Detection Of Surface Waves In The Ground Using An Acoustic Method

104 Pages·2002·2.46 MB·English

DETECTION OF SURFACE WAVES IN THE GROUND USING AN
ACOUSTIC METHOD

A Thesis
Presented to
The Academic Faculty

By

Fabien Codron

In Partial Fulfillment
of the Requirements for the Degree
Master of Science in Mechanical Engineering

Georgia Institute of Technology
July 2000
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DETECTION OF SURFACE WAVES IN THE GROUND USING AN
ACOUSTIC METHOD

Approved:

______________________________
Peter H. Rogers

______________________________
Waymond R. Scott

______________________________
Yves Berthelot

Date Approved
_________________

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ACKNOWLEDGEMENT

This work was accomplished in an environment very new to me. I had to
adapt to the language, the culture, and discover the campus, the city and the
people. Many people helped me go through this experience. Their knowledge,
their advice and their support made my research easier. I would like to thank
them all.
First, my advisor P. Rogers for giving me the opportunity to contribute to
this research.
W. Scott for managing the landmine detection project at Georgia Tech,
and also for his very useful debugging visits, and advice on electronics
Y. Berthelot for teaching me the basis on acoustic transducers and
supporting the exchange with Georgia Tech Lorraine.
J. Martin for his availability, his everyday advice and teaching. His
contribution to the good running of the acoustical laboratory was essential.
I would also like to thank the graduate students of the acousto-dynamic
group for their availability. They maintained a nice work environment.

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TABLE OF CONTENTS

ACKNOWLEDGMENTS iii
LIST OF TABLES vi
LIST OF FIGURES vii
SUMMARY ix

CHAPTER
I BACKGROUND 1
A. General 1
B. An acoustic method to detect the surface waves in the ground 10
C. Litterature Review 12
II THE TRANSDUCER SYSTEM 19
A. Presentation of the Transducers 20
B. Reflection of the Ultrasonic Waves from Soil 23
C. Focusing the Transducer Sound 26
III PHASE DEMODULATION 37
A. Investigation of Digital Demodulation 42
B. Analog Demodulation 52
IV NOISE AND FILTERING 65
A. Noise Measurement 65
B. Acoustic Noise 66
C. Electromagnetic Cross Talk 67
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D. Source Noise 68
E. Filtering 73
F. Digital Signal Processing 76
V CONCLUSIONS 79
VI RECOMMENDATIONS 82
VIII APPENDICES 85
VII REFERENCES 93

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LIST OF FIGURES

Page
Figure 1.1 - Schematic diagram of acousto-electromagnetic experimental system 3
Figure 1.2 - Top view of the experimental system 4
Figure 1.3 - Two dimensional finite difference model 7
Figure 1.4. - Numerical model results for the mine interaction with surface wave 8
Figure 1.5 -experimental and numerical results for the surface wave propagation 9
Figure 1.6 - spatial resolution 12
Figure 1.7 - Spectrum of a pure tone modulated by a sine wave 15
Figure 1.8 - schematic of the waves phases 15
Figure 1.9 -schematic of the phase demodulation signal processing 17
Figure 1.10 - Phase demodulation for a small amplitude modulation and signal in
quadrature 18
Figure 2.1 - Schematic of the transducer assembly 20
Figure 2.2 - Cross section of a transducer 21
Figure 2.3 - Generation of the transducer signal 22
Figure 2.4 - Transmit response of the capacitance transducer 22
Figure 2.5 - Picture of transducer facing a sand sample 24
Figure 2.6 - Graph of the pressure on axis, in the nearfield 27
Figure 2.7 - sound field generated by piston at 50kHz 29
Figure 2.8 - on axis pressure generated by a 50kHz spherically focused transducer
31
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Figure 2.9 - pressure field generated by a 50kHz spherically focused transducer
32
Figure 2.10 On axis pressure generated by a 200kHz spherically focused
transducer 33
Figure 2.11 - Normalized pressure maximums versus true focusing distance for
50, 100 and 200 kHz focused transducers 33
Figure 2.12 - Cross-section of a spherically focused transducer 34
Figure 3.1 - influence of the transducer axis angle on sensitivity 38
Figure 3.2 - Square wave with jittered edges 42
Figure 3.3 - Schematic of bit coding 43
Figure3.4 - Fourier transform of the 50kHz pure tone acquired 44
Figure 3.5 - Fourier transform of the digitized signal coded on 32 bits 47
Figure 3.6 - Fourier transform of an ideally sampled signal coded on 12bits 47
Figure 3.7 - Fourier transform of a ideally sampled signal coded on 16bits 48
Figure 3.8 - Fourier transform of signal sampled with 5.10-8s jitter 49
Figure 3.9 - Fourier transform of signal sampled with 5.10-9s jitter 50
Figure 3.10 Measurement setup 53
Figure 3.11- Picture of the transducer facing the sound projector 54
Figure 3.12 Schematic of the signal processing 62
Figure 3.13 - Operational amplifier circuit for quadrature 63
Figure 4.1 - Spectral density of a pure tone 69
Figure - 4.2 three stages passive low pass filter 71
Figure - 4.3 One stage of the Chebyshev active low pass filter 74
Figure - 4.4 Amplitude response of the active filter implemented on board 75
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Figure 4.5 LabView diagram of the digital signal processing 77
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LIST OF TABLES

Page
Table 1.1 - acoustic waves velocities in sand and mines 6
Table 2.1 - received signal level and signal to noise ration 25
Table 3.1 - results of the mixer test 57
Table 3.2 - Transfer function of the K-H 3 poles Bessel filter for fc=3000Hz 58
Table 3.3 - Calibration results for the TM3 configuration 60
Table 3.4 - Calibration results for the AD534 configuration 60
Table 3.5 - Results for the AD630 configuration 60
Table 4.1 - Noise floor levels in dBm after filtering of the multiplier output 72
Table 4.2 - values of the resistors and capacitors for the filter 74
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SUMMARY

Land mine detection techniques currently in use are not reliable for
modern plastic mines. An acousto-electromagnetic technique that has the
potential to detect such mines is being investigated at Georgia Tech. It uses an
acoustic source for generating waves in the ground, which are detected with a
radar, which scans the surface to be cleared of mines. The radar system visualizes
the surface wave and its interaction with the mine by measuring the surface
vibration. This radar ground vibration measuring system is expensive and may
not be effective in all environments.
The purpose of this thesis is to investigate an ultrasonic vibrometer that
could be used to supplement the radar or replace it. An ultrasonic system was
implemented and tested with several different demodulation techniques.
Emphasis was laid on getting a sensitivity of 1-nanometer, equal to that of the
radar sensor. In order to obtain such sensitivity , design and optimization of the
source, the transducer signal, the electronic filtering and the demodulation were
conducted. The focusing of the ultrasound and the effects of spot size were also
considered.
The system presented in this thesis has good potential characteristics for
surface waves detection at a low cost. It achieves the required resolution with
transducers running at 50kHz for a vibration of the soil in the frequency range
400-1200Hz. Placing the transducers a couple inches away from the vibrating
surface produces a satisfactory spot size
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