Thank you all for the great responses!
Dennis certainly make a good point about making students aware of the
reference model!
Greg
On 3/17/2011 1:07 PM, Dennis Brown wrote:
> Greg
> You have most of the answers in other e-mails, but none say that what the
> colors mean in tomography is pertubations from a reference velocity model
> that changes with depth. The color scale, which is given in %, is the
> variation from that reference model.
>
> If you want to play with the relations between seismic velocities,
> density, temperature, pressure and rocks that you design yourself, then
> there is a great Exel spreadsheet on the webpage of Bradley Hacker called
> "Physprops" that is free. You design the rocks from mineral end members
> (e.g., modal garnet with the percent almandine etc. in garnet plus the
> same for pyroxene and so on), set the T and P and the program will give
> you Vp, Vs and density that has been calculated using a number of
> different methods (i.e. Voight-Reus-Hill and so on). There is a table with
> the physical properties, shear modulos etc. that it uses to do this. If
> you want, I have added a bit that calculates the major element composition
> (in Wt.%) of the rock you make that I can give you as well.
>
> There is a page with examples of rocks, so this spread sheet is very easy
> to use and you can learn a lot.
>
> Cheers
> Dennis
>
>
>> Dear Greg
>> See www.mantleplumes.org, there are several discussions and opinions in
>> the section “seismology”, also about the “red = hot, blue = cold” your
>> students ask
>>
>>
>>> Awesome answer! Thank you, Ernie! That is just what I was looking for!
>>>
>>> Best regards,
>>> Greg
>>>
>>> On 3/17/2011 11:58 AM, Ernest Rutter wrote:
>>>> It is a fact that velocity increases with the density of a suite of
>>>> (different) rocks, but for constant elasticity velocity decreases with
>>>> density. It is just that going from say, granite to peridotite, the
>>>> increase in density is more than offset by the increase in stiffness.
>>>> Denser rocks tend to be stiffer. Hotter rocks have lower densities AND
>>>> lower stiffnesses. Stiffness is a much more variable property than
>>>> density, so you can have denser rocks being faster than less dense
>>>> rocks.
>>>>
>>>> Ernie Rutter
>>>>
>>>> -----Original Message-----
>>>> From: Tectonics& structural geology discussion list
>>>> [mailto:[log in to unmask]] On Behalf Of Gregory Dumond
>>>> Sent: 17 March 2011 16:45
>>>> To: [log in to unmask]
>>>> Subject: P-wave velocity, Density, and Modulus
>>>>
>>>> There is a lot of remarkable work being done in North America, Tibet,
>>>> and all over the globe using seismic tomography! Consequently, I
>>>> receive
>>>> questions from many students that are similar to "What do the red and
>>>> blue colors mean?!"
>>>>
>>>> I first admit my unfortunate lack of anything but a basic education in
>>>> geophysics. In my non-expert opinion, the equation for P-wave velocity
>>>> (e.g., in Fowler (2005-2nd ed. on p. 102) appears at first to be
>>>> nonintuitive, based on the following:
>>>>
>>>> (1) Classic experiments by Christensen and Mooney (1995: JGR-Solid
>>>> Earth) and others demonstrate a general trend of P-wave velocity
>>>> increase with increasing density. One of my illustrations for students
>>>> is to hand them similarly-sized hand samples of basalt and eclogite to
>>>> convey to them a sense of how phase changes and densification can occur
>>>> in a fixed bulk composition. Coincidentally, the average P-wave
>>>> velocity
>>>> and density for mafic eclogite (7.984 km/s for a density of 3515 kg/m3)
>>>> is greater than basalt (5.873 km/s for a density of 2926 kg/km3), at
>>>> 762C and 50-km-depths (Christensen and Mooney, 1995).
>>>>
>>>> (2) The density term is in the denominator of the P-wave velocity
>>>> equation, implying that density is (by definition) inversely
>>>> proportional with seismic velocity. Yet, Birch's Law tells us that
>>>> denser rocks DO have higher seismic velocities. This inspired me to
>>>> look
>>>> for references that have plotted Modulus as a function of Density. Web
>>>> of Science provided few results. The most intriguing one involve a 2007
>>>> publication in Nature Materials on the direct relationship between
>>>> modulus and density in bone and some nanomaterials (Fan et al., 2007).
>>>>
>>>> I really like Professor Fowler's explanations on pp. 102-104 in her
>>>> awesome "The Solid Earth..." textbook, but I have still encountered
>>>> difficulties with regards to getting junior-senior undergraduate
>>>> students (and some colleagues) to fully understand these relationships.
>>>>
>>>> I would sincerely appreciate any advice or guidance on this topic.
>>>>
>>>> With warmest regards,
>>>> Gregory Dumond
>>>> Department of Geosciences
>>>> University of Arkansas, USA
>>> --
>>> Gregory Dumond
>>> Assistant Professor
>>> Department of Geosciences
>>> University of Arkansas
>>> 18 Ozark Hall
>>> Fayetteville, AR 72701
>>> Phone: 479.575.3411
>>> Email: [log in to unmask]
>>> Web: http://geosciences.uark.edu/3858.php
>>>
>>> --
>>> Este mensaje ha sido analizado por MailScanner
>>> en busca de virus y otros contenidos peligrosos,
>>> y se considera que está limpio.
>>>
>>
>> Rubén Somoza
>> Dpto. Ciencias Geológicas, FCEyN, UBA
>> Ciudad Universitaria, Pabellón 2
>> C1428EHA, Buenos Aires, Argentina
>>
>>
>> --
>> Este mensaje ha sido analizado por MailScanner
>> en busca de virus y otros contenidos peligrosos,
>> y se considera que está limpio.
>>
--
Gregory Dumond
Assistant Professor
Department of Geosciences
University of Arkansas
18 Ozark Hall
Fayetteville, AR 72701
Phone: 479.575.3411
Email: [log in to unmask]
Web: http://geosciences.uark.edu/3858.php
|