Info

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Time (s)

Figure 9.10 Under-ice profiles (solid line), and measured (dashed line) and modeled (dotted line) under-ice monostatic reverberation time series for three Arctic sites (Bishop, 1989b).

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Time (s)

Figure 9.10 Under-ice profiles (solid line), and measured (dashed line) and modeled (dotted line) under-ice monostatic reverberation time series for three Arctic sites (Bishop, 1989b).

are indicated by the dashed and dotted lines, respectively. The time scale (in seconds) is indicated at the bottom of the figure. These results support the intuitive notion that relatively high reverberation levels are associated with areas of high ice roughness. More specifically, average keel draft determines the average scattering facet surface area (which determines the average amplitude of a specular return) and the keel frequency determines the total number of scattering facets (which determines the probability of scatter occurrence). Differences between measured and modeled results are thought to be due to significant scatterers present in the under-ice surface but not present in either the measured or modeled data.

9.7 Numerical model summaries

A summary of stand-alone underwater acoustic reverberation models is presented in Table 9.1. The models are segregated according to cell-or point-scattering approaches. Numbers within brackets following each model refer to a brief summary and appropriate documentation. Model documentation can range from informal programming commentaries to journal articles to detailed technical reports containing a listing of the actual computer code. Abbreviations and acronyms are defined in Appendix A. This summary does not claim to be exhaustive.

Table 9.1 Summary of underwater acoustic reverberation models

Cell scattering

Point scattering

Monostatic

Bistatic

Monostatic Bistatic

REVGEN [16] Under-ice reverberation simulation [17]

REVMOD [5]

OGOPOGO [13]

Notes

Cell scattering

Monostatic

1 DOP divides the ocean into time-Doppler cells, sums the received energy incoherently, and produces a spectrum for the surface, volume or bottom reverberation at a given time after transmission (Marsh, 1976).

2 EIGEN/REVERB is a series of programs used to calculate ambient noise, reverberation-versus-time signals, and transmission-loss values (Sienkiewicz et al., 1975). These programs are based on NISSM.

3 MAM was designed as a monostatic companion to the bistatic BAM model. For monostatic operation, it is required that the source and receiver be located at the same horizontal location, although they may differ in depth (Bartberger, 1991b; Vendetti et al., 1993b).

4 PEREV (Tappert's PE reverberation model), together with the UMPE propagation model, are described by Smith et al. (1993, 1996).

5 REVMOD calculates reverberation power spectra (C.L. Ackerman and R.L. Kesser, 1973, unpublished manuscript; Hodgkiss, 1980, 1984). [The REVMOD model is discussed in detail in Section 9.4.]

6 REVSIM generates coherent, multi-beam, non-stationary reverberation time series (Chamberlain and Galli, 1983).

7 TENAR is a subroutine that uses the sonar equation to calculate underwater target echoes, reverberation and noise (Luby and Lytle, 1987).

8 BAM predicts the performance of a sonar system consisting of a sound source and a receiver separated in both range and depth (Bartberger, 1985, 1991a; Vendetti et al., 1993a). The model computes the echo-to-background ratios for targets located at a set of bipolar grid locations in a horizontal plane at a specified target depth. The bipolar grid points are defined at the intersections of circles of constant range about the source and receiver. [BAM is discussed in detail in Section 9.5.2.]

9 BiKR is a bistatic reverberation model (Fromm, 1999) based on the KRAKEN propagation model.

10 BiRASP extended the RASP model to handle arbitrary (bistatic) source and receiver configurations in a 3D, range-dependent environment (Fromm et al., 1996). RASP had been previously modified to predict range-dependent, monostatic reverberation at higher frequencies (up to 10 kHz) and in water shallower than originally intended. This modification was referred to as the Shallow Water RASP Upgrade (Fulford, 1991).

11 BISAPP uses an integral fast eigenray model to calculate point-by-point echo levels to facilitate analysis of bistatic and multistatic scenarios (Pomerenk and Novick, 1987).

Bistatic

12 BISSM computes bistatic bottom-scattering strengths (Caruthers etal., 1990; Caruthers and Novarini, 1993; Caruthers and Yoerger, 1993).

13 OGOPOGO is based on the Bucker-Morris method for computing shallow-water boundary reverberation using normal modes to calculate the acoustic energy propagating from the source to the scattering area and back to the receiver. Ray-mode analogies and empirical scattering functions are used to compute the scattered energy at the scattering area (Ellis, 1995). The normal-mode model PROLOS computes the propagation loss. Travel times of the reverberation signals are derived from the modal-group velocities. Volume reverberation from either the water column or the subbottom is not currently included, but boundary reverberation is computed using empirical scattering functions and ray-mode analogies. Both monostatic and bistatic geometries can be handled, and horizontal or vertical arrays can be specified for the source and receiver. OGOPOGO was used to interpret reverberation measurements from shallow-water sites in the frequency range 25-1,000 Hz (Desharnais and Ellis, 1997).

14 RASP is a sequence of computer programs using multipath propagation and scattering processes to predict the long-range, low-frequency boundary reverberation and target returns that would be received in real ocean environments (Franchi et al., 1984; Palmer and Fromm, 1992).

15 RUMBLE calculates reverberation as a function of time for bistatic sonars (Bucker, 1986; Kewley and Bucker, 1987).

Point scattering

Monostatic

16 REVGEN produces digital baseband samples of transducer or beamformer signals for use by active or passive sonars (Princehouse, 1977).

Bistatic

17 Under-Ice Reverberation Simulation is a bistatic, high-frequency (>2kHz), under-ice acoustic-scattering model to evaluate the scatter produced by a pulse, originating from an arbitrarily located source, as detected by an arbitrarily located receiver (Bishop et al., 1986, 1987; Bishop, 1987, 1989a,b). [This model is discussed in detail in Section 9.6.2.]

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