### STUDY QUESTIONS FOR GEOPHYSICS

State the Law of Universal Gravitation

State Newton's Second Law.

Using the Law of Universal Gravitation and Newton's Second Law, derive an expression for the acceleration of gravity.

What is the approximate value for the acceleration of gravity?

What is the unit used for the acceleration of gravity? How many cm/sec2 is it equal to?

How do we know that the interior of the Earth must be composed of rocks denser than those on the Earth's surface?

Define and give formulas for angular momentum, moment of inertia?

What causes the Earth's oblateness?
Define:

• spheroid
• geoid

What is the name of the formula used to calculate the theoretical value of "g" at any latitude?

In what two ways may one measure the actual value of g at a particular location?
What are the disadvantages to using a pendulum to measure g?

How high are "solid Earth" tides?

What is a gravity anomaly? a positive gravity anomaly? a negative gravity anomaly?

Name the reasons why the actual value of g measured at a particular place is not the same as the theoretical value.

What corrections are applied to:

1. "correct" actual values of g to remove that portion due to the mass between sea level and the elevation at which g was measured,
2. correct actual values of g to remove the effects of nearby mountains and valleys,
3. correct actual values of g to sea level?

Define:

• Free Air anomaly
• Bouguer anomaly

Which of the three most commonly applied corrections is the least important and is often ignored?
If not ignored, how is the corrections usually made?
Why is the topographic correction always added to the measured value?

What are the characteristic values for Bouguer Anomalies in the following places?

• mountain regions
• continental shields
• oceans

How are positive anomalies produced? negative anomalies?

Describe Pratt's and Airy's concepts of isostasy. Which is probably correct?

How can the viscosity of the mantle be determined?

Where are regional anomalies produced (how far down?)

What is another name for small scale anomalies which remain after regional anomalies are removed?

Outline the procedure for interpreting gravity anomalies.

What kinds of anomalies (sign and shape) would be produced by the following geologic features?

• salt domes
• sedimentary basin
• mass of granite
• ultramafic mass
• buried ridges and valleys
• anticlines
• synclines
• oceanic trenches
• oceanic ridges
• fault
• buried stream channel
• continental crust/oceanic crust boundary

What evidence is there from gravity studies that all granite cannot have been produced by differentiation?

What is a gravity gradient?
How can gravity gradients be useful in locating anomalous bodies?

What is geochronology?

What is radioactivity? What changes take place during radioactive decay?
Define half life, parent element, daughter element.

What assumptions are routinely made in radiometric dating procedures?

What are isochrons? How are they obtained?

How can Rb, Sr dating and K, Ar dating be used to determine age of metamorphism?

How can Rb, Sr be used to determine crustal vs. mantle origin of rocks?

How can the 87Sr/86Sr ratio be determined for current mantle rock? for older mantle rock?

In a 238U -> 206Pb study, the original ratio of 206Pb/204Pb must be known. How can this be determined?

What are concordant ages? discordant ages?

What is a concordia curve? How can one be used in radiometric dating?

What is the lead-lead method? Why is it particularly useful?

How can 238U -> 4He be used to determine solidification ages?
Where has this method been used? What problems are there with this method?

How are U and/or Pb used to determine the following:

• the age of the Earth's crust
• the age of the Earth
• the age of a lead ore
• the age of the universe

Explain how fission track dating is done.

List some advantages in using K, Ar for radiometric dating, list some disadvantages.

Discuss the use of 14C in radiometric dating.

How can natural radioactivity be used in geophysical exploration?

What determines the direction of heat flow?

Describe three methods of heat transfer.
Where in the Earth might radiation be an important mechanism for heat flow?

Upon what does the temperature at the Earth's surface depend?

How does the average heat flow from the Earth's interior compare with the average heat flow from the Sun?

List and describe the sources for the Earth's internal heat.
Which of the above sources is the most important?

How can the temperature of the Earth at its core-mantle boundary be deduced?

Where on the Earth do areas of low heat flow occur? areas of high heat flow?

What is the temperature gradient near the Earth's surface?
How does the temperature gradient change with depth in the Earth?

How does temperature change with depth in the Earth?
What is the estimated temperature at the Earth's center?

What does the thermal conductivity of a rock depend upon?

Give an expression for heat flow.

Define the coefficient of thermal conductivity.
What does the thermal conductivity of a rock depend mostly upon?
What 2 other things can affect the thermal conductivity of rocks and why?

Name a mineral that is a good heat conductor.

Define thermal diffusivity and give a formula for it.

How fast does heat flow through soils and rocks by conduction? Give examples to illustrate your answer.

List 3 possible causes of high heat flow over young orogenic areas.

Why do Precambrian shield areas have lower than average heat flows?

What is the evidence from heat flow data that convection currents bring heat to the Earth's surface?
Where do these convection currents exist?

What is the relative ratio of heat flow due to convection and heat flow due to conduction through continents?

through ocean basins?

List occurrences or processes that can produce temperature anomalies.

What is a dipole?

How are magnetic fields produced?

What makes a substance diagmagnetic? paramagnetic? ferromagnetic?

Give examples of diamagnetic, paramagnetic, and ferromagnetic substances.

What happens when an external magnetic field is applied to a diamagnetic substance? to a paramagnetic substance? to a ferromagnetic substance?

What happens to diamagnetic substances when applied fields are removed? to paramagnetic substances? to ferromagnetic substances?

How can "permanent" magnetization be removed?

Define magnetic susceptibility.

How can magnetic susceptibility data be used to determine bedding or foliation directions in rock samples?

How are NRM, TRM, DRM, IRM, and VRM produced in rocks?

Define Curie temperature.

Give 5 examples of the use of paleomagnetism in geology.

What two parameters must be given to describe the Earth's magnetic field?

Define declination, inclination.

What is the dipole component of the Earth's magnetic field? the non-dipole component?

What percentage of the Earth's magnetic field originates within the Earth?

How strong is the Earth's magnetic field?
Where is it strongest? weakest?

Where does the Earth's internal field originate? How do we know?

How do we know that the whole Earth isn't permanently magnetized?

What hypothesis is the most generally accepted explanation for the Earth's internal magnetic field?

How is the external field produced?

What changes occur in the Earth's magnetic field?

What are magnetic storms?

What are diurnal changes and how are they produced?

What are secular changes? What is a possible explanation for secular changes?

What is westward drift? How might it be produced?

What evidence do we have for magnetic reversals?

How often do magnetic reversals occur?
How long does a reversal take?
When was the most recent reversal?

What happens to the strength of the Earth's magnetic field during a magnetic reversal?

What might be the effects on life of a magnetic reversal?

What effects beside faunal extinctions correlate with magnetic reversals?

Why are magnetic prospecting methods more complicated than gravity prospecting methods?
How are they easier?

What are magnetic prospecting methods most commonly used for?

What three things can produce magnetic anomalies?

What can the spacing of contour lines on magnetic maps tell you?

What is the significance of a sudden change in spacing in contour lines on a magnetic map?

What kind of magnetic anomaly (+ or -) do salt domes usually produce? Why?

Describe and give an explanation for the magnetic anomaly pattern observed on the ocean floors.

What are polar wandering curves? For what are they used?

Define potential difference.
What is the difference in potential difference and voltage?
What is the unit of voltage?

Describe 4 situations where natural potential differences occur.

Define:

• conductivity
• resistivity
• resistance
• insulator
• conductor
• apparent resistivity

What are the units for resistivity?

Give a formula for resistance.

What is the most important thing that determines conductivities of rocks?

When a current is passed through the ground, what effects do bodies of different conductivity produce?

How can you map the electric field produced when a current is passed through the ground?

Define equipotential lines.

What happens to equipotential lines in the presence of a good conductor? in the presence of a good insulator?

How can you study vertical diffenences in resistivity in the Earth?

Explain the origin of telluric currents.

What is Faraday's Law of Induction?

How are each of the following techniques used in electrical prospecting?

1. telluric methods
2. magnetotelluric methods
3. electromagnetic induction methods
4. induced polarization methods

State Hooke's Law.

• Define
• stress
• strain
• elastic substance
• plastic substance
• elastic limit
• Young's modulus
• Poisson's ratio
• bulk modulus
• compressibility
• modulus of rigidity
• shear modulus

List some of the elastic constants.

What causes earthquakes?

Define

• focus
• epicenter

How deep are shallow focus earthquakes? intermediate focus earthquakes? deep focus earthquakes?

Define

• wavelength of a wave
• frequency of a wave
• period of a wave
• amplitude of a wave

Give a formula which relates wavelength and frequency.

Distinguish between a seismograph and a seismogram.

What determines whether a seismograph will measure the displacement, v elocity, or acceleration associated with earth motion?

Explain in very simple terms how a seismograph works.

What are the 4 principle types of waves produced by an earthquake?
Which of these are body waves?
Which are surface waves?

List 3 other names for P-waves. List 3 other names for S-waves.

Which waves are the fastest? Gives equations for the velocities of P-waves and S-waves.

What is the most important factor in determining velocity in rocks?

What are SV and SH waves?

Why can't S-waves travel through liquid (give a mathematical proof)?

Show mathematically why P-waves are faster than S-waves.

What condition is necessary for the transmission of Love waves?

Describe the motion of a particle of rock as each of the four principle seismic waves passes.

What is the effect of depth of focus on the production of surface waves?

What happens when a seismic wave meets a surface of discontinuity within the Earth?

What is the relation between the angle of reflection and the angle of incidence?

Give Snell's Law.

What happens to a wave when it meets a surface of discontinuity at the critical angle?

Be able to trace the paths of earthquake waves such as PS, PcS, PKP, PKIKP, pPcP, etc.

List some reflected and/or refracted waves which might be mistaken on seismograms for surface waves and for aftershocks.

What are free oscillations?

Distinguish between magnitude and intensity.

Define magnitude mathematically, explaining the significance of each term.

State the relationship between magnitude and total energy of an earthquake.

What scales are used for magnitude? for intensity?
What was used for intensity prior to 1931? Who modified the Mercalli Scale?

Explain how the epicenter of an earthquake can be located.
How accurately can epicenters be located?

Explain how the direction of movement along a fault can be determined by studying seismograms.

What are transform faults? How were they detected?

How can the depth of focus of an earthquake be determined?

How do we know that the upper mantle is partially molten?

At what depths do major discontinuities exist within the Earth? What are the names of these discontinuities?
Name 2 minor discontinuities in the Earth.

What are the two methods commonly used in seismic prospecting?
Which is most often used? Which gives the most information?

What sources of energy are most often used in seismic exploration?

Describe in detail how refraction profiles would be produced, and what information could be obtained from them.

What kind of time/distance graph would be produced where velocity increases continuously with depth?

What kind of graph would be produced where velocity first increases continuously with depth and then abruptly changes?

What kind of graph would be obtained if a seismic profile was taken perpendicular to the strike of a vertical fault?
How can the throw of the fault be determined?

List 3 factors which help determine seismic wave velocity.

How does one recognize a reflecting horizon on seismograms?

How can the depth to a reflecting horizon be determined?

How can the dip of a reflecting horizon be determined?

How is the average velocity at a particular depth ordinarity determined?

What is seismic tomography? What information can it provide?

Have earthquakes been successfully predicted?
List and discuss possible geophysical properties which may be indicators of forthcoming earthquakes.

Describe one theory which explains why P-wave velocity would decrease prior to an earthquake.