Apparent Wave Velocity Research Articles

Errors in the measurement of hydraulic input impedance

It is shown that significant erros may be made in obtaining the input impedance of an arterial bed when pressure and flow are not measured at the same site. These errors manifest themselves mainly in the phase of the input impedance. The sign of the phase errors depends on whether flow is measured proximal or distal to pressure. The deviation in the absolute value is generally in the same direction for both cases. If the apparent wave velocity in the region between the pressure and flow transducers as well as their distance are measured, the phase error can be corrected easily. By means of a model of the systemic arterial tree the errors in the input impedance of the systemic arterial tree are quantified. At 10 Hz these errors may be in the order of 30 deg in the phase of the input impedance. The effect of applying the correction for phase is illustrated. The correction procedure used here can be successfully applied to avoid this difficulty in the future. This correction resolves most of the differences in the phase of the input impedance as reported by various investigators.

Dispersion bandwidth deduced from coherency of wave recordings from spatially separated sites

In an earlier paper it was pointed out that temporal variability in the dispersive properties of the propagation medium can cause a band of wave velocities to be associated with each frequency component in the time series of a sampled quantity. This results in degrading the coherence between stations separated in the direction of propagation and can cause significant error in the apparent wave velocity as deduced from the phase of the cross spectra. This bandwidth is complementary to the concept of beamwidth, which principally degrades the coherence between stations separated perpendicular to the direction of propagation. The present paper uses numerical procedures to extend the earlier results to larger beamwidths and bandwidths.

Possible forms of seismic anisotropy of the uppermost mantle under oceans

The apparent body wave velocities υP of the oceanic mantle immediately below the Mohorovicic discontinuity (M), obtained from oceanic refraction shooting, can depend on azimuth ϕ; because of real mantle anisotropy or horizontal variations in an isotropic crust and mantle. A small anisotropy of the uppermost mantle leads to υP(ϕ)=∑n=04An cosn ϕ sin4-n ϕ=∑n=−22Bneinϕ The An determine 5 of the 21 elastic parameters of the uppermost mantle. Curvature in the M discontinuity has the same effect on velocities υP(ϕ), but the azimuth dependence found by Hess in Raitt's and Shor's velocities, if real, is too large to be due to curvature. A sloping M discontinuity adds to velocities υP(ϕ) a term a cos ϕ + b sin ϕ. In a refraction survey of anisotropy, only two lines need be reversed to find a and b. If the ϕ dependence of velocities υP is due to a small mantle anisotropy and a small slope of the M discontinuity, velocities υP(ϕ) is determined for all ϕ by shooting five different lines through a point and reversing two of them or simply by shooting seven different lines. The theory is compared with Raitt's and Shor's data as reported by Hess.

Streaklines in a Perturbed Shear Flow

Streaklines are numerically obtained for a shear flow perturbed by an unamplifying traveling sinusoidal wave. It is shown that, if a dye is injected from near the critical layer, the streaklines have an appearance of amplification and rolling as if to indicate that the flow develops into discrete vortices. The rolling-up of a streakline cannot therefore constitute a positive identification of the presence of discrete vortices. Away from the critical layer, on the other hand, the streakline appear to show an alternating amplification and damping. Apparent wavelength and wave velocity are both incorrect except in the critical layer. Further, any information due to pathlines or obtained by tracing marked particles is equally improper in regard to the wave motion. These results strongly suggest that a great deal of caution must be exercised in interpreting flow phenomena based upon flow-visualization techniques.

Exploration seismic techniques applied to oceanic crustal studies

One unreversed seismic refraction profile, 33 km in length, was made northeast of the Bahama Islands to demonstrate and evaluate the feasibility of utilizing conventional shallow water seismic exploration techniques and equipment for oceanic crustal studies. This procedure requires the recording boat, towing a 1-mile-long, 24-hydrophone-group cable, to move away from a shooting boat which is held on a fixed position. Charges are detonated electrically each time the towed cable traverses its own length. Seismic velocities can be identified with a single shot at a relatively short distance by observing arrivals over the known length of cable. Dip determination from a reflection profile permitted an unambiguous refraction interpretation. Apparent compressional wave velocities of 2.18, 4.98, 5.74, and 7.04 km/sec were observed and tentatively interpreted, respectively, as a semiconsolidated sedimentary layer 1.67 km thick, a consolidated sedimentary rock or crustal rock layer 1.14 km thick, a granitic layer 1.85 km thick, and an underlying layer of undetermined thickness of gabbro or oceanic crust. Seismic wave velocities and calculated depth and thickness figures agree favorably with previously published seismic data.