Electrical and Electromagnetic Methods
Variations in natural electric field or induced (artificially generated)
electric currents at or near the Earth’s surface
Objective
Map variation in electrical response of rocks, minerals and pore
content. That is to measure the electrical conductivity, or its
inverse 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) Resistivity
Measures 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 as
the 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 Law
Empirical relationship between the current (I) flowing through a wire,
of resistance R and the voltage potential (V) required to propagate
the current.
V ? IR
Further
L
A
R ?
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.
? ? RA
L
or
? ? VA
IL
Geoelectric Soundings, Geology and Hydrogeology
Ranges of Electrical Resistivity
10-1
Dry Sand
Wet Sand
Sandy-silt
Silt
Clay
Peat
Sandstone
Siltstone
Coal
Salt water
1
Resistivity ? m
101
102 103
104
105
Current Flow in the Ground
+I
Equipotential
Surface (surface of
constant voltage)
-I
I
Current flow
lines
It is the potential difference between the equipotential surfaces that
causes the current to flow
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
I
V
Potential electrodes
Current electrodes
DC Resistivity
The most common electrical method used
Resistivity 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, i
in the form shown below.
– where a is the electrode separation.
?V
? ? ? 2? a
i
– 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.
Common Electrode Spacing
Pole-Dipole
a
Schlumberger
a or na
I >10a
Pole-Pole
V, >10a
a
b
a
Wenner
a
I, >10a
a
Potential electrodes
Current electrodes
a
a
Wenner Array Sounding
I
V
?1
a
a
a
?V
? a ? 2? a
I
?2
• 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
?2
?a
?1
a
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 specific
target 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
?1
?2
h1
? 1/ ? 2= 20
? a/ ? 1
5
1/5
1/20
L=AB/2
L/h1
A
From Milsom, 1989
m
n
B
“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- Mcohm
2D
• AMEM - Lund
• OYO - Mcohm 21 (DAP21)
• Advanced Geosciences Inc., - Sting/Swift
Initial Equipment Cost
Productivity
Maintenance
Operation budget
Experience
£4K to£6K; £10K to £15K
Low for 1D; medium for 2D
(5 sections per day)
Low
Low
Low for 1D; medium for 2D
Electrical Equivalence
•
An electric unit is characterized by
– resistivity (conductance)
– thickness
• the combination of these gives the layer equivalence
the 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
1
frequencies
? ?2
• Strength diminishes rapidly with depth d ? 503?
? ?
World Transmitter Locations
?f ?
? ?
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
Productivity
Maintainance
Operation budget
Experience
£3K to £15K
High along regional
transects
Low
Low
Low for acquisition,
processing and
interpretation
Differences and Similarities between Electrical and Electromagnetic
Methods
Electrical
• Time invariant (or slow)
electrical currents
• Electrical currents directly
applied to earth (galvanicelectrode 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
Spacing (s)
Tx
Rx
›
›
›
Time varying
magnetic field
Where
s = intercoil spacing
? = 2? frequency
µ0 = permeability of free space
Tx - transmitter coil
Hp -time varying primary
magnetic field induces,
Hs - secondary magnetic field
At low induction numbers, Hs/Hp
is linearly proportional to
terrain conductivity such that
the apparent conductivity (? a)
4 ?? H s ??
?a?
2 ?
? ? 0 s ? H p ??
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) with
soundings 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
Productivity
Maintenance
Operation budget
Experience
£8K to £15K
High along lines and for
grids
Low
Low
Low level necessary
FDEM Limitations and Advantages
Limitations
• 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 resolution
Typical FDEM Measurement Ranges for Geonics Equipment
Instrument
Coil Spacing
Horizontal Dipole
Vertical Dipole
EM38
1m
0.75m
1.5m
EM31
3.7m
3m
6m
EM34
10m
7.7m
15m
EM34
20m
15m
30m
EM34
40m
30m
60m
FDEM Examples
See: Arch-geophysics presentation
FDEM Examples
Electromagnetic Methods - Comparison of FDEM and TDEM System Waveforms
Time Domain
Frequency 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 field
on 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
Central horizontal receiver loop
Transmitter 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) with
soundings 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
Productivity
Maintenance
Operation budget
Experience
£30K to £40K
Medium to low
High
Medium
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
Explanation
Sedimentary Rocks
Carboniferous
Upper Devonian
St Andrews
Bay
Lower Devonian
Igneous Rocks
Ordnance Survey Map (Brown, 1980)
Drift geology around St
Andrews
Explanation
Recent and Pleistocene
Man-made deposits
Wind blown sand
Peat
Alluvium
Present beach & intertidal deposits
St Andrews
Bay
Raised marine deposits - post glacial
Raised marine deposits - late glacial
Raised marine delta - late glacial
Feature marking former coastline
Glacial meltwater deposits
Till
Ordnance Survey Map (Brown, 1981)
Bedrock
Boundary of superficial deposits
Glacial drainage channel
Geophysical Techniques
Electrical Soundings
• Result: geo-electric soundings or cross-sections of
the earth
Techniques
• 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
10-1
Dry Sand
Wet Sand
Sandy-silt
Silt
Clay
Peat
Sandstone
Siltstone
Coal
Salt water
1
Resistivity ? m
101
102 103
104
105
TDEM - Geoelectric Section
Borehole
Line 1 and Line 2
Line 1
Line 2
BH3
BH2
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
Traverse across links using EM31
dry
saturated
after Tapias and Casas