Ronald L. Dicus

Elements of a geoacoustic model of the upper crust

Elements of a geoacoustic model of the geologically young (< 10 million years) upper crust, the top few hundred meters of the basalt subbottom, are described. The model is based on analysis of low‐frequency reflectivity versus angle data at a 1 million‐year‐old sediment‐free site, as well as limited Deep Sea Drilling Project downhole logging results, and extrapolated seismic refraction measurements. The upper crust at this site, which is in close proximity to the crest of the East Pacific Rise, may be characterized by low interfacial velocities (Vp∼2800 m/s and Vs∼800 m/s), large gradients (3–5 s−1), and substantial rms roughness (∼5 m). The low‐inferred shear speed implies no shear critical angle. Hence, low‐frequency energy incident on the bottom at small grazing angles is in large part transmitted into the rock, refracted by the gradient, and reradiated into the water. Scattering loss occurs at both the initial incidence and the interaction of the refracted energy at the boundary. Low interfacial shear...

Impulse response estimation with underwater explosive charge acoustic signals

A procedure is presented for estimating the impulse response of a linear system from noise contaminated measurements of random input and output signals. The estimation requires ensemble averaged estimates of input and output noise spectra and a priori assumed spectra of the system function and input signal. A minimum mean‐square‐error estimator is derived and its theoretical signal‐to‐noise ratio is shown to be a slowly increasing function of the measured signal‐to‐noise ratios. The procedure is applied to a set of measured underwater explosive charge acoustic signals. For this application additional techniques are presented for synthesizing source replica signals and for accurately determining bubble pulse periods. The result of the deconvolution is a single sharp spike with at least 20‐dB bubble pulse suppression. With approximately 20‐dB signal‐to‐noise ratio in the measured signals the processed signal‐to‐noise ratios range from 10 to 13 dB and are all within 1 dB of theoretical values. Peak‐signal to...

Very-low-frequency under-ice reflectivity

This paper describes a direct method to model under‐ice reflection loss from analysis of ice draft data taken from a region of the Arctic near the FRAM IV experiment site. The water–ice boundary is modeled as a random distribution of infinite elliptical half‐cylinders fixed to a free surface. Burke and Twersky’s theory [J. Acoust. Soc. Am. 40, 883–895 (1966)] of scattering from a single cylindrical protuberance is used to calculate reflectivity from a distribution of scatterers. Individual ice ridge keels are identified from ice draft data resulting in a ridge keel depth distribution function spatially coincident with FRAM IV acoustic propagation paths. The scattering amplitude is weighted by the keel depth distribution function providing an effective ridge keel depth which is used to calculate the under‐ice reflectivity. The predicted reflection loss is in good agreement with those inferred from normal‐mode methods applied to FRAM IV acoustic field data received on a larger aperture vertical array.

Effects of Upper Crustal Geoacoustic Parameters on Low Frequency Sound

A seismo-acoustic paradigm of the upper crust is proposed, based on analysis of low-frequency reflectivity versus angle data at a 0.5 million-year old sediment-free site, and on spatially well sampled seismic refraction measurements at sediment-free and thinly sedimented sites in the Pacific Ocean. Crustal velocity measurements at thick sediment covered sites are evidently substantially higher, and are excluded from the analysis presented here. Interfacial compressional and shear speeds of thinly sedimented, geologically young upper crust (between 0.5 and 5 million years) at this site are estimated to be about 2800 m/s and 800 m/s respectively. At such sites sub-basement gradients are approximately 4 s-1. The rms roughness of the basement, which is Fresnel zone size dependent, has significant effects at frequencies as low as 10 Hz. Low shear speeds imply no shear critical angle. Hence, at low shear speed sites, low-frequency energy incident on the bottom at small grazing angles is in large part transmitted into the rock, refracted by the gradient, and reradiated into the water. Scattering loss occurs at both the initial incidence and the interaction of the refracted energy at the boundary. Low interfacial shear speeds lead to large grazing angles at the boundary for the transmitted shear waves, large wavenumbers and, hence large boundary scattering losses; small changes in interfacial shear speed produce large changes in subsurface boundary-scattering loss, and hence in the reflection coefficient. Sediment-free/thinly sediment-covered crustal shear speeds are projected to increase with age, eventually (at an unknown age) becoming faster than the speed of sound in water, resulting in critical angle reflection.

Compressional and Shear Wave Velocities in the Upper Crust

New reflectivity vs. grazing angle data at a number of thinly sedimented sites in the Pacific Ocean spanning 0.5 to 64 m.y. reveal that compressional and shear wave velocities, Vpand Vsof the uppermost crust increase with geologic age. Measurements between 40 and 64 m.y. reveal compressional and shear critical angles of 60 ± 3° and 9° ± 3° respectively. The corresponding compressional and shear speeds are 3090 ± 300 m/s and 1564 ± 15 m/s respectively. The relatively high estimated shear speeds are in sharp contrast to inferred shear speeds in 0.5 m.y. old crust, viz., 800 m/s, whereas the estimated compressional speeds in the older crust only slightly exceed Vpin young crust, which is about 2400 m/s.

Incoherent ocean bottom scattering from non‐Gaussian slopes in the Pacific.

Kirchhoff scattering theory was applied to measurements of bottom scatter to explain the time and angle spread of signals received on a horizontal array. The data were collected during ‘‘Pacific Echo II,’’ a joint experiment conducted by the U.S. Naval Research Laboratory and the Canadian Defence Research Establishment Pacific. Source signals were standard MK‐61 SUS charges detonated at 244‐m depth. The array was 300 m long, towed at 4 kn and 200‐m depth. The 64 hydrophone groups of the array were equally spaced. The site was characterized by an average sediment thickness of 25 m. Water depth was 5000 m permitting time separation of arrivals having different numbers of bottom bounces. A slope density function was constructed partially from seabeam measurements at the site and partially from documented deep tow measurements at other Pacific thin sediment sites. Predictions of omni‐ and beam‐formed time series and an angle spread function were calculated and compared with the data. Results show that the obs...

Low‐frequency measurements of bottom reflectivity at a thin sediment site in the North Pacific

Bottom reflectivity measurements were made in the North Pacific, during Pacific Echo I, using a low‐frequency, towed source. A MK VI source transmitted 50‐s FM sweep signals (5 to 15 Hz) to a vertical array of 16 hydrophones. The FM ramps were deconvolved and beamformed to separate direct and bottom arrivals in time and angle. In addition, ray theory predictions of the arrival structure, scaled by spreading loss, were beamformed. A comparison between the time integrated energy in the measured and theoretical bottom arrivals, led to a measure of the bottom reflectivity. FM ramps were processed along the source ship track to vary the bottom grazing angle from 10 to 75 deg. Reflectivity as a function of bottom grazing angle, smoothed over angle to reduce variance, will be presented. Theoretical computations of reflectivity from a simulated bottom that includes a sediment layer with sound‐speed gradient and an elastic basement, with shear and compressional speed gradients and attenuations, will be presented. ...

Towed array measurements of bottom time/angle spreads during Pacific Echo

Pacific Echo was a joint United States/Canadian experiment conducted in the Pacific in June 1986. As part of the exercise, explosive charges (MK‐61 SUS) were detonated along a closing‐range track near broadside to a towed array. A high‐level source (MK‐V1, 193 dB at 15 Hz) transmitted linear frequency‐modulated signals, sweeping from 5–15 Hz and back in 50 s. Measurements reported here are for a thin‐sediment site just south of the Tufts Abyssal Plain. Previous omnidirectional measurements in thin‐sediment areas are characterized by bottom time extensions on the order of a second. Time (angle) spread is defined here by identifying a two‐dimensional area in time/angle space containing an arrival, integrating over angle (time), convolving the resulting one‐dimensional sequence with a rectangular window, and determining the length of the window giving a peak −3 dB down from the total energy of the particular arrival. Observed time and angle spreads increased with grazing angle and frequency to values on the ...

An evaluation of matched field processor performance with environmental mismatch

The matched field processor is enjoying a technical renaissance because of increased computer computation speeds and environmental modeling capability. Its performance, however, is degraded by error or mismatch in waveguide steering vectors (theoretical fields) and by ambient noise. The effects of error in sound velocity profile, water depth, array depth, and sensor positions will be discussed in relation to their effect on source location bias, peak height, and peak width as a function of mismatch strength, frequency, range, and array length. Noise effects are not considered. To gain insight, the processor output was investigated for a simple multipath signal environment in which acoustic energy travels along straight line trajectories but with allowance for different effective wave speeds. At sufficient ranges it can be shown that the processor is equivalent to a linear sum of focused beamformers, each steered to a particular multipath. Error in the waveguide steering vectors has the effect of different...

An experimental study of the coherent under‐ice reflectivity of sound in the Greenland Sea Marginal Ice Zone (MIZ)

The coherent component of acoustic under‐ice reflectivity was investigated as a function of frequency and grazing angle in the MIZ. Measured reflectivity was compared with scattering theory predictions to test the hypothesis of ridge scattering dominance in the MIZ. Explosive source acoustic signals were received on a vertical array, during the Marginal Ice Zone Experiment (MIZEX 84), and deconvolved to separate the direct and ice‐reflected arrivals. Ice‐reflected signals received on individual hydrophones at different ranges and depths, and on different days, were aligned in time and coherently ensemble averaged within a moving 10‐deg grazing angle window. The resulting reflectivity was found to decrease with increasing frequency from 64–256 Hz and with increasing grazing angle from 12–35 deg. Comparison was made with predictions from smooth elastic plate theory, perturbation theory for a rough pressure release surface, perturbation theory for a rough elastic surface, Twersky theory for hemispherical bos...