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Introduction to Geophysics

Applied to Archaeology

Dr. Laurent Marescot

Course given at the University of Fribourg (2009)

Contact: laurent@tomoquest.com

www.tomoquest.com

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Geophysics: Definition

„Taking measurements at or near the Earth´s surface that

are influenced by the internal distribution of physical

properties. The analysis of these measurements reveals

information on the Earth´s interior (e.g., archaeological

remains)“

This could be the discovery of the century.Depending, of course, on how far down it goes…

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• Exploration of the structure and dynamic of the solid Earth

using physical methods

• Non-expensive and non-invasive methods

• Geophysical information is less localised than excavation

information (and often less expensive…) and gives a better

global picture. Geophysics can often optimize excavation

operations in archaeology

What is Geophysics?

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What is Geophysics?

Geophysical investigations at various scales...

• Global geophysics (e.g., geodynamics) 103 to 106 m

• Exploration geophysics (e.g., oil industry) 102 to 104 m

• Environmental and engineering geophysics <102 m

And using various sources...

• Natural field sources (e.g. gravitational field) or artificially generated sources (e.g. seismic waves)

• Natural field source methods are more simple to carry out but give less resolved pictures than artifical source methods

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• Hydrogeology

• Exploration of waste sites and pollutant

• Exploration for bulk mineral deposits (sand and gravel)

• Ground stability, permafrost, glaciology

• Archaeology

• Cavity detection, cable location etc

• Subsurface geological imaging for civil engineering

• Bulk modulus and shear modulus for civil engineering

• Alternative energy sources

Environmental/Engineering Geophysics

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Geophysical methods

Potential methods

• Gravity methods

• Magnetic methods

• Electrical methods

Diffusive methods

• Electromagnetic methods

Wavefield methods

• Georadar (GPR)

• Seismic methods

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Aim of the course

• Give an overview on geophysical methods applied in

archaeology

• Outline the possibilities and limitations of geophysical

techniques for archaeological applications

• Outline the basis in qualitative and quantitative

interpretation of geophysical data

• Give a general basis for more advanced studies

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Definition: Model

A model is a simple and ideal view of a physical reality

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Definition: contrast

To characterize different material using geophysics, a

contrast must exist (i.e. a difference in the physical

properties)

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Definition: Anomaly

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Noise in Geophysics

Forward and Inverse Problems

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Material properties

Rock = Matrix + Porosity + Fluid

1b ma fd d d

with :

db rock density

dma matrix density

df fluid density

porosity in %.

For example for density:

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Operative Physical Properties

• Seismic: density and elastic moduli (velocity)

• Gravity: density

• Magnetic: magnetic susceptibility and remanence

• Electrical resistivity: electrical conductivity (or resistivity)

• Electromagnetic: electrical conductivity and inductance

• Radar (GPR): dielectric constant

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Points of views

Archaeologists point of view:

• Problem: time, money

• Need: target location, extent and nature

Geophysicists point of view:

• Need:

– A priori information on target (probable shape, size, depth and composition)

– A priori information in the local geology

– Size of the survey and required accuracy for the survey

– Existing noise in the field

• Solution: several geophysical techniques are often used!

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Electrical Surveys: Mapping

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Electrical Surveys: Tomography

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Seismic Surveys

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Gravity Surveys

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Magnetic Surveys

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Electromagnetic Surveys

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Ground penetrating radar (GPR)

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Literature

BOOKSP. Keary, M. Brooks, I. Hill

An Introduction to Geophysical Exploration

2002, 3rd Edition, Blackwell Science

I. Scollar, A. Tabbagh, A. Hesse, I. Herzog

Archaeological Prospecting and Remote Sensing

1990, 1st Edition, Cambridge University Press

JOURNAL“Archaeological Prospection”, Wiley Interscience

www.interscience.wiley.com

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Web Links

Link to these documents and more:

http://www.tomoquest.com

Link to electrical surveys techniques on-line course, including animations (in

French, English and Spanish):

http://www-ig.unil.ch/cours/

Link to a database of geophysical techniques including applicability, limits

and definitions (in English):

http://www.gr.sgpk.ethz.ch