Time series photographs of the seafloor in



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TIME SERIES PHOTOGRAPHS OF THE SEAFLOOR IN

WESTERN MASSACHUSETTS BAY


Bradford Butman1, John Evans1, William N. Lange2, and Chris Polloni1
1U.S. Geologic Survey, Woods Hole, Massachusetts

2Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

Introduction


Time series photographs of the seafloor were obtained at a site in western

Massachusetts Bay (420 22.6' N 700 47.1' W) between June 1, 1996 and October 2, 1996 at water depth of 31 m. The long term monitoring station is just to the south of the location of the new ocean outfall that will begin discharging treated effluent in 1999 (see plan view map and



perspective map). The photographs are part of a suite of observations obtained by the U.S. Geological Survey and others to monitor sediment resuspension and transport in Massachusetts Bay. The observations are made in cooperation with the Massachusetts Water Resources Authority.
Several hundred similar sequences of bottom photographs have been obtained at a wide variety of locations on the U.S. continental margin since 1975. This is a pilot project to develop a mechanism to digitally view, archive, and distribute the time series bottom photographs.
Bottom Tripod System
The bottom photographs shown in this time-series movie were obtained by means of a 35 mm Benthos camera mounted approximately 2 m above bottom on a tripod frame (photo). The pictures were obtained every 4 hours. The field of view is approximately 1 m by 1.5 m. True north is to the top of the screen. A compass and vane mounted on a swivel, showing the orientation of the tripod and instantaneous direction of flow, are partially visible in the photos (the vane is often out of the field of view, and the glare of the strobe on the glass dome housing the compass limits the time that the magnetic north arrow is visible).
In addition to bottom photographs, instruments on the tripod measured current, temperature, light transmission, pressure, and conductivity. These provide direct measurements of the processes causing sediment resuspension and transport. The tripod also supports a sediment trap that collects sediments suspended in the water column.
Image processing
The Benthos camera places the images on a 100-foot roll of 35 mm film in a non-standard format. The images are placed along the long-axis of the film (this allows a larger image size of the bottom 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 results in the distance between frames varying slightly with each image. A LED data display, showing a record number and the day, hour, minute and second of the photo, is placed on each image.
To register each image, the film was scanned and digitized using a scanner developed by the Woods Hole Oceanographic Institution. The digitizing system adjusts the gain for low light levels, so the light levels between images are not directly comparable. Especially during times of sediment resuspension, the digitized images appear bright, where in fact, the light levels are extremely low due to the high level of suspended sediments.
The digitized images were registered using a MATLAB routine that searched the image for the data chamber (that is placed in the same position on each frame) and adjusted the image position accordingly. This procedure removed most of the ‘jitter’ resulting from the varying distance between frames when the images are shown in a time-series mode.
Display
The movie displays a cropped version of the bottom images (the data chamber time and date are not shown, as well as the perimeter of the image which was underexposed). Along the bottom of the photographs is additional data collected by sensors mounted on the bottom tripod. Beam attenuation (show in yellow) is a measure of suspended sediment concentration and the standard deviation of the bottom pressure (shown in green) is a measure of the currents caused by surface waves. The arrow on the left is the low passed current. This arrow points in the direction of the net flow and the direction of transport of water and sediment when the tidal currents are averaged out.
Highlights of bottom processes
A major process illustrated in this movie is the resuspension of sediments caused by waves. Note the episodic increases in suspended material, which obscures the bottom from view, which occur during times of large pressure standard deviation. The seafloor in this region of Massachusetts Bay is gravel and coarse sand. However, there are patches of fine grained sediment nearby, and a fine veneer of fine sediments accumulates on the sea floor during non storm times. These fine sediments are resuspended by the strong oscillatory currents associated with surface waves. In some of the events, a small rock or boulder shifts. Other observations and modeling suggest that fine sediment resuspend during storms in this region of western Massachusetts Bay is transported to the south toward Cape Cod Bay, and offshore into Stellwagen Basin. These regions are the long term depositional sites in the Massachusetts Bay region.
For more information:
Butman, B. and Bothner, M.H., 1998, Predicting the long term fate of sediments and contaminants in Massachusetts Bay, USGS Fact Sheet, 172 97.
Bothner, M.H., Bucholtz ten-Brink, M., Butman, B., Knebel, H.J., Manheim, F.T., and Signell, R.P., 1994, Circulation and contamination transport in Massachusetts Coastal waters: A summary of achievements and future plans, U.S. Geological Survey Open-file Report, 94-649, 43 p.
Geyer, W., Gardner, G., Brown, W., Irish, J., Butman, B., Loder, T., and Signell, R., 1992, Physical Oceanographic Investigation of Massachusetts and Cape Cod Bays, Technical Report MBP-92-03, Massachusetts Bays Program, U.S. EPA Region I/Massachusetts Coastal Zone Management Office, Boston, Massachusetts, 497 pp.

Figure caption:


The bottom tripod system being recovered aboard the U.S. Coast Guard Cutter WHITEHEATH. The camera points directly downward and a strobe light provides illumination for each photograph. The other cases on the tripod contain electronics and batteries for the current, pressure, light transmission and conductivity sensors. The tripod is recovered using a line that is released from a ‘rope cannister’ and pulled to the surface by a small float.


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