Common MidPoint (CMP) Records and Stacking

Transcription

1 Evromeal ad Explorao Geophyscs II Commo MdPo (CMP) Records ad Sackg om.h.wlso Deparme of Geology ad Geography Wes rga Uversy Morgaow, W Commo Mdpo (CMP) gaher, also ofe referred o as Commo Deph Po (CDP)

2 Commo Mdpo Gaher Doug Smh s sesmc Daa Processg se hp://www-geo.phys.ualbera.ca/~doug/g438/assgmes07/lecures/commo_mdpo.pdf elocy Aalyss

3 4.) Deerme he velocy ad hckess of he maeral above he frs reflecg horzo. 73 X (meers) meer geophoe erval races 33 meers from 40 o 73 meers Dscusso of problems 4. Read over ad hk abou how you are gog o solve problems 4.5 ad 4.8

4 Sack Trace Pure sgal If we sum all he osy races ogeher - sample by sample - we ge he race ploed he gap a rgh. Ths summao of all 6 races s referred o as a sack race. Noe ha he sack race compares que well wh he pure sgal. = a j = Where s he race umber ad j s a specfc me Greebrer Huro Oodaga Radom ose ca come he form of wd, ra, mg acves, local raffc, mcrosesmcy... Nose comes several forms - boh cohere ad radom. Cohere ose may come he form of some uwaed sgal such as groud roll. A varey of processg ad acquso echques have bee developed o reduce he fluece of cohere ose. - + The basc aure of radom ose ca be descrbed he coex of a radom walk - See Feyma Lecures o Physcs, olume.

5 The radom walk aemps o follow he progress oe acheves by akg seps he posve or egave dreco purely a radom - o be deermed, for example, by a co oss. - + Does he walker ge aywhere? Our uo ells us ha he walker should ge owhere ad wll smply woder abou her po of org. However, les ake a look a he problem form a more quaave vew. I s easy o keep rack of he average dsace he walker depars from her sarg poso by followg he behavor of he average of he square of he deparure. We wre he average of he square of he dsace from he sarg po afer N seps as D N The average s ake over several repeaed rals.

6 Afer sep D wll always equal ( he average of + or - s always. Afer wo seps - D ( D +) or ( ) = D whch s 0 or 4 so ha he average s. Afer N seps D ( D + ) or ( D ) N = N N ( D ) = D + D N + N N + ( D ) = D D N N N + ( D ) = D + D N + N N + ( D ) = D D N N N + Averaged over several aemps o ge home he wayward woderer ges o average o a dsace squared Sce = D N D N + from he sarg po. D =, follows ha D N = N ad herefore ha D N = N

7 The resuls of hree ses of radom co oss expermes See Feyma Lecures o Physcs, olume. The mplcaos of hs smple problem o our sudy of sesmc mehods relaes o he resul obaed hrough sackg of he races he commo mdpo gaher. The radom ose prese each race of he gaher (ploed a lef) has bee parly bu o erely elmaed he sack race. Jus as he case of he radom walk, he ose appearg repeaed recordgs a he same ravel me, alhough radom, does o compleely cacel ou

8 The relave amplude of he ose - aalogous o he dsace raveled by our radom walker- does o drop o zero bu decreases amplude relave o he sgal. If N races are summed ogeher, he amplude of he resula sgal wll be N mes s orgal value sce he sgal always arrves a he same me ad sums ogeher cosrucvely. The amplude of ose o he oher had because s a radom process creases as N N Hece, he rao of sgal o ose s or jus N N where N s he umber of races summed ogeher or he umber of races he CMP gaher. I he example a lef, he commo mdpo gaher cosss of 6 depede recordgs of he same refleco po. The sgal-o-ose rao he sack race has creased by a facor of 6 or 4. The umber of races ha are summed ogeher he sack race s referred o as s fold.e. 6 fold.

9 If you had a 0 fold daase ad wshed o mprove s sgal-o-ose rao by a facor of, wha fold daa would be requred? Square roo of 0 = 4.47 Square roo of N(?) = 8.94 Wha s N To double he sgal o ose rao we mus quadruple he fold To double he S/N we have o crease he fold by ; o rple, by 3 ; ec. The relably of he oupu sack race s crcally depeda o he accuracy of he correco velocy.

10 Sack = Summao Average Amplude Accurae correco esures ha he same par of adjace waveforms are summed ogeher phase. If he sackg veloces are correc. he he refleco respose wll be smeared ou he sack race hrough desrucve erferece bewee races he sum.

11 Refraco o hgh velocy layers brgs he eves alog pahs ha have o-hyperbolc moveou. Greebrer Lmesoe The reaso for hs becomes obvous whe you hk of he earh as cossg of layers of creasg velocy. A larger ad larger cdece agle you are lkely o come a ear crcal agles ad he wll ravel sgfca dsaces a hgher ha average (or RMS) velocy. Bg Iju The real world: mullayer reflecos Are hey also hyperbolc? The wo-erm approxmao o he mullayer refleco respose s hyperbolc. The velocy hs expresso s a roo-mea-square velocy.

12 A seres of fe erms bu we jus gore a buch of hem The sum of squared velocy s weghed by he woway erval ras mes hrough each layer. The approxmao s hyperbolc, whereas he acual s o. The dsagreeme becomes sgfca a loger offses, where he acual refleco arrvals ofe come earler ha hose predced by he hyperbolc approxmao.

13 RMS, A ad NMO are dffere. NMO does o equal RMS. Each of hese 3 veloces has dffere geomercal sgfcace. The NMO s derved form he slope of he regresso le f o he acual arrvals. I acualy he moveou velocy vares wh offse. The RMS velocy correspods o he square roo of he recprocal of he slope of he -x curve for relavely shor offses.

14 The geeral relaoshp bewee he average, RMS ad NMO veloces s show a rgh. Geomercally he average velocy characerzes ravel alog he ormal cdece pah. The RMS velocy descrbes ravel mes hrough a sgle layer havg he RMS velocy. I gores refraco across dvdual layers.

15 Normal Icdece Tme Seco. Waer Boom Refleco Refleco from Geologc erval Waer Boom Mulple

16 Ierbed Mulples D E P T H Ierbed Mulples

17 The Power of Sack exeds o mulple aeuao eloces assocaed wh prmary reflecos are hgher ha hose assocaed wh mulples. The prmares are flaeed ou whle resdual moveou remas wh he mulple refleco eve. The NMO Correced CDP gaher

18 Mulple aeuao Prmary Reflecos Mulple Mulple Examples of mulples mare sesmc daa Bured grabe or mulple

19 Mulples are cosdered cohere ose or uwaed sgal Ierbed mulples or Sacked pay zoes

20 Waerboom ad sub-boom mulples

21 Oher forms of cohere ose wll also be aeuaed by he sackg process. The dsplays a rgh are passve recordgs (o source) of he backgroud ose. The hyperbolae you see are assocaed wh he moveme of a auger alog a pael face of a logwall me. Sackg helps aeuae radom ad cohere oses Mulples Refracos Ar waves Groud Roll Sreamer cable moo Scaered waves from off le

22 Table (rgh) lss refleco arrval mes for hree refleco eves observed a commo mdpo gaher. The offses rage from 3 o 36 meers wh a geophoe spacg of 3 meers. Coduc velocy aalyss of hese hree refleco eves o deerme her NMO velocy. Usg ha formao, deerme he erval veloces of each layer ad her hckess. Offse (m) Refleco Refleco Refleco3 x (ms) (ms) 3 (ms) Noe hyperbolc moveou of he hree refleco eves. Source Recever Offse (meers) Arrval Tme (ms)

23 Recall - x 0 + rms The varables ad x are learly relaed. 0 s he ercep & s he slope rms Tme X Esmaes of RMS veloces ca be deermed from he slopes of regresso les fed o he -x resposes. Keep md ha he fed velocy s acually a NMO velocy!

24 Sar wh defo of he RMS velocy = = = RMS The s are erval veloces ad he sare he wo-way erval ras mes. = = = RMS = = 0 Le he wo-way ravel me of he h reflecor

25 = = RMS 0 = + = 0 RMS hece 0 0 = RMS RMS = = 0 RMS Sce = RMS RMS s he erval velocy of he h layer hs case represes he wo-way erval ras me hrough he h layer

26 = RMS 0 RMS 0 RMS 0 RMS 0 = Hece, he erval veloces of dvdual layers ca be deermed from he RMS veloces, he -way zero - offse refleco arrval mes ad erval ras mes. RMS 0 RMS 0 = The RMS erms represe he veloces obaed from he bes f les. Remember hese veloces are acually NMO veloces. 0 = he wo-way ravel me o he h reflecor surface = ( ) = Δ = 0 0 he wo-way erval ras me bewee he ad - reflecors See Berger e al. page 73 s he erval velocy for layer, where layer s he layer bewee reflecors ad -

27 The erval velocy ha s derved from he RMS veloces of he reflecos from he op ad base of a layer s referred o as he Dx erval velocy. However, keep md ha we really do kow wha he RMS velocy s. The NMO velocy s esmaed from he - x regresso le for each refleco eve ad ha NMO velocy s assumed o represe a RMS velocy. You pu hese deas o applcao whe solvg problems 4.4 & m 0 x See page Use dp-moveou approach (pages 97 hrough 99)

28 Problem 4. s due ex Moday Dx erval veloces. We ll have addoal me for quesos ex Moday 4.4 ad 4.8. They wll be due he followg Moday (March ). Quesos abou Exercses I-? Exercse I? Exercses I-I due March 7 h