erth2404 2013 l21 exploration_geophysics
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EXPLORATION GEOPHYSICS
Guest Lecturer: Raymond Caron
Ph.D. Candidate, M.Sc., B.Sc.H., B.Sc.
Photo:C.
Samson
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Lecture objectives and Contents
To understand the physical concepts behind thegeophysical exploration methods
What is it?
Common methods: Seismic Gravity
Magnetics
Data acquisition and processing techniques Differences and commonalities between methods
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Exploration geophysics
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Exploration geophysics
Making and interpreting measurements of
physical properties of the Earth to
determine subsurface conditions, usually
with an economic or environmental
objective
Sheriff, R.E. 1984.
Encyclopedic dictionary of exploration geophysics.
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What is Geophysics?
Its an applied science
The use of applied physics to investigate &
explain the natural world
Two types of geophysics
Active-source
Passive-source
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What is Geophysics?
Types of geophysical methods:
Seismic shallow A
Seismic deep P
Gravity P
Magnetics P
Many other methods A&P
EM, IP, MT, GPR
Associated methods A
DGPS, RADAR, Laser, LIDAR
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What is Geophysics?
Multiple applications:
Resource exploration (oil, gas, ore, fresh
water, aggregate)
Science (planetary, space)
War (mine detection, detection, defence)
Industrial (earthquake risk, corrosion, waste
management)
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What is Geophysics?
Multiple platforms:
Ground (vehicles, by foot, bore-holes, UGS)
Sea (boat, ship, submarine, USS)
Air (aircraft, balloon, UAS)
Space (satellites, probes)
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Common Features
Data Acquisition Sample rate
Recording time
Signal and Noise
What isnt a signal is noise Error is noise Instrument, position, assumptions
Signal : Noise Stacking
Averaging Subtraction
Filters
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Common Features
In general:As distance increases, resolution decreases
Tradeoff: High frequencies provide more information
Lower frequencies travel further
Information from deep targets or fartargets are low in resolution.
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Exploration seismology
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Exploration seismology
Mainly based on the reflection andrefraction of primary (P) waves on
boundaries between different media in the
subsurface P-waves:
Are easy to generate
Travel fastest
Propagate in liquids and solids
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Seismic Waves
= +
=
K=bulk modulus =shear modulus =density
(fluid compression) (rigidity)
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Exploration seismology
Controlled approach: specializedequipment is used to generate seismic
waves at pre-set times
Local scale Applications:
most widely used method for oil and gas
Scientific crustal studies (Crust, Mantle, Core) Ground water investigations
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Seismic Survey Layout
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Seismic survey layout
Source
Sharp, High-amplitude, High bandwidth Vibrosis (frequency wave-train)
Measuring (geophones & hydrophones) High bandwidth (high sample rate)
Sensitive
Sturdy
Recording (computer)
Accurate (time stamp) Broadband
Large hard-drive
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Seismic survey layout
Identify target
Shot location
Shot offset Geophone spacing
Geophone coverage
Data sample rate
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Seismic wave propagation &
processing
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Physical properties
Seismic impedanceZ [ kg/m2s]:
Z = v
v [m/s] : P-wave seismic velocity [kg/m3] : density
In general, harder the rock, higher Z
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Ray geometry
(Medium 1) Z1 = 1 v1
(Medium 2) Z2 = 2 v2
Z2 > Z1 Transmitted P
2 > 1
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3-types ofwaves
1) Reflection2) Refraction3) Direct
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z [m] v1 [m/s]
x [m]
v2 [m/s]
cc
0
200
400
600
800
1000
1200
0 1000 2000 3000
Shot-receiver distance [m]
Two-waytraveltime[ms]
Direct wave
Reflection
Refraction
critical
distance
crossover
distance
intercept
time
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Seismic Assumptions
1) Density (rigidity) increases with depth.
a) Seismic wave velocity increases with depth
i. When incorrect leads to hidden layers
2) Each layer of rock (or sediment) is
homogeneous.
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Sources of error
Hidden layers
Velocity inversions with depth
Incorrect geophone spacing
Low-sampling rate
Low-impedance contrast Thin layers (also depends on depth and
Dipping layers
Skewed 2-way travel times Edge effect
No signal
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Effect ofnoise on
seismic data
With noise
Bandpassfiltered
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Seismic field data acquisition
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Seismic sources
Objective: sample the subsurface at awide range of frequencies
High frequency high resolution
Low frequency deep penetration
According to Fourier decomposition:
An impulse in time is equivalent to the sum ofseveral sine waves
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Seismic sources
Two approaches Impulsive sources
Dynamite
Hammer
Waterguns, airguns
Vibratory sources: generating a sweep of sine
waves increasing from 10 to 80 Hz over a few
seconds Vibroseis
most modern, fastest, and controlled method
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Photos: C Samson
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Photo: C. Samson
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Photo: C. Samson
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Movie Link!Start watch ing at 1:40
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http://www.youtube.com/watch?v=hxJa7EvYoFIhttp://www.youtube.com/watch?v=hxJa7EvYoFI -
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ERTH2404 Winter 2013 Exploration geophysicsPhoto: C. Samson
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ERTH2404 Winter 2013 Exploration geophysicsRef.: GLIMPCE project
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Photo:C.
Samson
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ERTH2404 Winter 2013 Exploration geophysicsPhoto:C.
Samson
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Gravity
g
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Physical basis
Newtons law of gravitation
F = G m1 m2 / r2
Force [N] of attraction between masses
m1 and m2 [kg] separated by a distance r [m]
G : grativational constant
G = 6.67 x 10-11
N m2
kg-2
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The gravity method measuresspatial variations in gravitational acceleration
Units: [m/s2] , [cm/s2] , [m/s2] , [g.u] , [mGal]
Gravity varies because of the Earths: Ellipsoid shape Rotation Irregular surface relief
Heterogeneous subsurface density distribution
Objective:interpret these effects in terms of geology
Gravitational attraction
PlanetEarth
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GEOID, Ellipsoid, MSL
GEOID is the locationwhere gravity = 9.8 m/s2
IGF
Ellipsoid = Geodetic
Datum (approx. shape
of the Earth)
MSL = Mean Sea Level
= GEOID
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Gravity anomalies results from thedensity contrast between:
a rock body of density 1 and
its surroundings of density 2
= 1 - 2 [kg m-3]
Positive anomaly >0 1 >2Negative anomaly
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Measuring gravity
Units
Gravity anomalies due to geological sources are
tiny compared to the Earths gravitational
acceleration
100 ms-2
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Gravity anomalies
When several bodies are present in the
subsurface, the total observed gravity is
the sum of the gravities of each body
Shallow body short-wavelength anomaly Deep body long-wavelength anomaly
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Gravity field data acquisition
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Measuring gravity
The gravity method requires very precise:
Field measurements
Corrections for all effects not related to
subsurface density distribution
The instrument used to make gravity
measurements in the field is a gravimeter
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Measuring gravity
Gravimeter
Gravimeters measure:
Gravitational acceleration via the extension
of a spring
Relative values of gravity(i.e. difference in gravity between locations)
Only the vertical component of gravity
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ERTH2404 Winter 2013 Exploration geophysicsPho
to:A.
Snider
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to:A.
Snider
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Gravity data processing
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Data reduction
Data reduction: process of correcting
gravity data to isolate only the effects due
to subsurface density distribution
Apply corrections for:
The mass of the Earth
International Gravity Formula (IGF)
Instrument effects Drift
Temporal and spatial effects
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Data reduction
Correcting for temporal and spatial
effectsTidal Correction- influence ofsolid Earth tide and oceanic tide
Latitude Correction
- variation with latitude
Free Air Correction- elevation above a datum
Bouguer Correction
- density / thickness of rocks situated between survey and
reference level
Terrain Correction
- topographic relief
Eotvos Correction
- gravimeter movement on ship / aircraft
Elevation Corrections
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Instrument effects
During a survey, gravimeter readings tendto change gradually with time
Effects corrected for by repeating
measurements at base station throughoutthe day
Correct also tidal effects
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Drift
Causes:Anelastic spring behavior
Change in spring elastic properties with T
Ref.:
ReynoldsFig.
2.1
4
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Drift correction
Assuming linear drift between base stationreadings
corrected gravity = observed gravity drift * (t t0)
(in this example, apply a negative correction)
drift = gravity
time
t0
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Latitude correction
Correcting for the rotation of the Earth
Centrifugal acceleration with latitude
Resultant of centrifugal and gravitational
accelerations, gravity is weaker. Where?
Re
f.:
Reyno
lds
Fig
.2
.4
Re
f.:
Kearye
tal.Fig.
6.1
1
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Latitude correction
Correcting for the ellipsoid shape of the Earth
Polar radius 21 km shorter than equatorial radius
Gravity 0.7% higher at the poles
Ref.:
Reynolds
Fig.
2.1
62
E
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Elevation corrections
Ref.: Musset 2000 Fig. 8-15
Free-Air CorrectionBouguer Correction
Terrain Correction
The objective is to reduce to datum
an observation taken at elevation h
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Elevation corrections
Free-air correction (FAC)
Correcting for the difference in gravity between adatum and a station at an elevation h [m]
FAC [mGal] = 0.3086 [mGal/m] * h [m]
FAC requires precise topographic information
The datum usually chosen is the mean sea level
FAC > 0 for a station above the datum (h>0)
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0
20
40
60
80
3655 3660 3665 3670 3675 3680 3685
Observed gravity [mGal]
Elevation[
m]
Difference in gravity between
basement and top floor of Dunton Tower:
22 mGal
Gradient = 0.3067 mGal/m
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E e t e t
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Elevation corrections
Bouguer correction (BC)
Correcting for topographic mass Assumption:
Topography is represented by a horizontal rocklayer extending to infinity in all directions Layer thickness = elevation
Constant density
BC [mGal] = 0.04191 [mGal.m2/Mg] * [Mg m-3] * h [m]
BC requires precise topo and density information On land, BC < 0
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Interpretation
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Interpretation
Bouguer anomalies (BA)
Basis for interpretation of gravity data on landand in shallow waters
BA = observed gravity
tidal and drift corrections
- latitude correction
+ free-air correction
- Bouguer correction
+ terrain correction Eotvos correction
BA reflects density distribution below datum
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Gravity method
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Gravity method
Applications
Petroleum exploration- Delineation of structural trends, faults- Mapping of salt domes
Mineral exploration- Detection of ore bodies and mass determination
Hydrogeology
-Aquifer location- Mapping of soil-bedrock contact
Gravity Example 1:
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Ref.: N. Sneeuw, U. of Calgary (shown with permission)
July 2000 March 2002 August 2006
Continental scale surveysSatellite gravity map of Canada
Gravity Example 2:
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Overburden Topography
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Magnetics
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Magnetic Surveying
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Magnetic Surveying
Oldest geophysical method (since 1640) Used a magnet hanging from a string to find
iron ore.
Aeromagnetic surveying (WWII) Used to detect submarines
Now used for surveying for:
Ore, oil, gas, science
Fluxgate m agnetometer, 1965 GSC
Magnetic Surveying
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Magnetic Surveying
Magnetic permeability
The response of a material to an externalmagnetic field.A. Paramagnetic
e- are miss-aligned, small magnetisation
B. Ferromagnetic e- spins are aligned, large magnetisation
C. Anti-Ferromagnetic e- spins are opposite, no magnetisation
D. Ferrimagnetic
Some e- spins are opposite, small magnetisation
E. Diamagnetic Unbalanced crystal lattice, e-is small and negative
There are others
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Magnetic surveying
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Magnetic surveying
The intensity is governed by Coulombs
Law
Fis the force between poles m1 and m2
is the magnetic permeability of the mediumseparating the poles
ris the distance between the poles.
=12
42
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Magnetization
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Magnetization
B = your measured field (nT)
H = Earths field (nT)
= magnetic permeability of the
subsurface (no units)
B=H
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Magnetic field data acquisition
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Magnetic anomalies
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Magnetic anomalies results from thepermeability contrast between rock units
Assumption!
Its assumed that variations in the magneticfield are induced from Earths field, and there isNO REMNANT magnetization.
Although remnant magnetization is present.
Magnetic anomalies
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Measuring gravity
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Measuring gravity
Units
Magnetic anomalies due to geological sourcesare tiny compared to Earths magnetic field.
In Ottawa the magnetic field ~ 60,000 nT
An anomaly = 100 5000 nT
Magnetic units: Tesla (T) Most commonly presented as nanoTesla (nT) 1nT = 10-9 T
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Measuring Magnetics
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Measuring Magnetics
The instrument used to measure themagnetic field is a magnetometer There are many types:
Fluxgate Tiny, low resolution, durable
Proton-procession Small, portable, durable
Alkali-vapour (cesium vapour) Small, high resolution
SQUID (high & low temperature) Very high-resolution, requires cooling
Essence of magnetics
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Essence of magnetics
Measure the magnetic field
Subtract the modelled magnetic field of the
Earth
The International Geomagnetic Reference
Field (IGRF)
Because youre only interested in local variations
Present your data
TMI = Total magnetic Intensity
Gradient
X, Y, Z component
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Magnetic Surveying
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Magnetic Surveying
What you see is what you get (for the mostpart)
Positive anomalies are areas of high magnetic
susceptibility Negative anomalies are areas of low
magnetic susceptibility.
Doesnt hold true when using a movingplatform (aircraft) over an area that has a
rugged topography.
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Magnetic data processing
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Magnetic noise
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Magnetic noise
Diurnal sources
Solar wind (solar storms)
Tidal noise
Instrument noise
EM noise from electronics
Platform noise
A moving platform (airplane, ship) generate
an EM field as it moves and maneuvers
through the Earths magnetic field
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Magnetic noise
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Magnetic noise
reduction
Ground stations areused to measure
the diurnal changes
of the magneticfield.
Changes are
subtracted fromsurvey data.
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Corrections
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Corrections
Lag correction Corrects for a moving platform
GPS says youre at x, your magnetometer is at x+2
IGRF correction
Removes the bulk of Earths magnetic field inorder to isolate a regional anomaly
Subtract diurnal
Level the data
So all measurements are taken at the same (orclose) altitude as another measurement.
To avoid artifacts in the data due to altitude.
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Refe e ces
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References
DAndrea, W.R. 1998. Seismic and gravity prospecting Whats it allabout? DeMille Technical Books.
Grant, F.S. and West, G.F. 1965. Interpretation theory in geophysics.McGraw-Hill.
Keary, P., Brooks, M. and Hill, I. 2002. An introduction to geophysicalexploration. Blackwell Science. 3rd Edition.
Samson, C. 1991. Reprocessing and interpretation of GLIMPCEmarine crustal reflection seismic data from Eastern Lake Superior.Ph.D. Thesis. U. of Toronto.
Samson, C., Barton, P.J., and Karwatowski, J. 1995. Imagingbeneath an opaque basaltic layer using densely-sampled wide-angle OBS data. Geophysical Prospecting, 43: 509-527.
Sheriff, R.E. 1984. Encyclopedic dictionary of exploration geophysics.Society of Exploration Geophysicists. 2nd Edition.