Geophysics, and in particular magnetics, is highly valued as an integral component of modern geological mapping. This section looks at the general background of geophysics as applied to discrimination between rock types in place. The main objective is to enable the eventual integration of geophysical data with other geological data, especially in mapping.

This page is part of an edited version of a class presentation, not a full set of subject notes.

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A different view

Gravity and topography

• The previous view shows upper mantle temperatures, estimated from seismic wave speeds, shaded with gravity field variations (next)
• Continent/Ocean and Plate/Plate differences are evident
• Plate tectonics mechanisms must explain these!

Gravity Variations

General Scheme

• Overview of Geophysics
• Modern Geological Mapping
• Basics of Magnetics method
• Magnetic Properties of Rocks
  

What is it?

• Geophysics is
  • "The use of observations of physical phenomena to infer the structure of the Earth."
• Geophysics uses
  • the effects of differences in physical properties of rocks of different kinds.

Rock Property Examples and Physical Fields illustrated in simple tables.

Geophysical Methods

• allow "remote sensing" of rocks below the surface, by measuring these physical fields.
• Fields are usually measured at the surface, but give rise to a third-dimensional view.

Anomalies

• We can usually predict the response of a "standard" Earth
• Anomalies are differences between observation and expectation
• Anomalies result from differences in rock properties

Mathematics

• Calculations are needed, to
  • predict background response
  • predict response of inferred structures
• The maths can be very heavy, but
• There are many simple short cuts, and
• Imaging methods make data immediately useful

Way to go - the main geophysical methods

• Gravity
  • Simple physics, wide application, simple calculations
• Magnetics
  • Very wide application, more complicated physics, qualitative approach (here)
• Electrical Methods
  • Range of methods, applications, usually for specific purposes
• Seismic Methods
  • Simple physics, important applications

   Changing attractions

An image from the Australian Geological Survey Organisation website

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