Geoscience Reference
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are differences in the nature of discontinuities
between different tectonic regions. In addition,
we try to link the seismic properties with
results from mineral physics experiments on
rocks under similar pressure and temperature
conditions and use this to make inferences about
mantle dynamics.
seismic velocity. Reflected waves can only see a
discontinuity if its width is less than a quarter
of the wavelength of the seismic wave (Richards,
1972). The reflection will get too small to be
observed when the discontinuity thickness be-
comes half the wavelength (Shearer, 2000). Thus,
if a discontinuity is seen in short period data, it
must be sharp. If, however, it is only seen in long
period data, the impedance or velocity change
associated with the seismic discontinuity is prob-
ably spread out over a wider depth range. Receiver
functions are usually filtered at much shorter pe-
riod than SS precursors; thus, we can study much
smaller scale shear wave structure using receiver
functions. The smallest scale P wave structure is
observed with P
P
precursors, which have tradi-
tionally been the main data type to determine the
sharpness of the transition zone discontinuities.
Reflected and converted seismic waves from
discontinuities are usually too small to be seen in
a single seismogram. Therefore, large numbers of
seismograms are stacked to make the discontinu-
ity interactions visible (Shearer, 1991). Stacking
reduces incoherent noise and makes the reflected
and converted waves visible. Standard stacking
techniques assume that the discontinuities are
horizontal interfaces. However, if steep topogra-
phy is present, then migration techniques need to
be applied to correct for nonspecular reflections
of the discontinuities. Migration is most com-
monly applied in array studies (Rost & Thomas,
2009); Shearer
et al
. (1999) experimented with
using migration for SS precursor data, and PP pre-
cursors have been used for single event migration
(Thomas & Billen, 2009). Recent work has imple-
mented the Radon transform to comprehensively
include the effects of scattering on SS precursors
and also improve the resolution of the measured
discontinuity topography (Gu & Sacchi, 2009;
Cao
et al.
, 2011).
Discontinuity observations are often used to
determine topography on the transition zone dis-
continuities at 410 and 660 km depth. Arrival
times are measured, which are then converted
to discontinuity depth. These arrival times also
depend on the mantle structure that the seismic
waves have travelled through, and thus usually
10.2 Seismological Data and Methods
Seismologists use a range of different data types to
study mantle discontinuities, which can roughly
be divided into three groups depending on the
type of interaction between the seismic wave and
the discontinuity. Understandably, many studies
focus on one data type only. When making con-
nections with mineral physics, the real interest
lies in knowing all discontinuity characteristics,
including its compressional velocity, shear wave
velocity and density contrasts in addition to its
thickness and shape. No single data type can
provide all characteristics, and we require a com-
bination of different seismic data types to obtain
all information (e.g. Lawrence & Shearer, 2006a).
This is also where the difficulty is, as each data
type relies on a different source-receiver geome-
try and finding a location where all different data
types can be observed at once is challenging. In
fact, no such studies currently exist.
When making connections between seismic
observations and mineral physics, it is also im-
portant to realize that each seismic data type
has a different sensitivity to mantle properties.
Converted waves mainly depend on either the
compressional or shear wave velocity contrast at
the boundary. Reflected waves, on the other hand,
depend on the impedance contrast which is a com-
bination of velocity and density. No data type is
uniquely sensitive to density only, so to obtain
information on density requires combining differ-
ent data types (e.g. Shearer & Flanagan, 1999).
Another important issue is that these data
types are studied using seismic waves at different
frequency ranges (or periods). The resolution of
the seismic waves depends on the wavelength of
the wave, which is a function of its period and the
