Lecture 1c - Geophysical fields, natural and artificial sources

Flashcards based on lecture notes about geophysical methods, focusing on active and passive techniques, potential fields, and radiometry.

What are geophysical methods using 'active sources'?

Methods that require a particular signal or source to be generated for measurements.

What are 'passive' geophysical methods?

Methods that record a naturally generated background geophysical field.

Give an example of an artificially generated active source.

Seismic waves generated by a hammer or explosion.

Besides using explosions, what other methods require active sources?

Ground penetrating radar and some electromagnetic methods.

What is an advantage of using an active source in geophysical surveys?

Its location and size can be planned as part of the optimum practical survey design.

Give an example of natural sources used to probe the deep structure of the earth.

Large earthquakes.

Name two examples of geophysical potential fields measured by passive techniques.

Gravity and the magnetic field.

What does 'potential' imply in the context of geophysical potential fields?

Moving around that field may require energy.

In physics, what is a vector field?

It has both a magnitude and a direction at any point in space.

Give an example of a scalar field.

Height or a topographic surface.

What is the conventional value commonly used for gravity at the Earth's surface?

9.81 meters per second squared.

What is one method for highlighting anomalies in geophysical field measurements?

Using radiometry.

What is gradiometry?

A way of measuring the field gradient rather than the field magnitude.

In general, what does radiometry enhance the detectability of?

Enhances the detectability of short wavelength signals.

What is the technique where the difference between two magnetometer measurements represents the field gradient?

Magnetic radiometry.

How can gradiometry be represented mathematically?

Local gradient delta g by delta x.

What does the radiometry approach emphasize and suppress?

Emphasizing short wavelength anomalies and suppressing long wavelength variations.

In which geophysical methods can the radiometry approach be used?

Magnetic and gravity methods.

What was unique about the radiometer used in the Gotye mission?

It comprised three pairs of sensors which allowed gravity gradients to be simultaneously measured in three orthogonal directions.

What did the Svigocce data show about gravity measurements?

They showed the full vector field and allowed changes and rates of change in gravity to be assessed globally.