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Woods Hole Coastal and Marine Science Center

The Massachusetts Bay Internal Wave Experiment, August 1998: Data Report

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Field Experiment

The MBIWE98 was carried out from August 3 to September 2, 1998 and consisted of a moored array experiment and hydrographic surveys. Sediment sampling and bottom photography was carried out in July 1999.

Moored Array

The moored array consisted of instrumented moorings deployed at 3 stations spaced across Massachusetts Bay aligned along 55-245° T (fig. 1, fig. 2, table 1). Site A, located about 5 km from the western edge of Stellwagen Bank at 95 m water depth, was placed to provide a description of the waves near the generation region. Site A was instrumented with temperature and conductivity sensors at 10, 20, 30, 40 and 50 m deployed on a surface mooring. Site B was located approximately 12 km from the western edge of Stellwagen Bank in the center of Stellwagen Basin at 85 m water depth. Site B was heavily instrumented from 10 m from the surface to 1 m above bottom with an array of temperature, conductivity, current, light transmission and pressure instruments deployed on a surface mooring, 2 subsurface moorings, and a bottom tripod (fig. 3). One of the subsurface moorings at the central site (B) was a horizontal mooring to provide closely spaced measurements in both the horizontal and vertical. Site C was located at 50 m water depth, just offshore of the transition from fine to coarse-grained sediments. Measurements at Site C included temperature, conductivity, light transmission, pressure, and current from sensors deployed on a surface and subsurface mooring and on a bottom tripod (fig. 4). Time-series photographs of the sea floor were also made at Site C from a camera mounted on the bottom tripod. In order to resolve the high-frequency motions caused by the internal waves, instruments in the array sampled every 15 - 120 s, depending on instrument type (table 1 (in .xls format) and table 2).

The horizontal mooring (fig. 5, fig. 6) deployed at site B is a new technology, under development by WHOI in cooperation with ONR, designed to provide closely spaced (order 20 m) measurements in both the horizontal and vertical in the coastal ocean (Grosenbaugh and others, 2002). The mooring consisted of a 160 m long horizontal wire tensioned between two 48" subsurface floats at 20 m below the surface. The floats are kept in position by anchors attached to each float and set at approximately a 45° angle. Along the horizontal wire, 5 vertical wires spaced 30 m apart supported temperature and conductivity sensors 5 m apart in the vertical from 20 to 45 m water depth. In addition, three current meters were mounted on the horizontal wire, one at the center, and one at each end 55 m from the central meter.

Moorings were also in place during MBIWE at two additional sites in western Massachusetts Bay (fig. 1) as part of a long-term monitoring program carried out by the USGS in cooperation with the Massachusetts Water Resources Authority (Butman and others, 2004a). Instrumentation at site LT-A included an ADCP, subsurface mooring and a bottom tripod with current, temperature, conductivity, pressure, and light transmission. Instrumentation at site LT-B included an ADCP. The sampling rate of the near-bottom instruments at LT-A was 3.75 minutes; the ADCP's at LT-A and LT-B sampled for 5 minutes every 15 minutes.

Hydrography

Three hydrographic cruises were conducted during the moored array experiment to map the temperature, salinity, and suspended-sediment fields across Massachusetts Bay. Sections of temperature and salinity were made along a transect through the moorings (fig. 7) on August 6 and August 20, 1998 on the FV Christopher Andrew, and sections of temperature, salinity, and light transmission were made between August 31 and Sept. 2, 1998, during mooring recovery, on the RV Argo Maine. Temperature and salinity on the RV Argo Maine were measured with a SeaBird 9/11CTD and light transmission with a 0.25 m path length Chelsea AlphaTrackA transmissometer.

Site Characterization

The sea floor of Massachusetts Bay has been mapped using a multibeam sea floor mapping system (fig. 8), providing detailed bathymetry and backscatter intensity in the vicinity of the moorings (Valentine and others 2003a, b, c, Butman and others, 2004b).

Bottom photographs, videos and sediment samples of the sea floor were obtained along the MBIWE transect in July 1999 on the FV Isabel S. These observations were made using the SeaBed Observation and Sampling System (SEABOSS) (Valentine and others (2000), Blackwood and Parolski (2002)). SEABOSS is equipped with two video cameras, a still camera, a pressure-depth sensor, and a modified Van Veen sediment grab sampler. At each station, the SEABOSS was lowered to a target elevation about 76 centimeters (30 inches) above the sea floor and the ship allowed to slowly drift, typically for a few hundred meters. As the ship drifted, typically at a knot or less, bottom video was recorded continuously and individual photographs of the sea floor were triggered remotely based on the video available in real time at the surface. Typically 5-20 still photographs were obtained at each station. At 76 centimeters above the bottom, the still photographs show a 76 x 51 centimeter area of the sea floor. At most stations, a single sample of the sediment was obtained using the Van Veen sampler, typically at the end of the transect. At some locations, cobbles and boulders on the sea floor made sampling impossible. The upper 2 cm of the sediment were skimmed from the grab sample and analyzed for texture using the methods outlined in Poppe and others (2000). Photographs from these and other stations in western Massachusetts Bay may be found in Gutierrez and others (2001).

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The
following
figures are
in PDF
or Excel
format.

Figure 1, backscatter intensity and shaded relief topography of regaion east of Boston.
Figure 1.


Figure 2, section along moored array transect showing distribution of instruments deployed at sites A,B,C,D.
Figure 2.


Instrument Table
Table 1.


Sampling Table
Table 2.

Figure 3, USGS bottom tripod instrument system.
Figure 3.


Figure 4, small tripod frame used to make near-bottom measurements.
Figure 4.


Figure 5, illustration of WHOI horizontal moored array.
Figure 5.


Figure 6, engineering drawing of WHOI horizontal moored array.
Figure 6.


Figure 7, target location for CTD.
Figure 7.


Figure 8, Location of sediment grab samples.
Figure 8.


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