Seismic Exploration Methods PDF

SeismicExploration Methods 1. SeismicReflection method(Kaedah seismik pemantulan) 2. Seismic Refraction method(Kaedah seismik pembiasan) SeismicReflection Method Seismic ReflectionMethods  Themost widely used.  Inmany casestheassumption oflayered mediais valid .  Raw datacanbeprocessed...

SeismicExploration Methods 1. SeismicReflection method(Kaedah seismik pemantulan) 2. Seismic Refraction method(Kaedah seismik pembiasan) SeismicReflection Method Seismic ReflectionMethods  Themost widely used.  Inmany casestheassumption oflayered mediais valid .  Raw datacanbeprocessed sothat an“image ”of the subsurface emerges.  Dataprocessing isdone bycomputers. A) SingleHorizontal Reflector • Ahorizontal boundary(zero-dip), 2-dimensional • Reflected layerABand depth h 1 • Thearrival timest x ofreflections fromaninterface at depth h 1 asafunction ofoffset xare given as: • Thent(travel time)=2(SR)/V 1 • Reflected RayPaths Geometry 2 1 2 V  1   1  t (x) =   . ( x )+ 4h ReflectedRayPaths Geometry ReflectedRayPaths Geometry • Reflection method ReflectedRayPaths Geometry (1) • Inx,can bewritten as: V 2t2 = x 2 +4h 2 (V 2t2 /4h 2) – (x 2/4h 2)= 1 ⇒The traveltimetcurve ahyperbola • Geophone atG also records directwaves move along SGwhich isSG <SR +RG ⇒ travel time: t D = x/V ⇒ time curve isastraight line. ReflectedRayPaths Geometry • Position ofreflector (layerthickness, h)can be determined bymeasuring t o,travel timeatshot point . • When x=0,then h= ½Vt o or t o =2h/V • Then eq.(1 ) can bewritten as: t 2 = x 2 /V 2 + 4h 2 / V 2 t2 = x 2 /V 2 + t 2 (2 ) o • To find t,can besolved fromeq.(1 ). ReflectedRayPaths Geometry • So,ifplot t 2 vsx 2,we can determine V 1 andh 1 from theslope andintercept . • Thedifferent (additional correction) inthe travel time ofareflection for2geophones positionis known asmoveout, Δt. ReflectedRayPaths Geometry • Ift 1 & t 2 are travel timeandx 1 &x 2 are offset, so: Δt =t 2 –t 1 2 2 2 ~(x 2 –x 1 )/ 2V t o (3) The time difference betweenthearrival atx 1=0 and arrival atx, Δt known as normal moveout (NMO),Δt n then, Δt n ~x 2 /2V 2to (4) The importance ofNMO: •Having determined thelayer velocity, wecan usethepredicted quadratic shape toidentify reflectors • Thencorrect (shifttraces) andstack toenhance signaltonoise ReflectedRayPaths Geometry Graphical demonstration ofNMO Singlehorizontal reflector QuickTime ™and a TIFF (Uncompressed) decompressor are needed tosee thispicture. Singlehorizontal reflector Singlehorizontal reflector QuickTime ™and a TIFF (Uncompressed) decompressor are needed tosee thispicture. MultipleHorizontal Reflector B) Ifthere are2or more reflector layers,we want t(x) for each reflected event: t 1 ~ 2h 1/V 1 t 2 ~2[(h 1/V 1)+ (h 2 /V 2 )] And sofor t 3 ….until t n. • Ifthe average velocity foreach layer asV 1,V 2 --- - V n is known , then thethickness forlayer h 1,h 2,--- - h n can becalculated. Theaverage velocity V av can be given as: V av = Σh j/Σ(h j/V j) Multiplehorizontal layers QuickTime ™and a TIFF (Uncompressed) decompressor are needed tosee thispicture. Multiplehorizontal layers StandardReflection plot MultipleHorizontal Reflector • SeismicVelocities: • Weusevelocities veryoften inseismic reflection work:  Velocity controlstraveltimes  Velocity controlsimpedance contrasts  Velocitiesarenecessary forNMO corrections  Velocities areneeded forconversion fromtimeto depth. MultipleHorizontal Reflector • Therearemany different kindsofseismic velocities:  Interval velocity  Stacking (NMO)velocity  RMS(root-mean square) velocity  Average velocity • Intervalvelocity: V int =h i ti (at zero offset) i i i • Average velocity: V ∑ tt = ∑ viti = ∑ h i ave 0 ,i Seismic ReflectionMethods •Interval velocity  Velocity ofone layer oraverage velocity overa depth section. Defined as: V i = z i/t i • The average velocity: V av = Σz j/Σt j =Σ v jtj/Σt j=Z n/T n MultipleHorizontal Reflector • Agood approximation isby using root-mean square (RMS) velocity, thent(x): 2 22 2 ti (x)=t 0i +x /(V rms,i ) • root -mean square (RMS) velocity: V av = (Σv j2 tj/Σt j)½ where v j isinterval velocity, and t j is one -way travel time. Weassume hereoffset distances thatare small compared tothe reflector depths. MultipleHorizontal Reflector • Theindividual NMOcanbeused toestimate RMS velocity downtoacertain depthandvalues at different depthstoestimate theinterval velocity V n (Dixequation): • where t o,n istwo way travel time.V n is the interval velocity between then-1 and nthreflector. • The interval velocitycanbe determined fromthe RMS velocities layerby layer startingat thetop. MultipleHorizontal Reflector • Dix’sformula allowsustotake themeasured RMS velocities andconvert thembacktoreal interval velocities. • Once wehave velocities, thicknesses areeasy to obtain by: DippingLayers (5) C) Dippingreflector, dipmoveout • Ifθ= dip angle, h= depth • For wave pathSCR, t = (SC+CR)/V • By using cosain law, V 2t2 = IR 2 = x 2+4h 2-4hx cos(π/2+θ) = x 2+4h 2+4hx sinθ • Then, canbewritten as: V 2t2/(2hcosθ) 2 – (x+2h sinθ) 2/(2hcosθ) 2 =1 DippingLayers DippingLayers • Traveltimecurve isahyperbola ,but asymmetric where theaxial plane isat x= -2hsinθ • t has different valueforgeophones atboth sideof shotpoint • when x= 0, h = ½Vt 0 but his not avertical depth To find dipangle θ,we need tosolve tvalue from Eq. (5). DippingLayers t= 2h/V [1+(x 2+4hxsinθ) /4h 2]½ »to find θ,which isfrom thedifferent oftravel time for2geophones atboth sidewith the same distance. x for down -dip =+s and forup-dip =-s, Then travel timet 1 andt 2 canbecalculated: DippingLayers t1 ~t o [1+ (s 2+4hs sinθ) /8h 2] t 2 ~ t o [1+ (s 2-4hs sinθ) /8h 2] » Δt d =t 2 -t 1 ~ to (s sinθ /h) ~ 2ssinθ /V Value Δt d dip moveout If we replace Δxfor the distance 2s,dip can be estimated: sinθ~ V(Δt d /Δx) DippingLayers t1 ~t o [1+ (s 2+4hs sinθ) /8h 2] t 2 ~ t o [1+ (s 2-4hs sinθ) /8h 2] » Δt d =t 2 -t 1 ~ to (s sinθ /h) ~ 2ssinθ /V Value Δt d dip moveout If we replace Δxfor the distance 2s,dip can be estimated: sinθ~ V(Δt d /Δx)