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

EG&G SMS 990 59-kHz Digital Sidescan Sonar and SMS 960 Modem:
An EG&G SMS 990 high-resolution, digital sidescan sonar towfish and EG&G SMS 960 digital modem and recorder were used for the sidescan sonar survey. The system operates at 59 kHz, with a DC to 600­Hz bandwidth, and was set at a 1 km swath. The towfish was maintained at a nominal altitude of 50­100 m above the seafloor. Trackline spacing was 800 m providing a 20% overlap of imagery between adjacent lines. Advertised spatial resolution of the system is up to 1/800 of the selected swath width, equating to about 1.3 m for the 1­km swath employed. The system generates orthorectified images onboard ship in real time, that were used for quality control during the survey. The corrected real-time imagery was displayed on a graphic recorder having 16 gray tone levels, and uncorrected images were displayed on a Raytheon 800 TDU recorder. Unprocessed digital sonar data is acquired through USGS-developed MudScan software (Gann and others, 1993), merged with concurrently collected bathymetric data, and the digitally acquired data are archived on magneto-optical disc for post-cruise, full-scale digital processing and digital mosaicking.

The sidescan sonar system was deployed on 052/0411 and retrieved on 055/1412, following the survey. The system performed well, with only two periods of down time owing to one failed circuit board and one corroded connector.

Following the survey, the uncorrected sonar data were processed and digitally mosaicked using the USGS Mini Image Processing System (MIPS), to remove geometric distortions and noise inherent to the collection of sonar data. Post-cruise processing began with removal of the water column, followed by radiometric (shading, destriping and debanding, speckle removal, and nadir tonal improvements) and geometric (slant­to­ground range projection, aspect ratio, and delta velocity) corrections. Details of the processing routines employed in producing the digital mosaic are described by Chavez and Soderblom (1974), Chavez (1986), and Gardner and Chavez, (1987).

Interpretation of the Sidescan Sonar Mosaic:
Sidescan sonar mosaics are images of the seafloor that are analogous to an aerial photograph. Acoustic energy transmitted from the sidescan sonar vehicle is backscattered from the seafloor and the backscatter strength is recorded on a shipboard recorder and archived on optical disc. These digital acoustic data are then computer processed following the survey, so that the final mosaic represents a corrected, plan view of the seafloor. Seafloor features viewed on the processed mosaic are in their correct spatial position and true geometric shape.

The mosaic is presented as a black and white image, with various shades of gray used to define the variety of seafloor features. These shades of gray represent the varying energy levels of acoustic backscatter, and, for the mosaic presented in this report, the lighter shades represent higher acoustic backscatter. Typically, hard substrate, steep slopes, and rough bottoms produce higher backscatter (lighter features), but, many complex variables combine to determine how sound is backscattered and reflected from the seafloor. One assumption made by researchers is that the sidescan sonar is only imaging the seafloor. This is likely true for the system employed in this study, however, some sound energy may penetrate below the seafloor up to a few tens of centimeters. The amount of subsurface penetration is linked to the frequency of the sonar (the higher the frequency, the shallower the penetration), and nature of the seafloor substrate. Consequently, mosaic interpretation is not as straightforward as aerial photo interpretation, and, other data sets (i.e., high-resolution seismic reflection profiles, bottom photographs, and seafloor samples) are required to supplement the imagery so that accurate interpretations can be made.

The mosaic presented in this report (plate 1) is presented in Mercator projection at about 1:40000 scale, has a resolution of 1.3 m per pixel, and shows two principal types of high­backscatter features: dredged spoil deposits and submerged reefs. The dredged spoils form two major high backscatter deposits in the central portion of the mosaic (figure 6 and plate 1), that comprise circular to subcircular mounds or footprints that coalesce to form the larger deposits. The second set of high­backscatter features represent submerged (drowned) reefs. These reefs are primarily coralline debris and limestone. A large reef is visible as a broad area of high backscatter on the western side of the mosaic (figure 6). Native seafloor sediment has a typical low backscatter and is visible as the darker background on which the higher backscatter dredged material is resting.