Amplitude Distribution of Storm Microseisms 



41 



Discussion 



William L. Donn 

 Brooklyn College and Columbia University 



Abstract — Although the application of empiri- 

 cal amplitude data to the problems of location 

 and intensity of hurricanes eliminates some im- 

 portant sources of error inherent in tripartite 

 operations there are still several factors which 

 are at present adverse to operational success. 

 These factors are considered here. 



A new method of amplitude study is pre- 

 sented which employs resonant seismographs. 

 These instruments permit discrimination 

 among microseisms having different period 

 bands and presumably different areas of gen- 

 eration. 



Studies of simultaneous microseism and 

 ocean wave and weather data together with 

 local sensitive atmospheric pressure instru- 

 ments suggest that atmospheric impulses re- 

 sulting from the single or combined effects of 

 turbulence, gustiness or pressure oscillations 

 are the ultimate source of microseisms. The 

 observations given all negate ocean waves or 

 swell as being significant in the generation of 

 the type of microseisms under discussion. 



Introduction — A study of the intensity and 

 variations of microseism amplitudes affords 

 much information applicable to the practical 

 use, and the problem of origin of microseisms. 

 The use of such data has been presented in de- 

 tail by Gilmore but the theoretical signifi- 

 cance bearing on microseism origin, although 



VELOCITY (I0 3 ft/sec) 



— N> Oi 



o o o 



PERIOD (sec) AMPLITUDE (mm) 



<n * 



m 



Figure 1. Paths of the hurricanes of August 

 27-Sept. 6, and August 31-Sept. 14, 1950. 



Figure 2. Bermuda microseism data for Sept. 

 3-9, 1950. 



stressed, was not supported strongly by quan- 

 titative or correlative data. The discussion 

 given here is hence organized in three parts : 

 (1) an evaluation of the use of amplitude data 

 in describing the location and intensities of 

 hurricanes or violent storms; (2) the introduc- 

 tion of new instrumentation and resulting data 

 for the study of microseism amplitudes ; and (3) 

 a discussion of the mechanisms of origin of mi- 

 croseisms based on empirical amplitude data. 

 Part I — Amplitude Data Applied to the Posi- 

 tion and Intensity of Storms — The amplitude 

 distribution and micro-ratio methods of Gil- 

 more seem to have definite, although limited 

 success for the two cases given. It is noted 

 that storm fixes by his method have an ac- 

 curacy which varies from about 35 to 300 nau- 

 tical miles, with the average being higher than, 

 but closer to the lower value. Since this is 

 strictly an empirical procedure the question 

 now is whether continued success can be ob- 

 tained or whether the cases shown are ideal 

 for the procedure. Even these cases show 

 "fixes" over relatively small portions of the 

 hurricanes' paths. Hurricane No. 1, 1950 was 

 plotted on weather charts as early as August 

 13 but the first fix is given on August 18. The 

 procedure could not be applied to Hurricane 

 "How," 1951, during the early and more criti- 

 cal part of its course. Prolonged experience 

 in microseisms study suggests that a number 

 of the difficulties that beset tripartite opera- 

 tion still have an adverse effect on the applica- 

 tion of amplitude distribution, together ' 

 some that are not involved in tripartite stud 

 However, the micro-ratio technique eliminates 

 the effects of refraction, which has been a ma- 

 jor obstacle in tripartite work. It also elim- 

 inates any effect of possible short-crested mi- 

 croseisms, the presence of which would intro- 

 duce serious tripartite errors. A discussion 

 of the sources of error that should be consid- 

 ered in connection with the further applica- 

 tion of the micro-ratio method is given below, 

 together with suggestions for minimizing or 

 removing them when possible. 



1. The presence of two storms within 

 range of the recording seismographs will ren- 

 der the micro-ratio method subject to consid- 

 erable ambiguity. This is a very real prob- 

 lem. During the 1950 hurricane season in the 



