ES 2220b Environmental and Exploration Geophysics I Introduction and Outline

Contact Information Instructor: Dr. Karen Assatourians Office: B&GS 1012b E-mail: kassatou@uwo.ca Office hours: Tuesday, 2:30 – 4:00 pm. TA’s: Ryan Kahue (rkahue@uwo.ca); Derek Kouhi (dkouhi@uwo.ca)

Course Description This course provides a brief introduction to the discipline of exploration geophysics with emphasis on seismology. Seismology - the branch of geophysics that investigates the Earth’s structure, and earthquakes using sound waves in rocks. Seismology is to the Earth Sciences what radiology is to Medicine; it is our window into the Earth's interior, providing a way to map and study the inner workings of the Earth and planets at different scales.

Outline Seismic wave theory: Wave propagation, seismic energy sources and recording devices, stress and strain. Seismic refraction method: Basic principles and techniques. Seismic reflection method: Data acquisition, basic processing, interpretation, 3-D methods and case histories. Earthquakes and global seismology. Introduction to Ground Penetrating Radar.

Textbooks Required: Mussett, A.E. and Khan, M.A., Looking into the Earth: An Introduction to Geological Geophysics, Cambridge University Press, 2000. Recommended: Kearey, P., Brooks, M., Hill I., An Introduction to Geophysical Exploration, Blackwell, 2002. Reynolds, John M., An Introduction to Applied and Environmental Geophysics, Wiley, 2011.

Textbooks Recommended: Sheehan, H.R., Jones, A.F., and Burger C.H., Introduction to Applied Geophysics: Exploring the Shallow Subsurface, W. W. Norton & Company, 2006. Lay, T. and Wallace, T.C., Modern Global Seismology, Academic Press, 1995. Sheriff, R., and Geldart, L., Exploration Seismology, Cambridge University Press, 1995.

Course Work Assignments will consist of examination-style answer questions, and require no formal writeup. Late submissions will be accepted with a 5% per day penalty. In many cases, assignments will require the use of Matlab. Under exceptional circumstances, late submissions will be accepted with no penalty, provided that adequate documentation is given. With a few exceptions, only SI units should be used to report any physical quantities.

Course Work The project will involve a written report (5 pages + figures, single-sided, double-spaced). The topic will be chosen by the student and approved by the instructor before February 11. Research topics must be in any area of geophysics covered during the course. The project must include references to the scientific literature. Projects are due April 9.

Course Work The midterm exam will be held during the class period on Thursday, February 12 or Tuesday, February 23. (lets decide this week). The final exam will be two hours in length and will take place during the April examination period. For both exams, a single-sided hand-written crib sheet and a non-programmable calculator may be used. Grades, assignments, etc. are going to be posted on OWL (http://owl.uwo.ca).

Evaluation Assignments – 20% Project – 20% Midterm Exam – 25% Final Exam – 35%

What is Geophysics? Overview

What is Geophysics? The science of geophysics is the application of the principles of physics to the study of the Earth and planets. A huge subject that includes the physics of space and atmosphere, of the oceans, the interior of the Earth and planets. At the heart of geophysics is the theory of the solid Earth.

What is Geophysics? Geophysical investigations – taking measurements at or near the Earth’s surface. These measurements are influenced by the internal distribution of physical properties. Analysis of these measurements reveal how the physical properties of the Earth interior vary vertically and laterally.

What is Geophysics? Internal structure of the Earth: Crust: 0-60 km Mantle: 60-2900 km Core: 2900-6370 km

What is Geophysics? Examples of application of physical concepts: Magnetism  Earth magnetic field; presence of the ores, etc. Gravitational forces (Newton’s and Einstein theories)  Shape of the Earth and planets, existence of the solar system. Earthquakes  internal composition of the Earth, plate tectonics, etc.

What is Geophysics? Three aspects to Geophysics: “Pure” geophysics (ex. how the Earth’s magnetic field is produced). Understanding the physical principles. Exploration geophysics as an aid to geology (such as finding ores, extracting oil and gas, etc.). Earth, planets, and the solar system as a giant laboratory (earthquakes, etc.).

Geophysical methods Geophysical exploration methods (geophysical surveying) may be applied to a wide range of investigations: Prospecting of hydrocarbon and mineral deposits; Global geophysics; Exploration of the subsurface for engineering and environmental purposes; Archeology, forensic investigations, etc.

What is Geophysics? Advantages of geophysics to geologist: Make observations about subsurface using measurements taken at the surface. Can truly “look” into the Earth’s interior (ex. using earthquakes to study internal composition of the Earth, detect buried bodies and structures, etc.). Geophysics can add the third dimension.

What is Geophysics? A geologist describes rocks and structures mainly visually. A geophysicist measures physical properties of rocks: density, porosity, elastic moduli, etc; whether they are magnetic, electrically conducting, radioactive, and so on.

What is Geophysics? Martian topography and gravity anomaly (Zuber et al. Science, 2003)

What is Geophysics? A block of subsurface from the Forties oil field in the North Sea: Mussett and Khan, 2000

What is Geophysics? In summary: Geophysical methods are the investigation of the rocks and structures of the subsurface, by making (usually) physical measurements at the surface, or – in the case of radiometric dating and paleomagnetism – on samples. Geophysics does not replace geological investigation but complements it.

What is Geophysics? The exposed strata has the same features at the surface. Geophysical methods can detect various structures at depth. Such as faults, intrusions, salt domes, etc. Mussett and Khan, 2000

What is Geophysics? Limitations of geophysical surveying: Most methods give information only about the present. Some problems have non-unique solutions. Additional assumptions and constraints have to be taken into account. Data analysis and interpretation can be computer intensive and complex.

What is Geophysics? Advantages of geophysical surveying: Some geophysical methods reveal dynamic processes operating in the Earth. The measured physical properties are as much as vital as a visual description and provide information hard to find in any other way. For example: magnetism of a rock can be used to determine at which latitude it formed.

Seismology Overview

Seismology Seismology is concerned with determining the internal structure of the Earth – on all scales. It involves the analysis of the propagation of subsurface acoustic or elastic waves generated by different sources (natural and artificial).

Exploration Seismology Overview

Exploration Seismology Use the propagation of seismic waves to infer the geometry of geological structures at depth, such as: Top of the water table. Geometry of a salt diapir. Depth of the Moho (boundary between crust-mantle) etc. The techniques involved: Refraction / reflection of seismic waves, Use of active sources and earthquake data.

Exploration Seismology Salt diapirs and domes and oil trappings.

Refraction Seismology

Refraction Seismology Powerful and relatively cheap method for finding the depths to approximately horizontal seismic interfaces on all scales: local site investigations, continental studies. The method allows the estimation of seismic velocities. This method uses a particular case of refraction in which the refracted ray runs along an interface, while sending rays back up to the surface.

Refraction Seismology Crustal model for the Western U.S.: Mooney and Weaver, 1989

Crustal Thickness North American crustal thicknesses. The Caribbean margin in Colombia. Pimiento, 2009

Reflection Seismology

Reflection Seismology Seismic reflection surveying is a widely used geophysical technique. The most important tool for detailed imaging of layering within the Earth. It can reveal structural features such as folding and faulting. Its predominant applications are hydrocarbon exploration and research into crustal structure. Hydrological studies of aquifers. Shallow coal exploration.

Reflection Surveying Seismic reflection survey of the crustal structure of the Newfoundland Appalachians (from Lithoprobe 2005).

Global Seismology Overview

Internal Structure of the Earth Mechanical layers: Lithosphere Asthenoshere Mesosphere

Seismic Tomography Seismic structure of the Earth’s mantle van der Hilst and Karason, 1999

Reading for the next Lecture Mussett, A.E. and Khan, M.A., pages 7-12.