Post on 09-Jul-2020
Indian Journal of Geo-Marine Sciences
Vol. 39(4), December 2010, pp. 631-645
Role of oceanography in naval defence
J Swain, P A Umesh & M Harikrishnan
Naval Physical & Oceanographic Laboratory, Kochi 682 021, India
[E-mail: tsonpol@vsnl.com]
Received 20 August 2010; revised 21 December 2010
The study of oceans is an essential aspect from strategic, economic and ocean engineering points of view. Recent
advancements in SONAR (Sound Navigation and Ranging) technology for under water applications including naval defence
demand a clear understanding of the sound propagation in the ocean which is vital for detection of a target. As compared to
deep waters, the detection in coastal waters is relatively a challenging task. Detection becomes still complex in the littoral
waters. To ensure success in these highly complex oceanic regions, high resolution descriptions/predictions of past, current
and future conditions (hindcast, nowcast and forecast) as well the analysis of open ocean, coastal, and nearshore/littoral
zones around the Indian continent is an essential pre-requisite. At any given situation, the detection and discrimination of an
underwater target is highly dependant on the propagation characteristics of the medium, the surface and bottom boundaries,
leaving apart the efficiency of the SONAR system and the type of the target. It can be either a SONAR performance model
or an operational model for tactical warfare; one has to have the predicted sound speed for the 3-dimensional ocean
environment concerned since it is not always feasible to depend only on measurements. There are on-going programs of
collecting and analyzing ocean and atmospheric data and a wide range of research and development activities, some of
which are reviewed/presented in this paper with typical examples. In addition to in-situ measurements, significant progress
has been achieved in India by demonstrating the utility of satellite based remote sensing data for oceanographic research and
applications. By utilizing the existing and on-going experimental data from the regions of interest, it has become feasible for
implementation of an integrated “Naval Operational Ocean Prediction System” consisting of wave, tide, circulation and
internal wave models. The ocean environmental information predicted by these ocean models shall provide necessary inputs
to the SONAR range prediction models for routine operational use, tactical operations and simulating warfare scenarios.
[Keywords: Oceanography in naval defence, Naval operational prediction, Ocean prediction system, Nearshore
prediction]
Introduction
The oceans that make up three-quarters of the Earth's surface are realms of boundless energy. Oceans have been a source of food, the birthplace of weather systems that affect the continents, pathways for commerce, and fields for battle. Studying the world oceans, the atmosphere above it, and air-sea interaction
processes is the science of oceanography. Oceanography has been recognized as a formal scientific discipline for nearly one hundred and fifty years. However, finding practical applications (inventions) for commerce and war at sea, dates much further. Hence, oceanography means more than an
understanding of how the weather and climate changes over the globe. The history of both ocean exploration and ocean warfare is filled with examples of “environmental intelligence” with the invent of new weapons, sensors, ships and submarines from time to time. Nevertheless, understanding of underwater
environment is also the new frontier for major future discoveries for the benefit of the world at large
1.
Materials and Methods
Recent advancements in SONAR (Sound Navigation and Ranging) technology for under water applications including naval defence demand a clear understanding of the sound propagation in the ocean which is vital for detection of an enemy target. As a thumb rule, the winner is the one who detects first; no
matter how latest are the weapon systems the opponent is in possession. In the 1920s and 1930s, the scientific understanding on the behaviour of sound in the sea and its application to SONAR systems for anti-submarine warfare advanced slowly, and it was only with the emergence of a vastly increased
submarine threat at the onset of Second World War in 1939 that a major national effort was undertaken for the study of underwater acoustics. What emerged was a series of results that showed that the transmission of sound in the sea and in particular how effectively it could be used to detect submarines depended crucially
on how the temperature and salinity of the seawater varied with range and depth. It was found that sound
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rays bend underwater in ways that are intimately linked to the variation of the speed of sound from place to place, and that this could create "shadow zones" in which a target could hide.
These discoveries significantly widened the range
of oceanic phenomena which created interest among
oceanographers. In addition to concerns with water
depth, winds, waves and currents, the need to
measure and interpret underwater physical
parameters such as water temperature, salinity, and
sound speed at increasing depths, assumed major
importance. This required the development for new
kinds of instruments, new analysis techniques, new
ways of interpreting data, new modeling approaches,
state-of-the-art operational prediction capabilities
and in general, a substantial broadening of the
scientific disciplines needed in practice of
oceanography for naval applications. The present
paper shall highlight some of the aspects with
reference to the Indian waters.
Results and Discussion
Naval oceanography involves several major areas
of science: oceanography, meteorology, mapping,
charting, geodesy, astrometry (the science of
accurate astronomical measurements); and precise
time-keeping. There are on-going programs in
collecting and analyzing ocean and atmospheric
data spread over a wide spectrum of research
and development activities. The oceanographers
are investigating the nature and behaviour of the
oceans from every aspect. In addition, to customary
bathymetric surveys for bottom mapping, they also
collect data on the sub-bottom layers, the composition
and roughness of the ocean floor, as well the physical
and biological properties of seawater. Specialized
instruments are used to measure currents, waves,
ocean fronts, local variations of the Earth's
magnetic/gravitational fields, and acoustical
background noise. While these measurements have
traditionally been made from aircraft, buoys, and
ships at sea, there is an increasing emphasis on
the use of available space based platforms for a
wide variety of observational support. Ocean
research related laboratories and civilian technical
institutes/universities are major contributors to the
ocean environmental sciences, and dedicated efforts
are under way to translate their findings into research
applications for improving the accuracy and
timeliness of ocean prediction. The various issues
relating to the role of oceanography for naval defence
are further discussed below with some relevant
examples based on field measurements.
Oceanography and Naval Defence
In future, underwater acoustics will remain as the
principal means of detecting targets as the feasible
non-acoustic methods have limited range. When
compared to deep waters, the detection in coastal
waters is relatively a challenging task. The detection
becomes still complex in the littoral waters.
Therefore, the importance of “littoral warfare” today
is increasingly felt worldwide. At any given situation,
the detection and discrimination of an underwater
target highly depend on the propagation charac-
teristics of the medium as well as its surface and
bottom boundaries, leaving apart the efficiency of the
SONAR system and the type of target. To a large
extent, the geo-acoustic properties of the sea bottom
can vary in space, but can be considered as constant in
time although there can be short-term and seasonal
variability of the surface sediment layer, and the
suspended matter in particular. However, the surface
and the medium characteristics are highly variable in
coastal and littoral waters both in spatial and temporal
scales.
The sea surface wind, wind-induced surface gravity
waves and the associated sea-surface roughness are
of primary concern dealing with propagation of
sound in the surface duct. In terms of medium
characteristics, the spatio-temporal variations of
temperature, salinity with depth (or sound speed) are
the essential pre-requisites for the SONAR range
prediction model. Further, it could be either a
SONAR performance model or an operational model
for tactical warfare, where one needs to have the
predicted three-dimensional sound speed structure as
it is not always possible/feasible to depend solely on
measurements. However, measurements are required
for satisfying the initial and boundary conditions of
ocean environmental models which can supplement in
the post analysis of model predictions to support
ASW (Anti Submarine Warfare), surface warfare,
amphibious warfare and coastal surveillance.
Oceanographic Parameters for Naval Operations
To ensure success in the highly complex oceanic
regions, high-resolution descriptors of the past,
current and future conditions (hindcast, nowcast and
forecast) as well as analysis/monitoring of open
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ocean, coastal, and nearshore/littoral zones around
the Indian continent are very much essential. The
capability to derive and/or measure data in remote and
inaccessible part of our ocean providing suitable
analysis and prediction is a real need. Environmental
parameters which are of importance (direct and
indirect) for naval defence includes:
(i) Atmospheric: Weather (clouds, precipitation,
wind speed, wind direction and air temperature)
and marine boundary layer properties
(temperature, humidity, refractivity).
(ii) Oceanographic: Sea-state, tide, current (surface
and subsurface), internal waves, temperature,
salinity, turbidity, optical properties (vertical and
horizontal) and surf conditions.
(iii) Geophysical: Depth, beach slope, beach and
bottom sediment type/composition, sediment
layering and their geo-acoustic characteristics.
(iv) Acoustic: Scattering, ambient noise, reverbe-
ration and transmission loss.
(v) Gravity and Magnetic: Gravity anomaly,
bottom roughness, bottom strength/stability,
ambient magnetic and electrical fields.
(vi) Biological and Anthropogenic: Noise, optical
scattering, bioluminescence, fishing, offshore
activities, shipping and pollution.
In general, the above parameters directly or
indirectly contribute towards the oceanographic and
acoustic studies supporting the naval warfare.
However, the following discussions shall deal only
with the most important selected oceanographic
parameters.
Waves The wind-induced surface gravity waves have
several applications both in civilian and defence
sectors. In the context of military perspective, wave
information is vital in design of coastal and offshore
installations, jetties/harbors, optimum tracking of
surface and sub-surface vehicles, aircraft landing and
take off, towing of underwater arrays/bodies,
detection and discrimination of underwater targets/
objects, search and rescue operations during rough
weather and several other operations at sea. The
sea-state has a dominant role in the air-sea interaction
processes of the coupled ocean atmosphere system
for operational prediction of weather in advance.
Keeping all these in view, it is necessary to predict
the sea-state or the evolution of sea surface waves on
a routine basis for the region of interest.
There has been a lot of progress in the field
of wave modeling and prediction all over the
world2-11, 41-45
. In India, with the launch of Oceansat-I
(IRS-P4), extensive validations were performed with
the third generation wave model 3G-WAM for the
Indian Ocean region using the analysed wind fields
provided by the National Centre for Medium Range
Weather Forecast (NCMRWF), New Delhi. The work
carried out at NPOL through a collaborative research
programme between NPOL and Space Application
Centre (SAC) was reviewed12
. It was a part of the
IRS-P4, MSMR utilization programme. Under this
collaborative programme, performance evaluation
studies of 3G-WAM were carried out for the Indian
Ocean bounded in the geographical coordinates
encompassing 30°E to 120°E and 30°S to 30°N using
the analysed winds of NCMRWF as the forcing
mechanism. WAM runs were performed using six
hourly input wind fields in a 1.5°×1.5° grid
resolution. The outputs from WAM model such as
significant wave height, peak wave period, mean
wave period and mean wave direction were compared
with the time-series buoy measurements obtained
from National Institute of Ocean Technology (NIOT),
Chennai with few buoys and other available
measurements which is shown in Fig. 1.
NPOL has implemented WAM at Naval Operations
Data Processing and Analysis Centre (NODPAC),
Indian Navy, Cochin for operational wave forecasting
using NCMRWF winds since 2003. A sample output
of significant wave height (Hs) is shown in Fig. 2.
Evolution of 1-dimensional wave spectra at a selected
buoy location (DS1) for the period June-August 2002
and monthly mean wave spectra predicted by 3G-
WAM at the buoy location DS1 for the periods
Fig. 1—Map showing locations of wave measurements in the
Indian Seas utilized for WAM validations.
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May-August 2000 are depicted in Figs 3 and 4
respectively. The Year 2002 was an unusual event,
where the wave activity in general during June was
significantly higher compared with rest of the
monsoon period as seen in Fig. 3. Low wave activity
persisted during the July month normally considered
as the peak of south-west monsoon period. The
predicted mean monthly directional wave spectra at a
given location shown in Fig. 4 are able to indicate the
sea state which prevails at that location. Comparisons
between the predicted and observed wave parameters
for various case studies were found very encouraging.
Fig. 5 is an example of the WAM validations
carried out at NPOL utilising three hourly buoy
measurements. One of the important results to be
noted is that the model predictions of significant
wave height overestimates during the extreme wind
and wave conditions13,14,15,16
. In general, excluding
the extreme weather events, WAM predictions were
reasonably good for the south-west monsoon season
(May-September) periods in spite of the limitations
which were clearly known. These validation studies
Fig. 2—A sample WAM output of significant wave height (Hs) for Indian Ocean.
Fig. 3—Evolution of 1-dimensional wave spectra at a selected buoy location (DS1) during June-August 2002 (Swain et. al., 2005).
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Fig. 4—Mean wave spectra predicted by 3G-WAM at the NIOT buoy site [DS1] for May-August 2000
Fig. 5—Comparisons of observed and predicted wave parameters during May-August 2000 at DS1 location in the Arabian Sea
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have revealed that performance of WAM is
satisfactory and its predictions can be safely utilised
for various user applications in deep waters beyond
30 meters.
Tide
Tide is one of the most important hydrodynamic
parameter in coastal waters. It induces internal tides
(fluctuations of the internal temperature/density
layers) and largely influences the flow pattern from
surface to bottom in the shelf region. The Indian coast
experience semi-diurnal tide whose magnitude
gradually increases from south to north. The effect of
tides on the thermodynamic structure is also being felt
in deep waters (AMREX-II). Time-series current
measurements off Paradeep, east coast of India
revealed strong tidal reversals up to the bottom where
water depth was 51.5 m17
. During one of the field
experiment of NPOL, an echosounder was operated
continuously while the ship was anchored in the
absence of a tide gauge. The observed tidal range as
shown in Fig. 6 was about 1.5 m at the time-series
location, which is about 25 miles away from the coast.
Two distinct and regular crests were observed during
the day. The tidal height and the associated current at
different depths are valuable inputs for estimating the
pre-dominant component of coastal circulation and its
reversal during a tidal cycle. The observed current,
associated thermal structure and their variability
during the time-series measurements is discussed
further in the following sections in relation with the
tidal reversals.
As indicated above, tide is the most dominating factor in the coastal waters, which can directly or indirectly affect the oceanographic features leading to a considerable change in the medium properties
during its cycle. There are several models for the prediction of tide (astronomical) while some of them also predict the component of the meteorological tide
18.
Temperature and Salinity
As indicated earlier, the underwater sound
propagation depends on the environmental properties of the oceanic medium in addition to the surface and bottom boundary conditions. The spatio- temporal variations of temperature and salinity which determine the sound speed profile regulates the propagation of underwater sound. It is not
feasible to collect temperature and salinity profiles simultaneously from several locations by research ships. The simultaneous observations of temperature and salinity from different stations of a given location are limited to the number of ships involved in the field experiment (or scientific cruise). Generally,
when a single ship is involved spatial observations are taken in synoptic time intervals which depend on the speed of the ship and also the distance of separation between two stations. However, during analysis of the spatial data collected by a single ship or few ships, it is assumed that the observations are taken at a given
time simultaneously compromising on the short-term variability. Such measurements are alternatively possible by using moored buoys/sensors or through deployment of sensors similar to ARGO floats since the satellite data is restricted only to the sea-surface. Many such measurements and analysis is evident
from most of the published literature. However, temporal variations of temperature and salinity at a given location are possible from variety of measure-ment platforms (moored/anchored). One such measurement (Conductivity Temperature and Depth: CTD-yoyo) onboard INS Sagardhwani off Cochin,
west coast of India during 10-15 June 2003 (Fig. 7) demonstrates the variability of temperature and salinity during two tidal cycles (48 hours) at a coastal station. Such data sets can effectively improve the detection potential of SONAR stated above as sound is the best means of underwater detection for the
coming future. This was the first attempt by NPOL
Fig. 6—Time-series variability of water depth showing semi-diurnal
tide off Cochin, west coast of India
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(Mini-CTD yoyo) during which about 960 profiles of temperature and salinity were collected within a period of 48 hours by operating two Mini-CTD systems (measurement accuracies: depth ±0.02% of range 500 m, temperature ±0.01°C and salinity
±0.02 PPT) simultaneously. The water fall diagram as shown in Fig. 7 clearly
indicate the evolution of the temperature profiles due to the combined effect of diurnal heating/cooling, coastal currents, long-period waves, seasonal/climatic variations and other processes which occurs in the coastal ocean (tidal current appears to be the most predominant among all). The mixed layer depth (MLD) or the so called SONIC layer depth varies from almost 5 to 20 meters during the tidal cycle (semi-diurnal) which is very significant in acoustic detection using underwater systems. The effect is clearly seen up to the bottom (total depth 67 meters) as seen from the time-series currents, which is known to be very strong and a permanent feature in enclosed, semi-enclosed and estuarine waters
19,20.
The above time-series shown in Fig. 7, is a typical example of the short-term variability in coastal waters, the effect of which on SONAR range prediction is discussed later in this paper. Both temporal and spatial ocean variabilities are predictable using the state-of-the art ocean circulation models
21-24 with the help of measurements from all
available sources including satellites.
Currents
The sound speed profile in the ocean evolves with
the current. The following and opposing currents have significant influence on sound propagation and the effective detection range of a target. The ocean currents which are observed at various frequencies (periodicities) are responsible for the spatio-temporal variations of ocean properties like temperature and
salinity as discussed above. It is interesting to note that, the spatio-temporal variabilities of the ocean properties are in turn responsible for the varying ocean currents. It means that, the ocean is a dynamic system intimately coupled to the atmosphere where
the causes and effects of all its processes are dynamic and quasi-cyclic.
The variations of currents are shown in Fig. 8 (arrows/current vectors indicating speed and direction). This corresponds to the temperature variations as in Fig. 7 and the tidal cycle as in Fig. 6,
which is a typical example of coastal ocean variability in the Indian coasts. The current patterns clearly reveal the temperature variation particularly the MLD which need not exactly resemble each other since the flow vector is three dimensional depending on various other factors. The variability of currents due to the
tidal reversals both in magnitude (2 to 50 cm/second) and direction is the principal causative factor governing the sound speed profile.
Fig. 7—Water fall diagram showing variation of temperature from surface to 65m of water depth for a period of two days (X-axis), off
Cochin, west coast of India.
Fig. 8—Variations of observed temperature (minimum 19.7 near
bottom, maximum 30.2 degree Celsius at surface) and current for
two days (vectors indicate relative aspect ratio), off Cochin, west
coast of India
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Internal Waves
Oscillations of internal thermal layers within the ocean are very common in all the oceans, both in deep as well as shallow waters. These waves significantly
influence the sound transmission in the ocean causing fluctuations
25,26. The shallow water internal waves are
very complex and not well understood as the case in deep waters
27.
The variations of temperature and currents (speed and direction) as shown in Fig. 9 corresponding to the tidal cycle (Fig. 6) discussed before, are embedded
with low and high frequency variations (called as low and high frequency internal waves) which were demonstrated using band pass filters as indicated in the respective plots. The time-series temperature, current speed and direction (Figs 9a, 9c & 9d) clearly reveal the internal waves following the semi-diurnal
tide. Fig. 9b suggests the presence of high frequency internal waves with periods less than 3 hours (up to 15-30 minutes). These waves influence the propagation of sound in coastal waters.
A CTD yoyo experiment was conducted onboard ORV Sagar Kanya during her second cruise (17 July to 16 August 2002) - the Arabian Sea Monsoon experiment (ARMEX), Phase-I
28 an observational
study under the Indian Climate Research Programme (ICRP). The study of internal wave oscillations
(ARMEX programme onboard ORV Sagar Kanya) using CTD yoyo data, coincides the period where acoustic transmission experiment by NPOL onboard INS Sagardhwani was conducted. The objective of the acoustic transmission experiment was to study the effect of high-frequency (<1 cph) and low frequency (0.04 to 0.16 cph) internal waves which cause acoustic intensity fluctuations during its transmission in deep waters and prevailing rough weather conditions (south-west monsoon). The observed acoustic fluctuations in the surface duct could be due to wind, waves, internal waves and the prevailing current at the monitoring station. The depth variations of some typical isotherms plotted in Fig. 10 is based on the dataset collected during ARMEX which indicate internal wave periods ranging from 15 minutes to less than an hour (excluding higher frequencies) and also their vertical displacements (2 to 20 m) arising due to internal tides. The computed sound velocity using the observed temperature and salinity data reveals similar fluctuations as that during the yoyo period deciphering the presence of oscillations caused by internal waves. These oscillations are likely to influence the sound propagation
29 evident from the
observed internal wave magnitudes (Fig. 11) which shows variation predominantly from 2 to 5 m in a short span of 2 hours.
Fig. 9—Variations of observed isothermal layers (a) low passed time-series of temperature, (b) high passed time-series of temperature;
and observed time-series of current (c) speed & (d) direction for two days, off Cochin, west coast of India.
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Fig. 10—Fluctuation of selected isotherms plotted as time-series based on CTD yoyo at a selected location in the Arabian Sea
(Swain et. al., 2005).
Fig. 11—Frequency spectra for six selected isotherms (Swain et. al., 2005)
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The frequency spectra shown in Fig. 11 has
predominate peaks centered around 8 s with maximum
energy for all cases excepting the top 26°C isotherm.
In context of 26°C isotherms, high frequencies
oscillations are predominant. Average wave height and
the peak periods for the spectra shown in Fig. 11 varied
from 3.2 to 4.4 m and 7 to 40 s although the energy
levels are low for higher wave periods. This clearly
revealed the presence of high frequency internal
oscillations having intermediate periods between
the sampling frequency in the total length of the
time-series (CTD yoyo). Considerable progress has
been made to predict these waves with proper scientific
reasoning that was understood recently30
.
Oceanic Fronts and Eddies
Oceanic fronts and eddies are common
oceanographic phenomena which persists in various parts of the worlds oceans and play an important role in acoustic propagation
31,32. They have been reported
in the Arabian Sea and Bay of Bengal and also along the east and west coast of India by various investigators. A small scale front and eddy in the
coastal waters also play a significant role when the SONAR ranges of the order 5 to 15 kilometers. Large scale eddies are important for long range low frequency sound propagation. The density fronts that result due to variations in temperature and salinity in the coastal, nearshore and estuarine waters
influence the acoustic sensor performance. Suspended sediments may become trapped on these fronts resulting in turbid water columns which affect acoustic properties of the medium and thus the performance of acoustic sensors.
The above features are well identified and can be
routinely monitored using the satellite data. The spatial measurements (CTDs and hydrocasts) onboard research ships also form an important part of the database collected since several years of effort by many research organizations which have contributed towards the studies of fronts and eddies in the coastal
as well as deep waters. Presently, the most promising method for monitoring and predicting these phenomena is satellite measurements and models which can assimilate and predict their movements thereby its temporal evolution.
Nearshore & Estuarine waters
As stated earlier, the nearshore waters are highly
dynamic and mostly governed by the water
characteristics of the region/location and local
bathymetric conditions. Prediction in this region
(littoral zone) is a high challenging task. Waves feel
the bottom and break producing surf. The water depth
varies during the tidal cycle, which allow higher
waves to propagate during high tide from deep waters
and the water properties change accordingly. The
local winds (land and sea breezes), prevailing weather
conditions and freshwater discharges from land also
contribute towards the variability of the nearshore
waters considerably.
During one of the experiments, time-series and spatial observational data were collected from the coastal waters off Wheeler (up to 30 m water depth), north-east coast of India and the observed variabilities for a period of 24 hours is shown in Fig. 12. It may be
seen that the properties including water depth have varied dramatically during the semi-diurnal tide
20.
The observed salinity inside the estuarine waters varied from 15 to 31 PSU (includes all measurements time-series and spatial) which is due to the combined effect of diurnal as well as seasonal variations, tide
and mixing of fresh water discharged from the nearby rivers. The maximum current observed in the estuary was 1.2 m/s showing semidiurnal reversal along with the tide. The maximum tidal range measured at the outer Wheeler jetty was 3.1 m. The observed variations indicate a complex scenario for acoustic
propagation and detection in this region. Observed sea-state in the estuary was low
compared with the open coast/sea. The significant wave height varied from 0.2 to 0.9 m. One of the interesting features was the waves in the estuary were considerably high during the high tide compared to
low tide exhibiting a drastic variation of the sea-state during the day. This could be attributed due to the increased water depth as the waves are propagating from the deep waters. The spectral characteristics reveal single and multiple peaks (up to 3 peaks) with one or two high frequency peaks. The single peaked
swell spectra as shown in Fig. 13(f) was collected from a location which is open to the Bay of Bengal. The spectra show two prominent peaks which could be due to waves approaching the estuary from north-east and south-east directions, as there exists two open channels to the estuary from either side of the
observational point. The third peak noticed in the high frequency side of the spectrum is due to the presence of locally generated wind-waves within the estuary. The field experiment provided useful insights on the estuarine hydrodynamics and the predominance of the tidal currents. This information is essential for
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implementation in the hydrodynamic models and to validate the flow pattern within the estuary in presence of tidal variations (ebb and flood phase) prevalent inside the estuary.
Remote Sensing of Oceans Significant progress has been achieved in India
in demonstrating the utility of satellite based
remote sensing data for oceanographic research and
Fig. 12—Time-series plots of water depth, temperature, salinity, current speed, current direction at a location for 24 hours in Dhamara
estuary, north-east coast of India
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applications33
. Remote sensing of the oceans using
space based platforms is proven to be a feasible
alternative for obtaining the data in real-time for
operational utilization34
. Moreover, the present day
models are capable of providing information in the
three-dimensional ocean and atmosphere system over
a desired spatio-temporal resolution scales. This
cannot be obtained based on climatic database alone
real-time data alone or as simple the combination of
these two. Therefore, model based satellite data
assimilation for a variety of atmospheric and
oceanographic parameters can play a significant role
for naval defence, sea operations, routine exercises,
system evaluations, tactical operations and training of
personnel at sea.
In addition to the above, satellite remote sensing
can have several other feasible applications which are
well proven. The two-dimensional ocean wave
spectrum can only be measured from satellites (SAR
data) and it can provide the bathymetric data through
inverse modeling of waves and current besides the
surface signature of a submerged object or a
stationary underwater submarine most importantly in
the littoral waters. The blue-green laser based
measurements provide bathymetric information in
coastal waters and information on underwater moving
targets, submerged vessels in littoral and near shore
waters. Hydrodynamic signatures like surface wakes
and surface temperature anomaly due to moving ships
and submarines are detected by the satellites. The
satellites also provide information on ship traffic,
offshore operations/platforms, fishing boats, which is
the source of noise in the open sea. The measured sea-
bed microseism along the coast can provide
information on ships and submarines (coastal
surveillance) with the aid of satellite data on wind and
waves (source of ambient noise and surface
reverberation). Satellite based gravity and magnetic
anomaly detection can identify the movement of ships
or submarines. The satellite measurements can be
Fig. 13—Observed wave spectra from a selected location in Dhamara estuary, north-east coast of India
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effectively used to model the biological population in
the sea and its movement which contribute towards
biological noise in the sea. In the near future, satellite
remote sensing of the oceans will revolutionize our
understanding of the oceans.
High-resolution Ocean database As compared to deep waters, the detection of
underwater target in the coastal waters is relatively a
challenging task which demands high resolution
spatio-temporal information due to the rapid
variability in water properties35
. At any given
situation, the detection and discrimination of an
underwater target is highly dependent on the
propagation conditions of the medium as well its
surface and bottom conditions, leaving apart the
efficiency of the SONAR system and the type of the
target. It can be either a SONAR performance model
or an operational model for tactical warfare, one
needs to have knowledge on the predicted sound
speed for the 3-dimensional ocean environment. This
is true as it is not always feasible to depend only on
measurements as the surface and the medium
characteristics are highly variable both in space and
time scales in coastal and littoral waters. However,
field based measurements are essential for specifying
the initial and boundary conditions in ocean
environmental models which can supplement the post
analysis part of model predictions. Therefore, the
requirement of a high resolution database in space and
time need not be over emphasized for the coastal
ocean environment along the east and west coast of
India. One of the case studies utilizing the same data
as discussed before (Fig. 7) is used to study the short-
term variability of the predicted SONAR range.
The estimated/predicted SONAR ranges for three
selected frequencies (3, 9 and 14 KHz) for a source
depth of 5 m, FOM (Figure of Merit) 80 dB and target
depths 10 and 30 m as shown in Fig. 14 have revealed
that the dependence of SONAR detection range is
based on observed sound speed variations during the
two tidal cycles. Obviously, the detection range is
higher for the lower frequencies (3 and 9 KHz) as
compared with the higher frequency (14 KHz) but the
diurnal variations of the predicted range remains more
or less similar for all the three cases. The predicted
drastic changes in the detection range during tidal
reversals as observed from time-series data is very
essential for the operational requirements using under
water systems. It may be noted that, the over all
predicted range for three selected frequencies have
varied from 0 to over 50% of the average range
during a tidal cycle.
The composite plot of temperature (color
contours), tidal variation and SONAR detection range
(source 9.0 KHz, FOM 80 db, source depth 5 m,
target at 15 m) for 48 hours (Fig. 15) summarizes the
effect of environmental variations mostly the
temperature and salinity with the predicted detection
range assuming the bottom and surface parameter
unchanged. Kindly note that the graph shown for the
tidal variation in Fig. 15 indicates higher tidal level
which corresponds with higher depth scale, as the
water depth is plotted in negative scale. The observed
tidal range, while the ship was anchored is not
considered as the absolute tide at the observation
point. However, it clearly indicated the tidal cycle in
the coastal waters has certain time-lag compared with
the tides at Cochin. Comparing and considering the
extent of variations in temperature (and salinity) for
the corresponding tidal variation and the estimated
detection range demonstrates the necessity of short-
term predictions of the ocean parameters for
deployments of any under water systems and sensors.
Naval Operational Ocean Prediction System Unprecedented world political changes are
redefining national defense interests and altering the
research and development priorities of a nation.
Therefore, knowledge of the ocean, especially the
ocean acoustic properties of the Indian Seas (Arabian
Sea and Bay of Bengal) and coastal areas, is very
crucial and critical for our national defense. By
Fig. 14—Hourly variations of predicted SONAR range during two
tidal cycles for three selected source frequencies and source-target
depths
INDIAN J. MAR. SCI., VOL. 39, No. 4, DECEMBER 2010
644
utilizing the existing and ongoing experimental data
from the regions of interest, it is now feasible for the
implementation of an integrated “Naval Operational
Ocean Prediction System” which comprises the
waves, tides, circulation and internal wave models
being followed36-40
. The ocean environmental
information predicted by the integrated suite of ocean
models shall provide necessary inputs to the SONAR
range prediction models for routine operational use,
tactical operations and simulating warfare scenarios.
Acknowledgement
Authors express their sincere thanks to Director
and Group Head, Ocean Science Group, NPOL for
the facilities and support provided for this study.
Special thanks to all the cruise participants who were
responsible for the collection of field data utilized in
this paper.
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