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From: fisher@nosc.mil (Alvan Fisher)
Subject: Shallow Water ASW
Date: Mon, 7 Nov 1994 17:13:17 GMT

As a tactical oceanographer and former naval officer I thought
I'd add my two cents re ASW in shallow water.

1) Shallow water regions are generally complex, both with respect
to the water itself and the underlying bottom.  The normal rule
is one of high variability over short ranges.  In areas where
little variability is found, the problem is really not too bad. 

2) This causes multiple problems for both ASW units and subma-
rines.  Sensors, weapons, and tactics designed for deep water
generally do not work well in shallow water despite claims to the
contrary.  The USN has allocated considerable funding to overcome
these problems and some improvement has been made.  Problems for
submariners include loss of control (broaching, sudden dives)
because of sudden changes in water density, navigation (marginal
bathymetric charts in many Third World regions), and the same
hardware problems as ASW units.  Slow speed will be the rule for
a sub.  A well-trained crew has no problem bottoming a diesel-
electric boat, but a nuke won't want to bottom for fear of 
contaminating their coolant water with silt.  The scope must be
used frequently, both for navigation and attack.  Running aground
can spoil your whole day!     

3) The water structure -- and thus the acoustic structure -- is
influenced by fresh water runoff from the mainland and melt water
from ice, the overlying atmosphere (solar radiation, wind
stirring), greater than normal particular matter (silt,
biologics), considerable tidal flow and range, and increased
merchant and fishing traffic.  Influx of water from deeper
regions may combine with local water as influenced by runoff to
cause quasi-stationary, seasonal ocean fronts (similar to
stationary weather fronts) that may have considerable effect on
acoustic transmission.  The thermocline does exist, and while it
may not permit a sub to hide below it in winter, it certainly
does so in summer.  In any event, the t/c does strange things
both to sonar and radiated energy from the submarine.  Secondary
sound channels (aka near-surface sound channels) may created
near-bottom areas near the shelf break that can be successfully
exploited by placing a sensor within them.  Convergence zone
propagation will not exist.  Rapid changes in salinity causes
sudden changes in water density . . . thus the stability problem
mentioned above.

4) The bottom may be littered with wrecks and rock outcrops that
have both the acoustic and doppler (from tidal flow) of
submarines.  A sandy bottom -- such as that found in the Gulf of
Tonkin -- can project energy for considerable distance downrange. 
A mud bottom -- as in the Yellow Sea -- may absorb the energy 
completely.  Energy may be scattered in many directions with a
rock bottom, while thick layers of sediment may refract the
energy through the sub-bottom only to reappear in the water many
kilometers downrange. 

5) So what needs to be down to improve the situation.  The
easiest . . . which, unfortunately, requires years to accomplish
. . . is to collect first class data from the region of interest
for all seasons and conditions.  This includes measurements such
as bathymetry, oceanographic and meteorological data, marine
flora and fauna, shipping information, fishing patterns, ad
nauseam.  The bathymetry problem is being addressed where
possible, but as you can imagine, it is costly and not all
countries want their territorial waters surveyed.  Oceanography
and weather data are collected on a catch as catch can basis, but
this is not being done particularly well.  New systems, including
satellite imagery, have been most helpful.  Unfortunately,
current satellite sensors don't penetrate cloud coverage, making
it difficult to obtain decent imagery during much of the year.

6) the USN and other technologically sophisticated navies have
increased the number of shallow water ASW exercises.  Many NATO
navies (unlike the USN) consider ASW as their prime mission. 
Unfortunately, experience in the Baltic may not equate to the
needs of the IO . . . but there are some areas where analogies
can be made.  Scientific analysis of these data is increasing,
but more needs to be done.  Not all lessons learned are
interpreted correctly by the operating forces, and scientists
often infer the wrong things.

The final word . . . we've got to get smarter and use the
information better.  The cost of coming up with new sensors and
weapons is beyond current funding.  We are making progress, but
it's difficult to change old concepts.  Sorry, but I cannot
discuss system performance and tactics in this arena.  However, I
can say that an environmentally astute ASW commander can use the
environment to improve his chances.  Remember, ASW forces don't
need to get a hard kill -- they can succeed in their mission
merely by neutralizing a sub by keeping it away from prospective
targets.

Naturally, the usual disclaimers prevail . . . these above is not
necessarily the views of my employer (a Navy lab).  I've had a
lot of fun since getting into the ASW world in the mid-50's!

Al  

 
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