U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Methods
This study presents an interweaving of four types of data, each with a different resolution: (1) Sea Beam bathymetry, (2) SeaMARC I sidescan sonar backscatter imagery, (3) towed-camera photogeology and microbathymetry, and (4) submersible observations and sampling. A brief description of the methods used in collecting and processing these data follows.
Sea Beam
The original Sea Beam data set was collected using LORAN-C navigation during 19811984 from the NOAA ship Surveyor. Data from additional surveys by the Atlantis II in 1987 and the NOAA ship Discoverer in 1990 and 1992 extended coverage to the flanks of the ridge and corrected the bathymetry for depth changes caused by the volcanic eruption during the mid 1980s at the northern end of the Cleft segment (Figure 2) [Fox et al., 1992]. The operational characteristics of the Sea Beam system have been described by Renard and Allenou [1979], and de Moustier and Kleinrock [1986], and the processing techniques used to produce the maps in this Special Section are described by Embley et al. [1990a]. The map coordinate system was obtained by applying a geodetic corrector to the original LORAN-C navigation (but note that LORAN-C was used in previously published maps for this area of Chadwick et al. [1991], Embley et al. [1991], and Fox et al. [1992].
SeaMARC Sidescan
A single 5-km swath of SeaMARC I sidescan sonar (30 kHz) of the neovolcanic zone over the Juan de Fuca Ridge was collected in 1982 [Kappel and Ryan, 1986; Kappel and Normark, 1987]. Unfortunately, the digital logging system on this cruise was inoperative, so only analog records were obtained. In August 1987 the NOAA ship Discoverer resurveyed the axial valley of the Cleft segment with a deep-towed SeaMARC I sonar, and three overlapping 2-km-wide swaths of digital sidescan data were obtained along the entire segment between 44°28'N and 45°02'N. The initial processing techniques used for this data are described by Appelgate [1990].
Towed Camera
A towed-camera system designed and constructed at the Pacific Marine Environmental Laboratory (PMEL) was used to collect the photogeologic and microbathymetry data. Data from the NOAA PMEL camera system include pressure depth, altitude, light attenuation, temperature, salinity, 35-mm photographs, and real-time black and white video. The camera was flown by monitoring the real-time video, the altimeter, and an obstacle avoidance sonar. Because the repetition rate of the still camera can be adjusted from the surface, the timing interval of the photographs varies from tow to tow, depending on the complexity of the geology, tow speed, etc. The 35-mm photographs are analyzed according to the general scheme described by Fox et al. [1988] and Embley et al. [1990a]. These merged data can then be plotted as map views or cross sections for analysis. Most of the towed-camera data collected for this study were used long-baseline acoustic transponder navigation. The crossing errors are estimated at about 1020 m for data collected during a given field season.
Alvin Submersible Dives
Forty-two Alvin dives have been made along the Cleft segment between 44°54'N and 45°10'N during three expeditions in 1988, 1990, and 1991. About half of the dives concentrated on vent fluid sampling and conducting experiments on the dynamics of buoyant hydrothermal plumes and did not provide extensive geologic observations. The 1988 dive series was severely hampered by the absence of a gyro compass and poor navigation. An in-sub navigation system became operational about halfway through the 1990 dive series and on most of these dives, the relative position of the submersible is known to less than 5 m. The Alvin data were processed in a similar manner to the towed-camera data.
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