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Figure 5.9. Example of SPI images from the Hudson River. Upper, left to right: oyster shells and shell hash, worm burrows and a thin oxidized layer, a thicker and pitted oxidized layer. Lower, left to right: wood and detritus, coal on a sandy surface, and a brick (from Iocco et al., 2000).

(5) Shallow Water Geophysics - Radar and Chirp: Significant portions of the Hudson River are shallower (<3 m) than can be mapped with the large boats used for the majority of this program. In April 1999, as part of this program, radar and chirp sonar sub-bottom data were collected in collaboration with the USGS Water Resources Division from afreshwatermarshinArea3 (Haenietal., 1999). An Edge Tech Xstar topside system with the SB216-S fish was used to acquire the Chirp sonar data. For the radar acquisition a Mala Geoscience radar system comprised of a pair of 200 MHz unshielded antennas was used. Radar data were acquired with a sampling frequency of 2,012 MHz. Other radar acquisition parameters were adjusted accordingly with water depth, and varied between 8 and 16 fold stack, and 600-1,500 samples. Radar triggering was done on time, and varied between 5 and 10 traces s-1 (roughly corresponding to a spatial distance of 10-20 cm between traces). The goal of this pilot effort was to assess the imaging capability of radar versus sonar methods within shallow freshwater settings. Results of this study indicate that radar methods provide better penetration and resolution

Figure 5.10. Example of ground penetrating radar (GPR) record from a shallow-water section of the freshwater Hudson River. Upper: original record. Lower: labeled record. This record, collected from water less than two feet (0.6 m) deep, shows the location and shape of an earlier channel about 5 feet (1.5 m) deep as well as layering within the sediment sequence. The record is 1,500 feet (460 m) long.

1,500 ft

Figure 5.10. Example of ground penetrating radar (GPR) record from a shallow-water section of the freshwater Hudson River. Upper: original record. Lower: labeled record. This record, collected from water less than two feet (0.6 m) deep, shows the location and shape of an earlier channel about 5 feet (1.5 m) deep as well as layering within the sediment sequence. The record is 1,500 feet (460 m) long.

than sonar systems in the freshwater portion of the river for water depths shallower than 5 m (Fig. 5.10). The majority of the radar data showed significant sub-bottom structure, while only 35 percent of the coincident Chirp data provided any sub-bottom penetration. This difference in penetration arises because sound penetration is hindered by both gas and sand in the sediment, whereas neither of these properties affect radar penetration.

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