Method
Bathymetry source, sonar simulation and image formation
The bathymetry is a 1.12 m grid of a submarine channel about 1,000 m
deep off Morro Bay, California. It was surveyed by an autonomous
underwater vehicle of the Monterey Bay Aquarium Research Institute and
released by the U.S. Geological Survey (Kennedy et al. 2021,
doi:10.5066/P97QM7NF,
public domain). Survey grids carry the marks of their own processing,
and at the low grazing angles of a sonar image a few centimeters of
that structure cast stripes of shadow. The grid is therefore smoothed
with a Gaussian of 3 m standard deviation before use, which removes most
relief shorter than about 10 m. The seabed's centimeter-scale roughness
comes from the simulator's own sand model; relief between that and
about 10 m, such as the walls of small gullies, is not represented.
Each line is 1,040 m long and flown as thirteen 80 m swaths that tile
it edge to edge. The vehicle follows the bottom at 20 m above the bed
beneath it and looks to starboard to 200 m of range. The array is the
one used elsewhere on this site, at 100 kHz: 32 receivers 40 mm long
with 30 kHz of bandwidth. The seabed is simulated with ApertureLab's
point-scatterer backend at scatterers
per square meter on the terrain the grid gives, with acoustic shadows
cast by the simulator. Every swath is formed by time-domain
back-projection and written as a GeoTIFF in the grid's own map
projection (UTM zone 10N), so the swaths land on the bathymetry in a
GIS.
The images are formed on one flat surface at the height of the bed
beneath the vehicle. Terrain far above or below that surface is drawn
at the wrong range: on line 1, the channel floor 60 m below the bench
is drawn about m too far out. Two
readouts describe what a line misses: "Hidden by terrain" is the share
of each image that no seabed reaches along a line of sight from the
sonar, computed over the same terrain. "Too dark to see" is the share
of 1 m patches more than 10 dB darker than the survey's lit seabed. It
includes everything hidden, and also slopes that tilt away from the
sonar and return too little at their low grazing angle. Every swath of
every line is beamformed with one fixed time-varying gain and drawn
with one brightness scale, so a swath that is half shadow is not drawn
brighter than its neighbors.
The side-scan is a single 400 kHz element with the acoustics of an
EdgeTech 4125i: a 0.46° horizontal beam, 2.3 cm range resolution
and 200 m of range. Its 50° vertical beam is tilted 10° down.
The 4125i towfish itself is rated to 200 m of depth; here the same
acoustics ride the survey vehicle along the same lines, at the same
altitude and speed. One ping must return from 200 m before the next
leaves, so it pings 3.7 times a second and its pings land 0.54 m apart.
Each ping is a line of slant range, drawn on the same flat surface as
the SAS images, with its own fixed gain and its own brightness scale.
The toggle above the map swaps the two at the same place and zoom.
The map zooms from the whole line down to the sonar's own pixels:
2.5 cm on line 1, and 5 cm on the other three lines, which keeps the
page's download smaller. Every zoom level is cut from the same
full-resolution images with the same brightness scale.
The effect of the image surface on objects standing above it is the
subject of The surface of focus; the imaging
chain is described in Image formation.