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What is Q?

This text explores different interpretations of Q, the relationship between real and imaginary frequency under attenuation, and the effects of Q on physical dispersion. It also discusses the frequency dependence of Q and provides examples of Q extraction from seismic waveforms.

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What is Q?

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  1. What is Q? Interpretation 1: Suppose A0 represents wave amplitudes, then ln(A) intercept slope t Interpretation 2: Suppose u represents displacement, then w =“modified” or “instantaneous freq” Suppose: small attenuation, the We can define b = w*, where w* ---> 0 as Q increases (imaginary freq due to attenuation),

  2. Relation with velocity: Imaginary velocity due to attenuation So Q is a quantity that defines the relationship between real and imaginary frequency (or velocity) under the influence of attenuation. Interpretation 3: Q is the number of cycles the oscillations take to decay to a certain amplitude level. So amplitude at time tn (after n cycles)

  3. Attenuation and Physical Dispersion (continued…) • Different interpretations of Q (quality factor): • As a damping term Q = w0/g • As a fraction between imaginary and real frequencies (or imaginary velocity to real velocity) • As the number of cycles for a wave to decay to a certain amplitude. If n = Q, then • Connection with t* (for body wave). • Energy formula 1 number that describes Q structure of several layers

  4. Effects of Q (assume the SAME Q value) dependencies (2) Physical dispersion: different frequency component travels at different times, hence causing broadening of phase pulse. Lets assume a Dirac delta function: area = 1, infinite height. u(x,t) spectrum t

  5. F(w) Rough sketch of the real part of spectrum w Key: The Fourier spectrum of a delta function contains infinite number of frequencies/velocities, not a singlet nor a constant nor zero! Now: Lets find out what the delta function becomes IF there is attenuation (or, finite Q value). Inverse FFT to get time domain signal Eventually

  6. Noncausal Attenuation Operator Causal Attenuation Operator elastic Observation: pulse is “spread out” which means dispersive (different frequencies arrive at different times!) Problem: envelope of the function is nonzero before t=x/c (it is like receiving earthquake energy before the rupture, not physical!)

  7. How to make this process causal? One of the often-cited solutions: Azimi’s Attenuation Law: Where does it come from? Answer: Derived under the following causality condition: Physical Models of Anelasticity In an nutshell, Earth is composed of lots of Viscoelastic(or Standard Linear) Solids

  8. Spring const k1 Standard Linear Solid (or Viscoelastic Solid) m Spring const k2 Specification: (1) consist of two springs and a dashpot (2) h = viscosity of fluid inside dashpot

  9. Governing Equation (stress): The response to a harmonic wave depends on the product of the angular frequency and the relaxation time. viscous elastic 100 100 The left-hand figure is the absorption function. The absorption is small at both very small and very large frequencies. It is the max at wt = 1!

  10. A given polycrystalline material in the Earth is formed of many SLS superimposed together. So the final Frequency dependent Q is constant for many seismic frequencies ---- ~constant 100 0.0001 Question: Is the fact that high frequencies are attenuated more a contradiction to this flat Q observation?? Frequency(Hz) Frequency dependence of mantle attenuation Answer: No. High frequencies are attenuated more due to the following equation that works with the same Q. So it is really “frequency dependent amplitude”, NOT “frequency dependent Q”.

  11. Eastern USA (EUS) and Basin-and-Range Attenuation Mitchell 1995 (lower attenuation occurring at high frequencies, tells us that frequency independency is not always true Low Q in the asthenosphere, Romanowicz, 1995

  12. An example of Q extraction from differential waveforms. Two phases of interest: sS-S sScS - ScS Flanagan & Wiens (1990)

  13. Key Realization: The two waveforms are similar in nature, mainly differing by the segment in the above source (the small depth phase segment) Approach: Spectral division of S from SS, then divide out the Crustal operator (a function in freq that accounts for the of the additional propagation in a normal crust) Spectral dividing S and R will leave the attenuation term A(w) (slope = =t/2Q, t = time diff of sS-S)

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