Darrell R. Jackson

The validity of the perturbation approximation for rough surface scattering using a Gaussian roughness spectrum

The validity of the perturbation approximation for rough surface scattering is examined (1) by comparison with exact results obtained by solving an integral equation and (2) through comparison of low‐order perturbation predictions with higher‐order predictions. The pressure release boundary condition is assumed, and the field quantity calculated is the bistatic scattering cross section. A Gaussian roughness spectrum is used, and the surfaces have height variations in only one direction. It is found, in general, that the condition kh≪1 (k is the acoustic wavenumber, h is the rms surface height) is insufficient to guarantee the accuracy of first‐order (or higher‐order) perturbation theory. When the surface correlation length l becomes too large or too small with h held fixed, higher‐order perturbation terms can make larger contributions to the scattering cross section than lower‐order terms. An explanation for this result is given. The regions of validity for low‐order perturbation theory are also given. Th...

Phase conjugation in the ocean: Experimental demonstration of an acoustic time-reversal mirror

An experiment conducted in the Mediterranean Sea in April 1996 demonstrated that a time-reversal mirror (or phase conjugate array) can be implemented to spatially and temporally refocus an incident acoustic field back to its origin. The experiment utilized a vertical source–receiver array (SRA) spanning 77 m of a 125-m water column with 20 sources and receivers and a single source/receiver transponder (SRT) colocated in range with another vertical receive array (VRA) of 46 elements spanning 90 m of a 145-m water column located 6.3 km from the SRA. Phase conjugation was implemented by transmitting a 50-ms pulse from the SRT to the SRA, digitizing the received signal and retransmitting the time reversed signals from all the sources of the SRA. The retransmitted signal then was received at the VRA. An assortment of runs was made to examine the structure of the focal point region and the temporal stability of the process. The phase conjugation process was extremely robust and stable, and the experimental resu...

Phase conjugation in underwater acoustics

Phase‐conjugate mirrors are used in optics to compensate for aberrations caused by inhomogeneities in the propagation medium and by imperfections in optical components. In acoustics, analogous behavior can be achieved by a time‐reversed retransmission of signals received by an array. Compensation for multipath propagation and array imperfections is automatic and does not require knowledge of the detailed properties of either the medium or the array. The behavior of acoustic phase‐conjugate arrays is illustrated in several examples, some highly idealized and some more realistic. The effects of aperture size and inhomogeneities in the propagation medium are treated for both the near‐field and far‐field regions. It is concluded that phase‐conjugate arrays offer an attractive approach to some long‐standing problems in underwater acoustics.

Application of the composite roughness model to high‐frequency bottom backscattering

The composite roughness model is applied to bottom backscattering in the frequency range 10–100 kHz. For angles near normal incidence, the composite roughness model is replaced by the Kirchhoff approximation which gives better results. In addition, sediment volume scattering is treated, with account taken of refraction and reflection at the randomly sloping interface. In applying the model to published data it is found that sediment volume scattering is dominant in soft sediments except at small and large grazing angles. For coarse sand bottoms, roughness scattering dominates over a wide range of grazing angles. Implications for acoustic remote sensing are discussed.

Underwater acoustic communication by passive-phase conjugation: theory and experimental results

A new method for coherent underwater acoustic communication called passive phase conjugation is evaluated. The method is so named because of conceptual similarities to active phase conjugation methods that have been demonstrated in the ocean. In contrast to active techniques, however, the array in passive phase conjugation needs only receive. The procedure begins with a source transmitting a single probe pulse. After waiting for the multipathed arrivals to clear, the source then transmits the data stream. At each element in the distant receiving array, the received probe is cross-correlated with the received data stream. This cross-correlation is done in parallel at each array element and the results are summed across the array to achieve the final communication signal suitable for demodulation. As the ocean changes, it becomes necessary to break up the data stream and insert new probe pulses. Results from an experiment conducted in Puget Sound near Seattle are reported. Measurements were made at multiple ranges and water depths in range-dependent environments.

A LONG-RANGE AND VARIABLE FOCUS PHASE-CONJUGATION EXPERIMENT IN SHALLOW WATER

A second phase-conjugation experiment was conducted in the Mediterranean Sea in May 1997 extending the results of the earlier time-reversal mirror experiment [Kuperman et al., J. Acoust. Soc. Am. 103, 25–40 (1998)]. New results reported here include (1) extending the range of focus from the earlier result of 6 km out to 30 km, (2) verifying a new technique to refocus at ranges other than that of the probe source [Song et al., J. Acoust. Soc. Am. 103, 3234–3240 (1998)], and (3) demonstrating that probe-source pulses up to 1 week old can be refocused successfully.

High‐frequency bottom backscattering: Roughness versus sediment volume scattering

High‐frequency bottom acoustic and geoacoustic data from three well‐characterized sites of different bottom composition are compared with scattering models in order to clarify the roles played by interface roughness and sediment volume inhomogeneities. Model fits to backscattering data from two silty sites lead to the conclusion that scattering from volume inhomogeneities was primarily responsible for the observed backscattering. In contrast, measured bottom roughness was sufficient to explain the backscattering seen at a sandy site. Although the sandy site had directional ripples, the model and data agree in their lack of anisotropy.

Narrow‐band performance of phase‐conjugate arrays in dynamic random media

Phase‐conjugate arrays in underwater acoustics are of interest because of their ability to produce retrodirective fields. The theoretical narrow‐band performance of acoustic phase‐conjugate arrays in the presence of static and dynamic random media will be presented. For a static random medium, our results suggest that random refraction allows phase‐conjugate arrays to ‘‘super‐focus,’’ that is, produce a focal region smaller than the free‐space diffraction limit. For a dynamic random medium, the round‐trip time delay between the source and the array degrades phase‐conjugate array performance. When the dynamics of the medium are characterized by the statistics of oceanic internal waves, analytical results can be obtained for the intrinsic or ‘‘maximum‐possible’’ signal‐to‐noise ratio produced by the array at the source location. For a typical deep‐water oceanic medium, internal wave dynamics do not preclude interesting applications of phase‐conjugate arrays. [Work supported by ONR.]

An overview of SAX99: acoustic measurements

A high-frequency acoustic experiment was performed at a site 2 km from shore on the Florida Panhandle near Fort Walton Beach in water of 18-19 m depth. The goal of the experiment was, for high-frequency acoustic fields (mostly In the 10-300-kHz range), to quantify backscattering from the seafloor sediment, penetration into the sediment, and propagation within the sediment. In addition, spheres and other objects were used to gather data on acoustic detection of buried objects. The high-frequency acoustic interaction with the medium sand sediment was investigated at grazing angles both above and below the critical angle of about 30/spl deg/. Detailed characterizations of the upper seafloor physical properties were made to aid in quantifying the acoustic interaction with the seafloor. Biological processes within the seabed and the water column were also investigated with the goal of understanding their impact on acoustic properties. This paper summarizes the topics that motivated the experiment, outlines the scope of the measurements done, and presents preliminary acoustics results.

Studies of scattering theory using numerical methods

Abstract Numerical simulations, using both exact and approximate methods, are used to study rough surface scattering in both the smd and large roughness regimes. This study is limited lo scattcring lrom rough one-dimensional surfaces that obey the Dirichlet boundary condition and have a Gaussian roughness spectrum. For surfdces with small roughness (kh≪1, where k is the radiation wavenumber and h is the root-mean-square (RMS) Surface height), perturbation theory is known to be valid. However, it is shown numerically that when kh≪1 and kl≳6 (where I is the surface correlation length) the Kirchhoffapprorimation is valid except at low grazing angles, and one must sum the first three orders of perturbation theory obtain the correct result. For kh≪1 and kl≅1, first-order perturbation theory is accurate. In this region, the accuracy of the first two terms of the iterative series solution of the exact integral equation is examined; the first term a1 this series is the Kirchhoff approximation, It is shown numeric...