temperature with depth. Figure 2-9 depicts the three
types of temperature gradients as they would appear on
a bathythermograph trace.
Selection of Significant Data
Certain criteria have been established for selecting
data points from a bathythermograph trace. Remember
that the objective of selecting these points is to provide a
fairly accurate representation of the temperature profile
of the water column. The following points on a
bathythermograph trace are always considered
significant and must be reported:
The surface (or the first readable temperature in
the upper 10 meters or 30 feet).
The Mixed Layer Depth (MLD).
Tops and bases of isothermal layers.
Inflection points in the trace; that is, significant
points on the trace where the temperature
changes from positive to negative or vice versa.
The deepest point of the trace. If the BT strikes
the bottom, be sure to encode the value using the
00000 indicator group.
Following these procedures, there should never be a
need to report more than 20 points in the upper 500
meters (1,640 feet) of the trace. Once the significant
levels are selected, the sounding may be entered into
onboard computer systems for processing, and then
encoded in the bathythermograph log. Figure 2-10
shows an unusual bathythermograph trace from an
AN/SSQ-56 recorder. Significant levels have been
Some airborne expendable bathythermograph
(AXBT) systems produce digital printouts of
depth/temperature data vice actual depth/temperature
traces. Although these can be difficult to work with, the
same criteria is applied when selecting significant data
What might cause the stylus on an AN/BQH-7A
to make erratic excursions to the right on the
Other than equipment or probe malfunction,
what might be the cause of anomalous features
on a XBT recording trace?
What term describes an increase in temperature
What significant points on a BT trace must be
evaluated and encoded?
What is the maximum number of points that
should be encoded in a BT observation report in
the upper 500 meters (1640 feet) of the trace?
LEARNING OBJECTIVES: Describe how
elements observed during a bathythermograph
sounding are properly encoded in the BATHY
code. Identify the meaning of each element in
the BATHY code.
The CNMOC 3167/2 Bathythermograph Log
contains a foldout cover sheet with complete
instructions for completing the log and encoding the
observation in the proper International Code, which is
WMO Code FM 63-X BATHY. The Bathythermo-
Figure 2-9.Types of temperature gradients.