High frequency seismic and underwater acoustic wave propagation and imaging techniques

138 Pages·2008·17.85 MB·English

Alma Mater Studiorum
Universita` di Bologna
Dottorato di ricerca in Geofisica - XX Ciclo
Tesi di Dottorato
Settore scientifico-disciplinare FIS/06
High frequency seismic and underwater
acoustic wave propagation and imaging
techniques
Dottorando: Tutor:
Dr. Tony Alfredo Stabile Prof. Aldo Zollo
Coordinatore:
Prof. Michele Dragoni
Esame finale anno 2008
2
i
To my wife and my son,
my everlasting love.
ii
Contents
Introduction ix
1 Wave propagation theory 1
1.1 Elastodynamic equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Synthetic Seismogram Methods . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Ray-Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3.1 Mathematical derivation . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3.2 Ray theory general validity conditions . . . . . . . . . . . . . . . . 13
1.4 Rayleigh Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 Method for rapid high-frequency seismogram calculation: the COMRAD
code 19
2.1 Aim of a multiphase dynamic ray-tracing code . . . . . . . . . . . . . . . . 19
2.2 Method description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.1 General concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.2 Selection Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.3 Structure of the COMRAD code . . . . . . . . . . . . . . . . . . . . 27
2.3 Validation of the method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.4 Results and discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3 Applications of the COMRAD code in seismology 37
3.1 Forward modelling of active seismic data: the case study of Campi Flegrei
caldera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.1.1 A brief description of Serapis active seismic survey . . . . . . . . . 39
3.1.2 Comparisons between synthetic sections and real sections . . . . . . 39
3.1.3 Conclusions and discussions . . . . . . . . . . . . . . . . . . . . . . 44
3.2 Forward modelling of seismic sources . . . . . . . . . . . . . . . . . . . . . 45
3.2.1 Faulting sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.2.2 Synthetic seismograms for a vertical strike-slip point source . . . . . 47
3.2.3 Conclusions and discussions . . . . . . . . . . . . . . . . . . . . . . 50
4 High frequency underwater acoustic propagation in the Gulf of Naples 51
4.1 Physical properties of the Gulf of Naples . . . . . . . . . . . . . . . . . . . 52
iv CONTENTS
4.1.1 Analysis of the weather conditions . . . . . . . . . . . . . . . . . . . 53
4.1.2 One-dimensional velocity and geoacoustic model of the Gulf of Naples 57
4.1.3 Noise sources up to a frequency of 100 kHz . . . . . . . . . . . . . . 64
4.2 Simulations of acoustic signal propagation at 100 kHz . . . . . . . . . . . . 65
4.2.1 Methodology used for the simulation of the signal propagation . . . 65
4.2.2 How to calculate the signal-to-noise ratio (SNR) . . . . . . . . . . . 66
4.3 Calculation of the SNR in the Gulf of Naples at 100 kHz . . . . . . . . . . 67
4.4 Conclusions and discussions . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5 Very high frequency Submarine Acoustic Imaging 75
5.1 The STSS-500 Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
5.2 Development of an Acoustic Imaging Numerical Simulator . . . . . . . . . 77
5.2.1 Forward modelling: Rayleigh scattering . . . . . . . . . . . . . . . . 78
5.2.2 Imaging: Beamforming techniques . . . . . . . . . . . . . . . . . . . 81
5.3 Three-dimensional imaging of submerged objects . . . . . . . . . . . . . . . 83
5.4 Conclusions and discussions . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Conclusions 91
Acknowledgements 93
A The COMRAD Code 95
A.1 User Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
A.1.1 What the program does . . . . . . . . . . . . . . . . . . . . . . . . 96
A.1.2 Program Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
A.2 The source code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
B Fundamentals of underwater acoustics 113
B.1 The sonar equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
B.2 Sound attenuation in seawater up to a frequency of 1 MHz . . . . . . . . . 115
B.3 Calculation of the transmission reliability . . . . . . . . . . . . . . . . . . . 117
List of Figures
1.1 An example of seismogram recorder by a three component station. . . . . . 5
1.2 Subcritical, critical, and postcritical angles of incidence. . . . . . . . . . . . 14
2.1 Tree structure of ray strings stopped to the fourth generation. . . . . . . . 22
2.2 Tree structure of ray strings stopped to the fourth generation as in Figure
2.1, but using all of the (a) to (g) constraints . . . . . . . . . . . . . . . . . 23
2.3 Integral of scattering coefficients for an incident P-wave. . . . . . . . . . . 26
2.4 Integral of scattering coefficients for an incident S-wave. . . . . . . . . . . . 26
2.5 Block scheme of the Comrad.f computer code. . . . . . . . . . . . . . . . . 27
2.6 TFEM(t,f) and TFPM(t,f) plots for a receiver at 1 km distance from
the source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.7 As in Figure 2.6 but referring to the receiver at 30 km distance from the
source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.1 An image of the Campi Flegrei caldera. . . . . . . . . . . . . . . . . . . . . 38
3.2 Comparison between a real section and a synthetic section for a 1-D Campi
Flegrei model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.3 Three different real sections in which the multiple reflection of Figure 3.2 is
clear. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.4 Comparison between the observed and synthetic sections for the average
1-D model of Table 3.1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.5 AVO analysis (PS-to-PP ratio) to the second interface of the velocity model. 44
3.6 Extended source and point source conditions in far-field approximation. . . 45
3.7 Standard definition of fault-plane and slip vector orientation parameters. . 46
3.8 Synthetic seismograms for R and R receivers. . . . . . . . . . . . . . . . 48
2 4
3.9 Focal mechanism obtained from FPFIT program. . . . . . . . . . . . . . . 49
4.1 Bathymetric and topographic image of the Gulf of Naples. . . . . . . . . . 53
4.2 Mean air temperature and wind speed in the Gulf of Naples during one year. 54
4.3 Monthly frequency of the Wind direction in the Gulf of Naples. . . . . . . 56
4.4 Salinity profiles in the Gulf of Naples from the sea surface to the bottom. . 58
4.5 Temperature profiles in the Gulf of Naples from the sea surface to the bottom. 59
4.6 Velocity profiles in the Gulf of Naples calculated from the salinity and tem-
perature data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
vi LIST OF FIGURES
4.7 Mean velocity profiles for the summer and winter seasons. . . . . . . . . . 62
4.8 The signal-to-noise ratio for the summer and winter months in the Gulf of
Naples with the transmitter positioned at a depth of 1 m. . . . . . . . . . . 69
4.9 The signal-to-noise ratio for the summer and winter months in the Gulf of
Naples with the transmitter positioned at a depth of 190 m. . . . . . . . . 70
4.10 The signal-to-noise ratio for the summer and winter months in the Gulf of
Naples with the transmitter positioned at a depth of 299 m. . . . . . . . . 71
4.11 Monitoring system prototype developed during SisMa Project. . . . . . . . 73
5.1 Sound attenuation in seawater against range and frequency. . . . . . . . . . 80
5.2 Scheme of the beamforming technique used for the imaging process. . . . . 82
5.3 Source-receiver geometry used for the first example. . . . . . . . . . . . . . 84
5.4 Synthetic data and images obtained for the first example. . . . . . . . . . . 85
5.5 Two different configuration used for the acquisition system. . . . . . . . . . 86
5.6 Images of the target obtained using the configurations of Figure 5.5. . . . . 87
5.7 Real shape of the object and its image obtained by the imaging process. . . 88
List of Tables
2.1 Different velocity models to study the effect of the boundary between a
half-space that contains the incident wave and a half-space that contains
the transmitted wave. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2 Weights assigned for each region, defined by z parameter, and for each type
of phase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.3 The crustal velocity model used for the simulations. . . . . . . . . . . . . . 30
2.4 Computing time of COMRAD and core codes . . . . . . . . . . . . . . . . 30
2.5 Computing time of COMRAD code with constraints . . . . . . . . . . . . . 31
3.1 Average 1-D model for Campi Flegrei as derived by Maercklin and Zollo
(2008). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.2 Model used to compute synthetic seismograms for a vertical strike-slip point
source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.1 Mean annual temperature in the Gulf of Naples from 1997 to 2006. . . . . 54
4.2 Monthly total rain fallen in the Gulf of Naples (mean value from 1997 to
2006). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.3 Values of the physical parameters of the propagation medium for the Gulf
of Naples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.4 Sound velocities of the summer and winter models for increasing depths. . 63
5.1 Physical properties of the cubic block and the background medium used for
the second example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
B.1 The sonar parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
viii LIST OF TABLES