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Assigning Geometry

At a source interval of 50 m, a 96-channel line need only be about 13 km long to exceed 5,000 total data traces!"learly, #e have a real need to identify each trace #ith res$ect to its source-$oint, its ground $osition, its cm$family, and its offset from the shot! %&hese four variables are the various coordinate axes of a stacking diagram!'(etting u$ an identification scheme re)uires that #e define the geometry of the line!

&he first consideration is the origin, or *ero reference $oint, of the line! &his is often the first source-$oint % +igure

1 ' this means that some lines may have negative geo$hone grou$ coordinates!

Figure 1

Alternatively % +igure 1', #e may take as the origin the first ground $osition %source or grou$' this means thatfor some lines, the first source-$oint has a large $ositive coordinate! &he coordinate system is, ho#ever,arbitrary a *ero $oint is taken as convenient!

nce #e have defined the origin, #e su$$ly the source interval, the source-number increment, and thecoordinate of the first source-$oint! &he source-number increment is im$ortant because, in the field, source-$oints and geo$hone grou$s are numbered according to the same system % +igure ,In this example, the source

interval is equal to two group intervals (or ground position intervals' Therefore, the second source-point is SP3'!

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Figure 2

&herefore, if the first source is at ground $osition 1, and the grou$ interval is 5 m, and the source interval is 50

m, then the second source-$oint is at ground $osition 3, and is thus numbered! (o it is im$ortant that thegeometry $rogram understand that (.3 is the second source-$oint, and that it is 50 m from the first!

n land, #here the line $rogression is likely to be less regular, it may be best to define the ground $ositioninterval, and then tell the com$uter that sources are located at, for exam$le, $ositions 1, , 3, !!!n % +igure 3 '!

&his does not re)uire an ex$licit s$ecification of the source interval!

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Figure 3

&here is also the chance that a source-$oint is laterally offset! &his offset must be noted by the observer and dulyin$ut by the $rocessor, for it affects the locations of the mid$oints for that source! /here offsets are infre)uentand short, #e generally make no correction for them! ut #hen many offsets move many mid$oints a$$reciablyoff the line % +igure 3', #e may need to use an a$$ro$riate crooked-line $rogram!

+or each source-$oint, #e next describe the s$read % +igure 3'! &his is done by s$ecifying the ground-$ositionnumbers of the near and far offsets, and the ground-$osition interval bet#een geo$hone grou$s! %&he sourceinterval is sometimes half the grou$ interval! &his re)uires ground $ositions defined by the source-$oints, and as$ecification that the geo$hone grou$s are at alternate ground $ositions!'

f course, in doing all this, #e remember the im$ortance of checking the field notes and all the )c $lots thus fargenerated! And #e make $articularly sure of our o#n in$ut to the $rogram, lest a ty$ogra$hical error cause

geometry headaches later!

+or land data, the offset of the near grou$ is easy to determine, for the $hysical $ositions of the source andreceivers are fixed! &he situation is different at sea, and it often ha$$ens that the near offset changes from oneline to the next, and even #ithin a line!

n a nominal sense, of course, the near offset is $redetermined! &he $roblem lies in the fact that #e have stretch

sections in the streamer designed to isolate the active hydro$hone grou$s in the streamer from the $itching andya#ing of the shi$ #ith variations in the s$eed of the shi$ through the #ater, they stretch more or less! &hisnecessarily affects the near offset!

&he solution is to have a series of #aterbreak hydro$hones, one near the near grou$ and the others regularly

s$aced along the streamer! &hen the #aterbreak times %+igure 2 , For a straight streamer

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Figure 4

The distance from the source arra! ("# to the near group water$rea% detector ( varies $ecause of the stretch

sections The distance $etween two detectors (& and '# does not, $ecauseca$le sections have negligi$le stretch'at t#o such detectors %of kno#n distance a$art' can be used to calculate the velocity in the #ater! &his velocity,in turn, can be used in conunction #ith the #aterbreak time of the near grou$ to determine the distance fromthe source to the near grou$! (ometimes, cross-currents or evasive maneuvering cause the streamer to snake%+igure 2, For a $owed streamer The travel path of the water$orne wave is less compass sections () and *#allow us to calculate the arc of the streamer, and correct the offset calculations' com$ass sections allo# us, if #echoose, to calculate the deviations and correct our velocity calculations!

(o #e check that the #aterbreaks have been recorded, and #e check them for constancy %or for the smoothnessof their variations'! (ometimes, ho#ever, #e find that #e have only one #ater-break hydro$hone! n this case, itbecomes necessary to rely on the velocity that the observer has $rogrammed into the shi$4s de$th sounder! &hisvelocity is normally obtained from monthly tables that take into account tem$erature and salinity of course, theobserver should have recorded the value of velocity used!

/aterbreak information should al#ays be used to determine or to verify the near offset! &he s$read diagramsu$$lied by the cre# sometimes assumes no stretch in the stretch sections, and this cannot be correct! /e shallfind in later modules that o$timum stacking and meaningful velocity analysis de$end critically on correct offsets!

+rom the source $ositions and the calculated offsets, the com$uter can calculate the mid$oints and absolutereceiver coordinates for each trace! &his information is carried in the trace headers, and is u$dated as traces aredeleted or arrays are formed! ater, these coordinates #ill also allo# the com$uter to gather all those traces

#hich have the same mid$oint coordinate!

At the trace-labeling stage, #e also su$$ly certain survey information, such as elevations %including that of thedatum', source de$ths, u$hole times, and #eathering and datum velocities! &hese, too, are carried for#ard #iththe traces, ready for the a$$lication of datum corrections!

Education Inventory Tasksheet: SPS Format1

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SPS-11J - e!uired

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The aim of this module is to gain a good understanding of the SPS le format.

T"S$S % ST&'IES

- Read the Shell Processing Support Format document.

- Explain the purpose of the S, X and R le, what information is contained in each ofthem,and where does TR!"# get the information to create these les.

- Explain the purpose of a $% point code and where does TR!"# get the information tocreate these codes in the resulting SPS.

- &ene the following point codes 'which are normall( used on )* +* sure(s/ "0,"1, 20, 30, 31, 34.

- 2ow man( columns are re5uired in the S, X and R les and where is the (ear in the Sand R point records 6

- Explain what 7indexing7 is and gie an example of when multiple indexes are used.

8hat is the highest index that ma( 9e used in an SPS le 6

- *reate the S, X les for one sail line and the appropriate R le on (our current :o9.

- 8or; through the header of one of the les. &iscuss with (our superisor an( specialcomments '21< record used on (our current :o9 and the sources of all the otherinformation contained in the header.

EFEE()ES

SPS Format &escription 'nTouch & 41=>=?4

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