Bathymetry is the study of underwater depth of the
third dimension of lake or
ocean floors.
In other words, bathymetry is the underwater equivalent to
hypsometry. The name comes from Greek βαθυς,
deep, and μετρον,
measure. Bathymetric (or
hydrographic) charts are typically
produced to support safety of surface or sub-surface navigation,
and usually show seafloor relief or
terrain
as
contour lines (called
depth
contours or
isobaths) and selected depths
(
soundings), and typically also provide surface
navigational information. Bathymetric
maps (a more general term where
navigational safety is not a concern) may also
use a
Digital Terrain Model
and artificial illumination techniques to illustrate the depths
being portrayed.
Paleobathymetry is the study of
past underwater depths.
Measurement
Originally, bathymetry referred to the measurement of
ocean depth through
depth
sounding. Early techniques used pre-measured heavy
rope or cable lowered over a ship's side. The greatest
limitation of this technique is that it measures the depth only a
single point at a time, and so is inefficient. It is also subject
to movements of the ship and currents moving the line out of true
and therefore is inaccurate.
The data used to make bathymetric maps today typically comes from
an echosounder (
sonar) mounted beneath or over
the side of a boat, "pinging" a beam of sound downward at the
seafloor or from remote sensing
LIDAR or LADAR
systems . The amount of time it takes for the sound or light to
travel through the water, bounce off the seafloor, and return to
the sounder tells the equipment what the distance to the seafloor
is. LIDAR/LADAR surveys are usually conducted by airborne
systems.
Since the early 1930s, and more commonly from the 1940s onwards,
the occasional pings of a single-beam sounder might be averaged to
make a map. Today, a
multibeam
echosounder (MBES) may be used, featuring hundreds of very
narrow adjacent beams arranged in a fan-like swath of perhaps 90 to
170 degrees across. The tightly packed array of narrow individual
beams provides very high
angular
resolution and accuracy. In general the wide swath, which is
depth dependent, allows a boat to map more seafloor in less time
than a single-beam echosounder by making fewer passes. The beams
update many times per second (typically 0.1-50
Hz
depending on water depth), allowing faster boat speed while
maintaining 100% coverage of the seafloor. Attitude sensors allow
for the correction of the boat's
roll,
pitch and yaw on the ocean surface, and a gyrocompass provides
accurate heading information to correct for vessel yaw. (Most
modern MBES systems use an integrated motion-sensor and position
system that measures yaw as well as the other dynamics and
position.) The
Global
Positioning System (or other Global Navigation Satellite System
(GNSS)) is used to position the soundings with respect to the
surface of the earth. Sound speed profiles (speed of sound in water
as a function of depth) of the water column correct for refraction
or "ray-bending" of the sound waves owing to non-uniform water
column characteristics such as temperature, conductivity, and
pressure. A computer system processes all the data, correcting for
all of the above factors as well as for the angle of each
individual beam. The resulting sounding measurements are then
processed either manually, semi-automatically or automatically (in
limited circumstances) to produce a map of the area. A number of
different outputs are currently generated, including a sub-set of
the original measurements that satisfy some conditions (e.g., most
representative likely soundings, shallowest in a region, etc.) or
integrated
Digital Terrain
Models (e.g., a regular or irregular grid of points connected
into a surface). Historically, selection of measurements was more
common in
hydrographic applications
while DTM construction was used for engineering surveys, geology,
flow modeling, etc. Since ca. 2003-2005, DTMs have become more
accepted in hydrographic practice.
Satellites are also used to measure bathymetry. Satellite radar
maps deep-sea topography by detecting the subtle variations in sea
level caused by the gravitational pull of undersea mountains,
ridges, and other masses. On average, sea level is higher over
mountains and ridges than abyssal plains and trenches.
Most
surveys of navigable waterways in the United States
are performed or commissioned by the United States Army Corps
of Engineers, for inland waterways, and the National Oceanic
and Atmospheric Administration for oceans. Coastal
bathymetry data is available from
NOAA's
National
Geophysical Data Center (NGDC) . Bathymetric data is usually
referenced to tidal vertical
datum.
For deep water bathymetry, this is typically Mean Sea Level (MSL)
but most data used for nautical charting is referenced to Mean
Lower Low Water (MLLW) in American surveys, and Lowest Astronomical
Tide (LAT) in other countries. Many other
datum are used in practice, depending on the
locality and tidal regime.
Some occupations or careers related to bathymetry are the study of
oceans and rocks and minerals on the ocean floor, and the study of
underwater earthquakes or volcanoes. The measurement and analysis
of bathymetric measurements is one of the core areas of modern
hydrography, and a fundamental component
in ensuring the safe transport of goods worldwide.
See also
References
- NGDC-Bathymetry, Topography, & Relief
- NGDC/WDC MGG, Boulder-Coastal relief model
development
External links
Wikis
Quiz
Map
Search