MPL-U-20/67 

 Northrop 



where Ij^ is the level in dB re 1 dyne/cin2 at a distance R in nauti- 

 cal miles from a shot of intensity I^, the effective source strength 

 (intensity at a range of 1 yd) . For spectral energy within a 

 specific frequency band, an auxiliary formula may be usedi^'-' 



lo [fL to fu')^ Iq + 10 log 2/77 tan-lff^f'j- tan-lff^/f") (2) 



where f„ and f-r are the upper and lower frequencies respectively, 

 and f ' IS the half -power frequency at the range where the pressure 

 is 100 psi. 



Using these formulas and assuming spherical spreading to a 

 distance of 10 miles and semi- spherical spreading {k.'^ dB per 

 distance doubled) beyond (Equation (l)), a calculated level of 

 69 dB re 1 dyne/cm2 is computed in the frequency band 3-20 cps. It 

 is difficult to speculate on the observed signal level for this 

 direct arrival because of recording system overload. From the data 

 available, it appears that levels near 66 dB above ambient noise 

 were recorded for the low-frequency band. The level for intermedi- 

 ate frequencies was about 20 dB lower because the greater part of 

 the energy for large explosions is in the low frequencies and 

 greater attenuation is experienced at higher frequencies (O.OO5 

 instead of O.OO3 dB per nautical mile). 



Discussion 



Recording of topographic reflections from CHASE V was 

 selective at the FLIP site in that the Hawaiian Arch prevented 

 reception of reflected signals from the South Pacific. Even with 

 this reduction of reflectors, it was still difficult with the 

 method used to identify all of the arrivals noted. The first few 

 returns after the decay of the main arrival were fairly easy to 

 identify, but after that the multiplicity of targets that satisfied 

 the travel time considerations increased. For example, the round- 

 trip travel time shot to Vancouver to FLIP is the same as that from 

 shot to Hawaii Ridge to FLIP. Therefore, even though multiple 

 reflection paths (shot to Hawaii to Aleutians to FLIP for example) 

 were excluded because of travel time considerations, the multiplic- 

 ity of arrivals was such that individual targets were difficult to 

 identify. Further identification by arrival time differences 

 between FLIP, TERITU, and YAQUIM data were attempted to resolve 

 these reflectors. However, the YAQUIM recorder was secured because 

 of poor signal-to-noise ratio shortly after the Hawaiian reflection 

 was received, and the TERITU in the interval between the Hawaiian 

 and Aleutian reflections. Therefore, groups of arrivals with 

 nearly the same signal level were isolated and labeled A, B, C, D, 

 E, F, and G on Fig. 1. These general groupings are believed to be 

 correct, although as the total travel time increased, the number of 

 possible reflectors multiplied rapidly and errors in the later 

 identifications are more probable than in the early ones. 



33 



