Modeling the conversion of hydroacoustic to seismic energy at island and continental margins: preliminary analysis of Ascension Island data Page: 4 of 12
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OBJECTIVE:
Two types of recording stations are being used for hydroacoustic monitoring of the
Comprehensive Nuclear-Test-Ban Treaty (CTBT): hydroacoustic and T-phase. Hydroacoustic stations
consist of hydrophones in the ocean sound channel. T-phase stations are seismic stations on islands or near
continental margins that record acoustic signals, which convert to elastic waves (the "T-phase") at the
ocean-land boundary. T-phase stations are much cheaper to install and maintain than the hydrophone
stations. However, this cost-savings is offset by the reduction in signal quality due to energy loss at the
acoustic-to-seismic conversion and subsequent propagation of the T-phase through the solid Earth. Because
of local site conditions, T-phase amplitudes will vary with back-azimuthal (direction-of-approach).
The objective of this research is to perform numerical simulations of T-phase conversion to
identify the factors that most strongly impact T-phase signals and to gain predictive capabilities of T-phase
behavior. Simulations of T-phases will be compared with onshore recordings of airgun blasts from the
May 1999 Ascension Island Hydroacoustic Experiment (Harben et al., this volume). We performed
simulations of acoustic-to-seismic conversions using a two-dimensional finite difference code that is
accurate for both the acoustic and elastic wave propagation.
RESEARCH ACCOMPLISHED
Background
The propagation of acoustic waves in the ocean has been studied for some time. Acoustic waves in
the ocean travel very efficiently for long distances due to propagation in the sound speed channel and low
attenuation. The sound speed profile of the ocean decreases to a minimum at about 1000 m depth then
increases to the ocean floor (Figure la). The sound speed profile for Ascension Island is derived from the
World Ocean Atlas (Levitus et al., 1994) using the formula developed by Mackenzie (1981). Notice that
the sound speed does not vary much (~-6% peak-to-peak). Nonetheless, the low-velocity region of the
profile acts as a waveguide to channel acoustic waves without energy loss due to reflection at the ocean
surface or floor. When ocean propagating acoustic waves strike an island or continental margin they are
converted to elastic (seismic) waves. Such hydroacoustic-to-seismic conversions can be observed by on-
shore seismic stations and are called "T-phases". It is known that the efficiency of T-phase conversion is
impacted by such factors as local bathymetry (especially topographic slope) and the land path to the
seismic station (e.g. Cansi and Bethoux, 1985; Talandier and Okal, 1998).
The Ascension Island Experiment
The International Monitoring System (IMS) specified in the CTBT calls for a network of eleven
(11) hydroacoustic stations to detect and locate possible explosion events in or near the oceans. Six of
these stations will consist of hydrophones in the ocean and five will be seismic stations recording T-phases.
One of the hydrophone stations currently operating is Ascension Island (station code ASCH) in the
southern Atlantic Ocean. A field experiment in May of 1999 recorded offshore air-gun blasts with both
ocean hydrophones and T-phase seismic stations on-shore. This experiment, described in detail by Harben
et al. (1999, this volume), provided a unique data set for which to investigate T-phase behavior. Air-gun
blasts were used as active sources for an investigation of crustal structure in and around Ascension Island.
These shots occurred at distances from 1-45 km and circled the island. Analysis of the shots recorded as
acoustic waves in the ocean and T-phases recorded on shore is presented in Harben et al. (1999, this
volume).
Numerical Experiments
The following sections will describe a series of numerical experiments we performed to
investigate various aspects of the modeling procedures and hydroacoustic-to-seismic energy conversion.Validation of the Modeling: Particle Motion and the Convergence Zone
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Harben, P. & Rodgers, A. Modeling the conversion of hydroacoustic to seismic energy at island and continental margins: preliminary analysis of Ascension Island data, article, July 26, 1999; California. (https://digital.library.unt.edu/ark:/67531/metadc626039/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.