AN INVESTIGATION OF P-WAVE SCATTERING IN THE CRUST AND UPPER MANTLE USING TRAVEL-TIME FLUCTUATIONS AND ARRAY SIGNAL COHERENCE
AN INVESTIGATION OF P-WAVE SCATTERING IN THE CRUST AND UPPER MANTLE USING TRAVEL-TIME FLUCTUATIONS AND ARRAY SIGNAL COHERENCE
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Date
1981-03
Authors
Ojo, Samuel Bakare
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Abstract
The propagation of P-waves through models of the
Earth's crust and upper mantle with slightly random, lateral
inhomogeneities superimposed on one composed of layers with
vertical velocity gradients was investigated. The maximum
deviation of velocity from a mean value at a given depth,
and a correlation distance derived from a two-dimensional
smoothing filter were two parameters used to vary the
amplitude and size of the velocity anomalies. The resulting
models show short discontinuous reflectors scattered about
at various depths throughout the models, and are thus in
agreement with many deep seismic reflection experiments.
On the other hand, numerical experiments using
ray-tracing techniques showed that the effect of the lateral
and vertical velocity anomalies is to scatter the energy,
and break up the continuous travel-time lines from vertical
gradient models into travel-time segments with different
slopes similar to those observed in many long range seismic
refraction experiments, and to those resulting from layering
effects in the media. The travel-time segments produced an
apparent intermediate layer at depths which could be
correlated to the correlation distance.
In a similar manner, the effect of the velocity
anomalies in the upper mantle models is to cause several
arrivals of distinctly different slowness (dT/dA) to be
observed over very short distances similar to observations
obtained from a number of array studies, thereby suggesting
that it might not always be necessary to invoke major
discontinuities in the upper mantle to explain these rapid
variations in slowness and travel-time observations.
On the basis of some long-range refraction data from
the 1979 COCRUST experiment in southern Manitoba and
Saskatchewan, a random model was produced for the crust of
that region, and showed that the optimum correlation
distance and r.m.s. velocity fluctuation that best describe
the travel-time fluctuations are 5 km and 1% respectively.
A method was also developed to extract the information
about the scattering characteristics of different P-wave
travel paths from the ratio of the incoherent energy to the
total incident energy of the signal recorded at an array
station. It is based on the assumption that if a ray passes
through a homogeneous earth, the energy arriving at an array
station should be relatively coherent, whereas if the ray
encounters lateral and vertical inhomogeneities, its energy
will be incoherent and much more complex.
A series of coherence measurements were made on more
than 300 earthquake recordings (distance 15 -36 , azimuth
00 -3600 ; and distance 400 -670 , azimuth 00 -1300 ) at the
Gauribidanur medium aperture ( 25 km) seismic array (GBA) in
southern India, and over 65 earthquake recordings (distance
12°~30°, azimuth 165 -285 ) at the Yellowknife seismic array
(YKA) in western Canada.
The analyses showed that there is relatively more
heterogeneity for the continental upper mantle region around
India than for the corresponding oceanic structure, with the
structure above 400 km tending to be more complex than the
structure below that depth. The YKA analyses showed that
the upper mantle structure under the north-western part of
the North American continent could be progressively more
heterogeneous with depth between 400 km and 750 km.
Description
A thesis submitted in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
Faculty of Graduate Studies
The University of Western Ontario
London, Ontario
March, 1981.
Keywords
INVESTIGATION,, P-WAVE,, SCATTERING,, CRUST,, UPPER MANTLE,, TRAVEL-TIME,, FLUCTUATIONS,, ARRAY SIGNAL, COHERENCE