2.0 METHODS 



2.1 Navigation and Bathymetry 



The SAIC Integrated Navigation and Data Acquisition System (INDAS) provided the 

 precise navigation required for all field operations. DAMOS Contribution No. 48 (SAIC 

 1985) includes a complete description of this system. Shore stations for the 1992 field 

 operations were Marblehead Neck Light (42° 30.320' N and 70° 50.051' W) in Marblehead, 

 and Eastern Point Light (42° 34.809' N and 70° 39.899' W) in Gloucester, Massachusetts. 

 Previous MBDS surveys were also conducted with these shore stations. Repeated use of 

 these stations allows accurate comparisons of past and present surveys. 



The stated objective of the 1992 bathymetric survey was to map that portion of the 

 dredged material mound that was greater than 20 cm thick. Forty-nine lanes were run east to 

 west at 25 m spacing on 31 March 1992 to cover a 1200 x 1200 m grid centered at the 

 "MDA" buoy (42° 25.039' N and 70° 34.505' W). This same grid has been used in 

 bathymetric surveys since January 1987. 



An Odom DF3200 Echotrac® Survey Fathometer with a narrow-beam 200 kHz 

 transducer recorded depth. The fathometer recorded depth to a resolution of 3 cm (0.1 ft.). 

 However, the acoustic records could reliably detect changes in depth on the order of 20 cm 

 due to the accumulation of errors introduced by the positioning system, tidal corrections, the 

 calibration of the fathometer (speed of sound through the water column), the slope of the 

 bottom, and the vertical motion of the vessel. Water temperature and salinity data measured 

 by a Seabird model 19-01 CTD were used to calculate the speed of sound. 



During the 1990 bathymetric survey, the survey team had used a different Odom 

 DF3200 Echotrac® Survey Fathometer to record depth. This particular fathometer was 

 rented to temporarily replace identical equipment normally used because of a malfunction in 

 the in-house fathometer. The raw data for this survey were noted to contain a higher 

 variance which was most likely due to lower maintenance standards on the rental equipment. 

 The result is an apparent higher level of "noise" in the contoured bathymetric chart in 

 comparison to the 1992 survey. It is important to note that this variance does not obscure 

 the general correspondence of contours between the two surveys. 



2.2 REMOTS® Sediment-Profile Photography 



A REMOTS® survey was used to map the distribution of thin (1 to 20 cm) layers of 

 recently disposed dredged material not detectable widi bathymetry. The REMOTS® survey, 

 performed on 1, 2, and 4 April 1992, generated triplicate photographs for each of the 43 

 disposal site stations surrounding the "MDA" buoy and the 13 stations at each of the three 

 reference areas (Figure 2-1). Analysis of the benthic community in the REMOTS® 



Monitoring Cruise at the Massachusetts Bay Disposal Site. March 31 - April 4. 1992 



