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
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
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
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
Imperial Barrel Seismic Resources
http://www.geo.wvu.edu/~wilson/IBCWorkshop/Kingdom_for_IBC.htm
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.
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
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
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
Tom Wilson, Department of Geology and Geography
Audible frequency range
See http://en.wikipedia.org/wiki/Audio_frequency
Tom Wilson, Department of Geology and Geography
Translating up and down surface vibrations
into electromagnetic signals
Class Demo
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 binary 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!
Recording Amplitude
5
0
-5
0
5
10
Sample Number
Tom Wilson, Department of Geology and Geography
15
20
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
Profile data - Processed GPR profile
Liner and Liner, 1995
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
Very shallow high
resolution “hammer”
data
Redstone Coal
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 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
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
Breaking seismic disturbances
down into their component parts
Some nomenclature
Tom Wilson, Department of Geology and Geography
time and frequency
Tom Wilson, Department of Geology and Geography
wavelength and wavenumber
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
Amplitude = sin
2 x

or sin
2 t

through substitution for
x  vt or  =v , we get two additional forms
Amplitude = sin
Tom Wilson, Department of Geology and Geography
2 vt

2 x
or sin
v
The seismic wavelet
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
Tom Wilson, Department of Geology and Geography
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
Any time series can be
represented as a sum of sinusoids
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
Fruitland
coalsSan Juan
Basin, NM
The geologist usually immediately starts to see layers,
stratigraphy, depositional history, structure…
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
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
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, 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
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