INSTRUMENTED TOW CABLE MEASUREMENTS OF
TEMPERATURE VARIABILITY OF THE WATER COLUMN
ANTHONY A. RUFFA AND MICHAEL T. SUNDVIK
Naval Undersea Warfare Center Division, 1176 Howell Street, Newport RI 02841, USA
E-mail: ruffaaa@npt.nuwc.navy.mil; sundvikmt@npt.nuwc.navy.mil
The Instrumented Tow Cable (ITC) measures the temperature variability of the water
column with a spatial resolution of ½ meter along the cable and a temporal resolution
on the order of 100 seconds. The ITC is a modification of a conventional steel-armored
tow cable, involving the replacement of three outer steel armor wires with stainless steel
tubes containing optical fibers. The overall cable diameter is unchanged, and there is
otherwise no significant impact to the mechanical properties of the cable. The ITC
survived all standard mechanical ruggedness tests, including long stroke cyclic bending
over a 46-inch diameter sheave under tensions up to 22,500 lb. Temperature sensing is
derived from Raman scattering effects in the optical fibers. In lake tests, the standard
deviation against XBTs was found to be approximately 0.3 °C for repeated runs. Sea test
data from the shelf-slope front south of New England mapped the location of the front
within 150 m of the sea surface and showed indications of internal waves.
1
Introduction
The Office of Naval Research is funding the development of an environmentally adaptive
upgrade to U.S. Navy active sonar systems under the EA89 program. This will lead to a
capability to both measure and adapt to environmental variability. A key component of
this effort is the Instrumented Tow Cable (ITC), an otherwise conventional steel-armored
cable integrated with optical fibers, equipping it to measure the temperature of the water
column with an order of magnitude improvement in both spatial and temporal resolution.
The ITC was originally implemented in the LBVDS (Lightweight Broadband Variable
Depth Sonar) tow cable, a steel armored cable having a diameter of 1.6 inches. Three of
the outer steel armor wires were replaced with stainless steel tubes, each containing an
optical fiber. The fiber temperature is measured by processing scattered laser pulses
utilizing Raman scattering effects. The optical fibers survived long stroke cyclic bending
sheave mechanical tests up to 22,500 lb tension (over a 46” diameter sheave). The
system supports temperature measurements every 100 seconds in time, and every ½
meter along the cable. In lake test measurements [1], the standard deviation against XBT
data was 0.3 °C.
The purpose of this paper is to show the potential of the ITC for assessing
environmental variability. This will primarily be done by showing evidence of internal
wave activity taken from ITC sea test data.
43
N.G. Pace and F.B. Jensen (eds.), Impact of Littoral Environmental Variability on Acoustic Predictions and
Sonar Performance, 43-48.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
44
2
A.A. RUFFA AND M.T. SUNDVIK
Description of sea tests
Although there are several versions of the ITC, the two sea tests discussed here were both
conducted with the LBVDS tow cable, shown in Fig. 1. The first sea test took place on
June 26–28, 2000 on the U.S. continental slope, South of New England (in the vicinity of
40 deg N, 71 deg W) in water depths of 200 m to 850 m. Site selection was based on
previously well-sampled acoustic propagation and water column studies using
instrumented tow body technologies and moorings as a part of the PRIMER experiments,
conducted in 1996 by a team of researchers from WHOI, URI, and the Naval
Postgraduate School [2].
Figure 1. Cross-section of armored cable showing location of tubes containing optical temperature
measurement fibers.
The cable was towed at a nominal speed of 5 knots for 24 hours, while the ITC
system automatically collected temperature data every 2.5 minutes (with some gaps).
During the same 24-hour period, a T-10 XBT measurement was taken approximately
every fifteen minutes to provide a comparison.
The second sea test was conducted in April-May 2001 near the Hudson Canyon.
Here the ITC was towed and was also deployed from a stationary ship. The data set
consisted of four separate events, a shallow water tow at 3 knots along the 80-m
bathymetric contour, two separate overnight deployments while the ship was at mooring,
and a 7 knot tow further offshore in the vicinity of the shelf break front. Figure 2 shows
the location of these measurements.
3
Data conditioning
For both sea tests, ITC and XBT data were compared as a check on the accuracy of the
ITC data. ITC measurements are different from XBT measurements in two fundamental
ways: (1) each ITC measurement is averaged over 100 seconds along an inclined tow
cable while the ship is transiting (while XBT measurements are taken along a vertical
line); and (2) the error from one ITC measurement cell to the next is independent, in
contrast to XBT data [3]. The latter difference means that spatial averaging of ITC
measurements will reduce the scatter or jitter. An 11-point smoothing in depth was
INSTRUMENTED TOW CABLE MEASUREMENTS
45
performed, reducing the standard deviation of the error (Fig. 2). This is practical because
the ½-meter spatial resolution is more than needed: at 5 knots, and with slightly over 300
meters tow cable deployed, the critical angle of the tow cable allows the water column to
be sampled to a depth of 120 m. Thus, 600 measurements are made of the 120 m depth
profile, or one measurement every 20 cm in depth.
Calibration of the ITC bias error was not complete at the time of the sea tests,
requiring a 1.5 °C adjustment of the ITC temperature to remove the offset. This is also
evident in Fig. 3
Figure 2. Location of measurements from second sea test conducted in April-May 2001. Orange
lines indicate the location of the towing vessel’s tracks. Moorings where measurements occurred
overnight were at either end of the shallow track. The yellow box indicates the location of the
New Jersey STRATAFORM geophysical survey region where very high-resolution bathymetry is
available [5].
4
Results
Figures 4 and 5 are temperature plots from the first and second sea test, respectively. In
both plots, the cable depth varied with tow speed, and was determined by matching
appropriate ITC and XBT temperature profiles (there was no depth sensor).
Both plots appear to show strong evidence of internal waves. In Fig. 4, internal wave
packets are seen in the data when the vessel was over the shelf waters, as variations in the
depth of the isotherms with a period of about 10 minutes, where the dip in the isotherm is
sharp. The time that the internal wave activity was most evident was also in agreement
with one of the portions of the tidal cycles where solitons have previously been observed
for that area [4].
46
A.A. RUFFA AND M.T. SUNDVIK
ITCVS. XBT
22
Temperature (deg C)
20
18
XBT20:39
ITC20:34
16
ITC20:37
ITC20:40
ITC20:43
14
12
10
0
10
20
30
40
50
60
70
80
90
100
Depth(m)
Figure 3. Comparison of XBT to ITC measurements collected nearly at the same time. Raw ITC
measurements are at the bottom, the XBT at the top. As a result of these comparisons, an 11-point
depth smoothing and an offset of 1.5 °C was added to the ITC measurements.
Figure 4. False color image plot of temperature from first sea test. Oscillations in the upper water
column imply the presence of solitons at approximately 6 pm.
INSTRUMENTED TOW CABLE MEASUREMENTS
47
Figure 5 shows a false color plot from the second sea test. These temperature data
comprise the whole of the shallow tow along the 80-meter bathymetric contour. The
depth of the tow was inferred strictly from the speed of the tow, assuming a critical angle
(cable was assumed to follow a straight line in the water). Therefore fluctuations in the
depth of the bottom of the cable are due to variations in estimates of the ship’s speed.
Therefore, some error is induced by the uncertainties in ship speed as well as fluctuations
in the water column itself. The average tow speed was 3 knots. Fluctuations in the depth
of the thermocline are evident in the data, on the order of 1 to 5 meters in amplitude.
These fluctuations are most likely temporal in nature, and are of much higher frequency
than tidal fluctuations. They are most like produced by internal wave activity.
Figure 6 shows measurements taken while at anchor. These measurements have no
uncertainty in the depth of the tow, as the cable was suspended vertically while on a
mooring, and the currents and winds were small. (< 1 knot current, winds < 10 knots).
They depict only temporal variability. Note the strong similarity in amplitude and
frequency of these fluctuations to that of the slow tow. The measurements are a candidate
for estimating the internal wave field at frequencies below the half the sampling
frequency of the temperature measurement, or up to 1/120 Hz.
Figure 5. False color plot of temperature from second sea test. Oscillations implying presence of
internal wave activity are evident.
5
Summary
The Instrumented Tow Cable has demonstrated strong evidence of measuring the temperature
variability due to internal waves of the water column both in South of New England in the area of
the shelf-slope front, and in the Hudson Canyon area. The ITC accomplished this as a
straightforward upgrade to conventional cables commonly used by the Navy for towed systems,
and all measurements can be made during ship transit. Thus, the ITC has the potential to provide
a capability for directly measuring internal waves whenever the ship is in appropriate locations.
48
A.A. RUFFA AND M.T. SUNDVIK
Figure 6. False color image of temperature data collected during R/V Endeavor Cruise EN-353a
(second sea test) while at mooring. Colors representing degrees Celcius are shown at right side.
Fluctuations are similar to those observed during the shallow tow of Fig. 5. These fluctuations are
temporal indications of internal wave activity in the area.
Acknowledgement
This work is supported by the Office of Naval Research (ONR 321SS, Mr. Ken Dial)
under the Environmentally Adaptive Sonar Technology and Lightweight Broadband
Variable Depth Sonar Programs. The authors wish to acknowledge the helpful technical
discussion with Dr. Norman Toplosky (NUWC Division Newport) in modeling the
critical tow angle to obtain estimates of depth for the measurement cells. The assistance
of Walter Paul (WHOI) and Jessica Mary Donnelly (MIT), and the seamanship of the
crew of F/V Nobska, are also gratefully acknowledged. For the second sea test, the
authors gratefully acknowledge the assistance of Darren Blier, Alyssa Cosmo, Jason
Bard, the Officers and Crew of R/V Endeavor, and the data analysis and display work of
Ronald Regnier.
References
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measurement, Sea Technology 41, 38–43 (November 2000).
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Acoustics portion of the New England shelfbreak front PRIMER experiment,
http://acoustics.whoi.edu/AO/topics/Primer/Primer.html (1996).
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Atmos. Oceanic Technol. 4, 128–136 (1993).
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D.W. (MIT Press, Cambridge, MA, 1987) pp. 100–107.
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