Applied Geophysics An Introduction
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APPLIED GEOPHYSICS AN INTRODUCTION
FRED BEEKMANJEANNOT TRAMPERT
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COURSE OVERVIEWCourse contentOverview of the classical potential field and seismic methods. The course will review the basic physical principles underlying the various exploration techniques and will show how the field data are acquired and interpreted.
AimUnderstanding the fundamental concepts of geophysical exploration techniques and being able to put these concepts into practice.
Exercises Through paper exercises and computer practicals the students will solve a range of realistic problems. Students will acquire an appreciation for which techniques are appropriate for which application.
Grading•30% exercises+practicals•70% written exam
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COURSE OVERVIEW
Literature
“Introduction to Applied Geophysics” by Burger, Sheehan and Jones. Norton, 2006.
“An introduction to Geophysical Exploration” by Keary, Brooks and Hill. Blackwell, 2002.
“An introduction to Applied and Environmental Geophysics” by Robinson. Wiley-Blackwell, 2011.
“Applied Geophysics” by Telford, Geldart and Sheriff. Cambridge UP, 1990.
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APPLIED GEOPHYSICS Geophysics is the application of physical principles and methods to problems in Earth Sciences
There is no clear distinction between general and applied geophysics…
…but in general geophysics methods are typically applied to solve academic questions, whereas applied geophysics can be characterized by the application of geophysical methods for commercial purposes (“making money”).
Examples: oil, gas, coal, minerals, … groundwater geo-engineering (tunnels, dams, …) archeology pollution …
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APPLIED GEOPHYSICSPhysical methods used in applied geophysics:
Seismic methods: refraction and reflection seismics; VSP; seismic surveys
Gravity surveying: density contrasts; anomalies; gravimeters; corrections
Geomagnetic surveying: induced rock magnetism; anomalies
Electromagnetic surveying: EM fields; survey methods; ground penetrating radar (GPR)
Electrical surveying: resistivity; VES soundings;
Borehole petrophysics: well logging; core sampling;
Radiometrical surveying: radioactive decay; isotopes; dating;
Geothermal methods: heat flow; conduction;
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APPLIED GEOPHYSICSPassive versus Active methods
Passive methods:Use naturally present sources/fields to investigate properties of the subsurface
Examples:GravityEarth’s magnetic fieldEarthquakes
Active methods: Use man-made source to image structure of subsurface
Examples:Dynamite, air guns, …EM wavesElectrical currents
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APPLIED GEOPHYSICSTo select the most appropriate geophysical method to investigate a certain task/problem, following aspects need to be considered:
What are the relevant physical properties? (porosity, pemeability, seismic
velocity, density, …)
What spatial scales are relevant?
What are the field conditions? (e.g. urban, offshore, …)
Which acquisition geometries are optimal? (e.g. 2D vs 3D seismics)
Is there useful a priori information?
Is there a cheaper alternative?
The answer to these questions will depend strongly on the particular task/problem
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What is applied geophysics ?
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Why applied geophysics ?
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Applied geophysics infers property contrasts inside the earth from surface (borehole) measurements Remote sensing
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Be aware of biased data!
The data need to record the true signal
Noise is also an issue
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Be aware of a biased interpretation!
A physical theory calculates observables given appropriate model parameters and theory forward modellingThe inverse problem is often non-unique!
Several geophysical methods should be used together
Interpretation requires geological knowledge
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How many dimensions?
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How many dimensions?
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How many dimensions?
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CLASSIFICATION OF GEOPHYSICAL METHODSSeismic methods (active)• Reflection seismics• Refraction seismicsPotential field methods (passive)• Gravity• GeomagneticElectrical methods (active)• Electrical resistivity• Spontaneous (Self) Potential• Induced Polarisation
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CLASSIFICATION OF GEOPHYSICAL METHODSElectromagnetic methods (active)• Many methods among which ground penetrating radarWell logging (active and passive)
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SEISMIC METHODSMAINLY MEASURE TRAVEL TIMES AND AMPLITUDES WHICH DEPEND ON DENSITY AND ELASTIC MODULI
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SEISMIC METHODSMAINLY MEASURE TRAVEL TIMES AND AMPLITUDES WHICH DEPEND ON DENSITY AND ELASTIC MODULI
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POTENTIAL FIELD METHODSGravity measures spatial variations of the gravitational field due to lateral variations in density.
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POTENTIAL FIELD METHODSGeomagnetics measures spatial variations of the intensity of the magnetic field due to lateral variations in magnetic susceptibility.
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ELECTRO MAGNETICS
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GROUND PENETRATING RADAR
MEASURES TRAVEL TIMES OF REFLECTED RADAR WAVES
VELOCITY IS CONTROLLED BY THE DIELECTRIC CONSTANT