Transcript Intro Lecture (Fall 05) - West Virginia University

Environmental and Exploration Geophysics II
Intro Lecture (Fall 05)
Wave Types and Travel Paths Example Applications
tom.h.wilson
[email protected]
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
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!
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.
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 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
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
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
The
wavelet
Tom Wilson, Department of Geology and Geography
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
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
Tom Wilson, Department of Geology and Geography