APPLIED GEOPHYSICS AN INTRODUCTION

FRED BEEKMAN

JEANNOT TRAMPERT

COURSE OVERVIEW

Course 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

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.

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 …

APPLIED GEOPHYSICS

Physical 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;

APPLIED GEOPHYSICS

Passive 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

APPLIED GEOPHYSICS

To 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

What is applied geophysics ?

Why applied geophysics ?

Applied geophysics infers property contrasts inside the earth from surface (borehole) measurements Remote sensing

Be aware of biased data!

The data need to record the true signal

Noise is also an issue

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

How many dimensions?

How many dimensions?

How many dimensions?

CLASSIFICATION OF GEOPHYSICAL METHODS

Seismic methods (active)

• Reflection seismics

• Refraction seismics

Potential field methods (passive)

• Gravity

• Geomagnetic

Electrical methods (active)

• Electrical resistivity

• Spontaneous (Self) Potential

• Induced Polarisation

CLASSIFICATION OF GEOPHYSICAL METHODS

Electromagnetic methods (active)

• Many methods among which ground penetrating radar

Well logging (active and passive)

SEISMIC METHODSMAINLY MEASURE TRAVEL TIMES AND AMPLITUDES WHICH DEPEND ON DENSITY AND ELASTIC MODULI

SEISMIC METHODSMAINLY MEASURE TRAVEL TIMES AND AMPLITUDES WHICH DEPEND ON DENSITY AND ELASTIC MODULI

POTENTIAL FIELD METHODS

Gravity measures spatial variations of the gravitational field due to lateral variations in density.

POTENTIAL FIELD METHODS

Geomagnetics measures spatial variations of the intensity of the magnetic field due to lateral variations in magnetic susceptibility.

ELECTRO MAGNETICS

GROUND PENETRATING RADAR

MEASURES TRAVEL TIMES OF REFLECTED RADAR WAVES

VELOCITY IS CONTROLLED BY THE DIELECTRIC CONSTANT