3 4 5 6 n

Single Contacts

0 Some simplified responses to boundaries

1 > 2

Wenner

Sclumberger

TransverseDouble Dipole

Double Contacts

2 > 1

WennerDouble Dipole

Sclumberger

Profiling Summary

0 Symmetric arrays give symmetric anomalies

0 Assymmetric arrays give assymetric anomalies

0 A rule of thumb is that the boundary usually lies under the steepest slope in the anomaly curve

Resistivity

Equipment, Fieldwork,

and sample applications

Operation for Constant Separation Traversing

0 Pick electrode spacing (e.g. 10m)0 Electrodes in ground at 0,10, 20, 30 m (assuming 10m station spacing)0 “Measure point” middle of array so 15m0 Take measurement move on 1 spacing so 10, 20, 30, 40 >>

measurement point 250 repeat as needed, if you like you can always ‘infill’ extra data points at

critical places so 15,25,35,45 would give an extra data point at 30m along traverse

Operation for Vertical Electrical Sounding

0 Pick location from other surveys0 Place two tapes on the ground back to back0 Electrodes in ground at 0,10, 20, 30 m (assuming 10m station spacing)

4 3 32 101 2 4

Operation for Vertical Electrical Sounding

0 Spacings

0(0.25), 0.5,1, 2, 3, 4, 6, 8, 12, 16, 24, 32,

64 metres

C1 P1 P2 C2

Try not to get the wires crossed !

Start of line GRID REFERENCE End of LineGRID REFERENCE

ConditionsGround surface dry but soil damp should be no problems with connection

MethodResistivity CST survey using an electrode separation of 10m in a standard WENNER configuration

Equipment used

SAS 300 Terrameter

OPERATOR GKT

Distance - Midpoint of

ArrayReading Apparent

resistivity

15 45.4 2852.625 37.8 2375.035 35.6 2236.845 12.6 791.755 9.45 593.865 9.31 585.075 6.54 410.985 2.345 147.395 2.136 134.2

105 1.965 123.5115 1.456 91.5125 1.23 77.3135 1.115 70.1145 1.345 84.5155 1.689 106.1165 2.43 152.7175 5.56 349.3185 12.3 772.8195 45.6 2865.1

0 50 100 150 200 2500

5

10

15

20

25

30

35

40

45

50

Reading

Reading

Start of line GRID REFERENCE End of LineGRID REFERENCE

ConditionsGround surface dry but soil damp should be no problems with connection

MethodResistivity CST survey using an electrode separation of 10m in a standard WENNER configuration

Equipment used

SAS 300 Terrameter

OPERATOR GKT

Distance - Midpoint of

ArrayReading Apparent

resistivity

15 45.4 2852.625 37.8 2375.035 35.6 2236.845 12.6 791.755 9.45 593.865 9.31 585.075 6.54 410.985 2.345 147.395 2.136 134.2

105 1.965 123.5115 1.456 91.5125 1.23 77.3135 1.115 70.1145 1.345 84.5155 1.689 106.1165 2.43 152.7175 5.56 349.3185 12.3 772.8195 45.6 2865.1

0 50 100 150 200 25010.0

100.0

1000.0

10000.0

Apparent resistivity Line 1

Distance (m)

Log

App

. Res

. rh

o.m

Equipment

Sub-Surface Imaging or Electrical Tomography

0 A combination of V.E.S. and C.S.T. That provides an image conveying information both vertical and horizontal changes in resistivity

0 Needs many readings > must be fast > must be automated

C6.1 Wenner array

C6.2 Wenner pseudosections of some simple 2-D resistivity modelsForward modelling Example 1

C6.2 Wenner pseudosections of some simple 2-D resistivity modelsInversion Example 1

Figure courtesy of M.H. Loke

Case Studies - Mineralisation

St. Erth Formation, Cornwall Evidence for an unconformity, evidence of the clay/brickworks workings >Undisturbed site for future excavation

Engineering Applications

C6.3 Dipole-dipole array

C7.1.1 Cavity detection

C7.1.2 Environmental geophysics


3-D DC resistivity inversion


C7.1.3 Hydrocarbon exploration

Shallow gas exploration with DC resistivity. Data courtesy of KOMEX

http://iga.igg.cnr.it/geo/geoenergy.php

C7.1.4 Geothermal exploration

Low resistivityreservoir

Low resistivity clay cap

C7.1.4 Geothermal exploration

More details http://geothermal.marin.org/GEOpresentation/

Tongonan geothermal field, Leyte Bacman geothermal field, Bicol

Mayon Volcano, Bicol

C7.1.5 Geotechnical exploration


Time lapse variations

Case Studies V

Summary

0 All fieldwork requires ground contact so relatively slow0 provides quantitative results in terms of depths/location0 modern equipment provides for more rapid surveying and

2D and 3D approaches

electrical resistivity methods 13

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