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Tom Wilson, Department of Geology and Geography
Environmental and Exploration Geophysics II
Department of Geology and GeographyWest Virginia University
Morgantown, WV
IntroIntroWave Types and Travel Paths Wave Types and Travel Paths
-- Example ApplicationsExample 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
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Tom Wilson, Department of Geology and Geography
Grading25% on computer labs,
30% problem sets,
10% mid term exam,
20% Expl project/term report and class presentation,
15% final exam.
Tom Wilson, Department of Geology and Geography
For more information on the IB competition see http://www.aapg.org/iba/schedule.cfm
& http://www.geo.wvu.edu/~wilson/geol554/Barrel09Descript.pdf
Exploration Project Option
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.
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Tom Wilson, Department of Geology and Geography
http://www.geo.wvu.edu/~wilson/IBCWorkshop/Kingdom_for_IBC.htm
Imperial Barrel Seismic Resources
Tom Wilson, Department of Geology and Geography
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.
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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 assessmentIII. 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
format of the course, course content, grading … etc?
Check your final exam schedule at http://registrar.wvu.edu/current_students/spring_exams
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Tom Wilson, Department of Geology and Geography
How are seismic data sets like those shown below created?
Tom Wilson, Department of Geology and Geography
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
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Tom Wilson, Department of Geology and Geography
See http://en.wikipedia.org/wiki/Audio_frequency
Audible frequency range
Tom Wilson, Department of Geology and Geography
Translating up and down surface vibrations into electromagnetic signals
Class Demo
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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
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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 binary idea.
Tom Wilson, Department of Geology and Geography
Sample Number0 5 10 15 20
Rec
ordi
ng A
mpl
itude
-5
0
5
The 8 bit recording range corresponds to a -128 to 127 range of integer values – no decimals!
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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
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Tom Wilson, Department of Geology and Geography
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
Shot records – Upshur Co., WV
Redstone Coal
Very shallow high resolution “hammer”
data
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Tom Wilson, Department of Geology and Geography
Different kinds of waves … Body Waves
Tom Wilson, Department of Geology and Geography
Surface Waves
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Tom Wilson, Department of Geology and Geography
Body vs. “Surface” Waves
Tom Wilson, Department of Geology and Geography
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.
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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.
Single component vertical motion detectors
Tom Wilson, Department of Geology and Geography
Breaking seismic disturbances down into their component parts
Some nomenclature
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Tom Wilson, Department of Geology and Geography
time and frequency
Tom Wilson, Department of Geology and Geography
wavelength and wavenumber
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Tom Wilson, Department of Geology and Geography
Changing the sinusoid arguments allows us to translate time or distance of travel into a position on the sinusoid
2 2Amplitude = sin or sin
through substitution for or =v , we get two additional forms
2 2Amplitude = sin or sin
x t
x vtvt x
v
π πλ τ
λ τπ πλ τ
=
Temporal
form
Spatial
form
Tom Wilson, Department of Geology and Geography
The seismic wavelet
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Tom Wilson, Department of Geology and Geography
The wavelet or pulse is a transient disturbance – it comes and goes
Tom Wilson, Department of Geology and Geography
Spatial view
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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
See SumofCosines.xls on the class page
Tom Wilson, Department of Geology and Geography
The wavelet
Any time series can be represented as a sum of sinusoids
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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 …
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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
The
migrated
stack
Tom Wilson, Department of Geology and Geography
Those reflection events start off looking completely different than you are used to
seeing in the migrated stack section
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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
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Tom Wilson, Department of Geology and Geography
Ground roll = noise to the exploration geophysicist
Tom Wilson, Department of Geology and Geography
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, groundpenetrating radar.
Please look through chapters 2 through 4 and note that geophysics has substantial quantitative underpinnings.
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