Abstract:
An acoustic-sensing underwater tow cable includes a cable core for transmission of power/signals there along, a first jacket encasing the cable core, and discrete regions of carbon nanotubes affixed to the first jacket. The carbon nanotubes at each of the discrete regions define an acoustic sensor. A second jacket encases each acoustic sensor and any electrical conductors coupled thereto.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates generally to underwater tow cables, and more particularly to a cable that includes a cable core for transmission of power and data signals, a first jacket encasing the cable core, and discrete regions of carbon nanotubes (CNT) layers affixed to the first jacket and covered by a second jacket. The layers of carbon nanotubes at each of the discrete regions define an acoustic sensor. The positioning of the layers of carbon nanotubes provides specific acoustic information about the water column between the towing vessel and the towed sensor system as well as at other locations in the vicinity of the towed sensor system. 
     (2) Description of the Prior Art 
     Underwater surveillance is frequently performed using acoustic arrays/systems that are towed through the water. In general, these systems comprise a towing vessel, an electro-optical mechanical tow cable coupled on one end to the towing vessel, and a towed sensor system coupled to the other end of the tow cable. The towed sensor system includes an array of hydrophones designed to sense a variety of underwater acoustic signals based on a particular surveillance mission. 
     The sensor system is deployed at a generally horizontal orientation at some underwater depth as the tow cable traverses the distance/depth between the towing vessel and the towed sensor system. The tow cable provides the mechanical strength needed to tow the sensor system; the electricity required to power the sensor system; and for data signal transfer between the sensor system and the towing vessel. However, this type of acoustic surveillance system provides a limited amount of acoustic information about the water column between the towing vessel and the towed sensor system. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general purpose and primary object of the present invention to provide an acoustic-sensing underwater tow cable with the ability to provide detailed acoustic information about the water column between a towing vessel and a towed sensor system as well as at other locations in the vicinity of the towed sensor system. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, an acoustic-sensing underwater tow cable includes a cable core for transmission of power and data signals there along, a first jacket encasing the cable core, and discrete regions/layers of carbon nanotubes affixed to the first jacket. The CNT layers at each of the discrete regions define an acoustic sensor. At least one electrical conductor is coupled to each acoustic sensor. A second jacket encases each acoustic sensor and the electrical conductors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: 
         FIG. 1  is a prior art schematic view of a towed sensor system being towed by a vessel; 
         FIG. 2  is a cutaway view of an acoustic-sensing underwater tow cable in accordance with an embodiment of the present invention; 
         FIG. 3  is a side view of a carbon nanotube acoustic sensor region backed with acoustic isolation material in accordance with an embodiment of the present invention; and 
         FIG. 4  is a cutaway view of an acoustic-sensing underwater tow cable construction in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Prior to describing the acoustic-sensing underwater tow cable of the present invention; reference is made to  FIG. 1  in which a surface-deployed vessel  100  tows a sensor array  102  in a horizontal orientation at some depth under the water surface  200 . As is known in the art, the sensor array  102  can be a hosed towed array or may be a rigid body equipped with wings, fairings, etc., (not shown) that maintain the proper towed body orientation during towing. 
     The sensor array  102  is mechanically and electrically (and/or optically) coupled to the towing vessel  100  via an electro-optical and mechanical tow cable  104 . The tow cable  104  includes strength members and data-carrying conductors/fibers as well as electrical power transmission wires. 
     Referring now to  FIG. 2 , an embodiment of an acoustic-sensing underwater tow cable construction in accordance with the present invention is shown and is referenced by numeral  10 . The tow cable  10  could be used to replace the above-described tow cable  104 . The tow cable  10  is illustrated in a cut-away view in order to illustrate the various portions thereof. 
     The central portion of the tow cable  10  is a cable core  12  that can include an outer jacket  120  disposed about a number of power and signal transmission lines  122 . Typically, the power and signal transmission lines  122  include one or more electrical conductors  122 A and one or more optical fibers  122 B. In addition to providing power, the transmission lines  122  support signal transfer between a towing vessel and a towed sensor system that would be coupled to either end of the tow cable  10 . The particular design of the cable core  12  is not a limitation of the present invention. 
     While the outer jacket  120  may provide sufficient mechanical strength for some (non-towing) applications, armor wires  14  are provided about the jacket. The particular type of the armor wires  14 , configuration, and size, are not limitations of the present invention. By way of non-limiting examples, the armor wires  14  include metal wires made from materials such as galvanized plow steel or synthetic fibers made from commercially-available materials such as KEVLAR or SPECTRA fibers. 
     Encasing the cable core  12  and armor wires  14  is a jacket  16  that extends along the length of the tow cable  10 . The jacket  16  is generally extruded in place. The jacket  16  is made from a flexible waterproof material such as polyurethane, nylon, or high-density polyethylene. The jacket  16  serves as the substrate for a number of relatively inexpensive acoustic sensors as will be explained further below. 
     In accordance with the present invention, acoustic sensors are defined on the jacket  16  by regions  18  of carbon nanotube (CNT) layers such as single-walled and multi-walled CNTs. In particular, research has shown that thin-film acoustic transducers may be built by using single-walled carbon nanotubes (SWNTs) that are thin, transparent, lightweight, durable and have an exceptional acoustic response. 
     More specifically, the regions  18  of CNTs are affixed to the jacket  16 . Each region  18  is a thin layer of CNTs that can be sprayed or rolled on masked-off regions of the jacket  16 , or affixed to the jacket via film transfer techniques. The well-known electrical properties of CNTs allow each region  18  to function as an acoustic sensor or hydrophone. The size/shape of the region  18  can be used to tune the region for sensitivity to specific acoustic frequencies. 
     For example, sensitivity to lower frequencies could be achieved by increasing the size/length of the region  18  along the tow cable  10 . Each region  18  of CNTs has one or more electrical conductors  20  coupled thereto with conductors extending back along the length of the cable  10  to the end thereof that is to be coupled to a towing vessel. The number of conductors  20  may be optimized by using electrical signal techniques such as multiplexing. 
     Encasing the regions  18 , the conductors  20 , as well as exposed portions of the jacket  16 , is another waterproof jacket  22 . The jacket  22  can be similar to the jacket  16  in that it can be extruded in place and can be made from polyurethane, nylon, or high-density polyethylene. 
     The advantages of the present invention are numerous. Since a conventional underwater tow cable typically includes a cable core  12 , armor wires  14  and a jacket  16 ; the conventional underwater tow cable can be readily modified to incorporate a number of CNT-region acoustic sensors. In this way, an underwater tow cable can provide acoustic information within the water column between a towing vessel and a towed sensor system as well as other locations in the vicinity of the sensor system. 
     Although the present invention has been described relative to a particular embodiment thereof, the scope of the present invention is not so limited. For example, as shown in  FIG. 3 , each region  18  of CNTs could be disposed on a layer of acoustic isolation material  24 . The goal of the isolation material  24  is to isolate the region  18  from mechanical vibrations emanating from the cable core  12  and the armor wires  14 . The isolation material  24  can be an acoustic isolation material (e.g., rubber, nylon, or polyurethane) that is applied prior to affixing the regions  18  or simultaneously with the regions. 
       FIG. 4  illustrates another embodiment of the present invention where a layer  26  of CNTs are affixed to the jacket  16  along the length thereof in such a way that the discrete regions  18  are still defined while the layer  26  covers the remainder of the outer surface of jacket. That is, a CNT-free gap  28  surrounds each CNT region  18 . Each gap  28  can be defined by masking the jacket  16  prior to the affixing of CNTs thereto. As a result, the regions  18  still function as acoustic sensors while the remaining contiguous CNT layer  26  provides additional mechanical strength for the core of the tow cable  10 . 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.