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This 36 page Class Notes was uploaded by Jessica Braun MD on Saturday September 12, 2015. The Class Notes belongs to GEOL 554 at West Virginia University taught by Thomas Wilson in Fall. Since its upload, it has received 9 views. For similar materials see /class/202712/geol-554-west-virginia-university in Geology at West Virginia University.
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Date Created: 09/12/15
Geol 554 Wilson Environmental and Exploration Geophysics 11 Wave attenuation relationships At a distance of 100 m from a source the amplitude of a Pwave is 01000 mm and at a distance of 150 m the amplitude diminishes to 00665 mm What is the absorption coefficient of the rock through which the wave is traveling Consider both absorption and spherical divergence losses Specify the units of your result Remember the spherical divergence term translates into the following relationship A A7 S where r 1s the d1stance traveled and AS and Ar refer to amplltude at the source r and distance r respectively The absorption term is A Ase39m where at is the absorption coefficient The equation representing the combined spherical or geometrical divergence and A absorptlon losses 1s A Se 1 In the above questlon you know what the amplltudes r are at two distances from the source say r1 and r2 These amplitudes A1 and A2 are assumed to be solely the result of absorption and spherical divergence losses Take a more thorough look at this for Monday Exploration Geophysics II Don t forget to Visit the web site for slides and other info httpwwwgeowvueduwilsongeol554lect3lec3pdf T medistance relationships Raytracing tomhwi1son tomwi1sonmai1wvuedu Department of Geology and Geography West Virginia University Morgantown VN V M sd39mzlruauniw iry Surface Waves In general VR ltVL ltVS ltVP vueamumm ent vertical motion detectors I Love waves tend not to be recorded in the conventional seismic survey where the interest is primarily in the recording of Pwaves The geophones used in such surveys respond to vertical ground motion and thus do not respond to the sidetoside vibrations produced by Love waves Rayleigh waves produce large vertical displacements and are a significant source of noise in the conventional Pwave reflection seismic survey Vwammmi rmwn ent Vertical motion detectors Also since the velocity decreases in the near surface raypaths tend to bend toward normal incidence With the surface Thus shear waves also have relatively small amplitudes They can be processed out but they have relatively small amplitude memmuummy en onal PWave dta New MexiQQ I mgm mm PIAVE 439 Y s gt S 13ch Fc m R R camm mm 75 k nmncuar R menu v K squLdeUmeml n um um i Tuna W w VM wmquotAhmagi I wagmaumq 3 9139340 qu no G P 3 39LL39 Tgt r EJIRV 1139quot if i 4 61 L5 Skip to 15 Smwwmumq As time goes by re ectign a linearly with time They appm39 V meL39mvusly mumumly VWmeUmmky a mo l dk apz39hgrl llvdlo V X s VurgmaUnh suy 4 V cRchL REFRAch t This timedistance record shows everything coming in with different shapes sometimes almost at the same time and sometimes earlier sometimes later A real mess I nvrynltUnlmlly Snell s law 111 sm 61 n2 sm 82 where 11 is the index of refraction c velocity of light in vacuum 12 velocity of light in medium Skip to 30 ngvmummg Sway LAW Mg Mg 1 vi 1 SA V2 1 M ArgmmUnnmiy SAF K IXC Q V1 V VIE U V3 7 MCFEASe WhL39k 39 96134 Assume V1 lt VZ lt V3 W Lin 2 7 y Sine Vi 42451 m e GUTa 5mm AV J lt I 9 gt 9 mWIk I 1e QPP oaclxes and 3mg VI Va because sin1I2 1 wvmlaummq I X Em ec m 1 Cr an SW90 T11 QC an IQ as WVmgmaUnwmm The critical refraction problem is still a simple distance over velocity function but the velocity changes on us me Vf lo 07orhfal Tum mun Daymm meleYArd 6091th MmMmemmuq Vuscwwnumm SP COSQQ gtQL L c139Q W vugmumvumy vwwginmUmmmly 3 Ql gHraneg VHDEQQ jL 21h 3 Q11 IICOSQQ fL 4AmL Dir1043mm I Hm F Wlxmmuuvcsuy Variables39andCon 1 4 th 7 QA f Gu V1 WsdughuaUnlmmw have the correct relationship bti on to the direct arrival O X source 40 receiver offset 39 Oz distance 7 5 9 7 g 1 2 Trig simpli cations vwiMimuml vunmn I 39es of the critical refraction sin9 sin9 V1V2 quhmmUmwmv Trig transformations ti 2h1 72111tan6 V2 V1 cos 6 V2 The c tical or mjnjmmn refraction distance The refraction timeintercept Sklp to 39 Vmwapmmm C Y HA cram 39quot q quot Weg mq gt C 7 Q 5 39Wks ymallmmml ww 532 355 a s a E 5 a 9 ildg 3 x Uems gt J ue mzyuwu 1 lt v Haas gt f IL SIMZN Nummbb gt g S wmmzmmr V CTFLHJ CL FM GITrm r l at ang are 00 me wf Scares KEVEquot d antes IFEMVL 7416 Girllt49 d quot ner 19p munHam Glmc39zu rwu wmwmmmr vakhgin lmwmly 1 straight line 2 Refraction and re ection arrivals coincide at one offse 3 Refraction arrivals follow a straight line With 4 slope lVZ Where 5 lVZ is less than lV1 wwwmqu 20 caIncAL 5g 1 5 ashramquot l l l t l ll W Given that the geophone interval is 10 feet determine the velocities of events A B and C V A Mngannlwmry 5 I xz H quot p121 Density range 2 to 3 Y 012 to 11 X1011Nm2p 004 to 03 Which have the greates in uence on Vp Use Table 1 see HD1ive to help you assess this problem 0 How will you handle this problem Get together in groups and talk about this Describe your approach and hand in 26 will be due next Tuesday and should 7 from here out 7 be worked independently Brief class discussion to follow unuwumn mum 21 Continue your reading of Chapter 2 Hand in problems 21 and 23 Look over problems 27 2 12 and the absorption problem handed out today We ll discuss next Monday Vmmmml lnnuqu 22 VelocityAnalysis and Pitfalls in Seismic Interpretation tomhwilson tomwilsonmailwvuedu Department of Geology and Geography West Virginia University Morgantown WV V VislilymltUnlwisll Hand back problem 41 and discuss Exercise VI will is due this Wednesday Questions Problems 44 and 48 are also due this Wednesday Questions Hand in Exercises IVV today Hand in your Mid Term report today vwvlnilemull 3 mei ers on The oufer Trace Shoi record shows refleci39ion eveni Wl l l l near o fsei39 arrival Time of approximafely 80 ms wa mu p rai her Than single layer You were asked To freai f is as a singlel er refleci39ion problem Bui one mighi argue l l lCl l There is a shallower refleci39ion eveni l l lCl l has been overlooked Can you vwmiuum Anyi hing else Of course if is fempi39ing To simplify problems bui jusi as wif The refraci39ion analysis if we miss a shallower layer error is infroduced info our resuli39 yxwrwuim The V s are inferval velocifies and The 135 are The Twoway inferval fransif Times YWangimaUmm i 2 V1 ti 2 11 VRAISn ti 11 The Twoway Travel Time of The n reflec ror39 v VisAianuUmmsuy n hence V1 Msnt0n ZZVizti 11 11 2 2 2 VRAISnton Vn tn ZVi If gt1 1 39VMAmmLmm n71 2 2 smce VRMsn1t0n1 ti i1 2 2 2 VRAISnIOn Vn tn VRMSn1t01 2 2 2 Vn tn VRAISnton VRMSn1t01 Vquot is The inTer vaI velociTy of The n layer Tquot in This case represenTs The Twoway inTer39vaI TransiT Time Through The n layer V Visd wwummsu 2 2 2 Vn tn VRAISnton VRA1Sn1t0n1 2 2 VRAISnton VRA1Sn1t0n1 2 Vquot t n Hence The inTer39vaI velociTies of individual layers can be deTer mined from The RMS velociTies The 2way zer39o offSeT reflecTion arrival Times and inTer vaI TransiT Times VWxslnmiLimLLn 39m iaJmJM Table 1 righT IisTs reflecTion arrival Times for Three reflecTion evenTs observed in a common midpoinT gaTher The offseTs range from 3 To 36 meTers wiTh a geophone spacing of 3 meTers ConducT velociTy analysis of These Three reflecTion evenTs To deTermine Their NMO velociTy Using ThaT informaTion deTermine The inTervaI velociTies of each layer and Their Thickness V uwnIJUnlwisiA Source Receiver Offset meters USIDISZDZSZUZSAU Arrival Time ms vgtlngmiLimLmu 2 x NI 2 Irmr X Z R330 53 4 35H The variables t2 and x2 are linearly related 54m zsnn 441 nnnn 5544 mun t is the intercept amp may nnnn 72m mun 4475 mun sang nnnn IS the Slope 1295 nnnn nsnn Mun 4533 zann mus sann 2 Irmr mmmpm New WHWEHMUHIMHW c e We have nice straight lines Estimates of PMS velocities can be determined from the slopes of regression lines fitted to the tz x2 responses Keep in mind that the fitted velocity is actually an NMO velocity 39 mlEmm See Berger er ei page 173 2 2 V2 VRAISnton VRMS1t0n1 n tn The VRMSK Terms represenT The velociTies obTained from The besT fiT lines Remember These velociTies are acTually NMO velociTies tn gift The Twoway Travel Time To The N reflecTor surface I N t t The Twoway inTerval TransiT Time quot quot 0quot 039 beTween The n and n1 reflecTors Vquot is The inTerval velociTy for layer n where layer n is The layer beTween reflecTors n and n1 v m uwnmUnmisln Terminology review X2 u 2m Ammunicmumum ReflecTor 1 m 2 H In At ton ito 1 A13 to 102 3 an unueermmu lilimlagiialualgsllw The inTerval velociTy ThaT39s derived from The RMS velociTies of The reflecTions from The Top and base of a layer is referred To as The Dix inTerval velociTy However keep in mind ThaT we really don39T know whaT The RMS velociTy is The NMO velociTy is esTimaTed from The T2 x2 regression line for each reflecTion evenT and ThaT NMO velociTy is assumed To represenT an RMS velociTy You puT These ideas inTo applicaTion when solving problems 44 V me iiwnlJLlnlwhll The purpose of common midpoinT is mulTi fold IT enhances signal IT suppresses noise BoTh coherenT and incoherenT IT provides useful informaTion abouT subsurface physical properTies yuvigiiiLiLbi i 1 Random N01se Recall ThaT The relaTive ampliTude of The noise is analogous To The disTance Traveled by The random walker We acTually geT somewhere when we Take sTeps aT random In The seismic analogue if N Traces are summed TogeTher The ampliTude of The resulTanT signal will be N Times iTs original value The signal always arrives aT The same Time and sums TogeTher consTrucTively The ampliTude of random noise on The oTher hand increases N N Hence The raTio of signal To noise is T or jusT 1IN N where N is The number of Traces summed TogeTher or The number of Traces in The 6MP gaTher v Mmmn uanmw GeTTing The velociTies correcT is criTical To aTTenuaTing noise and enhancing signal AfTer NMO CorrecTion V MsrX nmanmniy SDZE 3MB 3U29 EDZS EUZS I 5 r MO correcTed CDP gaTher are summed e Tr dCe in The sTacked seismic daTg seT If The signal is summed TogeTher in reflceTion evenTs in The sTac ked seismic secTion STacking also helps aTTenuaTe coherenT noises MulTiples RefracTions Air waves Ground Roll STr eamer cable moTion ScaTTer ed waves from off line noise or unwanfed signal WHEMWW Mulfiples are considered coherenfquot x wum npmxwsm
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