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. A complete description of this system 

 can be found in DAMOS Contribution No. 48 (SAIC 1985). Shore stations for the 1990 

 field operations have been used in previous MBDS surveys and were established at 

 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 (Figure 2-1). 

 Repeated use of these stations allows accurate comparisons of past and present surveys. 



An Odom DF3200 Echotrac® Survey Recorder with a narrow-beam 208 kHz 

 transducer recorded depth. This particular fathometer was rented to temporarily replace 

 identical equipment used in 1988 because of a malfunction in the in-house fathometer. 

 Analysis of the data and comparison with the 1988 results indicated that the gridded depths 

 were reliable, in general, but the raw data contained a higher variance. This higher variance 

 was due most likely 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 

 1988 survey. It is important to note that this variance does not obscure the general 

 correspondence of contours between the two surveys. 



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. The speed of sound is determined from the water 

 temperature and salinity data measured by an Applied Microsystems CTD probe. However, 

 for this survey the correction factor was calculated based on historic depth/temperature 

 profiles obtained for August 1985 (SAIC 1987a) due to a malfunction of the CTD probe. 

 Depth/temperature profiles for August 1985 were obtained at the "A" buoy, 42° 25.671' N, 

 70° 35.004' W. Any discrepancy with the actual speed of sound during the bathymetric 

 analysis for 1990 was resolved when the 1990 survey was corrected to areas in the 1988 

 survey unaffected by disposal (an accepted method for normalizing to a benchmark survey). 



The bathymetric survey conducted on 13 and 14 August 1990 encompassed a 1200 x 

 1200 m grid centered around the "MDA" buoy at coordinates 42° 25.086' N and 

 70° 34.457' W. Forty-nine lanes were run east to west at 25 m spacing. The bathymetric 

 survey on 4 November 1988 utilized this same grid. This configuration provided adequate 

 coverage to assess the distribution of dredged material at the site. The stated objective of the 

 1990 survey was to map areas of the dredged material mound exceeding 1 m in thickness. 

 This objective assumed a substantial decrease in bathymetric measurements in deeper water 



Monitoring Cruise at the Massachusetts Bay Disposal Site, August 1990 



