Definition of Seismic Data Acquisiton

7/24/2019 Definition of Seismic Data Acquisiton

1/15

December 22, 2010

Definition:

. Seismic data acquisition is a process for mapping

geologic structure by observation of seismic waves, especially bycreating seismic waves with artificial sources and observing thearrival time of the waves reflected from acoustic-impedancecontrasts or refracted through highvelocity members.

Why we do seismic data acquisition: We do seismic data acquisition for the following purposes; Delineation of near-surface geology for engineering studies, and

coal and mineral exploration within a depth of up to 1km: the seismicmethod applied to the near surface studies is known asengineering seismology

!ydrocar"on exploration and de#elopment within a depth of up to 1$km: seismic method applied to the exploration and de#elopment ofoil and gas fields is known as exploration seismology.

%n#estigation of the earth&s crustal structure within a depth of up to

1$$ km: the seismic method applies to the crustal and earthquakestudies is known as earthquake seismology

'u"surface geologic structures containing hydrocar"ons arefound "eneath either land or sea 'o there is a land data-acquisitionmethod and a marine data-acquisition method (he two methods ha#e acommon-goal, imaging the earth )ut "ecause the en#ironments differ, soeach required unique technology and terminology1) Land Data Acquisition: %n land acquisition, a shot is fired *ie, energy is transmitted+ andreflections from the "oundaries of #arious ithological units within the su"-surface are recorded at a num"er of fixed recei#er stations on the surface(hese geophone stations are usually in-line although the shot source may

not "e When the source is in-line with the recei#ers at either end ofthe recei#er line or positioned in the middle of the recei#er line a two-dimensional *D+ profile through the earth is generated %f the sourcemo#es around the recei#er line causing reflections to "e recorded formpoints out of the plane of the in line profile, then a three-dimensional *.D+image is possi"le *the third dimension "eing distance, orthogonal to the in-line recei#er-line+ (he ma/ority of land sur#ey effort is expended in mo#ingthe line equipment along and 0 or across farm fields or through populated

1

SIESMIC DATA AC!ISITI"#

(ypes of 'eismic data acquisition


2/15

December 22, 2010

communities !ence, land operations often are conducted only duringdaylight thus making it a slow process

$) Marine Data Acquisition: %n a marine operation, a ship tows one or more energy

sources fastened parallel with one or more towed seismic recei#er lines %nthis case, the recei#er lines take the form of ca"le called 'teamercontaining a num"er of hydrophones (he #essel mo#es along and fires ashot, with reflections recorded "y the streamers %f a single streamer and asingle source are used, a single seismic profile may "e recorded in likemanner to the land operation %f a num"er of parallel sources and0orstreamers are towed at the same time, the result is a num"er of parallellines recorded at the same time %f many closely spaced parallel lines arerecorded, a .D data #olume is recorded ore than one #essel may "eemployed to acquire data on 2-hour "asis, since there is no need to

curtail operations in nights%) Transition & 'one (ecordin: )ecause ships are limited "y the water depth in which theysafely can conduct operations, and "ecause land operations mustterminate when the source approaches the water edge, or shore lines,transition-3one recording techniques ha#e "een de#eloped to pro#ide acontinuous seismic co#erage required o#er the land and then into the sea4eophones that can "e placed on the sea "ed or used with "oth marineand land shots fired into them

(echniques ha#e "een de#eloped to use "oth 4eophones and

hydrophones in the surface area where the shore line 0 water edge is likelyto migrate towards land and sea depending on the tide of sea a day (hecom"ination of such hydrophone 0 geophones is called a 5Dual 'ensor6(he ad#antage of why this is to see that either of the recei#er of Dual'ensor pickups the sur#eyed from the slots recorded using a land ormarine source and data gaps all along the coast within the area ofprospect

SEISMIC S"!(CE7 seismic source is a de#ice that generates controlled seismic energy usedto perform "oth reflection and refraction seismic sur#eys 7 seismic sourcecan "e simple, such as dynamite, or it can use more sophisticatedtechnology, such as a speciali3ed air gun 'eismic sources can pro#idesingle pulses or continuous sweeps of energy that generate seismicwa#es, which tra#el through a medium such as water or layers of rocks'ome of the wa#es then reflect and refract and is recorded "y recei#ers,such as geophones or hydrophones

2


3/15

December 22, 2010

'eismic sources may "e used to in#estigate shallow su"soil structure, forengineering site work, or deeper structures, usually in the search forpetroleum or mineral deposits, or for scientific in#estigation (he returningsignals from the sources are detected "y geophones, laid in knownlocations relati#e to the position of the source (he recorded signals are

then su"/ected to specialist processing and interpretation to yieldcomprehensi"le data a"out the su"surface

7 seismic source signal has the following characteristics:

1 generated as an impulsi#e source "and-limited. the generated wa#es are time-#arying

'ources of seismic energy come in a #ariety of si3es and shapes 8irtually

anything that impacts, or causes motion on, the surface of the earth will "ea source of seismic energy 9nfortunately, most sources areuncontrolla"le, such as road traffic, wind *this causes noise "y making"ushes and trees mo#e+, aircraft, people walking, etc the storagecapacitors were placed in a steel container *the "ang "ox+ on the sur#ey#essel (he high #oltages used, typically .,$$$ 8, required hea#y ca"lesand strong safety containers ecently, low #oltage "oomers ha#e "ecomea#aila"le (hese use capacitors on the towed sled, allowing efficientenergy reco#ery, lower #oltage power supplies and lighter ca"les (he low#oltage systems are generally easier to deploy and ha#e fewer safetyconcerns

7 geophysicist should select a source "ased on the following fi#e criteria:

enetration to the required depth


4/15

December 22, 2010

energy at the higher frequencies to maintain a "road reflection"andwidth 'ignal-to-noise- characteristics

Different areas ha#e different noise pro"lems (hey may dictate thesource selection

>n#ironmentWhen working in populated areas, there are special safetyrequirements to which geophysicists must adhere 7#aila"ility and ?ost

(hree types of sources are most commonly used for "oth refraction andreflection in#estigations of the near surface

Im*act Sources

'ources that generate seismic energy "y impactingthe surface of the >arth are pro"a"ly the most common type employed

7lthough impact sources can "e rather sophisticated in their construction,

the most commonly used type of impact source is a simple sledgehammer%n this case, an operator does nothing more than swing the sledgehammerdownward onto the ground %nstead of striking the ground directly, it is mostcommon to strike a metal plate lying on the ground, to a#oid dissipatingenergy in plastic deformation of the earth (he sledgehammer is usuallyconnected to the recording system "y a wire (he moment thesledgehammer strikes the plate, the recording system "egins recordingground motion from the geophones

(he principle ad#antages to using a sledgehammer source are primarily

ow ?ost and

'imple to operate and maintain

(he principle disad#antages of this source are

%t can "e difficult to assure that the source is operated in a

repeata"le fashion,

4


5/15

December 22, 2010

@peration is manually strenuous,

'ource outputs relati#ely small amounts of seismic energy

(herefore, it can "e difficult to record relia"le o"ser#ations at greatdistances, and

'ource outputs seismic energy that tends to "e low frequency in

nature *ie this source generates a lot of surface wa#es+

+un Sources

ike impact sources, gun sources generate seismic energy"y transferring the kinetic energy of a mo#ing o"/ect into seismic energy %nthis case, the mo#ing o"/ect is a "ullet or shotgun slug 'ome sources use"lanks instead of "ullets or slugs %n this case, energy is transferred fromthe column of air in the gunAs "arrel that is set in motion "y the "lank to theground

ike the sledgehammer, gun sources must also "e connected to therecording system so that you can "egin recording ground motion from thegeophones at the instant the slug or shell hits the ground

(he principle ad#antages of gun sources are

!ighly repeata"le source,

>nergy imparted into the ground is larger than is possi"le from a

sledgehammer, and

4un sources generally output higher-frequency energy (his helps tominimi3e surface wa#e generation

(he principle disad#antages of gun sources are

'afety,

>quipment is more "ulky and expensi#e than simple impact sources,

and

4etting permission *permitting+ to use this source may "e more

difficult

E,*losive Sources

>xplosi#e sources can impart a large amount ofseismic energy into the ground gi#en their relati#ely small si3e (hesesources can #ary in si3e and type from small "lasting caps and shotgunshells to larger, two-phase explosi#es 7ll explosi#e sources are triggered

5


6/15

December 22, 2010

remotely "y a de#ise known as a "lasting "ox (he "lasting "ox isconnected to "oth the explosi#e and the recording system 7t the momentthe "ox detonates the explosi#e, it also sends a signal to the recordingsystem to "egin recording ground motion from the geophones

>xplosi#es, such as dynamite, can "e used as crude "ut effecti#e sourcesof seismic energy 4enerally the explosi#e charges are placed "etween $feet to B$ feet "elow ground (he charges are placed in a hole that isdrilled with dedicated drilling equipment for this purpose (his type ofseismic drilling is often referred to as C'hot !ole DrillingC

7 common drill rig used for C'hot !ole DrillingC is the 7D?@ ?-1$$$ drillmounted on an 7D?@ < 22 "uggy (hese drill rigs often use water or airin assisting the drilling

(he principle ad#antages of explosi#e sources are ound for pound, these types of sources impart the most amount of

seismic energy into the ground of any of the sources descri"ed here,

(he energy tends to "e #ery high frequency, and "ecause the

explosi#es are usually placed in a shallow "orehole, it tends not to"e contaminated "y surface wa#es, and

>xplosi#e sources are #ery repeata"le

(he principle disad#antages of explosi#e sources are

'afety *the person in the photograph a"o#e is "eing foolishlyirresponsi"le+,

ermitting andowners tend to "e ner#ous a"out allowing the use of

explosi#es on their property,

Data acquisition using explosi#e sources is much slower than using

impact or gun sources (his is primarily "ecause "oreholes must "edrilled within which the explosi#es are to "e placed, and

>xplosi#es tend to "e expensi#e to acquire and maintain

"T-E( S"!(CES

Tum*er truc/

7 thumper truck *or weight-drop+ truck is a #ehiclemounted ground impact which can used to pro#ide the seismic source 7hea#y weight is raised "y a hoist at the "ack of the truck and dropped,possi"ly a"out three metres, to impact *or CthumpC+ the ground (o

6


7/15

December 22, 2010

augment the signal, the weight may "e dropped more than once at thesame spot, the signal may also "e increased "y thumping at se#eralnear"y places in an array whose dimensions may "e chosen to enhancethe seismic signal "y spatial filtering

(humping might "e less damaging to the en#ironment than firingexplosi#es in shot-holes, though a hea#ily thumped seismic line withtrans#erse ridges e#ery few metres might create long-lasting distur"anceof the soil 7n ad#antage of the thumper *later shared with 8i"roseis+,especially in politically unsta"le areas, was that no explosi#es wererequired

Seismic vibrator

7 'eismic #i"rator, commonly known "y its

trademark name 8i"roseis, propagates energy signals into the >arth o#eran extended period of time as opposed to the near instantaneous energypro#ided "y impulsi#e sources (he data recorded in this way must "ecorrelatedto con#ert the extended source signal into an impulse (hesource signal using this method was originally generated "y a ser#o-controlled hydraulic #i"rator or shaker unitmounted on a mo"ile "ase unit,"ut electro-mechanical #ersions ha#e also "een de#eloped 8i"roseis wasde#eloped "y the ?ontinental @il ?ompany *?onoco+ during the 1EB$s andwas a trademark until the companyAs patent lapsed

0oomer sources

)oomer sound sources are used for shallow waterseismic sur#eys, mostly for engineering sur#ey applications )oomers aretowed in a floating sled "ehind a sur#ey #essel 'imilarly to the plasmasource, it stores energy in capacitors, "ut it discharges through a flat spiralcoil instead of generating a spark 7 copper plate ad/acent to the coil flexesaway from the coil as the capacitors are discharged (his flexing istransmitted into the water as the seismic pulse

@riginally the storage capacitors were placed in a steel container *the "ang

"ox+ on the sur#ey #essel (he high #oltages used, typically .,$$$ 8,required hea#y ca"les and strong safety containers ecently, low #oltage"oomers ha#e "ecome a#aila"le (hese use capacitors on the towed sled,allowing efficient energy reco#ery, lower #oltage power supplies and lighterca"les (he low #oltage systems are generally easier to deploy and ha#efewer safety concerns

7


8/15

December 22, 2010

7 geophysicist should select a source "ased on the following fi#e criteria:

enetration to the required depth

n#ironment

When working in populated areas, there are special safetyrequirements to which geophysicists must adhere 7#aila"ility and ?ost

(he time of arri#al of a crew can "e extremely important and >nergy

'ources are of two types: >xplosi#e sources and Fon >xplosi#esources

Seismic (eceivers

1 +eo*ones:

?on#entional geophones are "ased on =araday&s law of electromagneticinduction (his law states that relati#e motion of a conductor through amagnetic field induces an electromagnetic force *>=+ which causes acurrent to flow through the conductor, if the conductor is an element of anelectrical circuit (he two types of geophones widely used in geophysicalsur#eys are

1 mo#ing coil geophone and

8


9/15

December 22, 2010

mo#ing magnet geophone

Constrution: (he essential ingredients to make a geophone are apermanent magnet, a conductor and a spring which positions either the

conductor in the magnetic field space *in mo#ing coil geophone+ or thepermanent magnet in the electric field space *as in mo#ing magnetgeophone+ (he conductor in reality is a length of copper wire wrapped intoa cylindrical coil shape %t is often referred to as the coil or element (heconductor&s or the magnet&s motion through the magnetic0electrical field,according to =araday&s law, causes an >= to "e induced that isproportional to the #elocity of the earth&s motion !ence, such a geophoneis called a #elocity phone "ecause its output is proportional to the #elocityof the earth&s motion (he large amount of su"surface information carried"y seismic signal would "e fully a#aila"le for interpretation only if the

geophones follow ground mo#ement faithfully with minimum distortion

-ydro*ones: (he hydrophone is an electro acoustic transducer thatcon#erts a pressure pulse into an electrical signal "y means of thepie3oelectric effect %f mechanical stress is applied on tow opposite faces ofa pie3oelectric crystal, then electrical charges appear on some other pairof faces %f such a crystal is placed in an en#ironment experiencingchanges in pressure, it will produce a #oltage proportional to that #ariationsin pressure

Dual Sensors: =or ocean "ottom ca"le *@)?+ applications, com"ining theoutput of geophones and a hydrophone is now widely accepted techniquefor reducing the ghosting effect caused "y the water0air interface (oo#ercome the disad#antage of using two separate sensors, "oth geophoneand hydrophone are a#aila"le in a single unit known as dual sensors or the2-component *2?+ recei#ers consist of a hydrophone, two hori3ontalgeophones and a #ertical geophone installed in a single water proofenclosure for recording , '8 and '! wa#es

4>@!@F> '>7D%F4

)y spread we mean the relati#e locations of the source point and thecenters of the geophone groups used to record the energy form thesource%n split-dip shooting the source point is at the center of a line of regularlyspaced geophone group often results in a noisy trace *"ecause of ground

9


10/15

December 22, 2010

roll or truck noise with a surface source, or gases escaping from the shothole and e/ection of tamping material+; hence the source may "e mo#ed 1Bto B$ m perpendicular to the seismic line @ften the geophone groupsnearest the source are not used, which creates a sourcepoint *shotpoint+gap

@ften the source is at the end of the spread of acti#e geophonegroups to produce an endon s*read, and in areas of exceptionally hea#yground roll the source point is offset "y an apprecia"le distance along theline from the nearest acti#e geophone group to produce an inline o22sets*read 7lternati#ely, the sourcepoint may "e offset in the direction normalto the ca"le, either at one end of the acti#e part to produce a "roadside-Ls*reador opposite the center to gi#e a "roadside-T s*read >nd-on andin-line offset spreads often employ sources off each end to gi#e continuousco#erage and two records for each spread (he in-line and "roadsideoffsets permit recording reflection energy "efore the ground-roll energy

arri#es at the spread Cross s*read:which consist of two lines ofgeophone groups roughly at right angles to each other, are used to record.D dip information

Seismic Tecniques'eismic techniques generally in#ol#e measuring the tra#el

time of certain types of seismic energy from surficial shots*ie anexplosion or weight drop+ through the su"surface to arrays of groundmotion sensors or geophones %n the su"surface, seismic energy tra#els

in wa#es that spread out as hemispherical wa#efronts *ie the threedimensional #ersion of the ring of ripples from a pe""le dropped into apond+ (he energy arri#ing at a geophone is descri"ed as ha#ing tra#eleda ray pathperpendicular to the wa#efront *ie a line drawn from the spotwhere the pe""le was dropped to a point on the ripple+ %n the su"surface,seismic energy is refracted*ie "ent+ and0or reflectedat interfaces"etween materials with different seismic #elocities *ie different densities+(he refraction and reflection of seismic energy at density contrasts followsexactly the same laws that go#ern the refraction and reflection of lightthrough prisms Fote that for each seismic ray that strikes a density

contrast a portion of the energy is refracted into the underlying layer, andthe remainder is reflected at the angle of incidence (he reflection andrefraction of seismic energy at each su"surface density contrast, and thegeneration of surface waves*or ground roll+, and the sound *ie the aircoupled wa#e or air blast+ at the ground surface all com"ine to produce along and complicated sequence of ground motion at geophones near a

10


11/15

December 22, 2010

shot point (he ground motion produced "y a shot is typically recorded asa wiggle trace for each geophone

Seismic (e2raction'eismic refraction in#ol#es measuring the tra#el time of the component ofseismic energy which tra#els down to the top of rock *or other distinctdensity contrast+, is refracted along the top of rock, and returns to thesurface as a head wa#e along a wa#e front similar to the "ow wake of aship(he shock wa#es which return from the top of rock are refractedwa#es, and for geophones at a distance from the shot point, alwaysrepresent the first arri#al of seismic energy

'eismic refraction is generally applica"le only where the seismic #elocitiesof layers increase with depth (herefore, where higher #elocity *eg clay+layers may o#erlie lower #elocity *eg sand or gra#el+ layers, seismicrefraction may yield incorrect results %n addition, since seismic refractionrequires geophone arrays with lengths of approximately 2 to B times thedepth to the density contrast of interest *eg the top of "edrock+, seismicrefraction is commonly limited *as a matter of practicality+ to mappinglayers only where they occur at depths less than 1$$ feet 4reater depths

11


12/15

December 22, 2010

are possi"le, "ut the required array lengths may exceed site dimensions,and the shot energy required to transmit seismic arri#als for the requireddistances may necessitate the use of #ery large explosi#e charges %naddition, the lateral resolution of seismic refraction data degrades withincreasing array length since the path that a seismic first arri#al tra#els

may migrate laterally *ie in three dimensions+ off of the trace of thedesired *two dimensional+ seismic profile

ecent ad#ances in in#ersion of seismic refraction data ha#e made itpossi"le to image relati#ely small, non-stratigraphic targets such asfoundation elements, and to perform refraction profiling in the presence oflocali3ed low #elocity 3ones such as incipient sinkholes

Seismic (e2lection

'eismic reflection uses field equipment similar toseismic refraction, "ut field and data processing procedures are employedto maximi3e the energy reflected along near #ertical ray paths "ysu"surface density contrasts

12


13/15

December 22, 2010

eflected seismic energy is ne#er a first arri#al, and

therefore must "e identified in a generally complex set of o#erlappingseismic arri#als - generally "y collecting and filtering multi-fold or highlyredundant data from numerous shot points per geophone placement(herefore, the field and processing time for a gi#en lineal footage ofseismic reflection sur#ey are much greater than for seismic refraction!owe#er, seismic reflection can "e performed in the presence of low#elocity 3ones or #elocity in#ersions, generally has lateral resolution #astlysuperior to seismic refraction, and can delineate #ery deep densitycontrasts with much less shot energy and shorter line lengths than would"e required for a compara"le refraction sur#ey depth

(he main limitations to seismic reflection are its higher cost than refraction*for sites where either technique could "e applied+, and its practicallimitation to depths generally greater than approximately B$ feet 7t depthsless than approximately B$ feet, reflections from su"surface densitycontrasts arri#e at geophones at nearly the same time as the much higheramplitude ground roll *surface wa#es+ and air "last *ie the sound of theshot+ eflections from greater depths arri#e at geophones after the groundroll and air "last ha#e passed, making these deeper targets easier todetect and delineate

'eismic reflection is particularly suited to marine applications *eg lakes,ri#ers, oceans, etc+ where the ina"ility of water to transmit shear wa#esmakes collection of high quality reflection data possi"le e#en at #eryshallow depths that would "e impractical to impossi"le on land

=or shallow applications this will normally comprise a hammer and plate,weight drop or explosi#e charge %n most reflection sur#eys shots aredeployed at a num"er of different positions in relation to the geophone

array in order to o"tain reflections from the same point on the interface atdifferent geophones in the array >ach common point of reflection istermed a common mid-point *?+ and the num"er of times each one issampled determines the Afold co#erageA for the sur#ey (races relating tothe same ? are stacked together to increase the signal-to-noise ratio ofthe sur#ey "efore "eing com"ined with other ?As stacked traces toproduce a reflection profile %n order to stack related ? traces a stacking

13


14/15

December 22, 2010

#elocity is applied to each trace (his accounts for the difference in two-way tra#el time "etween the normal incidence reflection *#ertical tra#elpath "elow the shot+ and those at increasing offsets from the shot *knownas the normal mo#eout or F@+ (he stacking #elocity will #ary down thetrace to take account of the increase in #elocity with depth for each

reflection e#ent

(he simplest form of seismic reflection profiling is the constant-offsetmethod (his technique uses a single geophone offset from the source "ya fixed distance (he two are mo#ed along the sur#ey line in equal stepswith a single trace "eing recorded at each position (he main ad#antage ofthis technique is the limited amount of processing that needs to "e appliedto the data due to the almost #ertical orientation of each raypath !owe#er,in order to a#oid pro"lems with interference from groundroll and the shotairwa#e, the offset distance has to "e selected with care

Continuous Surface-Wave System CSWS!

(he continuous surface-wa#e method utilises a specific type of seismicwa#e, known as the ayleigh wa#e, in order to determine in situ shearmodulus-depth profiles to depths of "etween Gm to $m "elow surface(he #elocity of a ayleigh wa#e is related to the shear modulus *4+ anddensity of the ground through which it propagates 9nlike crossholeseismic methods, which are routinely used to determine geotechnicalparameters such as the shear modulus *and additionally oissonAs ratio+,the ?'W technique require no "oreholesayleigh wa#es are constrainedto propagate within a 3one approximately 1 wa#elength in depth, such thatincreasing the wa#elength *decreasing the frequency+ of the transmittedenergy will result in an increase in the depth of in#estigation (hewa#elength and phase-#elocity of the ayleigh wa#es generated at aparticular frequency are calculated "y determining the phase shift "etweenthe transmitted and measured signals at each geophone location

hase-#elocities are measured o#er a range of frequencies in order to"uild up a dispersion spectrum for the ground "elow the spread

14


15/15

December 22, 2010

(his is then in#erted to determine a #elocity-depth profile and finally a

stiffness-depth profile(he system comprises a porta"le frequency-controlled #i"rator and an array of low frequency geophones arranged co-linearly with the source 7 laptop computer controls "oth the #i"rator anddata acquisition ayleigh wa#es are generated at frequencies of "etweenB!3 and 1$$!3 in $1-B!3 increments in order to "uild up acomprehensi#e stiffness-depth profile

15

Definition of Seismic Data Acquisiton

Documents

Transcript of Definition of Seismic Data Acquisiton