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
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