Electrical and Electromagnetic Methods

Variations in natural electric field or induced (artificially generated) electric currents at or near the Earth’s surface

ObjectiveMap variation in electrical response of rocks, minerals and pore

content. That is to measure the electrical conductivity, or itsinverse resisitivity, of the subsurface

Factors affecting electrical response• Mineralogy• Fabric• Pore content (fluid) type and saturation Examples• Hydrogeological - ground water exploration • Environmental -brine contamination migration, clay aquitard mapping• Lithological discrimination• Mineral exploration

Electrical and Electro-magnetic Methods

Direct Current (DC) ResistivityMeasures the electrical potential associated with electrical current flow generated by a direct current applied to the ground. Used to discriminate between different lithologies and pore fluids.

Induced Polarisation (IP)Measurement of transient (short term) variations in potential asthe current is initially applied or removed from the ground. Used to locate concentrations of clay and other electrically conducting materials.

Self Potential (SP)Measurement of naturally occurring electrical potentials commonly associated with the weathering of (sulphide) ore bodies and certain ground water flows.

Electro-magnetic (EM)Measurement of time-varying magnetic field generated by induction through current flow within the Earth. Used for locating conductive base metals and buried ferro-magnetic sources.

Fundamental Principles

Ohm’s LawEmpirical relationship between the current (I) flowing through a wire,

of resistance R and the voltage potential (V) required to propagate the current.

Further

where L is the length and A the cross sectional area of wire.However, as we are not concerned with wires in the Earth, and

electrical current is not constrained, the resistivity, ? of a material is a more useful concept where.

V IR?

? ? RAL IL

VA??

AL

R ?

or

Geoelectric Soundings, Geology and Hydrogeology

Ranges of Electrical Resistivity

Dry SandWet SandSandy-siltSiltClayPeatSandstoneSiltstoneCoalSalt water

Resistivity ? m10-1 1 101 102 103 104 105

Current Flow in the Ground

I

EquipotentialSurface (surface ofconstant voltage)

Current flow lines

It is the potential difference between the equipotential surfaces that causes the current to flow

+I -I

Measurement of Current Flow in the Ground

• However, it is difficult to measure the current flow directly because of contact resistance between the electrodes and the ground.

• Therefore, the potential difference between a second set of electrodes not connected to the primary circuit is measured

IV

Potential electrodes

Current electrodes

DC Resistivity

The most common electrical method usedResistivity Profiling - used to determine lateral changes in resistivity

due to changes in geologic structure.Resistivity Soundings - used to determine vertical changes in

resistivity due to geologic structure assuming horizontal layering.• Wenner Array• Schlumberger Array

– Both techniques measure apparent resistivity (?) computed from measurements of voltage, change in voltage and current, iin the form shown below.

– where a is the electrode separation.

– A plot of apparent resistivity (? )versus electrode spacing is made by moving the electrodes to new (expanded) positions. From this an interpretation of resistivity (conductivity) variation with depth is made.

? ?? ? 2 aVi

?

Common Electrode Spacing

Pole-Dipole

Wenner

Schlumberger

a aa

Current electrodesPotential electrodes

a ab

Pole-Pole

a a or na

V, >10a a

I >10a

I, >10a

Wenner Array Sounding

• depth soundings are made by increasing the spacing between the electrodes

• at small spacing, current flows in upper layer

• at larger spacing, current flows in deeper layer

IV

a

aa a?1

?1

?2

? 2?a

? ?a aVI

? 2?

Typical DC Resistivity Survey

• Define Objectives• Review Regional Geology and Hydrogeology• Assess Noise• Forward Model - geo-electric section for sounding, design survey

sample interval, station spacing, desired resolution at specifictarget depth

• Test/calibrate sounding at or near ground truth (outcrop or borehole) with soundings at more than one orientation

• Execute design survey or adjust survey to meet objectives

Vertical Electrical Sounding Interpretation

Interpretation•2 Layer - use master curve•3 Layer - books of master curves•4 or more layers - must use computer programs

General Points•# turning points equals # of resolvable layer•equal log/log plot, no slope > 45°•relative magnitude of resistivity important

after Reynolds, 1997

Geoelectric Section

Borehole

Geoelectric Section

south-north from grid ref. (-120,20) to (150,20)

west-east from grid ref. (-70,30) to (-70,10)

2 Layer Vertical Electrical Sounding Interpretation Using Master Curves

From Milsom, 1989

?1/ ?2= 20

5

1/5

1/20

?1

?2

? a/ ? 1

L/h1

h1

A m n B

L=AB/2

“2D” Profiling

• Uses fixed array type - Wenner, Schlumberger or commonly pole-pole (“Time Team” style)

• Uses fixed array distance• Rapid coverage of large area

Electrical Surveying Equipment

1D• Syscal - Junior, R1 Plus, R2• ABEM - Terrameter 300b and300c• OYO- Mcohm2D• AMEM - Lund• OYO - Mcohm 21 (DAP21)• Advanced Geosciences Inc., - Sting/Swift

Initial Equipment Cost £4K to£6K; £10K to £15KProductivity Low for 1D; medium for 2D

(5 sections per day)Maintenance LowOperation budget LowExperience Low for 1D; medium for 2D

Electrical Equivalence

• An electric unit is characterized by – resistivity (conductance)– thickness

• the combination of these gives the layer equivalencethe layer equivalence (a range of resistivity vs. thickness models) is

usually calculated based on the % fit of the data to the best fit model

Norton Farm Raised Beaches

S N

Limestone Bedrock Channel

Landfill Site

Pit Head

EM, Resistivity and Microgravity

3D Resistivity

Very Low Frequency(VLF) EM

• Hybrid of Electrical and EM• Earth acts like a resistor but current induced by varying

magnetic field• Field supplied by transmitter stations of different

frequencies• Strength diminishes rapidly with depth

World Transmitter Locations

21

503 ???

????

??

fd

?

Uses and Limitations of VLF Surveying

Advantages• Rapid - modern instruments can record 3 signals

simultaneously• Cheap - equipment, data acquisition and data processing

Disadvantages• Sometimes Non-repeatable dependent on signal strength• Reliant on the transmitter being on which is out of the

operators control• Effected by topography - if results are a mirror of

topography then beware

Response to a Vertical Sheet Conductor(water filled fractures)

Tilt angle profile Schematic of amplitude variation with depth

Response to a Vertical Sheet Conductor(water filled fractures)

Response to a Vertical Conductors

VLF Equipment

• Geonics - Em16, EM16R• ABEM - WADI• EDA - Omi IV• Scintrex - VLF3, VLF4• Phoenix - VLF2

Cost £3K to £15KProductivity High along regional

transectsMaintainance LowOperation budget LowExperience Low for acquisition,

processing andinterpretation

Differences and Similarities between Electrical and Electromagnetic Methods

Electrical• Time invariant (or slow)

electrical currents• Electrical currents directly

applied to earth (galvanic-electrode contact)

• inexpensive equipment• ease (low cost) of data

processing• poor lateral resolution• high sensitivity to geologic

noise• transmitter array

approximately 7-10 times greater than depth of penetration

Electromagnetic• Time variant

(quickly)electrical and magnetic fields

• Electrical currents induced by remote electrical and magnetic fields

• expensive equipment• complex processing

• high lateral resolution• low sensitivity to geologic

noise

• transmitter array approximately equal to depth of penetration

Generalized Schematic of Electromagnetic Surveying Method(Grant and West, 1965)

FDEM Principles

› Tx - transmitter coil› Hp -time varying primary

magnetic field induces,› Hs - secondary magnetic field

At low induction numbers, Hs/Hpis linearly proportional to terrain conductivity such that the apparent conductivity (? a)

Wheres = intercoil spacing? = 2? frequencyµ0 = permeability of free space

Spacing (s)

TxRx

Time varyingmagnetic field

???

????

??

p

sa H

Hs2

0

4? ?

?

Factors Affecting Conductivity

• Lithology - type (electrical properties of particles or crystals)• Porosity - shape, size, number of pores • Permeability - shape, size and extent of interconnecting passages• Pore fill type and amount - fluid or solid in pores? Conductivity of

pore fill• Concentration of dissolved electrolytes in the contained (bound)

moisture• Temperature and phase state of the pore fill content• Colloids/organic component? This is only partially understood and

beyond the scope of this course

Refer to Geonics Technical note TN-5 for a more complete discussion of these issues

Typical FDEM Survey

• Define Objectives• Review Regional Geology and Hydrogeology• Assess Noise• Forward Model - geo-electric section for sounding, design survey

sample interval, station spacing, line spacing, desired resolution at specific target depth, cost

• Test sounding at or near ground truth (outcrop or borehole) withsoundings at more than one orientation

• Layout survey grid or line (survey locations)• Execute design survey - acquire data (e.g. EM31 walk and measure

in horizontal or vertical dipole orientation)• Download data from data logger or transfer from field note book• Preliminary process and plot (line, or contour) before leaving field

at the end of each day and/or at regular intervals throughout the day

FDEM Equipment

• Geonics – EM38– EM31– EM34– EM39 (borehole)

• Geofyzika a.s.– CM-031

Cost £8K to £15KProductivity High along lines and for

gridsMaintenance LowOperation budget LowExperience Low level necessary

FDEM Limitations and AdvantagesLimitations• Measure of (very small) secondary field in presence of primary

field• Limited exploration depth• Very sensitive to cultural (electrical) noise• Limited vertical resolution

Advantages• No ground contact• High survey productivity• Direct measure of ground conductivity• High lateral resolutionTypical FDEM Measurement Ranges for Geonics Equipment

Instrument EM38 EM31 EM34 EM34 EM34Coil Spacing 1m 3.7m 10m 20m 40mHorizontal Dipole 0.75m 3m 7.7m 15m 30mVertical Dipole 1.5m 6m 15m 30m 60m

FDEM Examples

See: Arch-geophysics presentation

FDEM Examples

Electromagnetic Methods - Comparison of FDEM and TDEM System Waveforms

Frequency DomainTime Domain

After Hoekstra and Blohm (1989)

Electromagnetic Methods Comparison of FDEM and TDEM

Frequency Domain - Relative Response for Vertical and Horizontal Dipoles

Time Domain - Current Intensity with Depth and Time

After Hoekstra and Blohm (1989)

TDEM - behavior of emf

Behavior of emf due to vertical magnetic field in centre of 100m2

loop

Measured emf due to vertical and horizontal magnetic fieldon a profile through centre of 400m2 transmitter loop

After Mills et al. (1988)

TDEM - Eddy Current Distribution at Successive Times after Turnoff of Primary (transmitter) Current

Configuration for Central Loop TDEM sounding

Transmitter loopCentral horizontal receiver loop

Typical Central Loop TDEM Survey

• Define Objectives• Review Regional Geology and Hydrogeology• Assess Noise• Forward Model - geo-electric section for sounding, design survey

sample interval, station spacing, line spacing, desired resolution at specific target depth, cost

• Test sounding at or near ground truth (outcrop or borehole) withsoundings at more than one orientation

• Layout survey grid or line (survey locations for centre of loop)• Execute design survey - lay out square transmitter loop, stack data

at centre of loop. • Noise Check by acquiring data with receiver at 1/2 distance

between centre of loop and transmitter wire • Preliminary process and review data before leaving field at the end

of each day and/or at regular intervals throughout the day

TDEM Limitations and Advantages

Limitations• Sensitive to conductivity inhomogeneities around receiver coil• Sensitive to below and above ground EM noise (electrical/magnetic

storms)Advantages• High lateral resolution - small transmitter loop (0.75x depth of

penetration)• high vertical resolution• Low sensitivity to geologic noise• Ambient noise can be removed by stacking

TDEM Output• 1D geoelectic sections• 2D geoelectric sections

TDEM Equipment

• Geonics – EM47– EM37– EM42– EM61 (shallow metal detector)

• Geometrics– Stratagem

Cost £30K to £40KProductivity Medium to lowMaintenance HighOperation budget MediumExperience Medium to high level

necessary

Comparison of DC Resistivity Sounding and TDEM Sounding

Schlumberger Array Central Loop TDEM

After Hoekstra et al. (1992)

Bedrock geology around St Andrews

St Andrews Bay

Ordnance Survey Map (Brown, 1980)

Sedimentary Rocks

Carboniferous

Upper Devonian

Explanation

Igneous Rocks

Lower Devonian

Drift geology around St Andrews

Ordnance Survey Map (Brown, 1981)

St Andrews Bay

Recent and Pleistocene

Explanation

Man-made deposits

Wind blown sand

Peat

Alluvium

Present beach & intertidal deposits

Raised marine deposits - post glacial

Raised marine deposits - late glacial

Raised marine delta - late glacial

Feature marking former coastline

Glacial meltwater deposits

Till

BedrockBoundary of superficial depositsGlacial drainage channel

Geophysical Techniques

Electrical Soundings• Result: geo-electric soundings or cross-sections of

the earthTechniques• Direct Current Electrical Resistivity (DCR)

– Penetration depths to 20m• Time domain Electro-Magnetics (TDEM)

– penetration depths to 100m

Geoelectric Soundings, Geology and Hydrogeology

Ranges of Electrical Resistivity

Dry SandWet SandSandy-siltSiltClayPeatSandstoneSiltstoneCoalSalt water

Resistivity ? m10-1 1 101 102 103 104 105

TDEM - Geoelectric Section

Borehole

Line 1 and Line 2

Line 1

Line 2

BH2

BH3

Line 1

200 m

Line 2

200 m

Correlation of Geophysics and Geology

Borehole logging• Gamma ray• Resistivity

Potential of Future Geophysical Studies

• Monitoring Salt water Intrusion• Monitoring surface water/green quality

Saltwater Intrusion

from Blackhawk

Saltwater Intrusion

from Blackhawk

Green Quality- Watering Strategy

dry saturatedafter Tapias and Casas

Traverse across links using EM31