z? 



3.4.2 Sediment Acoustic Characterization System, Sediment Grab Sampling 

 Comparison 



As stated earlier, the July 1994 survey at CLIS provided an opportunity to evaluate 

 the use of acoustic remote sensors relative to traditional DAMOS Program monitoring 

 techniques. The Sediment Acoustic Characterization System, developed by SAIC, 

 collected bottom reverberation data over the southern half of the 2553 m x 2225 m 

 bathymetric survey area to map sediment types based on the relative density of the CLIS 

 sediments. Grain size data compiled as part of the sediment chemistry testing was used to 

 ground-truth the SACS returns and assess the effectiveness of this sensor. 



In general, SACS was able to differentiate between "harder" (sand, pebble, and 

 cobble sized grains) and "softer" (silt and clay) sediments on the seafloor. By comparing 

 the SACS remm signal strength over a 1260 m x 1100 m analysis area to point grain size 

 data, relationships between harder or stronger surface reflections and lower fme-grained 

 sediment fractions were observed (Figure 3-35). The three NHAV 93 mound grab 

 sampling stations that fall inside the concentrated analysis area were found to be composed 

 of high silt and clay content sediment (mean 72.9 ±1 %). As expected, the amount of 

 surface reflection was relatively low (86.0 dB to 96.0 dB) due to signal attenuation, or 

 dispersion, in the fmer grained material. The coarser grained material deposited over the 

 surface of the MQR mound appeared to be a better acoustic reflector, providing a stronger 

 signal return to the 24 kHz transducer. Surface reflections of 96.0 dB to 106 dB were 

 detected in close proximity to the MQR mound, correlating well with the percentage of 

 fme-grained material in the supplemental CDM (mean 51.0+18.6%). The variation in 

 fme-grained content of the MQR CDM is reflected in the overall SACS return from 

 surface sediments. However, due to the smoothing of the acoustic data set, a few 

 individual grain size samples did not correlate well with dB values. 



Although the SACS data showed significant agreement with the sediment grain size, 

 the results appeared to be directly affected by the amount of consolidation within the top 6 

 cm of the CLIS bottom. The area of most recent CDM deposition over the southwestern 

 flank of the NHAV 93 mound displayed a significant amount of signal loss (Morris et al. 

 1996). An unconsolidated marine sediment, such as fresh dredged material, typically has a 

 high water content, often approaching 200%. The increased volume of pore water 

 modifies the acoustic characteristics of the sediment deposit to be comparable to the 

 overlying seawater (LeBlanc et al. 1992). The sound wave generated by the 24 kHz SACS 

 transducer tends to pass through the unconsolidated dredged material deposit until it 

 reaches a stronger reflector, increasing signal attenuation. As a result, the sound wave 

 returning to SACS is considerably weaker than when it originated from the low frequency 

 transducer. 



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



