In two important papers, J. E. White and coauthors (White, 1975; White et al, 1976) have given an approximate theory for the calculation of attenuation and dispersion of compressional seismic waves in porous rocks filled mostly with brine but containing gas-filled regions. Modifications of White' s formulas for kE and Q in the case of gas-filled spheres brings the results into good agreement with the more exact calculations of Dutta and Ode (1979 a, b, this issue), who used Biot' s theory for porous solids. In particular, the modified formulas give the expected Gassmann-Wood velocity at very low frequencies.Inclusion of the finite gas compressibility in numerical calculations for gas-filled spheres shows an interesting maximum of the attenuation at low gas saturations which is not seen if the gas is ignored.A comparison of the attenuation calculated for the same rock and fluids but for three different geometries of the gas-filled regions suggests that the configuration of the gas-filled zones does not hav*e an important effect on the magnitude of the attenuation.

Harmonic distortion apparently arising from nonlinear processes, especially coupling of the vibrator to the ground, has often been observed in land seismic operations utilizing a frequency‐modulated mechanical vibrator as a surface source. The effect of harmonic distortion is to add a long oscillatory tail to the correlogram of a single sweep for the case in which the sweep frequency decreases with time. For up‐sweeps the distortion effect appears as a forerunner. It is shown analytically that for kth harmonic distortion, the zero‐to‐peak amplitude of the distortion tail (or forerunner) relative to the autocorrelation peak when the seismogram is crosscorrelated with, the undistorted outgoing signal is given approximately by √1/(k−1)TW times the amplitude ratio of harmonic to fundamental. W is the frequency range of the fundamental sweep and T is its length. Some examples computed by crosscorrelating a fundamental sweep (generated artificially) with its harmonic components are shown to demonstrate the effect of harmonic distortion. Some results on harmonic distortion observed in field seismograms recorded in the Val Verde basin of West Texas are presented. Second harmonic distortion of the order of 30 to 100 percent of the fundamental was observed.

In a recently published short note, F. K. Levin (1989) discusses the relation between the “moveout velocities” of P-P, P-SV, and SV-SV reflections from the bottom of a transversely isotropic layer with a vertical symmetry axis. We refer to such a medium as one exhibiting “polar anisotropy.” Levin’s note was prompted by a paper of Tessmer and Behle (1988), and it is relevant to a paper by Iverson and others (1989), both of which discuss the computation of shear velocities from moveout velocities obtained with P-P and P-S reflections. Levin’s note addresses the practically important question of the use of this method in the presence of polar anisotropy, a phenomenon which we believe occurs almost universally in the sedimentary layers of the real earth. Levin suggests that polar anisotropy of “typical” magnitude must be considered in this problem. He uses as an estimate of typical magnitudes data given by Thomsen (1986) and concludes from numerical examples that the method of estimating shear velocities proposed by Tessmer and Behle and by Iverson may be subject to unacceptably large errors in many real cases. Moreover, Levin suggests that the source of these errors is mysterious.

Thirty-four "forked tracks" similar to two previously reported by Rochester and Butler have been observed in 11,000 cloud-chamber photographs of cosmic-ray penetrating showers, confirming their conclusion that new unstable neutral and charged particles exist. The lifetime of the neutral particles is found to be about 3X10 -10 sec, and some information as to the nature of the decay products is given.

Recent observations of particle motion in vertical seismic profile surveys suggest the existence of borehole waves for which the formation horizontal particle motion is polarized in a specific azimuthal direction, possibly the direction of a local principal axis of stress. From an analysis of the static displacement of a pressurized vertical cylindrical borehole in a transversely isotropic formation with a horizontal axis of symmetry, we conclude that horizontal particle motion should not be azimuthally polarized for “tube waves” (lowest mode symmetric borehole wave) even in an ideal azimuthally anisotropic medium. Azimuthally polarized particle motion does exist for the “bending mode” of the borehole (lowest antisymmetric mode) even in isotropic formations; the polarization direction being determined by the source direction. Finite element calculations for a vertical borehole in a transversely isotropic formation with a horizontal axis of symmetry show that two bending modes with different velocities exist. The horizontal particle motion of one is polarized along the symmetry axis and the other is polarized orthogonal to that axis. At low frequencies the velocity of each approaches the velocity of a vertically propagating formation body shear wave with the same polarization direction. The recent experimental observations include one borehole arrival with a propagation velocity and a ratio of horizontal to vertical formation particle motion consistent with the hypothesis that it is a bending mode in an azimuthally anisotropic medium. The polarization direction and excitation of such a mode do not seem unreasonable. Other polarized arrivals cannot be easily explained.