Tripartite Stations and Direction of Approach of Microseisms 



11 



ments could be made accurately on the tripple 

 drum at the main vault, instead of recording at 

 each vault, with a resultant saving of consid- 

 erable time. 



In December 1944 it was decided that ad- 

 ditional tripartite stations should be estab- 

 lished, one at the Naval Air Station of Rich- 

 mond, Florida, (later moved to Opalocka) and 

 one at the Naval Operating Base of Roosevelt 

 Roads, Puerto Rico. The two new stations 

 were in complete operating condition by Au- 

 gust 1945. Thus a new technique was intro- 

 duced as a check by triangulation on the origin 

 of microseismic waves by means of the three 

 stations of the tripartite type. The bearings 

 and crossbearings obtained in 1945 with these 

 stations seemed convincing, and the results 

 were published in detail by Gilmore, Macelwane 

 and others. 



In 1946 a tripartite station was built at 

 Corpus Christi, Texas, (First report came out 

 17 August), and a single station at Antigua 

 (was ready for operation in October). In 

 1947 a single station was built at Trinidad 

 and a tripartite station at Swan Island (Feb- 

 ruary). At present tripartite or single sta- 

 tions include Bermuda, Whiting Field Jackson- 

 ville and Cherry Point. A tripartite station 

 was established in the Pacific area, on Guam 

 in the summer 1947, and single stations at 

 Okinawa and at other points. The station in- 

 stalled at Corpus Christi on loose sand was dis- 

 continued due to the unsuitable ground founda- 

 tion. Trinidad and Antigua were also discon- 

 tinued; Roosevelt Roads and Guantanamo Bay 

 are now single stations. 



Finally in 1949 a special tripartite station 

 was installed on the grounds of the Florissant 

 seismograph station, for the purpose of study- 

 ing the nature and origin of the 0.2-0.5 second 

 period microseisms by means of special capac- 

 ity seismographs developed at Saint Louis Uni- 

 versity under Dr. Joseph Volk and Dr. Florence 

 Robertson. One corner of the triangle was es- 

 tablished at the Florissant seismic vault, the 

 second corner was approximately 600 feet due 

 northeast and the third corner was about 800 

 feet due northwest of the Florissant vault. 



Regarding the direction of approach of 

 microseisms to a tripartite station, this was 

 the view point of Naval Aerology in 1947: "The 

 direction given by a tripartite station seems 

 to be accurate frequently within 10 or 20 de- 

 grees, but sometimes greater deviations are 

 to be expected due to geological idiosyncrasies, 

 error of the observers and the fact that the 

 source of the microseisms is not necessarily 

 in the center of the hurricane and even may 

 be an extended area with a different starting 

 point of the longest wave for each station at 

 given moment" (Navy report 1947, b). It was 

 also remarked that for a particular storm "all 

 the bearings obtained are through some por- 



tion of the storm. Some of them perhaps lead 

 the storm center, while most of them lag be- 

 hind." 



It has been my experience also that in a 

 particular storm the bearings obtained may 

 differ widely, but averaging several readings 

 leads to a truer indication of the direction of 

 approach. 



According to various authors this varia- 

 tion of the intervals of arrival of waves at the 

 corners of a tripartite station may be accounted 

 for as due to the possibility that the energy 

 source of microseisms waves may not be at a 

 point but that it is rather a wide source vary- 

 ing its maximum around the center of the 

 storm, or even that it is due to interference 

 from various other sources such as a new sys- 

 tem of microseismic disturbances coming from 

 a different direction. 



More recently Gilmore has found evidence 

 to state that microseisms "may not always be 

 propagated outward through the earth's crust 

 from the center of the storm in straight lines 

 because of refraction and reflection," and that 

 in order to track storms with microseismic 

 cross bearings from tripartite stations accur- 

 ately, charts are needed showing all refraction 

 around each station. Hence, a new method has 

 been developed by the Navy Microseismic Re- 

 search, called the micro-ratio technique of 

 storm tracking, which according to its author 

 Gilmore, may permit very accurate tracking 

 of storms that are far from land. 



Summing up, the tripartite station sys- 

 tem, either in the form of a tripartite station 

 for determining the bearing of the source or 

 in the form of a set or sets of tripartite sta- 

 tions for locating and following the origin of 

 the source of microseisms has been used and 

 still can be used successfully, provided the sta- 

 tions have a suitable ground foundation, a 

 proper distance between themselves, and as 

 much as possible, instrumental homogeneity, 

 accurate and identical time system, and simul- 

 taneous recording on a triple drum. 



The direction of approach of microseisms 

 can be rather accurately determined by each 

 tripartite station by averaging of several read- 

 ings. The crossbearings from tripartite sta- 

 tions can locate energy sources and track them 

 continuously from hour to hour and detect them 

 days before they can be detected by any other 

 method. 



REFERENCES 



HECKER, O., Versuche zur Bestimmung der Fortpflan- 

 zungsgeschwindigkeit der Bodenbewegung der mik- 

 roseismischen Unrhue., Mitteilungen der Zentral- 

 buros, Gerlands Beitr. z. Geoph., v. 14, pp. 28-33, 

 1915. 



