1605180084-geol-305-5-electrical-methods-1.ppt

ERT

2. 1. Introduction
1.1 Active and Passive
1.2 Advantages and Disadvantages
1.3 Electrical Methods Overview
2. Basics of Resistivity
2.1 Current Flow and Ohm's Law
2.2 Resistivity, NOT Resistance
2.3 Current Density & Electric Field
2.4 Rocks Resistivities
2.5 Current Flow bet . Electrodes
2.6 Measuring Resistivity
3. Resistivity and Geology
3.1 Sources of Noise
3.2 Depth vs. Electrode Spacing
3.3 Current Flow in Layered Media
3.4 Variation in Apparent Resistivity
3.5 Current Flow in Layered Media
4. Equipment and Field Procedures
4.1 Equipment
4.2 Soundings and Profiles
4.3 Soundings: Wenner and Schlumberger
4.4 Electrode Spacing and Resistivity Plots
4.5 Advantages and Disadvantages
4.6 Profiles
5. Interpretation
5.1 Curves for Soundings: One-Layered Media
5.2 Curves for Soundings: Two-Layered Media

3. 1. Introduction
A resistivity survey is the
observation of electric fields
caused by current introduced into
the ground as a means of studying
earth resistivity in geophysical
exploration.
Resistivity is the property
of a material that resists the flow
of electrical current. The term is
normally restricted to include only
those methods in which a direct
current, is used to measure the
apparent resistivity.

4. 1.1 Active versus Passive
Passive:
Passive geophysical surveys incorporate measurements of
naturally occurring fields or properties of the earth.
Example: Magnetic and Gravity
Active:
In active geophysical surveys, a signal is injected into the earth
and we then measure how the earth responds to this signal.
Example: Resistivity and Seismic

5. 1.2 Advantages and Disadvantages
Active Passive
Advantage Disadvantage Advantage Disadvantage
Better control of noise sources
through control of injected signal .
Because both sources and
receivers are under the surveyor's
control,, field equipment tends to
be more complex .
Surveyor need only record a
naturally occurring field;
therefore, he need supply only a
sensor and a data recorder .
Less control of noise because
source of the signal is out of the
control of the surveyor .
Because propagating fields are
generally measured, active
experiments usually provide
better depth control over source
of anomalous signal .
Field operations and logistics are
generally more complex and time
consuming than passive
experiments .
Field operations are generally
very time efficient. Thus, passive
experiments can be run over
wider areas in a more cost-
effective manner .
Because passive fields are
generally the result of integrating
anomalous geologic contributions
over wide areas, identification of
the source of an anomalous
reading can be difficult .
Many different source/receiver
configurations can be used
allowing for a wide variety of
survey designs. This allows survey
designers great flexibility in
customizing surveys for
particular problems .
The increase in the number of
field options inevitably leads to
greater survey design costs and
potentially to increased
probability of field mishaps .
One or two well-established field
procedures are generally used.
Contractors can provide these
surveys on short notice with
relatively easily quantifiable
results .
This limits the amount of
customization that can be done
for specific problems .
Once set up, active experiments
are capable of producing vast
quantities of data that can be used
to interpret subtle details of the
earth's subsurface .
The large quantity of data
obtained in many active
experiments can become
overwhelming to process and
interpret .
Interpretation of the limited set
of observations can be
accomplished with modest
computational requirements
quickly and efficiently .
The data sets collected in passive
experiments are smaller than
those collected in active
experiments and usually do not
allow for as detailed an
interpretation

6. 1.3 Electrical Methods Overview
Electrical methods employ a variety of measurements of the
effects of electrical current flow within the Earth. The phenomena
that can be measured include current flow, electrical potential, and
electromagnetic fields. A summary of the well-known electrical
methods is given below:
•DC Resistivity
•Induced Polarization (IP)
•Self Potential (SP)
•Electromagnetic (EM)
•Magnetotelluric (MT)

7. 1.3 Electrical Methods Overview
DC Resistivity
This is an active method that employs measurements of electrical
potential associated with subsurface electrical current flow generated
by a DC. Factors that affect the measured potential, and thus can be
mapped using this method, include the presence and quality of pore
fluids and clays. Our discussions will focus solely on this method.
Induced Polarization (IP)
This is an active method that employs measurements of the transient
(short-term) variations in potential as the current is initially applied
or removed from the ground. When a current is applied to the
ground, the ground behaves much like a capacitor, storing some of
the applied current as a charge that is dissipated upon removal of the
current. In this process, both capacitive and electrochemical effects
are responsible. IP is commonly used to detect concentrations of clay
and electrically conductive metallic mineral grains.

8. 1.3 Electrical Methods Overview
Self Potential (SP)
This is a passive method that employs measurements of naturally
occurring electrical potentials commonly associated with the
weathering of sulfide ore bodies. Measurable electrical potentials
have also been observed in association with ground-water flow and
certain biologic processes
Electromagnetic (EM)
This is an active method that employs measurements of a time-
varying magnetic field generated by induction through current flow
within the earth. In this technique, a time-varying magnetic field is
generated at the surface of the earth that produces a time-varying
electrical current in the earth through induction. Electromagnetic
method is used for locating conductive base-metal deposits, for
locating buried pipes and cables, for the detection of unexploded
ordnance, and for near-surface geophysical mapping.

9. 1.3 Electrical Methods Overview
Magnetotelluric (MT)
This is a passive method that employs measurements of naturally
occurring electrical currents, or telluric currents, generated by
magnetic induction of electrical currents in the ionosphere. This
method can be used to determine electrical properties of materials at
relatively great depths (down to and including the mantle) inside the
Earth. In this technique, a time variation in electrical potential is
measured at a base station and at survey stations. Differences in the
recorded signal are used to estimate subsurface distribution of
electrical resistivity.

10. Georg Ohm
In 1827, Georg 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 (R) of the material and has
the units of voltage (volts) over current
(amperes), or ohms.
2. Resistivity Basics
V=IR
2.1 Current Flow and Ohm's Law

11. 2.1 Current Flow and Ohm's Law
In principle, it is relatively
simple to measure the
resistance of a strand of
wire. Connect a battery to a
wire of known voltage and
then measure the current
flowing through the wire.
The voltage divided by the current yields the resistance of the wire.
This is how your multimeter measures resistance. In making this
measurement, however, we must ask two crucial questions.
1. How is the measured resistance related to some fundamental
property of the material from which the wire is made?
2. How can we apply this relatively simple experiment to
determine electrical properties of earth materials?

12. 2.2 Resistivity NOT Resistance
The problem with using
resistance as a measurement
is that it depends not only
on the material from which
the wire is made, but also
on the geometry of the wire.
So, we want to define a
property that describes a material's ability to transmit electrical
current that is independent of the geometrical factors. The
geometrically-independent quantity that is used is called resistivity
and is usually indicated by the Greek symbol ρ.
Resistivity (ρ) is a fundamental parameter of the material making
up the wire that describes how easily the wire can transmit an
electrical current.

13. 2.3 Resistivity of Earth Materials
Like magnetic
susceptibilities, there is a large
range of resistivities, not only
between varying rocks and
minerals but also within rocks
of the same type. This range of
resistivities, as described above,
is primarily a function of fluid
content. Thus, the target for
electrical surveys is to identify
fluid saturated zones. For
example, resistivity methods
are used in engineering and
environmental studies for the
identification of the water table.
Material Resistivity (Ohm-meter)
Air Infinite
Pyrite 0.3
Galena 0.002
Quartz 40,000,000,000- 200,000,000,000,000
Calcite 100,000,000,000- 10,000,000,000,000
Rock Salt 30- 10,000,000,000,000
Mica 90,000,000,000- 100,000,000,000,000
Granite 100 – 1,000,000
Gabbro 1,000- 1,000,000
Basalt 10 – 10,000,000
Limestones 50 - 10,000,000
Sandstones 1 - 100,000,000
Shales 20 – 2,000
Dolomite 100 - 10,000
Sand 1 - 1,000
Clay 1 - 100
Water 0.5 - 300
Sea Water 0.2

14. 2.4 Current Densities and Equipotentials
If a current is injected to the
ground and measured a
distance away, the voltage
would be constant along
circular lines centered at the
electrode. These circular
lines are referred to as
Equipotentials
Current density is defined as the amount of current passing
through a unit area of an equipotential surface. Thus, close to the
electrode, the current crossing any equipotential surface
normalized by the area of the surface will thus be high. Far away
from the electrode, current density is small. Current density has the
units of Amperes per meter squared (Ampere/m2).

15. 2.5 Current Flow and Ohm's Law
V is voltage, I is current, ρ
is resistivity, and r is
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 ρ over
2πr

16. 2.6 Current Flow From Two Electrodes
If we place two
current electrodes, current
distribution and equipotential
lines produced within a
homogeneous earth become
more complicated.

17. 2.7 A Practical Way to Measure
Resistivity
Knowing the locations of the
four electrodes, and by
measuring the amount of current
input into the ground (i) and the
voltage difference between the
two potential electrodes (ΔV)
we can compute the apparent
resistivity of the medium (ρa)
using the following equation:

18. 3. Resistivity and Geology
3.1 Sources of Noise
• Electrode Polarization
• Telluric Currents
• Nearby Conductors
• Low Resistivity Layer Near Surface
• Near Electrode Geology and Topography
• Induction in Measuring Cables

19. 3.2 Depth vs. Electrode Spacing
Small electrode spacing means small depth of penetration of electric
currents. Large electrode spacing means large depth of penetration of
electric currents. Thus, increasing the electrode spacing means larger
depth of penetration of currents BUT it needs larger electric source.

20. 3.3 Resistive Layer Over Conductive Layer

21. 3.4 Conductive Layer Over Resistive Layer

22. 4. Equipment and Field Procedures
4.1 Equipment
• Current Source
• Ammeter
• Voltmeter
• Electrodes
• Cables

23. 4.2 Soundings and Profiles
Resistivity Sounding is a method to detect variations in resistivity
that occur solely with depth. In principle, electrode spacing is
varied for each measurement. The center of the electrode array,
where the electrical potential is measured, however, remains fixed.
Depth to water table
Resistivity Profiling is a method to detect lateral variations in
resistivity like gravity and magnetic methods. Profiles employ
fixed electrode spacing, and the center of the electrode spread is
moved for each reading.
Detection of faults

24. 4.3 Soundings: Wenner and Schlumberger
In Wenner array,
AM = MN = NB
In Schlumberger array,
5 MN << AB

25. 4.4 Wenner vs. Schlumberger
Schlumberger Wenner
Advantages Disadvantages Advantages Disadvantages
Move the current
electrodes only for
most readings.
All four electrodes,
must be moved for
each reading.
Very sensitive
voltmeters are
required.
Less sensitive
voltmeters are
required.
Because the potential
electrodes remain in
fixed locations, the
effects of near surface
lateral variations in
resistivity are
reduced.
Because all
electrodes are moved
for each reading, this
method is susceptible
to near surface,
lateral variations in
resistivity.
Interpretation is
limited to simple,
horizontal layers.
Interpretation is
limited to simple,
horizontal layers.

26. 4.6 Profiles
When the electrode array is far
from the vertical fault, the
measured apparent resistivity is
equal to the resistivity of the
underlying rock. As the array
approaches the fault, the
resistivity varies in a
discontinuous fashion. That is,
the change in resistivity with
electrode position does not vary
smoothly. The discontinuities in
the resistivity profile correspond
to array locations where
electrodes move across the fault.

27. 5. Interpretation
For electrical soundings, electrode spacings commonly are
chosen so that they are evenly spaced in log distance rather than being
evenly spaced in linear distance to address the problem described
above. Shown below is a plot of log apparent resistivity versus log
electrode spacing, where the distance interval is now chosen to be
evenly spaced in log distance rather than linear distance. Now there
are approximately as many samples showing apparent resistivities of
500 ohm-m as there are of 50 ohm-m. In addition, the transition
between these two extremes is well-sampled.
The most common electrode spacing used is one that employs
6 soundings for every decade in distance. For this example, using six
points per decade would yield electrode spacings of 0.25, 3.67, 5.39,
7.91, 1.16, 1.70, 2.5, 3.67, 5.39, 7.91, 11.6, 17.0, 25.0, 36.7, 53.9,
79.1, 116.0, 170.0, 250.0.

28. 5.1 Curves for Soundings: One-Layered Media
A 10-meter thick, 5000 ohm-m layer
overlies a halfspace that has a resistivity less
than 5000 ohm-m. Shown below are
apparent resistivity curves computed
assuming various values of resistivity for the
halfspace (2500, 1000, 500, 50, 10, 5 ohm-
m). All of the curves approach the resistivity
of the layer, 5000 ohm-m, at small electrode
spacing. As electrode spacing increases, the
apparent resistivity curves approach the true
resistivity of the halfspace In addition, note
that the resistivity curves all tend to show the
greatest change in apparent resistivity with
electrode spacing when electrode spacings
equal the depth of the layer, 10 m.

29. 5.2 Curves for Soundings: Two-Layered Media
This structure consists of two
layers overlying a halfspace. A suite of
resistivity curves, each generated assuming
a different resistivity for the underlying
halfspace, is shown below. At small
electrode spacings, all of the curves
approach the resistivity of the top layer. As
electrode spacing increases, resistivity
decreases and then increases at larger
electrode spacings. At the curve's lowest
point, the apparent resistivity does not
approach 250 ohm-m, the resistivity of the
middle layer. It is still possible, though, to
discern the presence of the three layers and
qualitatively estimate their resistivities.