Geophysics Exam 2

Understanding: International Geomagnetic Reference Field (IGRF)

A theoretical model that is used to define the geomagnetic field at any point on the Earth’s surface.

Magnetic corrections: What do Diurnal corrections correct for?

Corrects for temporal variation in the Earth’s measured field.

Magnetic corrections: What do the IGRF corrections correct for?

Corrects for long period variation in the Earth’s measured field.

Magnetic corrections: Elevation correction???

It is not necessary

Sketch the magnetic anomalies of the dipole at equator

Sketch the magnetic anomalies of the dipole at the North Pole

Sketch the magnetic anomalies of the dipole at midlatitudes

What causes Secular variations of the Earth's magnetic field?

Long period variation (Internal)

How ?

Subtract IGRF from observed data

What causes Diurnal variations of the Earth's magnetic field?

Short period variation (External)

How ?

Base station readings

Base magnetometer

Magnetic observatory

Application of Poisson's relation in gravity and magnetic data interprelation:

It’s purpose is to compare magnetic and gravity surveys of the same area to determine whether the same body is producing both gravity and magnetic anomalies.

Induced Magnetization depends on ____________ and _____________.

Susceptibility and Magnetizing force

Remanent Magnetization depends on ____________ and _____________.

Physical history of rocks and Chemical changes

What are the effects of vertical derivative filters on geophysical signals?

1st:

• Enhances or Sharpens local anomalies masked by broader regional trends

• Can be considered as partial elimination of regional field

• High pass filter

2nd:

• Used to identify lateral extent of anomalous structures

• Enhances near surface effects at the expense of deep sources

• High pass filter

• Gives idea about lateral boundary of a causative body

What are some effects of maximum horizontal gradient on geophysical signals?

Enhances major geological trends (ex: Edges, lineaments, etc)

What are the Principles of operation of a Proton Precession Magnetometer (PPM)?

• A sensor bottle containing protons, is subjected to a direct current in a coil wound around the bottle.

• The current generates an induced field.

• The protons align in the direction of the induced field.

• When the current is shut off the protons precess along the earths magnetic field with a frequency, f that is proportional to the field strength, H_{e}.

What are the Principles of operation of a LaCoste & Romberg gravity meter?

Gravity Data Corrections

• Goal is to reduce differences in measurements to being due to geology only

Meter Reading Correction

• Convert to mGal

• Conversion is unique to each gravimeter - calibration table

Drift Correction

• The spring stretches with time

• Measurements are taken at the Base Station

• The more accurate our survey, the more often Base Station measurements are taken

• Meter is assumed to drift linearly

Free Air anomalies are used primarily at ___.

sea

Bouguer anomalies form the basis for gravity interpretation on ____.

land

Free-Air Correction math equation

FAC = 0.3086h mGal (h in meter)

g a 1/r²