Summary

Introduction 1, 2
  Study Area
  Previous Studies
  Oceanography
  Seafloor Materials

K1-93 Survey
  Methods
  Scope of Work
  Navigation
  Sidescan Sonar
  Bathymetry
  Profiling

Results
  Bathymetry
  Sonar, 3.5kHz 1, 2, 3

Conclusions

Figures
  1   2   3   4   5
  6   7   8   9 10
11 12 13 14 15
16 17 18 19 20

Plate 1

Apx 1: Statistics 1
Apx 1: Statistics 2
Apx 2: Equipment 1
Apx 2: Equipment 2
Acknowledgments

References 1, 2, 3

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.