1J_ 



2.3 Subbottom Profiling Operations 



An X-Star Model SB-216 Full Spectrum Digital Subbottom Profiler, manufactured 

 by Precision Signal, Inc., was used to acquire high-resolution subbottom profile data at 

 CLIS. Subbottom seismic profiling is a standard technique for determining changes in 

 acoustic impedance below the sediment/water interface. Acoustic impedance is the product 

 of the density of a layer and the speed of sound within that layer. The depth of penetration 

 and degree of resolution are dependent upon signal frequency, pulse width, and the 

 characteristics of the penetrated material. 



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

 body that trails approximately 15 meters behind a survey vessel. During a subbottom 

 survey, the X-Star system generates a frequency-modulated pulse that is swept over an 

 acoustic range from 2 to 10 kHz. The return signals are transmitted via a data cable 

 through an analog to digital (A/D) signal converter to an onboard Sun Sparc II Workstation 

 for data display and archive. The X-Star data acquisition system consists of computer 

 components for automatic data storage, real-time color data display, and hard-copy 

 printouts of profile data. Data were displayed on the screen in real time and ported to an 

 Alden thermal printer for a hard copy record (Figure 2-3). Data were also stored on 

 Exabyte tapes for further processing on shore. 



Following the survey, the subbottom profile data residing on Exabyte tapes were 

 digitized using a C-compiled program to read and analyze X-Star data. The subbottom 

 data were read and displayed on a personal computer (PC) monitor as both a continuous 

 profile, duplicating the shipboard display, and as individual pulses. The sediment/water 

 interface and subbottom layers were digitized manually and stored for further processing. 

 A continuous record of the surface reflection coefficient was also stored and processed. 



For subbottom analyses, each acoustic horizon or layer was digitized while the data 

 were played back on a PC monitor. Only lanes 53 through 74 of the survey were 

 processed, in the area of most recent disposal. The subbottom analyses concentrated on 

 the 2553 m x 525 m and 1600 m x 525 m survey areas over the northern portion of the 

 NHAV 93 mound (Appendix A, Table 1; Figure 2-4). Each acoustic horizon measurement 

 within the digitized layers was stored in a file as a depth from the sediment-water interface 

 and geodetic position. The depths were corrected using 1500 ms"' as a standard sound 

 velocity and were later modified with estimates of actual sound velocities in each layer 

 during postprocessing. 



Monitoring Cruise at the Central Long Island Sound Disposal Site, July 1994 



