IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 35, NO. 3, JULY 2010 603 A Geoacoustic Bottom Interaction Model (GABIM) Darrell R. Jackson, Robert I. Odom, Michael L. Boyd, and Anatoliy N. Ivakin Abstract—The geoacoustic bottom interaction model (GABIM) has been developed for application over the low-frequency and midfrequency range (100 Hz to 10 kHz). It yields values for bottom backscattering strength and bottom loss for stratified seafloors. The model input parameters are first defined, after which the zeroth-order, nonrandom problem is discussed. Standard codes are used to obtain bottom loss, uncorrected for scattering, and as the first step in computation of scattering. The kernel for interface scattering employs a combination of the Kirchhoff approximation, first-order perturbation theory, and an empirical expression for very rough seafloors. The kernel for sediment volume scat- tering can be chosen as empirical or physical, the latter based on first-order perturbation theory. Examples are provided to illustrate the various scattering kernels and to show the behavior predicted by the full model for layered seafloors. Suggestions are made for improvements and generalizations of the model. Index Terms—Acoustic scattering, reverberation, seafloor scat- tering, underwater acoustics. I. INTRODUCTION T HE geoacoustic bottom interaction model (GABIM) uses a combination of published results in theoretical and empir- ical modeling to predict bottom loss and bottom backscattering strength given a set of geoacoustic input parameters. The distin- guishing attribute of GABIM is the ability to model and predict scattering in seafloors having arbitrary stratification. Both rough- ness and volume scattering are included, and shear effects are treated to a limited degree. While GABIM exists as a computer code, this paper focuses on the set of seafloor scattering models that are used in GABIM, and the methods by which they are com- bined so that each takes effect in its preferred regime. The discus- sion of the scattering models also provides a brief survey of many of the approximations that are commonly applied to seafloor scat- tering. In particular, the applicability of the various approxima- tions is discussed, and features and shortcomings that may not be widely appreciated are pointed out. The problem of propagation in a stratified (plane-layered) medium has been considered extensively in the literature. Codes are available for calculations of reflection and transmission coefficients and wave fields in an arbitrarily stratified medium, including both fluid and fully elastic cases [1]–[3]. Develop- ment of models for scattering in stratified media has lagged the Manuscript received January 26, 2010; accepted April 01, 2010. Date of pub- lication July 29, 2010; date of current version September 01, 2010. This work was supported by the Space and Naval Warfare Systems Command (SPAWAR), Code PMW 120. The work of A. N. Ivakin was supported by the Office of Naval Research (ONR). Associate Editor: N. Chotiros. The authors are with the Applied Physics Laboratory, University of Wash- ington, Seattle, WA 98105 USA (e-mail: drj@apl.washington.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JOE.2010.2050170 development of propagation codes, in part because the latter provide the starting point for most scattering approximations. The current version of GABIM partly, but not fully, exploits the capability of available propagation codes. Possible upgrades to fully incorporate shear effects will be indicated as the discussion proceeds. An early model for scattering from arbitrarily stratified sed- iment was presented in [4]. In this paper, only one mechanism of scattering, volume heterogeneity, was considered. Special- ized profiles of sound speed and density were considered that allowed an analytic solution for the acoustic field in the sedi- ment, and the effects of internal boundaries in the seafloor on the frequency-angular dependencies of the bottom scattering strength were demonstrated. Additionally, the effects of contin- uous stratification (gradients) were considered and some gen- eral properties of the backscattering strength were established by using the WKB approximation. It was shown, e.g., that Lam- bert-like angular dependence can sometimes result from the ef- fects of surficial gradients on sediment volume scattering. In [5], staircase/step functions were used to approximate arbitrary profiles, corresponding limitations in computation algorithms were discussed, and model/data comparisons were presented. An approach similar to [4] was adopted in [6], where contin- uous stratification was also considered using a specialized sed- iment sound-speed profile that allowed an analytic solution for the field in the seafloor at the expense of generality. The next stage of development included scattering by rough interfaces of layered sediments. In [7], first-order solutions were presented for stratified fluid sediments with an arbitrary number of interfaces, and the bottom bistatic scattering strength was expressed through auto- and cross spectra of roughness of the different interfaces. In [8]–[11], both roughness and volume scattering mechanisms were considered and compared. It was shown, in particular, that the roughness and volume scattering amplitudes for layered sediments can be presented in a similar form. A somewhat different approach was taken in [12] and [13] where the rough sediment–water interface provides the only scattering mechanism, but the effect of internal reflections due to stratification is included. Volume and roughness scattering in a two-layer system were treated with some simplifications in [14]. A treatment of roughness scattering in a multilayered seafloor was given in [15], where the limit of close stacking of weakly scattering interfaces was shown to be indistinguishable from volume scattering. In [8], a unified perturbation approach to volume and roughness scattering was presented, and, in [16] and [17], this approach was described in detail, and generalized to the case where the stratified fluid sediment covers an arbitrary scattering basement. Numerical examples given in [5], [7], [10], and [18] assumed that the basement was fluid (but rough and/or heterogeneous), 0364-9059/$26.00 © 2010 IEEE