1 / 15

Measureable seismic properties

Measureable seismic properties . Seismic velocities – P & S Relationship to elastic moduli Seismic anisotropy -- directional variation in seismic velocity Seismic Attenuation – 1/ Q p & 1/Q s -- What is seismic attenuation ? -- What causes seismic attenuation?. l .

kuper
Download Presentation

Measureable seismic properties

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Measureable seismic properties • Seismic velocities – P & S • Relationship to elastic moduli • Seismic anisotropy -- directional variation in seismic velocity • Seismic Attenuation – 1/Qp & 1/Qs -- What is seismic attenuation? -- What causes seismic attenuation?

  2. l = Lame’s lambda constant Seismic velocities k = Bulk modulus  = Poisson ratio = Shear modulus m = density

  3. Measuring both Vp and Vs is useful The ratio of Vsto Vp depends on Poisson’s ratio: A good approximation is often that k = m; then s = 0.25 and Vp/Vs = √3 • We also sometime calculate the Seismic Parameter: • = Vp2 - 4/3 Vs2 = k/r • Shows variations in the bulk modulus (compare to Vs2 = m/r)

  4. Seismic Anisotropy Shear velocity of olivine Relationship of anisotropy and strain - xenoliths Mainprice & Silver [1993] Data from Kumazawa & Anderson [1969]

  5. Shear Wave Splitting

  6. Seismic Attenuation • In a perfectly elastic medium, the total energy of the wavefield is conserved • Seismic attenuation is the absorption of seismic energy, or the deviation from perfect elasticity Surface waves Body waves Coutier & Revenaugh[2006] Widmer & Laske[2007]

  7. Normal Modes • Different Modes show different rates of amplitude decay • So we can determine a Q for each mode • Different Qs result from how each mode samples the earth

  8. Attenuation variation in the Earth Gung & Romanowicz[2004] Pozgay, Wiens, et al. [2009]

  9. Q – Quality Factor • Attenuation is quantified by 1/Q, in analogy to the damped harmonic oscillator (underdamped) • Smaller Q results in faster damping (greater deviation from elastic case) • Frequency-independent Q damps high frequencies more than low frequencies • Q = 2π (total energy/energy lost during one cycle)

  10. Shear and Bulk Q • Shear wave attenuation results from relaxation of the shear modulus (μ) • P wave attenuation results from the relaxation of both the shear (μ) and bulk (κ) moduli • In general bulk attenuation is thought to be very small in the earth (Qκ> 1000) • If Qκ~ ∞ and assuming a Poisson Solid (λ = μ), QP= 2.25 QS

  11. Anelasticity

  12. Absorption Band & Velocity Dispersion • A single relaxation time gives an absorption peak at ω = 1/τ • Velocity increases from relaxed to unrelaxed values at about the same frequency • A spectrum of relaxation times superposes these effects

  13. Frequency Dependence of Attenuation Lekic et al. [2009] • Q is observed to be weakly frequency dependent in the “seismic band” • Described as Q = Q0ω-α • Interpreted as a broad spectrum of relaxation times

  14. Possible Attenuation Mechanisms Another Mechanism: Dislocation Damping (Farla et al., 2012) • Identification of mechanism is necessary to scale results from lab to earth • Scaling in grain size, temperature, pressure, etc.

  15. Attenuation and Velocity Anomalies are Highly Correlated Q model S Velocity Model Dalton et al. [2009]

More Related