Applied Geophysics An Introduction Fred Beekman Jeannot Trampert

Course overview Course content Overview 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. Aim Understanding 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: Gravity Earth’s magnetic field Earthquakes Active methods: Use man-made source to image structure of subsurface Dynamite, air guns, … EM waves Electrical 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 modelling The 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 methods Mainly measure travel times and amplitudes which depend on density and elastic moduli

Seismic methods Mainly 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