Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Ocean Engineer at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1982
Description:
Seismic refraction experiments have been used extensively in the past
thirty five years in investigations of the structure of the oceanic crust. The
longer range of the refraction or wide angle reflection technique, on the
order of tens of kilometers, permits a deeper and wider area of examination,
although with less resolution, than the spatially limited seismic reflection
experiment. Observations of arrivals from the Mohorovicic discontinuity, at an
average depth of seven kilometers below the sea floor, are routinely made.
The major focus in interpreting refraction data has been the analysis of
travel time/range data and the "inversion" of this data for the purpose of
determining a velocity versus depth profile of the crust. The most frequent
application of this procedure is the geophysicist's use of velocities for
postulating geologic structures and rock types below the sediment (Christensen
& Salisbury, 1975). Another area using refraction data, less widely seen,
falls into the ocean acoustician's domain. In studying the behaviour of sound
in the ocean, the sea floor is often modelled as a boundary with a half space
below, and with some form of reflection characteristic and/or loss mechanism.
If acoustic energy, upon encountering the bottom, was either reflected or
transmitted directly, this would be appropriate, and the determination of
reflection and transmission coefficients for the sea-sediment interface would
probably be sufficient. However, sound energy does penetrate beneath the sea
floor and is both reflected and refracted back to the water. In an active
acoustical experiment, especially at longer ranges, a significant amount of
the received energy may come from waves that have interacted with the earth's
crust and have been reinjected into the water. Since these arrivals can be
detected in the ocean, their study is of concern for the acoustician.
The role of bottom interaction, especially at low frequencies, is now an
area of intense research activity in modelling acoustic propagation. In
particular, in the language of the sonar engineer, the TL, or transmission
loss, of this energy is of major importance for i) predicting the character of
the sound field at a receiver in future experiments, ii) for comparing crustal
loss with the better known TL of paths remaining primarily in the water layer,
and iii) expanding the role of arrival amplitudes in inversion theory. Just as
there may be a number of possib1e paths in the sea between a source and
receiver, each with a different loss characteristic, trajectories in the crust
are variegated and exhibit different TL behaviors. It is important to be able
to differentiate the energy partitioned among the different paths, and to
determine which paths are most important.
Resolving the locus of a particular acoustic path is intimately tied to
the problem of determining the velocity structure of a medium. To the limits
of the geometrical optics approximation of acoustic behaviour, sometimes
sorely pressed at low frequencies, a completely detailed knowledge of sound
speed variations, both laterally and with depth, plus known source
characteristics and attenuation losses in the medium, enables one in principle
to predict signals observed at a receiver. For an ocean acoustician, the
requirement of environmental knowledge of the sound speed profiles, both in
water and crust, needed to predict the amplitude and timing of data, is
clearly very burdensome. In the past twenty five years, however, models of the
oceanic crust have been formulated which are statistically consistent over
much of the oceans. These models divide the crust into three or more
horizontal layers with certain average thicknesses and velocities (Raitt,
1963). At least within the confines of these models, if a typical transmission
loss were known for each of these layers, an acoustician can make predictions
of the expected strength and timing of crustal arrivals at other stations.
Most of this environmental information has been obtained from refraction
and/or wide angle reflection data, usually via travel time analysis. Little
has been done in developing models accounting for amplitude dependence.
Description:
This work was supported under ONR contract N-00014-75-C-0852.
Keywords:
Seismic refraction method
;
Data processing
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
Permalink
