|
The
original data acquisition system ("PHONE"), installed on August
29, 1991, accessed individual hydrophones from arrays throughout
the North Pacific and provided omni-directional coverage of
low-level seismicity (mb > 2.4) in the entire North Pacific
basin. Two channels were allotted for each array, collecting
individual hydrophone elements from the two ends of each array.
The omni-directional nature of the hydrophones allow detection
of sources at any bearing, and the availability of the pair
of signals allows calculation of the arrival bearing. By combining
arrival time and arrival bearing on several arrays, common
events can be associated automatically within software.
By
combining all of the hydrophone element signals with predefined
phase lags for each element based on their relative locations
and local sound speed, an acoustic "beam" can be formed, oriented
in any direction. The "BEAM" system, installed June 22, 1993,
accessed formed-beams from six hydrophone arrays trained on
the northeast Pacific seafloor spreading system from the Mendocino
to the Sovanco Fracture Zones. The signal gain obtained through
beam forming allows the detection of seismic events with source
magnitudes as low as 1.8. This system was continuously analyzed
to provide immediate detection of significant seismo/acoustic
events of potential interest, including volcanic dike injections
and eruptions, and was the basis of several successful event
response efforts.
On
October 12, 2000, changes in the Navy's electronics configuration
resulted in the loss of beamformed outputs to dual users.
At that time, the two NOAA/PMEL data acquisition systems were
combined into a single system ("WHIDBEY") that collects multiple
hydrophone elements at 256 Hz sampling rate. The data are
transferred, digitally beamformed, and displayed at PMEL in
Newport. Although more effort, the new configuration allows
complete flexibility in beamforming. In performing this upgrade,
PMEL and University of Washington Applied Physics Laboratory
engineers made accommodations to allow future dual users to
easily assess all hydrophone elements from all systems.
|
|