Geoscience Reference
In-Depth Information
Chapter 11
Geotomography: heterogeneity of the mantle
You all do know this mantle.
Shakespeare (Julius Caesar)
Tomography
Seismic tomography can be used to infer the
three-dimensional structure of the Earth's inte-
rior, and, under certain conditions, the min-
eralogy. It is more difficult to infer tempera-
ture and composition. Although seismology and
tomography are quantitative sciences, the result-
ing maps and cross-sections are often inter-
preted in a visual or intuitive way. Conclusions
based on visual inspection of color tomographic
cross-sections [see
mantleplumes
] can be called
Qualitative chromo-tomography
or QCT. Quantitative
tomographic interpretations involve
probabi-
listic
tomography
maps of chemical het-
erogeneities, understanding how the
power is distributed in the wave-
number domain,
anisotropy and derived quan-
tities such as
V
p
/
V
s
ratios, correlations, spec-
tral densities vs. depth, matched filters, statis-
tics and so on. Anelasticity, anharmonicity and
mineral physics and geodynamic constraints,
are also used in quantitative interpretations of
tomographic models. Quite often a tomographic
cross-section is misleading because of ray cov-
erage, color saturation, cropping, data selec-
tion, bleeding and smearing. There are also arti-
facts associated with source, receiver and finite
frequency effects, lateral refraction, anisotropy
and projection. Global tomography gives only
the long-wavelength components of heterogene-
ity. Seismic scattering and high-frequency reflec-
tion experiments provide details of small-scale
structure.
We must now admit that the Earth is not like an
onion. It is time for some lateral thinking. One-
dimensional radial variations in the mantle were
responsible for what are now the standard one-
and two-layer models of geodynamics and man-
tle geochemical reservoirs. The lateral variations
of seismic velocity and density are as important
as the radial variations. The shape of the Earth
tells us this directly but provides little depth res-
olution. The long-wavelength geoid tells us that
lateral density variations -- and probable chemi-
cal variations -- occur at great depth. Heat flow
tells us that there are pronounced shallow vari-
ations in heat productivity, structure and phys-
ical properties. Lateral variations in the mantle
affect the orientation of Earth in space and con-
vection in the mantle and core. This property of
the Earth is known as
asphericity
. It is best stud-
ied with seismic tomography. In the following
chapters we further recognize that the Earth is
neither elastic nor isotropic; it is
anelastic
and
anisotropic
. Long-wavelength lateral variations are
revealed by global tomography. High-resolution
seismic studies and scattering of high-frequency
seismic waves complement the long-wavelength
studies but are not consistent with the simple
dynamic and chemical models based on older 1D
or long-wavelength studies. Scattering may con-
tribute to the anisotropy and attenuation of seis-
mic waves in the upper mantle, and may help
resolve the fates of recycled materials and the
question of homogeneity of the upper mantle.
