GPH 321-Survey Design
GPH 321-Survey Design
GPH 321-Survey Design
Cont.
( 2 weeks )
Cont.
MIDTERM EXAM
ELECTROMAGNETIC METHODS
Classification of electromagnetic systems
Principles of electromagnetics
Magnetotelluric Methods
Vertical loop (VLEM)
Slingram & Turam Systems
Very Low Frequency (VLF)
Audio Frequency Magnetics (AFMAG)
Time-Domain systems ( TDEM )
Airborne Method
Ground Penetrating Radar
FINAL EXAM
( 4 weeks )
1-2-3-5-7-9-13-14-19-21-23-24
Midterm exam.
Lab.
Homework Assignments
Final exam.
TEXT :
25
20
15
40
%
%
%
%
Robinson & Coruh (1988 ) . Basic Exploration Geophysics. John Wiley & Sons
Lowrie, W. ( 1997). Fundamentals of Geophysics. Cambridge University Press.
INSTRUCTOR
:
ABDULLAH M. S. AL-AMRI
OFFICE HOURS
:
Sun & Tues
1 - 2
Ohms Law
Ohms Law describes the electrical properties of any
medium. Ohms Law, V = I R, relates the voltage of a
circuit to the product of the current and the resistance.
This relationship holds for earth materials as well as
simple circuits. Resistance( R), however, is not a
material constant. Instead, resistivity is an intrinsic
property of the medium describing the resistance of
the medium to the flow of electric current.
Resistivity is defined as a unit change in resistance
scaled by the ratio of a unit cross-sectional area and a
unit length of the material through which the current is
passing (Figure 1). Resistivity is measured in ohm-m
or ohm-ft, and is the reciprocal of the conductivity of
the material. Table 1 displays some typical resistivities.
1 Metals :
Conduction by the flow of electrons depends upon the
availability of free electrons. If there is a large number of
free electrons available, then the material is called a
metal, the number of free electrons in a metal is roughly
equal to the number of atoms.
The number of conduction electrons is proportional to a
factor
n E/KT
E 1/n
Tn
: Dielectric constant
K: Boltzmans constant
T: Absolute Temperature.
E Activation Energy.
Resistivity
For a uniform wire or cube, resistance is proportional
to length and inversely proportional to cross-sectional
area. Resistivity is related to resistance but it not
identical to it. The resistance R depends an length,
Area and properties of the material which we term
resistivity (ohm.m) .
Constant of proportionality is called Resistivity :
t is defined by .
H is the total thickness
unit
1) Layer Resistivity ( i )
2) Lager Thickness( ti )
is
characterized
by
two
Ice .
C) Materials whose connate water is clean (free
from salinity ) will show high resistivity such as :
Archies Law
Empirical relationship defining bulk resistivity of a saturated
porous rock. In sedimentary rocks, resistivity of pore fluid is
probably single most important factor controlling resistivity of
whole rock.
Archie (1942) developed empirical formula for effective
resistivity of rock:
0 = bulk rock resistivity
w = pore-water resistivity
a = empirical constant (0.6 < a < 1)
m = cementation factor (1.3 poor, unconsolidated) < m < 2.2
(good, cemented or crystalline)
= fractional porosity (vol liq. / vol rock)
Formation Factor:
Effects of Partial Saturation:
Sw is the volumetric saturation.
n
is the saturation coefficient (1.5 < n < 2.5).
Archies Law ignores the effect of pore geometry, but is a
reasonable approximation in many sedimentary rocks
Resistivity survey instruments:
a)High tension battery pack (source of current).
b)Four metal stakes.
c)Milliammeter.
d)Voltmeter.
e)Four reels of insulated cable.
AC is preferred over DC as source of current. The advantage
of using AC is that unwanted potential can be avoided.
2.
3.
Sources of Noise
There are a number of sources of noise that can effect our
measurements of voltage and current.
1- Electrode polarization.
A metallic electrode like a copper or steel rod in contact with
an electrolyte groundwater other than a saturated solution of
one of its own salt will generate a measurable contact
potential. For DC Resistivity, use nonpolarizing electrodes.
Copper and copper sulfate solutions are commonly used.
2- Telluric currents.
Naturally existing current flow within the earth. By
periodically reversing the current from the current electrodes
or by employing a slowly varying AC current, the affects of
telluric can be cancelled.
3- Presence of nearby conductors. (Pipes, fences)
Act as electrical shorts in the system and current will flow
along these structures rather than flowing through the earth.
always
perpendicular
to
Example
If target depth equals electrode separation, only 30% of
current flows beneath that level.
1. To energize a target, electrode separation typically
needs to be 2-3 times its depth.
2. High electrode separations limited by practicality of
working with long cable lengths. Separations
usually less than 1 km.
The fraction of the total current (if) penetrating to depth
Z for an electrode separation of d is given by :
if = 2 / tan -1 ( 2 Z / d )
ELECTRODE CONFIGURATIONS
The value of the apparent resistivity depends on the
geometry of the electrode array used (K factor)
1- Wenner Arrangement
Named after wenner (1916) .
The four electrodes A , M , N , B are equally spaced along a
straight line. The distance between adjacent electrode is
called a spacing . So AM=MN=NB= AB = a.
a= 2 a
V /I
AB 2 MN
V 2
2
a
I
MN
a = [ ( r2 / a ) r ] v/i
Or . if the separations a and b are equal and the distance
between the centers is (n+1) a then
a = n (n+1) (n+2) . a. v/i
5- Pole-Dipole Array .
The second current electrode is assumed to be a great
distance from the measurement location ( infinite
electrode)
a = 2 a n (n+1) v/i
6- Pole-Pole .
If one of the potential electrodes , N is also at a great
distance.
a= 2 a
V /I
1 tan 2 = 2 tan 1
If 2 < p1 , The current lines will be refracted away from
the Normal. The line of flow are moved downward because
the lower resistivity below the interface results in an easier
path for the current within the deeper zone.
B. Distortion of Potential
Consider a source of current I at the point S in the first
layers P1 of Semi infinite extent. The potential at any point
P would be that from S plus the amount reflected by the
layer P2 as if the reflected amount were coming from the
image S/
V1 (P) = i 1 / 2 [ (1 / r1) + ( K / r2 ) ]
K = Reflection coefficient = 2 1 / 2 + 1
Method of Images
Potential at point close to a boundary can be found using
"Method of Images" from optics.
In optics:
Two media separated by semi transparent mirror of reflection
and transmission coefficients k and 1-k, with light source in
medium 1. Intensity at a point in medium 1 is due to source and
its reflection, considered as image source in second medium,
i.e source scaled by reflection coefficient k. Intensity at point in
medium 2 is due only to source scaled by transmission
coefficient 1-k as light passed through boundary.
1> 2> 3
1> 2< 3
1< 2> 3
1< 2< 3
Calculated
curve does
not match
data,
but
can
be
perturbed to
improve fit.
2- Principle of Suppression.
This states that a thin layer may sometimes not be
detectable on the field graph within the errors of field
measurements. The thin layer will then be averaged into
on overlying or underlying layer in the interpretation. Thin
layers of small resistivity contrast with respect to
background will be missed. Thin layers of greater
resistivity contrast will be detectable, but equivalence
limits resolution of boundary depths, etc.
The detectibility of a layer of given resistivity depends on its
relative thickness which is defined as the ratio of
Thickness/Depth.
In Sch.
MN
1/5
AB
Wenner
MN =
1/3
AB
(2)
In Sch. Sounding, MN are moved only occasionally.
In Wenner Soundings, MN and AB are moved after each
measurement.
(3)
The manpower and time required for making Schlumberger
soundings are less than that required for Wenner soundings.
(4)
Stray currents that are measured with long spreads effect
measurements with Wenner more easily than Sch.
(5)
The effect of lateral variations in resistivity are recognized
and corrected more easily on Schlumberger than Wenner.
(6)
Sch. Sounding is discontinuous resulting from enlarging
MN.
Mise-A-LA-Masse Method
This is a charged-body potential method is a development of
HEP technique but involves placing one current electrode
within a conducting body and the other current electrode at a
semi- infinite distance away on the surface .
either or both
pos/neg ions
neutral ion
reverse process
made
continuous
by
reactions
involving ferrous and ferric hydroxide
with presence of H+
Equipment:
- potentiometer or high impedance voltmeter
- 2 non-polarizing electrodes
- wire and reel
Non-polarizing
electrodes
were
described
in
connection with resistivity exploration although
they are not usually required there. Here, they are
essential. The use of simple metal electrodes would
generate huge contact or corrosion potentials
which
would mask the desired effect. nonpolarizing electrodes consist of a metal in contact
with a saturated solution of a salt of the metal .
Contact with the earth can be made through a
porous ceramic pot.
3. Nernst Potential
EN = - ( RT / nF ) Ln ( C1 / C2 )
Where Ia = Ic in the diffusion potential Equation .
4. Streaming potentials due to subsurface water flow are the
source of many SP anomalies. The potential E per unit of
pressure drop P (The streaming potentials coupling
cocfficent) is given by :
EK = CE P
4
Electrical Resistivity of the pore Fluld.
Ek
Electro-kinetic potential as a result from an electrolyte
flowing through a
porous media.
Dielectric constant of the pole fluid.
Viscosity of the pore fluid
P pressure difference
CE electro filteration coupling coefficient.
Interpretation
Usually, interpretation consists of looking for
anomalies.
The order of magnitude of anomalies is
0-20 mv
normal variation
20-50 mv
possibly of interest, especially if
observed over a fairly large area
over 50 mv definite anomaly
400-1000 mv very large anomalies
Applications
Groundwater applications rely principally upon potential
differences produced by pressure gradients in the
groundwater. Applications have included detection of leaks in
dams and reservoirs location of faults, voids, and rubble
zones which affect groundwater flow delineation of water flow
patterns around landslides, wells, drainage structures, and
springs, studies of regional groundwater flow
Other groundwater applications rely upon potential
differences produced by gradients in chemical concentration
,Applications have included outline hazardous waste
contaminant plumes
Thermal applications rely upon potential differences
produced by temperature gradients.
Applications have included geothermal prospecting map
burn zones for coal mine fires monitor high-temperature
areas of in-situ coal gasification processes and oil field steam
and fire floods.
Four systems of IP .
Time domain
Frequency domain
Phase domain
Spectral IP
< 10 HZ
10-3 to 4000 HZ
Sources of IP Effects
1 ) Normal IP
Membrane Polarization
Over-voltage effect
3 ) IP is A bulk effect.
Grain (electrode) polarization. (A) Unrestricted electrolytic flow
in an open channel.
(B) Polarization of an electronically conductive grain, blocking a
channel
( unitless )
Metal Factor
= A ( a1 a0 )
a0 &
a1
siemens / m
apparent resistivity.
Time constant
M IP chargeability
.
This is called cole cole
relalaxation
IP Survey Design
Limitations of IP
Adventages of IP
Introduction
Electromagnetic methods in geophysics are distinguished by:
1. Use of differing frequencies as a means of probing the
Earth (and other planets), more so than source-receiver
separation. Think skin depth. Sometimes the techniques
are carried out in the frequency domain, using the
spectrum of natural frequencies or, with a controlled
source, several fixed frequencies (FDEM method
---frequency domain electromagnetic). Sometimes the
wonders of Fourier theory are involved and a single
transient signal (such as a step function) containing, of
course, many frequencies, is employed (TDEM method time domain electromagnetic). The latter technique has
become very popular.
Adventages
Cultural Noise
Applications
1. Mineral Exploration
2. Mineral Resource Evaluation
3. Ground water Surveys
4. Mapping Contaminant
Plumes
5. Geothermal Resource
investigation
6. Contaminated Land Mapping
7.Landfill surveys
8.Detection of Natural
and Artificial Cavities
9.Location
of
geological faults
10.Geological Mapping
Type of EM Systems
- EM can be classified as either :
1 - Time Domain (TEM) or
2 - Frequency- Domain (FEM)
- FEM use either one or more frequencies.
- TEM makes measurements as a function of
time .
- EM can be either :
a - Passive, utilizing natural ground
signals (magnetotellurics)
b - Active , where an artificial
transmitter is used either in the nearfield (As in ground conductivity meters)
or in the far-field (using remote highpowered military transmitters as in the
case of VLF Mapping 15-24 KHZ ).
Principle of EM surveying
EM field can be generated by passing an
alternating current through either a small coil
comprising many turns of wire or a large loop of
wire .
The frequency range of EM radiation is very wide,
from < 15 HZ ( atmospheric micropulsations) ,
Through radar bands (108 1011 HZ) up to X-ray
and gamma >1016 HZ .
Depth of Penetration of EM
Skin Depth : is the depth at which the amplitude of a plane wave has
decreased to 1/e or 37% relative to its initial amplitude Ao .
Amplitude decreasing with depth due to absorption at two frequencies
Az = Ao e-1
The skin depth S in meters = 2 /
= 503 f
= 2 f
= 503 / f
= 503 / v
: conductivity in s/m
: magnetic permeability (usually 1)
: wavelength ,
f : frequency , v : velocity , p : Resistivity thus, the
depth increases as both frequency of EM field and conductivity decrease..
Ex. In dry glacial clays with
conductivity 5x 10-4 sm-1 ,
S is about 225 m at a
frequency of 10 KHZ .
Skin depths are shallower for
both higher frequencies and
higher conductivities (Lower
resistivities ).
Magnetotelluric Methods ( MT )
Telluric methods: Faraday's Law of Induction: changing magnetic
fields produce alternating currents. Changes in the Earth's magnetic
field produce alternating electric currents just below the Earth's
surface called Telluric currents. The lower the frequency of the
current,
the
greater
the
depth
of
penetration.
Telluric methods use these natural currents to detect resistivity
differences which are then interpreted using procedures similar to
resistivity methods.
MT uses measurements of both electric and magnetic components of
The Natural Time-Variant Fields generated.
Major advantages of MT is its unique Capability for exploration to
very great depths (hundreds of kilometers) as well as in shallow
Investigations without using of an artificial power source
Natural Source MT uses the frequency range 10-3-10 HZ , while
audio frequency MT (AMT or AFMAG) operates within 10-104 HZ
The main Application of MT in hydrocarbon Expl. and recently in
meteoric impact, Environmental and geotechnical Applications.
Typical equation:
apparent resistivity = where Ex is the strength of the
electric field in the x direction in millivolts
Hy is the strength of the magnetic field in the y direction in
gammas
f is the frequency of the currents
Depth of penetration =
This methods is commonly used in determining the
thickness of sedimentary basins. Depths are in kilometers
Field Procedure
MT Comprises two orthogonal electric dipoles to measure the
two horizontal electric components and two magnetic sensors
parallel to the electric dipoles to measure the corresponding
magnetic components .
1. Two orthogonal grounded dipoles to measure electric components
2. Three orthogonal magnetic sensors to measure magnetic
components.
Thus, at each location, five
parameters
are
measured
simultaneously as a function of
frequency. By measuring the
changes in magnetic (H) and
electric (E) fields over a range of
frequencies
an
apparent
resistivity curve can be produced.
The lower the frequency, the
grater is the depth penetration.
Survey Design
EM data can be acquired in two configurations
1) Rectangular grid pattern
2) Along a traverse or profile .
EM equipment Operates in frequency domain. It allows
measurement of both the .
1) in-phase (or real ) component .
2) 900 out of phase (or quadrature ) component.
Slingram System
slingram is limited in the size of TX coil. This system
has the Transmitter and Receiver connected by a
cable and their separation kept constant as they are
moved together along a traverse.
Magnetic field Through The receiver has two sources :
The primary field of The Transmitter .
The secondary field produced by The Target .
Turam system
More powerful system than Slingram. It uses a very large
stationary Transmitter coil or wire laid out on the ground,
and only The receiver is moved . TX 1-2 km long, loop over
10 km long. The receiver consists of two coils and kept a
fixed distance between 10-50 m apart.
Ground Surveys of EM
Amplitude measurement
1- Long wire
Receiver pick up horizontal component of field
parallel to wire .
Distortions of Normal field pattern are related to
changes in subsurface conductivity.
Dip-Angle
Measures combined effect of primary and secondary
fields at the receiver.
AFMAG : Dip-angle method that uses Naturally
occurring ELF signals generated by Thunder storms.