Surface Wave Analysis for Near Surface Applications - Giancarlo Dal Moro

Surface Wave Analysis for Near Surface Applications (eBook)

Giancarlo Dal Moro (Autor)

eBook Download: PDF | EPUB

2014 | 1. Auflage
252 Seiten
Elsevier Science (Verlag)
978-0-12-801140-9 (ISBN)

Seismic Wave Analysis for Near Surface Applications presents the foundational tools necessary to properly analyze surface waves acquired according to both active and passive techniques. Applications range from seismic hazard studies, geotechnical surveys and the exploration of extra-terrestrial bodies. Surface waves have become critical to near-surface geophysics both for geotechnical goals and seismic-hazard studies. Included in this book are the related theories, approaches and applications which the lead editor has assembled from a range of authored contributions carefully selected from the latest developments in research. A unique blend of theory and practice, the book's concepts are based on exhaustive field research conducted over the past decade from the world's leading seismologists and geophysicists. - Edited by a geophysicist with nearly 20 years of experience in research, consulting, and geoscience software development - Nearly 100 figures, photographs, and examples aid in the understanding of fundamental concepts and techniques - Presents the latest research in seismic wave characteristics and analysis, the fundamentals of signal processing, wave data acquisition and inversion, and the latest developments in horizontal-to-vertical spectral ratio (HVSR) - Each chapter features a real-world case study-13 in all-to bring the book's key principles to life

Giancarlo dal Moro, PhD is a Senior Geophysicist and software developer for Eliosoft in Italy, a software development firm for the geosciences. Dr. dal Moro received his doctorate in geophysics from the University of Trieste. He specializes in Geophysical methods for site characterization (academic research and field practice): joint inversion of geophysical/seismic data, surface wave analysis (dispersion and attenuation), site effect assessment, software implementation, remote sensing, and seismic-data processing. Giancarlo is a contributing author and reviewer for several journals in the field of Geophysics including the Journal of Applied Geophysics, Journal of Geophysics and Engineering, and the Journal of Remote Sensing.

Front
1
SURFACE WAVE ANALYSIS FOR NEAR SURFACE APPLICATIONS 4
Copyright 5
CONTENTS 6
PREFACE 8
Chapter 1 - Introducing Surface Waves 10
1.1 A BRIEF INTRODUCTION 10
1.2 LORD RAYLEIGH AND PROF. LOVE 10
1.3 DISPERSION FOR DUMMIES 14
1.4 DISPERSION, VELOCITY SPECTRA, AND DISPERSION CURVES 17
1.5 ATTENUATION IN SHORT 23
1.6 SURFACE WAVES, GEOLOGY, NONUNIQUENESS, AND ANISOTROPIES 27
Chapter 2 - Data Acquisition 32
2.1 INTRODUCTION 32
2.2 ACTIVE METHODOLOGIES 34
2.3 PASSIVE METHODOLOGIES 45
2.4 FEW FINAL REMARKS 50
Chapter 3 - Understanding Surface-Wave Phenomenology 52
3.1 INTRODUCING THE PROBLEM 52
3.2 MORE ABOUT MODES AND COMPONENTS 58
3.3 ABOUT PASSIVE METHODS 66
3.4 FEW FINAL REMARKS 70
Chapter 4 - Horizontal-to-Vertical Spectral Ratio 74
4.1 INTRODUCTION 74
4.2 DATA ACQUISITION AND HVSR COMPUTATION 75
4.3 SOME PROBLEMS 81
Chapter 5 - Inversion and Joint Inversion 96
5.1 INTRODUCTION 96
5.2 MISFIT, INVERSION, AND MODELING: CONCEPTS AND MISCONCEPTS 97
5.3 LOCAL MINIMA AND NONUNIQUENESS OF THE SOLUTION 102
5.4 JOINT ANALYSIS 105
Chapter 6 - Full Velocity Spectrum Inversion and Other Unconventional Approaches 112
6.1 INTRODUCTION 112
6.2 FULL WAVEFORM AND FULL VELOCITY SPECTRUM INVERSIONS 116
Chapter 7 - Some Final Notes 122
7.1 THE ADOPTED PERSPECTIVE 122
7.2 A BRIEF MISCELLANEA ON MODES AND SHEAR-WAVE VELOCITIES 123
7.3 SURVEY PLANNING AND RESULT EVALUATION 130
7.4 SUMMARIZING FEW FINAL RECOMMENDATIONS 136
Appendix—A Collection of Commented Case Studies 140
Case Study 1 - A Simple ZVF Analysis for Geotechnical Purposes 142
Case Study 2 - A Simple (but Educational) Case Study 148
Case Study 3 - Inverse Dispersion by the Book 158
Case Study 4 - When the Joint Analysis of Love and Rayleigh Waves Is Necessary 162
CPT DATA 167
HVSR DATA 167
Case Study 5 - Joint Analysis of Rayleigh-Wave Dispersion and P-Wave Refraction 168
Case Study 6 - A Comprehensive Survey in the Swiss Alps 172
. ACKNOWLEDGMENTS 178
Case Study 7 - Joint Analysis of Rayleigh and Love Waves via Full Velocity Spectrum Analysis 180
Case study 8 - A Civil Engineering Job 186
. SITE#2 187
. SITE#8 189
Case Study 9 - A Landslide Area 194
Case Study 10 - Back to the Swiss Alps 198
. ACKNOWLEDGMENTS 204
Case Study 11 - Modes and Components (A Very Tricky Site) 206
Case Study 12 - Analyzing Phase and Group Velocities Jointly with Horizontal-to-Vertical Spectral Ratio 214
A12.1 JOINT INVERSION OF HVSR AND LOVE-WAVE GROUP VELOCITIES 214
A12.2 ESAC DATA 214
Case Study 13 - Some Focus on Horizontal-to-Vertical Spectral Ratio Computation 220
A13.1 SPECTRAL SMOOTHING 220
A13.2 SESAME CRITERIA FOR MULTIPEAK HVSR CURVES 222
Case Study 14 - Surface Waves on the Moon 228
A14.1 A BRIEF COMPULSORY FORWARD 228
A14.2 THE CONTEXT 228
A14.3 APPROACHING THE ANALYSIS 230
A14.4 THE APOLLO 16 DATASET 230
A14.5 SOME FINAL REMARKS 233
REFERENCES 236
INDEX 244

Chapter 1

Introducing Surface Waves

Abstract

In this first chapter, we introduce basic aspects regarding surface wave generation and propagation. The comprehension of these facts will be crucial to then carry out the analysis of their dispersion (and attenuation) and, consequently, define a good subsurface model.

For properly identifying each “object” relevant for our analyses, the use of a proper terminology is crucial. The central concepts of dispersion curve and velocity spectrum will be defined giving the necessary emphasis to their different meaning in terms of data analysis.

Keywords

Anisotropy; Attenuation; Attenuation curve; Dispersion; Dispersion curve; Effective dispersion curve; Love waves; Modal dispersion curve; Nonuniqueness; Rayleigh waves; Scholte waves; Surface waves; Velocity spectrum

We begin where we are.

Robert Fripp

1.1. A Brief Introduction

As very well known from basic seismology courses, fundamentally there are two kinds of seismic waves: those propagating inside a medium (body waves) and those traveling along the very shallow part of it (surface waves (SWs)). Compressional waves (commonly indicated as P waves) and shear waves (S waves) are body waves while Rayleigh, Scholte, Stoneley, and Love waves are different kinds of SWs.

In the last decades, a number of papers dealing with SWs have been published but it must be recalled that their theoretical description and first applications date back to almost a century ago.

SWs have been in fact used for a number of applications since the 1920s: Nondestructive testing (even for medical applications), geotechnical studies, and crustal seismology (e.g., Gutenberg, 1924; Evison et al., 1959; Viktorov., 1967; McMechan and Yedlin, 1981; Kovach, 1978; Roesset, 1998; Stokoe et al., 1988; Stokoe and Santamarina, 2000; Jørgensen and Kundu, 2002; O'Neill et al., 2003; 2004; Gaherty, 2004; Pedersen et al., 2006; Luo et al., 2007; O'Connell and Turner, 2011; Prodehl et al., 2013).

Recently the interest toward their application has increased both for the increasing demand for efficient methodologies to apply in geotechnical studies and because the recent regulations addressing the assessment of the seismic hazard (see for instance the Eurocode8) are giving the necessary emphasis to the determination of the shear-wave velocity vertical profile.

Because of their practical importance and wide use in a number of near-surface applications, we will focus our interest on Rayleigh and Love waves in the following.

1.2. Lord Rayleigh and Prof. Love

There are two kinds of SWs actually relevant while analyzing seismic waves propagating on land: Rayleigh and Love waves. The first ones were described mathematically by Lord Rayleigh in 1885 (Rayleigh, 1885), while it was Prof. Love who, in 1911, described the kind of waves that were then named after him (Love, 1911).

The fundamental characteristics of Rayleigh waves are represented in the sketch reported in Figure 1.1. The wave (traveling in the direction of propagation) induces an elliptical (retrograde) motion (see the blue ellipse drawn at time T = 3) whose amplitude exponentially decreases with depth. Such elliptical motion is the result of the superposition of the vertical and horizontal (more specifically radial) components (Figure 1.2).

Figure 1.1 Rayleigh waves. T represents the time (the wave motion is depicted at three moments successive to the wave generation). The particle motion determined by the traveling Rayleigh wave occurs both on the vertical and horizontal planes (retrograde elliptical motion). On the horizontal plane the motion is along the radial component (see also Figures 1.2 and 1.3). From http://www.geo.mtu.edu/UPSeis/waves.html.

Love waves are somehow simpler than Rayleigh waves because (Figures 1.3 and 1.4) they move only on the horizontal plane, transversally with respect to the direction of propagation. Incidentally, this simplicity also mirrors in both the computational load necessary to solve their constitutive equations (and describe their propagation), both in their phenomenology which, how we will broadly see in the next chapters and in several presented case studies, will result extremely useful (even necessary) to solve puzzling interpretative issues related to complex Rayleigh-wave velocity spectra.

Let us now summarize further basic facts:

• While considering a surface normal load, the energy converted into Rayleigh waves is by far predominant (67%) with respect to the energy that goes into P (7%) and S (26%) waves (Miller and Pursey, 1955);

• Rayleigh and Love waves are called SWs because their amplitude exponentially decreases with depth, thus the motion induced by their passage is limited to a shallow portion (whose depth depends on the considered wavelength λ—see later on);

Figure 1.2 Normalized vertical and radial displacements of Rayleigh waves as a function of depth (normalized with respect to the considered wavelength): (a) the individual displacements of the vertical and radial components and (b) the elliptical motion resulting from the composition of the vertical and radial movements. From Gedge and Hill (2012).

• Just because their energy is confined to a shallow layer, while expanding from the source (geometrical spreading), their amplitude decreases fundamentally according to the square root of the distance from the source, while body waves (whose propagation involves a semisphere and not just a circle) lose their energy (thus amplitude) according to the distance (because of this, the amplitude of the body waves decreases much more with respect to SWs and consequently SWs tend to dominate the data);

• Compared to body waves, their amplitude is remarkably larger and, for this reason, in the low-frequency range they dominate the data and are therefore often referred to as ground roll (Figure 1.5 reports a classical common-shot gather giving evidence of this);

• Rayleigh waves move along a radial plane (they have both a radial and vertical component) according to a retrograde movement (that means that the elliptical particle motion is on the opposite direction with respect to the direction of propagation—see Figures 1.1, 1.2 and 1.4); Love waves (Figures 1.3 and 1.4) move only on the horizontal plane, with the particle motion perpendicular to the direction of propagation.

The fact that Rayleigh waves have both a vertical and a horizontal component means that they can be acquired in a so-to-speak alternative way with respect the common practice represented by the use of vertical geophones: using horizontal geophones oriented radially with respect to the source (for further details see next Chapter). This can have a relevant series of theoretical and practical consequences that, in the following chapters and case studies, will be described in some detail.

Figure 1.3 Love waves. T represents the time (the wave motion is depicted at three moments successive to the wave generation). The particle motion determined by the traveling Love wave lies only on the horizontal plane, transversally (i.e., perpendicularly) to the direction of propagation (see also Figure 1.3). From http://www.geo.mtu.edu/UPSeis/waves.html.

Figure 1.4 Ground motion associated to Rayleigh and Love waves: Rayleigh waves induce a motion along the vertical and radial axes, while Love waves along the transversal one.

Land acquisition is surely the most common, but what happens while considering marine (or lacustrine) seismic data (i.e., data traveling at a solid–fluid interface)?

While the characteristics of Love waves remain the same (e.g., Winsborrow et al., 2003), the so-to-speak “marine Rayleigh waves” are following slightly different equations that describe the so-called Scholte waves (Scholte, 1947).

Figure 1.5 Example of common-shot gather containing both ground roll and reflections/refractions: (a) filtered from 0 to 15 Hz; (b) from 15 to 30 Hz; and (c) unfiltered. (From Cary and Zhang (2009).) Please notice that in the low-frequency range (0–15 Hz) the dataset is largely dominated by the ground roll (Rayleigh waves). On the other side, in the 15–30 Hz frequency range (high frequencies), data are dominated by refractions and reflections.

Scholte waves are actually quite similar to Rayleigh waves. The particle motion is absolutely analog (an elliptical motion on the radial–vertical plane) but, because of the influence of the water, the velocities are slightly different (Scholte waves tend to be slower). The difference between Rayleigh and Scholte waves results proportional to the thickness of the...

Erscheint lt. Verlag	4.11.2014
Sprache	englisch
Themenwelt	Naturwissenschaften ► Geowissenschaften ► Geologie
	Naturwissenschaften ► Geowissenschaften ► Geophysik
	Naturwissenschaften ► Physik / Astronomie
	Technik
ISBN-10	0-12-801140-8 / 0128011408
ISBN-13	978-0-12-801140-9 / 9780128011409

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