Analysis of a Sub-Bottom Sonar Profiler for Surveying Underwater Archaeological Sites Amy ...

101 Pages·2014·9.23 MB·English

Analysis of a Sub-Bottom Sonar Profiler for
Surveying Underwater Archaeological Sites
by
Amy Vandiver
Submitted to the Department of Electrical Engineering and Computer
Science
in partial fulfillment of the requirements for the degree of
Master of Engineering in Electrical Engineering and Computer Science
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
May 2002
© Amy Vandiver, MMII. All rights reserved.
The author hereby grants to MIT permission to reproduce and
distribute publicly paper and electronic copies of this thesis document
in whole or in part.
A uthor .................
Department of Electrical Engineering and Compyter Science
Oay 24, 2002
Certified by..........
. . %, ... . . . . . . . . . . . . . . .
David A. Mindell
Professor,
Thesis Supervisor
Accepted by.........
Arthur C. Smith
Chairman, Department Committee on Graduate Students
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Analysis of a Sub-Bottom Sonar Profiler for Surveying
Underwater Archaeological Sites
by
Amy Vandiver
Submitted to the Department of Electrical Engineering and Computer Science
on May 24, 2002, in partial fulfillment of the
requirements for the degree of
Master of Engineering in Electrical Engineering and Computer Science
Abstract
Imaging buried objects with bottom penetrating sonar systems is a research problem
of interest to archaeologists as well as the defense community and geologists. The
deep sea archaeology setting brings a unique set of design constraints to this field,
namely high resolution imaging and limited depth of penetration. A prototype high-
frequency sub-bottom profiler was designed and built by David Mindell and Marine
Sonic Technologies,Inc. The characteristics and limitations of this prototype are
analyzed in this thesis with the intent of improving our ability to interpret the data
that it collects. By characterizing the transducer and the signal processing electronics
it was possible to collect quantitative field data with the sensor and compare it with a
model of the system. In addition, several sources of error are identified and suggestions
for improving the system are made.
Thesis Supervisor: David A. Mindell
Title: Professor, Science Technology and Society
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Acknowledgments
I would like to acknowledge my thesis advisor David Mindell and the DeepArch
research group at MIT including Brian Bingham, Brendan Foley, Aaron Broody,
Katie Croff, Johanna Mathieu, and the students in STS.476. I would also like to
thank my academic advisors Gill Pratt and Frans Kaashoek for their guidance during
the time I have spent at MIT.
My Mother, Father and Step-Mother have been a great source of inspiration and
motivation for me over the years and I would not be where I am today without them.
Finally, I would like to thank Terry Smith for his endless patience with me this spring
and Ben Vandiver for keeping me on track and providing moral support.
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Contents
1 Introduction 13
1.1 Acoustic Profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2 Deep Water Archaeology . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.3 Precision Navigation . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2 Related Work 21
2.1 Sub-Bottom Profilers ........ . . . . . . . . . . . 22
2.1.1 Chirp Signals . . . . . . . . . . . . . . . . . . . 22
2.1.2 Buried Object Detection . . . . . . . . . . . . . 23
2.1.3 Scattering, Attenuation and Acoustic Modeling 24
2.2 Medical Ultrasound . . . . . . . . . . . . . . . . . . . . 25
3 Description of the Existing System 29
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2 Ashkelon Shipwreck Data..... . . . . . . . . . . . . . . . . . . . 30
3.3 Monitor Turret Survey . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4 Electronics and Signal Processing in the Current System 37
4.1 O verview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.2 Pulse Shape and Bandwidth . . . . . . . . . . . . . . . . . 39
4.3 Power Consumption . . . . . . . . . . . . . . . . . . . . . 40
4.4 Time Varying Gain . . . . . . . . . . . . . . . . . . . . . . 41
4.5 Enveloping and Sampling . . . . . . . . . . . . . . . . . . . 46
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4.6 Improvements to the Existing System . . . . . . . . . . . . . . . . . . 48
4.6.1 Analog to Digital Conversion . . . . . . . . . . . . . . . . . . 49
4.6.2 Digital Signal Processing . . . . . . . . . . . . . . . . . . . . . 50
5 Analysis of the Transducer 55
5.1 R esolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.1.1 Wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.1.2 Beam Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5.2 Depth of Penetration . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.2.1 Reflection Coefficients . . . . . . . . . . . . . . . . . . . . . . 61
5.2.2 Attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.2.3 Estimate of the Depth of Penetration . . . . . . . . . . . . . . 64
5.3 Model of the Sub-Bottom Profiler . . . . . . . . . . . . . . . . . . . . 64
6 Experimental Results 71
6.1 Experiment Description . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.2 R esults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
6.3 A nalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
6.4 Further Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
7 Design Recommendations for Future Systems 87
8 Conclusion 91
A Schematics 95
A.1 Amplification and enveloping . . . . . . . . . . . . . . . . . . . . . . 95
A.2 TVG suface mount board . . . . . . . . . . . . . . . . . . . . . . . . 96
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List of Figures
3-1 A vertical cross section taken by the prototype sub-bottom profiler of
the Tanit shipwreck (circa 750 BC) located in 400 meters of water off
the coast of Ashkelon, Israel . . . . . . . . . . . . . . . . . . . . . . . 32
3-2 Photomosaic of the Tanit shipwreck. . . . . . . . . . . . . . . . . . . 32
3-3 Expanded view of the cross-sectional imaged produced by the sub-
bottom profiler. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3-4 Sub-bottom profiler data collected during the survey of the Monitor. . 35
4-1 Block diagram of the electronics . . . . . . . . . . . . . . . . . . . . . 38
4-2 The uniform pulse shape and bandwidth. . . . . . . . . . . . . . . . . 39
4-3 Shape and bandwidth of the Gaussian pulse. . . . . . . . . . . . . . . 40
4-4 Predicted and measured gain as a function of numerical gain parameter. 42
4-5 Time varying gain values as a function of time . . . . . . . . . . . . . 44
4-6 Measured and modeled time varying gain and corresponding average
error as a function of step size . . . . . . . . . . . . . . . . . . . . . . 45
4-7 Signal processing performed by the current electronics. . . . . . . . . 46
4-8 Signal processing of 2 pulses separated by 10 microseconds . . . . . . 47
4-9 Alternative enveloping options . . . . . . . . . . . . . . . . . . . . . . 48
5-1 The far field beam pattern . . . . . . . . . . . . . . . . . . . . . . . . 57
5-2 Data collected during a swimming pool test to estimate the beam width
2.3 meters away from the sensor. . . . . . . . . . . . . . . . . . . . . 59
5-3 Data collected during a swimming pool test to estimate the near field
sidelobes of the transducer . . . . . . . . . . . . . . . . . . . . . . . . 60
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5-4 Diagram of a boundary between two materials . . . . . . . . . . . . . 61
5-5 Estimated maximum depth of penetration for various sediment types. 65
5-6 A sample object field and the corresponding modeled data for a sensor
with a beam width of 1 cm. . . . . . . . . . . . . . . . . . . . . . . . 67
5-7 The top figure shows the coefficients of the moving average filter. The
bottom figure is the modeled profiler data including the moving average
approximation of the beam width. . . . . . . . . . . . . . . . . . . . . 68
5-8 Modeled data including the effects of a low-pass filter. The colors are
displayed on a log scale and time varying gain is not modeled. .... 69
6-1 Pictures of the trench (top) and gantry structure over the trench after
the objects were buried (bottom). . . . . . . . . . . . . . . . . . . . . 73
6-2 Top view of objects buried at the test site . . . . . . . . . . . . . . . 74
6-3 Vertical cross-section of the test site . . . . . . . . . . . . . . . . . . . 74
6-4 Predicted data displayed on a log scale. . . . . . . . . . . . . . . . . . 75
6-5 Two data sets collected at the test site with constant gain. . . . . . . 77
6-6 Swimming pool test to verify the hypothesis that the second double
bounce was caused by a reflection off of the surface of the water. . . . 78
6-7 Model data including the low pass filter and plotted on a linear scale 79
6-8 Two data sets collected at the test site with time varying gain. ..... 81
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