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UWE SEND
Strengths and weaknesses:
A main limitation of gliders is the maximum current they can stem due to
their low speed. While the range is already a few 1000km, this still limits the
ability to reach a mission area from shore, to carry out survey work, and
return to a base. A very strong point is the flexible useage, both in terms of
sampling and sensors, of being able to choose tracks that are defined by
science not by merchant ships, of mission type, and the ability to steer the
glider from shore.
Further readings: Davis et al (2002), Seaglider website, Spray website.
2.6 Integrating techniques
Description:
A few methods exist which inherently provide spatially integrated
information, rather than data at the location of the platform. One is acoustic
tomography, which samples the ocean horizontally with sound over large
distances. This is usually done between pairs of moorings, which are fixed,
but the information extracted from the traveltime of the sound (temperature,
current along the path) represents an average over the entire section between
the moorings. This technique has been used successfully in a number of
experiments.
The other approach exploits the geostrophic relation and the principle
that the average (or integrated) geostrophic current can be determined alone
from the pressure distribution (as a function of depth) at the endpoints of a
section. This is usually calculated from density profiles, traditionally
collected with shipboard CTD’s, but can now be done with self-recording
sensors on a mooring. One thus obtains timeseries of mass transport,
integrated over the section between the moorings again. As in the traditional
geostrophic method, there is still a reference level problem, since the
pressure field determined from the density measurements is relative to a
pressure level whose depth and inclination is not known. For this, highprecision bottom pressure measurements are now possible to within a few
millimeters of equivalent sea surface elevation, which at least give the
fluctuations of the pressure gradient at a reference level.
Application:
Tomography is not much used in the “imaging” sense anymore, i.e.
trying to extract horizontal mapping resolution from the integrals along the
transmission paths. Instead, it is most useful where heat content or currents
along a section are of interest (water mass formation regions, straits). Over
long ranges it is also sometimes called “thermometry” and can then provide
basin-scale temperature changes.
UWE SEND
Strengths and weaknesses:
A main limitation of gliders is the maximum current they can stem due to
their low speed. While the range is already a few 1000km, this still limits the
ability to reach a mission area from shore, to carry out survey work, and
return to a base. A very strong point is the flexible useage, both in terms of
sampling and sensors, of being able to choose tracks that are defined by
science not by merchant ships, of mission type, and the ability to steer the
glider from shore.
Further readings: Davis et al (2002), Seaglider website, Spray website.
2.6 Integrating techniques
Description:
A few methods exist which inherently provide spatially integrated
information, rather than data at the location of the platform. One is acoustic
tomography, which samples the ocean horizontally with sound over large
distances. This is usually done between pairs of moorings, which are fixed,
but the information extracted from the traveltime of the sound (temperature,
current along the path) represents an average over the entire section between
the moorings. This technique has been used successfully in a number of
experiments.
The other approach exploits the geostrophic relation and the principle
that the average (or integrated) geostrophic current can be determined alone
from the pressure distribution (as a function of depth) at the endpoints of a
section. This is usually calculated from density profiles, traditionally
collected with shipboard CTD’s, but can now be done with self-recording
sensors on a mooring. One thus obtains timeseries of mass transport,
integrated over the section between the moorings again. As in the traditional
geostrophic method, there is still a reference level problem, since the
pressure field determined from the density measurements is relative to a
pressure level whose depth and inclination is not known. For this, highprecision bottom pressure measurements are now possible to within a few
millimeters of equivalent sea surface elevation, which at least give the
fluctuations of the pressure gradient at a reference level.
Application:
Tomography is not much used in the “imaging” sense anymore, i.e.
trying to extract horizontal mapping resolution from the integrals along the
transmission paths. Instead, it is most useful where heat content or currents
along a section are of interest (water mass formation regions, straits). Over
long ranges it is also sometimes called “thermometry” and can then provide
basin-scale temperature changes.