Henry W. Kutschale

Quaternary sedimentation in the Arctic ocean

Abstract A distinct boundary between sediment types usually occurs at a depth of about 10 cm in bottom cores raised from the Alpha Rise in the Arctic Ocean. The sediment between the tops of the cores and the 10 cm boundary is a dark brown, foraminiferal lutite mixed with ice-rafted sand and pebbles. The sediment between the 10 cm boundary and a depth of about 40 cm is a light brown sand with ice-rafted material but few Foraminifera. The 10 cm boundary apparently represents the most recent change in pelagic deposition in this region and must be connected with climatic changes. Foraminifera from a zone between 7 and 10 cm have been dated by the C14 method as 25,000 ± 3000 and as 30,000 years BP in two different samples. The 10 cm boundary itself has been dated as 70, 000 years BP by a uranium series method. If these dates are accepted, a low sedimentation rate of 1 1 2 to 3 mm/1000 years is indicated for the Alpha Rise and for the Arctic Ocean as a whole if pelagic sedimentation has been uniform over the entire ocean. Cores from the Canada Abyssal Plain differ in character from the Alpha Rise cores consisting primarily of olive-gray lutite without Foraminifera or ice-rafted material. This sediment was probably deposited by turbidity currents. A 3 mm layer of dark brown, foraminiferal lutite occurs at the top of the Canada Abyssal Plain cores. This layer is similar to the upper layer in Alpha Rise cores and apparently represents continued pelagic deposition since the last turbidity current. Foraminifera from this upper 3 mm layer have been dated as 700 ± 100 years BP by the C14 method. The conclusion is that pelagic sedimentation has continued unchanged in the Arctic Ocean from about 70,000 years ago to the present. This implies that the present ice cover has existed for that length of time.

Shallow‐Water Propagation in the Arctic Ocean

Dispersion characteristics of underwater sound on the Arctic continental shelf north of Alaska were investigated at ranges between 2 and 250 km and for frequencies between 3 and 250 cps. Explosive charges were used as sources, and geophones were used as detectors. Observations were interpreted in terms of normal‐mode theory, and good agreement between theory and experiment was found for both phase and group velocity. Portions of the first and second modes were recognized at all ranges, and, at short ranges, “leaking modes,” associated with the ice layer, were also noted. For long ranges, the water wave amplitude varied as the −1.85 power of range.

Bottom‐interacting acoustic signals in the Arctic channel: Long‐range propagation

Hydroacoustic signals from underwater explosions that have propagated over the Arctic abyssal plains commonly display marked frequency dispersion in pulses that are bottom‐interacting and that arrive after the SOFAR signal. In the infrasonic band of 2 to 20 Hz, the temporal dispersion for each pulse that has interacted with the flat bottom of the plain can be nearly as strong as that observed in the SOFAR signal for the first mode. However, the bottom‐interacting pulses correspond to a coherent summation of many higher‐order normal modes in a channel bounded above by the ocean surface and below by the upper 400 m of the bottom sediments, where the velocity increases with depth. Using normal‐mode theory and the Multiple Scattering Pulse Fast Field Program (MSPFFP), we have analyzed the dispersion and pulse shapes and have derived the acoustic properties of the bottom in the Pole, Barents, and Mendeleyev Abyssal Plains. The principal properties of the bottom controlling the propagation are compressional velocity, density, and attenuation. In contrast, the ice layer has a negligible effect on the dispersion of the observed waves. The effect on pulse compression of this frequency dispersion of the bottom‐interacting signals was simulated numerically, using predistorted waveforms matched to the dispersion of the SOFAR channel at specified ranges.

Pulse propagation in the Central Arctic Ocean

Wave‐theoretical computer codes have been developed to model pulse propagation in the central Arctic Ocean. Pulse shapes as a function of range and depth are computed from the Pulse Fast Field Program (PFFP) and the pulse parabolic equation (PPE) code. Group‐ and phase‐velocity dispersion and eigenfunctions are computed from the PFFP or from a corresponding normal‐mode program. Good agreement has been obtained between measured and computed SOFAR signals. Computer simulations of pulse compression show in detail the possibility of achieving significant signal gain by pulse compression in the Arctic channel. This is accomplished by using predistorted waveforms matched to the dispersion of the channel at specified ranges. The effect of ice roughness on Arctic SOFAR propagation is illustrated from field data and the PFFP. Recent experiments of long‐range transmission of explosion sounds over the Pole Abyssal Plain show marked frequency dispersion of bottom‐interacting signals following the SOFAR signal. The di...

Pulse propagation in the ocean. II: The parabolic equation method

The Fast Field Program (FFP) is a convenient method to compute the exact integral solution of the wave equation derived from harmonic sources in multilayered media. Solid layers as well as liquid layers may be included in the model. Thus, elastic subbottom layers are included in the model and, for Arctic propagation, an elastic ice sheet is also included. The sound field is computed as a function of range for each detector depth employing the fast Fourier transform (FFT) algorithm. The FFP has been extended to compute waveforms from broadband sources as a function of range and depth by Fourier synthesis employing the FFT algorithm. The input for this synthesis as a function of frequency is computed by the FFP. The computer program has been used to model SOFAR propagation in the central Arctic Ocean. Computed waveforms for the Arctic channel are in excellent agreement with field data. The computations clearly show the evolution in range of the dispersive signals observed in the Arctic channel. At any selec...

Pulse propagation in the ocean. I: The fast field program method

Long‐range propagation experiments are often conducted in range‐dependent environments. The prediction of pulse propagation characteristics in such environments is possible with the parabolic equation (PE) method, using the Tappert—Hardin split‐step Fourier algorithm. A program has been developed to compute temporal waveforms from broadband sources as a function of range and depth, using Fourier synthesis to coherently add up the frequency response as obtained from the PE code at each range step. The effects of range‐dependent ice roughness are modeled by a modified formula of Marsh and Mellen, using as input the rms ice roughness along the propagation path. The computed waveforms have been tested against field data on SOFAR propagation in the central Arctic Ocean, and excellent agreement has been obtained. Numerical simulations of pulse compression in the Arctic channel using predistorted waveforms to match the dispersion of the channel at specified ranges show in detail the possibility of successfully a...

Sound radiation from multilayered structures

The exact integral solution of the wave equation is derived from a vertical harmonic point force within a solid layer of a laminated structure of interbedded fluid and solid layers resting between two fluid half‐spaces. Structural damping is included in the theory by introducing complex compressional and shear velocities in each solid layer and a complex compressional velocity in each fluid layer. The farfield radiation pattern is rapidly computed from the integral solution by application of the principle of stationary phase. Sample computations illustrate coincidence effects associated with flexural and extensional waves traveling in the layered structure. For a single solid layer resting on a fluid, the radiation patterns over a wide frequency range are generally in good agreement with results derived by Feit [J. Acoust. Soc. Am. 40, 1489–1494 (1966)] from the Timoshenko‐Mindlin theory of flexural motions on thick plates. Exceptions to this agreement are noted for coincidence effects associated with extensional waves traveling in the structure. The present theory is applied to problems of sound radiation from floating pack ice of the central Arctic Ocean.

Some Effects of a Solid Bottom on Propagation Loss at Low Frequencies

The fast field program (FFP) is a fast, convenient method employing the fast Fourier transform to compute propagation loss as a function of range. The FFP integrates directly the exact integral solution of the wave equation derived from point harmonic sources in multilayered media. In the past, the bottom has been represented by a layered liquid, but here the FFP is used to investigate some effects at low frequencies of a layered solid bottom on propagation loss as a function of range. Computations of propagation loss are compared between bottoms with the same layer thicknesses, compressional‐wave velocity, and density but with and without rigidity. In deep water, the main effect in the band 8–30 Hz on propagation loss of a solid bottom is to alter the detailed fluctuations of loss with range in response to the multipath interference of the waves, but the smoothed loss with range commonly measured from explosive sources is not significantly different for the test cases over the range intervals up to 200 k...

The Integral Solution of the Sound Field in a Multilayered Liquid‐Solid Half‐Space with a Liquid Bottom

The integral solution of the wave equation is derived by matrix methods for point sources of harmonic waves in a liquid layer of a multilayered half space of interbedded liquid and solid layers. The solution is in a form convenient for numerical computation on a high‐speed digital computer. The integral over wavenumber has singularities in the integrand for guided propagation and is conveniently transformed into the complex plane. By a suitable choice of contours, complex poles are displaced to an unused sheet of the two‐leaved Riemann surface, and the integral solution for the multilayered system reduces to a sum of normal modes plus the sum of two integrals, one along the real axis and the other along the imaginary axis. These integrals are evaluated by a Gaussian quadrature formula. Numerical computations are presented for propagation in the ice covered Arctic Ocean.

Effect of an Ice Layer on Arctic SOFAR Propagation

The effect of an ice layer on Arctic SOFAR propagation is investigated from the normal‐mode solution of the sound field in multilayered media. For an ice layer 3 m thick, typical of the central Arctic Ocean, the effect on pressure amplitudes at depth and dispersion is small, but even this thin ice sheet causes a large change in particle motions near the surface. Waves are elliptically polarized in the ice in the plane of propagation. Particle motion is retrograde elliptical at the surface and prograde elliptical at the bottom of the ice. This orbital motion is similar to that of flexural waves in the ice. Flexural waves generated by large‐scale ice movements at the boundaries of floes are the principal background noise in the ice. It is significant that a node of horizontal particle motion often does not occur at the same depth in the ice for both hydroacoustic waves and flexural waves. This effect may be useful to improve the signal‐to‐noise ratio when horizontal component geophones are used as listening...