The transducer was well insulated so that thermal variations, which 

 would cause changes in pressure within the two enclosed volumes, would have 

 a period considerably greater than the instrument's passhand. The input of 

 the microbarograph was simply a tube passing from the transducer through the 

 wall of the building to the open air. 



e. The Records 



The outputs of the LaCoste-Romberg gravimeter and the microbarograph 

 were recorded continuously on Esterline-Angus strip charts generally oper- 

 ated at a speed of three-fourths of an inch per minute. The output of the 

 gravimeter was expressed in milligals (l milligal = 10-3 cm/sec2) as a 

 function of time, and full-scale deflection was adjusted to either 28 or 35 

 milligals. 



The gravimeter records vertical acceleration of the ice as a fvuiction 

 of time; a seismograph records displacement directly. Displacement of the 

 wave with respect to time is most often observed in wave studies. Gravimeters 

 have been used in this work because of their portability and ease of operation 

 on ice in comparison to seismographs. In the analysis of these records the 

 freq.uency characteristics of the narrow-band energy peaks will be essentially 

 the same whether displacement or acceleration is recorded for the period range 

 being considered. A conversion is necessary, however, when spectral power 

 density in terms of displacement is desired. This is accomplished by dividing 

 the acceleration spectrum by the fourth power of the angular freq.uency ( ^^ ^) . 



If the ice were not in motion the output of the gravimeter woiild be a 

 straight line in the center of the chart, representing a constant downward 

 acceleration of gravity with time. Oscillation above or below this line repre- 

 sents a positive or negative change, respectively, of the downward acceleratior 

 at a given instant about the constant gravity value at that point in space. 

 Positive peaks on the strip chart, therefore, correspond to positive displace- 

 ments of the ice. 



The output of the microbarograph is recorded in units of dynes/cm^. 

 Full-scale deflections of 270 and 500 dynes/cm^ were used in this work with 

 pressiure increasing in the positive y direction. 



Samples of simultaneous pressure and wave records for 5 and 6 July 

 1961 are shown in figures 4a, b, and c. 



3. DATA REDUCTION 



a. Design Considerations 



Power spectrum analysis (Blackman and Tukey, 1958) and cross-spectrum 

 techniques discussed by Munk et al. (1959) are probably the most powerfiiL 

 tools available for analyzing statistical properties of wave motion with a 

 stationary gaussian distribution. The gravity waves and micropressure fluc- 

 tuations under consideration fall in this category. These techniques rely 

 on data processing by high-speed digital computers. The data collection was 

 undertaken with these processing techniques in mind. 



