Lecture 8

Question 1

Define gravity surveying

Answer 1

Gravity surveying measures variations in the Earth’s gravitational field caused by differences in density of subsurface rocks

Question 2

Define Magnetic Surveying

Answer 2

Geomagnetic surveying measure variations in the Earth’s magnetic field caused by differences in magnetic properties of subsurface rocks

Question 3

Definition of Geodynamo

Answer 3

The rotation and slow convective motions within the conductive liquid outer core result in electrical currents which generate the geomagnetic fluid

Question 4

Is the magnetic field stationary?

Answer 4

No

Question 5

What does inclination vary with?

Answer 5

Latitude

90 degrees @ poles, 0 degrees @ equator

Question 6

What can the geomagnetic field be described in terms of?

Answer 6

Inclination
Declination
Total magnetic force vector (F)

Question 7

What does the Earth’s Geomagnetic field protect us from?

Answer 7

Solar wind

• continuous bombardment of charter particles coming from the sun

Question 8

What can the strength of the geomagnetic field be expressed in terms of?

Answer 8

H (magnetic field strength)
and
It can be expressed in terms of the closeness of magnetic flux lines which is called Flux Density. This related to Tesla and has the SI unit of Nm/A

Question 9

Can the intensity of the geomagnetic field be calculated for the whole globe?

Answer 9

Yes - international geomagnetic reference field

Question 10

Describe the relationship between density, porosity and velocity

Answer 10

When Density increases, porosity decreases and velocity increases

Question 11

Describe the three types of remnant magnetism

Answer 11

Thermo-remnant
Detrital-remnant
Chemical remnant

Question 12

What is the main component of the geomagnetic field?

Answer 12

Dipolar Field, can be approximated by a dipolar magnet at the centre of the earth

Question 13

What can seismometers detect?
5-10s
10-20s
20-100s

Answer 13

5-10s Secondary microseism
10-20s primary microseism
-swell of waves, impact solid earth as waves hit bottom of sea
20-100s Earth’s hum

Question 14

Define gravity data reduction

Answer 14

data needs to be corrected before gravity anomalies a

can be interpreted

Question 15

Define instrumental drift

Answer 15

Gravimeter readings change with time (drift) due to elastic creep. Drift can be measured by repeating measurements at the same location throughout the day. Measurement taken at a certain time (t) has to be reduced by the amount of drift at that time.

Question 16

Define free-air correlation

Answer 16

When a measuring system is located above or below sea level the magnitude of gravity will be less (or more) than at sea level due to changes in distance to the centre of the Earth. Free air correlation accounts for this. It is positive above sea level and negative below sea level.

Question 17

Terrain Correction

Answer 17

In areas with considerable variations in elevation, particularly closer to a gravity measuring station a terrain correction must be applied.

Question 18

Eotvos correlation

Answer 18

When a gravimeter is moving whilst measuring, measured gravitational acceleration will be increased or reduced by accelerations caused by the vehicle.

Question 19

What does the bouguer Correction do?

Answer 19

Accounts for extra rock mass

Question 20

What doe the bouguer anomaly do?

Answer 20

Correlated with lateral variations in density at appropriate depths within the subsurface.

Question 21

Name three types of magnetometers

Answer 21

Torsion/Balance Meters, Fluxgate meters, resonance meters

Question 22

What does the diurnal correction do?

Answer 22

Measurements need to be corrected for the daily (diurnal) variation in Earth’s magnetic field.
Day to day variability