Scope of Work
Sonar, 3.5kHz 1,
Apx 1: Statistics 1
Apx 1: Statistics 2
Apx 2: Equipment 1
Apx 2: Equipment 2
APPENDIX 2: SCIENTIFIC EQUIPMENT METHODS
Geophysical and Navigation Systems (2)
3.5-kHz HighResolution Subbottom Profiling System:
High-resolution 3.5kHz subbottom profiles were collected concurrently
with sidescan sonar imagery. The system comprises an Ocean Data Equipment
Corporation (ODEC) Bathy 2000, chirp signal correlator and Raytheon PTR
transceiver, driving a 3.5kHz subbottom profiler having four Raytheon
TR109 transducers (mounted in a towfish). Pulse repetition rates were 0.25,
0.5 and 1.0 sec. The 3.5kHz return signals were displayed on a 16bit
format color monitor and on analog HP inkjet color paper records.
The monitor displays the acoustic profile, date, time (JD/GMT), location,
ship speed, pulse repetition rate and duration, water depth, and gain.
All data are digital, merged with navigation and archived on optical disc.
The theoretical resolution of the system is 11 cm in the vertical and 5
to 30 meters in 30 to 1000 meters water depth. Owing to printer and monitor
resolution, the practical resolution is on the order of 0.5 meters. A correction
was made for offset of the tow fish/transducers from the sea surface, and
the acoustic velocity of sea water was assumed to be 1500 m/sec.
The 3.5kHz tow fish was deployed on 051/2335 (March 20, 1993).
The system was tested, found operational, and official logging commenced
at 052/0000. The system performed well with no maintenance required throughout
the course of the survey. The 3.5kHz profiling terminated on 057/1937
(March 23, 1993).
Datasonics Chirp Sonar HighResolution Subbottom Profiler:
A Datasonics CAP6000A chirp sonar subbottom profiling system was used
for this study, and is described in detail by Mayer and LeBlanc (1983),
Schock and others (1989), and Schock and LeBlanc (1990a, 1990b). The chirp
system produces very high resolution subbottom profiles from a precise,
computer generated, swept frequency output whose reflected returns are
match filtered to compress the pulse and suppress noise. The acoustic profiles
are displayed real time on a super VGA graphics monitor and on inkjet
color paper copies. The raw-data is archived as a full wave form return
signal on 4-mm DAT tape; this allows the received signals to be replayed
through the CAP6000A system at scales and gain settings that allow
optimum observation of subbottom reflections.
The chirp sonar system was deployed for testing on 051/2252 and tested
as operational. Chirp sonar profiling commenced on 055/1955, and concluded
on 057/1936. The system performed poorly throughout the survey, owing to
noise inherent to our specific chirp sonar and to noise generated by the
winch. When the system did operate properly, we were unable to resolve
any subbottom layers that approached the advertised resolution (20 cm).
The poor quality of the chirp data may also result from the nature of the
seafloor sediment in Mamala Bay. Communications with colleagues who have
employed chirp sonar in Mamala Bay indicate similar results with other
chirp systems (James Barry, MMTC, Look Laboratory, University of Hawaii;
and Mark Erickson, Sea Engineering Inc., Waimanalo, Hawaii; oral communication).
YoNav Navigation System:
The primary shipboard navigation system employed was autonomous
or single- receiver GPS (the Global Positioning System), and shipboard
navigation was provided by a Trimble 4000AX GPS receiver. GPS is a 3dimensional
location system, the foundation of which is the Department of Defense¹s
(DOD) NAVSTAR satellite constellation. The system is based on observations
of signals emitted from the satellite constellation. Satellite range observations
are then processed by GPS receivers that determine geodedic latitude, longitude,
and height relative to a reference ellipsoid (Georgiadou and Doucet, 1990;
Wells and Kleusberg, 1990). The single-receiver GPS has an accuracy of
100 meters, 2D RMS, which occurs when the U.S. Government DOD program "Selective
Availability" (SA) is implemented. SA denies GPS users the full position
accuracy of GPS. However, when SA is not implemented, observed accuracy
is about 50 m, 2D RMS.
LORANC and transit satellites were the primary backup positioning
systems. Navigational data was collected with the USGS-designed YoNav Navigation
system, capable of collecting a variety of navigation signals including
GPS, LORANC (either hyperbolic or rhorho), transit satellites,
and micro-wave frequency shorebased transponder systems. The YoNav
system is a PCbased data acquisition and display program written in
Microsoft C/C++ designed by the USGS to provide navigation services on
almost any DOS platform. The YoNav system incorporates a real-time trackline
display and line generating software for both the ships' bridge and scientific
personnel and is described in detail in Gann (1992). The display shows
the ships¹ position relative to the desired survey line, allowing
bridge personnel to more easily stay on line. The GPS system worked well,
providing 24 hours per day navigation.