Intro Wave Types and Travel Paths -Example...

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Tom Wilson, Department of Geology and Geography

Environmental and Exploration Geophysics II

[email protected]

Department of Geology and GeographyWest Virginia University

Morgantown, WV

IntroIntroWave Types and Travel Paths Wave Types and Travel Paths

-- Example ApplicationsExample Applications


Environmental and Exploration Geophysics II

Course Syllabus

•Prerequisites

•Grading

•Exploration Project

•Term Report

•Computer Software/ Computer Labs


Grading25% on computer labs,

30% problem sets,

10% mid term exam,

20% Expl project/term report and class presentation,

15% final exam.

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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.


http://www.geo.wvu.edu/~wilson/IBCWorkshop/Kingdom_for_IBC.htm

Imperial Barrel Seismic Resources


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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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.


format of the course, course content, grading … etc?

Check your final exam schedule at http://registrar.wvu.edu/current_students/spring_exams


Seismic Ears or ground vibration sensors

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Translating up and down surface vibrations into electromagnetic signals

Class Demo


These signals are sampled at discrete points. They are not continuous or analog recordings


Counting in base 2

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Although recording is no longer done on magnetic tape, the tape analogy is a good way to visualize the idea.


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!


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

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Liner and Liner, 1995

Profile data - Processed GPR profile


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.


Shot records – Upshur Co., WV

Redstone Coal

Very shallow high resolution “hammer”

data

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Different kinds of waves … Body Waves


Surface Waves


Body vs. “Surface” Waves

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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.


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


Breaking seismic disturbances down into their component parts

Some nomenclature

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time and frequency


wavelength and wavenumber


Sinusoid arguments

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

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The seismic wavelet


The wavelet or pulse is a transient disturbance – it comes and goes


Spatial view

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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


The wavelet

Any time series can be represented as a sum of sinusoids


How do mechanical waves get from point A to B

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You can go straight there or …


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


Those reflection events start off looking completely different than you are used to

seeing in the migrated stack section

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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!


The seismic diffraction event may seem different that it’s optical cousin

But it all boils down to a point


Ground roll = noise to the exploration geophysicist

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Things to do

• Skim through chapter 1

• Read chapter 2 – pages 7 through 39


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.

Intro Wave Types and Travel Paths -Example...

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