Receiver Depth Research Articles

Characteristics of a set of noises used to test effects of underwater explosions on sea mammal hearing

As part of a study of temporary threshold shifts (TTS) expected to occur in sea mammals near underwater explosions, a set of ten explosion-like noises were synthesized [J. A. Clark, J. A. Young, and J. B. Gaspin, J. Acoust. Soc. Am. 105, 1048 (1999)]. The ten transient noises differed principally in maximum peak-to-peak pressure and were used in the TTS study as an ascending series of levels presented to two beluga whales and two bottlenose dolphins. The set of levels were selected from a larger set of predicted signatures of underwater explosions corresponding to a variety of charge weights and ranges in an underwater environment of interest which was characterized by charge depth, receiver depth, water depth, and sound-speed profile. This talk describes the ten transients used for the TTS studies. Time histories, frequency spectra, time-frequency distributions, and a variety of characteristics derived from these graphical descriptions will be presented.

Seismic attenuation above 10 Hz in southern California from coda waves recorded in the Cajon Pass borehole

We estimate the attenuation around Cajon Pass, southern California, for frequencies above 10 Hz, and we find total Q to exhibit only weak frequency dependence, ranging from ∼800 at 10 Hz to ∼1500 at 100 Hz. The intrinsic attenuation is approximately twice the level of the scattering attenuation. Measurements are made using earthquake seismograms recorded at 0–3 km depth using the multiple lapse time window method. The results are not dependent on receiver depth and are consistent with previous estimates of Q made from direct waves recorded at 2.5 km depth. Our Q values are therefore thought to be representative of the seismogenic crust, and the technique used is uncontaminated by the highly attenuating near surface, at least above ∼10 Hz. We also calculate Q from surface data between 1 and 10 Hz and find a clear change in the frequency dependence of both intrinsic and scattering attenuation at ∼10 Hz. Q exhibits strong frequency dependence below 10 Hz ( ∝ƒ1.8), consistent with previous studies in active tectonic regions, and only weak frequency dependence at higher frequencies ( ∝ƒ0.34). This change in behavior renders it unwise to extrapolate Q measurements outside the frequency range from which they were derived, for example, in earthquake‐source studies. Possible factors responsible for the apparent change in frequency dependence of Q are considered. Further work is required, however, to resolve the causes of this change.

Environmental constraints on sound transmission by humpback whales

Singing humpback whales in Hawaii produce a variety of sounds at high source levels (ca. 185 dB re: 1 μPa), in water 15–500 m deep. The utility of these sounds is limited by their detectability and discriminability (which is a function of their acoustic features and the channel through which they are transmitted). To investigate how shallow water propagation might constrain humpback whales use of sound, simulations were run to calculate optimum frequencies for environments frequented by singers [here optimum frequency is the f that maximizes maxzrI(zr,f), where I is intensity, and zr is receiver depth]. Simulations were performed using ORCA, an acoustic propagation model for multilayered acousto-elastic ocean environments based on normal mode theory [Westwood et al., J. Acoust. Soc. Am. 100, 3631–3645 (1996)]. The frequency band tested was 20–4000 Hz; the sub-bottom was taken to be a 50-m-thick sandy/silty layer with gradient, above a basalt half-space. Water depth, source depth and thermocline axis depth were varied across simulations. Source depth was found to be a major determinant of which frequencies propagated the farthest. Optimal frequencies decreased as source depth increased; they were typically above 500 Hz for source depths of 30 m or less. Humpbacks could affect transmission range (by factors of 2 to 3) by adjusting their position and sounds in response to environmental factors.

Monitoring the growth and mortality rates of pelagic fish with absorption spectroscopy measurements

The experimental geometry for absorption spectroscopy measurements, which was employed during Modal Lion, can be adapted for monitoring the growth and mortality rate of pelagic fish. During the juvenile stage, in which sardines grow from 3.5 to 13.5 cm over a period of about 1 year and mortality rates are relatively high, absorption spectroscopy measurements (together with complimentary biological and oceanographic measurements) conducted for a few days approximately once per month would permit investigation of the environmental determinants of these parameters. Measurement sites would have to follow the migration route of juvenile sardines over a period of about 1 year. To minimize cost and simplify logistics, a broadband omnidirectional source (instead of a parametric source) would be suspended from a moored ship or buoy. A relatively low source level (∼170 dB) would be adequate. Simulations will be presented of the temporal evolution of the resonance frequency, absorption coefficient, and transmission loss due to sardines between 0.6 and 6.0 kHz. Simulations will illustrate the sensitivity of transmission loss to source and receiver depth, and the average depth and thickness of absorbing layers. In addition, calculations that illustrate the potential of estimating the depth and thickness of absorbing layers by varying the depth of an omnidirectional source will be presented. [This work was supported in part by the Office of Naval Research.]

Mode function extraction from a VLA using singular value decomposition

A method is developed for extracting the depth-dependent mode functions from single-frequency measurements on a vertical line array as a source moves out in range. A matrix of the complex pressure field versus receiver depth and source range is formed, and the cross-spectral density matrix (CSDM) is computed. A singular value decomposition (SVD) is performed on the CSDM to obtain the orthonormal eigenvectors and the eigenvalues. Rearrangement of the normal mode equations reveals that under ideal conditions, the eigenvectors correspond to the mode functions and the eigenvalues are proportional to the modal source excitation and wave number. When two or more eigenvalues are nearly equal, the eigenvectors are not unique, and the correct mode functions are linear combinations of the eigenvectors. Other complications arise when the pressure field is not sampled adequately in depth or range. The procedure is applied to simulated data for a Pekeris waveguide and a realistic geoacoustic profile. Extracted mode functions from sources of opportunity are intended to be used for geoacoustic inversion of the sound-speed profile and bottom properties. [Work supported by the U. S. Army Research Office, AASERT Grant No. DAAH04-95-1-0486, and ONR.]

BROADBAND SOURCE LOCALIZATION IN THE GULF OF MEXICO

The Gulf of Mexico experiment involved a broadband source (100–600 Hz) transmitting pseudorandom noise. The acoustic field was received by five hydrophones positioned vertically in the water column. By performing a correlation between the received time series and the transmitted signal, an approximation of the impulse response for the ocean channel is obtained. This impulse response is readily interpretable to yield information about the experiment, especially its geometry including receiver and bottom depths. With this refined environmental information, we then apply matched-field processing to obtain the source position.

Simulated performance of an acoustic modem using phase-modulated signals in a time-varying, shallow-water environment

Underwater acoustic modems using coherent modulation, such as phase-shift keying, have proven to efficiently exploit the bandlimited underwater acoustical communication channel. However, the performance of an acoustic modem, given as maximum range and data and error rate, is limited in the complex and dynamic multipath channel. Multipath arrivals at the receiver cause phase distortion and fading of the signal envelope. Yet, for extreme ratios of range to depth, the delays of multipath arrivals decrease, and the channel impulse response coherently contributes energy to the signal at short delays relative to the first arrival, while longer delays give rise to intersymbol interference. Following this, the signal-to-multipath ratio (SMR) is introduced. It is claimed that the SMR determines the performance rather than the signal-to-noise ratio (SNR). Using a ray model including temporal variations of the shallow-water environment, the performance of the acoustic modem may be estimated. Simulations indicate that optimum performance is not necessarily found at receiver depths yielding the maximum total signal level, since the SMR may correspondingly be low due to strong intersymbol interference. [Work sponsored by the Danish Technical Research Council.]

Application of a stochastic inversion method for estimating the sound speed in the ocean bottom

A stochastic inversion method is applied to the problem of determining sound-speed profile in bottom sediment from modal eigenvalues, which can be observed from the FFT beamformed output of a horizontal array in a range-independent shallow-water waveguide. Under a noisy field, each observed eigenvalue has different variance depending on the signal-to-noise ratio (SNR) of modal peaks in the output wave-number spectrum; this difference in SNR originates from the relation between mode shape and source/receiver depths. Thus by using those variances in a covariance matrix involved in stochastic inversion method, it is numerically demonstrated that we can improve the estimation for the solution as compared to that obtained by using a uniform covariance matrix. It is also shown that combination of the same modal eigenvalues observed at the different source or receiver depth can reduce the variance of estimated solution. Application of the stochastic inversion method to real data is also discussed together with comparison with a regularization method. [Work support by JDA.]

Approximations for surface-wave propagation in laterally varying media

A number of techniques which exploit the waveforms of seismic surface waves depend on simple approximations for the character of the propagation process from source to receiver based on the representation for a stratified medium. Commonly the propagation path is assumed to lie along a great circle and to be representable by a path-averaged structure. The influence of structure near the source and near the receiver is included by using local modal formulations. However, the terms that depend on source depth and receiver depth in the stratified medium results are not purely local in character, and so care has to be taken to ensure a simple mapping between the modal shapes for the different structures. For frequencies less than 0.03 Hz, different crustal structures can be used at the source, near the receiver, and along the propagation path, provided that the change in crustal thickness is not more than 10 km between contiguous structures. Furthermore, for frequencies up to 0.035 Hz, it should be possible to use a single modal set in non-linear waveform inversions for perturbations of up to 5 per cent in lithospheric velocities along the propagation path. For propagation paths of length from 1000 to 4000 km, typical of a continental scale, the path-averaged structure approximation should be suitable for waveform fitting for frequencies in the range 0.01-0.03 Hz. The lower limit depends on the use of asymptotic approximations and the upper on the influence of heterogeneity on the modal content of the seismograms. Where surface waves cross a major structural boundary such as the continent-ocean transition, some aspects of the wavefield can still be represented using the path-averaged approximation.

VSP normal moveout stretch

The vertical seismic profiling, common depth‐point (VSPCDP) transformation is a technique used to image the reflected waves in an offset VSP. This coordinate transformation procedure maps reflection amplitudes from receiver depth and reflection times to lateral offset and reflector depth (Wyatt and Wyatt, 1981). The process involves the application of moveout correction followed by stacking. This moveout correction results in a distortion of the original waveform and a smear of the reflection event that consequently degrades the lateral resolution of the transformation (Owusu, 1991). Therefore, to determine the optimum processing parameters for the transformation we need to understand the effects of the VSP moveout stretch.

Theory and experiment of pulse propagation in shallow water

An experiment was conducted in shallow water with strong thermocline, located between the depths from 11 to 15 m. Explosive sources were detonated at 7 and 25 m, with receivers also at 7 and 25 m. Band‐limited signals with 1/3 oct bandwidth for different center frequencies and ranges were analyzed. The waveform structures are strongly dependent on source and receiver depths, center frequency, and range. A ray‐mode theory including beam displacement was used to calculate the waveform structures and to invert for the bottom‐loss versus grazing angle. The calculated waveform structures are consistent with the measured ones well.

An efficient method for computing Green's functions for a layered half-space with sources and receivers at close depths

AbstractWe propose an analytical method to compute efficiently the displacement and stress of static and dynamic Green's functions for viscoelastic layered half-spaces. When source and receiver depths are close, it is difficult to compute Green's functions of the layered half-space, because their integrands, whose variable of integration is the horizontal wavenumber, oscillate with only slowly decreasing amplitude. In particular, when the depths are equal, it is extremely difficult to compute the stress Green's functions, because their integrands oscillate with increasing amplitude. To remedy this problem, we first derive the asymptotic solutions, which converge to the integrands of Green's functions with increasing wavenumber. For this purpose, we modify the generalized R/T (reflection and transmission) coefficient method (Luco and Apsel; 1983) to be completely free from growing exponential terms, which are the obstacles to finding the asymptotic solutions. By subtracting the asymptotic solutions from the integrands of the corresponding Green's functions, we obtain integrands that converge rapidly to zero. We can, therefore, significantly reduce the range of wavenumber integration. Since the asymptotic solutions are expressed by the products of Bessel functions and simple exponential functions, they are analytically integrable. Finally, we obtain accurate Green's functions by adding together numerical and analytical integrations. We first show this asymptotic technique for Green's functions due to point sources, and extend it to Green's functions due to dipole sources. Finally, we demonstrate the validity and efficiency of our method for various cases.

Wavefield separation by 3-D filtering in crosshole seismic reflection processing

In processing crosshole seismic reflection data, it is necessary to separate the upgoing and downgoing primary reflections from each other, from the direct waves, and from other wave types in the data. We have implemented a 3-D f-k-k filter for wavefield separation that is applied in a single pass. The complete data set is treated as a data volume, with each sample defined by the three coordinates of source depth, receiver depth, and time. The filter works well because upgoing primaries, downgoing primaries, and direct waves lie in different quadrants in f-k-k space. The strongest multiples, including mode‐converted multiples, lie in the same quadrants in f-k-k space as the direct waves, so they are readily rejected together. Tube waves and mode‐converted primaries are also suppressed as most of the energy in these wave types lies outside the pass volume for P‐wave primaries. Some head wave and S‐wave primary energy will be passed by the filter; however, these waves tend to have low amplitudes and late arrival times, respectively, and will be smeared out on imaging with the P‐wave velocity field. We have processed a real crosshole data set using two different methods of wavefield separation: applying 2-D f-k filtering to common source gathers and applying a 3-D f-k-k filter to the whole data set. The migrated image produced after 3-D f-k-k filtering contains less coherent noise and consequently shows improved continuity of reflectors.

Vertical noise versus wind speed

The outpost sunrise exercise was conducted at a deep, high bottom loss area in the Pacific Ocean. Vertical 24-element arrays, nested for 75 and 150 Hz, were moored at the seabottom and at critical depth. The exercise was conducted during the fall, when high wind speeds are likely; wind speed (w) was recorded from a nearby buoy. For w≥4.5 m/s, omnidirectional noise depended on receiver depth and frequency band. At 300 Hz, ambient noise recorded below the critical depth was well correlated with wind speed, but at lower frequencies, this was not so. Correlation of beam noise level [BNL(φ)] for w≥4.5 m/s depended on vertical angle φ. Horizontally (φ=0°), the field was dominated by distant noise. BNL(φ) for φ≤15° closely tracked w. BNL(φ) was estimated at 65 and 130 Hz. BNL(90°) increased from 55 to 70 dB//μPa2 Hz-sr as w increased from 4.5 to 11 m/s. Theoretical BNL(φ) with source directionalities of a dipole, plane of monopoles, and volume of monopole planes are compared with data. A source modeled as a of a plane of monopoles between λ/5 and λ/3 depth best fist the data. [Work supported by the Office of Naval Research.]

July 1993 ARSRP acoustics experiments: Some results from R/V Al l i a n c e

In July 1993 SACLANTCEN participated in an experiment for the Acoustic Reverberation Special Research Program (ARSRP). The primary objective was to take high-resolution measurements to shed light on the detailed physical processes dominating the low-frequency scattering from rough topographic features and from deep sediment pond areas. A very detailed set of monostatic and bistatic scattering experiments were conducted (using frequencies from 150–1500 Hz) just west of the Mid-Atlantic Ridge near 26 °N and 47 °W. Results are presented that show monostatic and bistatic scattering from selected bathymetric features using both pulses and shots as sources. Area water depths ranged from 3300 to greater than 5200 m. The receivers were a horizontal array of 128 elements spaced at 0.5, 1, and 2 m, and a vertical array with 32 elements and the same spacings. Source depths varied from 120 to 1200 m and receiver depths ranged from 400 to 500 m. 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 are given using parabolic equation propagation models to interpret the data. Finally, comparisons of measured reverberation and modeled reverberation are presented.

On describing pulse signals from explosives in a waveguide

SUS charge data are examined in a 300-m waveguide off the straits of Florida. A realistic spectrum was employed for the explosive, and time arrivals and pulse forms were calculated for bandwidths of 200 and 400 Hz centered about 300 and 600 Hz, respectively. The waveguide is strongly downward refracted and data exist for arrivals at 6.7 and and 13.7 nautical miles. It is possible to predict the pulse widths and arrival times as well as the pulse forms correctly. In addition to describing the data, trends for the arrivals as a function of receiver depth are examined. Due to the ducting effect, depth dependence yields interesting information. [Work sponsored by the Naval Research Laboratory 6.1 Program Element 0601153N.]

Reply to comment on ‘Aspects of 1D seismic modelling using the Goupillaud principle ‘by Dan Loewenthal1

We thank Dan Loewenthal for his comments on our paper. It is very comforting to know that there are other scientists who recognize the importance of determining the source signature. We agree that it is possible to determine the source signature if the right measurements are made. In the case of a monopole source, Loewenthal points out that by measuring the pressure field at two depths the source signature may be extracted from the data. Using our own notation, in particular equations (2), (3), (4) and (S), in which the wavefield is decomposed into plane-wave components, this may be seen as follows. In the w p domain the pressures at the two receiver depths r , and r , are

Low‐frequency acoustic transmission at a shallow water site in the Gulf of Mexico

During Area Characterization Test I (ACT I), conducted in September 1992 in 100‐fathom water on the West Florida Shelf, acoustic transmission measurements were made covering the band 50 Hz to 1 kHz using explosive charges detonated at 90‐m depth. The test site had a sand–silt–clay bottom with a gentle slope of 0.135°, and up, down, and cross‐slope transmission data to ranges of 40 km were collected on collocated 25‐element vertical and horizontal hydrophone line arrays. The low sea states and downward refracting acoustic conditions prevalent during ACT I ensured that transmission loss was controlled by geometric spreading and bottom interactions. Transmission loss as a function of range, frequency, and receiver depth has been determined and compared favorably with predictions for both the flat and sloping bottom cases. In addition, the multipath temporal structure and the vertical arrival structure of the received signals have been analyzed and have indicated the role of bottom and subbottom characteristics to the properties of the received signal. These analyses of the signal structure have highlighted transmission mechanisms not readily apparent in transmission loss data and have emphasized the importance of knowing subbottom structure to a detailed understanding of shallow water transmission over sand bottoms. [Work supported by the ARPA Adverse Environments Program.]

Experimental Techniques in VSP Recording and Tube Wave Suppression as Applied in the Northern Mount Isa Basin

This paper reports the first implementation of a downhole airgun and tube wave baffle to VSP recording in Australia. A downhole airgun, offset from the wells, was recently applied to the acquisition of VSPs in petroleum exploration in the northern Mount Isa Basin. Despite excellent source-to-ground coupling, good bandwidth and repeatability of shots, the recorded data were of poor quality. This is considered to be a consequence of both interference from tube wave energy and geology, rather than VSP source characteristics. In one of the wells, recording of the full VSP wavelet could not be reliably achieved for receiver depths greater than 1000 m, despite using a high powered airgun with a volume of 120 cubic inches. Field displays showed that the data were severely contaminated by tube wave energy which, in some cases, was 60 dB above reflected signal strength. Attempts were made to reduce the tube wave in the field by lowering the mud column in the well and by attaching a tube wave baffle above the receiver. In both cases there was little improvement in relative signal strength. However, the lowered mud column did attenuate the tube wave at high frequencies. A significant improvement in data quality was achieved by aligning, enhancing and subtracting the tube wave in processing. Unfortunately the requirement for pre-drilled pilot holes negated offsetting the source further from the well to delay and attenuate tube waves during recording. This would obviously have been a better approach to tube wave reduction so that the dynamic range of the recording system was tuned principally to signal. Extensive fracturing above targets and multiple velocity inversions within the sequence drilled also contributed to the relatively poor signal strength.

Monostatic and bistatic reverberation results using linear frequency-modulated pulses

Estimates of reverberant scattering strength are presented as a function of range and bearing for selected monostatic and bistatic geometries of a reverberation experiment. Diffuse scattering strengths were found to be comparable (within ∼3 dB) for both monostatic and bistatic geometries. This experiment was performed off the west coast of Sardinia in approximately 2800 m of water during February of 1990. The receiver was a horizontal array of approximately 20 wavelengths. Source and receiver depths ranged from 50 to 120 m. The signals used consisted of linear frequency-modulated (FM) pulses centered near 1 kHz with durations ranging from 8 to 16 s and bandwidths ranging from 32 to 64 Hz. Monostatic and bistatic scattering strength values are estimated using uncorrelated (i.e., not match-filtered) pulses. The scattering strengths presented vary from −29 to −48 dB for ranges up to 40 km (but not data near fathometer returns). A method is described that shows different correlator gains against noise and reverberation. Comparisons of measured and modeled reverberation are presented using the generic sonar model (GSM) and curve fits with the decay in monostatic diffuse backscatter show good agreement with Lambert’s rule using a −32 dB constant. Finally, GSM is used to estimate grazing angles from which plots of measured scattering strength versus grazing angle in the 10°–26° interval are presented. Using this method, a Lambert’s rule constant of ∼−30 dB was a better fit to the data.