This report presents time-series photographs of the sea floor obtained from an instrumented tripod deployed at Site A in western Massachusetts Bay (42° 22.6' N., 70° 47.0' W., 30 m water depth, figure 1) from June 1997 through June 1998. Site A is approximately 1 km south of an ocean outfall that began discharging treated sewage effluent from the Boston metropolitan area into Massachusetts Bay in September 2000. Time-series photographs and oceanographic observations were initiated at Site A in December 1989 and are anticipated to continue to September 2005. This is the first in a series of reports planned to summarize and distribute these images in digital form. The objective of these reports is to enable easy and rapid viewing of the photographs and to provide a medium-resolution digital archive. The images, obtained every 4 hours, are presented as a movie (in .avi format, which may be viewed using an image viewer such as QuickTime or Windows Media Player) and as individual images (.tif format). The images provide time-series observations of changes of the sea floor and near-bottom water properties.
The photographs obtained at Site A are part of a long-term study to understand the transport and long-term fate of sediments and associated contaminants in the Massachusetts bays (see the Web site Boston Sewage Outfall: The Fate of Sediments and Contaminants in Massachusetts Bay, http://woodshole.er.usgs.gov/project-pages/bostonharbor/). This long-term study is carried out by the U.S. Geological Survey (USGS) in partnership with the Massachusetts Water Resources Authority (MWRA)( http://www.mwra.state.ma.us/) and the U.S. Coast Guard (USCG) (http://www.uscg.mil). Long-term oceanographic observations at Site A were obtained to document seasonal and inter-annual changes in currents, hydrography, and suspended-matter concentration, and the importance of infrequent catastrophic events, such as major storms or hurricanes, in sediment resuspension and transport. (See Butman and others (2002) for a description of the oceanographic measurements at Site A and Butman and Bothner (1997) for discussion of sediment transport in Massachusetts Bay.)
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 (figure 2 and figure 3). The camera (figure 4) was mounted about 1.5 m above the sea floor and aimed downwards; a strobe (figure 5) illuminated the sea floor from one side of the image. The field of view of the camera is 54 degrees, resulting in an image area measuring approximately 1.5 m x 1 m. A compass and vane (figure 6) mounted in the field of view of the camera show direction of current flow, scale, and orient the images to north. Instrumentation mounted on the same tripod frame measured current speed and direction, temperature, light transmission, conductivity, and pressure every 3.75 minutes. (See Butman and others, 2002 for a description of these instrument systems.)
The images were taken on Kodak Ektachrome Professional Film (E200, a daylight-balanced 200-speed color transparency film) (100 foot roll, about 700 images/roll). The Benthos camera places the images in a nonstandard format along the long axis of the film. This allows a larger image size of the sea floor than if the image 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 result in the distance between frames varying slightly with each image.
The camera and strobe were controlled by a timer set to obtain an image every 4 hours. The time of each photograph differs from a uniform spacing by a few minutes because of drift in this analog controller. A digital LED clock, separate from the controller, places the hour, minute, second, and day on each image. The day counter counts to 31 and then rolls back to 1.
Instrumented tripods are deployed and recovered at Site A three times each year, typically in February, May-June, and September, as part of the USGS study. This report presents images from three deployments: deployment 501 beginning in June 1997, deployment 507 beginning in September 1997, and deployment 516 beginning in February 1998 (501, 507 and 516 are USGS mooring identification numbers). The time-series photographs in deployment 507 terminated in November 1997 due to an instrument failure. Each tripod was located 100-150 m southwest of the USCG B Buoy (National Ocean Service, 1997) on the southern flank of a ridge in water about 30 m deep (figure 7). This location was selected for the long-term observations because the USCG Buoy marked the site and provided some protection from human disturbance.
Time-lapse movies were created from the individual bottom photographs. Each image on the 35 mm film was digitized as a .tiff image at a resolution of 4238 by 2626 pixels by Amaranth Photo Imaging (www.amaranthimaging.com). The exposure was kept constant for all frames, so that changes in light intensity reflect changes in water clarity, instrument fouling, or ambient light. The images were reduced to 600 by 372 pixels using PolyView (www.polybytes.com); this image size was chosen to balance resolution and publication size. The camera controller was set to obtain an image every 4 hours. However, this controller (separate from the digital LED clock that placed a time stamp on the images) drifts by a few minutes over the course of the deployment. For the purposes of the movie, the images were distributed at an even interval between the start and stop time of the photographic record, as determined from the LED clock. The camera controller occasionally missed an image or exposed multiple frames; missed images were filled with blanks and multiple images deleted. MATLAB software (www.mathworks.com) was used to align the images, make the data plots, and create the .avi movie.
Description of Movie Frames
Each movie frame includes an image of the sea floor at the top and shows oceanographic data collected at the same time at the bottom. The movie plays at 3 frames/second or 2 seconds/day. The field of view of the image is approximately 1.5 m wide and 1 m high. The vane on the compass in the images swings with the current and points in the direction of current flow. (Sometimes the vane swivel gets stuck preventing the vane from aligning with the current.) The triangular black arrow on the compass points toward magnetic north (16 degrees west of true north at Site A). The white arrow in the upper left of the frame points toward true north. The file name of the image is in the upper left corner of each frame. (This number provides a key to the individual .tif images; see 'Images' in the table below.) The red digits in the lower left corner of the image show time (in Eastern Standard Time): HR.MM (hour and minute) on the upper line and SS.DD (second and day) on the second line. The day (DD) counter rolls over every 31 days and thus does not indicate true day of month for the entire deployment; the correct day of the month is below the data panel (see below). The third line of red digits is a record identifier (NN, the last two digits of the USGS mooring number).
Below the image to the right is a data panel that shows a plot of current speed (in cm/s at 1 m above bottom (mab) (in red); beam attenuation (in m-1), a measure of water clarity, at about 2 mab (in blue); standard deviation of bottom pressure (in mb), a measure of wave intensity, at about 2 mab (in black); and water temperature (in degrees C), at about 2 mab) (in green) obtained every 3.75 minutes. The values of current speed, beam attenuation, standard deviation of pressure, and bottom temperature at the time of the photograph are also listed in the data panel. The vertical black line in the center of the data panel is the time of the photograph. The plot shows 4 days of data, 2 before and 2 after the image displayed was obtained. Below the image to the left is a vector plot showing current speed (length of line) and direction toward which the current flows (true north is up in this display).
The time below the data panel is the time for each image (in Greenwich Mean Time), computed by evenly spacing the images over the deployment. The best estimate of the time of the image is obtained from the hour and minute recorded by the digital clock on the image (in EST), and the day from the data panel.
Play Movies and View Images
The movies may be viewed using an image viewer such as QuickTime or Windows Media Player. Click on Movie in the table below to open the movie, or navigate to the file on the DVD (in the subdirectory IMAGES in each individual tripod directory) and open with an image viewer. Click on Images to open thumbnails of individual images; click on an image to view the image, in .tif format, at a resolution of 600x372 pixels. Each set of images contains several hundred images and may take a few minutes to load.
Highlights of Bottom Processes
The time-lapse movies show changes in the sea floor and bottom water characteristics. A major process illustrated in these photographs is the resuspension of sediments caused by currents associated with waves. Increases in beam attenuation (decreased water clarity) usually coincide with periods of increased wave intensity (caused by storms) and visible cloudiness in the photographs. The sea floor in this region of Massachusetts Bay is mostly cobbles, pebbles, and coarse sand. However, there are patches of fine-grained sediment to the west of this site (figure 7), and a thin veneer of fine sediments is likely to accumulate at the tripod site during times when it is not stormy. The fine sediments are resuspended by the strong oscillatory currents associated with surface waves. In some of the events, a pebble or cobble shifts position on the sea floor. Other observations and modeling suggest that fine sediment resuspended during storms in this region of western Massachusetts Bay is transported to the southeast toward Cape Cod Bay and offshore into Stellwagen Basin (Butman and Bothner, 1997). These basins are long-term depositional sites in the Massachusetts Bay region. Note that objects, such as fish, close to the camera lens, appear larger than they are in relation to features on the sea bottom.
The time series data of current, temperature, light transmission, beam attenuation, and pressure shown in the movies obtained at site A are available in Butman and others (2002).
MATLAB software (www.mathworks.com) was used to align the images, make the data plots, and create the .avi movie and thumbnail Web pages. M-files may either be opened within a text viewer or within the MATLAB editor. To download the m-files click on the links below.
W. Strahle and M. Martini prepared the instruments for deployment. F. Lightsom processed the time-series data. We thank the officers and crew of the USCG Cutter White Heath and USCG Cutter Marcus Hannah for deployment and recovery of instruments at sea. This work was supported by the U.S. Geological Survey Coastal and Marine Geology Program and by the Massachusetts Water Resources Authority.
Butman, Bradford. and Bothner, M.H., 1997, Predicting the long-term fate of sediments and contaminants in Massachusetts Bay: U.S. Geological Survey Fact Sheet 172-97. (Also available online at http://pubs.usgs.gov/fs/fs172-97/.) (Accessed May 18, 2004.)
Butman, Bradford, Bothner, M.H., Lightsom, F.L., Gutierrez, B.T., Alexander, P.S., Martini, M.A., and Strahle, W.S., 2002, Long-term observations in western Massachusetts Bay offshore of Boston, Massachusetts; Data report for 1989 - 2000: U.S. Geological Survey Digital Data Series DDS-74, 1 DVD-ROM. (Also available online at http://pubs.usgs.gov/dds/dds74/.) (Accessed May 18, 2004.)
Butman, Bradford, Hayes, Laura, Danforth, W.W., and Valentine, P.C., 2003, Backscatter intensity, shaded relief, and sea floor topography of Quadrangle 2 in western Massachusetts Bay offshore of Boston, Massachusetts: U.S. Geological Survey Geologic Investigations Series Map I-2732-C, scale 1:25,000. (Also available online at http://pubs.usgs.gov/imap/i-2732c/.) (Accessed May 18, 2004.)
National Ocean Service, 1997, Massachusetts Bay: National Oceanic and Atmospheric Administration, National Ocean Service, Chart 13267, scale 1:80,000.
U.S. Geological Survey, 2004, Boston sewage outfall: The fate of sediments and contaminants in Massachusetts Bay: U.S. Geological Survey Web site at http://woodshole.er.usgs.gov/project-pages/bostonharbor/. (Accessed May 18, 2004.)
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The following figures are in PDF format.