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  • Geophysics aims to interpret the subsurface by combining observations of the Earth Materials (geology) with observations of physical phenomena (physics). A cross section illustrating the surface geology, with no subsurface information. Model of a physical parameter, the change in Earth’s gravity field that would result from a sphere of radius R and density contrast of ( δρ  The mathematical equation predicts the change in gravity caused by such a mass buried at a depth, z, below the surface. Observed change in gravity field, along with a model of subsurface density distribution that might cause such a change. The model agrees with the observed surface geology and shows density distributions that results in a predicted gravity anomaly close to that observed.
  • Both forward and inverse methods are used to interpret geophysical observations. In each case, we want to know “what the cause of our observations”. A forward modeling method assumes a subsurface model and calculates its predictions. By adjusting the model parameters, we try to find the best model that fit the predictions with observations.
  • While inversion uses mathematical equations to calculate a subsurface model from observation data. For example, shown in the upper left are observed seismograms. The arrival times of both direct and critically refracted waves can be picked from seismograms. (b) A two-layer model showing parameters that can be read from the observed data: slope of the direct arrival; T-axis intercept time; slope of the refracted arrival. c) The mathematical equations that related the observed data with model parameters (v1, v2, h). d) The result from solving the equations in c).

    1. 1. Introduction to Geophysics Ali O. Oncel [email_address] Department of Earth Sciences KFUPM Lecture 1 Overview Introduction to Seismology-KFUPM
    2. 2. Dr. Ali O. Oncel: Building 26, Room:319 (oncel@kfupm.edu.sa) Saturday : 8:00-9:00 PM room 319 Monday : 8:00-9:00 PM room 319 Wednesday : 8:00-9:00 PM room 319 Any time you can ask me an appointment through e-mail. Scheduled Office Hours Introduction to Seismology-KFUPM
    3. 3. Parasnis, P principles of Applied Geophysics 5E , 1997 R.J. Lillie , Whole Earth Geophysics , Prentice Hall, 1999. Recommended Text and Grading Introduction to Seismology-KFUPM GEOP 204: % Final Exam 35 Comprehensive Exam 30 Quiz 15 Homework 15 Class Participation on time 5 Monday is day for weekly exam. Wednesday is day for field-trip or work.
    4. 4. <ul><li>Homework is due one week after it is assigned </li></ul><ul><li>Late homework will not be accepted one week after the deadline. </li></ul><ul><li>Late homework will be penalized: </li></ul>Homework Introduction to Seismology-KFUPM 80% for 1 day late 60% for 2 day late 40% for 3 day late 20 % for 4 day late 10% for 5 day late.
    5. 5. Scope of Course <ul><li>Providing knowledge more about the mathematical and physical aspects of Applied and solid Earth geophysics such as the currently accepted physical models used to describe the Earth's interior and evolution ………………………….. </li></ul><ul><li>Providing knowledge about the most important datasets and methods which have been used to learn about Applied and Solid Earth Geophysics ………………... </li></ul>Introduction to Seismology-KFUPM
    6. 6. Geology + Physics Geophysics Geophysics aims to interpret the subsurface by combining observations of the Earth Materials (geology) with observations of physical phenomena (physics). Introduction to Seismology-KFUPM
    7. 7. Forward Modeling (1) what the cause of our observations? By adjusting the model parameters, we try to find the best model that fit the predictions with observations. Introduction to Seismology-KFUPM
    8. 8. Applied Seismology Refraction Reflection Seismic refraction • Used to study large scale crustal layering: thickness and velocity Seismic reflection • “ Imaging” of subsurface reflectors • Difficult to determine accurate velocities and depths Introduction to Seismology-KFUPM
    9. 9. Inversion (2) Introduction to Seismology-KFUPM
    10. 10. Interpretations As for the interpretations of geophysical data, I just want emphasize that most interpretations are actually “ non-unique”. That means more than one interpretation can explain the data. The problem is like that of the “blind men and the elephant;” Thus, all the available information must be synthesized in order to obtain a better picture of the subsurface structure. It is wise to use the words like “uncertain, possibly, unclear suggestions…” when you describe your interpretations. However in the real literature, you may see that a lot of authors still use the words like “ certain, undoubtedly, positive proof, strongly suggested …” Introduction to Seismology-KFUPM
    11. 11. Seismic refraction profile recorded in ocean basin, Sheriff and Geldart, 1995 S – Reflections from bottom of water Refraction Interpretation D D - Direct Arrival P P - Lower Crustal Refraction within the crust and from the Moho S Introduction to Seismology-KFUPM
    12. 12. Fowler, 1990 Seismic Refraction Interpretation Introduction to Seismology-KFUPM
    13. 13. Reflection Interpretation Bowen and White, 1986 A migrated seismic-reflection profile across the Vema Transform Fault on the Mid-Atlantic Ridge at 6°N, approximately midway between the African and South American plates. Introduction to Seismology-KFUPM