Vertical Array Of Receivers Research Articles

Applying the time-reversal operator to detection in a shallow-water environment

The decomposition of the time-reversal operator (DORT) is a method of isolating scatterers with a multiple-source/multiple-receiver system [C. Prada et al., J. Acoust. Soc. Am. 99, 2067–2076 (1996)]. A series of simulations is shown that elucidates the resolution of a DORT system in a reverberant, shallow-water environment. The system consists of vertical line arrays of sources and receivers in monostatic and vertically bistatic configurations. Two basic resolution problems are considered: the ability to discriminate two scatterers in the water column and the ability to detect a single scatterer in the presence of bottom clutter. The performance of the DORT system is shown relative to array configuration, location of scatterer in the water column, and the level and type of bottom clutter. [Work supported by ONR.]

Geoacoustic inversion techniques applied to field data

Reliable and efficient techniques for solving geoacoustic inverse problems have been developed. A coordinate rotation technique can be used to identify the best resolved combinations of parameters for a given frequency and configuration of hardware [M. D. Collins and L. Fishman, J. Acoust. Soc. Am. 98, 1637–1644 (1995)]. Problems involving a vertical array of receivers at short range can be solved in a few seconds with an implementation of the self starter that is based on the method of undetermined coefficients [D. K. Dacol and M. D. Collins, J. Acoust. Soc. Am. (accepted for publication)]. These techniques are applied to data that were obtained in the Straits of Florida during the KWIX ’98 experiment. [Work supported by ONR.]

Wave number sampling at short range

The self-starter is an efficient approach for solving geoacoustic inverse problems involving a vertical array of receivers located on the order of ten wavelengths from a source [R. J. Cederberg and M. D. Collins, J. Acoust. Soc. Am. 22, 102–109 (1997)]. With this approach, accurate solutions can be obtained by sampling the wave number spectrum at approximately ten points. There have been unsubstantiated claims that the spectral solution provides similar efficiency when implemented with an approach that involves perturbing the integration contour off the real line [F. B. Jensen et al., Computational Ocean Acoustics (American Institute of Physics, New York, 1994), pp. 231–240]. It is demonstrated that this quadrature scheme breaks down at short ranges and that the self-starter solution is about an order of magnitude faster. These conclusions are based on comparisons of the rational approximations associated with the self-starter and the quadrature scheme, comparisons of acoustic fields for particular problems, and a simple analysis of the spectral integral. These conclusions are not surprising since the rational approximation associated with the self-starter is based on the analytic evaluation of the spectral integral while the other rational approximation is based on numerical integration. [Work supported by ONR.]

The Use of Wave-field Directivity for Velocity Estimation: Moving Source Profiling (MSP) Experiments at KTB

—Within the "Integrated Seismics Oberpfalz 1989 (ISO89)" a three-component Moving Source Profiling (MSP) experiment, also named walk-away VSP, was carried out at the drilling site of the "Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland (KTB)" in Germany. Analysis of transmitted waves traveling from the source locations at the surface down to the receiver array in the borehole reveals velocity information about the illuminated part of the subsurface. Complementary to the widely used evaluation of travel-time perturbations to locate velocity inhomogeneities we suggest the use of the directivity of transmitted wave types down in the borehole. To determine the wave-field directivity we focus on transmitted arrivals by employing principles of "Controlled Directional Reception (CDR)." We calculate local slant-stacks for three different depth positions as a function of the source offset, thus obtaining the variation of the vertical slowness (vertical ray parameter) of incident waves along the horizontal source profile and the vertical receiver array. The slowness data combined with travel times are interpreted by forward modeling taking into account geological information of the survey area. Our findings confirm results from gravity measurements which suggest the existence of large amphibolite/metabasite complexes in the vicinity of the borehole. The described method is also used to identify P-to-S converted energy originating from fracture zones above the receiver array and to locate the region in which conversion occurs.

Acoustic tomography of a coastal front in Haro Strait, British Columbia

An experiment validating the concept of acoustically focused oceanographic sampling (AFOS) was recently implemented in Haro Strait, British Columbia (Canada). Four 16-element vertical receiver arrays were moored around the location of a coastal front driven by estuarine and tidal forcing. Various signals were transmitted from array to array and from a moving source to the arrays over a period of five weeks. Tomographic signals were transmitted over a wide frequency band (150 Hz to 15 kHz). The novelty of the Haro Strait data set resides in its unusual tomographic features: ranges are short (less than 3 km), sound speed perturbations are small (2 to 3 m/s), and currents are relatively strong (3.5 kts). Light-bulb-generated wideband acoustic signals are used in this paper in conjunction with local nonacoustic measurements in order to image the three-dimensional sound speed and current fields within the water mass enclosed by the moored arrays. The combined use of integral and local data leads to a significant decrease of the field estimate uncertainty while maintaining a coverage of the area not achievable by nonacoustic means.

Fluctuations in acoustic propagation seen in the SWARM 95 experiment

Fluctuations in acoustic propagation as seen in data collected during the SWARM 95 experiment are analyzed and related to the oceanographic environment. Acoustic data were collected on a 32 element vertical receiver array which spanned the 88 m water column from depths of 23 to 85 m. The receiver array was located 42 km seaward from two fixed acoustic projectors which transmitted a 224 Hz PRN signal (16 Hz bandwidth) and a 400 Hz PRN signal (100 Hz bandwidth). Intensity statistics, such as scintillation index (SI) and the probability distribution function (PDF), are discussed as a function of depth and time for narrow-band and broadband (replica correlation) processing. Broadband SI is shown to be less than the narrow-band SI which has a typical value of 2. The PDF is shown to be neither a Rayleigh nor a log-normal distribution and evidence is presented that unsaturated scattering processes are involved. [Work supported by the Office of Naval Research.]

Comparison of the performance of global optimization methods with perturbative methods in the estimation of ocean bottom properties from multifrequency data

During 1988 a series of acoustic propagation experiments was conducted in the Hudson Canyon area off the coast of New Jersey. It included synthetic aperture experiments in which a source transmitting multiple tones was towed toward/away from a vertical array of receivers. By this means, a two-dimensional structure of the acoustic field was obtained at eight different frequencies. This data set is used to estimate the bottom acoustic properties using global optimization approaches such as simulated annealing and genetic algorithm and to compare their performance with that of perturbative methods, which because of their simplicity have some advantages over the global methods.

The extraction of a target scattering response from measurements made in a range-dependent shallow-water environment

The free-field scattering response of a target may be extracted from measurements made over long ranges in a range-independent shallow-water environment by the use of vertical array of sources and vertical array of receivers. The procedure requires the generation of a low-order shallow-water mode by the source array and the extraction from the received echo at the receiver array of a low-order shallow-water mode. The algorithm is generalized to range-dependent environments where additional complications exist due to the mode coupling produced by the range dependence in the shallow-water environment, which exists in addition to the mode coupling produced by the scatterer. Results of numerical simulations are presented for a ribbed cylindrical shell with hemispherical end caps. The target scattering response is computed using the finite elements and infinite elements method, and the propagation is computed using the parabolic equations method. [Work supported by ONR.]

Imaging of crustal structures from vertical array measurements

Walk-away or offset Vertical Seismic Profiling (VSP), a technique widely used in oil and gas exploration, was carried out in crystalline rock at the KTB (Kontinentales Tief-Bohrprogramm) drill site in Germany. The walk-away VSP experiment comprised a shot profile of 10 km length at the surface across the well and a vertical receiver array in the borehole between 3210 m and 3560 m depth with geophone intervals of 25 m. To identify reflected/refracted seismic energy from crustal structures the ‘Controlled Directional Reception’ (CDR) method was used. The key element is the vertical geophone array which allows the determination of the vertical slowness with great accuracy due to the excellent coherence of waveforms. Slant stack processing and subsequent directional migration produced a clear image of mid-crustal reflectors at 10 to 12 km depth. Additionally, a number of dipping faults were detected in the upper crust, that can be connected to surface geology. As the walk-away VSP experiment was part of an integrated seismic measurement program, the results were compared with those from 2D and 3D surface surveys and borehole measurements. This led us to conclude that hydraulic fracture zones play an important role in the reflectivity of crystalline rocks at KTB.

Modal evolution and inversion for seabed geoacoustic properties in weakly range-dependent shallow-water waveguides

In a shallow-water ocean environment, the range dependent variation of the geoacoustic properties of the seabed is one of the crucial factors affecting sound propagation. Since the local modes of propagation depend on the spatial changes in the bottom sediments, the local eigenvalues of these modes are useful as tools for examining the range dependence of the sediment properties. In order to extract the local eigenvalues from measurements of the pressure field in a laterally inhomogeneous waveguide, the zeroth-order asymptotic Hankel transform with a short sliding window is utilized. The local peak positions in the output spectra differ from the local eigenvalues due to both the range variation of the local modes and the interference of adjacent modes. The departure due to the former factor is evaluated analytically by using the stationary phase method. In order to reduce the error induced by the latter factor, mode filtering is utilized by incorporating data from a fixed vertical array of receivers. The methods developed are applied to simulated pressure field data as well as experimental field data, and it is shown that the range evolution of the local modes can be successfully estimated. In addition, field measurements are used to demonstrate that the modal trajectories in range can be used to infer the range-dependent geoacoustic properties of the seabed.

Acoustic propagation experiments in the Hudson Canyon: Inversions

In September 1988 a series of acoustic propagation experiments was conducted in the Hudson Canyon area. It included a synthetic aperture experiment in which a source which transmitted a set of four pure tones was towed toward/away from a vertical array of receivers. The data obtained at 50 and 175 Hz are used to determine the compressional wave-speed profile for the bottom using the modal inverse method. The ability of the inferred model to predict the measured field at 50 Hz and at other higher frequencies is examined. Further, the applicability of the model to a contiguous area is examined to investigate the range-dependent nature of the bottom in the region. [Work supported by ONR.]

Obtaining the ocean index of refraction spectrum from the acoustic amplitude fluctuations

Ewart and Reynolds [Wave Propagation in Random Media (Scintillation), edited by Tatarski, Ishimaru, and Zavorotny (SPIE, Bellingham, WA, 1993), pp. 100–123] provide a convincing comparison between the ocean spectral model determined from a stochastic inverse using the acoustic phase spectrum (measured in the Mid-ocean Acoustic Transmission Experiment) and the same model derived from oceanographic data. The success of using the acoustic phase for such inverses is based on the high accuracy of Rytov theory to predict the phase spectrum. Accurate measurement of the acoustic phases (travel times) requires fixed sources and receivers, an experimental complication. If it were possible to use Rytov theory for the amplitude fluctuations, the experiment complexity is much reduced and simpler moorings could be used. In this talk results are presented from a study of the viability of this approach using numerical simulations of propagation through internal waves to a vertical array of receivers. With the only requirement being applicability of the Rytov approximation, two-dimensional (depth/time) correlations of the oceanographic model are obtained by stochastic inversion and compared to the input internal wave model. The importance of this result to augmenting oceanographic measurements, and a discussion of the future possibilities, are presented. [Work supported by ONR.]

Extraction of target characteristics from the response of a ribbed cylindrical shell in a shallow water environment

Bistatic and monostatic target strength values of a ribbed cylindrical shell contain various scattering phenomena such as Bloch waves, rib resonances, specular reflections and other effects due to waves traveling on the surface of the shell. When the target is placed in a shallow water environment, the response is further cluttered by the presence of the various multipaths that a signal travels in a bounded medium. By using a vertical line array of sources and receivers, the target response may be extracted. A vertical array of sources is used to suppress certain shallow water modes in the incident field. A receiver array is used to remove certain shallow water modes from the scattered field. Time-frequency analysis is a final filter for selecting the modes of interest for which the target response is extracted. The extracted target response is correlated against the free field target characteristics previously determined by time-frequency synthesis techniques.

Inversion of acoustic field data using genetic algorithms: Shallow-water results

Precise knowledge about environmental parameters is highly desirable for precise acoustic modeling of the underwater channel. Simulations have shown that global inversion methods such as simulated annealing and genetic algorithms are effective for the estimation of both geoacoustic and geometric (source location, receiver locations, and water depth) parameters. In this paper genetic algorithms are applied to acoustic field observations. The field observations were collected at a shallow-water site in the Mediterranean Sea near the Italian coast, where the depth was approximately 130 m. The source was stationary and produced a broadband signal at 170 Hz; a vertical array of receivers which spanned most of the water column was employed. The inversion process used the Bartlett processor for the object function. A range-independent version of the SACLANTCEN normal-mode acoustic propagation model snap was used for forward modeling. Inversion was carried out simultaneously over source location parameters, range and depth, and environmental parameters. A 20-min sample of observations was considered; at 1-min intervals estimates of the parameters were computed. As a function of both time and frequency, over a limited frequency band, it was shown that the genetic algorithms produced stable estimates for both the environmental and source location parameters.

Scattering from an object in a stratified medium. II. Extraction of scattering signature

This paper investigates methods to extract and/or characterize the scattering signature of an object in a stratified medium based on data received on a vertical array. Scattering by an object in free space transforms the incident (plane-wave) field into a scattered field with a characteristic angular distribution. The scattering function can be estimated by correlating the scattered field with the incident field over a domain of incident and scattered angles. In an ocean waveguide, the acoustic field incident on the (submerged) object involves many multipath arrivals. Likewise, the scattered field from the object is modified by multipath propagation from the scatterer to the receiver array. Hence, extraction of the scattering function requires a method to deconvolve the channel transfer function from the received data. As the scattering function is a function of both the incident and scattered angles, a comprehensive characterization requires data covering a range of incident and scattered angles. For a distant target, a vertical array of sources and receivers is considered in order to invert for the scattering function. But the data are limited to shallow incident and scattering angles since high angle arrivals are severely attenuated in a shallow water waveguide. For practical problems, a holographic image of the scatterer is proposed for target classification.

A single slice tomography experiment using a long vertical array of receivers

Broadband signals were transmitted from a near-axial source to a long vertical receiving array 1000 km distant in the North Pacific for 9 days in July 1989. The signals were received at 50 hydrophones spaced 60 m apart in the vertical, starting near the surface. Pulses were recorded at 10-min intervals for two periods, totaling 24 h, while hourly pulses were recorded for the duration, giving a total of about 320 pulses. Individual ray arrivals were resolved and their arrival times measured with a precision of a few milliseconds using phase-coded transmissions centered at 250 Hz with a bandwidth of 83 Hz (12-ms resolution). The pulse arrivals as a function of time and depth correspond well with predictions based on geometric (ray) and physical (WKBJ) optics. In each transmission, 600 to 900 ray arrivals can be identified with rays from numerical analysis, distributed along up to 24 distinct wave fronts. The ray arrivals evolve at the time scales of ocean processes. The longest period travel time changes can be inverted to infer changes in the mean sound-speed field between the source and receiver and in high-wave-number ocean features with wavelengths correspondng to ray loop lengths and their harmonics (70 km and shorter wavelengths). Simultaneous CTD and XBT surveys are available for comparison. Temporal and vertically lagged fluctuation statistics of the individual arrivals quantify internal wave and fine-structure effects. [Work supported by ONT.]

Broadband acoustic‐field simulations from standard ray theory

The complete wave field over a small region around 1000 km from a pulsed source is reconstructed in two ways. First, all the rays from the source to a vertical array of receivers at 1000 km are found, along with their travel times, number of caustics, arrival angles, and intensities. The pattern of wave fronts in a space at a given time is then reconstructed on a closely spaced grid surrounding 1000 km by treating these rays in an appropriate way. Second, the parabolic equation method is used at multiple frequencies to synthesize a pulse. The two fields are compared. Finally, the effect of internal waves is simulated by use of the first method, introducing random fluctuations on the travel times and arrival angles of each ray. [Work supported by ONR, Code 1125OA.]

A long-range acoustic experiment with sampling in the internal wave fluctuation band

Transmission at 460 Hz between two moorings centered about 600 km east of Miami is scheduled for August 1988. The 10-day, 260-km experiment has been designed to measure fluctuations induced by ocean internal waves (IW). The use of a 64-element, 150-m vertical receiver array, 6/h pulse rate, and high bandwidth allows measurement of pulse travel time (phase) to a fraction of a carrier period, and allows estimation of phase and intensity coherence times and the vertically lagged correlation function of the acoustic field, including the rms vertical wave front tilt. Path-integral solutions of the parabolic wave equation yield predictions of these statistical properties. For the Garrett-Munk IW model, rms travel-time predictions are 2–6 ms, and rms vertical angle predictions are 0.2–0.7 deg, with the lowest values for shallow turning rays and the largest for near axial rays. Expected phase coherence time scale is 5 min., with intensity time scale varying between 1 and 30 min., indicating the presence of both partially and fully saturated rays. These quantities yield a depth-dependent measure of rms vertical IW displacement, and some information about the IW spectrum.

Source localization in a waveguide using a matched‐field technique

The ability to localize a source in a waveguide by correlating measured and predicted acoustic field values at a vertical array of receivers depends at least on (1) the number of dominant acoustic modes that have been adequately sampled and (2) the accuracy of the field predictions. Several attempts to localize sound sources, using experimental and model data for shallow and deep water environments, are discussed. The results of these attempts are presented as well as observations regarding the number of depth/range replica source points required to localize the source for complicated (many modes) versus simple (few modes) acoustic environments. [Work supported by NORDA.]

Sound field fluctuations in a shallow water waveguide

Experimental measurements of sound transmissions in a laboratory waveguide are analyzed. The transmissions were from a single source to a vertical array of receivers that operated as a mode filter. Water waves on the surface caused the sound fields to fluctuate at the receiver. We used theories of coherent mode transmission to describe the experiments. Within a mode, sound field fluctuations have three components: (1) phase fluctuations due to fluctuations of the horizontal component of the wave number, (2) fluctuations of depth-dependent eigenfunctions at source and receiver positions, and (3) mismatch of the mode filter to the local eigenfunction. Analysis showed that most of the fluctuations of the mode filtered signals were due to components (2) and (3).