Important Qu Erries

8/10/2019 Important Qu Erries

1/52

Two major emphasesof geophysics:

1. "pure"2. "applied"

1. pure geophysics- study of the physics of the Earth

Examples:

variations in temperature with depth

causes of reversals in Earth's magnetic field

2. applied geophysicsalso called exploration geophysics! - to find economic deposits

ll methods depend fundamentally on the presence of #odies with contrasting physical

properties$ such as density$ magnetic suscepti#ility$ heat conductivity$ elastic constants$ etc.

ctive methods - stimulate response ex. - setting off dynamite #last!

%assive mehtods - simply measure property ex. - density!

Part 1: Gravity

AssumeEarth does not rotate and has uniform density distri#ution.

&etermine acceleration of gravity usually ust called "gravity" #y geophysicists! at point on

Earth's surface.

Law of Universal Gravitation:

GMeM F = ------- R2

( ) Universal Gravitational Constant) *.*+, x 1-dyne cm2/gm20/- ., dyne ) 1 gmcm/sec2!

ewton!s "nd Law: ) a

for earth$ use sym#ol "g" instead of "a$" so # $ %g

GMeM GMe

Since F = F; then Mg = ------ and g = ---- R2 R2

g $ approximately &'( cm)sec"or 3. m/sec2!

1 cm/sec2is called a gal.

4ormally use milligals *1)1((( gal or a+out 1 millionth g, or gravity units *g- u-. (-1 mgal,


2/52

Complication /1:

0arth rotates

5esult: Earth not round #ut #ulges at e6uator and is flattened at poles.E6uatorial radius is 21 7ilometers greater than at poles.

Complication /":Earth's mass is not symmetricala#out the e6uatorial plane - Earth is "pear-shaped."

Complication /:

The e2uator isn!t perfectly spherical#ut only varies #y a few meters.

The regular surface which most nearly approximates the surface of the actual Earth is a surface

called the geoid.

The geoid surface is everywhere perpendicular to a plum# #o#.The geoid corresponds to mean sea level.

8n land covered areas$ the geoid is the surface that would #e determined #y the level to which

water would rise in narrow canals cut through the continents.

9ince g depends on distance from center of Earth radius!$ g varies with latitude.

3nternational Gravity #ormulacan #e used to determine g at a particular latitude:

g ) 3.+,1 1 0 .,2; sin2 - .3 sin22 ! where is the latitude< units are m/sec2

=alculated value for g "corrected" for latitude is called the theoretical gravity and a##reviated gt

4ow measure actual valueof gravity at any spot.

1. can use pendulum

formula from physics: where > is length of pendulum and T is period!

ccuracy ) 1. mgal< ta7es a#out , minutes per measurement

2. can experimentally measure acceleration of o+ject droppedat Earth's surfaceccuracy ) .1 mgal< measuring apparatus not porta#le although one of the latest models

availa#le is said to #e porta#le #ecause it weighs less than one ton!

,. most commonly measure differences in gravityfrom place to place #y using a "gravity

meter" ass suspended from spring!.ccuracy ) .1 mgal

verage density of Earth is .2 gm/cm,.

verage density of surface roc7s is much less.

Therefore interior of 0arth must +e of much higher density than surface roc4s .


3/52

=an get some idea of 0arth!s density distri+utionfrom study of its angular momentum:

ngular omentum ) oment of 8nertia x ngular ?elocity

The moment of inertia of any o#ect depends on its mass distri#ution.

0xamples:

solid cylinder revolving a#out its axis$ 8 ) . 52< where is mass and 5 is radius of

cylinder

sphere$ 8 ) .; 52

spherical shell$ 8 ) .*+ 52

0arth!s moment of inertia) .,,+ 52

@est fitting model is series of nested ellipsoids of different densities$ #ut generally denser towardcenter.

easured value of g called "actual" value and a##reviated ga! is not usually the same as g t.

&ifference in gaand gtcalled a gravity anomaly.

Actual not same as theoretical +ecause:

1. actual not measured at sea level where theoretical is calculated2. actual not measured on a flat surface

,. solid 0arth has tides of 5617 cm

;. density distri+ution in 0arth not uniform

To adust for difference A1$ we apply two "corrections" to the measured value #efore comparingit to the theoretical value:

1st : adust for elevation distance from center of Earth$ h!

called the #ree Air Correction< ) .,* h when h is in meters

2nd: remove that portion of g due to the mass #etween sea level and the point where

measurement made

called the 8ouguer Correction) -.;13 h is density in 7g/m,!

To adust for difference A2$ we then add another "correction" to the measured value #eforecomparing it to the theoretical value #y removing the influence of near#y mountains and valleys.

called the 9opographic or 9errain Correction9ince this correction rarely exceeds 1 mgal except in mountainous areas$ it is fre6uenty ignored.

To adust for difference A,$ formulas are availa#le to determine the necessary correction. This

tidal correctionis very fre6uently ignored.

inally$ any difference +etween the corrected values of actual gravity and theoreticalgravity should +e due to density variationsA;!.


4/52

;igher than average density roc4will cause the measured value of g to #e greater than the

theoretical value and produces a positive anomaly while less dense roc4 produces a

negative anomaly.

=onsider a plum# #o# hanging near a tall mountain.

The mass of the mountain pulls it sideways.Bnowing the density and volume of the mountain allows us to calculate its mass and ena#les us

to determine how much force it should exert on the plum# #o#.easurements show mountains exert only a#out 1/, of the expected amount.

Cuestion: Dhy

%ountain supposedly has low density roots-

Theory of 8sostasy - the total mass of roc7 and sea! in any vertical column of unit cross section

is constant

?arious models have #een developed to descri#e this root Airy< Pratt$ etc.!

Where sedimentation occurs, the weight of the sediment may cause the crust belowto sink. Similarly, where erosion occurs the crust may rebound.

Cuestions:

re roots permanent features

Dhy do mountains have roots

>arge scale gravity anomalies are called regional anomalies.

Fsually due to density variations in lower crust or variation in thic4ness of crust.

a7e it hard to recogniGe small or shallow features.Hften "removed" #y various processes.

%rocess so su#ective that 8 have sometimes thought that "the regional anomaly is what you ta7e

out in order to ma7e what's left loo7 li7e what you want it to."

9mall scale anomalies often called residual anomalies, produced +y ore +odies or geologic

structures.

9eldom more than a few milligals in siGe.

Fse trial and error to find a #ody of the right location$ shape$ siGe and density to produce theanomaly.

0xampleof a spherical ore #ody:

or a sphere$ g at a location x


5/52

where 5 is the radius of the sphere$ G is the depth to the center of the sphere$ x is measured from

a point on the surface directly a#ove the center of the sphere to the location$ and is the densitycontrast difference in densities of #ody and surrounding material!.

There is usually assumed to #e a constant density difference #etween an ore #ody and its

surroundings and a sharp$ well- defined #oundary separating them.4either assumption is li7ely to #e correct.inding thedensity contrast to use in the formula is very difficult if you don't 7now what lies

#elow ground. nd if you 7new what was down there$ why #other with exploration methods

li7e gravity surveys!

=ther shapescan #e modeled with similar #ut more complex formulas.

Complex formscan #e thought of as com#inations of simple forms.

Fsually use computers.

9ome general ruleshave #een found.

Circular anomaliesproduced #y:

compact mineral #ody

salt dome gravity low with small high due to dense cap roc7 in center!

0longated anomaliesproduced #y:

gra#en

#uried folds

#uried channels su#duction Gones

oceanic ridges

egative anomalies:

>ess dense roc7 such as in sedimentary #asins$ #atholiths$ su#duction Gones$ oceanic

ridges

Positive anomalies:

ore dense roc7 such as ultramafic masses

Fplifts of denser roc7 in structures such as anticlines or reverse faults.

The deeper the +ody< the +roader and lower in amplitude will #e the anomaly profile.

>apid change in amplitude or gradientshould suggest density change in su#surface - such as a

fault or edge of a #uried #asin.


6/52

There is no uni2ue answer.

9everal models can produce exactly the same anomaly.

?ery important to use 4nowledge of area!s geologyto limit possi#le solutions.

Part ": >adioactivity< >adiometric ?ating and atural Gamma %ethods

Geochronology- concerned with determining age and history of geologic materials #y studying

their isotopes

>adioactivity

&iscovered in 13*

4atural change from one element to another #y emission of particles from nucleus or addition of

particles to nucleus

Particles include:

helium nuclei alpha particles!

electrons #eta particles!

high energy electromagnetic waves gamma rays!

&ecay occurs at constant rate and is not affected #y temperature$ pressure$ chemical com#inationor any other 7nown thing

>adioactive isotopes- an element capa#le of spontaneously changing into another element #y

the emission or addition of particles to its nucleus@ta+le isotopes- an isotope which is not radioactive

>adiogenic isotopes- an isotope produced #y radioactive decay

on6radiogenic isotopes- an isotope not produced #y radioactive decay

;alf6life- time for half of element to decay

Parent - the radioactive element which decays

?aughter- an element formed from another #y radioactive decay

9 *half life,) ln 2/ ) .*3,1/

The e6uation which represents radioactive decay is derived in most geophysics texts for those

who are interested and 7now a little calculus!:

solved fort *age of roc4,:


7/52

Assumptions made in radiometric dating:

no loss or gain of parent or daughter decay rate constant

half life 7nown

can measure amounts of parent and daughter accurately usually use mass spectrometer!

>+@r dating

5#

+

-I 9r

+

could also write

+

5#$ etc.!5# commonly su#stitutes for B in minerals< so method used on B-#earing minerals or roc7s

which contain them

&ecay e6uation reads:

9u#script m stands for measured$ or in other words$ now< o stands for original!

8t is easier to measure ratios of atoms rather than a#solute num#ers so expression usually written:

=ould solve for t age of mineral!:

4ow measure @r'5)@r'and >+'5)@r'ratios and for reaction ) 1.,3 x 1-11/yr!

Then estimate *@r'5)@r',o=an measure this ratio in coexisting undistur#ed minerals whichcontain no 5#!

4ote: 9r*m) 9r*

osince 9r*is sta#le and non-radiogenic

9r*$ 9r;$ and 9rare all sta#le and non-radiogenic.

ny could #e used< 9r*most a#undant and therefore most often used.


8/52

0asier mathematics and more accurate way of determining *@r '5)@r',original:

E6uation for straight line is y ) ax 0 #$ where a is slope$ # is intercept on y axis!

E6uation is in that form actually y ) # 0 ax! when t is

constant for several minerals in a roc7 or several roc7s of the same age!

8f we plot *@r'5)@r',mvs *>+'5)@r',m$ the values should #e different for different roc7s and

minerals #ecause they would have different initial amounts of 5#.

The slope of the line o#tained #y connecting these points is -1 and the intercept is 9r+/9r*!oThus we can o+tain +oth the age of the suite and the initial strontium ratio-These plotted lines are called isochrons.

3sochrons can also +e used to determine age of metamorphism-

8f whole roc7 hasn't lost 5# or 9r$ #ut minerals have passed them around during metamorphism$

two ages will #e o#tained - one from dating whole roc7 and one metamorphism age! fromdating individual minerals in roc7.

Another @r isotope use:

irst must 7now 9r+/9r*in material that made up primitive Earth.

Fsually assume it was same as non-5#+#earing meteorites or a#out .*33

&uring differentiation of crust$ #ehavior of 5# and 9r would #e different different charge$

different siGe!.

5# concentrated in crust$ 9r evenly distri#uted #etween crust and mantle.

%roduction of 9r+should thus #e faster in crust than in mantle and 9r+/9r*ratios should #ehigher for crustal material.

?ifference in @r'5)@r'ratios< then< is a means for distinguishing igneous roc4s that have

formed +y partial melting of crustal roc4s from those that have their origin in

differentiation or partial melting of mantle material

%resent 9r+/9r*ratio for mantle roc7 estimated from analyses of recent #asalts and ga##ros from

oceanic environments direct origin from mantle assumed and no contamination #y continental

material!?alue is a#out .+;

Extrapolation #etween .*33 and .+; gives reasona#le estimate for ratio in mantle at any time

in past.

Loo4 at @r'5)@r'ratios for roc4s when they formed to determine origin .

ratio a#ove or consistent with expected mantle ratio!

5emem#er can get 9r+/9r*ratios from isochrons.!

Uranium< 9horium 6 Lead dating:

U"'6B P+"(


9/52

U"6B P+"(5

9h""6B P+"('

=ommonly use ratio with sta#le %#2;

Hne e6uation might #e written:

or:

ust determine ratio %#2*/%#2;!oand .

=an find original ratiofrom associated lead minerals such as galena! or can use mineral forstudy that wouldn't have had any original lead Gircon$ uraninite$ sphene$ apatite$ monaGite$ etc.!

@y using F2,$ F2,$ and Th2,2$ theoretically you get three age determinations and they should

agree concordant ages!.

8f disagreement$ ages are said to #e discordant-This is pro#a#ly due to gain or loss of material.

Lead6lead method

8f e6uation for F2,is divided #y e6uation for F2,$ we get another e6uation:

Fse of this e6uation called lead-lead method.

Jandy #ecause F2,/F2,ratio 7nown$ as are decay constants.=an't solve remaining e6uation directly for t #ut ages corresponding to different isotope ratios

have #een plotted and can #e o#tained from pu#lished graphs or ta#les

Fse of %#-%# method is good chec7 on F2,$ F2,$ and Th methods #ecause if lead lost$ the ratio

of isotopes of remaining lead should not #e changed and valid age should still #e given.


10/52

=an also directly use ratio:

These two 6uantities increase with time at different rates and if plotted against each other$ a

curved line is formed called a concordia curve#ecause all points on the curve have concordant

F2,/%#2*and F2,/%#2+ages!.

3f a roc4 sample has lost no P+$ calculated ages from F2,and F2,would #e concordant and apoint representing the ratio of the a#ove 6uantities would lie on the concordia curve.

3f P+ has +een lost$ the ages will #e discordant and the point representing the ratio will lie #elow

the curve.

9ince lead loss would presuma#ly #e different for different areas in the sample$ several differentanalyses from different locations in the sample should give several different ratios and thus

several different points #elow the concordia curve.

8t can #e determined mathematically that these several points will lie on a straight line called a

discordia!.

8f the discordia line is extended to intersect the concordia curve$ upper intersection gives age of

roc4-

Lower intersectionsupposedly gives time lead lost #ut almost never accurate since lead almostnever lost all at once #ut gradually over long time.

Technically could use U"'6B ;e7< U"6B ;e7< or 9h""6B ;e7

@ut$ helium may #e lost since a gas.

ssume that any Je present when roc7 was molten escaped

Therefore$ any Je present now formed from F or Th after solidification.Je ages thus give solidification ages

Example: how long it ta7es for granite #atholith to solidify!.

=ther P+ uses

1. =an measure average amounts of F2,and %#2*$ or F2,and %#2+in roc7s at the Earth'ssurface usually use recent marine sediments!.

ssume no radiogenic lead to start with$ can calculate age of 0arth!s outer portion.

2. @egin with primeval lead lead present when Earth formed!: %#2;$ %#2*$ %#2+$ %#2in certain

ratios for Earth as whole usually assume this to #e same as ratios in meteorites without F$ Th!.


11/52

Dith time$ radiogenic lead increases$ thus higher %#2*/%#2;$ etc.$ ratios with time.

=an get age of 0arth;-;+ my!.

,. variation on 2!fter a time$ ore might form example: galena!.

This ore would "sample" the lead at time of formation$ which would consist of the primeval leadplus all radioactive lead formed #efore the time of ore formation total lead called the common

lead!.Thus$ age of orecan #e determined #y comparing its lead ratios to the ratios which would have

existed at various times.

;. 9ta#le nuclei atomic weight a#out ; and a#ove are present in a#out same a#undance.ssume when elements formed$ same rule applied to unsta#le elements.

4ow F2,is 1; times as a#undant as F2,.

8f #oth once e6ually a#undant$ would ta7e * #illion years to reach present proportion.

Age of UniverseD of our part of UniverseD of our @olar @ystem ne+ulaD

#ission6trac4 dating:

F2,spontaneously +rea4s down +y fissionsplits into two large parts!.

This is a rare occurrence.

These fission particles pass through the surrounding material with very high energy and leavetu#e-shaped damage trac4s.

These trac7s can +e countedetch mineral with Jl$ loo7 at under microscope! and thus the

num#er of spontaneous fissions may #e counted.This gives amount of daughterproduct in sample.

=an determine generally from measurement of amount of radiation #eing emitted! current U"'

contentin sample.

Essentially have num+er of daughter atoms and num+er of remaining parent atoms and can

thus determine age.

Fseful #ecause can #e used on wide variety of su+stances of wide range of ages.

&isadvantage which turns out to #e an advantage:

ission trac7s are "healed" #y prolonged heating millions of years!.9emperature at which healing occurs is different for each mineral-

Each different mineral thus can yield a different age apparent disadvantage! #ecause each

mineral has its cloc7 "restarted" #y healing at different temperatures and thus different times.

@ut temperature history of samplecan #e determined #y comparing different minerals insample.


12/52

Potassium6Argon dating:

B;undergoes " principal 4inds of decay$ to =a;and to r;.&ecay to =a;not useful$ #ecause =a;most common isotope of =a and small amount produced

radiogenically would #e undetecta#le.

Therefore$ use B-r.

9ince 2 separate decay types are possi#le$ decay e2uation somewhat more complicated.

>et #e total decay constant$ r#e decay constant for B-r reaction$ and =a#e decay constant

for B-=a reaction.

Then decay e6uation can #e written:

r;original ) for all #ut very exotic minerals original r a gas$ wouldn't survive formation exceptunder very unusual circumstances$ such as enormously high pressures!.Therefore$ su#stituting for original r and also su#stituting decay constants:

t = 1.88 x 109ln (1+ 9.07 Ar40/K40)

8f metamorphism occurs$ r;already formed will pro#a#ly #e lost and cloc7 reset.

B-r methods can therefore #e used to date metamorphic events.

?isadvantage to method:

r is gas and will often escape

Advantages to method:

can #e applied to very common and a#undant roc7s and minerals$ since B one of maor

elements in Earth's crust

(lauconite in sedimentary roc7s can #e used and other methods not generally useful for

sedimentary roc7s

schists and slates can #e dated

since 5# usually found with B$ 2 independent ages can usually #e o#tained from same

sample and compared

wide range of ages #ecause of length of halflife from age of Earth to a#out years

old!< no other methods allow dating of roc7s a few tens of thousands of years oldimportant for esta#lishing chronology of recent magnetic reversals!


13/52

@amarium6eodymium dating:

9echni2uessame as for 5#-9r or B-r.

Jasadvantagethat #oth elements are mem#ers of rare-earth group and have virtually identical

chemical properties.@oth similarly affected #y weathering and metamorphic processes.

9m/4d ratios would remain unchanged$ giving relia#le date for original crystalliGation.

Car+on dating:

Car+on 17 datingalso called radiocar+on dating!

=1;formed in upper atmosphere #y reaction of 42with neutrons produced #y cosmic rays.

5eaction is: 410 +41;-I *=1;0 1J1

then =1;decays -I +41; 0-1

Thus$ total amount of C17 in atmosphere constant.

=ar#on in organism has same =1;/=12ratio as air or water does as long as organism alive.

Dhen organism dies$ =1;not replenished$ disappears$ and =1;/=12ratio decreases to Gero.

C17)C1"ratio thus gives age since death.

>imited to very young samples less than +$ years! #ecause of short half6life+, years!.

8nstead of measuring =1;/=12ratio in material directly$ normally we compare =1;in sample to =1;

in air #y comparing radioactivity of the 2 samples num#er of decays per minute per gram ofcar#on!.

is activity of =1;in material to #e dated and ois activity of air.

*Age of sample, t $ 1&


14/52

atural Gamma

Concentrations of radioactive su+stancessuch as uranium and thorium can #e detected +y

measuring the products of their decay$ especially gamma rays.

=ther mineralssuch as titanium and Girconium are often associated with radioisotopes soradioacivity surveying may also #e used in their search. 4onradioactive minerals especially

those formed #y mineral replacement processes! are sometimes associated with depletionsaswell as with concentrations of radioisotopes.

easurements may #e made from the air$ along a ground traverse or in #oreholes.

?ifferent roc4soften have different radioactivityand these differences can #e utiliGed in

geologic mapping.

5adioactivity is often concentrated along faults.

5adioactivity lowsare sometimes associated with oilfields#ut the reason is not 7nown.

%art ,: Jeat

Jeat flows from points of high temperature to points of low temperature.

%ethods of heat transfer:

radiation may occur in Earth's core!

conduction

convection

;eat flow due to conduction $ F x temperature gradient

where F is coefficient of thermal conductivityof su#stance and temperature gradient )

T/thic7ness.

The thermal diffusi#ility of a su#stance

where is the density and =pis the specific heatof the su#stance at constant pressure.

9hermal conductivity determined +y:

composition most important!

whether saturated with water open crac7s don't conduct!


15/52

pressure closes crac7s!

8f B is large$ then material is a good conductor of heat.

CuartG is the #est conductor of heat among minerals usually encountered.

Jeat travels extremely slowly through soil and roc4s#y conduction.Typical values would #e 1-* 7m2per million years.

8f transfer due to conduction alone$ a thermal event originating at a depth of 1 7m will not #e

percepti#le near the surface for 1 million - 1 million years

0xamples:

cm #elow surface - daily changes are seldom more than 1 degree and are 1/2 to 1 day

late

few meters down - only seasonal changes detecti#le and arrive months late

few thousand meters down - effects of last ice age still detecti#le

%liocene and %leistocene lavas are warmer than the average lava

Temperature at Earth's surface depends mainly on radiation from 9un.Jeat flow from interior is 1/1 as much as that from 9un.

Temperature in Earth rises with depth.

9emperature gradientnear surface is a#out 1- o=/7ilometer #ut decreases with depth.

=an use mantle/core #oundary conditions to estimate internal temperature.Temperature on #oth sides must #e same.

aterial at #ottom of mantle solid< material at top of core li6uid.

=onsidering all possi#le materials$ maximum is 2+oB.

9ome sources of 0arth!s internal heat:

radioactivity #y far most important!

left over potential energy from formation

recrystalliGation

heat of fusion if outer core solidifying

chemical reactions$ including oxidation at surface and exothermic reactions #etween sea

water and #asalt

compression of roc7s and friction along fault planes

Jeat flow a#out the same all over the Earth< average heat flow for continents same as that for

oceans.

Jowever$ continental materials much richer in radioactive materials and thus should give off

more heat.


16/52

0xplanation:9ome heat flow in ocean #asins due to conduction.

Total surface heat flow:

Hceans - small amount due to conduction< large amount due to convection

=ontinents - mostly due to conduction

8nteresting speculation: 8s it a coincidence that oceanic heat flow e6uals continental heat flow

0xamplesof large scale anomalies:

1. lower than average heat flow:

continental shields 1.2 x 1-*cal/cm2sec!

due to low concentrations of radioactive elements or cold underlying upper mantle

seaward of oceanic trenches

2. higher than average heat flow

island arcs 1. x 1-*cal/cm2 sec!

oceanic ridges 1. x 1-*cal/cm2sec!

other areas of recent volcanic activity as high as + x 1-*cal/cm2sec!

young orogenic regions

as a result of crustal thic7ening

0xamples of local heat anomaliesuseful for prospecting:

chemical reactions which give off heat ex. - oxidation of sulfide ores produces detecta#le

heat!

presence of local radioactive heat sources ex. - granite intrusions!

differences in heat conductivity of roc7s ex. - salt is highly conductive!

presence of volcanic and hydrothermal sources

Part 7: %agnetism

9implest magnetic structure is called a dipole. dipole consists of 2 poles of e6ual strength and opposite sign separated #y a small distance.

Electrons and nuclei are dipoles.


17/52

@peculation:

&o poles always exist in pairs

0arth is a magnet.4orth-see7ing pole of a magnet also called positive! is one that is attracted to the Earth's north

pole.Earth's north pole is a south-see7ing pole.

The Earth's magnetic field is defined #y giving its strength and direction.

The magnetic field strengthJ! at a point in the field of a magnet is the force per unit of polestrength which would #e exerted on a pole at that point.

agnetic field strength is also sometimes given in terms of the density of imaginary lines of

force representing the field.1 =ersted) 1 line of force per cm2called a gauss!

Typical la#oratory magnet has field strength of 1$ Hersteds

The field strength of the Earth varies from a#out ., Hersteds at the e6uator to a#out .*Hersteds at the poles.

&irection given #y specifying declination and inclination.

?eclination- deflection of a north-see7ing pole from geographical north< positive if toward east

3nclinationor dip - deflection of north-see7ing pole from horiGontal< positive if down

9ome terminology:

%agnetic e2uator- curve around the Earth connecting points where inclination is

horiGontal

%agnetic dip poles- points on the Earth's surface where inclination is vertical severalin polar region< also occur where strong local fields exist!

3somagnetic charts- plots of Earth's magnetic field

3sodynamics- contours of e6ual intensity

3sogonics- contours of e6ual declination

3soclinics- contours of e6ual inclination

=omponent's of the Earth's field:

internally generated33K of total!< called the dipole component

externally generated1K of total!< called the non-dipole component

8nternal field can #e mostly accounted for #y a fictitious magnetic dipole displaced from the

center of the Earth a#out ; 7ilometers southward toward 8ndonesia! and tilted 11 1/2 degreeswith respect to the axis of rotation.


18/52

uestion:Dhere does Earth's internal field originate

9ince a uniformly magnetiGed sphere gives the same magnetic field as a dipole at center< there

are two possi#ilities:

1. Dhole earth is magnetiGed

2. ield comes from Earth's center

8f A1$ ield strength should decrease with depth

8f A2$ ield strength should increase with depth.Experimental evidence supports A2

uestion: Jow is Earth's internal field produced

Two possi#ilities:

1. permanently magnetiGed material will discuss process later!

2. electric currents

%ro#lem with possi#ility A1:

ll materials lose their a#ility to #ecome permanently magnetiGed at temperatures which are

reached in the lower crust.

9upport for possi#ility A2:Experimental studies show that relatively simple motions of a conducting fluid such as a nic7le-

iron alloy! can produce a magnetic field.

%ichael #araday!s experiment:

=onducting dis7$ spinning a#out an axle in a magnetic field.

5esult is voltage difference #etween axle and rim of dis7.8f we connect wire from axle to rim$ a current will flow.

The current in the wire generates its own magnetic field which can add to the original.4ow remove original magnetic field.

8f dis7 continues to spin 6uic7ly enough$ the current 7eeps flowing through the wire and a

magnetic field still exists.=alled a self-exciting dynamo.

4otice 2 things necessary:

must supply energy continually to spin dis7

must have small initial applied magnetic field

%ossi#le initial fieldfor Earth's dynamo

some 7ind of primitive #attery action produced #y variations in chemical composition

and temperature in Earth's interior

the 9un


19/52

9ource of energyto 7eep dynamo "spinning"

thermal convection

8f so$ source of heatDhy doesn't the convection distur# the layering of the outer core called fine structure!

solification of inner core

roc7ing of Earth as it moves around 9un precession! setting li6uid in outer core in

motion

try roc7ing a #ottle of li6uid to see similar effect

agnetic fields which will spontaneously reverse polaritycan #e produced #y a com#ination

of dis7 generators.Dill examine significance of this fact later!

9ource of external field is mostly circulating electric currents in the ionosphere.

Earth's magnetic field not constant.

Changes:

1. magnetic storms2. diurnal changes

,. secular variation

;. westward drift

. reversals

=ontinuous recordings of changes are called magnetograms.

1. %agnetic storms:

last several days

change of a#out 1 gamma 1 gamma ) 1-3 Hersteds!

produced #y charged particles emitted #y the 9un.

2. ?iurnal changes:

last a#out a day

change of a#out 2 gamma

produced #y:

o effect of radiation from 9un on ionosphere varies with latitude!

o tidal pulls of 9un and oon on atmosphere


20/52

,. @ecular variation:

regional changes

occur over decades or centuries

possi#le cause

variations in core motions$ especially eddies near the core #oundary

;. Hestward drift:

entire magnetic field "drifts" around Earth in period of a#out 2 years

possi#le cause

core rotates slower than rest of Earth

. %agnetic reversals:

4orth magnetic pole #ecomes a south pole and vice versa.

There are no reasons why the Earth's field should have a particular polarity and there is nofundamental reason why its polarity should not change.

agnetic reversals are 7nown to occur in the 9un and have #een o#served in other stars.

aor groupings of normal and reversed se6uences are called magnetic epochs.

@riefer fluctuations in polarity are called events.verage of three reversals per million years.

5eversals occurred in the pre=am#rian and have #een found in all su#se6uent periods except the

%ermian.

Cuestion: Dhy were there no reversals in the %ermian

The most recent periodof reversed polarity was a#out - 2 years ago.

5eversal process ta4es a+out ((( years.

8n one area in southeastern Hregon$ a gradual transition from normal to reverse magnetiGation

can #e o#served across a section made up of * individual flows.

?uring a reversal$ the dipole field strength decreases to near Gero.The strength is currently dropping K per century and has #een dropping for the past 2 years.

De may #e approaching a reversal.

Earth's magnetic field shields surface from cosmic radiation.=osmic radiation produces mutations.

8n general$ there is a rough agreement +etween faunal extinctions and reversals.

The pro#a#ility of a correlation occurring #y chance is 1 in +.

=ther correlationsfound:


21/52

Jigher magnetic field strengths correlate with colder climates.

Cuestion: =ould climatic changes cause extinctions

5eversals correlate with te7tite increases in deep sea sediments.

Cuestion: &o violent meteorite impacts produce reversals

LenI!s law:Dhen a su#stance is placed in a magnetic field$ little extra currents are generated inside the

atoms #y a process called induction.These currents produce a magnetic field opposite in direction to the applied field.

or details$ loo7 up >armor precessions in a 6uantum mechanics #oo7.!

This induced field is called the 3ntensity of %agnetiIation8! and is proportional to the applied

field: 3 $ 4;7 is called the magnetic suscepti+ilityof the su#stance

0xamplesof direct uses of magnetic suscepti#ility measurements:

maximum in direction of #edding planes and foliation planes

earth6ua7e prediction will discuss later!

The total new field in the su#stance is the applied field plus the induced field.This is called the %agnetic 3nduction @!: 8 $ ; J 3@ is usally given in Tesla 1;Hersteds!.

Gammaor nonotesla$ 1-3Hersteds! are usually used in exploration geophysics.

otions of electric particles including electron spin and or#ital motion! produce magneticfields.

9hree types of magnetic +ehavior:

1. diamagnetic

2. paramagnetic

,. ferromagnetic

1. 8n diamagnetic su+stances$ small magnetic fields produced #y particle motions are randomly

oriented and cancel each other out$ leaving atoms and ions with no net magnetic field.

Examples: salt$ gypsum$ mar#le$ 6uartG$ graphite

2. 8n paramagnetic su+stanceswhich include most su#stances!$ the small fields don't cancel

each other out #ut leave the atoms or ions with net magnetic fields.

Jowever$ since the atoms are randomly arranged$ the su#stance as a whole has no net magneticfield.

,. 8n ferromagnetic su+stances$ the atoms have net magnetic fields and the atoms are arranged

in regions called domainsin such a way that each domain has a magetic field.


22/52

&omains can only #e explained #y using 6uantum theory.!

Jowever$ normally the domains are randomly oriented and there is no net magnetic field in the

su#stance.Examples: iron which is technically ferrimagnetic!$ magnetite$ hematite technically canted anti-

ferrimagnetic!$ ilmenite$ pyrrhotite$ goethite$ many other iron compounds

Dhen each of these 7inds of su#stances is placed in an external magnetic fieldli7e the Earth's

field$ for example!$ additional small magnetic fields are induced.

1. ?iamagnetic su+stances:

9mall induced field produced opposite to applied field.

Thus total field is slightly less than the applied field.%roduces small negative magnetic anomaly.

5emove applied field< induced field disappears.

2. Paramagnetic su+stances:

Two effects occur:

1. 9mall induced field produced opposite to applied field.2. 9mall magnetic fields already existing are partially lined up in same direction as applied

field.

&on't line up completely #ecause of thermal agitation< so the lower the temperature$ the stronger

the effectEffect 2 is greater.

4et effect is total field larger than applied field.

%roduces small positive magnetic anomaly.

5emove applied field< induced field disappears$ thermal agitation randomly distri#utes the atoms

,. #erromagnetic su+stances:

Three effects:

1. 9mall induced field produced opposite to applied field.2. &omains which are oriented in a favora#le direction grow larger.

,. &omains may rotate to a more favora#le direction.

Effects 2 and , are very large effects.5esult is a total field is considera#le larger than applied field.

5emove applied field$

effect 1 disappears

effect , disappears #ecause of thermal agitation

effect 2 remains and su#stance #ecomes "permanently magnetiGed"

Exceptions:


23/52

Dhen temperature of su#stance is a#ove the Curie 9emperature$ domains #rea7 downate %aleoGoic roc7s have a declination ,odifferent from Europe< less difference with time

;. %aleomagnetic correlation of deep-sea cores

. %aleomagnetic inclinations allow the determination of past latitudes

Examples:

trace 8ndia's path distinguish among terrains

*. &etermine former fit of continents and time of plate #rea7-up #y use of "polar wandering"

curves which are identical until the time of #rea7-up and then diverge or convergence of plates

if curves merge!

+. arine anomalies will examine later!

Earth's magnetic field shows little relationship to +road featuresof geography and geologyogging

;. Electromagnetic ethods

1. @elf6 Potential %ethods:Fses Potential ?ifference or Koltage- the difference in electrical potential energy #etween twoplaces. Fnit is volt.

%otential differences occur naturally within the Earth and can #e measured.

These potential differences are caused +y

a. ore #odies #ehaving li7e natural "#atteries" with separation of positive and negative

charge called 0lectrolytic Potential!Jow this wor7s is not understood.


27/52

The most accepted theory for sulfides suggests that the portion of the ore #ody a#ove the water

ta#le is #eing oxidiGed losing electrons! while the portion #elow is #eing reduced$ setting up a

flow of electrons from one end of the ore #ody to the other.This theory cannot explain anomalies where the ore #ody is completely #elow the water ta#le$

explain why a clay over#urden prevents a self-potential from forming$ or explain how self-

potentials form in poor conductors.#. differences in salt concentration in water called 0lectrochemical Potential!

c. solutions flowing through permea#le roc7s called @treaming Potential!

d. electric activity caused #y life processes of plants and animals such as differences

#etween open ground and #ush! called 8ioelectric Potential!

2. >esistivity methods:a7e use of the fact that some materials are good conductors of electricity and some are poor

conductors

where 8 is the amount of current flowing through a #ody

is the cross sectional area through which the current flows? is the voltage

> is the distance the current flows

is the conductivityof the material of which the #ody is made

The reciprocal of the conductivity is the resistivity.5esistivity is measured in ohm cm or ohm m.

5esistance 5esistivity x >/!$ in ohms$ is more commonly used #y physicists.%oor conductors have high resistivities.4ote: for inhomogeneous #odies$ we actually measure a sort of average resistivity along the path

of current flow$ called the apparent resistivity.

(ood conductors include metals$ graphite$ most sulfides.

8ntermediate conductors called semi-conductors! include most oxides and porous roc7s.%oor conductors insulators! include most common roc7-forming minerals.

=urrent in most roc7s is carried #y ions in fluids in the roc7's pores called electrolyticconduction!.

small change in water content affects resistivity enormously.lso$ the salinity of the water is highly important in determining conductivity.

The shapes and arrangements of the pores can result in greater current flow in some directions

than in others.aults$ oints$ etc.$ can produce "structural" conductors.

Procedure:

=urrent driven through ground using 2 electrodes


28/52

%otential distri#ution mapped with 2nd set of electrodes to determine potential difference pattern

voltage distri#ution! and directions of current flow.

nomalies conducting #odies$ for example! distur# regular patterns that would normally #eproduced

=ommon methods loo7 for:

1. variation of resistivity with depth

2. variation of resistivity horiGontally

1.to measure variation of resitivity with depth:current penetrates to deeper depths with increasing separation of current electrodes

can determine approximate depths to layers #ut not thic7nesses of layers

pro+lem 1- the deeper you go$ the wider the electrodes must #e spaced and the more powerful

the current supply necessary.

This limits the method to a few hundred feet.

pro+lem "6 a layer with intermediate resistivity #etween layers of high and low resistivitywill

not show up.Example - loo7ing for groundwater where layer of wet alluvium lies #etween layer of dry

alluvium and layer of shale

Hften used for +asement depth determinations:

sedimentary section generally has range of resistivities su#stantially lower than #asement roc7s$so can #e thought of as a 2-layer pro#lem

uantitative method for first approximations< rough wor4:gives reasona#le estimates for shallow depths< does not give good results on thic7 #eds!

sum all apparent resistivity values up to and including present reading and plot vs electrodespacing

Example: 8f readings are 1$ 2$ , ohm m for spacings of 1$ 2$ , m< plot 1$ ,$ *

ohm m vs 1$ 2$ , mthen draw segments of straight lines through as many readings as possi#le

cross-overs of segments gives depths to interfaces

2. to measure horiIontal variations in resistivityplace current electrodes great distance apart and move closely spaced potential electrodes along

grid #etween themplot resistivity vs. locations of potential electrodes

can use map or profile to display data< profiles are most common.

3nterpreting maps:

=an use either current lines or e6uipotential lines on maps

>ines of current flow always perpendicular to e6uipotential lines lines along which potential is

constant!


29/52

Fsually interpret maps 6ualitatively to simply identify locations of good conductors or goodresistors

3nterpreting profiles:

0stimate of depth to conducting +odyto 0/- 1K! can #e made #y the shape of theprofile- depth is half of the width of the curve at half its maximum height.

@teep gradientsin resistivity curve are characteristic mar4ers of structures with near6

vertical +oundaries$ such as faults$ di7es$ veins$ stream channels$ etc.

lac4 of symmetryin the profile implies a dipping +ody$ with steeper slope and

positive tail on the downdip side.

,. Hell Logging:8n well logging$ #oth potential differences and resistivities are used.

Example:

Jigh resistivity could #e due to limestone or oil #earing sand.

potential difference indicates flow of water into or out of well and/or difference in saltconcentration.

Therefore indicates oil #earing sand.

ain value of well logging lies in the possi#ility of correlation#etween wells.

;. 0lectromagnetic %ethods:

a. Telluric methods#. agnetotelluric methods

c. Electromagnetic 8nduction methods

d. 8nduced %olariGation methods

a. 9elluric methods:#araday!s Law of 3nduction: changing magnetic fields produce alternating currents.=hanges in the Earth's magnetic field produce alternating electric currents ust #elow the Earth's

surface called Telluric currents.

The lower the fre6uency 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 those descri#ed earlier under resistivity methods.

#. %agnetotelluric methods:The changing magnetic fields of the Earth and the telluric currents they produce have differentamplitudes.

The ratio of the amplitudes can #e used to determine the apparent resistivity to the greatestdepth in the 0arth to which energy of that fre2uency penetrates.


30/52

Typical e6uation:

apparent resistivity )

where Exis the strength of the electric field in the x direction in millivoltsJyis the strength of the magnetic field in the y direction in gammas

f is the fre6uency of the currents

&epth of penetration )

This methods is commonly used in determining the thic4ness of sedimentary +asins.

c. 0lectromagnetic 3nduction methods:=hanging magnetic fields are produced #y passing alternating currents through long wires or

coils.

These changing magnetic fields induce electric currents in #uried conductors such as ore #odies

which then produce their own induced magnetic field.There are a huge variety of techni6ues which use either the induced electric currents or the

induced magnetic field which these currents in turn produce.

This method is especially important in mineral exploration and surveys are easy to conduct formairplanes.

dvantages to using an airplane to conduct geophysical surveys:

not necessary to get permits from landowners

straight$ evenly spaced survey grid pattern easier to o#tain!

d. 3nduced polariIation methods:Dhen a current is applied to a formation containing metallic minerals$ each metallic mineral

grain has a small voltage produced across it in the direction of current flow.

---------> ----------> [ mineral grain ] ---------->

current negative negative

charge charge

added removed

Dhen the current is turned off$ the separation of charge remains for a short time and the voltagecan #e measured.

The total voltage for the formation depends on the percentage of metallic mineralsit contains.


31/52

Part : @eismology

@tress- specifies the nature of the internal forces acting within a mineral

@train- defines the changes of siGe and shape deformation! arising from those sources

n elastic su+stanceis one in which stress is proportional to strain Joo7e's >aw!

The constants of proportionality are 7nown as the elastic constantsand are different for different

7inds of stress twisting$ compressing$ stretching! and for different materials.Examples:

8f wire is stretched and #ecomes thinner$ the proportionality constants are E$ oung!s

modulusand $ Poisson!s ratio.

8f wire twisted$ the proportionality constant is $ the modulus of rigidity or shear

modulus.

8f a sphere is compressed$ the proportionality constant is B$ the +ul4 modulus.

8n a plastic su+stance$ under a given stress$ strain is not constant #ut is dependent on time.

The Earth is constantly undergoing stress.The roc7s of the Earth sometimes #ehave elastically and sometimes plastically.

8f the stress #ecomes large enough the elastic limitis reached!$ fracturing will occur$ suddenly

releasing stress and producing elastic waves which travel through the Earth earth6ua7e!

#ive most important types of waves:

8ody waves 6

o compressional *longitudinal< primary or P6waves,

o transverse *shear< secondary or @6waves,

@urface waves *@6waves, 6

o Love waves *transverse< horiIontal,

o >ayleigh waves *circular< reverse of water wave motion,

#ree oscillations

P6waves:

usually have the smallest amplitude

?elocity can #e calculated from elastic constants of material through which wave is traveling -one formula is:


32/52

vp) where is density

@6waves:

8f the particles in an 9-wave all move in a parallel line$ the wave is said to #e polariIed.n 9-wave with all vertical particle motion is called 9?< one with all horiGontal motion is 9J.

The velocity of 9-waves is given #y the formula:

?s)

uestion: Hhy can!t @6waves travel through fluidsD

8n a fluid$ rigidity ! is Gero$ therefore ?smust also #e Gero.

uestion: Hhy are P6waves always faster than @6wavesD

@ecause B and are always positive num#ers$ the ratio of ?pto ?swill always #e greater than 1.

Love waves:transverse and horiGontal

possi#le only in a low6speed layer overlying a medium in which elastic waves have a higher

speed

>ayleigh waves:

particle motion in circles li7e water waves$ #ut in opposite direction

travel only along the free surface of an elastic solid

amplitude decreases with depth#elow surface

slower than >ove waves

Dhen there is a low speed layer overlying a much thic7er layer of material in which the speed ofelastic waves is higher$ the surface wave velocity varies with wavelength.This variation of velocity with wavelength is called dispersion.

or deep focus earth2ua4es$ surface waves are either non-existent or have very low amplitudes.

#ree =scillations:

motions of the Earth as a whole


33/52

Beppler >aws

the Nrst law are a conse6uence of the conservation of energy of a planet or#iting the 9un under

the effect of a central attraction that varies as the inverse s6uare of distance. The second

law descri#ing the rate of motion of the planet around its or#it follows directly from theconservation of angular momentum of the planet. The third law results from the #alance #etweenthe force of gravitation attracting the planet towards the 9un and the centrifugal force away

from the 9un due to its or#ital speed. The third law is easily proved for circular or#its

The energy of a seismic wave is proportional to the s2uare of its amplitude.

s a wave spreads out from its source$ the energy spreads out over a large area and therefore theamplitude decreases.

There is also a loss of energy due to friction converting the elastic energy into heat$ leading to an

additional reduction in amplitude.

The loss of amplitude is called attenuationof the wave.

4eed many seismographs to completely record motion of groundduring an earth6ua7e$

including one each to record 4-9 motion$ E-D motion and up-down motion.

The relation +etween the natural period of a seismograph and the period of the waves +eing

recordeddetermines whether the instrument will measure the displacement$ the velocity or theacceleration associated with the Earth motion.

8f the natural period of a seismograph is much less than that of the earth vi#ration

fre6uency greater!$ the displacement of the seismograph #ecomes proportional to the

acceleration of the Earth and the instrument acts as an accelerometer. 8f the two periods are approximately e6ual$ the instrument reading will #e proportional to

the velocityof the Earth motion.

8f the natural period is much greater than the period of Earth vi#ration$ the reading

#ecomes proportional to the actual displacementof the Earth.

Hhen a wave meets a surface of discontinuity< part of it will +e reflected and part refracted

#ent!.

Every reflection or refraction generates additional waves$ producing an incredi#ly complexsituation and seismograms which are extremely confusing.

The recognition of the several different arrivals is a s7ill ac6uired #y long practice.

8t is often easier to follow reflected and refracted waves #y viewing them as raysmoving at right

angles to the wave front.

>eview of physics:

Hhen a wave is reflected< the angle to reflection is e2ual to the angle of incidence-


34/52

Hhen a wave is refracted< @nell!s Law applies:

where v1is the velocity in the 1st medium< v2is the velocity in the 2nd mediumichter %agnitude @calecan #e descri#ed #y the following formula:

% $ log1(*a)9, J f * < h, J C


35/52

a is the amplitude of the ground motion for surface waves from a 9outhern =alifornia

earth6ua7e recorded on a Dood-nderson seismograph in microns$ .1 mm!

T is the dominant wave period in seconds!

is the distance measured as the angle su#tended at the center of the Earth! #etween the

earth6ua7e and the seismometer

h is the depth of focus

f $ h! is a term found from a study of many recordings. 8t is #asically an expression for

the attenuation of the waves and has the effect of reducing all o#servations to a standard

distance

= is a station correction to adust for local peculiarities of seismometer siting.

The 5ichter agnitude 9cale did not originally specify which wave type used.

4ow we commonly use %-waves for deep focus earth6ua7es and the horiGontal component of

5ayleigh waves for shallow focus earth6ua7es.

Hne #ig pro+lem with the >ichter %agnitude @caleis that it doesn't directly measure anythingrelated to fault mechanics.

relatively new scale$ called the %oment %agnitude @cale$ which attempts to address this

pro#lem is now #ecoming widely used.

9he seismic moment is defined as: %o$ A u

is the shear modulus

is the area of the fault

u is the average displacement on the fault

9he %oment %agnitude is: %w$ ") log %o6 1(-5

formula often used to give the relationship +etween magnitude and total elastic waveenergyof an earth6ua7e is:

log1( 0 $ 1"-"7 J 1-77 %E is in ergs!

,. #irst %otion @tudies:

or simplification$ we will choose simple horiGontal stri7e-slip motion and choose axes parallel

and perpendicular to fault. Hther cases more complicated.8n 2 of the 6uadrants$ first motion will #e away from the epicenter< in other 2 6uadrants$ 1stmotion will #e toward epicenter.

otion away from the epicenter and toward the o#server! appears as an upward movement on a

seismic record.t right angles to the fault$ the motion would #e at a minimum$ while at small angles to the fault$

motion would #e maximum.

There will #e a reversal in the direction of first motion as one crosses the trend of the fault.


36/52

9ransform faultswere found to #e different from regular stri7e-slip faults #y loo7ing at their

relative movement as determined #y irst otion 9tudies.

;. locating areas of molten or partially molten roc4:The formulas for the velocities of % and 9 waves indicate

the lower the rigidity< the lower the velocity

@6waves don!t travel through fluids-

aor regions:

the molten outer core

the partially melted Gone in the upper mantle a#out 1 7m down! called the >ow

?elocity Mone or asthenosphere

a.determining depths to discontinuities

Travel times for % and 9 waves depend primarily on the distance they travel and therefore thedepth to which they penetrate into the Earth.The velocities of seismic waves depends on roc7s' elastic properties and can #e determined.

Bnowing velocities and timing the arrivals of reflected and refracted waves at 7nown distances

from source allows the calculation of the depths to discontinuities.

Dithin the Earth$ maor discontinuities occur at depths of , to * 7m the %ohorovicic

discontinuity!$ 23 7m the Guten+erg discontinuity! and 7m.

These discontinuities are used to divide the Earth into the crust< mantle< outer core and innercore.

8n addition$ there are many minor discontinuties.4ota#le ones are:

=rustal layers

>ow ?elocity Mone in upper mantle discussed previously!

The Earth can #e thought of as #eing made up of an infinite num#er of layers$ each with greater

density than the one a#ove. This results in an infinite num#er of refractions and is responsi#le forthe general curved nature of the paths of seismic waves through the Earth.

&iagrams which trace the paths of seismic waves through the Earth usually use sym+olsas

follows:

reflection at surface of Earth indicated #y succession of chief sym#ols ex. %%$ %9$ 99!

reflection at the outer surface of the core is shown #y interposing = ex. %c%$ 9c9$ %c9!

B is used for a %-wave refracted through the outer core %B%! and is often a##reviated %'

8 is used for a %-wave refracted through the inner core.


37/52

L is used for an 9-wave refracted through the inner core.

or deep focus earth6ua7es$ a small preceding s or p is used to indicate a wave moving

up from the focus to the surface ex. p%$ p9$ p%c%!

#. determining compositional variationsBnowing the velocities of seismic waves at different locations allows us to determine densities

and elastic properties at those locations.

Exploring the Earth's interior with % and 9 waves is sometimes called seismic tomography#yanalogy with =T scans =athode pplied Tomography! which use x-rays to study the interior of

a human #ody.

c. @eismic prospecting methods:

Explosions$ vi#rations and dropped o#ects often used to produce artificial earth6ua7es.@asic procedure is to set up seismic waves and time their arrivals at 4nown distances.

The waves may travel along direct paths$ or may #e refracted or reflected.

lmost always use only the first arrivals of P6wavesregardless of the path ta7en!.

Two commonly used types of methods:

1. 9eismic refraction methods

2. 9eismic reflection methods

1. @eismic refraction:

=an #e used to detemine thic4nesses and dips of layers and seismic velocities in each layer $ma7ing identification of roc7 types possi#le.

Example of one layer case:%lot time of arrival of waves T! versus distance to detector x!.

Dill o#tain a straight line with a slope of dT/dx which is e6ual to 1/velocity!$ allowingcalculation of velocity of %-waves in layer.

Hf limited usefulness$ o#viously.

Example of two layer case:Daves can travel from source to the detectors directly or #y critical refraction along the #oundary

#etween the layers.

Those that travel directly will produce the same type of plot as in the one layer case.

The travel time versus distance plot for refracted waves will also produce a straight line #ut onewhich has an intercept on the T axis.

The mathematical proof for this statement and the associated calculations can #e found in any

introductory geophysics text$ generally occupying a num#er of pages of manipulations offormulae. (o loo7 it up if you are interested.!


38/52

The depth to the #oundary$

where Tiis the intercept on the T axis and ?2is the velocity in the lower layer.

The slope of the line is 1/?2.

8n reality$ since we measure only first arrivals$ at distances less than a certain distance called the

critical distance!$ the direct wave is recorded and at distances #eyond the critical distance$ the

refracted wave is recorded.

The plot we o#tain is thus made up of segments of two straight lines and allows us to o#tain thevelocities in #oth layers and the depth to the interface.

or multi6layer cases$ the procedure is similar #ut more complicated.

The plot is made up of one line segment for each layer.?elocities can #e read off the graph fairly easily #ut the e6uations used to o#tain the depths to the

interfaces are horrendous and generally impossi#le without the use of a computer.

Example of a situation where the higher velocity layer is on topvery rare in nature!:

4o critical refraction occurs>ayer missed and thic7ness not accounted for

>eads to depth calculation errors

Example where velocity increases continuously with depth:

@asically the same as a multi-layer case with an infinite num#er of layers.%lot will loo7 li7e a curve with the shape of the curve dependant upon how the velocity varies

with depth.

Example of case of fault:

8f a #ed is faulted vertically$ the plot o#tained perpendicular to the stri7e of the fault will consistof 2 parallel #ut displaced linear segments.

The throw vertical displacement! of the fault can #e calculated from the difference #etween the

T intercepts of the the two linear segments.

Example of dipping layers:8f layers are horiGontal$ the same plot will #e o#tained #y reversing positions of the energy

source and the detector.

This will not #e true if layers dip.

The apparent dip and velocities in the layers can still #e determined #ut the procedure isextremely complicated. =onsult geophysics text if interested.

2. @eismic reflection:

the most widely used and valua+le geophysical exploration method and one of the easiest to

interpret 2ualitatively


39/52

9eismic waves traveling down from a source are reflected upward from each interface

encountered.

3nterfacesare not necessarily #oundaries #etween layers #ut could #e any of a num#er oflithologic changes which cause velocity contrasts.

5eflections from a single shot are usually recorded #y groups of geophones - fre6uently as many

as 3*.

Dhen several closely spaced detectors are laid out along a line$ each will record a reflection fromeach interface.

8f the seismograms from these detectors are recorded parallel to each other$ the waves

corresponding to a reflection will all line upacross the records in such a way that the crestsand troughs on adacent traces will appear more or less to fit into one another.

To ma7e a record easier to analyse$ we usually ma7e a dynamic correction *also called normalmoveout!.

The different geophones were at different distances from the shot point and therefore the waves

had longer distances to travel.The dynamic correction has the effect of mathematically placing all geophones at the same

distance from the shot point.

Hther corrections might involve:

elevation variations

removing the effects of the surface layer #ecause it is generally very varia#le and not of

particular interest

correcting for the fact that we are assuming vertical paths for the incident and reflecting

rays and this would not #e true for dipping or irregular surfaces and correcting for

diffraction effects #oth corrections called seismic migration!

removing multiple reflections called deconvolution!

After reflections have +een identified< they are timed$ using the trough of the 1st wave.

or horiGontal #eds$ where T is the travel time$ x is the distance #etween the shot point and the

receiver$ and ? is the average velocity in the section a#ove the interface$ the depth to the

interfaceis:

The average velocityin an area is often determined #y exploding charges of dynamite in a

shallow drill hole alongside a deep exploratory #orehole and recording the arrival times of waves

at detectors at a num#er of depths in the hole.The average velocity is simply the total vertical distance divided #y the total time.

The difference #etween the times of a pea7 or a trough for the same reflection at successive

detector positions gives information a#out the dip of the reflecting interface.


40/52

=hanging the distance #etween the shot point and the geophones gives several readings for thesame reflecting surfaces.

This results in the same reflection signal #eing recorded #ut different "noise" signals$ ena+ling

us to remove the noise signalsor at least to minimiGe them! with the use of various techni6ues.

#iltersused in geophysics can #e compared to maps of different scales

Hne geophysicist's noise is another's music. 5ayleigh waves disparagingly called ground roll!get in the way of exploration geophysics #ut are very important in crustal studies.

4oises are due to many things and we could devote an entire course to the techni6ues used to

deal with them.

3nterpretation:

Fnow thic4nesses and 4now velocities-

;ave at least some 4nowledge of the geology of the area-

3n addition to type of roc4< several other factors also affect velocity< including porosity and

water content-

Guess a little-

@eismic 9omography

@eismic tomographyuses data from hundreds of earth6ua7es and recording stations to generatea sort of CA9 scanof the Earth in a way that is similar to the whole-#ody scanning method used

for medical purposes.

The computer modeling methods are very complex. The end result is a three6dimensionalmodel of the shear6wave velocitywithin the Earth.

These 9-wave variations provide information a+out temperature conditions and mantle flow-

0arth2ua4e Prediction

(eophysical properties used in earth2ua4e predictionattempts:

1. slowing down of seismic waves

@efore an earth6ua7e$ the %-wave velocity drops to a minimum and then returns tonormal.

Cua7e occurs in a#out 1/1 time that anomaly lasted.

9iGe of 6ua7e correlates to duration of anomaly

%ossi#le explanation: Dhen crac7s first #egin to open$ %-waves slow down #ecause they

don't travel as fast through open


41/52

space as they do through solid roc7. (round water then seeps in and %-wave velocity

returns to normal< also roc7s are lu#ricated.

%ro#lems:

o usually doesn't occur

o 9ometimes when it occurs$ earth6ua7es don't

2. roc4 deformation

characterised #y tilting or vertical changes

,. increase in electrical resistivity

%ossi#le explanation: air in crac7s is not a good conductor

;. local magnetic field changes

>a#oratory experiments show that compression in direction of magnetiGation reduces

suscepti#ility and remanence< perpendicular compression increases it. Effect pro#a#ly

due to rotation of magnetic domains.

=ould #e related to increase in stress #efore 6ua7e or release of stress at time of faulting.

. electromagnetic noise

*. earth2ua4e lights

&ue to 9nellOs law$ the ray is thus #ending at the interface if the two velocities are

different. 9ince this relation holds for all interfaces$ the 6uantity sini!/v will #e constant

and the ray parameter is constant. The 6uantity 1/p is also the apparent velocity along

%: =ompressional wave

9: 9hear wave

B: % wave through outer core

8: % wave through inner core

%% and 99: % or 9 wave reflected at the surface


42/52

%%%: 5eflected , times etc.

9% and %9: 9 reflect as % or % as 9 at the surface

p% $ p9$ s9 or s%: % or 9 wave upgoing from the focus and reflected at the surface

c: Dave reflected at the core-mantle #oundary

%dif: % wave diffracted along core-mantle #oundary

%g t short distances$ either an upgoing % wave from a source in the upper crust or a % wave

#ottoming in the upper crust. t larger distances

also arrivals caused #y multiple %-wave rever#erations inside the whole crust with a group

velocity around . 7m/s.

%# alt:%P! Either an upgoing % wave from a source in the lower crust or a % wave #ottoming in

the lower crust

%n ny % wave #ottoming in the uppermost mantle or an upgoing % wave from a source in the

uppermost mantle

%n%n %n free surface reflection

%g%g %g free surface reflection

%m% % reflection from the outer side of the oho

%m%4 %m% multiple free surface reflection< 4 is a positive integer. or example$ %m%2 is

%m%%m%

%m9 % to 9 reflection from the outer side of the oho

9g t short distances$ either an upgoing 9 wave from a source in the upper crust or an 9 wave

#ottoming in the upper crust. t larger distances

also arrivals caused #y superposition of multiple 9-wave rever#erations and 9? to % and/or % to

9? conversions inside the whole crust.

9# alt:9P! Either an upgoing 9 wave from a source in the lower crust or an 9 wave #ottoming inthe lower crust


43/52

9n ny 9 wave #ottoming in the uppermost mantle or an upgoing 9 wave from a source in the

uppermost mantle

9n9n 9n free surface reflection

9g9g 9g free surface reflection

9m9 9 reflection from the outer side of the oho

9m94 9m9 multiple free surface reflection< 4 is a positive integer. or example$ 9m92 is

9m99m9

9m% 9 to % reflection from the outer side of the oho

>g wave group o#served at larger regional distances and caused #y superposition of multiple9-wave rever#erations and 9? to % and/or % to

9? conversions inside the whole crust. The maximum energy travels with a group velocity

around ,. 7m/s

5g 9hort period crustal 5ayleigh wave

4T>E %J9E9

% longitudinal wave$ #ottoming #elow the uppermost mantle< also an upgoing longitudinalwave from a source #elow the uppermost mantle

%% ree surface reflection of % wave leaving a source downwards

%9 %$ leaving a source downwards$ reflected as an 9 at the free surface. t shorter distances the

first leg is represented #y a crustal % wave.

%%% analogous to %%

%%9 %% to 9 converted reflection at the free surface< travel time matches that of %9%

%99 %9 reflected at the free surface

%c% % reflection from the core-mantle #oundary =@!

%c9 % to 9 converted reflection from the =@

%c%4 %c% multiple free surface reflection< 4 is a positive integer. or example %c%2 is %c%%c%

%G0% alt:%G%! % reflection from outer side of a discontinuity at depth G< G may #e a positive

numerical value in 7m. or example %**0% is a %


44/52

reflection from the top of the ** 7m discontinuity.

%GQ% % reflection from inner side of discontinuity at depth G. or example$ %**Q% is a %

reflection from #elow the ** 7m discontinuity$ which

means it is precursory to %%.

%G09 alt:%G9! % to 9 converted reflection from outer side of discontinuity at depth G

The

%n and 9n travel times are used for processing. The second 9-wave fits with #oth >g

and 9g and again it is difficult to udge which one it is. 8f an 9g$ a %g should #e

expected where the R%g O is mar7ed difference 9g S9n is 1.+ times the difference %g-

%n!$ #ut no clear phase is seen. 9o most li7ely the second 9-phase is >g.

What is Migration?

Migration is a tool used in seismic processing to get an accurate picture of

underground layers. It involves geometric repositioning of return signals

to show an event (layer boundary or other structure) where it is being hit

by the seismic wave rather than where it is picked up.

migration methods are: prestack and poststack migration

!ow to determine focal depth

seismic wave used to determine focal depth is the s% phase - an 9 wave reflected as a % wave

from the Earth's surface at a point near the epicenter. This wave is recorded after the p% #y a#out

one-half of the p%-% time interval. The depth of an earth6ua7e can #e determined from the s%phase in the same manner as the p% phase #y using the appropriate travel-time curves or depth

ta#les for s%.

8f the p% and s% waves can #e identified on the seismogram$ an accurate focal depth can #e

determined.


45/52

"oulomb#s $aw

a force exists between 2 magnetic poles:

where

is the force

is the permeability of free space, =

, are the magnetic pole strength

is the distance separating the poles

is the unit radial ector

unlike graity, poles come in 2 !aors:

o " #north$seeking%

o $ #south$seeking%

o like poles repel #%is ", force is outward%

o unlike poles attract #%is $, force is inward%

Magnetic Induction& '


46/52

o as with graity, we are interested in force arthexerts on a unit pole

#like acceleration, with g%

o or, &magnetic 'eld intensity&

o analogous to graitational acceleration #but not acceleration units(%

o force per unit pole strength #force exerted on unit magnetic pole%

#)n our analogy with graity, m here is the *arth&s +monopole+ 'eld, which is a

'ction Stacey incorrectly calls '+magnetic 'eld, which is !%

Magnetic %ield trength& !

o if we only had to deal with a acuum #or een air, since it has

negligible magnetic susceptibility%, we could always deal with !

#magnetic 'eld strength%..

o howeer, in presence of +magneti-able+ material, there is a magnetic

polari*ation#or, simply, magneti*ation% of material which produces

an additional 'eld #+% which adds to !

o combining the 'eld strength, !, and the magnetic polari-ation

#magneti-ation%,+, is call the magnetic induction, '

ow much bigger and stronger is a /.0 mag earth1uake compared to a .0

earth1uake

magnitude .+ earth6ua7e is +3; times @8((E5 on a seismogram than a magnitude .

earth6ua7e. The magnitude scale is logarithmic$ so

(10!"#$%(10&"!$ = (&"0110'$%(")110$

= "#'*10)

= #'* +R

= 10(!"#-&"!$

= 102"'

= #'*")2!


47/52

nother way to get a#out the same answer without using a calculator is that since 1 unit of

magnitude is 1 times the amplitude on a seismogram and .1 unit of magnitude is a#out 1.,

times the amplitude$ we can get$

10 10 10 % 1") = #' time,

[not eact. /ut a decent aroimation]

The magnitude scale is really comparing amplitudes of waves on a seismogram$ not the

9T5E4(TJ energy! of the 6ua7es. 9o$ a magnitude .+ is +3; times #igger than a . 6ua7e asmeasured on seismograms$ #ut the .+ 6ua7e is a#out 2,$ times 9T5H4(E5 than the .

9ince it is really the energy or strength that 7noc7s down #uildings$ this is really the more

important comparison. This means that it would ta7e a#out 2,$ 6ua7es of magnitude . toe6ual the energy released #y one magnitude .+ event. Jere's how we get that num#er:

Hne whole unit of magnitude represents approximately ,2 times actually 1PP1. times! the

energy$ #ased on a long-standing empirical formula that says logE! is proportional to 1.$

where E is energy and is magnitude. This means that a change of .1 in magnitude is a#out 1.;times the energy release. Therefore$ using the shortcut shown eartlier for the amplitude

calculation$ the energy is$

)2 )2 )2 % 1"* = 2).*0& or a/out 2).000

*lectronics and seismic instrumentation

Hhms law$ use of multimeter$ internal resistance$ power supply.

- 9eismic sensors: echanical and electrical constants$ how to measure constants$ overview of

main types.

Bnowledge of approximate values of generator constants and free period for different types of

sensors.

- 9ignal conditioning: mplifier general properties$ filters.


48/52

- &igital instruments: Dhat is an /& converter$ what does num#er of #its mean$ typical units on

the mar7et$ anti alias filters$ (94 station$ 9E89>H( station.

- Triggered systems: &etector and data storage.

- 9eismic noise: (enerally expected shape of noise curves$ noise measured as displacement or

power spectral density of acceleration.

diodeis an electrical component acting as a one-way valve for current.Dhen voltage is applied across a diode in such a way that the diode allows current$ the diode

is said to #eforward-biased.Dhen voltage is applied across a diode in such a way that the diode prohi#its current$ the

diode is said to #e reverse-biased.The voltage dropped across a conducting$ forward-#iased diode is called the forward voltage.

orward voltage for a diode varies only slightly for changes in forward current and temperature$

and is fixed #y the chemical composition of the %-4 unction.9ilicon diodes have a forward voltage of approximately .+ volts.

(ermanium diodes have a forward voltage of approximately ., volts.

The maximum reverse-#ias voltage that a diode can withstand without U#rea7ing downV is

called thePeak Inverse Voltage$ orPIVrating.

%urpose of a Mener &iodeexceeding a normal diode&s 3)4 usually results in destruction of the diode. Specialtypes of diodes, though, which are designed to 5break down6 in reerse$bias modewithout damage #calledzener diodes%

3urpose of 7ecti'cation

Rectificationis the conversion of alternating current =! to direct current &=!.

half-waverectifier is a circuit that allows only one half-cycle of the = voltage

waveform to #e applied to the load$ resulting in one non-alternating polarity across it. Theresulting &= delivered to the load UpulsatesV significantly.

full-waverectifier is a circuit that converts #oth half-cycles of the = voltage

waveform to an un#ro7en series of voltage pulses of the same polarity. The resulting &=

delivered to the load doesn't UpulsateV as much.

%olyphase alternating current$ when rectified$ gives a much UsmootherV &= waveform

less ripplevoltage! than rectified single-phase =.

Jow a rectifier is used with capacitor


49/52

8peak detectoris a series connection of a diode and a capacitor outputting a 9oltage e1ual to the peak alue of the applied 8 signal.

lipper , lamper and 4oltage ;ultiplier ircuits acting as speciali-ed circuits toproduce a certain output 8 oltage multiplier produces a 9 multiple #2,*9: emission of speci'c$fre1uency radiant energy wheneer electrons fall from ahigher energy leel to a lower energy leel.

n ohmmeter may #e used to 6ualitatively chec7 diode function. There should #e low

resistance measured one way and very high resistance measured the other way. Dhen using anohmmeter for this purpose$ #e sure you 7now which test lead is positive and which is negativeThe actual polarity may not follow the colors of the leads as you might expect$ depending on the

particular design of meter.

9ome multimeters provide a Udiode chec7V function that displays the actual forward voltage

of the diode when its conducting current. 9uch meters typically indicate a slightly lower forward

voltage than what is UnominalV for a diode$ due to the very small amount of current used during

the chec7.

;aximum 9 reerse oltage

;aximum #aerage% forward current

;aximum total dissipation

?perating @unction temperature

;aximum reerse current

Aypical @unction capacitance = B, the typical amount of capacitance intrinsic to the

@unction, due to the depletion region acting as a dielectric separating the anode and

cathode connections.

7eerse recoery time = trr, the amount of time it takes for a diode to 5turn off6

when the oltage across it alternates from forward$bias to reerse$bias polarity.

>E&unctionsglowwhen forward #iased. diode intentionally designed to glow li7e a lamp is

called a light-emitting diode$ orLED.


50/52

orward #iased silicon diodes give off heat as electron and holes from the 4-type and %-type

regions

9H>5 =E>>9 operates in photovoltaic mode %?! #ecause it is forward #iased #y the voltagedeveloped across the load resistance

%84 diodes are used in place of switching diodes in radio fre6uency 5! applications$ %84 diode

is manufactured li7e a silicon switching diode with an intrinsic region added #etween the %4unction layers. This yields a thic7er depletion region$ the insulating layer at the unction of a

reverse #iased diode. This results in lower capacitance than a reverse #iased switching diode.

T58H&E :thermionic diode$ the heated cathode either directly or indirectly #y means of a

filament! causes a space charge of electronsthat may #e attracted to the positively charged plateanode in FB parlance! and create a current. pplying a negative charge to the control grid will

tend to repel some of the also negatively charged! electrons #ac7 towards the cathode: the larger

the charge on the grid$ the smaller the current to the plate. 8f an = signal is superimposed on the&= #ias of the grid$ an amplified version of the = signal appears inverted! in the plate circuit.

9ifference between 8 and 9

Alternating Current ?irect Current

mount of energy that

can #e carried

9afe to transfer over longer city

distances and can provide more power.

?oltage of &= cannot travelvery far until it #egins to lose

energy.

=ause of the direction of

flow of electrons5otating magnet along the wire.

9teady magnetism along the

wire.

re6uencyThe fre6uency of alternating current isJG or *JG depending upon the

country.

The fre6uency of direct currentis Gero.

&irection8t reverses its direction while flowing

in a circuit.

8t flows in one direction in the

circuit.

=urrent 8t is the current of magnitude varyingwith time 8t is the current of constantmagnitude.

low of ElectronsElectrons 7eep switching directions -

forward and #ac7ward.

Electrons move steadily in one

direction or 'forward'.

H#tained from .= (enerator and mains. =ell or @attery.

%assive %arameters 8mpedance. 5esistance only

%ower actor >ies #etween W 1. it is always 1.

Types 9inusoidal$ TrapeGoidal$ Triangular$ %ure and pulsating.
http://www.wikipedia.org/wiki/Diodehttp://www.wikipedia.org/wiki/Electronhttp://www.wikipedia.org/wiki/Electronhttp://www.wikipedia.org/wiki/Diodehttp://www.wikipedia.org/wiki/Electron


51/52

Alternating Current ?irect Current

96uare.

Jow do you measure free period of seismometer

sensitive seismometer$ connect a multimeter in &= mode most sensitive range!

8dentifying polarity of a seismometer

first step is to identify the positive terminal on the seismometer #y rapidly tilting the

seismometer$ which means that the seismometer moves F% when using a vertical seismometer orhoriGontally in direction of rotation for a horiGontal seismometer.

@road#and seismometer

#road#and #and sensor can #e understood as a usual velocity sensor with an extended fre6uency

range in the low fre6uency end

8dentifying dominant period of seismometer

- =onnect a voltmeter to the output of the sensor

re6uency response of multimeter and oscilloscope

%lotting fre6uency vs voltage output

ccelerometer

The accelerometer measures the ground acceleration from &= to a#out JG without significant

change in gain. The instrument therefore can measure static changes in the gravity field as wellas dynamic changes. Dhen the accelerometer is placed horiGontally$ theoretically output from all

, sensors should #e Gero.Cse of instrument noise

Aheoretically, a standard seismometer or geophone will output a signal in the whole

fre1uency range of interest in seismology. So why not @ust amplify and 'lter the

seismometer

output to get any desired fre1uency response instead of going to the trouble of

making


52/52

complex DD sensorsE Ahat brings us into the topic of instrument self noise. 8ll

electronic

components as well as the sensor itself generate noise. )f that noise is larger than

the signal

generated from the ground motion, we obiously hae reached a limit.

@9U? U0@93=@ #=> G0=P;@3C@

9tate the >aw of Fniversal (ravitation

9tate 4ewton's 9econd >aw.

Fsing the >aw of Fniversal (ravitation and 4ewton's 9econd >aw$ derive an expression for the

acceleration of gravity.

Dhat is the approximate value for the acceleration of gravity

Dhat is the unit used for the acceleration of gravity Jow many cm/sec 2is it e6ual to

Jow do we 7now that the interior of the Earth must #e composed of roc7s denser than those onthe Earth's surface

Scientists beliee that the oerall chemical composition of the *arth is ery similarto a kind of meteorite called chondrites, which formed at the same time the *arthwas formed. We know a lot about the composition of the *arths crust and mantle,because we can obsere those rocks that hae been brought to the surface bygeologic processes.

Date post:	02-Jun-2018
Category:	Documents
Upload:	datta-pkd-kumar-pushan
View:	223 times
Download:	0 times

Important Qu Erries

Documents