44 



Symposium on Microseisms 



6 6 Sec 



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60 120 



Figure 5. Comparison of Palisades resonant seismograph traces (A) and Benioff and Columbia 

 vertical records (B and C) for May 6-7, 1952. 



in width and position with the position of, and 

 environment beneath meteorologic disturb- 

 ances responsible for the microseism storms. 

 In order to select microseisms from a particu- 

 lar environment a series of sharply-tuned ver- 

 tical seismometers were designed at Lamont 

 Geological Observatory and recently put into 

 operation. Although this study is in a pre- 

 liminary stage and a later report will give a 

 full description of the instruments it seems 

 worth describing some of the data and results 

 here. 



During most of the time of operation the 

 resonant periods were set at 2, 3, 4.4 and 6.4 

 seconds, and 15 second Leeds and Northrop gal- 

 vanometers are used. Although not yet cali- 

 brated the instruments are considered to have 

 "very sharp" tuning, which seems supported 

 by a comparison of records. Also by compari- 

 son with other instruments at the Lamont Geo- 

 logical Observatory at Palisades, New York, 

 the magnifications have been estimated for the 

 2-second seismograph at about 35,000, and for 

 the 3-, 4.4-, and 6.4-second seismographs at 

 about 50,000. The seismometers are undamp- 

 ed with the pendulums having a logarithmic 

 decrement of about V^. In order to attempt 

 rapid study of the onset times of microseism 

 storms, and the envelope of amplitudes without 

 the laborious measurements heretofore used, 



these instruments record on drums making one 

 rotation in 25 hours. Although a time scale is 

 given on the illustrations to follow, since no 

 time marks were made on the early records, 

 automatic time marks are being imposed on 

 current records. 



A few cases are given below which illus- 

 trate the nature of the data and results ob- 

 tained so far, and compare them with other 

 records made at Palisades. 



Case 1. May 5, 1952 — Figure 3 shows a 

 portion of the records made (from bottom to 

 top) by seismometers whose free periods are 

 indicated on each trace. A 1.5- second instru- 

 ment has just been added to this array. On 

 the 2-second trace a detectable amplitude in- 

 crease occurs about 0120, with a slightly 

 stronger rise at 0200. The 3-second trace 

 shows a detectable rise at 0210 followed by a 

 stronger one about 0430. It can be noted that 

 the 2-second trace is returning to background 

 level while the 3-second trace is still high. Dur- 

 ing this interval, the upper-long-period traces 

 are at normal background level which appears 

 higher than background for the shorter-period 

 instruments. It is evident that real amplitude 

 variations of several minutes occur, some of 

 which show sharp peaks that may correspond 

 to the maximum wave or wave groups common 



