Abstract
Sound backscattered to a sonar from a seabed decreases in intensity with increasing range ( R ) due to geometrical spreading. As a far-range approximation, a geometrical spreading correction of + 30 log R decibels may be applied. A correction based on an accurate estimation of the area of the seabed ensonified by the sonar pulse incorporates additional terms that are a function of: range, sonic ray inclination angle, along- and across-trace components of seabed slope and sonar vehicle pitch. At near-normal incidence, the area of the seabed ensonified by the pulse lies within a circle truncated by the narrowness of the sonar beam. Beyond a critical range, the ensonified area separates into two areas disposed on opposite sides of an annulus, one being the principal and the other its conjugate. With increasing range, backscatter intensity from the conjugate area rapidly decreases. At steep inclination angles, the principal area of seabed ensonified is effectively increased by an estimable factor due to scattering from the conjugate area. Backscatter from the conjugate area leads the angle of incidence measured by swath interferometry requiring a correction for an estimate of the angle to the center of the pulse in the principal area.
Highlights
Acoustic intensity is the product of acoustic energy density and velocity, and is a vector with magnitude proportional to the square of the acoustic amplitude
For active sonar and on the assumption of simple geometries, the loss in signal intensity along paths due to geometrical spreading as a function of range can be expressed in decibels as (e.g., [1]): GSdB = −N log(R/R0) decibels where: N is a number associated with the way acoustic energy spreads geometrically; R the range or one-way travel distance; and R0 the reference distance (1 m)
There are effects on received intensity that are functions of inclination angle: the sonar beam function and the associated effect of sonar vehicle roll, and seabed backscatter functions and the associated effect of seabed slope. All of these effects need to be compensated for in order for the raw acoustic amplitude recorded by a sonar system to be reduced to an effect of the seafloor alone unaffected by anything else. This is important in multi-spectral sidescan sonar imaging, in which seabed acoustic response as a function of frequency is represented by color, in order for acoustic color to be an effect of the seabed unaffected by other factors (Tamsett et al [7,8])
Summary
Acoustic intensity is the product of acoustic energy density and velocity, and is a vector with magnitude proportional to the square of the acoustic amplitude. There are effects on received intensity that are functions of inclination angle: the sonar beam function and the associated effect of sonar vehicle roll, and seabed backscatter functions and the associated effect of seabed slope (de Moustier and Alexandrou [3], Hughes Clarke [4], Hughes Clarke et al [5], Tamsett and Hogarth [6]) All of these effects need to be compensated for in order for the raw acoustic amplitude recorded by a sonar system to be reduced to an effect of the seafloor alone unaffected by anything else. Accounting for sonar near-field effects would considerably complicate an otherwise relatively simple analysis These are ignored in the current paper and a sonar transducer treated as a point source so far as the function of intensity with range is concerned. Eng. 2017, 5, 54 geometrical spreading will allow more accurate corrections to be applied in post-acquisition sonar data processing
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