Mag 704
Mag 704
Mag 704
Geophysical prospecting
MAG 704
DEPARTMENT OF GEOSCIENCES
Recommended Textbooks
Environmental and Engineering
Geophysics By Prem Sharma
Introduction to Geophysical Prospecting By
Milton Dobrin
An Introduction to Applied and
Environmental Geophysics By John M.
Reynolds
Electrical Methods
There are many electrical and
electromagnetic methods used in
geophysics. These methods are most
often used where sharp changes in
electrical resistivity (resistance in the
ground) are expected - particularly if
resistivity decreases with depth.
Applied current Methods: when a
current is supplied by the geophysicist.
Currents are either DC or low frequency
A.C
In the Electrical resistivity method, the
potential difference (voltage) is measured at
various points;
In the Induced Polarization method, the
rise and fall time of the electric potential are
measured.
In Spontaneous Potential method, natural
potential that may arise from electrochemical
reactions due to many ore bodies is measured.
The electromagnetic method applies an
alternating current with a coil and the
resulting field magnetic field is measured
with another coil.
Natural Currents: when natural currents in the earth
are measured. Movement of charge in the
ionosphere and lightning cause telluric currents to
be generated in the earth.
Variation of the spectra of these current fields
and their magnetic counterparts yield information
on subsurface resistivity.
Concept of Electricity
resistor
V
volt meter
Resistance tells us the total drag on the current, but not the
property of the material that is generating the drag. The
value of resistance depends upon both the material and its
shape ( e.g., Cu and Pb)
Resistivity is the resistance in ohms between the opposite
faces of a unit cube of a material.
The resistivity of a material is a measure how well the
material retards the flow of electrical current. It varies
from one material to another. A good conductor like
Cu is on the order of 10-8Ohm-m, the resistivity of an
intermediate conductor such as wet topsoil is ̴ 10Ωm,
and the resistivity of poor conductors such as sandstone
is ̴ 10Ωm.
Due to this great variation, measuring the resistivity of an
unknown material has the potential for being very useful
in identifying that material, given little further
information.
Resistivity of different Materials
C1 P1
current
current dr
equipotential
If we measure the potential difference between
two shells at some a distance D from the
electrode, we get
l dr
dV iR i i 2
A 2r
i dr i
V D dV D r 2 2D Eq *
2
IF the resistivity of the ground is UNIFORM.
This is the basic equation of resistivity, in that
we can add the potentials from many sources to
obtain a "potential" map of a surface.
By contouring that map, we have equipotential
lines, along which no current flows. Current
flows in directions perpendicular to equipotential
lines.
TWO CURRENT ELECTRODES:
i i i 1 1
VP1 ( )
2r1 2r2 2 r1 r2
What is the potential between the electrodes vs. depth?
We can express r1 & r2 in terms of the x-z coordinate
i 1 1
VP1
2 d
2 2
x z 2 d x z 2
2 2
Current flow…
We have seen that the current flow lines are
perpendicular to the equipotential lines, but
we do not know how the current is
distributed.
A simple equation was established by Van
Nostrand and Cook, 1966 that provides
current distribution as a fraction of the total
current between the two electrodes.
Along a vertical plane midway between the two
electrodes, fraction of the total current (if)
penetrating to a depth z for an electrode separation
d is given by:
2 2z
1
if tan
d
We can use the equation above to investigate the
current distribution for various current electrode
separations ( d) and depth (z).
If several values for the current electrode
separation are used, it will be observed that the
values in depth electrode separation and the % of
Total current do not change.
Percentage of current penetrating a homogeneous Isotropic earth
10%
40%
70%
potentia l
The potential
C1 + P1 P2
- C2 difference between
r1 r2
P1 and P2 is:
r3 r4
i i i i i 1 1 1 1
VP1 P2
2r1 2r2 2r3 2r4 2 r1 r2 r3 r4
ip 1 1 1 1
V Vp1 Vp2
2 r1 r2 r3 r4
2V 1
i 1111
r1 r2 r3 r4
Thus, we can measure the current, voltage, and
appropriate distances and solve for resistivity.
Factors Affecting Electrical Resistivity
Resistivity of water
Porosity of the formation
Pore geometry
Lithology of the formation
Degree of cementation
Type and amount of clay in the
rock
Rock containing pores saturated
Saturation
with water and hydrocarbons Equation Formation
Non-shaly rock, 100% saturated
Factor
with water having resistivity, Equation
Rw
Rt Cube of water
= 20% having resistivity,
Sw = 20% Rw
Ro
= 20%
Sw = 100%
Rw
= 100%
Sw = 100%
(1) Rock
Conductivity
Increasing
Resistivity
Increasing
(2) Gas
(3) Oil
(4) Fresh Water
(5) Salt Water
Electrode configurations/ Arrays
The electrode patterns used in resistivity
surveying almost always are the Wenner,
Schlumberger, dipole-dipole, pole-dipole
and pole-pole.
In conducting an expanding-spread Wenner
survey, we move all electrodes along a
straight line after every reading so the
spacing between electrodes remains equal
and takes on certain prescribed values
Wenner array
In Wenner array four electrodes are arranged
collinearly and the separations between adjacent
electrodes are equal.
The apparent resistivity of the medium measured
with this array is given by:
Schlumberger Array
The Schlumberger array is arranged with two current
electrodes on the outside of the array, set apart by a
distance at least five times the spacing between the
two interior potential electrodes.
The potential difference measurement is believed to
lie at the mid span of the interior potential electrodes,
at a depth approximately one half of the length
between the exterior current electrodes.
The Schlumberger Array is preferred for VES
applications due to the strong horizontal resolution
and ease of setup in the field
The apparent resistivity measurement for the Schlumberger
Array can be represented by equation given above.
In the equation, the spread length or distance between
current electrodes is L, and the length between the potential
electrodes is expressed by the variable l.
With respect to L and l, the apparent resistivity measurement
is valid as long as the spread length, L, does not exceed 5
times the potential electrode spacing, l
Fig. Some commonly used electrode arrays
Electrical Resistivity Methods
Resistivity surveying investigates variations of
electrical resistance (or conductivity, the inverse
of resistivity), by causing an electric current to
flow through the ground, using wires connected
to current electrodes pushed into the ground.
In resistivity surveying, we are concerned with
the movement, or flow, of charges through rocks
Electrical resistivity techniques
The resistivity method involves the measurement of the ability of
soil, rock and ground water to resist the flow of an electrical
current.
It is a function of the soil and rock matrix, percentage of fluid
saturation and the conductivity of the pore fluids.
The electrical resistivity method is used to map the subsurface
electrical resistivity structure, which is interpreted by the
geophysicist to determine geologic formations and/or physical
properties of the geologic materials.
The electrical resistivity of a geologic unit or target is measured in
ohm-meters (Ω-m), and is a function of porosity, permeability,
water saturation and the concentration of dissolved solids in pore
fluids within the subsurface.
Techniques in ER
Vertical Electrical Soundings (VES)
Constant Separation Traversing (CST)
VES
When the ground consists of a number of more or less
horizontal layers, knowledge of the vertical variation of
resistivity is required.
The vertical electrical sounding (or drilling) deduces the
variation of resistivity with depth below a point on the
ground surface and the procedure is based on the fact
that the current penetrates deeper with increasing
separation of the current electrodes.
The two most common arrays for electrical resistivity
surveying in the sounding made are the Wenner and
Schlumberger arrays (Reynolds, 1997).
CST cont’d
Resistivity profiling involves moving an array of
electrodes while keeping the array arrangement and
spacing fixed (Sheriff, 1989).
Lateral changes in resistivity are mapped, allowing
for the delineation of contaminant plumes and of
lateral changes in hydrogeologic conditions.
Profiling can allow for rapid data interpretation by
mapping apparent resistivity values and noting
anomalous features relative to background.
CST cont’d
If the layers or boundaries are vertical,
rather than horizontals planes, the electrical
profiling or constant separation traversing is
usually employed.
The objective of this method is to detect
lateral variation in the resistivity of the
ground (Reynolds, 1997).
The two most common arrays for electrical
resistivity surveying involving profiling
mode are the Wenner and dipole-dipole
arrays (Reynolds, 1997).
CST cont’d
The electrode geometry for the Wenner
array is the same as in sounding, the
difference is that in the profiling mode, the
entire array is moved laterally along the
profile while maintaining the potential and
current electrode separation distances.
In the dipole-dipole array the distance
between the potential and current dipoles (a
dipole consists of a pair of electrodes) is
maintained while the array is moved along
the profile
CST’D
The data listed below were acquired using a constant-spread, Wenner
array was used at a-spacing = 3m. Interpret the data as completely as
possible
Horizontal Resistivity (ρa)Ωm Horizontal Resistivity (ρa)Ωm Horizontal Resistivity (ρa)Ωm
Position (m) Position (m) Position (m)
900
800
700
Apparent resistivity (Ωm)
600
500
Resistivity Ωm
400
300
200
100
0
0 5 10 15 20 25 30 35 40 45
Horrizontal position of spread centre (m)
Electrical Resistivity (ER)
Various problems can hinder the collection of accurate
resistivity data. Dry surface material (high resistivity) can make
injection of current very difficult.
Common problems include: Coupling between wires and reels.
Poor electrical contact with the ground.
Cultural noise, including stray currents, potential fields and
electromagnetic currents as a result of power lines, man-induced
ground currents, fences, railroad tracks, and buried metallic pipes.
Inverse problem
Computer Modeling
Inversion scheme in geoelectric
sounding
Inversion cont’d
For 1D VES, inversion is carried out using
geophysical inversion softwares (Winresist,
Res1Dinv etc).
The layer resistivity and thicknesses
obtained after curve matching i.e. forward
modeling were used as input during the
inversion process are input.
The layer parameters are altered until a
good fit is achieved between the observed
and the calculated values.
Inversion cont’d
The iteration process of a curve can go as
far as 30 times of achieving an effect match,
after which the computer displays the final
result of the iteration and the layer
parameters.
In CST, forward modeling is used to
calculate the apparent resistivity values of
2-D resistivity data using DIPPRO
software or
Res2Dinv a popular inversion program
(Geotomo)
Inversion cont’d
Resistivity of each block is then calculated
to produce an apparent resistivity pseudo
section.
A pseudo section is obtained when a good
model misfit is obtained between the
calculated and observed apparent resistivity
values (Optimization process).
A measure of this difference is given by the
root-mean-square (RMS) error.
Possible interpretation errors
Equivalent models
Resistivities and thicknesses of each layer can be derived
from the apparent resistivity curve clearly.
In the field measurement errors occur .
The apparent resistivity curve can be interpreted by
different resistivity models.
The principle of equivalence: The thickness and resistivity
can not be derived independently
Dar Zarrouk Parameters
Fig. Schematic diagram of the construction and inversion of DZ curves, hj, layer
thickness; pj layer resistivity; T, transverse resistance; S, longitudinal conductance, ρm,
DZ resistivity; Lm, DZ depth
VES1 133.3 VES2 147.3
A 118.4 A'
1356.5 354.9
5
152.5
10
1640.8
DEPTH(m)
15
1740.3 LEGEND
20
TOPSOIL
145.7
25 SANDY CLAY
30 141.4 SAND
35 145.7 RESISTIVITY(Ohm-m)
(b)
Factors affecting induced polarization
Membrane polarization Electrode polarization
Show that
Metal factor of some earth materials
Survey principle with multi-electrode system
• Monitoring wells are usually installed within a site and its environ
to gauge the extent of contamination of the water resources.
SW BH 1 NE
0
-10
clay
-20 leachate sand
-10
-20
weak IP unit
-10
unsaturated clay
-20
section sand
-10
-20
weak IP unit
-10
-20 clay
sand
Elevation -30
(m) 20 40 60 80 100 120 140 160 180
Inverse Model Resistivity Section
0
-10
-20
strong IP units
-30
20 40 60 80 100 120 140 160 180
-20
-20
-40
unsaturated soil
-60
leachate without waste
Elevation 50 100 150 200 250 300
(m) Inverse Model Resistivity Section
0
-20
-40
waste with fraction of leachate
-60
50 100 150 200 250 300
Inverse Model Chargeability Section
Resistivity Chargeability
Horizontal Distance (m) (Ωm) (ms)
20
uncontaminated
0 leachate upslope saturated surface
leachate downstream
Elevation
(m) 20 40 60 80 100 120 140 160 180
Inverse Model Resistivity Section
20
0
waste with leachate waste with leachate
Resistivity Chargeability
20 40 60 80 100 120 140 160 180 (Ωm) (ms)
20
uncontaminated
saturated surface
0 leachate upslope leachate downstream
20
-20
saturated soil
(not clay) unsaturated soil
-40
20 40 60 80 100 120 140 160 180
Elevation
Inverse Model Resistivity Section
(m) 0
-20
weak IP unit
-40
20 40 60 80 100 120 140 160 180
Resistivity Chargeability
Inverse Model Chargeability Section (Ωm) (ms)
Leachate Plume Low (Guérin et al. 2004; Low (Gallas et al. 2011)
Kaya et al. 2007)
Saturated waste Intermediate High (Dahlin et al. 2010)
(without leachate)
Saturated soil Intermediate (Guérin et Low
(uncontaminated) al. 2004)
• In the joint application of resistivity and IP surveys, the
interpretation of resistivity models is more definite than analysis of
chargeability models. There is negligible controversy on resistivity
status of various subsurface sections of the waste disposal sites.
Electrochemical potential
If the concentration of the electrolytes in the ground
varies locally, potential differences are set up due to the
difference in mobilities of anions and cations in solutions
of different concentrations‐‐‐called liquid‐junction or
diffusion potentials
Electrical potential is also generated when 2 identical
metal electrodes are immersed in solutions of different
concentrations‐‐‐ called Nernst potential.
Diffusion + Nernst potentials = Electrochemical, or
static, self potential.
One of the most common natural electrolytes is NaCl.
For NaCl solutions of different concentration (C1,C2)
but at the same temperature ,T (°C), the amplitude of
the electrochemical potential (Ec) is given by
MINERAL POTENTIALS