Data cable

Abstract: Doping the jacket of a data cable reduces the bulk volume resistivity of the jacket material allowing built-up static electricity to discharge. This reduces the likelihood of static charge interfering with data transmission.

Description: This invention relates generally to a data cable and a method for production thereof and, more particularly, to the dissipation of static charge produced on the surface of a data cable as a result of dry air blowing across its outer surface. A data cable used in an outdoor environment (such as those found on seismic surveys) is subject to many harsh conditions. The cable typically must endure temperature, humidity, and ultra-violet radiation extremes that range from the desert, to the rain forest, to the Arctic. The jacket of the cable generally has to be abrasion resistant to withstand the rough handling on the cable that may be retrieved and deployed several times a day over all types of terrain in all ranges of climates. To successfully endure these conditions, traditional cables have used high-quality outer jacketing material such as polyurethane. In addition to the properties necessary to survive the adverse listed conditions, these materials typically are moderately good electrical insulators, with bulk volume resistivity values of about 10 12 ohm-cm. As is well known, a static electrical potential difference is produced between dissimilar materials when they are rubbed together. This effectively occurs when (dry, perhaps sand-laden) air blows across the surface of an insulative cable jacket, due to the accumulation of electrical charge. If there is an effective path or means for the static charge to dissipate then no harm is done to the signal being carried by the conductors in the cable. However, if there is no such effective means present that allows dissipation of the charge as rapidly as it is accumulated then eventually voltage breakdown will occur between the outer jacket of the cable and the surrounding ground plane thereby creating a path through which the charge dissipates. When there is such a voltage breakdown, this typically results in a relatively high current arc which is capacitively coupled as noise into the signals being carried by the conductors. The effectiveness of the path of dissipation is inversely proportional to the resistance values of the materials used to form the jacket of the cable; an insulator will retain a charge while a conductor will dissipate it. Therefore, to reduce noise due to the build up of static charge, there exists a need to provide a cable providing a better electrical path from the surface of a cable (where the charge accumulates) to the ground on which it is lying. For outdoor applications such as seismic surveying, this problem is typically worse on very dry, windy days and almost non-existent on rainy days. In severe cases in seismic applications, prior art has suggested a labour intensive method of wetting the cable and/or the nearby surrounding soil, which in remote and desert areas may be impractical. In any event, since seismic surveying is conducted over a vast area at a time such a solution can be very expensive. Furthermore, this solution provides only temporary relief, i.e., until the moisture has been evaporated. Thus, there remains a need for a data cable which effectively dissipates or discharges static charge built up on the surface of the cable jacket. According to a first aspect of the present invention there is provided a method of producing a cable comprising the steps of doping a polyurethane jacket with a semi-conductive polyethylene material thereby reducing the bulk resistivity of the jacket, and placing said jacket around an electrical conductor. It will thus be apparent that embodiments of the present invention may provide a method of increasing the conductivity of the cable jacket that does not compromise the other physical characteristics of the cable. The basic jacketing material to be modified is polyether based thermoplastic polyurethane, known generically simply as polyurethane. Polyurethane has wide use due to its ease of processing and its exceptional abrasion resistant properties. It is customarily blended with some additives such as UV radiation inhibitors to enhance the physical properties. It may be blended with small amounts of other polymers such as polyethylene colorants without seriously degrading its physical or electrical properties. For instance, doping the jacket with black colorant concentrate changes the colour of the jacket but does not seriously affect the bulk resistivity of the jacket material. Thus, the charge on a cable jacket may be removed at a sufficient rate that it will not reach the level that results in an arc. In so doing, the generation of electrical signal noise (spikes) transferred to the conductors within the cable may be significantly reduced. "Semi-conductive polyethylene" is a special classification of material that is used (see US-A-4317001) to limit peak insulation stress around the wires of high voltage cables. Homogeneously blending a small amount of this material into the polyurethane drastically reduces the bulk resistivity of the composite. Tests indicate that blending less than a quarter of a percent of the semi-conductive polyethylene material is sufficient to reduce the bulk resistivity of the composite enough to allow the dissipation of charge as rapidly as it is generated, thereby reducing static related electrical noise. The draining of the charge may be further enhanced by embedding small conductors in the jacket material. These conductors could then be intentionally connected to a ground stake. According to a second aspect of the present invention there is provided a data cable having an electrical conductor surrounded by a cable jacket, the cable jacket comprising a material doped to reduce the bulk resistivity of the jacket. According to a third aspect of the present invention there is provided a data transmission system comprising a data transmitter, a data receiver, and a data cable according to the above second aspect of the invention. While the present invention is particularly adapted to seismic survey systems, it is also broadly applicable to data communication systems having long cables with conductors carrying low signal levels and exposed jackets that are subject to a buildup of a static charge. Thus, according to a fourth aspect of the present invention there is provided a seismic survey system comprising a plurality of geophones, a geophone string electrically coupling the geophones to an analog to digital converter, and a data transmission cable according to the above second aspect of the invention for coupling the analog to digital converter to a recording means. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, wherein: Figure 1 depicts a system in which an embodiment of the present invention may be used. Figure 2 is a cross section of a typical cable constructed in accordance with an embodiment of the present invention. For the purpose of explanation and clarity, and not as a way of limitation, a cable is described for use in seismic surveying applications. Figure 1 depicts a part of a system for use in seismic surveying which utilizes a cable embodying the present invention. When a prior art cable is used in such a system, static charge builds up on the jacket of the cable thereby degrading the performance of the entire system. In the system of Figure 1, several geophones 10 or velocity sensors (sometimes several hundred or several thousand) are placed in a spaced apart relation on the earth's surface over a predetermined area which may cover several square miles. Groups of geophones are electrically coupled to separate remote signal processing units, such as remote unit 16 via cables 12. Cables 12 typically rest directly on the earth's surface. The placement of the geophones and their interconnection to each other and to the remote processing unit 16 depend upon various design criteria, including the number of desired data channels to be used for each such remote data processing unit, which are well known in the art of seismic surveying. The remote signal processing units may be single or multiple channel type and they typically serve as analog to digital converters, receiving control signals and power from remote equipment located on trucks (not shown), processing signals received from geophones and transmitting the processed signals to the equipment located on the trucks for storage and further processing. Other conductive cables 14, 18 and the like are also electrically coupled to the remote signal processing units to perform certain specified functions. Typically, like cable 12, cables 14 and 18 rest on the earth's surface and are used to provide power to the remote signal processing units and to transport electrical signals to and from the remote signal processing units to the equipment on the truck, which equipment controls the operation of the system of Figure 1 and stores the data or signals received from the remote processing units. The cables 14, 18 are often further connected to couplings 20 and 22 at one or more than one location. These couplings may optionally be connected to ground spike 26 via ground wires 24. In the system of Figure 1, cables 12, 14 and 18 are made according to the present invention. In operation, shock waves or sound waves are transmitted into the earth every few seconds. These shock waves are reflected and refracted from various formations under the earth and are returned to the earth's surface. The geophones 10 detect these returned shock waves, produce corresponding electrical signals, which are extremely small, and transmit them to the remote signal processing units. In dry operating environments, dry, perhaps sand-laden, air blowing develops a static charge on the surface of the jackets of cables 12, 14 and 18 and the like. This static charge, if allowed to build up, can discharge and create noise in the signals being transmitted through the cables, thus degrading or distorting the data. In very dry conditions, as noted earlier, it is common to wet the cable and the surrounding ground to provide a respective conductive path between the earth and jackets of the cables, 12, 14, 18 and the like which, in seismic surveying of large surface areas, can be very expensive and in many instances impractical. The present cable may provide a solution to a very longstanding problem in the industry. Figure 2 depicts a typical cross section of a cable embodying the present invention. The cable typically includes a pair of twisted conductors 28 and a jacket 30. However it may contain any number of conductors configured in any desired manner. Typically, the cable 12 may take the configuration shown in Figure 2, while cables 14 and 18 may take the form of a telemetry cable having multiple data and power conductors. The jacket 30 is preferably made from polyurethane doped with about 0.25% polyethylene semi-conductive material such as Union Carbide DHDA-7707 Black 55. Using the above-noted amount of the semi-conductive material provides a cable jacket having no more than about 10 9 ohm-cm bulk volume resistivity. Tests have shown that a jacket with 10 5 to 10 9 ohm-cm of bulk volume resistivity effectively reduces static electricity build up on cable jacket surfaces. Tests also have shown that doping polyurethane with up to ten percent (10 %) of a semi-conductive material does not materially degrade the physical characteristics of the jacket. It should be noted that if a jacket material other than polyurethane is used, such as neoprene rubber, a dopant other than semi-conductive polyethylene may be required.

Claims: A method of producing a data cable comprising the steps of doping a polyurethane jacket with a semi-conductive polyethylene material thereby reducing the bulk resistivity of the jacket, and placing said jacket around an electrical conductor. A method according to claim 1, comprising the step of embedding electrical conductors within the doped material. A method according to claim 1 or 2 wherein the step of doping comprises doping said jacket to a bulk resistivity of 10 5 to 10 9 ohm-cm. A method according to claim 1, 2 or 3 wherein the step of doping comprises doping the jacket with said semi-conductive material to 0.25%. A data cable having an electrical conductor surrounded by a cable jacket, the cable jacket comprising polyurethane doped with a semi-conductive polyethylene material thereby reducing the bulk resistivity of the jacket. A data cable according to claim 5 wherein the cable jacket has a bulk resistivity of 10 5 to 10 9 ohm-cm. A data cable according to claim 6 wherein the dopant comprises 0.25% of the jacket. A data cable according to any one of claims 5 to 7 comprising electrical conductors embedded within the cable jacket. A data transmission system comprising a data transmitter, a data receiver and a data cable according to any one of claims 5 to 8. A seismic survey system comprising a plurality of geophones, a geophone string electrically coupling the geophones to an analog to digital converter, and a data transmission cable according to any one of claims 5 to 8 for coupling the analog to digital converter to a recording means.

Inventors: SACKETT JAMES A, SACKETT,JAMES A.
Applicants: I O EXPLORATION PRODUCTS U S A, I/O EXPLORATION PRODUCTS (U.S.A.), INC.
Application year: 1993
Main classification: H01B11
IPC codes: H01B11