32 



the accumulation of errors introduced by the positioning system, vertical motion of the 

 survey vessel, changes in sound velocity through the water column, the slope of the 

 bottom, and tidal corrections. 



Observed tidal data were obtained through the National Oceanographic and 

 Atmospheric Administration (NOAA), Ocean and Lake Levels Division's (OLLD) 

 National Water Level Observation Network. This network is composed of 181 water level 

 stations that are located throughout the Great Lakes and coastal regions of United States 

 interest. These stations are equipped with the Next Generation Water Level Measurement 

 System tide gauges and satellite transmitters that have collected and transmitted tide data to 

 the central NOAA facility every six minutes, since 1 January 1994. 



Observed tide data are available 1 to 6 hours from the time of collection in a station 

 datum or referenced to mean lower low water (MLLW) and based on Coordinated 

 Universal Time (UTC). For the 1995 and 1996 PDS surveys, data from NOAA tide 

 station 8418150 in Casco Bay, Portland, ME, were used for tidal calculations. The NOAA 

 6-minute tide data were downloaded in the MLLW damm, corrected to local time, and 

 tidal differences based on Potts Harbor, South Harpswell Neck, were applied. 



During the bathymetric survey, a Seabird Instruments, Inc. SBE 26-03 Sea Gauge 

 wave and tide recorder was used to collect tidal data on site. The tide gauge, deployed in 

 the survey area, recorded pressure values every six minutes. After conversion, the 

 pressure readings provided a constant record of tidal variations in the survey area. These 

 observed tidal data were later used to compare and verify the corrected NOAA data 

 generated from the Portland station (Figure 3-2). 



A Seabird Instruments, Inc. SEACAT SBE 19-01 Conductivity, TemperaUire, and 

 Depth (CTD) probe was used to obtain sound velocity measurements at the start, midpoint, 

 and end of each survey day. The data collected by the CTD probe were bin- averaged to 1 

 meter depth intervals to account for any pycnoclines, rapid changes in density that create 

 distinct layers within the water column. Sound velocity correction factors were then 

 calculated using the bin- averaged values. 



3.3.2 Bathymetric Data Processing 



The bathymetric data were analyzed using SAIC's Hydrographic Data Analysis 

 System (HDAS), version 1.03. Raw bathymetric data were imported into HDAS, 

 corrected for sound velocity, and standardized to MLLW using the NOAA observed tides. 

 The bathymetric data were then used to construct depth models of the surveyed area. A 

 detailed discussion of the bathymetric analysis technique is provided in the DAMOS 

 Bathymetry and Navigation Reference Report (Murray and Selvitelli 1996). 



The Portland Disposal Site Capping Demonstration Project, 1995-1997 



