13 



Weeks Marine. Water depths are indicated with specific contour intervals noted on the 

 figures. Both contour plots and three-dimensional relief plots were produced using Golden 

 Software's Surfer® program; a vertical exaggeration has been added to the 3D plots shaded 

 relief in order to highlight small topographic gradients. 



2.3.2 Subbottom Data Collection and Analysis 



High resolution subbottom profile data were acquired with an Edgetech X-Star"^*^ 

 Model 216S Full Spectrum Digital Subbottom Profiler. Subbottom profile data were 

 collected simultaneously with bathymetry, and therefore along the same survey lanes 

 (Section 2.3.1). Subbottom seismic profiling is a standard technique for determining the 

 presence of sediment layers below the sediment/water interface. The X-star system emits a 

 swept-frequency pulse; the frequency of the transmitted pulse changes linearly with time, 

 and is therefore called a chirp system. The depth of penetration and the degree of 

 resolution is dependent on the frequency and pulse width of the seismic signal, and the 

 characteristics of the penetrated material. 



The narrow beam (13°) transducers of the X-Star system, mounted in a towfish, 

 were lowered using the winch aboard the survey vessel Cyprinodon, and trailed the vessel 

 by approximately 15 m. The X-Star system generated a frequency-modulated pulse that 

 was swept over an acoustic range of 2 to 10 kHz during the subbottom survey. The pulse 

 rate was set to 6 pulses per second for optimum performance of the output devices. At 6 

 pulses per second, traveling at an average vessel speed of 4-5 knots, a subbottom 

 measurement was acquired every 34-43 cm along the vessel track. The remm signals were 

 transmitted via a data cable through an analog to digital (A/D) signal converter to an on- 

 board Sun Sparc II Workstation for data display and archive. Data were stored on Exabyte 

 tapes, and continuous profile data were printed on an Alden thermal printer. 



Penetration of sound in sediment is both a function of system frequency and the 

 impedance contrast between the water column and sediment. Acoustic impedance, the 

 product of velocity and density of sound in a layer, is also affected by differences in 

 surface roughness, porosity, and grain size, among other factors (Hamilton 1970; LeBlanc 

 et al. 1992). In general, sound penetrates further into fine-grained sediment because the 

 impedance of high-water content silt and clay is closer to that of the water column. The 

 ability to detect subbottom layers is similarly dependent on the acoustic impedance contrast 

 between sediment layers. Subbottom has been used to accurately map the lateral and 

 vertical coverage of a sand cap over dredged material because of the contrast between the 

 sand cap and underlying fine-grained dredged material (e.g., Murray et al. 1994a). 



Subbottom layers were not digitized due to the difficulty in identifying continuous 

 reflectors below the surface of the dredged material. The thermal paper printouts were 

 scanned and several representative sections are included in this report. Although depths 

 are shown in the figures, these are not reliable for estimating actual layer thicknesses or 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



