Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics II tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Intro Lecture (Fall 05) Wave Types and Travel Paths - Example Applications

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

Tom Wilson, Department of Geology and Geography Grading 30% on computer labs, 30% problem sets, 10% mid term exam, 15% term report/project and class presentation, 15% final exam.

Tom Wilson, Department of Geology and Geography 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 Do you have any general questions about the format of the course, course content, grading … etc?

Tom Wilson, Department of Geology and Geography Seismic Ears or ground vibration sensors

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

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

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

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

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

Tom Wilson, Department of Geology and Geography 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). 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. Dynamic Range

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

Tom Wilson, Department of Geology and Geography Miller et al 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.

Tom Wilson, Department of Geology and Geography Shot records – Upshur Co., WV

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

Tom Wilson, Department of Geology and Geography Surface Waves

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

Tom Wilson, Department of Geology and Geography In general V R <V L <V S <V P 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 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

A Wave Packet or Wavelet Time domain and frequency domain a different way of viewing the time series

Tom Wilson, Department of Geology and Geography The wavelet

Tom Wilson, Department of Geology and Geography

How do mechanical waves get from point A to B

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

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

Tom Wilson, Department of Geology and Geography Those reflection events start off looking completely different

Tom Wilson, Department of Geology and Geography 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 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