water depths from 2,200 to 5,500 feet. Seventy percent of the seafloor 

 is covered by a thin veneer of sand, the remaining 30% by outcrops of 

 basaltic rock. (See Figure 1 for hydrophone configuration.) 



(2) The Azores Fixed Acoustic Range (AFAR) includes three tower 

 structures bearing on the basaltic rock tops of seamounts in the Azores 

 group. The towers are 120 feet high resting on an 8-foot square base 

 plate and held upright by buoyant upper sections. The 8-foot square 

 plate has at each of its corners a vertical spiked post; the set of 

 four posts are capable of some load re-distribution between them. (See 

 Figure 2.) The primary purpose of the spiked posts is to provide re- 

 sistance to lateral loads by engaging the rock surface. The AFAR 

 towers weigh 6 tons submerged; installation water depths are greater 

 than 1000 feet. 



(3) The Naval Underwater Center has established an underwater 

 sound source resting on bare volcanic rock atop a seamount off Southern 

 California. The steel frame, approximately ten feet square and weighing 

 5-7 tons, was placed at a 45-degree angle on the first try. The 

 second bottom contact was near level, and performance has been 

 satisfactory. The frame is supported by four steel legs capped with 

 plates. 



Related Study - Seafloor Rock Microrelief 



The Navy Electronics Laboratory (NEL) analyzed more than 700 NEL 

 seafloor photographs, obtained in a wide variety of environments." 

 The NEL study showed rock outcrops and buried boulders commonly occur- 

 ring on shallow and irregular underwater topographic features such as 

 banks, island shelves, sides of troughs, seamounts, cliffs, ridges, 

 canyons, marine terraces, and shelf breaks. Medium to coarse sands 

 are usually present near the outcrops and show ripple marks, scour, 

 banking, and other signs of water activity. Here the range of 

 "microrelief" is from three centimeters (about one inch) to possibly 

 as great as three meters (about ten feet) , although all the photos of 

 the NEL study showed "microrelief" of less than one meter (about three 

 feet) . "Microrelief" is defined as "the very small surf icial topo- 

 graphic features that are superimposed on relief and are distinguish- 

 able by examination of bottom photographs."" 



In summary then, even though exposed rock and near surface rock 

 (herein defined as less than 25 feet of sediment cover) probably con- 

 stitute only a very small percentage of the seafloor surface, that 

 rock is generally located in strategic areas such that the Navy has 

 already found it necessary to found seafloor structures on rock. 

 Studies have examined seafloor rock microrelief and geological signif- 

 icance; but, as yet, these studies have not been applied to the problem 

 of rock foundation design and performance prediction. 



