3. TURBULENT AND VORTEX WAKES 
A very common, in fact possibly the most frequently observed type of 
ship wake feature in SAR images is a dark, narrow line along the ship 
track. This feature occurs when the surface is sufficiently rough to yield 
a measurable background return, which is suppressed in the region near the 
ship track. The suppression is frequently more pronounced at L-band than 
at X-band, and is sometimes accompanied by a bright line on one or both 
edges of the dark area. An example is shown in Figure 8. This data was 
collected during pass 4 of the DREP 8 mission. Occasionally, the dark line 
is observed to split into two lines. 
The features described in the previous paragraph have been termed tur- 
bulent wakes (Shuchman et al., 1983), although it is not clear that turbu- 
lence alone is responsible for the appearance of these features. The 
effects of turbulent currents on surface waves are not well understood and 
the persistence of the phenomenon is somewhat problematic. Recently it has 
been suggested that horizontal vortices produced at the edges of the ship's 
hull, as well as a net rearward velocity within the turbulent wake, may be 
partially responsible for the suppression of surface waves near the ship 
track and an enhancement of the waves near the edges of the smoothed area 
(Swanson, 1984). These characteristics are shown graphically in Figure 9. 
The vortex model developed by Swanson (1984) was used to predict the 
surface current pattern associated with ship-generated vortices along the 
line A-A' shown on Figure 8. These currents were used to predict spectral 
perturbations of L-band Bragg waves using the method developed by Lyzenga 
(1984). Spectral perturbations of Bragg waves are directly proportional to 
radar backscatter changes and can be used to simulate the expected radar 
signature of ship-generated vortices. A comparison between the predicted 
and actual radar intensities along the line A-A' are presented in Figure 
10. The wind conditions assumed for the predicted values are shown graphi- 
cally on Figure 10. The vertical and horizontal positions of the two plots 
were adjusted to achieve the best agreement based on visual observations. 
The close agreement between the predicted and actual data is obvious. It 
should be emphasized that these results are preliminary in nature. We are 
presently analyzing this data using actual current measurements made by a 
research vessel which transected the wake of the Quapaw during this data 
collection. This will allow us to check the accuracy of the Swanson vortex 
model for predicting surface currents. A more complete discussion of the 
above results is given in Lyden, et al. (1985c). 
A coupling between the mechanisms responsible for the darkened area 
near the ship track and the bright, narrow wakes discussed in the previous 
section is suggested by the proximity of these features. Possibly, a 
resonant-type interaction occurs for surface waves whose group velocity is 
equal to the spreading velocity of the turbulent region (or vortex pair). 
Such an interaction might increase the amplitude of these waves and prolong 
their lifetime. It has also been suggested (Swanson, 1984) that vortex 
motions can generate internal waves under suitable conditions. The effects 
of these internal waves are discussed in the following section. 
4, SHIP-GENERATED INTERNAL WAVES 
When a strong, shallow thermocline (or halocline) is present, internal 
waves can be produced directly by the hull displacement of a ship or by 
vortices generated at the hull edges. Several documented examples of 
419