Receiver Depth Research Articles

The impact of backscatter angular dependency on reverberation levels in shallow water under downward refracting conditions

For downward refracting conditions in shallow water over a sandy bottom it was found that propagation loss was dependent on the source and receiver depth [P. D. Herstein etal., J. Acoust. Soc. Am. 92, 2302(A) (1992)]. The analysis is now extended to reverberation levels, considering the grazing angles corresponding to various source and receiver depths and both the first and second power sine of the angle dependencies of the backscattering. This is compared to a compilation of shallow water backscattering results.

Multipath structures of pulse signals in shallow water with a thermocline

It was observed that the pulse signals in shallow water with a thermocline have regular multipath structures, which are strongly dependent on the depths of source and receiver. When both the source and receiver are located above the thermocline, the received signal consists of a train of almost equispaced wave packets and the time interval T between packets is related to the speed-profile parameters. When the source is below the thermocline and the receiver is above, the received signal consists of a train of wave packets with time intervals τ and (T−τ), where τ is dependent on the distance from the source to the bottom. The formulas of intervals T and τ were deduced by the ray theory, and the calculated values of T and τ are consistent with measured ones. The normal-mode theory was used to model the waveforms of pulse signals, and the effects of speed-profile, geoacoustic parameters, signal parameters, and source and receiver depths on waveform structure were studied.

Modeling of the acoustic propagation through the Gulf Stream

Propagation loss modeling has been done in support of the experimental cruise ‘‘Maple Gelato’’ conducted in an area of the Gulf Stream, east of Cape May, New Jersey (by the SACLANTCEN, Italy, and the Defence Research Establishment Atlantic, Canada). The experiment was conducted in summer conditions (September 1992), and seamounts were also present in the area. Although the data will not be presented at this time, the modeling of the propagation through the front, done with the SACLANT parabolic equation model, will be presented for frequencies ranging from 10 to 300 Hz, and for sensors distributed along a 300-m vertical array. Preliminary results show that the effect of the front is to redistribute the energy in space, and to shift convergence zones. The acoustic levels within the two first convergence zones however are not affected substantially, although this depends on source and receiver depths. Seamounts, depending on their locations relative to the source, can counteract the effects of the fronts. The modeling results will be compared with results obtained in other frontal areas, to emphasize the particularities of the Gulf Stream.

Preliminary bistatic and monostatic scattering results from a steep basin feature in the Tyrrhenian Sea

A feature scattering experiment was conducted by SACLANTCEN off the island of Palmarola in the Tyrrhenian Sea in March 1991. Estimates of this feature scattering are presented as a function of range, source type, and source depth. Area water depths ranged from 200 to greater than 3800 m. The receivers were a horizontal array of 64 elements spaced at 0.5, 1, and 2 m and a vertical array with 32 elements and the same spacings. Source depths varied from 90 to 1800 m and receiver depths ranged from 90 to 150 m. Depth charges were used as sources, as were low-frequency cw pulses and linear FM pulses of various pulse durations and bandwidths. Basin wide received reverberation levels and scattering strength values are also presented as functions of the estimated scatterer position on area bathymetry maps. Comparisons of shot and pulse data reverberation levels are presented. Estimates of monostatic bottom scattering strengths versus grazing angle are given using generic sonar model (GSM), PAREQ, and KRAKEN to interpret the data. Finally, comparisons of measured reverberation and modeled reverberation are presented.

Detection and measure of the shear‐wave birefringence from vertical seismic data: Theory and applications

An original method is presented that allows us to measure the local shear‐wave birefringence properties over any depth interval. It requires the acquisition of two shear‐wave vertical seismic profiles (VSPs), each with different initial polarizations of the shear wave. The method is based on the estimation of a two by two matrix (called the propagator matrix) that represents a linear operator between two states of polarization. No information is required about layering above the zone of interest (in particular, about the weathering zone). If these two states of polarization correspond to the direct downgoing shear wave at two different depths [Formula: see text] and [Formula: see text], the operator represents the transmission properties between the two depths. Under the previous hypothesis, this operator is independent of the source polarization and can be accurately estimated by a least‐squares method in the frequency domain. Physically, this operator is a multicomponent deconvolution, whose column vectors represent the state of polarizations at a depth [Formula: see text] for two linear and mutually perpendicular polarizations at depth [Formula: see text]. This allows for the measurement of the birefringence properties in all azimuthal directions to determine the directions for which a linearly polarized shear‐wave propagates. In addition, the method can be applied to perform the deconvolution of the upgoing wavefield by the downgoing wavefield to obtain a reflection matrix. Then the matrix can be interpreted in terms of anisotropy below the receiver depths (particularly below the well bottom) and in terms of anisotropy of the reflector itself. The proposed method is validated on synthetic data and is applied to real data from the Paris basin. For this particular data set, the birefringence is located in two layers; the first layer consists of unproductive sands and clays while the second one corresponds to a carbonate oil reservoir from the Dogger formation. The natural directions in both layers are very close to the main directions known for the regional stress field. The presence of fractures in the reservoir layer can explain the strong birefringence ratio (>6 percent).

Sensitivity of shallow water transmission loss to source and receiver proximity to a hard bottom under downward refracting conditions

Under downward refracting conditions, raypaths from a shallow source will tend to have grazing angles at a hard bottom that are greater than the critical angle and therefore suffer a relatively large loss per bounce. As the source depth increases, lower grazing angles can be obtained. When the grazing angle becomes less than the critical angle, bottom loss per bounce is significantly reduced allowing a possible reduction in propagation loss. An analysis is made for a North Atlantic shallow water area south of Long Island under summer conditions to determine the sensitivity of transmission loss to changes in source and receiver depth for ranges up to 50 km. The results are compared to the previous results of Cole and Podeszwa [B. F. Cole and E. M. Podeszwa, J. Acoust. Soc. Am. 41, 1479–1484 (1967)].

Sound spatial correlations in shallow water

A method is presented for the estimation of the vertical and longitudinal correlations averaged over range and depths in a multipath channel. The resulting correlations are shown to be the integral of the vertical and longitudinal correlation factors weighted by the angular energy density. A correlation peak occurs when the receiver depth approaches either the source depth or its conjugate. The longitudinal correlation is shown to be reciprocal while the vertical is not. Numerical results are developed to gain additional insight into the physical features involved. Some field measurements of spatial correlations obtained in North China Sea are included.

Acoustic modeling through a Mediterranean water lens (Meddy).

Mediterranean water lenses (Meddies), as reported by Armi [L. Armi and W. Zenk, J. Phys. Oceanogr. 14, 1560–1576 (1984)], are small warm-core eddies with a large width (100 km) to thickness (0.9 km) ratio. Centered at a depth of approximately 1000 m, they were observed in the Canary Basin of the North Atlantic Ocean. These are unlike many warm-core eddies that extend to or near the surface [P. A. Nysen, P. Scully-Power, and D. G. Browning, J. Acoust. Soc. Am. 63, 1381–1388 (1978)], and hence may cause unique propagation anomalies for certain source–receiver configurations. A set of reported oceanographic specifications of a Meddy was converted to sound-speed profiles which were merged with historical data below a depth of 2000 m. Acoustic modeling runs at 200 Hz were made across the Meddy using the IFD/PE model. The results were compared to normal propagation predictions for the area. The characteristic shift of the convergence zones is observed, but as might be expected the impact is relatively small for shallow source–receiver configurations and greatest for source and receiver depths near the center of the Meddy.

Predictions and observations of seafloor noise generated by sea surface orbital motion.

A model is developed for the prediction of the seismo-acoustic noise spectrum in the microseism peak region (0.1 to 0.7 Hz). The model uses a theory developed by Cato [J. Acoust. Soc. Am. 89, 1096–1112 (1991)] for an infinite depth ocean in which the surface orbital motion caused by gravity waves may produce acoustic waves at twice the gravity wave frequency. Using directional wave spectra as inputs, acoustic source levels are computed and incorporated into a more realistic environment consisting of a horizontally stratified ocean with an elastic bottom. Noise predictions are made using directional wave spectra obtained from the SWADE surface buoys moored off the coast of Virginia and the SAFARI sound propagation code, with a bottom model derived using wave speeds measured in the EDGE deep seismic reflection survey. The predictions are analyzed for noise level variations with frequency, wave height, wind direction, and receiver depth. These predictions are compared to noise measurements made in ECONOMEX using near-bottom receivers located close to the surface buoys. Good agreement is found between the predictions and observations under a variety of environmental conditions. [Work supported by ONR.]

Air‐gun signatures and the minimum‐phase assumption

The air‐gun array signature is close to minimum‐phase as a function of continuous time, in the sense that for processing purposes its phase spectrum can be derived from the Hilbert transform of the logarithm of its amplitude spectrum. This phase spectrum is different, however, from the minimum‐phase spectrum that is estimated by spiking deconvolution for a sampled and time‐windowed version of the signature. As a consequence, there can be large phase errors when spiking deconvolution is applied to an air‐gun signature or to a recording instrument response. The errors can be shown to consist primarily of a time shift and, at least visually over a limited bandwidth, a phase rotation of the output wavelet. The time shift is introduced by time sampling, while the phase rotation is caused by the spectral smoothing generated by time windowing. If the seismic wavelet as a whole, and not just the air‐gun signature, is minimum‐phase, then the total residual phase error after spiking deconvolution, including also the error due to data noise, can also be shown to be close to a time shift and a phase rotation. This may be physical justification for the phase rotation schemes that are often successful in matching seismic data and well‐log synthetics. The minimum‐phase assumption can be used for statistical air‐gun array signature deconvolution, providing that a limited amount of deterministic information (the instrument slopes and the source and receiver depths in the approach used here) is available to guide the process in those areas of the spectrum that are critical to the phase computation. Date examples show that, with care, almost identical results can then be obtained from either purely statistical deconvolution or deterministic deconvolution plus statistical deconvolution of multiples and ghosting.

Bistatic and monostatic scattering results off the coast of Portugal.

Estimates of ocean-bottom scattering strengths are presented as a function of range and bearing for a number of monostatic and bistatic geometries achieved during a SACLANTCEN sea trial off the coast of Portugal in September 1990. Area water depths ranged from 200 to greater than 5000 m. The receiver was a horizontal array of 64 elements spaced at 0.5 and 2 m. Source depths were about 100 m and receiver depths ranged from 90 to 150 m. Depth charges were used as sources, as were low-frequency cw pulses and linear FM pulses of various pulse durations and bandwidths. Received reverberation levels and scattering strength values are presented as functions of the estimated scatterer position on area bathymetry maps. Estimates of monostatic bottom scattering strengths versus grazing angle are given using Generic Sonar Model (GSM) to interpret the data. Finally, comparisons of measured reverberation and modeled reverberation are presented using GSM.

Receiver depth selection for passive sonar systems

Since tactical acoustic systems such as towed arrays can be deployed at various depths, the authors address the question of what depth is optimal. This question is considered principally from the point of view of optimum propagation conditions, employing two deep-water scenarios representing summer and winter conditions in the western Mediterranean. Two simple rules-of-thumb are derived from these results: first, if the source depth is known, then the best receiver depth is either the source depth or the conjugate depth (where the sound speed is the same as that at the source). Second, if the source depth is unknown, then a receiver depth where the ocean sound speed is as low as possible is optimal. These two rules are qualified with a few disclaimers. In the first place they are derived under the assumption of a range-invariant environment. In addition, a definition of optimality requires numerous assumptions that may not always be appropriate. Both these guidelines and their domain of applicability are discussed. >

Acoustic propagation in the western Greenland Sea frontal zone

A study, utilizing numerical experiments, of the environmental acoustic effects of the western Greenland Sea Frontal Zone in the summer of 1983 along a 185-km west to east transect is presented. The transect begins at the edge of the marginal ice zone in the East Greenland Current, then crosses the East Greenland Polar Front and terminates in the water of the Greenland Sea gyre. The transect contains several mesoscale eddylike features and a sharp front between Return Atlantic Intermediate Water and Greenland Arctic Intermediate Water, whose effects on an acoustic field have not been previously reported. This front can impart >15-dB increase in loss in <10 km. The effects of the frontal zone on acoustic propagation as a function of location and depth of the acoustic source and depth of the receiver for a nominal frequency of 50 Hz are discussed. Significant differences, sometimes greater than 20 dB, in the acoustic field were observed and are explained in the analysis for variations in any of the source and receiver parameters. The sound-speed structure on either side of the frontal zone makes propagation loss and the dominant acoustic modes very dependent on direction and location. The direction of propagation with the lowest loss (as much as 15-dB difference along the entire transect) can change with a modest shift in source from a 10-m depth to 150 m. As compared to the nonfrontal environment, propagation in the presence of the frontal zone, for any of the source and receiver depths studied, can result in changes in the levels and distribution of energy in the acoustic field by as much as 15 dB as well as changes in the mode of propagation (e.g., refracted/surface reflected to SOFAR). It can also result in an increase (>10 dB) in the sensitivity of propagation loss to changes in receiver depth (10–150 m) compared to the nonfrontal zone environment. A significant finding is that a longer transmission range (93 km) through the waters of a frontal zone can reduce the effects of crossing the front itself by as much as 15 dB. It was also observed that 75 km of upslope propagation that included two oceanic fronts was unaffected by a decrease in bottom depth of 2000 m within the 75-km range.

Theoretical and measured underwater noise from surface wave orbital motion

This paper develops a theory of sound generation by orbital motion of sea surface waves (as distinct from motion directly resulting from wave breaking such as bubble oscillation) and compares the noise predictions with measurements in a carefully controlled experiment. Theory and measurement were found to agree within the experimental errors. The mechanism is also known as the nonlinear interaction of surface waves and has been addressed by a number of authors. The approach of this paper differs from other models in that it avoids the use of the commonly applied perturbation expansion, and calculates the total noise field whereas others have limited their estimates to sound production from standing waves and waves that closely approximate standing waves. It is shown that while standing waves result in distributed dipoles with vertical axes, other wave interactions result in dipoles with axes inclined to the vertical so that there are components with both horizontal and vertical axes. The relative contribution of the horizontal dipole components to the noise field is of the same order of magnitude as that of the vertical dipole components. This paper therefore predicts higher noise levels and different directionalities, and also determines the contribution from the evanescent or near field that dominates for receiver depths less than several hundred meters (depending on frequency) resulting in substantially higher noise levels at shallow receivers. On the basis of this and previous work there seems little doubt that this mechanism is a significant source of noise in the ocean, usually dominant from about 0.2 to 5 Hz.

Measured estimates of monostatic and bistatic reverberant scattering strength using low-frequency linear FM pulses

Measured estimates of ocean-bottom reverberant scattering strength are presented as a function of range and bearing for a number of monostatic and bistatic geometries achieved during two separate sea thais. One trial was performed off the west coast of Sardinia in approximately 2800 m of water during February of 1990, and the other was performed in the North Sea in approximately 1000 m of water during October 1988. The receiver in both cases was a horizontal array of 64 elements spaced at 0.5 m. Source and receiver depths ranged from 50–100 m. The signals used consisted of linear FM pulses centered near 1 kHz, with durations ranging from 4–16 s and bandwidths ranging from 32–64 Hz. Scattering strength values are estimated using uncorrelated (i.e., not match-filtered) pulses. The values presented vary from − 30 to − 45 dB for distances up to 20 km. Also presented are plots that show the difference between uncorrelated and correlated received levels. These plots demonstrate the extent of correlator gain against noise and reverberation.

Transversely isotropic phase velocity analysis from slowness estimates

Near‐offset vertical seismic profiling (VSP) is an established technique to obtain interval velocities for lithology characterization, time‐to‐depth calibrations, and seismic data processing. Unfortunately, these velocities are often insufficient to distinguish lithology types and can disagree with surface seismic stacking velocities. Multiple‐source offset VSPs provide additional travel time information for calculating interval anisotropic velocities that can help discriminate lithology. In this paper, a technique is developed for computing transversely isotropic phase velocities over intervals less than the dominant seismic wavelength. Independent estimates of vertical and horizontal slowness are made for each receiver depth and source offset pair. These provide the magnitude and direction of phase velocity for calculating the in situ elastic properties of a transversely isotropic medium. This technique requires the assumption of lateral homogeneity because horizontal slowness components are related to surface properties between sources and not to rock properties between receivers at depth. Factors that violate this assumption such as source elevation differences and dipping layers are evaluated with synthetic data. Six impulsive source P wave offset VSPs in a 663‐m well in east Texas are analyzed and a near offset SH wave vibrator VSP provides vertical shear wave phase velocities. Strong transversely isotropic velocity variations observed in the Tertiary and Cretaceous sediments penetrated by this well correlate with lithology types, suggesting that this technique is sensitive to local properties at the receiver. These results may not indicate exact intrinsic elastic properties and could be due in part to dipping layers, vertical heterogeneities, and unknown local surface variations. Percent “phase velocity” anisotropy appears to increase from 6 to 20% for P wave and 10 to 50% for SV wave with increasing amounts of calcareous material and shale in the rock. (Percentage anisotropy is defined as the ratio of the horizontal to vertical velocity for P waves and the ratio of velocity at 45° to vertical velocity for SV waves.) Sandstones and chalk exhibit the least amount of anisotropy, while the shales and calcareous shales exhibit the most.

Directivity of an acoustic monopole in a floating ice plate and implications to Arctic ambient noise

Predictions were made of the directivity of higher‐frequency (1‐kHz) acoustic radiation into the water from a monopole source located in a floating ice plate (2.4 m thick). Theoretical calculations were conducted using the SAFARI propagation routine [H. Schmidt and J. Glattetre, J. Acoust. Soc. Am. 78, 2105–2114 (1985)] and by the method of steepest descents (P. J. Stein, “Monopole Source in a Floating Ice Plate,” MIT/WHOI Ph.D. thesis, 1986). The models show close agreement with each other and predict a strong downward directivity with most of the energy radiated at grazing angles greater than 70°. This implies that (1) higher‐frequency impulsive ice noises detected at an omnidirectional hydrophone are likely radiated from a relatively small area of ice directly above the receiver (the area increasing with receiver depth) and (2) the nonimpulsive (background) ambient noise at higher frequencies is dominated by radiation from ice events located within a horizontal range radius equal to roughly one water depth. Modeling also shows that nulls can occur for some radiation angles for particular source depths within the ice. Also, only small effects of a more realistic ice strength vertical profile over a uniform profile are seen. [Work supported by ONR.]

Modeled time variability of acoustic propagation through a Gulf Stream meander and eddies

A numerical modeling study of the time variability of acoustic propagation through a Gulf Stream meander and eddies is presented. Both a cold-core (cyclonic) eddy and a warm-core (anticyclonic) eddy are included. Time periods of 2, 4, and 7 days; a frequency of 25 Hz; source and receiver depths of 150 m; and propagation out to 180 km are considered. Propagation conditions assumed an absorbing bottom and no azimuthal coupling of energy. Results indicate that variations in the locations and propagation loss levels of convergence zones (CZ) in 2 days can be significant. In the first CZ, the losses can change by more than 5 dB and shift in their pattern range location by as much as 10 km. Changes in the environment over a 2-day period can be such that perturbations to the acoustic field caused by the environment within the first CZ can be nullified by the environment between the first and second CZs. The differences between propagation with the eddies present and not present were also observed for ranges out to the first CZ. For a source located in the center of a cold-core eddy, the propagation loss in the CZ increased by 3 dB and the range decreased by about 5 km. With the source located within the eddy but off center, the loss showed little change in level but the range again decreased by about 5 km. For a source located in the center of a warm eddy, the propagation loss in the CZ increased by about 3 dB and the range increased by 7–10 km. With the source located off center, the loss showed little change in level and range location.

Depth dependence of noise resulting from ship traffic and wind

Under conditions of distantly generated noise, the noise level is found to decrease with depth in the mid-northeastern Pacific. These data show a decrease in noise level greater than 25 dB between critical depth and the ocean bottom. A result of this decrease is that locally wind-generated noise can be detected on near-bottom receivers for wind speeds less than 10 kn. It is shown that the noise level generated form local sources such as wind and nearby shipping is almost independent of receiver depth. The differences in spectra shape between the distant shipping noise and wind-generated noise and the low noise levels detected near the ocean bottom allow the measurement in the frequency band at 200-500 Hz of local wind noise level for wind speeds less than 10 kn.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Depth dependence of high‐frequency ambient ocean noise

Little or no published data exist on the depth dependence of ambient ocean noise at frequencies above 9 kHz. In this higher frequency region, absorption becomes a major effect in reducing surface‐generated noise as receiver depth increases. To investigate this effect, two experiments were conducted. The first, conducted in the Tongue of the Ocean, off Andros Island in the Bahamas, utilized bottom‐mounted hydrophones at a depth of 1500 m to observe ambient noise levels covering sea states 1–3 over a frequency span of 13 to 37 kHz. Near surface noise measurements were also made for comparison. The second experiment, conducted in Hawaii, covered a 9‐ to 25‐kHz span with bottom‐mounted hydrophones at depths of 751, 1173, 1769, 2481,3981, and 4505 m. Results in both cases show a reduction in ambient noise levels with increasing depth and frequency, as predicted from models developed by Urick and others. Closest agreement was observed with the assumption of a dipole surface intensity radiation pattern at the higher sea states.