This module covers the different extrusive geophysical methods that can be used to analyse areas for their different physical properties. Each lecture covers a new method and delves into the background, application and processes of that method.
Lecture notes
Geophysics: ‘the application of physics to the study of the earth from its surface to its centre
by looking at differences in physical properties.’ → focusing on applied geophysics which
focuses on up to 20km beneath the surface. → pure geophysics is more theoretical.
Geophysics is extrusive; compared to intrusive investigations it is cheaper, quicker, less
damaging, quieter, safer/ lower risk and creates continuous data.
Measures travel times of refracted and reflected seismic waves, so observes
differences in density of elasticity of moduli.
Electrical-
electromagnetics (A)
resistivity (A)
Measures earth resistance as a result of conductivity.
, induced polarisation (A)
Measures voltage or frequency of polarization which depends on ground resistance.
ground penetrating radar (A)
Measuring travel times of reflected radar pulses.
other-
gravity (P)
There are spatial variations in gravitational field strength determined by density
variations.
magnetics (P)
There are spatial variations in geomagnetic field strength determined by differences in
magnetic susceptibility and remanent magnetism.
self-potential (P)
Measuring electrical potential.
Not studied
magnetotellurics (P)
radiometrics (P)
Passive source- uses natural energy- more for deeper subsurface investigations
Active source- involves actively adding in a source of energy- usually for shallower
investigations as it dissipates but creates higher resolution data.
Usually a combination of methods will be used to avoid errors or bias. Method is selected
based on factors such as depth/ size of area, time/ cost limitations, environment, required
resolution and nature of the target and physical ground properties.
Technique→ survey→ data→ interpretation
Data Acquisition
-ground; slowest
-sea
-air; fastest therefore more cost effective
, W2
Data processing, analysis and modelling
Understanding of how geophysical data is processed and analysed
Introduction to time series analysis techniques in data processing
Understanding of basic principles in modelling and imaging geophysical data
Lecture 2
Digitization;
1. Dynamic range
2. sampling frequency
Aliase data- not sample frequently enough; sample at double the frequency
Waveforms
periodic- repeats itself at a fixed time period
transient- non-repetitive (e.g. a seismic pulse)
Fourier analysis; waveform can be decomposed into a series of sine or cosine waves. To
express these we must have:
-frequency -amplitude -phase
Waves are expressed in time domain or frequency domain.
Waveform Processing
Convolution- a mathematical operation that changes the shape of a waveform from its
passage through a filter.
‘Impulse response’ is the output when a spike function is the input.
Earth acts as a filter for seismic and other unnatural pulses through the ground.
y(t) = g(t) * f(t)
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