The final example we present is from a survey of several Sherman duplex drive (DD) tanks (Fig. 13a) sunk approximately 4 km off of Omaha Beach. The Sherman DD tanks were modified into amphibious tanks with the addition of twin propellers and a canvas skirt fitted around the hull to provide flotation (Fig.13b). The DD tanks were designed to be part of the first wave of the invasion, maintaining a low profile while "swimming" to shore and then providing covering fire for the first waves of infantry. The seas on 6 June were too rough for the fragile flotation mechanism of the DD tanks and of the first wave of 32 tanks launched approximately 5 km off Omaha Beach, all but 3 sank (Ambrose, 1994). Witnessing this disaster, the skippers of the LCT's carrying other DD tanks brought their tanks directly to the beach.

Figure 13a

Figure 13b

Figure 14a shows a remarkable image of what is unquestionably a Sherman tank (Figure 14b), located in approximately 19 m of water and pointed directly toward the beach. Again, the evidence of scour around the tank is quite clear. While the rendered image (based on data gridded at 25 cm) clearly has the characteristic shape of a Sherman tank, it appears that the gun barrel is missing. Closer examination, however, reveals that this is not the case and points out that in some circumstances, the gridding and rendering (formulation of the 3-D image) process can remove potentially important detail. If instead of looking at the rendered gridded surface, we look at the individual soundings rendered in 3-D, we can clearly see four hits of the sounder on the approx. 90mm-wide barrel of the 75mm gun.

Figure 14a

Figure 14b

Figure 15