Structural geologists are well aware of the fact that isotropic rocks
are quite exceptional in nature. Whicheverorigin, sedimentary,
metamorphicormagmatic, rocks are shaped with a plane of mineral
flattening, the foliation in geologists' jargon, and with a line
ofmineral elongation, the lineation. Just like a good quarryman, a
trained structural geologistwill detectapreferredorientationin an
apparently isotropic granite. Preferred mineral orientation and thus
structural anisotropy are the rule in nature. Consideringthe
largevariationsinelasticcoefficientsofrock-forming minerals, itcould be
predicted that, in turn, seismic anisotropy should exist and be
important, provided thatdomains withasimilarstructural signatureare
largeenough to affectseismic waves. This is why, in 1982 at a conference
held in Frankfurt, which was oneofthe fIrst meetings devoted to the
subject of seismic anisotropy, I asked Don Anderson the question of why
seismologists had not considered earlier in their models the obvious
constraint of anisotropy. I still remember Don's answer: "Adolphe, we
knew that our isotropic models were not very good but we had no other
choice. It is simply that, so far, computerswere not largeenough
tointegrate the anisotropy parameter". Changingisotropic glassesfor
anisotropic ones permits us to obtain betterand more realistic seismic
modelsofthe Earth's interior, but, maybe more importantly, it has, for a
seismologist, the farreaching consequenceofsteppinginto the fIeld
ofgeodynamics.
