The purpose of this study was to see if the 

 above water shape of icebergs was related in a 

 significant way to the height to draft ratios for 

 those bergs. Height to draft ratios were obtained 

 for a total of 30 icebergs. 



Methods 



Measurements of iceberg draft were taken with 

 a Kelvin-Hughes Transit Sonar during a cruise 

 aboard the CGC EDI8TO, July 1974. The 

 EDISTO was operating in the Davis Straits 

 area and along the west coast of Greenland. The 

 Kelvin-Hughes Transit Sonar was designed to 

 conduct bottom surveys; however, we were inter- 

 ested in vertical targets rather than in horizontal 

 ones. The sonar transducer produced a beam .5° 

 wide in the horizontal and 5^° wide in the verti- 

 cal, both being to the 3db level. For our purposes 

 the transducer was pointed down by 26°, so that 

 the top of the fan-shaped beam would just pass 

 under the surface of the water and the bottom 

 of the beam would be depressed at 52°. The 

 transit sonar was designed for use from a small 

 boat with only a few feet of freeboard. It was 

 mounted on the EDISTO's arctic survey boat 

 (ASB). This arrangement worked well, provid- 

 ing cover for the deck gear and personnel, along 

 with high maneuverability and good speed con- 

 trol. The first five bergs were surveyed from the 

 ASB with great success. Use of the ASB was 

 then discontinued because the single point bridle 

 used to raise and lower it was hazardous in any 

 but the calmest weather. For the next two bergs 

 the motor surf boat (MSB) was used. It was 

 inadequate because the equipment was exposed 

 to the weather and because the boat had such 

 little stability that it was difficult to maintain 

 the transducer orientation with respect to the 

 iceberg. The MSB was retired due to a failure 

 of the boat davit. 



Finally, a method for using the transducer 

 from the EDISTO itself was devised. The free- 

 board of the EDISTO was approximately 

 eighteen feet from the rail to the water line aft 

 of midship. A 21 foot pipe was fabricated that 

 would support the transducer three feet below 

 the water line. The transducer was mounted on 

 the. bottom of the pipe, and the pipe was man- 

 handled from the deck to the outboard position 

 for each run. Small chunks of ice were a con- 

 stant problem and once sheared the transducer 



off the supporting pipe. xV safety line attached 

 to the transducer prevented loss of equipment. 

 With the sonar on the EDISTO it was possible 

 to have the deck gear in the oceanographic lab- 

 oratory and also to operate from a very stable 

 platform. 



When the ship was positioned near enough to 

 the berg, the beam of the sonar was completely 

 intercepted by the iceberg. As the ship circled 

 the berg, it increased the distance from the berg 

 so that at some part of the sonar beam passed 

 under the berg. The distance was increased till 

 the ship was at maximum range (550 meters slant 

 distance from the bottom of the berg) or a good 

 echo was no longer received. 



Five assumptions were made in interpreting 

 the record. First, that the first echo was returned 

 from the near surface portion of the berg; second, 

 that the strong echos were reflected from vertical 

 surfaces on the underwater portion of the berg; 

 third, that weak returns came from walls which 

 slope away from the observer along a radial of 

 the sonar beam; fourth, that blank areas in the 

 return were the results of shadow areas caused by 

 caves, holes or ridges in the iceberg; and fifth, 

 that if the transducer was far enough away from 

 the berg the last return from the berg comes 

 from the deepest portion of the berg. 



The entire record of the iceberg sonar trace 

 was examined and points which were representa- 

 tive of the deepest point on the berg were chosen. 

 These points were plotted on radial grid so that 

 the radial distances to the various portions of the 

 berg could be, converted to vertical measurements 

 of berg draft. These estimates of draft were 

 plotted versus distance to the berg. As the dis- 

 tance to the berg increased, the draft estimates 

 approached an asymptote which was assumed to 

 represent the true draft of the iceberg. 



Discussion 



The subaerial shapes of icebergs are extremely 

 varied, sometimes displaying fantastic forms. 

 Some bergs have "windows" in high vertical 

 walls, while others are pockmarked like a piece 

 of Swiss cheese, and still others have huge grottos 

 or voids. As a means of organizing the shapes 

 of the visible portion of icebergs into some sys- 

 tem certain prominent characteristics have been 

 chosen and used for typing icebergs into classes. 

 These classes are based solely on visual identifi- 

 cation. 



71 



