Sunday, June 10, 2012

DISC 14 Stavanger (31 May)

We had an excellent DISC in Stavanger with 41 in attendance.  The venue was a nice lecture room at the University of Stavanger (that seemed like a good school).  In the audience was my friend Rian de Jong of Shell whom I met years ago on a consulting trip to Oman.  Back then, he and friends took me on a weekend jaunt along the coast road south of Muscat, then up into the high country to look at wadis and ancient villages.  Rian's hobby is jetting around the world to see total solar eclipses.  He put me on to NASA's Fred Espenak,  the utmost guru on such thins... have a look.

Anyway, we had some discussions that were quite interesting.  It was a very good group.


Feedback


From: Andrew James Carter 
Date: Thu, Jun 7, 2012 at 8:41 AM
Subject: Re: 2 quick feedback items

Hi Chris,

Two things arising from the [Stavanger] DISC that I can quickly send some feedback on:

1.   Transforming from Thomsen parameters to stiffnesses and back again.

I’m sure that by now someone else has already remarked on this, but in both your Backus Number article and the DISC book you don’t mention that whilst going from stiffnesses to Thomsen parameters is unique, going in the other direction is non-unique. Because of the quadratic relation between delta and the stiffness components it is necessary to choose the sign of (c13 + c55) (Voigt notation) in going from Thomsen’s parameters back to stiffnesses. This is dealt with in Tsvankin’s 2001 book on page 19. I think it’s a good idea to mention this when providing equations to go from Thomsen’s parameters to stiffnesses in case the unlikely event happens that one day someone stumbles upon a geological material of importance that violates this assumption.

2.  Definitions of Q. 

I agree with you in general about avoiding unnecessary approximations that can lead to hidden errors, although a high-Q approximation simplifies matters if you choose to use a different definition of Q than the one you and Kjartansson use! A particularly good reference on definitions of Q is O’Connell and Budiansky (1978). This paper explains the relation between the definition of Q that Kjartansson uses and Futterman’s choice of definition. I prefer Kjartansson’s choice as it is better behaved for strong attenuation and has a simpler relation to the complex modulus in the wave equation, and it allows the development of Kjartannson’s elegant CQ model. The point is though, as long as you are working in a high-Q approximation the difference between the two definitions is not especially significant, but once you choose to write everything out exactly, it is necessary to remember that you are using a different definition of Q than, for example, Aki and Richards. Aki and Richards used what I refer to as the “2pi definition” as opposed to the “4pi definition”, as did Futterman. I’m afraid I haven’t had time to check through the start of your attenuation chapter yet, but I will try to do so for my own satisfaction if I get time. I couldn’t see anything wrong after a quick look through though. I do think it might be useful for your readers to be aware that different attenuation laws e.g. Kolksy-Futterman and Kjartansson use different definitions of Q, and that these differences begin to matter at Q values that could actually be encountered locally within seismic sections containing highly attenuative zones.

Some evidence regarding 1d-scattering attenuation/stratigraphic attenuation that conflicts with some of your argument to follow when I get a bit more time.

Best regards,

Andrew



About town

Even in late May, Stavanger is a cold and windy place, although they tell me I saw it on a very balmy day.  Still, it was windy, damp and cold.  The Norwegians have a saying: "There is no such thing as bad weather, only bad clothes."

The big boats come in very close with Stavanger's deep water harbor.

This town is proud of it's role in oil and gas.  No such thing in Houston.

Sam and interesting wall art.

No idea, but someone thought she might be getting married.

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