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SPICEX
LOAPEX
BASSEX
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LONG-TERM GOALS
The ultimate limits of long-range sonar are imposed by ocean variability
and the ambient sound field. Scattering due to internal waves and other
ocean processes limits the temporal and spatial coherence of the received
signal. The objectives of the North Pacific Acoustic Laboratory (NPAL)
program are to understand the basic physics of low-frequency, long-range,
broadband propagation, the effects of environmental variability on signal
stability and coherence, and the fundamental limits to signal processing
at long-range imposed by ocean processes. The long-term goal is to enable
advanced signal processing techniques, including matched field processing
and other adaptive array processing methods, to capitalize on the three-dimensional
character of the sound and noise fields.
OBJECTIVES
The scientific objectives are:
- To study 3-D coherence (horizontal, vertical, and temporal) of long-range,
low-frequency resolved rays and modes
- To explore the range and frequency dependence of the fluctuation
statistics of resolved ray and mode arrivals and of the highly scattered
finale observed in previous experiments
- To understand the surprisingly large amount of acoustic scattering
into the geometric shadow zone beneath caustics previously seen with
bottom-mounted SOSUS receivers (shadow-zone arrivals)
- To elucidate the relative roles of internal waves, ocean spice,
and internal tides in causing acoustic fluctuations
- To improve basin-scale ocean nowcasts via assimilation of acoustic
travel-time and other data into models
APPROACH
NPAL employs a combination of experiment, data analysis, and simulations
to address the issues outlined above. The principal experimental effort
during the current phase of NPAL is a long-range ocean acoustic propagation
experiment with three main components, named SPICE04, LOAPEX (Long-range
Ocean Acoustic Propagation EXperiment), and BASSEX (Basin Acoustic
Seamount Scattering EXperiment) (Fig. 1). SPICE04 and
LOAPEX share a pair of closely spaced vertical receiving arrays
that together span a large fraction of the water column. The acoustic
signals transmitted during SPICE04 and LOAPEX will
also be received on a towed horizontal array during BASSEX
to study acoustic scattering and diffraction from Kermit Roosevelt seamount.
SIO is responsible for SPICE04 and for the vertical receiving
arrays. APL-UW is responsible for LOAPEX. MIT and OASIS are
responsible for BASSEX. The geometry was chosen to keep the
SPICE04 and LOAPEX paths entirely within the subtropical
gyre. The VLA moorings are located between the Subarctic and Northern
Subtropical Fronts and to the west of the complicated California Current
region.
Fig. 1. Overall geometry of the 2004 NPAL experiment.
The 250-Hz moored transceivers (black) are located 500 and 1000
km west of the SVLA/DVLA receivers (yellow). The LOAPEX transmission
stations (red) extend roughly westward from the VLAs to a maximum
range of about 3200 km. A final LOAPEX station is located near
the Kauai source. The BASSEX towed array will be deployed close
to Kermit Roosevelt Seamount (orange), as well as in the vicinity
of the Kauai source. U. S. Navy SOSUS receivers that will receive
the various transmissions are shown in white.
In addition, the transmissions during SPICE04 and LOAPEX,
as well as the continuing transmissions from the ATOC/NPAL source north
of Kauai, are being recorded at the U. S. Navy SOSUS receivers in the
North Pacific. APL-UW has primary responsibility for continued operation
of the Kauai source and SOSUS receivers. Ambient noise data are also
being recorded at the SOSUS receivers.
Theoretical issues raised by NPAL and other long-range propagation data
are being addressed by a number of our collaborators.
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