MEASUREMENTS, WINTER 1955 BERING 

 SEA EXPEDITION 



Equipment and Procedures 



MECHANICAL PROPERTIES 



The icebreaker USCGC NORTHWIND (WAGB-282) lay to in the ice from 

 23 March to 5 April. Again measurements were made of the longitudinal plate- 

 wave velocity and of the air-coupled flexural wave frequency. The seismic equip- 

 ment consisted of a Consolidated Engineering Corporation type 5-lOlB, l4-channel 

 recording oscillograph. The galvanometers used in the oscillograph had a resonant 

 frequency of 375 cps. Nine of the amplifiers were type GN amplifiers made by 

 Engineering Laboratories, Inc., modified to give a uniform frequency response 

 from 11 cps to 1000 cps. Five amplifiers were designed and built at NEL to pro- 

 vide a uniform frequency response from 3 cps to 5000 cps. Eleven geophones of 

 Engineering Laboratories, Inc., type GS-lOO were used. These geophones had a 

 resonant frequency of 27.5 cps. Two Brush C-23 hydrophones were used to detect 

 the water wave velocity. The shot-break signal was transmitted through field tele- 

 phone wire to the recorder. 



Three sets of geophones were placed 91.4 meters (100 yards) apart. Each set 

 consisted of three geophones, one for each vector direction. In addition, two geo- 

 phones were set on top of the ice sheet, and two hydrophones were placed under 

 the ice sheet in the water. The explosive charges were detonated at a distance 

 of 457.2 meters (500 yards) and 914.4 meters (1000 yards) from the nearest set 

 of geophones. The general surface of the ice sheet was flat; however, the over-all 

 ice sheet consisted of two sections which had been rafted together into one ice 

 sheet. Thus, two sets of geophones were not in the same sheet as the one in which 

 the explosive charges were set off. The results of these measurements are pre- 

 sented in tables 2 and 3. A schematic arrangement of geophones and shot-blast 

 stations appears in figure 3. 



Density profile measurements were made on a sample of ice taken from the area 

 of Site 1 (fig. 4). The density measurements were made immediately after taking 

 a core sample of ice by the direct method of weighing and measuring the dimen- 

 sions of the sample of ice. 



Salinity profiles were made at Site 1 and Site 2 (figs. 5, 6, and 7). The size of 

 each sample was selected to assure a representative quantity. Chlorinities were 

 determined by the titration method and then converted to salinities by the use of 

 the Knudsen hydrographical tables. 



It should be noted that errors exist in the determination of salinities by this 

 method.^"'^' The error in this method, which is well within practical limits, is 

 apparently less than 0.1°/oo. 



Vertical temperature profiles through the ice sheet were made at Site 2 at the 

 time of the explosive shots. The results are presented in figures 8, 9, and 10 for 

 each day the tests were conducted. Thermistors were used as the thermal elements. 



Profiles of the crystal grain size of a sample of ice were taken at Site 2. A graph 

 of the distribution of grain sizes is presented in figures 11 and 12. An Eastman 

 Kodak photo-copyer with 1.5-power magnification was used to photograph the 

 ice samples, which were placed under a calibrated grid between cross polaroid 

 plates. 



