The time-lapse bottom photographs were obtained by means of a Benthos 35-mm camera mounted on a tripod frame that rests on the sea floor (figs. 2 and 3). The camera (fig. 4) was mounted about 1.5 m above the sea floor and aimed downwards; a strobe (fig. 5) illuminated the sea floor from one side of the photograph. The field of view of the camera is 54 degrees, resulting in a photograph area on the sea floor measuring approximately 1.5 m x 1 m. A compass and vane (fig. 6) mounted in the field of view of the camera show the instantaneous direction of current flow and the scale and orientation of the photographs.
The photographs were taken on Kodak 35-mm Ektachrome Professional Film (E200, a daylight-balanced 200-speed color transparency film, 100-ft roll, about 700 photographs/roll). The Benthos camera places the photographs in a nonstandard format along the long axis of the film. This allows a photograph of the sea floor larger than would be possible if the photograph were placed across the film in a standard 35-mm format. The film is advanced using an O-ring drive; the loose drive and drive-motor inertia cause the distance between frames to vary slightly with each photograph. The unevenly spaced photographs required alignment to view them as a time-series movie without jitter.
The camera and strobe were controlled by a timer set to obtain a photograph every 4 or 6 hours, depending on the deployment. The time of each photograph differs from a uniform spacing by a few minutes because of drift in this analog controller. A digital light emitting diode (LED) clock, separate from the controller, places the hour, minute, second, and day on each photograph. The day counter on the LED clock counts to 31 and then resets to 1. The time on the LED typically differed from true time by only a few minutes at the end of the deployment and thus is assumed to provide a reasonably accurate time for each photograph.
Instrumentation mounted on the same tripod frame measured current speed and direction, temperature, light transmission, conductivity, and pressure every 3.75 minutes (Butman and others, 2004d). Light transmission is a measure of water clarity and was converted to beam attenuation (attenuation = - 4ln(percent transmission over 0.25 m)). Current was measured, typically at 2 Hz, and vector-averaged to provide a mean current every 3.75 minutes. Pressure also was measured at 2 Hz; the standard deviation of pressure (called PSDEV) was computed every 3.75 minutes as a measure of wave-induced pressure fluctuations at the sea floor. Plots of these data are included in the .avi movies.
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Instrumented tripod on deck of USCG Cutter Marcus Hanna.
Instrumented tripod being deployed from USCG Cutter Marcus Hanna.
Benthos 35-mm underwater camera mounted on tripod frame.
Benthos strobe mounted on tripod frame. The reflector is painted with white antifouling paint to discourage biological growth.
Compass and vane assembly mounted in the field of view of the camera.