Atmospheric Acoustic Remote Sensing - Chapter 1 ppt

Stuart BradleyCRC Press is an imprint of the Taylor & Francis Group, an informa business Boca Raton London New York ATMOSPHERIC ACOUSTIC REMOTE SENSING STUART BRADLEY CRC Press Taylor &

Atmospheric Acoustic Remote Sensing

Stuart Bradley

CRC Press is an imprint of the

Taylor & Francis Group, an informa business

Boca Raton London New York

ATMOSPHERIC

ACOUSTIC REMOTE SENSING

STUART BRADLEY

CRC Press

Taylor & Francis Group Boca Raton London New York CRC Press is an imprint of the

Taylor & Francis Group, an informa business

CRC Press

Taylor & Francis Group

6000 Broken Sound Parkway NW, Suite 300

Boca Raton, FL 33487-2742

© 2008 by Taylor & Francis Group, LLC

CRC Press is an imprint of Taylor & Francis Group, an Informa business

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Library of Congress Cataloging-in-Publication Data

Bradley, Stuart.

Atmospheric acoustic remote sensing / author, Stuart Bradley.

p cm.

Includes bibliographical references and index.

ISBN 978-0-8493-3588-4 (hardback : alk paper)

1 Atmosphere Remote sensing 2 Echo sounding I Title

Preface xi

Acknowledgments xiii

Author xv

Symbol List xvii

Chapter 1 What Is Atmospheric Acoustic Remote Sensing? 1

1.1 Direct Measurements and Remote Measurements 1

1.2 How Can Measurements Be Made Remotely? 2

1.3 Passive and Active Remote Sensing 3

1.4 Some History 3

1.5 Why Use Acoustics? 4

1.6 Direct Sound Propagation from a Source to a Receiver 4

1.7 Acoustic Targets 5

1.8 Creating Our Own Target 5

1.9 Modern Acoustic Remote Sensing 6

1.10 Applications 7

1.11 Where to from Here? 8

References 9

Chapter 2 The Atmosphere Near the Ground 11

2.1 Temperature Profiles near the Surface 11

2.2 Wind Profiles near the Surface 13

2.3 Richardson Number 16

2.4 The Prandtl Number 17

2.5 The Structure of Turbulence 18

2.6 Monin-Oboukhov Length 20

2.7 Similarity Relationships 20

2.8 Profiles ofCT2 andCV2 22

2.9 Probability Distribution of Wind Speeds 23

2.10 Summary 23

References 25

Chapter 3 Sound in the Atmosphere 27

3.1 Basics of Sound Waves 27

3.2 Frequency Spectra 30

3.3 Background and System Noise 32

3.4 Reflection and Refraction 34

3.5 Diffraction 36

3.6 Doppler Shift 37

vi Atmospheric Acoustic Remote Sensing

3.7 Scattering 39

3.7.1 Scattering from Turbulence 39

3.7.2 Intensity in Terms of Structure Function Parameters 43

3.7.3 Scattering from Rain 44

3.8 Attenuation 47

3.8.1 Losses Due to Spherical Spreading 47

3.8.2 Losses Due to Absorption 47

3.8.3 Losses Due to Scattering out of the Beam 48

3.9 Sound Propagation Horizontally 51

3.10 Summary 52

References 53

Chapter 4 Sound Transmission and Reception 55

4.1 Geometric Objective of SODAR Design 55

4.2 Speakers, Horns, and Antennas 56

4.2.1 Speaker Polar Response 56

4.2.2 Dish Antennas 57

4.2.3 Phased Array Antennas 61

4.2.4 Antenna Shading 66

4.2.5 Receive Phasing 69

4.2.6 Reflectors 70

4.3 Monostatic and Bistatic SODAR Systems 71

4.4 Doppler Shift from Monostatic and Bistatic SODARs 73

4.5 Beam Width Effects on Doppler Shift 82

4.6 Continuous and Pulsed Systems 83

4.7 Geometry of Scattering 89

4.8 The Acoustic Radar Equation 90

4.9 Acoustic Baffles 91

4.10 Frequency-Dependent Form of the Acoustic Radar Equation 96

4.11 Obtaining Wind Vectors 97

4.12 Multiple Frequencies 100

4.13 Pulse Coding Methods 100

4.14 Summary 103

References 103

Chapter 5 SODAR Systems and Signal Quality 105

5.1 Transducer and Antenna Combinations 105

5.1.1 Speakers and Microphones 105

5.1.2 Horns 108

5.1.3 Phased-Array Frequency Range 109

5.1.4 Dish Design 110

5.1.5 Designing for Absorption and Background Noise 111

5.1.6 Rejecting Rain Clutter 112

5.1.7 How Much Power Should Be Transmitted? 114

Contents vii

5.2 SODAR Timing 115

5.2.1 Pulse Shape, Duration, and Repetition 115

5.2.2 Range Gates 117

5.3 Basic Hardware Units 120

5.3.1 The Basic Components of a SODAR Receiver 120

5.3.2 Microphone Array 120

5.3.3 Low-Noise Amplifiers 121

5.3.4 Ramp Gain 122

5.3.5 Filters 123

5.3.6 Mixing to Lower Frequencies (Demodulation) 123

5.3.7 Switching from Transmit to Receive, and Antenna Ringing 126

5.4 Data Availability 127

5.4.1 The Highest Useful Range 127

5.5 Loss of Signal in Noise 128

5.5.1 Loss of Signal Due to Beam Drift 132

5.6 Calibration 134

5.6.1 Why Are Calibrations Required? 134

5.6.2 Effective Beam Angle 137

5.6.3 What Accuracy Is Required? 138

5.6.4 Calibrations against Various Potential Standards 138

5.6.5 The PIE Field Campaign Setup 140

5.6.6 Raw SODAR Data versus Mast 141

5.6.7 Numerical Filtering of Data 143

5.6.8 Correlation Method 145

5.6.9 Distribution of Wind Speed Data 147

5.6.10 Regression Slope 149

5.6.11 Variations with Height 152

5.6.12 Wind Direction Regressions 154

5.7 Summary 154

References 156

Chapter 6 SODAR Signal Analysis 157

6.1 Signal Acquisition 157

6.1.1 Sampling 157

6.1.2 Aliasing 157

6.1.3 Mixing 158

6.1.4 Windowing and Signal Modulation 160

6.1.5 Dynamic Range 160

6.2 Detecting Signals in Noise 162

6.2.1 Height of the Peak above a Noise Threshold 162

6.2.2 Constancy over Several Spectra 162

6.2.3 Not Generally Being at Zero Frequency 163

6.2.4 Shape 163

6.2.5 Scaling with Transmit Frequency 164

6.3 Consistency Methods 164

viii Atmospheric Acoustic Remote Sensing

6.4 Turbulent Intensities 166

6.4.1 Second Moment Data 167

6.5 Peak Detection Methods of AeroVironment and Metek 168

6.5.1 AeroVironment 168

6.5.2 Metek 168

6.6 Robust Estimation of Doppler Shift from SODAR Spectra 170

6.6.1 Fitting to the Spectral Peak 170

6.6.2 Estimation ofTw 174

6.7 Averaging to Improve SNR 175

6.7.1 Variance in Wind Speed and Direction over One Averaging Period 176

6.7.2 Combining Wind Data from a Number of Averaging Periods 177

6.7.3 Different Averaging Schemes for SODAR and Standard Cup Anemometers 180

6.7.4 Calculating Wind Components from Incomplete Beam Data 182

6.7.5 Which Gives Less Uncertainty: A 3-Beam or a 5-Beam System? 183

6.8 Spatial and Temporal Separation of Sampling Volumes 185

6.9 Sources of Measurement Error 188

6.9.1 Height Estimation Errors 188

6.9.2 Errors in Beam Angle 189

6.9.3 Out-of-Level Errors 190

6.9.4 Bias Due to Beam Spread 190

6.9.5 Beam Drift Effects 190

6.10 A Model for SODAR Response to a Prescribed Atmosphere 193

6.11 Summary 195

References 195

Chapter 7 RASS Systems 197

7.1 RADAR Fundamentals 197

7.2 Reflection of RADAR Signals from Sound Waves 198

7.3 Estimation of Measured Height 201

7.4 Deduction of Temperature 202

7.4.1 Doppler-RASS 202

7.4.2 Bragg-RASS 203

7.5 Wind Measurements 204

7.6 Turbulance Measurements 204

7.7 RASS Designs 204

7.8 Antennas 206

7.8.1 Baffles 207

7.9 Limitations .207

7.9.1 Range 208

7.9.2 Temperature 208

7.10 Summary 211

References 211

Contents ix

Chapter 8 Applications 213

8.1 Review of Selected Applications 213

8.1.1 Environmental Research 213

8.1.2 Boundary Layer Research 215

8.1.3 Wind Power and Loading 217

8.1.4 Complex Terrain 217

8.1.5 Sound Speed Profiles 220

8.1.6 Hazards 222

8.2 Summary 223

References 224

Appendix 1 Mathematical Background 227

A1.1 Complex Exponentials 227

A1.2 Fourier Transforms 228

A1.3 Autocorrelation and Convolution 230

A1.4 Least-squares Fitting 232

Appendix 2 Sample Data Sets and Matlab Code 235

Appendix 3 Available Systems 237

A3.1 AeroVironment Inc [California, USA] 237

A3.2 AQ Systems [Stockholm, Sweden] 237

A3.2.1 AQ500 SODAR 237

A3.3 Atmospheric Research Pty Inc [Canberra, Australia] 238

A3.4 Atmospheric Research and Technology LLC (ART) [Hawaii, USA] and Kaijo Corporation [Tokyo, Japan] 239

A3.5 Atmospheric Systems Corporation (ASC) [California, USA] 239

A3.6 METEK GmbH [Elmshorn, Germany] 240

A3.7 REMTECH SA [France] 241

A3.8 Scintec GmbH [Tübingen, Germany] 242

Appendix 4 Acoustic Travel Time Tomography 247

Appendix 5 Installation of a SODAR or RASS 249

A5.1 Guidelines for the Use of Sodar in Wind Energy Resource Assessment 249

A5.1.1 Calibration and Testing 249

A5.1.2 Operating Requirements 250

A5.1.2.1 Temperature 251

A5.1.2.2 Precipitation 251

A5.1.2.3 Vertical Range and Resolution 251

A5.1.2.4 Reliability Criteria 252

x Atmospheric Acoustic Remote Sensing

A5.1.3 Siting and Noise 253

A5.1.3.1 Acoustic Noise (Passive and Active) 253

A5.1.3.2 Electronic Noise 254

A5.1.3.3 Public Annoyance 254

A5.1.4 Power Supply and Site Documentation 254

A5.1.5 Data Collection and Processing 255

A5.1.5.1 Data Parameters and Sampling/Recording Intervals 255

A5.1.5.2 Calculation of Wind Shear 255

A5.1.5.3 Measurement Period 256

A5.1.5.4 Exclusion of Precipitation Periods 257

A5.1.6 Comparisons with Mechanical Anemometry 257

A5.1.7 Other Considerations for Incorporating SODAR Information into a Resource Assessment Program 258

A5.2 Analysis of The AeroVironment 4000 SODAR Data Processing Methods 260

A5.2.1 Parameter settings 260

A5.2.2 Interrelations of Parameters and Conditions for Parameter Values 260

A5.2.3 Use of an Artificial Signal to Verify Performance 261

References 265

In 2001 I was contacted by a consortium of research institutions and wind energy

interests with a request to provide some background information on the operational

characteristics of acoustic radars or SODARs The consortium partners had set

up and been funded for an European EU project to evaluate SODARs as a tool in

monitoring wind flows at wind turbine sites They felt reasonably confident in their

knowledge of SODARs and had purchased some instruments, but wanted to be able

to consult on any more complex issues which arose Ultimately this developed into a

relatively simple contract in which my colleague at the University of Salford, Sabine

von Hünerbein, and I delivered an intensive two-day short course on SODARs to a

small group of scientists and engineers at ECN headquarters in Amsterdam There

were two aspects of this short course which impressed themselves upon me The

first was the volume of information required to adequately cover the principles of

operation and data interpretation for SODARs performing wind measurements in the

atmospheric boundary layer The second aspect was that intelligent and extremely

technically capable people, already working in the area of wind measurement, did

not adequately obtain enough information about remote sensing instruments from

manufacturers’ information manuals and data sheets

The initial interaction with the ‘WISE’ EU consortium led to Sabine and me

being responsible for overseeing the major calibration work-package in the project

The final report from the group working on that work-package was arguably the

most comprehensive investigation of SODAR-mast calibrations But, of necessity,

that report was focused on wind energy applications and target goals for calibration

accuracy There still remained a need to make available a more general description

of SODAR and other atmospheric acoustic remote sensing principles for a wider

audience

There is a huge body of literature available in journal papers which covers

appli-cations of acoustic remote sensing methodology in sensing atmospheric properties in

the 1-km layer nearest the ground But the body of literature describing design and

operating principles is much more confined and also often rather specialized

Fre-quent requests from a range of scientists, engineers, local authorities, and other areas

indicate that there is a demand for a more comprehensive collection of information

on ‘how things work’

The difficulty in writing a book of this nature is to cover the principles of

opera-tion in detail sufficient enough that the reader is not left wondering about gaps in the

descriptions, but at the same time trying to give a more intuitive feel for interactions

between various atmospheric and instrumental components than might be found in a

pedantically accurate textbook Although the resulting book does contain

consider-able algebra, extensive use of diagrams makes for better readability and efforts have

been made to avoid the more abstruse mathematical treatments

SODARs and RASS instruments are endemic in monitoring atmospheric

bound-ary layer wind systems, turbulent transports, and thermal properties There is of

xii Atmospheric Acoustic Remote Sensing

course competition from other technologies such as mast-mounted cup anemometers

and sonic anemometers, scintillometers, radar wind profilers, LIDARs, and

radi-ometers But it is still difficult to comprehensively replace, using these alternative

technologies, the acoustic remote sensing capabilities of inexpensively providing

wind, turbulence, and temperature profiles As newer methods emerge, it is also

very important to be able to competently compare output products

For all these reasons, this book is aimed at providing a useful description of how

atmospheric acoustic remote sensing systems work and giving the reader insights

into their strengths and limitations

Stuart Bradley

Auckland, New Zealand, 2007

Getting to the point of writing a book is a long road involving many years of

per-sonal interest in the research area targeted by atmospheric acoustic remote sensing

Clearly this is not really an individual effort at all, but rests on the enthusiasm and

knowledge base of many colleagues as well as support and faith from family and

friends

The people at CRC Press, Taisuke Soda, Theresa Delforn, Jim McGovern and so

many others, have been astonishingly patient with my many excuses and delays

I am grateful to my many colleagues and friends in the “ISARS community” who

meet every second year to share their latest insights at the International Symposia for

Acoustic Remote Sensing In particular the various organisers have added so much

to this general field Of these, my friend Bill Neff of NOAA has been a never-ending

source of encouragement and inspiration (it is difficult to do something in this field

which is entirely original and which hasn’t already been visited by Bill in some way),

Margo Kallistratova adds such useful insights while maintaining a twinkle in her

eye, and Erich Mursch-Radlgruber is always so innovative with his latest hardware

developments as well as knowing all the best wine cellars in Vienna! A seminal

influence on this book has also been from the activities of the EU WISE consortium,

and in particular Sabine von Hünerbein, Ioannis Antoniou, Detlef Kindler, Hans

Jørgensen, and Manuel De Noord A number of these people have gone on to form

the nucleus of the EU UpWind remote sensing group

All the manufacturers of SODARs and RASS have been happy to discuss aspects

of their designs In particular I would like to thank Hans-Jürgen Kirtzel and

Ger-hard Peters of Metek, and Ken Underwood of ASC for their friendship and many

enjoyable analyses over a beer

Many students and Post-doctoral Fellows at the University of Auckland and the

University of Salford have worked happily with me in building my experience in

acoustic remote sensing I have really enjoyed the interaction with the group at

Uni-versity of Reading, and look forward to on-going interesting work with Janet Barlow

on urban meteorology

Sabine von Hünerbein of the University of Salford has worked extensively with

me Her pragmatic view of the science and questioning approach is refreshing and

keeps me ‘on my toes’ The two of us can seldom meet without getting into deep

discussion about some aspect of SODAR or RASS operations, and I greatly value

Sabine’s input into aspects of this book

Underpinning everything is Chrissie, my wife She is so very patient in the

panic times when deadlines loom, and endlessly encouraging in the quieter moments

When things become frustrating I know I can rely on talking with her and hearing

her words of wisdom on how to tackle priorities This book would not have been

possible without her always being there for me

Author

Stuart Bradley was born and educated in New Zealand, working as a researcher with

the New Zealand Meteorological Service and with CSIRO in Australia before taking

a position in the Physics Department at the University of Auckland He designed one

of the earliest phased-array mini acoustic radars and a number of more sophisticated

instruments, as well as consulting for major manufacturers

In 2000 Stuart accepted a Chair of Acoustics at the University of Salford in

Manchester, UK, with a highly regarded acoustics research group His work there

included profiling of winds and temperature structure to better understand outdoor

sound propagation; research relating to surface-atmosphere coupling in the

Antarc-tic; and acoustic noise in cellular phones generated by micro-turbulence He was

a partner in the EU-funded ‘WISE’ project, which aimed to use acoustic remote

sensing for evaluation of wind turbine performance Together with Dr Sabine von

Hünerbein at Salford and other collaborators in WISE, Professor Bradley produced

a comprehensive analysis of the state of the art of acoustic wind profilers

Stuart returned to the University of Auckland in mid-2004, where he is currently

Head of the Physics Department He holds a dual academic position in Salford

where he is a Professor in the Acoustics Research Centre, engaged through the EU

‘UpWind’ project in design of new acoustic technologies for wind energy

Symbol List

xviii Atmospheric Acoustic Remote Sensing

Symbol List xix

M dryair , M water molecular weights

xx Atmospheric Acoustic Remote Sensing

t time

Symbol List xxi

tdownwind, tupwindacoustic travel time downwind and upwind