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L15_SeismicReflectionII-page13
Course: PHYSICS 384, Spring 2009School: Stony Brook University
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Zone Tells Fresnel us about the horizontal resolution on the surface of a reflector First Fresnel Zone The area of a reflector that returns energy to the receiver within half a cycle of the first reflection The width of the first Fresnel zone, w: 2 w 2 d + = d + 4 2 w2 = 2d + 2 2 4 If interface an is smaller than the first Fresnel zone it appears as an point diffractor, if it is larger it appears as an...
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Zone
Tells Fresnel us about the horizontal resolution
on the surface of a reflector
First Fresnel Zone
The area of a reflector that returns energy to
the receiver within half a cycle of the first
reflection
The width of the first Fresnel zone, w:
2
w
2
d + = d +
4
2
w2 = 2d +
2
2
4
If interface an is smaller than the first Fresnel
zone it appears as an point diffractor, if it is
larger it appears as an interface
Example:
30 Hz signal, 2 km depth where = 3 km/s then = 0.1 km and the width of the
first Fresnel zone is 0.63 km
Applied Geophysics Seismic reflection II
13
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Stony Brook University - PHYSICS - 384
Gravity anomaly mapApplied Geophysics Corrections and analysisSimple shape anomaliesApplied Geophysics Corrections and analysis7
Stony Brook University - PHYSICS - 384
Profile Cusps Wenner Arrayi. Current lines converge towardboundary, decrease potentialgradient at potential electrodesii. C2 at boundaryiii. Current density increases adjacentto boundary in low resistivity unit,causes potential gradient betweenpot
Stony Brook University - PHYSICS - 384
Suppression Principle of suppression:Thin layers of smallresistivity contrast withrespect to background willbe missed. Thin layers of greaterresistivity contrast will bedetectable, but equivalencelimits resolution ofboundary depths, etc.Horizon
Stony Brook University - PHYSICS - 384
Equivalence: several models producethe same results Ambiguity in physics of 1D interpretation such thatdifferent layered models basically yield the sameresponse. Different Scenarios: Conductive layers between two resistors, wherelateral conductance
Stony Brook University - PHYSICS - 384
ConvolutionReflectivityseriesSourcewavelet1-Output10RecordedwaveformApplied Geophysics Seismic reflection IConvolutionReflectivityseriesSourcewaveletOutput1-101RecordedwaveformApplied Geophysics Seismic reflection I9
Stony Brook University - PHYSICS - 384
Reflectivity and convolutionThe seismic wave is sensitive tothe sequence of impedancecontrastsThe reflectivity series (R)We input a source wavelet (W) which isreflected at each impedance contrastThe seismogram recorded at the surface(S) is the con
Stony Brook University - PHYSICS - 384
ExamplesElectromagnetic surveyingApplied geophysics IntroductionExamplesElectromagnetic surveyingApplied geophysics Introduction14
Stony Brook University - PHYSICS - 384
Geometrical FactorsArray advantages and disadvantagesArrayAdvantagesDisadvantagesWenner1. Easy to calculate a in the 1. All electrodes moved eachsoundingfield2. Sensitive to local shallow2. Less demand onvariationsinstrument sensivity3. Long
Stony Brook University - PHYSICS - 384
Current density and equipotential linesfor a current dipoledfraction total current 2z 2i f = tan1 dif=0.5 atz=d2if=0.7 at z = dWider spacing Deeper currentsApparent ResistivityPrevious expression can berearranged in terms of resistivity:=
Stony Brook University - PHYSICS - 384
Two Current Electrodes: Source and Sink Why run an electrode to infinity when we can use it?sourcesinkrsourcePrsinkVsource =Total Voltage at P:i2rsourceVsin k =Vp = Vsource Vsin k =i2 rsinki 11r2 source rsink Measurement Practicalities
Stony Brook University - PHYSICS - 384
Generalized reciprocal methodProcedure:Velocity analysis function, TV Calculate TV as a function ofoffset AG for a variety of XYdistances The optimal XY is when E and Fconverge on H Optimal XY is identified by thesmoothest TV curveTV Refractor
Stony Brook University - PHYSICS - 384
Generalized reciprocal methodThe plus-minus methodassumes a planar interface andshallow dip between C and EThe generalized reciprocalapproach uses two geophones,X and Y, recording refractedarrivals originating from thesame point on the refractora
Stony Brook University - PHYSICS - 384
Alkali Vapor MagnetometerAdvantages Dont have to align bottle with field Very rapid, almost continuousDisadvantages Cant measure vector fieldMeasurement accuracy : 0.01 to 1nTMagnetic Gradiometer Takes differences between two measurements that ar
Stony Brook University - PHYSICS - 384
Alkali Vapor Magnetometer Basic Physics: Uses precession frequency of alkali vapor Quantum mechanics Magnetometer construction:Bottle filled with cesium or rubidium vaporPolarized light source of same elementCoil to generate radio frequency magneti
Stony Brook University - PHYSICS - 384
Suppression Principle of suppression:Thin layers of smallresistivity contrast withrespect to background willbe missed. Thin layers of greaterresistivity contrast will bedetectable, but equivalencelimits resolution ofboundary depths, etc.12
Stony Brook University - PHYSICS - 384
Equivalence: several models producethe same results Ambiguity in physics of 1D interpretation such thatdifferent layered models basically yield the sameresponse. Different Scenarios: Conductive layers between two resistors, wherelateral conductance
Stony Brook University - PHYSICS - 384
ExamplesSeismic reflectionApplied geophysics IntroductionExamplesSeismic reflectionApplied geophysics Introduction5
Stony Brook University - PHYSICS - 384
Reflection and transmissionSeismic rays obey Snells Law(just like in optics)The angle of incidence equals theangle of reflection, and the angle oftransmission is related to the angle ofincidence through the velocity ratio.But a conversion from P to
Stony Brook University - PHYSICS - 384
DeploymentImportant considerations Need good coupling to the ground spike Mini-arrays to reduce surface wave noiseOffset of geophonesSmall offsets Near-vertical incidence retains P-energy High resolution of subsurface reflectorsSeismic reflection
Stony Brook University - PHYSICS - 384
ExamplesGravityApplied geophysics IntroductionExamplesSeismic refractionApplied geophysics Introduction12
Stony Brook University - PHYSICS - 384
Definitions: Magnetic fieldstrength or intensity Biot-Savarts law definition:for a loop of wire of radius r that iscarrying a current I, H at center isgiven as:H=nI/2r [A/m]where n is a unit vector normal to theplane of the loop. The magnetic fie
Stony Brook University - PHYSICS - 384
Definitions: Magnetic fieldor flux density Definition: Vector quantity defining the magneticflux/unit area; i.e., the density of the magnetic field lines.Thus often called Flux Density Mathematical Definitions:o pp Wbr = c 2 r 2 = Tesla Air:4 r
Stony Brook University - PHYSICS - 384
Magnetization or magnetic polarization A measure of the pole strength/unit area along oneof the ends of magnetic material:J=(p/A) n [A/m]Magnetic moment Strength of a magnetic field generatorM=J V = p l [A m2]For a loop of current: M=(Ir2) n
Stony Brook University - PHYSICS - 384
Dipole nature of magnetic materialsBar MagnetN+ Although, no magnetic monopolesexist in nature, they are useful fortheory: magnetic monopoles of samesign repel, opposite signs attract.S Dipole created by two poles ofopposite sign and separated by
Stony Brook University - PHYSICS - 384
Definitions: Magnetic potential Remember that the potential is defined as thepotential to do work.o pp Wb =c W= Magnetic Potential:4 rrmwhere o=4 10-7 [H/m] is the magnetic permeability of free spaceand p [A/m] is magnetic pole strength Gravit
Stony Brook University - PHYSICS - 384
Applications Shallow (Engineering and Environmental):contaminants, toxic waste, pipes, cables and metalinclusions Military: location of UXOs Archeology: buried walls, old fire pits Mining: iron sulfide deposits Oil and groundwater: depth to magneti
Stony Brook University - PHYSICS - 384
Derivatives Emphasizing shorter wavelength features. First vertical derivative emphasizes near surface features. It canbe measured with gradiometer, or derived from corrected data Second vertical derivative emphasizes boundaries of targetzones.Reduc
Stony Brook University - PHYSICS - 384
Removal of Regional Use formula tosubtract off IGRFvalue. Filtering processes toget the regional formula(like gravity).General Guidelines16
Stony Brook University - PHYSICS - 384
Gravity Anomalies: 2D forward calculationfor rectangular parallelepipeds with greater vertical extent than horizontalSpreadsheet: Grav2Dcolumnsee Dobrin and Savit eq 12-34Gravity anom aly2.50Define density structureProfile 10.30.30.30.30.30.3
Stony Brook University - PHYSICS - 384
SpikingdeconvolutionRecordedwaveform1-1Deconvolutionoperator101-1Output0RecoveredreflectivityseriesApplied Geophysics Seismic reflection IISpikingdeconvolutionRecordedwaveform1-1Deconvolutionoperator11100Output0-Recovere
Stony Brook University - PHYSICS - 384
Spiking deconvolutionApplied Geophysics Seismic reflection IISpikingdeconvolutionRecordedwaveformDeconvolutionoperatorOutput11-10-11RecoveredreflectivityseriesApplied Geophysics Seismic reflection II3
Stony Brook University - PHYSICS - 384
Vertical Electric Sounding When trying to probe howresistivity changes withdepth, need multiplemeasurements that each givea different depth sensitivity. This is accomplished throughresistivity sounding wheregreater electrode separationgives great
Stony Brook University - PHYSICS - 384
Geometrical FactorsArray advantages and disadvantagesArrayAdvantagesDisadvantagesWenner1. Easy to calculate a in the 1. All electrodes moved eachsoundingfield2. Sensitive to local shallow2. Less demand onvariationsinstrument sensivity3. Long
Stony Brook University - PHYSICS - 384
Current density and equipotential linesfor a current dipoledfraction total current 2z 2i f = tan1 dif=0.5 atz=d2if=0.7 at z = dWider spacing Deeper currentsApparent ResistivityPrevious expression can berearranged in terms of resistivity:=
Stony Brook University - PHYSICS - 384
Horizontal interfaceTraveltime equationsDirect wave:T=HeadwavexV1Head wave:T = TSB + TDD ' + TBDT=2h1x 2h1 tan ic+V1 cos icV2T=22x 2h1 V2 V1+V2V2V1T = ax + bslope: 1/V2intercept: gives h1Applied Geophysics Refraction IHorizontal
Stony Brook University - PHYSICS - 384
Critical incidenceWhen rP = 90 iP = iC the critical anglesin iC =VP1VP 2The critically refracted energy travelsalong the velocity interface at V2continually refracting energy back intothe upper medium at an angle iCa head waveReflection and tran
Stony Brook University - PHYSICS - 384
Classifications of magnetic materials Anti-ferromagnetic Almost identical to ferromagnetic except that themoments of neighboring sublattices are alignedopposite to each other and cancel out Thus no net magnetization is measured Example: Hematite Fe
Stony Brook University - PHYSICS - 384
Classifications of magnetic materials Ferromagnetic Materials contain unpaired electrons in incompleteelectron shells. Magnetic moment of each atom is coupled to othersin surrounding domain such they all becomeparallel. Caused by overlapping electr
Stony Brook University - PHYSICS - 384
Station spacingMust ensure thatspacing is sufficient tosample anticipatedsignalApplied geophysics IntroductionLimitations Methods require contrast in physical properties NonuniquenessDirect modeling: calculate the result of a specific structureI
Stony Brook University - PHYSICS - 384
Analysis and interpretationOnce we have made our gravity observations,corrected for surface effects,we attempt to deduce sub-surface structureConsiderations: Anomaly profile (2D structure) or map (3D structure)?If anomaly length > twice the width a
Stony Brook University - PHYSICS - 384
Bouguer anomalyApply all the corrections:g B = g obs + C + C F C B + CTwatch the signs!Smaller scale engineering/environmental surveys: Not tied to absolute gravity Use corrections with accuracy necessarydetermined by the size of the target signal
Stony Brook University - PHYSICS - 384
Gravity and potentialsg is a vector field:g=GM EREGravitational potential:U=2r1where r1 is the unit vector pointingtoward the center of the EarthGmrU is a scalar field which makes iteasier to work withDefinition: The gravitational potential
Stony Brook University - PHYSICS - 384
Remote sensingConstraining the Earths sub-surface withobservations at the surfaceGeophysical techniques measure physical phenomena: Gravity Magnetism Elastic waves Electricity Electromagnetic wavesWhich are sensitive to sub-surface physical prope
Stony Brook University - PHYSICS - 384
P and S-velocitiesP-velocity+43VP =S-velocityVS =change of shape and volumechange of shape onlyFor liquids and gases = 0, thereforeVS = 0 and VP is reduced in liquids and gasesHighly fractured or porous rocks have significantly reduced VPThe b
Stony Brook University - PHYSICS - 384
Body wavesP and S-wavesApplied Geophysics Waves and rays - IElastic moduliYoungsmodulusShear modulus, Force per unit area tochange the shape ofthe materialdescribe the physical properties of the rockand determine the seismic velocityPoissons
Stony Brook University - PHYSICS - 384
ExampleRockhead determinationfor waste disposal siteRefractor velocity determination Velocity is 1/slope ofoptimal TV plot Decided on threerefractor velocities asdata fitted well withthree straight linesegmentsApplied Geophysics Refraction III
Stony Brook University - PHYSICS - 384
ExampleRockhead determinationfor waste disposal sitePhantomingIncreased velocityor reduced depth? Used to generate arefraction arrival timemuch longer that it ispossible to collectNear-constantvelocity refractor Could be used todetermine inte
Stony Brook University - PHYSICS - 384
Gravity:Analysis and examplesReading:Today: p39-64Next Lecture: p65-75Applied Geophysics Analysis and examplesGravity Anomalies: 2D forward calculationfor rectangular parallelepipeds with greater vertical extent than horizontalSpreadsheet: Grav2Dc
Stony Brook University - PHYSICS - 384
Concept of hysteresis Complex relationship betweenB and H that occurs inferromagnetic materials. B flattens off with increasing Hat saturation When H is decreased, B does notfollow same curve Will have remanent B value atzero H
Stony Brook University - PHYSICS - 384
AttenuationThe amplitude of an arrival decreases with distance from the source1. Geometric spreadingEnergy spread over a sphere: 4r2Amplitude 1/r2. Intrinsic attenuationRocks are not perfectly elastic.Some energy is lost as heat due tofrictional d
Stony Brook University - PHYSICS - 384
Velocity sensitivityThe amplitude of wave motiondecreases with depthRelated to depth/wavelengthLonger wavelengths sample deeper(This fig isfor water-waves)Seismic velocity generally increaseswith depth.Surface waves are dispersive,which means th
Stony Brook University - PHYSICS - 384
Analytic Signal Combination of derivatives:2 F(x , y, z ) + x2A(x , y ) = F(x, y, z ) +z F(x, y, z ) z2 Shape is independent ofinclination/ declination ofinduced field.Other analysis or filters Fourier transforms Wavelets Upward and downw
Stony Brook University - PHYSICS - 384
Derivatives Emphasizing shorter wavelength features First vertical derivative emphasizes near surface features. It canbe measured with gradiometer, or derived from corrected data Second vertical derivative emphasizes the plan view boundariesof target
Stony Brook University - PHYSICS - 384
Velocity and densityNafe-DrakecurveVPigneous andmetamorphic rockssediments andsedimentary rocksVSThis curve has beenapproximated usingthe expression = aVP14(a is a constant: 1670 when in km/m3 and VP in km/s)Applied Geophysics Waves and r
Stony Brook University - PHYSICS - 384
Head waveYou can see:a head wave, trapped surface wave, diving body waveApplied Geophysics Waves and rays - IFactors affecting velocityVP =Density velocity typically increases with density+43VS =( and are dependant on and increase more rapidly t
Stony Brook University - PHYSICS - 384
ExamplesArcheological investigationsP-wave refraction lines Sledge hammer source 2m geophone spacing 50m lines reversedS-wave refraction lines Rubber maul hammerstriking horizontally on avertical plate 2m horizontal geophonespacingFrom Sharma:
Stony Brook University - PHYSICS - 384
ExampleRockhead determinationfor waste disposal siteRefractor depth profile Using TG, the refractorand overburden velocity,we can calculate thedepth of the refractorbeneath each geophoneGround surface Note it is the distance ofthe refractor fro
Stony Brook University - PHYSICS - 384
Methodology of interpretationInverse modelingForward modeling:Make a skilled guess of the structure (the model)Calculate the anomaly this would produceCompare to the observations (the data)Adjust the model and recalculate etcInverse modeling essent
Stony Brook University - PHYSICS - 384
Gravity Anomalies: 2D forward calculationfor rectangular parallelepipeds with greater vertical extent than horizontalSpreadsheet: Grav2Dcolumnsee Dobrin and Savit eq 12-34Gravity anom aly0.000Define density structure24681012141618-0.3-0.
Stony Brook University - PHYSICS - 384
Seismic methods:Seismic reflection - IVReflection reading:Sharma p130-158; (Reynolds p343-379)Applied Geophysics Seismic reflection IVSeismic reflection processingFlow overviewThese are the mainsteps in processingThe order in whichthey are appli