Tripartite Stations and Direction of Approach of Microseisms 



13 



Pwnl 

 C 



Point 

 A 



Pant 

 B 



2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 

 Time (seconds) — *■ Time (seconds) *■ 



Figure 1. Illustration of two wave trains crossing a station at right angl< 



Perhaps the most serious factor which in- 

 troduces incoherency in the recorded micro- 

 seisms is the possibility of two or more wave 

 trains simultaneously crossing the station in 

 different directions. In this case the trains 

 will add together in different phases at each 

 seismometer with the net result that a false 

 direction and velocity will be computed. Figure 

 1 illustrates a very simple case of two wave 

 trains crossing a station at right angles. For 

 this example the apparent velocity is 11,200 

 ft/sec as compared to 8,000 ft/sec for the com- 

 ponent waves; and the direction is intermedi- 

 ate between the direction of the component 

 waves. Figure 2 shows a graph of the ratio 

 of apparent velocity to the real velocity as a 

 function of the angle between the direction of 

 propagation of two similar wave trains. 



This indication that the apparent velocity 

 of the recorded microseisms is increased if two 

 or more wave trains simultaneously cross a 

 network has suggested a method of selecting 

 the bearings computed from a tripartite sta- 

 tion. This method is reported by Kammer and 

 Dinger [1951] and later studied by Donn and 



--A(sin „, t+ft) 



"0 45 90 135 180 225 270 315 360 

 Angle * 



Figure 2. Ratio of apparent velocity to 

 real velocity as a junction of angle 



between direction of propagation of 



two similar wave trains. 



Blaik [1952]. Figure 3 shows a plot on a po- 

 lar graph of a typical series of bearings ob- 

 tained by the Naval Research Laboratory tri- 

 partite station on 21 November 1950. The 

 distance of each dot from the origin is a 



