Environmental and Exploration Geophysics II

1. Environmental and Exploration Geophysics II Intro Wave Types and Travel Paths - Example Applications tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Tom Wilson, Department of Geology and Geography

2. Environmental and Exploration Geophysics II • Course Syllabus • Prerequisites • Grading • Exploration Project • Term Report • Computer Software/ Computer Labs Tom Wilson, Department of Geology and Geography

3. Grading 25% on computer labs, 30% problem sets, 10% mid term exam, 20% (Expl project – IB or other)/term report and class presentation, 15% final exam. Tom Wilson, Department of Geology and Geography

4. Exploration Project Option vs. Term Report An exploration project option is centered around the AAPG Imperial Barrel data set used in their annual competition. If you wish to join other students in this competition it is possible to fashion a term report effort around the IB data sets that focuses on prospect development with an emphasis on seismic interpretation. If you wish to do that then you can sign up for the Imperial Barrel Seismic Lab. Lab participation requires that you attend the Barrel meetings and the regional competition in Pittsburgh (late March/early April). The data sets usually arrive early in January. Section and regional competition will be held sometime during the month of March. It all happens pretty fast and will consume a lot of your time. For more information on the IB competition see http://www.aapg.org/iba/schedule.cfm & http://www.geo.wvu.edu/~wilson/geol554/Barrel09Descript.pdf Tom Wilson, Department of Geology and Geography

5. Imperial Barrel Seismic Resources http://www.geo.wvu.edu/~wilson/IBCWorkshop/Kingdom_for_IBC.htm Tom Wilson, Department of Geology and Geography

6. Imperial Barrel Component Note that while Geol 554 will accommodate student interests to participate in the competition. IB competition is not a requirement and the class content is not focused on analysis of IB data sets. Tom Wilson, Department of Geology and Geography

7. Course Objectives I. Provide discussions of basic principles in reflection and refraction seismology and ground penetrating radar. II. Provide experience with the application of these principles to non-invasive investigations of subsurface conditions important to environmental assessment of hazardous waste sites and groundwater exploration, engineering studies, and resource exploration. Student choice of term project and term report help the student focus geophysical applications on their primary interest whether exploration or environmental assessment III. Provide an introduction to computer modeling as a problem solving tool. IV. Develop oral-presentation and report-writing skills. Tom Wilson, Department of Geology and Geography

8. Questions? format of the course, course content, grading … etc? Check your final exam schedule at http://registrar.wvu.edu/current_students/spring_exams Tom Wilson, Department of Geology and Geography

9. How are seismic data sets like those shown below created? Tom Wilson, Department of Geology and Geography

10. We’ll be covering a lot of the basics. Looking inside the box > Seismic Ears or ground vibration sensors Audible frequency range is about 20 to 10,000 Hz. When you look at a seismic section you are seeing ground vibrations that for conventional seismic data fall in the 10 to 120 Hz range Tom Wilson, Department of Geology and Geography

11. Audible frequency range See http://en.wikipedia.org/wiki/Audio_frequency Tom Wilson, Department of Geology and Geography

12. Translating up and down surface vibrations into electromagnetic signals Class Demo Tom Wilson, Department of Geology and Geography

13. These signals are sampled at discrete points. They are not continuous or analog recordings Tom Wilson, Department of Geology and Geography

14. Counting in base 2 Tom Wilson, Department of Geology and Geography

15. Although recording is no longer done on magnetic tape, the tape analogy is a good way to visualize binary idea. Tom Wilson, Department of Geology and Geography

16. The 8 bit recording range corresponds to a -128 to 127 range of integer values – no decimals! Tom Wilson, Department of Geology and Geography

17. Dynamic Range Dynamic range refers to the number of bits available to store information An 8 bit (1 byte) record allows one to store numbers in the range -128 to 127 A 32 bit record allows one to store numbers in the range -2,147,483,648 to 2,147,483,647. To see additional detail in the ground motion - to measure the “fractional” motion - you need to increase the dynamic range of the recording system. The engineering seismograph we demonstrated in class today is restricted primarily to the shallower applications since events that have traveled great distances will have very small amplitude (less than 1on the scale of ±128). Tom Wilson, Department of Geology and Geography

18. Profile data - Processed GPR profile Liner and Liner, 1995 Tom Wilson, Department of Geology and Geography

19. Miller et al. 1997 Seismic and GPR methods both record waves that have been reflected from subsurface interfaces. In the one case (GPR) these waves are electromagnetic (and much faster), in the other (Seismic) they are acoustic or mechanical waves (and much slower). Tom Wilson, Department of Geology and Geography

20. Shot records – Upshur Co., WV Very shallow high resolution “hammer” data Redstone Coal Tom Wilson, Department of Geology and Geography

21. Different kinds of waves … Body Waves Tom Wilson, Department of Geology and Geography

22. Surface Waves Tom Wilson, Department of Geology and Geography

23. Body vs. “Surface” Waves Tom Wilson, Department of Geology and Geography

24. In general VR <VL <VS <VP But this is not strictly true. The Love wave is a surface wave and its velocity will be equal to the shear wave velocity in the upper medium. The Love wave like the Rayleigh wave is also a dispersive wave. That means that deeper Love wave motion usually propagates more rapidly since velocity increases with depth. Shear waves beneath the surface layers are generally much faster than those in the surface, so in application, the shear waves that we are concerned with generally have higher velocity than the Love waves. Tom Wilson, Department of Geology and Geography

25. Single component vertical motion detectors Love waves tend not to be recorded in the conventional seismic survey where the interest is primarily in the recording of P-waves. The geophones used in such surveys respond to vertical ground motion and thus do not respond to the side-to-side vibrations produced by Love waves. Rayleigh waves produce large vertical displacements and are a significant source of “noise” in the conventional P-wave reflection seismic survey. Tom Wilson, Department of Geology and Geography

26. Breaking seismic disturbances down into their component parts Some nomenclature Tom Wilson, Department of Geology and Geography

27. time and frequency Tom Wilson, Department of Geology and Geography

28. wavelength and wavenumber Tom Wilson, Department of Geology and Geography

29. Changing the sinusoid arguments allows us to translate time or distance of travel into a position on the sinusoid Temporal form Spatial form Tom Wilson, Department of Geology and Geography

30. The seismic wavelet Tom Wilson, Department of Geology and Geography

31. The wavelet or pulse is a transient disturbance – it comes and goes Tom Wilson, Department of Geology and Geography

32. Spatial view Tom Wilson, Department of Geology and Geography

33. Time domain and frequency domain a different way of viewing the time series A Wave Packet or Wavelet See SumofCosines.xls on the class page Tom Wilson, Department of Geology and Geography

34. Any time series can be represented as a sum of sinusoids The wavelet Tom Wilson, Department of Geology and Geography

35. How do mechanical waves get from point A to B Tom Wilson, Department of Geology and Geography

36. You can go straight there or … Tom Wilson, Department of Geology and Geography

37. The reflection events we see in a seismic section don’t start off looking like this Fruitland coals- San Juan Basin, NM The migrated stack The geologist usually immediately starts to see layers, stratigraphy, depositional history, structure… Tom Wilson, Department of Geology and Geography

38. Those reflection events start off looking completely different than you are used to seeing in the migrated stack section Tom Wilson, Department of Geology and Geography

39. When we bang on the ground, the Earth speaks back in a variety of ways This time-distance record shows everything coming in with different shapes, sometimes almost at the same time and sometimes earlier, sometimes later. A real mess! Tom Wilson, Department of Geology and Geography

40. The seismic diffraction event may seem different that it’s optical cousin But it all boils down to a point Tom Wilson, Department of Geology and Geography

41. Ground roll = noise to the exploration geophysicist Tom Wilson, Department of Geology and Geography

42. The fine print! I should emphasize that this is not just a seismic interpretation course. Seismic interpretation will hopefully be an enjoyable part of the course, but the class will include considerable quantitative effort. The math level required to do work in this class is basic: algebra & trigonometry for the most part. This effort falls under the first objective of the course and that is for you to walk away from this course with an appreciation of the basic physics of seismic, and to much smaller degree, ground penetrating radar. Please look through chapters 2 through 4 and note that geophysics has substantial quantitative underpinnings. Tom Wilson, Department of Geology and Geography

43. Things to do • Skim through chapter 1 • Read chapter 2 – pages 7 through 39 • Work problems 2.1 and 2.2 and hand in Wednesday Tom Wilson, Department of Geology and Geography