IntroductiontoMATLABandSeismicData
1.1ScopeandPrerequisites
Thisisabookaboutacomplexanddiversesubject:thenumericalalgorithmsusedto processexplorationseismicdatatoproduceimagesoftheEarth’scrust.Thetechniques involvedrangefromsimpleandgraphicaltocomplexandnumerical.Moreoftenthannot, theytendtowardthelatter.Themethodsfrequentlyuseadvancedconceptsfromphysics, mathematics,numericalanalysis,andcomputation.Thisrequiresthereadertohavea backgroundinthesesubjectsatapproximatelythelevelofanadvancedundergraduate orbeginninggraduatestudentingeophysicsorphysics.Thisneednotincludeexperience inexplorationseismology,butsuchexperiencewouldbehelpful.
Seismicdatasetsareoftenverylargeandhave,historically,strainedcomputerstorage capacities.This,alongwiththecomplexityoftheunderlyingphysics,hasalsostrongly challengedcomputationthroughput.Thesedifficultieshavebeenasignificantstimulusto thedevelopmentofcomputingtechnology.In1980,a3Dmigration1 wasonlypossible intheadvancedcomputingcentersofthelargestoilcompanies.Atthattime,a50000trace3Ddatasetwouldtakeweekstomigrateonadedicated,multimillion-dollarcomputer system.Today,muchlargerdatasetsareroutinelymigratedbycompaniesandindividualsaroundtheworld,oftenoncomputerscostinglessthan$5000.Theeffectiveuseof thisbook,includingworkingthroughthecomputerexercises,requiresaccesstoasignificantmachine(atleastalate-modelPCorMacintosh)withMATLABinstalledandhaving significantmemoryanddiskdrivecapacity.
Thoughnumericalalgorithms,codedinMATLAB,willbefoundthroughoutthisbook, thisisnotabookprimarilyaboutMATLAB.Itisquitefeasibleforthereadertoplan tolearnMATLABconcurrentlywithworkingthroughthisbook,butaseparatereference workonMATLABishighlyrecommended.Inadditiontothereferenceworkspublishedby TheMathWorks(themakersofMATLAB),therearemanyexcellentindependentguides inprintsuchasEtter(1996),HanselmanandLittlefield(1998),RedfernandCampbell (1998),andthemorerecentHighamandHigham(2000).Inaddition,thestudenteditionof MATLABisabargainandcomeswithaverygoodreferencemanual.Ifyoualreadyowna MATLABreference,thenstickwithituntilitprovesinadequate.ThewebsiteofTheMathWorksisworthavisitbecauseitcontainsanextensivedatabaseofbooksaboutMATLAB.
ThoughthisbookdoesnotteachMATLABatanintroductorylevel,itillustratesavarietyofadvancedtechniquesdesignedtomaximizetheefficiencyofworkingwithlarge
1 Migration referstothefundamentalstepincreatinganEarthimagefromscattereddata.
datasets.AswithmanyMATLABapplications,ithelpsgreatlyifthereaderhashadsome experiencewithlinearalgebra.Hopefully,theconceptsofmatrices,rowvectors,column vectors,andsystemsoflinearequationswillbefamiliar.
1.1.1WhyMATLAB?
AfewremarksareappropriateconcerningthechoiceoftheMATLABlanguageasavehicleforpresentingnumericalalgorithms.Whynotchooseamoretraditionallanguagelike CorFortran,oranobject-orientedlanguagelikeC++orJava?
MATLABwasnotavailableuntilthelatterpartofthe1980s,and,priortothat,Fortran wasthelanguageofchoiceforscientificcomputations.ThoughCwasalsoapossibility,its lackofabuilt-infacilityforcomplexnumberswasaconsiderabledrawback.Ontheother hand,FortranlackedsomeofC’sadvantagessuchasstructures,pointers,anddynamic memoryallocation.
TheappearanceofMATLABchangedthefaceofscientificcomputingformanypractitioners,theseauthorsincluded.MATLABevolvedfromtheLinpackpackage,whichwas familiartoFortranprogrammersasarobustcollectionoftoolsforlinearalgebra.However, MATLABalsointroducedanewvector-orientedprogramminglanguage,aninteractive environment,andbuilt-ingraphics.Thesefeaturesofferedsufficientadvantagesthatusers foundtheirproductivitywassignificantlyincreasedoverthemoretraditionalenvironments.Sincethen,MATLABhasevolvedtohavealargearrayofnumericaltools,both commercialandshareware;excellent2Dand3Dgraphics;object-orientedextensions;and abuilt-ininteractivedebugger.
Ofcourse,CandFortranhaveevolvedaswell.ChasledtoC++andFortrantoFortran90.Thoughbothoftheselanguageshavetheiradherents,neitherseemstoofferas completeapackageasdoesMATLAB.Forexample,theinclusionofagraphicsfacilityin thelanguageitselfisamajorboon.ItmeansthatMATLABprogramsthatusegraphicsare standardthroughouttheworldandrunthesameonallsupportedplatforms.Italsoleads totheabilitytographicallydisplaydataarraysatabreakpointinthedebugger.Theseare usefulpracticaladvantages,especiallywhenworkingwithlargedatasets.
ThevectorsyntaxofMATLAB,oncemastered,leadstomoreconcisecodethaninmost otherlanguages.Settingonematrixequaltothetransposeofanotherthroughastatement like A=B’; ismuchmoretransparentthansomethinglike doi=1,n doj=1,m
A(i,j)=B(j,i) enddo enddo
Also,forthebeginner,itisactuallyeasiertolearnthevectorapproach,whichdoesnot requiresomanyexplicitloops.ForsomeonewellversedinFortran,itcanbedifficultto unlearnthishabit,butitiswellworththeeffort.
ItisoftenarguedthatCandFortranaremoreefficientthanMATLABandthereforemore suitableforcomputationallyintensivetasks.However,thisviewmissesthebigpicture. Whatreallymattersistheefficiencyoftheentirescientificprocess,fromthegenesisofthe
idea,throughitsroughimplementationandtesting,toitsfinalpolishedform.Arguably, MATLABismuchmoreefficientforthisentireprocess.Thebuilt-ingraphics,interactive environment,largetoolset,andstrictrun-timeerrorcheckingleadtoveryrapidprototypingofnewalgorithms.Eveninthemorenarrowview,well-writtenMATLABcodecan approachtheefficiencyofCandFortran.Thisispartlybecauseofthevectorlanguagebut alsobecausemostofMATLAB’snumbercrunchingactuallyhappensincompiledlibrary routineswritteninC.
TraditionallanguageslikeCandFortranoriginatedinanerawhencomputerswere room-sizedbehemothsandresourceswerescarce.Asaresult,theselanguagesareoriented towardsimplifyingthetaskofthecomputerattheexpenseofthehumanprogrammer.Their crypticsyntaxleadstoefficienciesinmemoryallocationandcomputationspeedthatwere essentialatthetime.However,timeshavechangedandcomputersarerelativelyplentiful, powerful,andcheap.Itnowmakessensetoshiftmoreoftheburdentothecomputerto freethehumantoworkatahigherlevel.Spendinganextra$100tobuymoreRAMmay bemoresensiblethandevelopingacomplexdata-handlingschemetofitaprobleminto lessspace.Inthissense,MATLABisahigher-levellanguagethatfreestheprogrammer fromtechnicaldetailstoallowtimetoconcentrateontherealproblem.
Ofcourse,therearealwayspeoplewhoseethesechoicesdifferently.ThoseindisagreementwiththereasonscitedhereforMATLABcanperhapstakesomecomfortinthefact thatMATLABsyntaxisfairlysimilartoCorFortranandtranslationisnotdifficult.Also, TheMathWorksmarketsaMATLAB“compiler”thatemitsCcodethatcanberunthrough aCcompiler.
1.2MATLABConventionsUsedinThisBook
ThereareliterallyhundredsofMATLAB functions thataccompanythisbook(andhundredsmorethatcomewithMATLAB).SincethisisnotabookaboutMATLAB,mostof thesefunctionswillnotbeexaminedinanydetail.However,allhavefullonlinedocumentation,andtheircodeisliberallysprinkledwithcomments.Itishopedthatthisbook willprovidethefoundationnecessarytoenabletheusertouseandunderstandallofthese commandsatwhateverlevelnecessary.
Typographicstylevariationsareemployedheretoconveyadditionalinformationabout MATLABfunctions.Afunctionnamepresentedlike plot referstoaMATLABfunction suppliedbyTheMathWorksaspartofthestandardMATLABpackage.Afunctionname presentedlike dbspec referstoafunctionprovidedwiththisbook.Moreover,thename NMESToolbox referstotheentirecollectionofsoftwareprovidedinthisbook.
MATLABcodewillbepresentedinsmallnumberedpackagesentitled“codesnippets.” Anexampleisthecoderequiredtoconvertanamplitudespectrumfromlineartodecibel scale(CodeSnippet1.2.1).
TheactualMATLABcodeisdisplayedinanuprighttypewriterfont,whileintroductory remarksare emphasizedlikethis.Thecodesnippetsdonotemploytypographicvariations toindicatewhichfunctionsarecontainedinthe NMESToolbox asisdoneinthetextproper.
CodeSnippet1.2.1
Thiscodecomputesawaveletanditsamplitudespectrumonboth linearanddecibelscales.ItmakesFigure1.1.Thefinallineprintsthefigureintoan“eps” file,adetailthatwillnotusuallybeincludedinsubsequentcodesnippets.
1 [wavem,t]=wavemin(.001,20,1);
2 [Wavem,f]=fftrl(wavem,t);
3 Amp=abs(Wavem);
4 dbAmp=20*log10(Amp/max(Amp));
5 figure
6 subplot(3,1,1);plot(t,wavem);xlabel(’time(sec)’);xlim([0.2])
7 subplot(3,1,2);plot(f,abs(Amp));xlabel(’Hz’);ylabel(’linearscale’);
8 ylim([01.01])
9 subplot(3,1,3);plot(f,dbAmp);xlabel(’Hz’);ylabel(’dbdown’)
10 prepfiga
11 bigfont(gcf,2,1)
12
13 print-deps.\intrographics\intro1a.eps
Ithasprovenimpracticaltodiscussalloftheinputparametersforalloftheprograms showninthecodesnippets.Thosegermanetothecurrenttopicarediscussed,butthe remainderareleftforthereadertoexploreusingMATLAB’sinteractivehelpfacility.2 For example,inCodeSnippet1.2.1, wavemin createsaminimum-phasewaveletsampledat 0.002s,withadominantfrequencyof20Hzandalengthof0.2s.Then fftrl computes theFourierspectrumofthewaveletand abs constructstheamplitudespectrumfromthe complexFourierspectrum.Finally,theamplitudespectrumisconvertedtodecibels(dB) online4.
Adecibelscaleisalogarithmicdisplayofamplitude,whichisextremelyusefulwhenthe amplituderangeextendsovermanyordersofmagnitude.Givenanamplitudefunctionlike A( f ),whereinthiscontextwearespeakingoftheamplitudeofaspectrumasafunction offrequency,thedecibelrepresentationcomesfromtheformula
( f ) = 20 log10
whereweassume A( f )> 0everywhere(ifitisnotobviouswhy,thenyoumightwantto reviewlogarithms),and Aref isareferenceamplitude.Oftenthechoice Aref = max(A( f )) ismade,whichmeansthat AdB ( f ) willbeallnegativenumbersexceptatthemaximum, whereitwillbezero.Suppose A( f ) = Aref /2;then AdB = 20 log10 (0.5) ≈−6dB.Using thepropertiesoflogarithms,if A( f ) = Aref /4,then AdB = 20 log10 ( 1 2 1 2 ) = 20[log10 1 2 + log10 1 2 ]≈−12dB.So,every 6dBrepresentsadecreaseinamplitudebyafactorof 1 2 . Similarly,ifthereferenceamplitudeisnotthemaximum,theneveryincreaseinamplitude
2 Tofindoutmoreaboutanyofthesefunctionsandtheirinputsandoutputs,type,forexample,“helpfftrl”at theMATLABprompt.
Figure1.1 Aminimum-phasewavelet(top),itsamplitudespectrumplottedonalinearscale(middle),anditsamplitude spectrumplottedonadecibelscale(bottom).
byafactorof2isa+6dBincrease.Sometimesweprefertothinkinfactorsof10,anditis similarlyeasytoshowthat 20dBrepresentsadecreasebyafactorof 1 10 ,while+20dB isanincreaseby10.
Thisexampleillustratesseveraladditionalconventions.Digitalseismictraces3 arediscretetimeseries,butthecommontextbookconventionofassumingasampleintervalof unityinarbitraryunitsisnotappropriateforpracticalproblems.Instead,twovectorswill beusedtodescribeatrace,onetogiveitsamplitudeandtheothertogivethetemporal coordinatesforthefirstvector.Thus wavemin returnstwovectors(thattheyarevectors isnotobviousfromthecodesnippet),with wavem beingthewaveletamplitudesand t beingthetimecoordinatevectorfor wavem.ThusthetopgraphofFigure1.1iscreatedby simplycross-plottingthesetwovectors: plot(t,wavem).Similarly,theFourierspectrum, Wavem,hasafrequencycoordinatevector f.Temporalvaluesmustalwaysbespecifiedin secondsandwillbereturnedinseconds,andfrequencyvaluesmustalwaysbeinhertz. (Onehertz(Hz)isonecyclepersecond.)Millisecondsorradians/secondspecifications willneveroccurincode,thoughbothcyclicalfrequency f andangularfrequency ω may appearinformulas(ω = 2π f).
Seismictraceswillalwaysberepresentedbycolumnvectors,whetherinthetimeorthe Fourierdomain.Thisallowsaneasytransitionto2Dtracegathers,suchassourcerecords andstackedsections,whereeachtraceoccupiesonecolumnofamatrix.Thiscolumn vectorpreferenceforsignalscanleadtoaclassofsimpleMATLABerrorsfortheunwary user.Forexample,supposeaseismictrace s istobewindowedtoemphasizeitsbehaviorin onezoneandde-emphasizeitelsewhere.Thesimplestwaytodothisistocreateawindow vector win thatisthesamelengthas s anduseMATLAB’s.*(dot-star)operatortomultiply
3 A seismictrace isusuallytherecordingofasingle-componentgeophoneorhydrophone.Lesscommonly,for amulticomponentgeophone,itistherecordingofonecomponent.
CodeSnippet1.2.2
Thiscodecreatesasyntheticseismogramusingthe convolutional model butthengeneratesanerrorwhiletryingtoapplyaboxcarwindow.(Aboxcar windowisatimeseriesthatisallzerosexceptforaselectedrange,whereitis1.0.)
1 [r,t]=reflec(1,.002,.2);%makeasyntheticreflectivity
2 [w,tw]=wavemin(.002,20,.2);%makeawavelet
3 s=convm(r,w);%makeasyntheticseismictrace
4 n2=round(length(s)/2);
5 win=0*(1:length(s));%initializewindowtoallzeros
6 win(n2-50:n2+50)=1;%100samplesinthecenterofwinare1.0
7 swin=s.*win;%applythewindowtothetrace
EndCode
introcode/intro2.m
eachsampleinthetracebythecorrespondingsampleinthewindow.Thetemptationisto writeacodesomethinglikeCodeSnippet1.2.2. MATLAB’sresponsetothiscodeistheerrormessage
Errorusing.*
Matrixdimensionsmustagree.
Theerroroccursbecausethecode 1:length(s) usedonline5generatesa1 × 501row vectorlike1,2,3, , n,where n=length(s),while s isa501 × 1columnvector.The.* operatorrequiresbothoperandstohaveexactlythesamegeometry.Thesimplestfixisto write swin=s.*win(:);,whichexploitstheMATLABfeaturethat a(:) isreorganized intoacolumnvector(seepage38foradiscussion)regardlessoftheactualsizeof a.
Asmentionedpreviously,two-dimensionalseismicdatagatherswillbestoredinordinarymatrices.Eachcolumnisasingletrace,andsoeachrowisa timeslice.Acomplete specificationofsuchagatherrequiresbothatimecoordinatevector, t,andaspace coordinatevector, x
Rarelywilltheentirecodefromafunctionsuchas wavemin or reflec bepresented. Thisisbecausethecodelistingsofthesefunctionscanspanmanypagesandcontainmuch materialthatisnotdirectlyrelevantorisoutsidethescopeofthisbook.Forexample,there areoftenmanylinesofcodethatcheckinputparametersandassigndefaults.Thesetasks havenothingtodowithnumericalalgorithmsandsowillnotbepresentedordiscussed. Ofcourse,thereaderisalwaysfreetoexaminetheentirecodesatleisure.
1.3SeismicWavelets
Intheprevioussection,mentionwasmadeofa minimum-phasewavelet,ofwhichthere isagreatvariety,andtherearealsomanyotherwavelettypes.Inseismology,awavelet typicallyrepresentsthewaveformemittedbyaseismicsourceandaspossiblymodified bydataprocessing.Allwavelettypeshaveaspecificmathematicalform,whichmayhave
variableparameterssuchasadominantfrequency,awaveletlength(intime),andperhaps aphaseoption.Themathematicalformulasthatthesewaveletsarebasedonwillbepresentedlater,inSection4.7.1.Fornow,wewillsimplyillustratehowtogeneratesomebasic waveletsanddiscusstheirmostobviousproperties.
Aminimum-phasewavelet(Figure1.1)referstoaspecifictypeofwaveletthatis thoughttobeagoodmodelforthewaveformfromanimpulsiveseismicsourcesuchas dynamite.Fornow,thereasonsforthename“minimumphase”willnotbeinvestigated; rather,itissufficienttorealizethatsuchwaveletsarecausal,4 haveashapedamplitude spectrumwithadominantfrequency,haveanamplitudespectrumthatmustnevervanish atanyfrequency,5 andhaveaphasespectrumthatislinkedtotheshapeoftheamplitude spectrum.(Ifthesetermssuchas“spectrum,”“amplitude,”and“phase”areunfamiliar, restassuredthattheywillbeexplainedfullyinChapter2.)Minimum-phasewavelets aretypicallyusedtomodelraworunprocesseddata.Whiletheyarisenaturally,theyare notusuallypreferredforinterpretation,becausetheirpeakenergyisalwaysdelayedfrom t = 0.Itisamajortaskofdataprocessingtoestimatethewaveletintherawdata(it isneverknownapriori)anddesignanoperatortoshapeittozerophase.Thusvarious zero-phasewaveletsaretypicallyusedtorepresentprocessedseismicdata.
Figures1.2aand1.2bshowfourtypicalwaveletsinthetimeandfrequencydomains. ThefigureswerecreatedbyCodeSnippet1.3.1.Theminimum-phasewavelet,created by wavemin ,showsthebasiccausalnatureandthedelayofthemaximumenergyfrom t = 0.TheRickerwavelet,createdby ricker ,isapopularwaveletforuseinseismic interpretation.Afrequenttaskistocreateasyntheticseismogramfromwellinformation tocomparewithafinalseismicimage.Thesyntheticseismogram, s,isusuallyconstructed fromaconvolutionalmodel s(t) = w (t) • r(t),where r isthereflectivityfunction(derived fromlogginginformation), w isusuallyazero-phasewavelet,veryoftenaRickerwavelet, and • representsconvolution.6 InFigure1.2b,theamplitudespectraofthesewaveletsare shown,andtheminimum-phaseandRickerwaveletsaresimilar,withsmoothlysloping spectraawayfromadominantfrequency.WhiletheRickerspectrumisonlyadjustableby shiftingthedominantfrequency,thedecayrateoftheminimum-phasewavelet’sspectrum canbecontrolledwiththeparameter m online8.Theothertwowavelets,Ormsby(from ormsby )andKlauder(from klauder ),havenosingledominantfrequencybutinstead haveabroad,flatpassband.Thedetailsofthesewaveletsandspectracanbecontrolled fromtheinputparametersinthevariousprograms.InCodeSnippet1.3.1,theseparameters havebeendeliberatelychosentomakethewaveletssimilar.
Comparingthedifferentzero-phasewavelets,itisapparentthattheRickerwavelethas theleastsidelobeenergybutalsohasagreaterwidthasmeasuredbetweenthefirstzero crossingsoneithersideofthemaximum.Thechoiceofwhichzero-phasewavelettouse increatingaseismogramwilldependuponthenatureofthedataandthedataprocessing. TheKlauderwaveletisintendedfor correlated Vibroseisdata.Vibroseisdataisusually
4 Acausalwaveletisonethatvanishesbeforeaspecifictime,usuallytakentobe t = 0,andpersistsarbitrarily longafterwards.
5 Exceptpossiblyatafewisolatedpoints.
6 ConvolutionisdescribedintuitivelyinSection2.3.1.
CodeSnippet1.3.1
ThiscodecreatesFigures1.2aand1.2b.Fourwaveletsarecreated: minimumphasewith wavemin ,Rickerwith ricker ,Ormsbywith ormsby ,andKlauder with klauder .Theminimum-phaseandRickerwaveletshavethesamedominantfrequency,specifiedonline2.TheOrmsbyandKlauderwaveletshavebroad,flatpassbands from fmin to fmax.Thewaveletsallhavethesametemporallengthof tlen,whichisset hereto2.0s.Thisismuchlongerthanusualandisdonesimplytoforcefinesamplingin thefrequencydomainandmakeabettergraph.Thewaveletsarecreatedonlines10–13 andtheirspectraarecalculatedonlines15–18using fftrl .Notetheuseof linesgray toplot,ratherthan plot .Thismakesfigureswithgraylinesofdifferentshadesratherthan color.Theequivalent plot commandsareshowncommentedoutforcomparison.
1 dt=.001;%timesamplesize
2 fdom=30;%dominantfrequencyforrickerandwavemin
3 fmin=10;%beginningofpassbandforormsbyandklauder
4 fmax=70;%endofpassbandforormsbyandklauder
5 tlen=2;%temporallengthofeachwavelet
6 slen=8;%sweeplength(klauder)
7 staper=.5;%sweeptaper(klauder)
8 m=3.5;%spectraldecaycontrolinwavemin
9
10 [wm,twm]=wavemin(dt,fdom,tlen,m);
11 [wr,twr]=ricker(dt,fdom,tlen);
12 [wo,two]=ormsby(fmin-5,fmin,fmax,fmax+10,tlen,dt);
13 [wk,twk]=klauder(fmin,fmax,dt,slen,tlen,staper);
14
15 [Wm,fwm]=fftrl(wm,twm);
16 [Wr,fwr]=fftrl(wr,twr);
17 [Wo,fwo]=fftrl(wo,two);
18 [Wk,fwk]=fftrl(wk,twk);
19
20 figure
21 inc=.1;
22 %plot(twm,wm,twr,wr+inc,two,wo+2*inc,twk,wk+3*inc);
23 linesgray({twm,wm,’-’,.4,0},{twr,wr+inc,’-’,.9,.3},...
24 {two,wo+2*inc,’-’,.9,.5},{twk,wk+3*inc,’-’,1.1,.7});
25 xlabel(’time(sec)’)
26 xlim([-.25.25]);ylim([-.1.45])
27 legend(’Minimumphase’,’Ricker’,’Ormsby’,’Klauder’)
28 grid
29 prepfig;bigfont(gca,1.8,1)
30
31 figure
32 %plot(fwm,abs(Wm),fwr,abs(Wr),fwo,abs(Wo),fwk,abs(Wk))
33 linesgray({fwm,abs(Wm),’-’,.4,0},{fwr,abs(Wr),’-’,.9,.3},...
34 {fwo,abs(Wo),’-’,.9,.5},{fwk,abs(Wk),’-’,1.1,.7});
35 xlabel(’frequency(Hz)’)
36 xtick([010305070100150])
37 legend(’Minimumphase’,’Ricker’,’Ormsby’,’Klauder’)
38 xlim([0150]);ylim([01.1])
39 grid
40 prepfig;bigfont(gca,1.8,1)
41
Figure1.2a (left)Fourcommonseismicwavelets.Theminimum-phasewaveletisacausalwavelet,whichisamodelforan impulsivesourcelikedynamite.TheRickerwaveletiszerophasewithminimalsidelobes.TheOrmsbywaveletisalso zerophasebutwithmuchlargersidelobes.Thezero-phaseKlauderwaveletisacommonmodelforVibroseisdata.The waveletsareplottedwithverticalshiftstoavoidplottingontopofeachother.
Figure1.2b (right)TheamplitudespectraofthewaveletsofFigure1.2a.Thespectraoftheminimum-phaseandRickerwavelets decaysmoothlyawayfromadominantfrequency.TheOrmsbyandKlauderwaveletshaveaflat,broadpassbandand thendecaysharplyatthelimitsofthepassband.TheearsattheedgesoftheKlauderpassbandarerelatedtothe lengthofthesweeptaper.
acquiredwithasourcesignatureknownasasweep.Asweepisatemporallylong7 signal inwhichthevibratorrunsthroughaprescribedrangeoffrequenciescalledthe sweptband Whenequalpowerisgiventoeachfrequency,thesweepisknownasalinearsweep,and thisistheusualcase.Suchdatacanbemodeledas sv (t) = σ(t) • I(t),where σ(t) isthe sweepand I(t) istheEarth’simpulseresponsethatwouldberecordedfromaperfectly impulsivesource.Afteracquisition,therawdataiscrosscorrelatedwiththesweep.We canrepresentthecorrelateddataas sc = σ(t) ⊗ sv (t) = wk (t) • I(t),where ⊗ means crosscorrelationand wk istheKlauderwaveletdefinedby wk (t) = σ(t) ⊗ σ(t).Thusthe Klauderwaveletisthecrosscorrelationofthesweepwithitself,orthe autocorrelation of thesweep.ThereisatheoreticalformulafortheKlauderwavelet,butitismorerealistic toactuallyconstructthesweepandnumericallyautocorrelateit,andthisiswhat klauder does.InFigure1.2b,theKlauderspectrumisnoticeablymorenarrowthantheOrmsby one.Thisisbecause,whilethesweepbeginsandendsat10and70Hz,respectively,its amplitudemustrisegraduallyfromzerotofullstrengthbecauseitisusedtodrivealarge machine.Thelengthoftimeoverwhichthishappensiscalledthesweeptaperandis appliedatbothendsofthesweep.Thetapercausesreducedpoweratthebeginningand endofthesweepandisalsoresponsibleforthe“ears”seenatabout15and65Hz.Reducing thelengthofthetaperwillbroadenthespectrumbutincreasetheheightoftheears.
TheOrmsbywaveletisoftenusedtorepresentseismicdatathathasbeendeliberately processedtohaveaflatpassbandoversomeprescribedrange.Thisproducesaverynarrow centralwaveletpeakbutwithquitealotofsidelobeenergy.ThespecificationofanOrmsby
7 Sweeplengthsrangingfrom4stoaslongas20sarecommoninpractice.
waveletisusuallydonebygivingthefour Ormsbyparameters,whicharecalled f1 , f2 , f3 , f4 , withthefollowingmeanings:
• f1 :highendofthelow-frequencystopband.Thereisnosignalfor f < f1
• f2 :lowendofthepassband.Theamplituderampsuplinearlyfor f1 ≤ f ≤ f2 .
• f3 :highendofthepassband.Theamplitudeisconstantfor f2 ≤ f ≤ f3 .
• f4 :lowendofthehigh-frequencystopband.Thereisnosignalfor f > f4
ThetheoreticalOrmsbywaveletisinfinitelylong,andanypracticalversionmustbe truncated.Thistruncationcausesthestopbandstohavesomenonzeroenergy.
Eachofthesewaveletsisnormalizedsuchthatasinewaveofthedominantfrequency isunchangedinamplitudewhenconvolvedwiththewavelet.
1.4DynamicRangeandSeismicDataDisplay
Seismicdatatendstobeachallengetocomputergraphicsaswellastocomputercapacity. Asingleseismicrecordcanhaveatremendousamplituderange.Inspeakingofthis,the term dynamicrange isused,whichreferstoaspanofrealnumbers.Therangeofnumerical voltagesthataseismicrecordingsystemcanfaithfullyhandleiscalleditsdynamicrange. Forexample,currentdigitalrecordingsystemsuseafixedinstrumentgainandrepresent amplitudesas24-bitintegercomputerwords.8 Thefirstbitisusedtorecordthesign,while thelastbittendstofluctuaterandomly,soeffectively22bitsareavailable.Thismeansthat theamplituderangecanbeaslargeas222 ≈ 106.6 .Usingthedefinitionofadecibelgiven inEq.(1.1),thiscorrespondstoabout132dB,anenormousspreadofpossiblevalues. This132dBrangeisneverusuallyfullyrealizedwhenrecordingsignals,foravarietyof reasons,themostimportantbeingtheambientnoiselevelsattherecordingsiteandthe instrumentgainsettings.
Inafixed-gainsystem,theinstrumentgainsettingsaredeterminedtominimize clipping bytheanalog-to-digitalconverterwhilestillpreservingthesmallestpossiblesignal amplitudes.Thisusuallymeansthatsomeclippingwilloccuroneventsnearesttheseismic source.Averystrongsignalshouldsaturate22–23bits,whileaweaksignalmayaffect onlythelowestseveralbits.Thusthe precision,whichreferstothenumberofsignificant digitsusedtorepresentafloating-pointnumber,steadilydeclinesfromthelargesttothe smallestnumberinthedynamicrange.
1.4.1Single-TracePlottingandDynamicRange
Figure1.3wasproducedwithCodeSnippet1.4.1andshowstworealseismictraces recordedin1997byCREWES.9 Thistypeofplotiscalleda wiggletracedisplay.
8 Previoussystemsuseda16-bitwordandvariablegain.Afour-bitgainword,an11-bitmantissa,andasign bitdeterminedtherecordedvalue.
9 CREWESisanacronymforthe ConsortiumforResearchinElasticWaveExplorationSeismology atthe UniversityofCalgary.
CodeSnippet1.4.1
Thiscodeloadsnearandfaroffsettesttraces,computesthe Hilbert envelopes ofthetraces(withadecibelscale),andproducesFigures1.3and1.4.
1 clear;loadtesttrace.mat
2 figure
3 subplot(2,1,1);plot(t,tracefar,’k’);
4 title(’1000moffset’);xlabel(’seconds’)
5 subplot(2,1,2);plot(t,tracenear,’k’);
6 title(’10moffset’);xlabel(’seconds’)
7 prepfig
8 bigfont(gcf,1.3,1)
9
10 print-depscintrographics\intro3.eps
11
12 envfar=abs(hilbert(tracefar));%computeHilbertenvelope
13 envnear=abs(hilbert(tracenear));%computeHilbertenvelope
14 envdbfar=todb(envfar,max(envnear));%decibelconversion
15 envdbnear=todb(envnear);%decibelconversion
16 figure
17 plot(t,[envdbfarenvdbnear],’k’);xlabel(’seconds’);ylabel(’decibels’);
18 grid;axis([03-1400])
19 prepfig
20 bigfont(gcf,1.3,1)
21 ht=text(1.2,-40,’Neartraceenvelope’);
22 fs=get(ht,’fontsize’);
23 set(ht,’fontsize’,2*fs);
24 ht=text(1,-100,’Fartraceenvelope’);
25 set(ht,’fontsize’,2*fs);
26
27 print-depscintrographics\intro3_1.eps
EndCode
introcode/intro3.m
Theuppertrace,called tracefar,wasrecordedontheverticalcomponentofathreecomponentgeophoneplacedabout1000mfromthesurfacelocationofadynamiteshot.10
Thelowertrace,called tracenear,wassimilarexceptthatitwasrecordedonly10m fromtheshot.(BothtracescomefromtheshotrecordshowninFigures1.10aand1.10b.)
Thedynamiteexplosionwasproducedwith4kgofexplosiveplacedat18mdepth.Such anexplosivechargeisabout2mlong,sothedistancefromthetopofthechargetothe closestgeophonewasabout √162 + 102 ≈ 19m,whilethattothefarthestgeophone wasabout √162 + 10002 ≈ 1000m.Theverticalaxesforthetwotracesindicatethevery largeamplitudedifferencebetweenthem.Iftheywereplottedonthesameaxes, tracefar wouldappearasaflat,horizontallinenexttotracenear.
10 Adynamitechargeisusuallyplacedbelowgroundinaboreholedrilledbyaspecialshot-holedrilling machine.Suchboreholesaretypically2inchesindiameterand3–20mdeep.Largerchargesareplacedin deeperholes.A1kgchargesizeplaced10mdeepiscommonandispackagedasacylinderabout1ftlong and2inchesindiameter.Thesurfacelocationofadynamiteshotreferstothelocationofthetopoftheshot holeattheEarth’ssurface.
Figure1.3 Top:Arealseismictracerecordedabout1000mfromadynamiteshot.Bottom:Asimilartracerecordedonly10m fromthesameshot.SeeCodeSnippet1.4.1.
Figure1.4(alsoproducedwithCodeSnippet1.4.1)showsamoredefinitivecomparison oftheamplitudesofthetwotraces.Herethe traceenvelopes arecomparedusingadecibel scale.Atraceenvelopeisamathematicalestimateofaboundingcurveforthesignal(this willbeinvestigatedmorefullylater,inSection2.4.6)andiscomputedusing hilbert, whichcomputesthecomplex,analytictrace(Taneretal.,1979;Cohen,1995)andthen takestheabsolutevalue.Theconversiontodecibelsisdonewiththeconveniencefunction todb (forwhichthereisaninverse, fromdb ).Thefunction todb implementsEq.(1.1) forbothrealandcomplexsignals.(Inthelattercase, todb returnsacomplexsignalwhose realpartistheamplitudeindecibelsandwhoseimaginarypartisthephase.)Bydefault,the maximumamplitudeforthedecibelscaleisthemaximumabsolutevalueofthesignal,but thismayalsobespecifiedasthesecondinputto todb .Thefunction fromdb reconstructs theoriginalsignalgiventheoutputof todb .
Thecloseproximityof tracenear toalargeexplosionproducesaverystrongfirst arrival,whilelaterinformation(at3s)hasdecayedby ∼72dB.(Togainfamiliaritywith decibelscales,itisusefultonotethat6dBcorrespondstoafactorof2.Thus72dBrepresentsabout72/6 ∼ 12doublings,orafactorof212 = 4096.)Alternatively, tracefar showspeakamplitudesthatare40dB(afactorof26.7 ∼ 100) weakerthan tracenear.
The firstbreaktime isthebestestimateofthearrivaltimeofthefirstseismicenergy. For tracefar,thisisabout0.380s,whilefor tracenear itisabout0.02s.Oneachtrace, energybeforethistimecannothaveoriginatedfromthesourcedetonationandisusually takenasanindicationofambientnoiseconditions.Thatis,itisduetoseismicnoisecaused bywind,traffic,andothereffectsoutsidetheseismicexperiment.Onlyfor tracefar isthe firstarrivallateenoughtoallowareasonablesamplingoftheambientnoiseconditions.In thiscase,theaveragebackgroundnoiselevelisabout120to130dBbelowthepeaksignals
Figure1.4 TheenvelopesofthetwotracesinFigure1.3plottedonadecibelscale.Thefaroffsettraceisabout40dBweakerthan thenearoffsetone,andthetotaldynamicrangeisabout120dB.SeeCodeSnippet1.4.1.
on tracenear.Thisisveryneartheexpectedinstrumentperformance.Itisinterestingto notethatthelargestpeakson tracenear appeartohavesquaretops,indicatingclipping, atanamplitudelevelof1.0.Thisoccursbecausethegainsettingsoftherecordingsystem weresettojustclipthestrongestarrivalsandthereforedistributetheavailabledynamic rangeoveranamplitudebandjustbeneaththesestrongarrivals.
Dynamicrangeisanissueinseismicdisplayaswellasinrecording.Itisapparentfrom Figure1.3thatbothsignalsfadebelowthevisualthresholdatabout1.6s.Checkingwith theenvelopesinFigure1.4,thissuggeststhatthedynamicrangeofthisdisplayisabout 40–50dB.Thislimitationiscontrolledbytwofactors:thetotalwidthallottedforthetrace plotandtheminimumperceivabletracedeflection.InFigure1.3,thiswidthisabout1inch andtheminimumdiscerniblewiggleisabout0.01inches.Thusthedynamicrangeisabout 10 2 ∼ 40dB,inagreementwiththeearliervisualassessment.
Itisveryimportanttorealizethatseismicdisplayshavelimiteddynamicrange.This meansthatwhatyouseeisnotalwayswhatyou’vegot.Forexample,ifaparticularseismic displaybeinginterpretedforexplorationhasadynamicrangeof,say,20dB,thenany spectralcomponents(i.e.,frequenciesintheFourierspectrum)thataremorethan20dB downwillnotaffectthedisplay.Iftheseweakspectralcomponentsaresignalratherthan noise,thenthedisplaydoesnotallowtheoptimaluseofthedata.Thisisanespecially importantconcernforwiggletracedisplaysofmultichanneldata,whereeachtracegets aboutone-tenthofaninchofdisplayspace.
Morepopularthanthewiggletracedisplayisthewiggle-trace,variable-area(WTVA) display.Thefunction wtva (CodeSnippet1.4.2)wasusedtocreateFigure1.5,wherethe twodisplaytypesarecontrastedusing tracefar.TheWTVAdisplayfillsinthe peaks oftheseismictrace(or troughs ifthepolarityisreversed)withsolidcolor.Doingthis
time(sec)
Figure1.5 AportionoftheseismictraceinFigure1.3isplottedinWTVAformat(top)andwiggletraceformat(bottom).See CodeSnippet1.4.2.
CodeSnippet1.4.2 Thesametraceisplottedwith wtva and plot.Figure1.5istheresult.
1 clear;loadtesttrace.mat
2 figure
3 plot(t,tracefar,’k’)
4 [h,hva]=wtva(tracefar+.02,t,’k’,.02,1,-1,1);
5 axis([.2.8-.02.04])
6 xlabel(’time(sec)’)
7 prepfiga
8 bigfont(gcf,1.7,1)
9
10 print-deps.\intrographics\intro3a
EndCode introcode/intro3a.m
requiresdeterminingthe zerocrossings ofthetrace,whichcanbeexpensiveifprecisionis necessary.Thefunction wtva justpicksthesampleclosesttoeachzerocrossing.Formore precision,thefinalargumentof wtva isaresamplingfactor,whichcausesthetracetobe resampledandthenplotted.Also, wtva workslikeMATLAB’slow-levelfunction line in thatitdoesnotclearthefigurebeforeplotting.Thisallows wtva tobecalledrepeatedlyin alooptoplotaseismicsection.Thereturnvaluesof wtva areMATLABgraphicshandles forthe“wiggle”andthe“variablearea”thatcanbeusedtofurthermanipulatetheirgraphic properties.(Formoreinformation,consultyourMATLABreference.)
Clipping wasmentionedpreviouslyinconjunctionwithrecordingbutalsoplaysarole indisplay.Clippingreferstotheprocessofsettingallvaluesonatracethataregreater thanacliplevelequaltothatlevel.Thiscanhavetheeffectofmovingthedynamicrange
Figure1.6 TheseismictraceinFigure1.3isplottedrepeatedlywithdifferentcliplevels.Thecliplevelsareannotatedoneach trace.SeeCodeSnippet1.4.3.
ofaplotdisplayintoloweramplitudelevels.Figure1.6resultsfromCodeSnippet1.4.3 andshowstheeffectofplottingthesametrace(tracefar)atprogressivelyhigherclip levels.Thefunction clip producesaclippedtrace,whichissubsequentlyrescaledsothat thecliplevelhasthesamenumericalvalueasthemaximumabsolutevalueoftheoriginal trace.Theeffectofclippingistomaketheweakeramplitudesmoreevidentatthepriceof severelydistortingthestrongeramplitudes.Clippingdoesnotincreasethedynamicrange; itjustshiftstheavailablerangetoadifferentamplitudeband.
Exercises
1.4.1UseMATLABtoloadthetesttracesshowninFigure1.3anddisplaythem.By appropriatelyzoomingyourplot,estimatethefirstbreaktimesasaccuratelyas youcan.Whatistheapproximatevelocityofthematerialbetweentheshotand thegeophone?(Youmayfindthefunction simplezoom useful.Aftercreating yourgraph,type simplezoom attheMATLABpromptandthenusethe left mousebuttontodrawazoombox.Adouble-clickwillunzoom.)
1.4.2UseMATLABtoloadthetesttracesshowninFigure1.3andcomputethe envelopesasshowninCodeSnippet1.4.1.Foreithertrace,plotboththewiggle traceanditsenvelopeandshowthatthetraceiscontainedwithinthebounds definedby ± envelope.
1.4.3Whatisthedynamicrangeofaseismicwiggletracedisplayplottedat10 traces/inch?Whatabout30traces/inch?
CodeSnippet1.4.3
ThiscodemakesFigure1.6.Thetesttraceisplottedrepeatedlywith progressivelyhighercliplevels.Line5definesthecliplevel,line6clipsthetraceby calling clip ,andline7rescalestheclippedtracetohavethesamemaximumamplitude astheoriginaltrace.Thelastlinecreatesaneps(EncapsulatedPostscript)filefromthe figure.
1 clear;loadtesttrace.mat
2 figure
3 amax=max(abs(tracefar));
4 fork=1:10
5 clip_level=amax*(.5)^(k-1);
6 trace_clipped=clip(tracefar,clip_level);
7 trace_adj=trace_clipped*amax/max(abs(trace_clipped));
8 wtva(trace_adj+(k-1)*3*amax,t,’k’,(k-1)*3*amax,1,1,1);
9 ht=text((k-1)*3*amax,-.05,[int2str(-(k-1)*6)’db’]);
10 set(ht,’horizontalalignment’,’center’) 11 end 12 flipy;ylabel(’seconds’);
prepfig
xlim([-.05.55])
1.4.2MultichannelSeismicDisplay
Figure1.6illustratesthebasicideaofamultichannelWTVAdisplay.Assuming ntr traces, theplotwidthisdividedinto ntr equalsegmentstodisplayeachtrace.Atraceisplotted inagivenplotsegmentbyaddinganappropriateconstanttoitsamplitude.Inthegeneral case,thesesegmentsmayoverlap,allowingtracestooverplotoneanother.Afteraclip levelischosen,thetracesareplottedsuchthatanamplitudeequaltothecliplevelgetsa traceexcursion totheedgesofthetraceplotsegment.Forhard-copydisplays,thetraces areusuallyplottedataspecifiednumberperinch.
Figure1.7aisasyntheticseismicsectionmadebycreatingasinglesyntheticseismic traceandthenreplicatingitalongasinusoidal(withrespectto x)trajectory.Figure1.7b isazoomedportionofthesamesyntheticseismicsection.Thefunction plotseis made theplot(seeCodeSnippet1.4.3),andclippingwasintentionallyintroduced.Thesquare troughsinseveralevents(e.g.,near0.4s)arethesignatureofclipping.Thefunction plotseis providesfacilitiestocontrolclipping,produceeitherwiggletraceorWTVA displays,changepolarity,andmore.Bydefault, plotseis plotsonthecurrentaxis,so hereweprecedeitwiththe figure commandtoopenupanewwindow.Thesubsequent bigfont commandmakestheaxisfontboldforimprovedreadability.Bydefault, plotseis putsthehorizontalaxisontop,butthiscanbeshiftedtothebottomwiththe command set(gca,’xaxislocation’,’bottom’).
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with dull red or yellow marks or tinges, and fatal only to little insects which were lured by the glittering bait of honey drops spread out for them.
Now let us introduce a native of Australia, called the stinging tree. No wasp, or hornet, ever had a sharper sting. Dreaded by all who know it, the stinging tree is yet a beautiful object. It is only twelve feet high and has dark, glossy, green leaves, and large bunches of scarlet berries. The tree grows always in a cone shape and reminds one of a Christmas-tree lighted with red candles. But at the tip of each projection of the green, saucer-shaped leaf grows a deadly thorn. The least touch of one of these thorns fills a man or beast with a terrible pain. Beasts act as if seized with hydrophobia; human beings stung by this tree are partly paralyzed, and suffer agony for days, or even weeks. The dry leaf, wind-blown, is able to effect as serious a sting as the leaf still on the cruel tree. Dogs and horses that have come in contact with one of these thorns must be killed at once, as in their pain they become dangerous.
Let us turn from this cruel stinging tree to one more useful and pleasant, the soap tree of Chili. This is a large, handsome tree with dark green leaves, casting a delightful shade. When the rough, outer bark is stripped away, behold under it a smooth, white bark, which, ground to powder, affords a soap nearly as good as any that you can buy. Who need have dirty hands or soiled clothing when a soap tree grows close beside his door? “Odd bark that,” do you say? Come to this next tree and we will show you a yet more wonderful bark.
Have you any jugs or bottles to cork? Do you want some cork soles? Here is the tree on which they grow. This is the great cork tree of Spain. It is found in many parts of South Europe and on the North African coast, but Spain is the land where cork collecting is a chief industry When you take a cork in your hand, let it be as the enchanted stone, to carry you away to the warm hills of Spain, where you may see the laughing, black-eyed Spanish boys and girls running about to help the cork gatherers.
Our cork tree is thirty feet high, with well-spread branches, and a trunk very stout in proportion to the size of the tree. This trunk is
covered with a thick, tough, elastic bark, which is constantly increased from within, growing thicker and thicker. When the trees are between fifteen and twenty years old, the bark is stripped off for the first time. This first bark is coarse and is used only for tanning, and for rough work. After eight or ten years the bark is stripped off again, and this time it is finer, and good enough for net-floats, and other coarse work, as life-preservers and buoys.
However, in eight or ten years more another coat can be stripped off, and this is fine and beautiful cork that will bring a good price. A cork tree will live for one hundred and fifty years, during which time the bark may be regularly stripped at intervals of eight or ten years. The stripping is done in July and August by making a cut entirely around the stem, just above the ground, and a second cut just below the branches, then three cuts are made lengthwise the tree, and the bark is taken off in long strips. The greatest care is used to avoid injuring the inner bark. These slabs of bark are heated and slightly scorched; this causes them to flatten, closes up the pores, and gives toughness to the whole material. What should we do without cork for jackets, cork legs and arms, cork apparatus in lifeboats, and other appliances for saving people from drowning? Our cork tree with its marvellous bark is by no means the least valuable specimen in our wonder grove.
What is this beautiful tree, tall and stately, with delicate oblong leaves, lovely flowers, and fruit like a plum? It is the tree from which our gutta percha comes, and without that valuable product what should we do for many instruments, for piping, and for a good covering for telegraph wires? As the Chinese and Malays always cut down these noble trees to get the juice which hardens into gutta percha, sometimes three or four millions of the trees are killed each year. At that rate they will all soon be gone.
What next? Why, the tree that bears india-rubber toys, overshoes, and water-proof coats. By good luck the people of South America, Central America, Asia, and Africa, where the varieties of indiarubber, or caoutchouc, producing trees are found, have learned to tap the trees and take care of them and do not kill them for a single harvest. The rubber is the hardened juice of the tree; when it first
runs out it is like milk or cream, and hardens very quickly It is sent to market in flat cakes called “biscuits.” Trees producing various kinds of “rubber” abound in most tropical countries.
Now, finally, here is yet another specimen in our grove, a small shrub, only a tree by courtesy, but one of the wonders of the world, nevertheless, and happily a harmless curiosity,—if let alone! This is the electric plant.
We have long known of the electric eel, and the electric fish, that can give a powerful shock to anything touching them. A French naturalist has found and investigated, in the forests of India, a shrub endowed with amazing magnetic powers. At a distance of six yards it affects the magnetic needle.
If you break a leaf from the plant, you receive a powerful shock. Touch it with the tips of your fingers, and you receive a shock as from an induction coil. No birds or insects light upon it, they know the result too well; it would be certain death. How do birds and insects learn what is safe and what is dangerous for them to do? I cannot tell you.
No electric metals are found in the vicinity of this shrub. The electricity belongs solely to the plant. At two in the afternoon the electricity is at its height; at night the plant nearly loses its curious properties. This is no doubt owing to the dewy dampness of night; for during a rain the electricity is also lost, and one can then break off a leaf with impunity. During an electric storm its intensity redoubles, so that the shrub becomes nearly as dangerous as an electric wire.
FOOTNOTES:
[62] Some of the highest authorities state that in its native waters the Regia blossom lasts but one day and two nights.
LESSON XXXI.
A NOISY FAMILY.
“When the hounds of spring are on winter’s traces, The mother of months in meadow or plain Fills the shadows and windy places With lisp of leaves and ripple of rain.”
A. C. S .
BROTHER HOP O’ MY THUMB.
Passing one day beside one of those shallow basins that are dug out of the clay soil of the west, to collect the rainfall and to serve as
watering-places for stock, I saw a strange sight. Above the brown and turbid water projected a row of green and yellow roundish knobs. These were placed at intervals eight or ten inches apart, nearly around the edge of the basin. Looking closer, I saw that at the base of each knob were two bright, prominent eyes. I laughed, as who would not, considering this living wreath encircling the pond. I had never before seen frogs drawn up in such orderly array.
For frogs these were, silent and alert. Whether fear or the heat of the day had driven them to this position I could not tell. Presently one of the bronze-green knobs elevated itself a little, became evident head and shoulders, rose above the surface of the water with a hop, and paused on the muddy edge of the basin for further observations. Watching carefully I saw that all the knobs moved now and again with a quick, bobbing motion. Connecting this motion with the sudden and final disappearance of numerous insects flying low over the water, I discovered that the neatly ambushed frogs were taking their supper. Presently I tossed a stone lightly into the pond, and at once, with a plash all the noses vanished. A little later, as twilight fell, a loud clangor of dissonant voices told that my frogs were engaged in their evening concert. A family of vocalists are these frogs.
The other day I passed this same pond. A winter of sunny weeks, with neither rain nor snow, had caused the water to dry away until only a few inches lay over the mud in the centre of the hollow.
“Where are all our frogs?” asked Hermie. “Frogs hibernate,” I said. “As winter closes in they bury themselves in masses in the mud under the water. Some degree of moisture is needful to them at all times, and the mud affords all the dampness and warmth necessary. If the pond dries away, and the mud hardens about them, so that they cannot get out, they may live in their imprisonment for a number of months.”
“I have heard,” said Hermie, “that frogs have lived shut up in lumps of coal, in rocks, and in trees, for a great many years. Do you believe that?”
“I should want more reliable witnesses and more accurate information than have ever yet appeared in behalf of these tales.
Experiments have shown that frogs and toads soon die if entirely shut away from air and food, though with a very little air and food they can live a long time shut up in wood or clay. An old man told me he had seen a frog taken out of a rock, where it had been buried five hundred years, and when it came out it howled. Now I know that a frog cannot howl,[63] and I concluded that one part of this marvellous tale was as untrue as the other.”
The frogs, whose scientific name is Ranidæ, belong to the lower vertebrates, and are the largest family of their order. They are scarcely known in Australia and South America, and reach their highest state in the East Indies.
Among the best known of the Ranidæ in England and North America is the bull-frog, noted for his large size, his noisy voice, his green coat laced with gold, and his silvery vest. The most curious forms of frogs are found in South America. The Oceanic Islands usually have very few examples of the order. Frogs can endure great changes of heat and cold, and can live on land as well as in water, provided they have the amount of moisture needed to preserve the suppleness of the skin; but salt water is fatal to the frog in any stage of its existence.
For the study of the Ranidæ let us return to the representative best known to us, the lusty croaker of the summer pond. The frog dressed in green, silver, bronze, and gold is an expert swimmer, a mighty leaper, an ear-splitting vocalist, aquatic in his tastes, never wandering far from his native pond except when sent to market, and thence served up at the table as a dainty. But on the table we should not know him for our frog; only his hind quarters appear, and these skinned and fried are much like the legs of a spring chicken.
A most harmless, timid, and interesting beastie is the frog, and often most unfortunate, being considered a legitimate mark for all the stones that can be thrown at him by urchins wandering around his native pool. Moreover, he is filled with mortal terror when with stately progress a swan or a goose sails over the water or searches in the herbage where he hides.
What can we do to give this victim of geese and small boys a certain human interest, and win for him from the boys respect and friendship, rather than pelting? Let us look at him. We have described the colors of his skin, which is smooth, having no scales or plates. The yellow color is laid upon the bronze-green ground in stripes and spots, the stripes extending from the nose to the end of the back with a shorter stripe on each side. The front feet or hands are divided into four fingers, and the front legs are much shorter, smaller, and weaker than the hind ones, which are largely developed, and are used in swimming and leaping. The feet of the hind legs are webbed and have five fingers or toes.
The frog has a backbone, but no ribs. As he is ribless he cannot expand and contract his chest in breathing, so that he must swallow what air he wants. In swallowing air he must close his mouth and take the air in only by the nostrils; therefore, oddly enough, if his mouth is forcibly kept open he may be smothered.[64] The frog’s breathing is partly through its skin, which gives off carbonic acid gas; and moisture is as needful to a frog’s skin as to the gills of a fish.[65] The frog likes damp, rainy weather, and is as fond as a child of playing in a puddle. As soon as rain falls out come the frogs. The frog’s skin absorbs moisture, which it stores up in an internal reservoir. When a frog is alarmed by being suddenly seized, it ejects some of this water. People have fancied that such water was poisonous, but it is not. Frogs have no poison sacs, and no weapons. Toads also give out a fluid from the skin.
Open our frog’s mouth. There are a few tiny teeth on the upper jaw and palate; these are for partially grinding up horny insects. See this odd tongue; it is fastened at the front end to the mouth, and the hinder part is free and hangs down the creature’s throat. This tongue is covered with a glue-like secretion, and when an insect is to be captured the tongue is snapped forward from the mouth, and the insect adheres as to bird-lime. Some frogs have cheek-pouches, and some drum-like throat-plates, wherewith to make their loud croak.
What is the life history of our frog? From seed to seed is plant-life story; from egg to egg is frog story. The eggs of the frog are
deposited in roundish masses attached to sticks lying in water, or to the stems and leaves of submerged water-plants. The creature which comes from this egg is no more like a frog, than a caterpillar is like a butterfly. It has a big head and no limbs, and what of it is not head seems to be tail. In fact, in this stage the creature is more like a fish than a frog, and has branched gills. The gills are nearly covered by a fold of skin. Have you seen it? It is a tadpole. This tadpole can live only in water, and swims and feeds from the first instant of its free life. Change in its shape begins almost immediately: the branched gills are drawn within the neck and hidden; a pair of forelegs begin to bud, and then a pair of hind-legs bud and push out faster than the fore-legs. As the legs grow the tail is gradually absorbed and disappears. Meanwhile the interior of the body changes; the lungs and heart become like those of a reptile. When first the tadpole emerged from the egg it ate the jelly-like egg cover: then, by means of a pair of little horny jaws, it ate soft animal or vegetable matter. When its gills and tail are gone, and its legs are fully formed, it hops out of the water a perfectly formed frog.
Tadpoles feed on insects and also on each other. They are admirable preparers of small skeletons. Try the tadpole at this trade; drop into his pond a dead mouse, or bird, or frog, or squirrel. Soon you will find a clean, white skeleton, nice enough to put in any museum.
The food of the adult frog is chiefly insects. Less hungry than when in the tadpole state,—for the tadpole like the child must not only eat to live but to grow—the frog is not often tempted to vary his diet.
The frog, seated in cool leaf shadows, watches with his great, black, gold-ringed eyes for such insects as good fortune shall send past his retreat. As one hovers near, out flies his limber, sticky, notched, ribbon-like tongue, true to its mark. The insect adheres to the viscous surface of the projected ribbon, and is gently deposited in the open throat. During this process the frog maintains a calm, superior, self-satisfied expression, as if in this still hunt he not so much satisfied an appetite, as fulfilled a mission of ridding nature of superfluous insects. He seems to have no malice toward the insects;
he is merely giving them an honorable burial, and saving them from the further perplexities of life.
The common frog attains a length of three or four inches.[66] He has glandular ridges down the skin of his back, and these, with his colors, singularly fit him for his home: the darker ridges imitating plant-stems; the green coat, leafage; the silver vest, the glimmer of water; the brownish feet and markings, the moist earth; while the yellow markings add to the protective display, in being like the stamens and pistils of surrounding flowers, and of the hue of many buds and blossoms. Thus the frog in his native haunts is protected by his garments, and is little likely to be seen unless he moves, or is betrayed by his full, bright eyes or the palpitations of his breast.
While the common frog represents the aquatic Ranidæ, his cousin, the wood-frog, represents a branch of the widely distributed family which prefers dry dwellings, except in the breeding season, when the eggs must be deposited in water. The wood-frog is smaller than the bull-frog, and is dressed in olive-green and shades of brown, like the colors of dead leaves and dry twigs. There is a large black patch on the side of the head around the big ear-drum. The wood-frog takes such enormous leaps that it is very difficult to catch. It is shy, and makes a prodigious jump at the first hint of danger. When it drops to the ground it is scarcely discernible from the dry vegetation about it. The wood-frog maintains the moisture of its skin by hiding in damp moss or in decayed logs, and in little hollows in the ground. It avoids the sunshine, and keeps close to the earth.
The tree-frog is another curious frog. Smaller than its cousins, it is dressed in bright green, spotted with black, and has a membrane stretched between its toes, which gives its feet a broad, flat surface, and helps to sustain it as it leaps from branch to branch, somewhat in the fashion of a flying squirrel. In tropical lands, where many trees are decked with gorgeous blossoms, tree-frogs appear very gaily colored, the splendor of the coat being protective in such surroundings.
The swamp-frog is dressed in black and light brown, and lives in marshes in the Eastern United States. Its voice is a prolonged croak.
The clamata, or “bawling-frog,” lives about cold, damp springs, and is very noisy. The resounding roar of the bull-frog is a well-known sound. No wonder that Horace, the Roman poet, wrote that “the noisy frogs from the marshes drive away sleep.”
Cats, geese, owls, vultures, hawks, otters, and other creatures eat frogs, and the luckless creatures can hardly appear without finding an enemy. To balance this destruction of their forces they produce great numbers of eggs. When tadpoles first reach the frog state they are black. I have seen hundreds of them together, so that the pond mud seemed alive and crawling.
FOOTNOTES:
[63] Frogs and toads sometimes squeal or cry with pain or terror.
[64] As these animals respire also through the skin they may live sometime without breathing through the nostrils
[65] See Nature Reader, No. 3, Lesson 40.
[66] Our large bull-frog is eight or twelve inches long Frogs even one or two feet long (Holder’s Zoölogy) have been found, but such a size is very unusual
LESSON XXXII.
THE FROG’S COUSIN.
“Sweet are the uses of adversity, Which like a toad, ugly and venomous, Wears yet a precious jewel in his head.”
As You Like It.
Belonging to the same class of vertebrates as the frog, to the same order also, but to a different sub-order, are the toads, the cousins of the frogs. As the frog is well known about the ponds, so the toad is a constant inhabitant of our groves and gardens. We introduce the frog to an admiring public as Mr. Rana, the toad as Mr. Bufo. The order to which the frogs and toads belong is divided according to the structure of the base of the skull and the shape of the shoulder girdle. In general anatomy the frogs and toads are alike. The eggs and young are closely similar, and the stages of growth from egg to the adult form are nearly identical. When the adult form is reached the frogs and toads are yet very tiny creatures, but small as they are the most casual observer can distinguish them from each other by the shape of the snout, and by the far larger development of the hind-legs of the frog.
The chief differences to be noted between Mr. Rana and his cousin, Mr. Bufo, are these: the toad has no teeth, but the frog, as we have seen, has teeth on the upper jaw and palate. The attachment of the tongue is the same, but the free end of the frog’s tongue is forked, and the toad’s is not. The toad’s skin is usually warty, the frog’s is smooth; the toad has a rounder body, shorter hind-legs, and its feet are less fully webbed; its snout is more rounded than that of the frog.
We find, let us say, a toad and a frog in our morning ramble. We notice the soft, moist skin and say, “These are amphibians.” We see that they are tailless, and we say they are anurans, or tailless amphibians. Then having placed them together thus far, we note their differences, and we say of the frog, “This belongs to the Ranidæ,” and of the smaller cousin, “Here is one of the Bufonidæ.”
Of the two, we may find the toad the more interesting animal, as among its kinsfolk are some very remarkable families.
Mr. Bufo is a citizen of the world, except of the severely cold polar regions. He hops through the tropics and the temperate zones, and goes pretty well up into the polar regions; an inoffensive, gentle, humble, useful, and generally utterly silent creature. No person is faultless, and Bufo is not superior to humans in that no evil may be laid to his charge. He does eat bees. Happy is Mr. Bufo when, brigand-like, he can stand by the highway of the bees returning laden with wealth of honey and entering their waxen city. He holds his captives for no ransom, he scarcely stays to kill them, but swallows them alive and whole, and digests them later.
As far as we know, this bee-eating is Bufo’s one fault; to counterbalance it he devours millions of harmful insects, and so assuredly saves the world thousands of bushels of fruit and vegetables. We will not say that Bufo is greedy in eating. It is true that he swallows insects ceaselessly and swiftly, so much so that a grasshopper’s legs, or some other creature’s antennæ, may sometimes be seen sticking out of a toad’s mouth, while the carcass is well down his throat—but we will not be hard on him; we call this— zealous attention to business. The French gardeners appreciate Bufo’s usefulness, and he is brought alive to market to be sold to those who need his help in their gardens.
Bufo can be tamed and taught to eat from your hand. He can be beguiled with sugar, and with bread soaked in milk. As to this last dainty, like a captious child, he eats out the middle of the slice and leaves the crust!
There are three myths about the toad: first, that he can live during centuries shut up in clay or stone; this is no more true of Bufo than of
his cousin Rana. The second myth is that his skin when handled causes warts, and that the fluid he discharges is poisonous; this is an idle tale. The third allegation is, that he has a jewel in his head. This has been believed from very ancient times; perhaps the story rose from the beauty of his eyes, with their iris of flame-color.
The eggs of the toad are deposited closely set side by side in a long transparent tube which is dropped into water, and sinking to the bottom coils up until the eggs hatch. The young tadpoles are jet black and very active. They make all their changes very early and in the same manner as the frog, and are quite small when they arrive at the perfect toad-shape. As soon as they have produced four legs and their tails have been absorbed, they leave the water and set off on long journeys; for the toad is a born vagrant, and not, like the frog, a home-stayer.
Avoiding the sun’s heat they travel chiefly by night, and by day hide under stones or herbage. If clouds cover the sky they take heart and hop forth on their pilgrimage. During a long drought they disappear, but if a rain comes they suddenly swarm out by hundreds, and thus have arisen the tales of a “shower of toads.”
Going one day into my garden, I saw under my favorite rose tree a little hollow in the loose earth. It looked just like the cup-shaped impression made by an egg half buried in the ground. An hour later, I found a toad seated in this place, which he had evidently fashioned for himself in order to preserve the moisture of his body, while he hunted for his dinner. His keen, black eyes were fixed on the drooping stem and leaves of the rose-bush, and out and in played his little ribbon-like tongue, capturing an insect at every dart.
The toad feeds on worms as well as flies, and when after a rain the worms and toads are mutually inspired to take their walks abroad, many a luckless worm finds its way into the toad’s maw. He never eats a dead insect.
In winter the toad hibernates like the frog, and since the young toad reaches its adult size in the autumn, it passes the first period of its grown-up life in a sleep, or coma, in some hole or burrow which it has found or fashioned in the earth. Sometimes toads creep into
rock-crevices or into hollows in logs and trees, and being found in these places early in the spring, are hastily supposed to have been prisoners for many years.
The toad, as well as the frog, casts its skin in the process of growth. When the skin has become too small, and the shedding approaches, the white, green, and brown colors of the coat become dull, and a peculiar dryness appears. A new skin is forming under the outgrown one, which presently splits in half down the middle of the back and the under part of the body. The toad now begins to twist and twitch, and the old skin wrinkles and folds along the sides. Then the toad tucks a hind-leg under his forearm, and gives a good pull, and lo, he is out of that leg of his trousers. Then the other leg comes off in the same style. Next he puts one of his hands in his mouth, and giving a jerk, off comes the covering of that hand and arm, like a discarded glove. Then off with the other. Now then, what? Why, he rolls the outgrown skin into a neat ball and swallows it. No second-hand clothes sold or given away by Mr. Bufo. The frog strips off and disposes of his skin in the same way.
Toads and frogs can change to some extent the color of their skins to suit their homes. Toads kept in the dark with dark surroundings become dark in color; and those kept in light with white or light accessories become lighter; but the color of the toad changes more slowly than that of the frog. The arrangement of the color does not alter; the change is merely from light to dark.
Let us now look at some curious specimens of the Bufonidæ. The pipa, or Surinam toad, does not lay its eggs in water. The mother toad places the eggs on her back, and a fold of skin rising up encloses each egg in a separate cell, until the young have not only hatched, but have passed through all their metamorphoses, and come out fully formed. Another toad, common in Europe and Asia, is largely colored with bright crimson, and the father toad carries the little ones in separate cells fastened to his hind-legs like chains. In these cells the little ones change to their perfect shape, when the cells wither away and the young toads hop out able to take care of themselves.
We said that toads were generally silent; a little toad about three inches long, called a “natter-jack,” is common in England and is a noted singer, for a toad. His “gluck-gluck, gluck-k-k,” can be heard any night. The “green toad,” well known on the continent, is not so noisy as the natter-jack, but has a low, moaning cry.
All the true tree-toads, or Hylidæ, have clear, shrill voices,[67] and are fond of singing serenades. Like tree-frogs, tree-toads have the ends of the toes dilated into flat plates, or disks, which render their footing more secure as they hop among the branches. Our American Hylidæ are small, green-colored with brown markings, but in Asia they are often brightly colored. One very pretty little Hyla lives near ponds, and seated on the hard leaves of water-plants sings its shrill monotonous song.
All summer, from early April, the Hylidæ trill and sing, but as the autumn frosts come, their songs begin earlier and last longer. As the red and yellow leaves fall in showers from the trees, and the goldenrod and asters begin to faint and die among the sere grasses, the Hylidæ seem to be singing their requiem, and bewailing the coming of winter.
Another toad with a voice is the spade-foot. This toad is rare though widely distributed; it is remarkable for its feet, shaped for digging, its subterranean habits, and its curious way of appearing and disappearing very suddenly. After a rainy season the spade-foot will come out from some hiding-place, attract attention by loud cries, swarm by hundreds about ponds, lay innumerable eggs,—and vanish. But while thousands of eggs are laid, scores only hatch, for most of them perish from being laid so near the water’s edge that the subsidence of the water leaves them to dry up. While scores hatch, only tens grow up; for the spade-foot tadpoles devour one another, and are so greedy that very few survive. These few are most ravenous in their appetites. A spade-foot tadpole will seize a grasshopper, and being too small to swallow over half his prey, will calmly lie waiting for the swallowed half to digest in order that he may gulp down the remainder. As soon as a spade-foot tadpole becomes a “hopper,” getting four legs and losing its tail, it leaves the pond and indulges its digging instincts. It scrapes out a house for
itself, from eight inches to two feet in length, and cheerfully takes up an underground residence.
Thus we find that toads have three different methods of life, some being arboreal, or tree dwellers, seldom appearing on the ground; some subterranean, or underground dwellers seldom appearing on the surface; some dwellers on the ground, hiding among grass and other herbage when asleep, or when the sun is too hot for their comfort. But all toads, except the two varieties mentioned above, which carry their young on their bodies, go to the water to drop their eggs, and the young live in the water until they reach the adult state.
FOOTNOTES:
[67] In the spring the common toad takes to the water and there sings loudly. The loud continuous trill that we hear in swamps in springtime is made by toads, not by frogs, as is commonly supposed.
