Using Space
Earth Observation satellites are now mainstream sources of
information in many areas and impact in numerous ways, mostly without us realising. This
impact is real and affects us all. For example, due largely to the availability of
satellite data for forecasting meteorological and ocean conditions, deep water oil
exploration in the North Atlantic is economically viable. This means countries with
offshore oil reserves receive significant revenues they would not otherwise obtain.
Radar satellites capable of imaging the Earth in all weather conditions allow regular
surveillance of environmentally sensitive regions such as the North Sea, the Mediterranean
and the Baltic Sea to ensure oil tanker operators do not illegally dump contaminants into
the environment. Satellite measurements provide precise sea surface temperature time
series on a global scale enabling scientists to obtain a better handle on whether
and how global warming is changing our planet.
These pages illustrate how the use of space affects us and
the people around us engaged in their daily activities. In these pages you can find:
- Background information on the various phenomena measured
- Information on Services available today using space based
measurements
- An overview of Research and Development presently underway both to
improve the service offering available and to use satellite data to better understand
environmental processes
What are internal waves?
The ocean is not homogeneous - sharp changes in density,
temperature, and salinity occur which give rise to a series of layers, each of which
differs sharply from its neighbours. On the interface where two such layers meet, waves
can exist. These are one type of “internal waves”. In most cases of
interest they are excited by the interaction between tidal currents and the sea bed
structure.
Why measure them?
They represent a poorly understood energy transfer mechanism across
continental shelf edges. in addition, they cause strong localised departures from the
surrounding ocean conditions causing shear stresses on underwater structures, large
variations in acoustic transmission properties of the ocean and transport of discharges in
unexpected directions.
Who needs to know?
- Oil companies need advance warning on internal wave
conditions to prevent damage caused by internal wave generated current shears on deep
water structures such as riser pipes and floating production systems
- Engineers need information on internal wave conditions to
ensure proper design of deep water waste outlets to prevent internal waves impeding the
efficient dispersal of pollutants
- Naval operations personnel need forecasts of internal wave
conditions to ensure Sonar systems are properly planned to minimise the performance
degradation that internal wave fields cause on Sonar instrumentation
- Subsea engineering contractors need accurate forecasts of
internal wave conditions to minimise the exposure of divers to hazardous currents and
sudden changes in pressure
Why use space?
Conventional methods to detect and measure internal wave properties
are expensive and limited to collection of data over a fairly localised region. However,
the modelling on which forecasts and nowcasts depend is not sufficiently precise without
correction by ocean data.
The use of remote sensing data when combined with additional knowledge such as tidal
properties a limited number of in-situ observations improves the accuracy of internal wave
models. Without the wide area data provided by the spaceborne SAR, forecasts are not
accurate enough for offshore operations management. The availability of satellite
instruments also reduces the requirement levels for in-situ instrumentation.
ERS SAR data represent very large cost savings over conventional techniques. Indeed, an
effective and reliable forecasting service would not be viable without such data.
Picture example -> 
Internal Waves Background
Problems
Internal waves cause problems in many areas, e.g.: Offshore
oil recovery - The greater part of the new offshore oil and gas fields are in deep
waters which are only economically viable due to the development of new recovery
technologies which involve cable and wire moorings or long, fragile structures such as the
riser pipes and drill columns. The concentration of energy within solitary internal waves
can result in strong localised current systems which generate high levels of shear stress
on these structures, causing significant levels of damage and representing a potential
safety hazard to operations personnel.
Acoustic propagation in the ocean - in the
ocean, sound is the only viable means of communication and range finding. However, just as
atmospheric turbulence causes stars to twinkle, so variations in the ocean structure cause
fluctuations to develop in the sound field as it travels. At typical Sonar operating
frequencies, the main cause of these fluctuations are internal waves. These can be
strong enough to completely prevent a signal from being detected. Clearly this is of
interest in naval operations but it is also of growing importance in marine
engineering where control of remote vehicles is increasingly based on acoustic
communication.
Deep water outfalls - Engineers generally take account
of prevailing and extreme conditions in designing structures such as waste outfalls to
ensure maximal dispersion. For outlets in deeper waters, this must include consideration
of the effects of internal waves. The intense local currents generated by internal waves
can prevent proper dispersal of effluent and can force such discharges back in the
direction of the coast. A complete analysis of the internal wave statistics for an area
requires measurements over a wide area for an extended period, something that would be
prohibitively costly using conventional measurement technology.
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Example riser system for deep water oil recovery |
What can
be done?
Forecasting services attempt to predict internal wave conditions at sites where their
occurrence would prove critical to operations activities. These forecasts depend on
regional models of the evolution of the internal wavefields. However, modelling
internal wave propagation is difficult due to the nonlinear nature of their evolution.
Initial errors in estimating their properties or the nature of the urrounding ocean medium
will grow with each time step of the model so that the effects at the site of interest can
differ significantly from the observed behaviour. These errors can only be
reduced by assimilating measurements of the internal wave field but, using conventional
techniques, this is extremely expensive and limits the provision of such services to the
military and a small number of priority areas
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US helicopter making Sonar and environmental measurements
(US NAVY PHOTO) |
How does
ERS SAR help?
Using ERS SAR combined with additional knowledge such as tidal
properties and a limited number of in-situ observations can reduce the errors in modelling
the internal wave behaviour to an extent where the model forecasts and nowcasts represent
an accurate analysis of the conditions at operations locations. Due to the
capability to view the internal wave field over a wide area and the reduced requirements
for in-situ instrumentation, use of ERS SAR data represents very large cost savings over
conventional techniques ERS SAR data are downlinked
to a local station as the images are acquired. In many areas of the world, these stations
have special “low resolution” processors capable of generating a SAR image with
a spatial resolution of approximately 100m by 100m (more than adequate for detecting
internal waves) in only a few minutes. These images are then transmitted to the
organisation providing the forecasting/nowcasting service. There, the images are
interpreted manually to identify internal waves. The spacing between successive packets is
measured and this information is combined with hydrodynamic data (in particular, the tidal
periodicity of the region of interest) to determine the velocity of the packets. Combining
this information with models of the propagation of internal waves allows their effects to
be accurately forecasted.
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How
does it work?
Satellite instruments cannot see below the sea surface so it is not
obvious how a spaceborne SAR can detect features such as internal waves. Indeed, when what
were clearly internal waves were first seen in SAR images from the US Seasat mission in
1978, oceanographers were extremely surprised. The
synthetic aperture radar generates an image of the surface roughness, integrated over a
short time interval. Where internal waves occur, they generate perturbations to the local
current pattern which propagate upwards towards the sea surface. These currents can then
cause one or both of the following effects:
- a sharp variation in surface roughness due to the currents
modulating the surface wave pattern
- concentration of surface algae into bands - where algae are
present, the surface is locally smoothed, thus appearing darker in a SAR image. This
generates a characteristic pattern in the SAR imagery.
Each of these effects can be clearly seen in an ERS
SAR image. This is mainly due to the polarisation of the ERS SAR being better suited to
detecting the modulation of the surface currents caused by the internal waves. Continuity
of such a data source is guaranteed by the Envisat satellite, due to be launched in 2000,
which carries a SAR instrument with a greater functionality, including a wider coverage in
each image.
Top of the page |
Example of the capability of ERS SAR to detect internal waves. This shows part of
an image acquired over the Galapagos islands |
Internal Waves Overview