In a recent paper Springer et al. [1968] present results concerning the Sterling nuclear decoupling experiment. At this time, I would like to present additional theoretical analyses verified with datg obtained over a wider frequency band. The additional data indicate that the alecoupling at low frequencies is somewhat less than that indicated by the data in the vicinity of 1 Hz. An analysis is performed that satisfactorily explains the observed frequency dependence in terms of a simple analytic expression. As shown in Figure 3 of the referenced paper, the seismic instruments used to record the events Salmon and Sterling are characterized by low-frequency rolloffs. In particular, the NC-21 velocity meter employed by the U.S. Coast and Geodetic Survey has a second-order rolloff at I Hz. Using an analog correction circuit [Watson, 1967], the inverse of this response function was programmed on an analog computer resulting in an effective instrument response that gives reliable information down to 0.3 Hz. We employed this circuit in the processing of the Salmon and Sterling measurements and thereby obtained data over the frequency range of 0.3-100 Hz. As mentioned by Springer et al. [1968], narrow bandpass filtering can be used to approximate Fourier analysis. We agree with this concept and routinely employ analog filters characterized by the following transfer function [Lynch, 1965]: + + S,o where $ is the Laplace transform variable, and •o is the center frequency of the filter. The

Salmon, an underground nuclear explosion in the Tatum salt dome in Southern Mississippi, was seismically recorded throughout North America and at some overseas stations. Data are presented from 143 seismograph stations recording signals from the Salmon event. Travel times at smaller distances generally confirm the high upper mantle velocities indicated by Gnome as recorded in the eastern United States. Travel times for the area east of the Rocky Mountain front may generally be represented by the equation t = Δ/8.3 + 8 seconds (Δ in kilometers); however, additional detail is shown indicating apparent velocities of up to 8.7 km/sec. In the 10° to 20° distance range multiple arrivals are apparent, and beginning near 16° late initial arrivals are observed which correspond to Jeffreys‐Bullen travel time. Amplitudes were generally higher than expected for regional and for teleseismic distances but lower in the 13° to 22° range in the western United States. A revision to procedures for determining magnitudes is shown to be necessary.

Low-and very-low-frequency air-coupled seismic waves were efficiently excited on different occasions by jet fighter planes flying at high altitudes and at Mach numbers greater than 1.2. The experiments presented in this investigation were clearly recorded on a geophone array containing up to 12 short-period vertical component stations, and a singular station recording the transverse and radial type of motion. A highermode, seismic, coupled wave from sonic booms has been observed for the first time. Correlation is made between the acoustical signal registered at the microphone stations in Cape Kennedy and the first impulsive onset of the seismic waves recorded at the array setup. The seismic waves coupled from sonic booms are explained as a constructive interference phenomenon in the surficial ground layers. Fourier-transform techniques are applied to some of the seismograms, and some interesting features are delineated. • A. P. Crary, "Seismic Studies on Fletcher's Ice Island, T-3," Trans. Amer. Geophys. Union 35, 293-300 (1954).

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Empirical equations derived from explosion data compare well with earthquake measurements. Maximum earth motion in cm, cm/sec, and gravity units, body wave earthquake magnitude, and distance in km, are included in the functions: a = 3.04 x 10°'74m-4R-1.4 v = 4.06 x 10 0.