ERTH2404 2013 L21 Exploration_Geophysics

7/28/2019 ERTH2404 2013 L21 Exploration_Geophysics

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ERTH2404 Winter 2013 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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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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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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30Photos: 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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ERTH2404 Winter 2013 Exploration geophysicsPho

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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L


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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

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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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E


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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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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

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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.

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