32 



Symposium on Microseisms 



ratio technique. The micro-ratio fixes were, 

 for the first time, released along with the 

 Weather Central advisories. The two tracks 

 are almost identical. Hurricanes ABLE and 

 BAKER in August and September 1952 gave 

 additional proof that the new technique of 

 storm tracking is a very valuable tool for the 

 forecaster of tropical storms. Accurate infor- 

 mation as to the location and intensity of these 

 storms were derived from microseismic charts. 



The validity of such amplitude and micro- 

 ratio charts is naturally dependent upon the ac- 

 curacy of the data used in determining the in- 

 tensity of past storms. Wind force is difficult 

 to estimate, especially the higher velocities, and 

 is often as much as 10 knots in error, even in 

 some aircraft reconnaissance reports. If it 

 were always correct, it would still be only a 

 rough gauge of the relationship between the to- 

 tal energy of the storm and the amplitude of 

 the microseisms produced at each station. Also 

 there is the stipulation that no other major 

 source of microseismic generation, except the 

 hurricane, be influencing the station. This, of 

 course, can be assured only by the placement of 

 additional single microseismic stations through- 

 out the entire hurricane belt in such numbers 

 that stations influenced by other sources would 

 not have to be depended upon. These data in- 

 dicate that, when a proper network of micro- 

 seismic stations is available, it will then be pos- 

 sible to detect and track any and all tropical 

 storms. 



Conclusions 



The data presented in this 



report clarify and solve to some extent some of 

 the problems involved in microseismic storm de- 

 tection and show what is necessary for the fu- 

 ture microseismic forecasting of tropical 

 storms. The primary problem of detecting and 

 tracking tropical storms with microseismic data 

 appears to be near solution with the newly de- 

 veloped micro-ratio technique. Even a few sec- 

 ondary problems concerning the actual method 

 by which energy is transmitted from a storm to 

 the ocean floor and then propagated in the form 

 of microseisms may be near solution. There 

 are three important items concerning the funda- 

 mental problem of complete microseismic storm 

 detection to which attention is invited. 



A. Early Tropical Storm Detection. 



From all the information obtained in the 

 research there is no longer any doubt that the 

 seismograph, when properly located in rela- 

 tion to a tropical storm, can detect the storm 

 when it is over water. The effect of apparent 

 irregularities in the earth's crust that tend to 

 impede microseismic transmission can be great- 

 ly minimized, if not altogether eliminated by 

 placing seismograph stations on each side of all 

 known "microseismic barriers." Early storm 

 detection is obviously a valuable aid to any hur- 

 ricane warning system and it is very probable 

 that the present operational network. A suffi- 

 ciently close network of such stations would, in 



addition to giving early warning of a storm, 

 greatly aid the weather centrals in scheduling 

 flights into suspicious areas. 



B. Tracking Tropical Storms. 



The newly developed micro-ratio technique 

 for tracking tropical storms is based entirely 

 upon an empirical use of microseisms and for 

 that reason is not influenced in any manner by 

 changes in geologic formations between the 

 storm and the observing station nor by the size 

 or intensity of the storm. The accuracy ob- 

 tained in tracking past hurricanes, if main- 

 tained with future storms, could fulfill opera- 

 tional needs. These hurricanes were the first 

 storms passing within range of three or more 

 microseismic stations since the new technique 

 was developed. If this high degree of accuracy 

 can be obtained again with this same group of 

 stations and with new stations in other areas, 

 it would, without doubt, be a long step towards 

 the answer to the original problem in tropical 

 storm tracking. 



C. Detecting Changes in the Intensity of 



a Storm. 

 It is just as important to know whether or 

 not a hurricane is intensifying, when approach- 

 ing a populated area, as it is to know its approx- 

 imate position. For this reason alone the ampli- 

 tude charts are of prime importance in hurri- 

 cane forecasting. The amplitude charts give 

 far more accurate information on storms than 

 merely moving them along at a constant for- 

 ward speed, when weather reports are not avail- 

 able. This is especially true at night when no 

 reconnaissance can be made. It is obvious that 

 information from such charts will become more 

 valuable as the charts are improved by increas- 

 ingly accurate data from future storms and as 

 the number of effective stations is increased. 



REFERENCES 



Macelwane, J. B., S. J., Practical application of micro- 

 seisms to forecasting, Compendium of Meteorology, 

 Amer. Met. Soc, 1951. 



U. S. Naval Air Station, Miami, Florida. 



Discussion 



D. S. Carder 

 U. S. Coast & Geodetic Survey 



The technique that was just presented by 

 Mr. Gilmore on the use of empirical amplitude 

 relationships of storm microseisms as a method 

 of tracking tropical storms is highly interesting 

 and shows promise. However, a large amount 

 of observational data is needed before its use- 

 fulness can be verified. The method, to be re- 

 liable, needs precision calibration of the instru- 

 ments, as was outlined by Mr. Gilmore ; and in 



