VẬT lý địa CHẤN seismic reflection

Seismic Reflection Surveying •The most widely used and well know geophysical technique •A seismic section looks similar to a geologic cross-section – a trap for the unwary •Only by understanding how the reflection method is used and seismic sections are created can geologists make informed interpretations •Today we will discuss some background theory and methods •Thursday we will collect some data •Next Tuesday we will discuss time series analysis •The following Thursday we will discuss and actually some processing Review: Body Waves There are two types of body wave (waves which travel through the earth) P-waves – Travel through the earth in a series of dilations and compressions Akin to sound through air S-waves Shear wave, not travel through fluids, travel at about half the speed of P-waves Review: Seismic Velocities velocity of waves = restoring force appropriate mass The velocity depends on two main things – the restoring force (analagous to the strength of a spring), and the mass (analagous to the mass of the spring) As the restoring force increases, the velocity increases However, as the mass increases, this will slow the spring, reducing the velocity The mass in the case of a rock is its density (mass per unit volume) S-waves involve a change in shape – this requires a shear force The size of the force depends on the shear, or rigidity modulus, μ A P-wave also involves a change in size, so the compressibility modulus κ is also involved vs = µ ρ Where ρ = denisty In a liquid, μ is zero, so vs is always zero From Mussestt and Khan, 2000 κ+ µ vp = ρ Velocities Rocks differ in their elastic moduli and densities and, hence, in their seismic velocities From Kearey, Brooks, and Hill, 2002 • Velocities •Many different ways of measuring velocity: •Refraction •Velocity analysis (conventional, PSDM) •Boreholes •Vertical Seismic Profiles •In situ logging using – measuring the travel time of a high frequency acoustic pulse •Hand samples •Travel time of a high frequency acoustic pulse •Anisotropy •Importance of confining pressure – P-wave velocity increases with confining pressure Velocities •Each layer is characterized by an interval velocity •If z1 is the thickness of layer i and ti is the oneway travel time through it then the interval velocity of that layer is: zi vi = ti i=1 i=2 i=3 •The root-mean-square velocity of the section down to the nth interface can be approximated by: vrms ,n  = ∑ vi2ti  i =1 n  ti  ∑ i =1  n Velocities vrms ,n  = ∑ vi ti  i =1 n  ti  ∑ i =1  n v1=1500 m s-1 t1=2.14 s v2=2000 m s-1 t2=1.21 s v3=2345 m s-1 t3=1.13 s What is vrms at the base of layer 3?  (1500 × 2.14) + (2000 ×1.21) + (2345 ×1.13)    14 + 21 + 13   = 1882.064 m s -1 2 2 Attenuation •The energy E transmitted outwards from a source becomes distributed over a From Kearey et al., 2002 spherical shell •If the radius of the wavefront is r, the amount of energy contained within a unit area of the shell is E/4πr2 •With increasing distance along a ray path, the energy contained in the ray falls of as r due to geometrical spreading of the energy •Wave amplitude, which is proportional to the square root of the wave energy, falls of as r-1 Attenuation •The ground is imperfectly elastic – energy is gradually absorbed by internal frictional losses •Absorption coefficient: proportion of energy lost during transmission through a distance equivalent to a complete wavelength – (dB λ-1) •Absorption coefficient is usually assumed to be independent of frequency •Higher frequency waves therefore attenuate more rapidly than lower frequency waves as a function of time or distance From Kearey et al., 2002 •Absorption produces a progressive lengthening of the seismic pulse Attenuation A 10 Hz seismic wave travelling at km s-1 propagates for 1000 m through a medium with an absorption coefficient of 0.2 dB λ-1 What is the wave attenuation in dB due solely to absorption? Repeat the above exercise for a 231 Hz seismic wave Comment on the differences λ=5000/10 = 500 m Attenuation = 1000/500 * 0.2 = 0.4 dB λ=5000/231 = 21.65 m Attenuation = 1000/21.65 * 0.2 = 9.24 dB References Used Kearey, P., M Brooks, and I Hill, An Introduction to Geophysical Exploration, 2002 Goodliffe, A.M., B Taylor, and G.D Karner, Correlations between seismic, sogging and core data from ODP Leg 180 sites in the western Woodlark Basin, in Proceeding of the Ocean Drilling Program, Scientific Results, Leg 180, edited by P Huchon, B Taylor, and A Klaus, Ocean Drilling Program, College Station, 2001 Mussett, A.E and M.A Khan, Looking into the Earth: An introduction to geological geophysics, 2000 http://www.ldeo.columbia.edu/res/fac/oma/sss (airgun schematics) http://www.ldeo.columbia.edu/res/fac/oma/sss/bubble.html (bubble pulse) http://www.ldeo.columbia.edu/res/fac/oma/sss/tuning.html (array tuning) [...]... velocity (v): Z = vρ •Generally speaking, the “harder” the rock the greater its acoustic impedance •Acoustic impedance contrast is the important factor •Maximum transmission of seismic energy requires a matching of acoustic impedances  Reflection coefficient R is a numerical measure of the effect of an interface on wave propagation It is the ratio if the amplitude A1 if the reflected ray to the amplitude... of the amplitude A2 of the transmitted ray to the amplitude A0 of the incident ray: T= A2 2 Z1 for a normally incident ray this becomes T = A0 Z 2 + Z1 From Kearey, Brooks, and Hill, 2002 Reflection and Transmission Reflection and Transmission •If R = 0, all the incident energy is transmitted •There is no acoustic impedance contrast •Velocity and density of the layers may still be different From Goodliffe.. .Reflection and Transmission From Kearey, Brooks, and Hill, 2002 •The total energy of a transmitted and reflected ray must equal the energy of the incident ray •The relative proportions are determined by... equation can be rearranged to sin i1 sin i2 = v1 v2 Snell’s Law From Mussestt and Khan, 2000 sin 37 o sin i2 = 4 5 5 sin i2 = sin 37 o 4 So i2 = 48.8o Vertical Resolution • • • • • • • • Dependant on seismic wavelength Wedge tests Individual reflectors clearly resolved when separated by > λ/4 v=f λ If v = 2000 m/s, and f = 30 Hz – Resolution = (66.67 m)/4 = 16.67 m If v = 8000 m/s and f = 20 Hz – Resolution... wavelength Parts of a reflector separated by less than the width of the Fresnel zone will not be resolved Wf ≈ (2z λ)1/2 z = depth If depth = 2000 m, λ = 60 m – Wf ≈ 490 m If depth = 100 m, λ = 1 m – Wf ≈ 14 m Seismic Sources Electrical Sources: Sparker •A spark is produced by the discharge of a high voltage capacitor bank through an underwater electrode •Produces a rapidly expanding bubble of ionized gas Electrical... place, the piston rapidly but gently moves downward, re-sealing the chamber, and readying the sound source for refilling Air Guns •Bolt Air gun From Kearey, Brooks, and Hill, 2002 •The most common marine seismic source •Very Repeatable signal Air Guns •Airguns suspended from stowed booms •Single Air gun – note air ports The Ideal Shot •An ideal pulse convolved with the seafloor creates a simple seismogram ... m s-1 t1=2.14 s v2=2000 m s-1 t2=1.21 s v3=2345 m s-1 t3=1.13 s What is vrms at the base of layer 3?  (1500 × 2.14) + (2000 ×1.21) + (2345 ×1.13)    14 + 21 + 13   = 1882.064 m s -1 ... progressive lengthening of the seismic pulse Attenuation A 10 Hz seismic wave travelling at km s-1 propagates for 1000 m through a medium with an absorption coefficient of 0.2 dB -1 What is the wave attenuation... earth) P-waves – Travel through the earth in a series of dilations and compressions Akin to sound through air S-waves Shear wave, not travel through fluids, travel at about half the speed of P-waves