Surface Wave Analysis for Near Surface Applications

234 Pages·2014·61.155 MB·English

by  Giancarlo Dal Moro

It’snotwhatyoulookatthatmatters,it’swhatyousee.
HenryDavidThoreau
SURFACE WAVE
ANALYSIS FOR NEAR
SURFACE APPLICATIONS
GIANCARLO DAL MORO
Institute ofRock Structure and Mechanics Academy
of Sciences of the Czech Republic,Prague,
CzechRepublic
&
Eliosoft,Udine,Italy
AMSTERDAM BOSTON HEIDELBERG LONDON NEWYORK OXFORD
PARIS SANDIEGO SANFRANCISCO SINGAPORE SYDNEY TOKYO
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PREFACE
Learntherulessoyouknowhowtobreakthemproperly
DalaiLama
Rulesareforfools
Americanfolklore
In the mind of the Author, this book represents an attempt to fill the gap between
academic research and field applications. Purely academic works sometimes risk to
become self-referential efforts without reflecting in a genuine support for the society
while, on the other side, practitioners and researchers facing the problem of field-data
analysis without a sufficiently-robust theoretical background risk to fail because of
erroneous comprehension of the data.
In fact, no methodology approach or software/hardware application will ever allow
proceeding in sound data analysis without a clear understating of the considered
phenomena by the side of the user.
Due to the several cross-references to different sections, the book should be consid-
eredinaholisticperspective(tosomedegree,theunderstandingofonepointrequiresthe
understanding of all the others). Equations and formulas (easily found in the mentioned
literature) were intentionally avoided while the focus was kept on the surface-wave
phenomenology so to train the eye in the reading of the velocity spectrum (the funda-
mental object to consider in surface-wave analysis).
The principles characterizing the considered active and passive methodologies are
necessarily presented without any claim or presumption to describe the whole universe
of possible techniques and approaches.
Ofcourse,asalwaysinthiskindofeditorialprojects,itisimpossibletomentionallthe
papersandstudiesthathavebeenpublishedonthesesubjectsandwethuspayhomageto
the comprehension of all those researchers that, being not cited, will practice their
patience and comprehension about that.
The overall goal is to provide an arsenal of conceptual and practical tools capable
(whenproperlyconsidered)to attack(asmathematiciansoftenliketosay)asiteandsolve
it in terms of reconstruction of its subsurface model.
No easy and cheap approach based on simplistic assumptions can in fact result in
universally valid analyses and no approach can be claimed as the ultimate one, suitable
and sufficient for all the seasons. It will be shown that some rigid rules often proposed
andadoptedforsurface-wavedataacquisitionandanalysiscanbeinadequateorevenfully
misleading,thusrealizingtheconcretenessofthewisdompervadingtheopeningquotes.
vii
viii Preface
Thebestapproach(andbythetermbestwecandefinetheapproachcapableofunam-
biguously solving a site ensuring at the same time the minimum effort in terms of data
acquisition and analysis) is in fact site-dependant and, in general terms, only the joint
analysis of different (but related) data sets can guarantee the success of a survey.
CasestudiespresentedintheAppendixrepresentavitalcomponentofthebookanda
majoreffortwasundertakeninordertoselectdatasetscapableofprovidingasufficiently
large outlook on the richness and complexity of the surface-wave phenomenology,
approached and solved by adopting the methodologies that best suited the data
themselves.
Giancarlo Dal Moro
CHAPTER 1
Introducing Surface Waves
We begin where we are.
RobertFripp
1.1 A BRIEF INTRODUCTION
Asverywellknownfrombasicseismologycourses,fundamentallytherearetwokindsof
seismicwaves:thosepropagatinginsideamedium(bodywaves)andthosetravelingalong
the very shallow part of it (surface waves (SWs)). Compressional waves (commonly indi-
cated as P waves) and shear waves (S waves) are body waves while Rayleigh, Scholte,
Stoneley, and Love waves are different kinds of SWs.
Inthelast decades,anumberofpapersdealingwithSWshavebeenpublishedbutit
mustberecalledthattheirtheoreticaldescriptionandfirstapplicationsdatebacktoalmost
a century ago.
SWshavebeeninfactusedforanumberofapplicationssincethe1920s:Nondestruc-
tive 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;JørgensenandKundu,2002;O’Neilletal.,2003;2004;Gaherty, 2004;Pedersen
et al., 2006; Luo et al., 2007; O’Connell and Turner, 2011; Prodehl et al., 2013).
Recentlytheinteresttowardtheirapplicationhasincreasedbothfortheincreasingdemand
forefficientmethodologiestoapplyingeotechnicalstudiesandbecausetherecentregulations
addressingtheassessmentoftheseismichazard(seeforinstancetheEurocode8)aregivingthe
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 ap-
plications, we will focus our interest on Rayleigh and Love waves in the following.
1.2 LORD RAYLEIGH AND PROF. LOVE
TherearetwokindsofSWsactuallyrelevantwhileanalyzingseismicwavespropagating
onland:RayleighandLovewaves.ThefirstonesweredescribedmathematicallybyLord
Rayleighin1885(Rayleigh,1885),whileitwasProf.Lovewho,in1911,describedthe
kind of waves that were then named after him (Love, 1911).
The fundamental characteristics of Rayleigh waves are represented in the sketch re-
ported in Figure 1.1. The wave (traveling in the direction of propagation) induces an
SurfaceWaveAnalysisforNearSurfaceApplications Copyright©2015ElsevierInc.
ISBN978-0-12-800770-9,http://dx.doi.org/10.1016/B978-0-12-800770-9.00001-7 Allrightsreserved. 1
2 SurfaceWaveAnalysisforNearSurfaceApplications
X Y Direction of Propagation
T = 0
Z RAYLEIGH WAVE
T = 1
T = 2
T = 3
Particle Motion
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
waveoccursbothontheverticalandhorizontalplanes(retrogradeellipticalmotion).Onthehorizon-
talplanethemotionisalongtheradialcomponent(seealsoFigures1.2and1.3).Fromhttp://www.geo.
mtu.edu/UPSeis/waves.html.
elliptical(retrograde)motion(seetheblueellipsedrawnattimeT¼3)whoseamplitude
exponentially decreases with depth. Such elliptical motion is the result of the super-
positionoftheverticalandhorizontal(morespecificallyradial)components(Figure1.2).
LovewavesaresomehowsimplerthanRayleighwavesbecause(Figures1.3and1.4)
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
theirphenomenologywhich,howwewillbroadlyseeinthenextchaptersandinseveral
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:
(cid:129) Whileconsideringasurfacenormalload,theenergyconvertedintoRayleighwavesis
by far predominant (67%) with respect to the energy that goes into P (7%) and S
(26%) waves (Miller and Pursey, 1955);
(cid:129) Rayleigh and Love waves are called SWs because their amplitude exponentially de-
creases with depth, thus the motion induced by their passage is limited to a shallow
portion (whose depth depends on the considered wavelength ldsee later on);
IntroducingSurfaceWaves 3
(a) (b)
0 0
Horizontal
displacement
h)
gt0.4 0.4
n
e
el
v
a
w
z/
h (0.8 Vertical 0.8
pt displacement
e
d
d
e
s
ali
m1.2 1.2
or
N
1.6 1.6
0 1 –0.2 0 0.2
Normalised displacement amplitude
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
verticalandradialmovements.FromGedgeandHill(2012).
(cid:129) Just because their energy is confined to a shallow layer, while expanding from the
source (geometrical spreading), their amplitude decreases fundamentally according to
thesquarerootofthedistancefromthesource,whilebodywaves(whosepropagation
involvesasemisphereandnotjustacircle)losetheirenergy(thusamplitude)accord-
ingtothedistance(becauseofthis,theamplitudeofthebodywavesdecreasesmuch
more with respect to SWs and consequently SWs tend to dominate the data);
(cid:129) Comparedtobodywaves,theiramplitudeisremarkablylargerand,forthisreason,in
thelow-frequencyrangetheydominatethedataandarethereforeoftenreferredtoas
groundroll(Figure1.5reportsaclassicalcommon-shotgathergivingevidenceofthis);
(cid:129) Rayleigh waves move along a radial plane (they have both a radial and vertical
component)accordingtoaretrogrademovement(thatmeansthattheellipticalparticle
motionisontheoppositedirectionwithrespecttothedirectionofpropagationdsee
Figures 1.1, 1.2 and 1.4); Love waves (Figures 1.3 and 1.4) move only on the hori-
zontalplane,withtheparticlemotionperpendiculartothedirectionofpropagation.
ThefactthatRayleighwaveshavebothaverticalandahorizontalcomponentmeans
that they can be acquired in a so-to-speak alternative way with respect the common
4 SurfaceWaveAnalysisforNearSurfaceApplications
X Y Direction of Propagation
T = 0
Z LOVE WAVE
T = 1
T = 2
T = 3
Particle Motion
Figure1.3 Lovewaves.Trepresentsthetime(thewavemotionisdepictedatthreemomentssucces-
sivetothewavegeneration).TheparticlemotiondeterminedbythetravelingLovewaveliesonlyon
the horizontal plane, transversally (i.e., perpendicularly) to the direction of propagation (see also
Figure1.3).Fromhttp://www.geo.mtu.edu/UPSeis/waves.html.
Figure1.4 GroundmotionassociatedtoRayleighandLovewaves:Rayleighwavesinduceamotion
alongtheverticalandradialaxes,whileLovewavesalongthetransversalone.
practice represented by the use of vertical geophones: using horizontal geophones ori-
ented radially with respect to the source (for further details see next Chapter). This
canhavearelevantseriesoftheoreticalandpracticalconsequencesthat,inthefollowing
chapters and case studies, will be described in some detail.
Land acquisition is surely the most common, but what happens while considering
marine (or lacustrine) seismic data (i.e., data traveling at a solidefluid 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 equa-
tions that describe the so-called Scholte waves (Scholte, 1947).
IntroducingSurfaceWaves 5
Figure1.5 Exampleofcommon-shotgathercontainingbothgroundrollandreflections/refractions:
(a) filtered from 0 to 15Hz; (b) from 15 to 30Hz; and (c) unfiltered. (From Cary and Zhang (2009).)
Please notice that in the low-frequency range (0e15Hz) the dataset is largely dominated by the
groundroll(Rayleighwaves).Ontheotherside,inthe15e30Hzfrequencyrange(highfrequencies),
dataaredominatedbyrefractionsandreflections.
Scholte waves are actually quite similar to Rayleigh waves. The particle motion is
absolutely analog (an elliptical motion on the radialevertical 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 water column so that, from the practical point of view, in shallow
waters to some degree it is possible to analyze Scholte waves while using the
Rayleigh-wave equations.
Analyzing marine datasets, some difficulties can actually depend on guided waves
travelingwithinthewatercolumn(e.g.,Kleinetal.,2005)and,dependingonthespecific
goals of the survey, it can be desirable to use multicomponent sensors deployed on (or
close to) the sea floor (e.g., Ritzwoller and Lavshin, 2003) rather than single-
component hydrophones floating on the water column.
StoneleywavesareafurthertypeofSWthatcreatealongasolidesolidinterfaceand
which are often exploited in borehole seismics to infer the shear-wave velocities (e.g.,
Stevens and Day, 1986).
1.3 DISPERSION FOR DUMMIES
It is well known that a seismic wavelet (actually any signal) is the result of several com-
ponents (frequencies) that, all together, create the specific wavelet which can be
describedintermsofamplitudeandphasespectra(anyelementarysignalprocessingtext-
book widely treats Fourier analysis and related topics).
ThecrucialpointaboutSWpropagationisthatthepropagationofaspecificcompo-
nent (that is frequency) that compose the traveling seismic wavelet, depends on the