EARS2232 Exploration Seismics. Exploration. 2006/2007 Semester 1. Teaching Team. Dr. Sebastian Rost (Module Leader)

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1 EARS3 Exploration Seismics Teaching Team Dr. Sebastian Rost (Module Leader) Dr. Graham Stuart (Seismic Interpretation) 006/007 Semester 1 Ben Dando (Demonstrator) Objectives On completion of this module students should be able to: 1. Understand the physical principles underlying the application of the seismic refraction and reflection techniques to the determination of shallow structure and the exploration for hydrocarbons. Appreciate the techniques and equipment used to undertake exploration seismic surveys on land and sea 3. Understand techniques for the processing and interpretation of seismic refraction data Books An introduction to Geophysical Exploration Kearey, Brooks and Hill Blackwell publishing Covers more than Seismology Good introductory textbook Easy to understand not very mathematical ~3 Exploration Seismology Seismic Data Analysis Sheriff and Geldart Cambridge Yilmaz SEG Very complete Graphics a bit outdated Mathematical background reprinted in 006 ~45 Way over the top for this course Probably most complete Must have if you stay in the field ~ $ (really US $) 1

2 3D Seismic Interpretation Bacon, Simm and Redshaw Cambridge Good for seismic interpret. Acquisition Processing last ¼ of course What is Exploration Seismology? Exploration seismology deals with the use of artificially generated elastic waves to locate mineral deposits (including hydrocarbons, ores, water, geothermal reservoirs, etc.), archeological sites, and to obtain geological information for engineering. (Sheriff and Geldart, 1995) ~80 Mintropkugel in Göttingen (first used in 1908 L. Mintrop) Wiechert Vertical Seismometer Seismological exploration stops long before unique answers are found Additional (better methods: drilling wells etc) Techniques are exchanged between exploration seismology and global seismology Basic techniques: measuring travel times from seismic time series Simple concept: Seismic waves are generated at a source such as an explosion, these waves propagate through an elastic medium by reflection and refraction and are recorded at a receiver. Amount of time taken and intensity (amplitude) holds information about both the source and the medium through which the wave has travelled.

3 BODY WAVES Wave types Seismic waves which travel through the body of the medium divided into P- and S- waves SURFACE WAVES Seismic waves which travel along or near the surface of a body And the motion decays rapidly with distance from the surface. Body Waves P waves S-waves compressional waves, particle motion in direction of propagation v p = 4 κ + µ 3 ρ transversal/shear waves particle motion perpendicular to direction of propagation v s = µ ρ κ = bulk modulus = incompressibililty µ = shear modulus = rigidity ρ = density SV-wave: S wave energy polarised so the the motion is in a vertical (saggital) plane which also contains the direction of wave propagation P and SV solutions are coupled. recorded on the radial component. SH-wave: S-wave which has only a horizontal component of motion SH waves are mathematically decoupled from P-SV solutions Important points: Elasticity increases at a greater rate than density so velocity (in general) increases with depth No shear waves in a fluid for perfectly elastic solid: V S V = 0 3 P V S Surface waves Seismic waves which travel along or near the free surface of a body and the motion or energy of the wave decays rapidly with distance from the surface. Surface waves travel with slower velocities than body waves 3

4 Rayleigh wave A surface wave whose particle motion is elliptical and retrograde in the vertical plane containing the direction of wave propagation. Its amplitude decreases exponentially with depth. Particle motion retrograde ellipse. In exploration know as Ground Roll are important as obscure signals of interest. Love waves A surface wave associated with the surface layer which is characterised by horizontal motion perpendicular to the direction of propagation with no vertical motion. They can be thought of SH waves trapped in a surface or channel; must have at least one layer to exist. They are dispersive and travel faster than Rayleigh but slower than S-waves In a layered Earth they are dispersive. Dispersion Phase and Group velocities Variation of velocity with frequency. Dispersion of a body wave is usually small* but surface waves show considerable dispersion. Group velocity refers to the velocity of energy propagation. Phase velocity refers to the velocity of a particular phase e.g. peak/trough *Typically just a few % difference between 10s of Hz and 10s of khz A dispersed Rayleigh wave generated by an earthquake in Alabama near the Gulf coast, and recorded in Missouri. Sheriff and Geldart, 1995 P-wave velocities Unconsolidated material: Dry sand km/s Wet sand km/s Clay km/s P-velocities (cont.) sedimentary rocks: Tertiary sandstone Carbon. sandstone Chalk Limestone Salt km/s km/s km/s km/s km/s Lithology - most obvious factor to control velocities Porosity: very important, depends on depth and pressure Velocity lowered, when gas/petroleum present More sensitive: V p /V s ratio Igneous/metamorphic rocks: Granite km/s Gabbro km/s Gneiss km/s Air: 0.33 km/s; Water: km/s; Petroleum: km/s 4

5 Basic definitions Frequency - number of times a wavelet repeats a second; measured in hertz (Hz = s -1 ) Period - time between peaks, troughs or zero crossing on a waveform; measured in seconds (s) Frequency = Period -1 A wavelet with a duration between peak and trough of 5ms has a period of 50 [ms] = 0.05 [s] Wavelength - distance between peak or troughs on the Ground - measured in meters (m) Wavelength = Velocity / Frequency A 50Hz wave traveling with a velocity of 4000 m/s has a wavelength of 4000/50 = 80m Amplitude - measure of the intensity of the wave ~ energy Frequency = 1/0.05 [s] = 0 [s -1 ] = 0 [Hz] As a seismic wave propagates through regions of changing velocity its ray direction will change. Wavefront: a surface over which the phase (travel-time) of a traveling wave is e e.g. one ripple on a pond Ray: the raypath is the direction of energy transport. In isotropic Media the ray is perpendicular to the wavefront. Travel time: the time for a wave to travel from one point to another along a ray path. This is known as refraction. Rays will refract towards regions of lower velocity and away from high velocity regions. In general velocity increases with depth. As a result seismic energy will turn as it propagates in to the Earth eventually arriving at the surface again (turning waves). It can be shown that a linear velocity gradient results in a ray path which is an arc of a circle. Snell s law When a seismic wave crosses a boundary between two media the wave changes direction such that the horizontal component of 1/velocity is conserved It is easy to prove using Fermats principle Snell s law: ratio of sine of angle of incidence and refraction angle are equals the ratio of velocities sin i sin r α = 1 = α β1 β sin i sin r = = p v v 1 5

6 Reflection/Transmission Reflection: a) normal incidence b) inclined incidence incident ray A I transm. ray A T reflected ray A R v1,ρ1 v,ρ Reflection coefficient (normal incident): A R = A R I ρv ρ v = ρ v + ρ v Acoustic Impedance Z = ρ V = density x velocity Note: when ρ 1 V 1 < ρ V then R is negative, i.e. the Reflected wave will undergo a phase change by π The polarity of the wavelet will undergo sign change Normal Incidence!! 6

7 The simple normal-incidence relations are a special case of the more complex equations which describe the reflection and transmission coefficients for elastic waves with arbitary angle of incidence form the boundary known as Zoeppritz equations (1919) Karl Zoeppritz Head waves Rays which enter or leave a high velocity medium at a critical angle are known as head waves or refracted waves 7

8 Using rays: assumption: reflection in one point. reality: waves energy turning from large area = Fresnel Zone F.Z. : area from which reflected energy arriving at the station has phase difference of less than half cycle energy interferes constructively nλh0 R n πλh0 S 1/ λ/4 criterion Radius of FZ R n nλh0 πλh S 0 1/ Area of annular ring Diffraction occurs at abrupt discontinuities or structures whose radius is shorter than a wavelength Huygens Principle every point on an advancing wavefront can be regarded as the source of a secondary wave and that a later wavefornt is the envelope tangent to all the secondary sources Cause: Huygens principle Diffracting edge 8