Copyright by Chad Allen Greene 2010

161 Pages·2011·3.05 MB·English

Copyright
by
Chad Allen Greene
2010
The Thesis Committee for Chad Allen Greene certifies
that this is the approved version of the following thesis:
Low-Frequency Acoustic Classification
of Methane Hydrates
APPROVED BY
SUPERVISING COMMITTEE:
Preston S. Wilson, Supervisor
Mark F. Hamilton
Richard B. Coffin
Low-Frequency Acoustic Classification
of Methane Hydrates
by
Chad Allen Greene, B.S.
THESIS
Presented to the Faculty of the Graduate School of
The University of Texas at Austin
in Partial Fulfillment
of the Requirements
for the Degree of
MASTER OF SCIENCE IN ENGINEERING
THE UNIVERSITY OF TEXAS AT AUSTIN
December 2010
To Dr. Wochner.
Acknowledgments
I stood on the stepstone when school days was o’er
And longed for the time to go by
Now that it’s gone, I stand here tonight
To bid this old stepstone goodbye.
Goodbye dear old stepstone, goodbye to my home
God bless the ones that I leave with a sigh
I’ll cherish fond mem’ries when I am gone
To ramble this wide world alone.
I stand on my stepstone at eventide now
The wind whistles by with a moan
The fields will be whitening, but soon I’ll be gone
Goodbye to my stepstone and home.
Goodbye dear old stepstone, goodbye to my home
God bless the ones that I leave with a sigh
I’ll cherish fond mem’ries when I am gone
To ramble this wide world alone.
’Tis sad to be parted from those that we love
Strange faces we see every day
Each heart string of mine is broken in time
When I think of those dear ones at home.
Goodbye dear old stepstone, goodbye to my home
God bless the ones that I leave with a sigh
I’ll cherish fond mem’ries when I am gone
To ramble this wide world alone.
Bascom Lamar Lunsford wrote this song about finishing school, leaving
home,andtakingontheworldbyhimself. Uponthecompletionofmymaster’s
v
degree,Ifeelmanyofhissamesentiments. However,IknowIwon’tberamblin’
this wide world alone. I know I will always have, as I have always had, the
abiding love and support of my parents. Mom and Dad, you laid down the
stepstones on my path toward this degree and I will forever appreciate and
love you for it. Thank you for all that you have done for me—I certainly
wouldn’t be here without you. Thanks also to my advisor, Preston Wilson.
Whether in class, in the lab, or at sea, you have always taken the time to help
me understand acoustics, to help me be a better scientist, and to help me find
my way in life. To all who have been there for me and helped me along this
sometimes toiling path, thank you. God bless all of you, whom I love; God
bless you, whom I leave with a sigh. Thank you to my office mates, my friends,
and my professors; thank you my dear stepstones, and goodbye.
Chad Allen Greene
vi
Low-Frequency Acoustic Classification
of Methane Hydrates
Chad Allen Greene, M.S.E.
The University of Texas at Austin, 2010
Supervisor: Preston S. Wilson
Methane hydrates are naturally-occurring ice-like substances found in
permafrost and in ocean sediments along continental shelves. These com-
pounds are often the source of cold seeps—plumes which vent methane into
aquatic environments, and may subsequently release the potent greenhouse
gas into the atmosphere. Methane hydrates and methane gas seeps are of
particular interest both for their potential as an energy source and for their
possible contribution to climate change. In an effort to improve location of
hydratesthroughtheuseofseismicsurveysandecho-soundingtechnology, this
work aims to describe the low-frequency (10 Hz to 10 kHz) acoustic behav-
ior of methane gas bubbles and methane hydrates in water under simulated
ocean-floorconditionsoflowtemperaturesandhighpressures. Productsofthe
experiments and analysis presented in this thesis include (a) passive acoustic
techniques for measurement of gas flux from underwater seeps, (b) a modified
form of Wood’s model of low-frequency sound propagation through a bubbly
vii
liquid containing real gas, and (c) low-frequency measurements of bulk mod-
uli and dissociation pressures of four natural samples of methane hydrates.
Experimental procedures and results are presented, along with analytical and
numerical models which support the findings.
viii
Table of Contents
Acknowledgments v
Abstract vii
List of Tables xii
List of Figures xiii
List of Symbols and Abbreviations xv
Chapter 1. Introduction 1
1.1 Road Map of Thesis . . . . . . . . . . . . . . . . . . . . . . . . 2
Chapter 2. Passive Acoustic Gas Flux Measurement 4
2.1 Review of Literature . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Development of Models . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Experimental Design . . . . . . . . . . . . . . . . . . . . . . . 10
2.4 Optical Measurements . . . . . . . . . . . . . . . . . . . . . . 11
2.5 Graduated Cylinder Measurements . . . . . . . . . . . . . . . 15
2.6 Acoustic Measurements . . . . . . . . . . . . . . . . . . . . . . 17
2.6.1 Low Flow Rate (LFR) Passive Recording . . . . . . . . 18
2.6.1.1 LFR Results . . . . . . . . . . . . . . . . . . . . 22
2.6.2 High Flow Rate (HFR) Passive Recording . . . . . . . . 23
2.6.2.1 HFR Results . . . . . . . . . . . . . . . . . . . 25
2.6.3 Simulated Natural Bottom (SNB) Passive Recordings . 28
2.7 Discussion of Passive Recording Measurements . . . . . . . . . 29
ix
Chapter 3. Low-Frequency Acoustics of Bubbly Liquids in a
Pressure Chamber 31
3.1 Review of Literature . . . . . . . . . . . . . . . . . . . . . . . 31
3.2 Development of Models . . . . . . . . . . . . . . . . . . . . . . 34
3.2.1 Wood’s Model . . . . . . . . . . . . . . . . . . . . . . . 34
3.2.2 Sound Propagation Models . . . . . . . . . . . . . . . . 36
3.2.3 Equations of State . . . . . . . . . . . . . . . . . . . . . 39
3.2.4 1D Acoustic Resonator . . . . . . . . . . . . . . . . . . 44
3.2.4.1 Modal Sound Speeds . . . . . . . . . . . . . . . 44
3.2.4.2 Slope Method of Calculating Sound Speed . . . 45
3.2.5 Elastic Waveguide Effect . . . . . . . . . . . . . . . . . 47
3.3 Experimental Design . . . . . . . . . . . . . . . . . . . . . . . 49
3.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.5 Discussion of Bubbly Liquids Experiment . . . . . . . . . . . . 53
Chapter 4. Low-Frequency Acoustics of Methane Hydrates in a
Pressure Chamber 56
4.1 Definition of Methane Hydrates . . . . . . . . . . . . . . . . . 56
4.2 Review of Literature . . . . . . . . . . . . . . . . . . . . . . . 58
4.3 Development of Models . . . . . . . . . . . . . . . . . . . . . . 61
4.3.1 Wood’s Model for a Two-Phase Mixture . . . . . . . . . 62
4.3.2 Wood’s Model for a Three-Phase Mixture . . . . . . . . 64
4.3.3 Hydrate Stability Models . . . . . . . . . . . . . . . . . 65
4.4 Experimental Design . . . . . . . . . . . . . . . . . . . . . . . 66
4.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.5.1 Acoustic Measurements of Bulk Moduli . . . . . . . . . 73
4.5.2 Acoustic Measurements of Dissociation Pressures . . . . 78
4.6 Discussion of Methane Hydrates Experiment . . . . . . . . . . 81
Chapter 5. Conclusions 84
Appendices 88
Appendix A. Supplemental Figures 89
x