Interference From Other Sonar Systems; Speed; Air Bubbles; Environment - Teledyne SeaBat T Series Operator's Manual

ACTIVE SONAR USAGE

6.5.3 Interference from Other Sonar Systems

Interference from other sonar systems can be seen as radial lines, typically moving away the
minimum to maximum range scale as these pings are not correlated with the ping repetition rate
of the SeaBat system. The most typical source of interference of this type is navigational sonars
(often 50kHz systems) and Doppler velocity logs.
Synchronization of acoustic systems is one means to mitigate this effect. The most effective
approach is to remove the source of interference entirely, either by moving the equipment away
from the SeaBat or turning it off.

6.5.4 Speed

Hydrodynamic flow noise can be a concern when operating at higher speeds. Externally mounted
acoustic sensors should have fairings that are as flush (or near-flush) mounted to the ship hull as
is practically possible. This way the hydrodynamic flow-induced vibration noise should be
minimal. For means of compensating for this, see subsections in 6.2 above.
A poorly designed sonar fairing can degrade the system performance by a factor of 4 for
speeds above 8-10 knots. Typically, this noise source is the most common problem on all
sonar installations.

6.5.5 Air Bubbles

Bubble sweep-down can be another potential noise problem. The significance of this problem is
associated with the noise generated as the bubbles cascade along the hull after being ingested in
the bow wave of the ship and, more importantly, with the baffling produced by the entrained air
layer between the face of the sonar sensor and the water environment.
If air bubbles are present between the sonar and the water, the system will not function properly. This
phenomenon is transient in nature occurring only for a few seconds per wave period. It is typically
most prevalent on shallow draft ships and increases in intensity at higher sea states and ship speeds.
Modeling of this problem only indicates that air bubble noise may be present by determining the
transmission vehicle of potential bubbles. Actual at-sea measurements are usually required to
fully determine the presence and severity of bubble sweep-down. At-sea diver viewing has been
very successful in the past to capture the bubble sweep-down characteristics of a particular ship
hull on film.
Air bubbles mask some or all of the view of the sonar where the affected area will completely
blank out. Common causes for bubble masking include:
 The vessel props when thrusting into reverse and pours a stream of bubbles over the
sonar.
 The sonar has not been deployed deeply enough and there is bubble wash from the
sea surface.
 Bubble ingestion as the vessel pitches into the sea.

6.5.6 Environment

A high-reverb environment where echoes of the previous ping are contaminating the current ping
may prove to be difficult. For means of compensating for this, see section 6.2.3 above.
SeaBat T-Series Subsea Operator's Manual
Version 7
NOTE
Page 51 January 31, 2019