Abstract:
A modification to designs of existing hard-wired electrical and electronic systems that extends the operating reach of these systems or improves signal quality, or both. Conventional hard-wired systems have communicated narrow broadband electrical signals only over electrically conductive media such as copper coaxial cable. A modification to the design using an embodiment of the present invention adds electrical-to-optical and optical-to-electrical transceivers, optical fiber, signal conditions and circulators to existing hard-wired systems to permit transmittal of narrow broadband pulses and FM-CW steps signals over a much longer landline than available for conventional systems. Embodiments of the present invention include sensor systems using RF pulses or FM-CW step signals and time domain reflectometry.

Description:
STATEMENT OF GOVERNMENT INTEREST  
       [0001]     Under paragraph 1(a) of Executive Order 10096, the conditions under which this invention was made entitle the Government of the United States, as represented by the Secretary of the Army, to the entire right, title and interest therein of any patent granted thereon by the United States. This patent and related ones are available for licensing. Contact Sharon Borland at 703 428-9112 or Phillip Stewart at 601 634-4113. 
     
    
     BACKGROUND  
       [0002]     Scour is a severe problem that results in millions of dollars of damage to infrastructure and contributes to loss of life annually. Scour occurs during high tides, hurricanes, rapid river flow and icing conditions when sediment, including rocks, gravel, sand, and silt are transported by the currents. Scour undermines bridge pier foundations, submarine utility cables and pipelines, and fills in navigational channels. Scour is dynamic. Ablation and deposition can occur during the same high-energy hydrodynamic event, so the worst-case net effect cannot be easily predicted nor monitored in real-time.  
         [0003]     Several bridge scour monitoring technologies exist, including patented electromagnetic sensors described in U.S. Pat. No. 6,526,189, Scour Sensor Assembly to Yankielun, Feb. 25, 2003 and incorporated herein by reference; U.S. Pat. No. 5,790,471, Water-Sediment Interface Monitoring System Using Frequency Modulated Continuous Wave, to Yankielun and Zabilansky, Aug. 4, 1998 and incorporated herein by reference; U.S. Pat. No. 5,784,338, Time Domain Reflectometry System for Real-Time Bridge Scour Detection and Monitoring, to Yankielun and Zabilansky, Jul. 21, 1998 and incorporated herein by reference; and a patent application by Yankielun and Zabilansky, U.S. patent application Ser. No. 09/293,781, A Scour Detection and Monitoring Apparatus for Use in Lossy Soils, filed Apr. 19, 1999 and incorporated herein by reference. Other work has been accomplished in this area, e.g., see Yankielun, N. E. and L. Zabilansky,  Laboratory Experiments with an FM - CW Reflectometry System for Detecting and Monitoring Bridge Scour in Real - Time,  (paper submitted to  Canadian Journal of Civil Engineering ), 1998; Yankielun, N. E. and L. Zabilansky,  Innovative Instrumentation Techniques for Detect and Measuring Effects of Sediment Scour Under Ice, ASCE Water Resources Engineering,  pp. 204-209, 1998; Yankielun, N. E. and L. Zabilansky,  Laboratory Investigations of a Time Domain Reflectometry System for Real - Time Bridge Scour Detection and Monitoring, Canadian Journal of Civil Engineering,  February 2000; Zabilansky, L.,  Ice Force and Scour Instrumentation for the White River, Vermont,  U.S. Army Corps of Engineers ERDC SR 96-6, 1996.  
         [0004]     While these Time Domain Reflectometry (TDR) instruments are successful in detecting, monitoring and measuring scour and deposition of sediments, their operational range like that of other hard-wired RF systems, is limited by the effects of bandwidth “dispersion” and amplitude attenuation of the short (narrow), broadband pulse that is applied. In typical installations, the distance between instrumentation and probe has not exceeded 300 meters. For larger bridges, dams and other scour-prone structures and for applications where it is desirable to centralize the instrumentation from several clusters of remote probes, this distance may not be practical.  
         [0005]     Current implementations of metallic TDR instruments  300  rely on long lengths of high quality, low-loss coaxial cable  102  to interconnect the above-the-surface TDR instrument with a submerged TDR probe (sensor)  301  typically buried in saturated sediments (not shown separately) such as may occur in a river bottom. The coaxial cable  102  propagates a short broadband pulse  101 , i.e., very narrow pulse in the time domain (often on the order of nanoseconds), which implies a wide bandwidth frequency spectrum. A difficulty with transmitting wide bandwidth signals over “coax” is the degradation of the signal due to electrical attenuation and dispersion of the transmitted signal.  
         [0006]     Refer to  FIG. 1 . Attenuation refers to the decrease in signal amplitude (strength) as it propagates down a transmission line  102 .  FIG. 1 A  illustrates the effect of attenuation on a short broadband pulse  101 . A strong signal (pulse)  101  applied to one end of a transmission line  102  may appear as a very weak signal  103  at the far end of the transmission line  102  due to the attenuating effects of the cable impedance (resistance and reactance). Even the best copper cable has some resistance, resulting in a voltage drop from input to output of the cable. This voltage drop is uniform for all frequencies comprising a wideband signal. The reactive component of the transmission line, i.e., effects of intrinsic capacitance and inductance of the wire, also contributes to attenuation. However, the losses due to this reactance are frequency dependent, i.e., the individual frequency components of “wideband” signals respond non-uniformly because of this frequency “dependence,” thus the amplitude relationship of the various frequency components of a wideband signal (or a step function having a fast rise time) is not consistent.  FIG. 1 B  illustrates the effect of this dispersion on a narrow pulse  101 , effectively “broadening” the pulse as shown by the wide pulse  104 . In frequency-dependent attenuation, the “velocity factor” of the copper cable may vary as a function of the applied frequency(ies), thus changing the phase relationships of the various frequency components that comprise the signal. This is particularly critical with a short, wideband pulse or a sharp rise-time step. Both of these factors (amplitude attenuation and frequency “broadening” or dispersion) contribute to the combined pulse dispersion and attenuation shown in  FIG. 1C  as the “broadened and attenuated” pulse  105 .  
         [0007]     Refer to  FIG. 2  depicting the effects of dispersion on the ability to resolve spatially close dielectric material boundaries. Specifically referring to  FIG. 2 A , the shaded box  200  represents bounded dielectric material. The solid lines  201  A, B represent pulse reflections of the two short wideband pulses  101  originally impressed on the transmission line  102  that communicates with the dielectric material  200 . The reflections  201  A, B are typical of those from leading and trailing boundary interfaces of the dielectric material  200  as transmitted over a relatively short transmission line of less than 300 m. Dotted lines  202  A, B represent reflected pulses as might be seen on a display of a TDR  300 . The two narrow pulses  201  A, B clearly define boundaries as shown by the pulses  202  A, B that are typical of those displayed on a TDR display  300 .  
         [0008]     Refer to  FIG. 2B . Two somewhat dispersed short wideband pulses  203 ,  205  that may have been similar to pulses  201  A, B upon initial impression on the transmission line  102  but traverse a greater distance of the transmission line  102  typically still maintain a marginal ability to discern the two boundaries of the dielectric material  200  after transmission over a moderately long transmission line, typically at least about 300 m. The twin peaks of the signals  204 ,  206  that would typically appear on the TDR display  300  are discernible but the overlap at  210  is considerable.  
         [0009]     Refer to  FIG. 2C . Two greatly dispersed pulses  203  A, B, typically the pulses  203  and  205  that have traversed a greater length of the transmission line  102 , make resolution of material boundaries nearly impossible because the two signals  203  A,  205  A have each now “dispersed” to “reflect” a distorted broad single signal  207  that is typical of what might appear on the TDR display  300 .  
         [0010]     Dispersion and attenuation affect the signal-to-noise ratio of a system as well as the ability to resolve two pulses adjacent in time. Thus, with long copper transmission lines of 300 m or more on which it is necessary to transmit short, wideband pulses, the ability to temporally resolve the peaks of two adjacent pulses diminishes. Of practical concern is the inability to measure changes in dimension of material that is being monitored by a TDR system if the data need be transmitted more than 300 m.  
         [0011]     Thus, what is needed is a system and technique that permits the direct transmission of a short wideband pulse (or step) over distances on the order of at least several kilometers. Preferably, the system employs COTS fiber optic components. The advantage of a fiber optic transmission line is significantly lower signal attenuation rate per unit length than coaxial cable and significantly lower pulse dispersion. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]      FIG. 1 A  depicts only the relative attenuation of a short wideband pulse after passing through a relatively long copper transmission line as would be employed in a prior art system.  
         [0013]      FIG. 1 B  depicts only the relative dispersion of the pulse of  FIG. 1 A  after passing through a relatively long copper transmission line as would be employed in a prior art system.  
         [0014]      FIG. 1 C  depicts both the relative attenuation and the relative dispersion of the pulse of  FIG. 1 A  after passing through a relatively long copper transmission line as would be employed in a prior art system.  
         [0015]      FIG. 2 A  depicts two short wideband pulses as initially reflected from a dielectric shape and transmitted on a relatively short copper transmission line to a TDR display and a trace indicating their most likely presentation on the TDR display as would be employed in a prior art system.  
         [0016]      FIG. 2 B  depicts two short wideband pulses as initially reflected from a dielectric shape and transmitted on a copper transmission line longer than that of  FIG. 2 A  to a TDR display and a trace indicating their most likely presentation on the TDR display as would be employed in a prior art system.  
         [0017]      FIG. 2 C  depicts two short wideband pulses as initially reflected from a dielectric shape and transmitted on a copper transmission line longer than that of  FIG. 2 B  to a TDR display and a trace indicating their most likely presentation on the TDR display as would be employed in a prior art system.  
         [0018]      FIG. 3  depicts a prior art embodiment of a TDR system used with a probe.  
         [0019]      FIG. 4 A  depicts a first embodiment of the present invention as used with a probe.  
         [0020]      FIG. 4 B  depicts a second embodiment of the present invention as used with a probe.  
         [0021]      FIG. 4 B  depicts a third embodiment of the present invention as used with a probe.  
         [0022]      FIG. 5  depicts a fourth embodiment of the present invention as used with a probe.  
         [0023]      FIG. 6  depicts an embodiment of the present invention as used with multiple probes and two multiplexers.  
         [0024]      FIG. 7  depicts an embodiment of the present invention that is an alternative to the embodiment of  FIG. 6 .  
         [0025]      FIG. 8  depicts an embodiment of the present invention that is another alternative to the embodiment of  FIG. 6 . 
     
    
     DETAILED DESCRIPTION  
       [0026]     In select embodiments of the present invention, an apparatus extends the operating reach of systems that have conventionally communicated narrow broadband electrical signals only over electrically conductive media. The apparatus comprises: means for converting first electrical signals to optical signals that retain at least one parameter of the first electrical signals; means for transmitting the optical signals, the means for transmitting communicating with the means for converting the first electrical signals; one or more optical fibers connected to the means for transmitting the optical signals; means for receiving the converted optical signals, the means for receiving connected to the optical fiber; means for converting the optical signals to second electrical signals that retain at least one parameter of the first electrical signals; and means for transmitting the second electrical signals, the means for transmitting the second electrical signals connected to the means for converting the optical signals, such that the apparatus permits system operation at a distance greater than conventional systems incorporating only electrically conductive media.  
         [0027]     In select embodiments of the present invention, the apparatus is a sensor system. In select embodiments of the present invention, the sensor system incorporates circuitry implementing time domain reflectometry (TDR).  
         [0028]     In select embodiments of the present invention, the electrical signals are radio frequency (RF) signals. In select embodiments of the present invention, the RF signals are pulses. In select embodiments of the present invention, the RF signals are FM-CW step signals.  
         [0029]     In select embodiments of the present invention, the electrically conductive media is coaxial cable. In select embodiments of the present invention, the means for converting electrical signals to optical signals is one or more electrical-to-optical converter/transceivers. In select embodiments of the present invention, the means for transmitting optical signals is one or more electrical-to-optical converter/transceivers. In select embodiments of the present invention, the means for converting the optical signals to second electrical signals is one or more optical-to-electrical converter/transceivers. In select embodiments of the present invention, the means for transmitting the second electrical signals is one or more optical-to-electrical converter/transceivers.  
         [0030]     In select embodiments of the present invention, a method for extending the operating reach of systems that have conventionally communicated narrow broadband electrical signals entirely over electrically conductive media comprises: providing means for converting first electrical signals to optical signals that retain at least one parameter of the first electrical signals; converting the first electrical signals to optical signals; providing means for transmitting the optical signals, the means for transmitting optical signals communicating with the means for converting the first electrical signals; providing one or more optical fibers connected to the means for transmitting the optical signals; transmitting the optical signals over the optical fiber; providing means for receiving the optical signals, the means for receiving the optical signals connected to the optical fiber; receiving the optical signals; providing means for converting the optical signals to second electrical signals that retain one or more parameters of the first electrical signals, the means for converting the optical signals communicating with the means for receiving the optical signals; converting the optical signals to the second electrical signals; providing means for transmitting the second electrical signals, the means for transmitting the second electrical signals communicating with the means for converting the optical signals; and transmitting the second electrical signals, such that the method permits electrical signals to be transmitted at a distance greater than conventional methods employing only electrically conductive media.  
         [0031]     In select embodiments of the present invention, the immediately above method is used with a sensor system. In select embodiments of the present invention, the above method is used while implementing time domain reflectometry (TDR) in a sensor system.  
         [0032]     In select embodiments of the present invention, the above method implementing TDR in a sensor system is accomplished by employing radio frequency (RF) signals as the electrical signals. In select embodiments of the present invention, the immediately above method employs the RF signals as pulses. In select embodiments of the present invention, the immediately above method employs RF signals as FM-CW step signals.  
         [0033]     In select embodiments of the present invention, the immediately above method uses coaxial cable for the electrically conductive media.  
         [0034]     In select embodiments of the present invention, the immediately above method employs one electrical-to-optical converter/transceivers as the means for converting the electrical signals to optical signals. In select embodiments of the present invention, the immediately above method employs one or more electrical-to-optical converter/transceivers as the means for transmitting the optical signals. In select embodiments of the present invention, the immediately above method employs one or more optical-to-electrical converter/transceivers as the means for converting the optical signals to second electrical signals. In select embodiments of the present invention, the immediately above method employs one or more optical-to-electrical converter/transceivers as the means for transmitting the second electrical signals.  
         [0035]     In select embodiments of the present invention, a method is employed for retaining the characteristics of narrow broadband electrical signals that conventionally are communicated entirely over electrically conductive media. The method comprises: providing means for converting first electrical signals to optical signals that retain at least one parameter of the electrical signals; converting the first electrical signals to the optical signals; providing means for transmitting the optical signals, the means for transmitting the optical signals communicating with the means for converting the first electrical signals; providing one or more optical fibers connected to the means for transmitting the optical signals; transmitting the optical signals over the optical fiber; providing means for receiving the optical signals, the means for receiving the optical signals connected to the optical fiber; receiving the optical signals; providing means for converting the optical signals to second electrical signals that retain one or more parameters of the first electrical signals, the means for converting the optical signals communicating with the means for receiving the optical signals; converting the optical signals to the second electrical signals; providing means for transmitting the second electrical signals, the means for transmitting the second electrical signals communicating with the means for converting the optical signals; and transmitting the second electrical signals, such that the method preserves characteristics of the electrical signals better than conventional methods employing only electrically conductive media.  
         [0036]     In select embodiments of the present invention, an apparatus is provided for retaining characteristics of electrical signals that have conventionally been communicated over electrically conductive media in a system, comprising: means for converting first electrical signals to optical signals that retain at least one parameter of the first electrical signals; means for transmitting the optical signals, the means for transmitting the optical signals communicating with the means for converting the first electrical signals; one or more optical fibers connected to the means for transmitting the optical signals; means for receiving the optical signals, the means for receiving the optical signals connected to the optical fiber; means for converting the optical signals to second electrical signals that retain at least one parameter of the first electrical signals, and means for transmitting the second electrical signals, the means for transmitting the second electrical signals communicating with the means for converting the optical signals; such that the apparatus preserves the characteristics of the first electrical signals better than systems not incorporating the apparatus.  
         [0037]     In select embodiments of the present invention, a time domain reflectometry (TDR) sensor system is provided. The TDR sensor system employed as an embodiment of the present invention communicates narrow broadband RF signals partially over electrically conductive media and partially over optical fiber and comprises: means for initiating one or more signals on electrically conductive media; means for facilitating simultaneous transmission of the signals and receipt of reflections of the signals, the means for facilitating connected to the electrically conductive media; means for conditioning the signals and reflections, the means for conditioning communicating with the means for facilitating simultaneous transmission; means for impedance matching the signals and reflections, the means for impedance matching communicating with one or more means for facilitating simultaneous transmission; one or more sensors communicating with the means for impedance matching; means for converting first electrical signals to optical signals that retain at least one parameter of the first electrical signals, the means for converting first electrical signals communicating with the means for conditioning; means for transmitting the optical signals, the means for transmitting the optical signals communicating with the means for converting the first electrical signals; one or more optical fibers connected to the means for transmitting the optical signals; means for receiving the optical signals, the means for receiving the optical signals connected to the optical fiber; means for converting the optical signals to second electrical signals that retain at least one parameter of the first electrical signals, the means for converting the optical signals to second electrical signals communicating with the means for receiving the optical signals, and means for transmitting the second electrical signals, the means for transmitting the second electrical signals communicating with the means for converting the optical signals, such that the system operates at a distance greater than conventional systems incorporating only electrically conductive media.  
         [0038]     In select embodiments of the TDR sensor system as described above, the means for initiating one or more signals is a TDR instrument.  
         [0039]     In select embodiments of the TDR sensor system as described above, the means for initiating one or more signals is a signal generator.  
         [0040]     In select embodiments of the TDR sensor system as described above, the means for facilitating simultaneous transmission is a microwave circulator.  
         [0041]     In select embodiments of the TDR sensor system as described above, the means for conditioning the signals and reflections is one or more amplifiers  
         [0042]     In select embodiments of the TDR sensor system as described above, the means for impedance matching the signals and the reflections is one or more impedance matching transformers.  
         [0043]     In select embodiments of the TDR sensor system as described above, the RF signals are pulses. In select embodiments of the TDR sensor system as described above, the RF signals are FM-CW step signals.  
         [0044]     In select embodiments of the TDR sensor system as described above, the electrically conductive media is coaxial cable.  
         [0045]     In select embodiments of the TDR sensor system as described above, the means for converting the electrical signals to optical signals is one or more electrical-to-optical converter/transceivers. In select embodiments of the TDR sensor system as described above, the means for transmitting the optical signals is one or more electrical-to-optical converter/transceivers.  
         [0046]     In select embodiments of the TDR sensor system as described above, the means for converting the optical signals to second electrical signals is one or more optical-to-electrical converter/transceivers. In select embodiments of the TDR sensor system as described above, the means for transmitting the second electrical signals is one or more optical-to-electrical converter/transceivers.  
         [0047]     In select embodiments of the TDR sensor system as described above, the system further comprises one or more means for data storage and display. In select embodiments of the TDR sensor system as described above, the data storage and display means is one or more TDR instruments. In select embodiments of the TDR sensor system as described above, the data storage and display means is one or more oscilloscopes.  
         [0048]     In select embodiments of the TDR sensor system as described above, the TDR sensor system further comprises one or more multiplexers for multiplexing the RF signals and reflections thereof from multiple sensors.  
         [0049]     In select embodiments of the TDR sensor system as described above, the TDR sensor system further comprises one or more lengths of coaxial cable, each length connecting one microwave circulator to a corresponding impedance matching transformer.  
         [0050]     Refer to  FIG. 3  depicting a conventional connection of a metallic TDR  300  to a scour sensor probe  301  via coaxial cable  102 . The transformer  302  shown between the coaxial cable  102  and probe sensor  301  provides a degree of impedance match between cable  102  and sensor  301 , minimizing the magnitude of a reflection at that boundary.  
         [0051]     While illustrated and explained in detail here for a metallic short wideband pulse (or fast-rise step pulse) TDR-based system, embodiments of the approach apply to a frequency-modulated continuous wave (FM-CW) reflectometer-based system.  
         [0052]     RF component suppliers, such as MINICIRCUITS, MITEQS, and the like, manufacture COTS electronic components, such as amplifiers, voltage-controlled oscillators, and the like, that may be used to build high-resolution FM-CW or pulse-based reflectometers.  
         [0053]     Refer to  FIG. 4A  depicting an embodiment of the present invention  400  employing a TDR instrument  300  suitable for launching and recovering a short, wideband RF pulse. The RF pulse  101  is generated at the TDR instrument  300  and propagates counter-clockwise around the electrical circulator  402 . The pulse exits the circulator  402  and is “conditioned,” e.g., either amplified or attenuated to a specified level at amplifier  411 , for input to a first electrical-to-optical converter/transceiver  409  that converts the RF (electrical) signal to a photonic signal that maintains the waveform and bandwidth characteristics of the original RF signal. The resultant photonic signal next propagates through the fiber optic path  413 , encountering an optical-to-electrical converter/transceiver  407  in which the original RF pulse  101  is re-created from the photonic signal. The “reconstituted” RF waveform is “conditioned” at amplifier  405 , e.g., either amplified or attenuated, as required. The RF pulse  101  is then applied to a second electrical circulator  403 , propagating counter-clockwise around the second electrical circulator  403 , and exiting to an impedance-matching transformer  302  prior to traveling down the parallel transmission lines constituting the sensor probe  301 .  
         [0054]     Upon interacting with dielectric boundaries (not shown separately) in the environment surrounding the probe  301 , one or more reflections (depending on the composition of the medium in which the probe  301  is inserted) propagate back up the transmission lines of the probe  301 . The reflection(s) propagate across the impedance matching transformer  302  and counter-clockwise around the second circulator  403 . Once through the second circulator  403  they are conditioned by the amplifier  404  and applied to a second electrical-to-optical converter/transceiver  406  that converts the reflected RF signals to photonic signals that maintain the waveform and bandwidth characteristics of the reflected RF signals. The resultant photonic signal next propagates through the fiber optic path  414 , encountering a second optical-to-electrical converter/transceiver  408  in which the reflected RF signals are re-created from the photonic signals. The “reconstituted” reflected RF waveforms are “conditioned” at amplifier  410 , e.g., either amplified or attenuated, as required, and input to the first circulator  402 . The conditioned reconstituted reflected RF waveforms propagate counter-clockwise through the first circulator  402  and are applied to the input of the TDR  300  where they are displayed, offset in time from an image of the originally transmitted pulse  101 . This “offset” represents the round-trip propagation time of the originally transmitted pulse  101  from each of the dielectric boundaries that it reflected from with sufficient “strength” to be recognized by the threshold set by the circuitry of the TDR system  400 . That is, the display of the TDR  300  shows all “recognized” reflections from the various impedance changes or mismatches (dielectric boundaries) in the pathways of the TDR system  400  and probes  301 .  
         [0055]     Each mismatch is displayed as a reflected pulse of diminished amplitude that is displaced in time proportional to the pulse&#39;s one-way propagation time plus the return time from the particular mismatch associated with the specific reflection. Selected of these diminished amplitude “reflection” pulses are due to reflections caused by discontinuities in the dielectric material that surrounds the probe  301 , e.g., an air/water or water/sediment boundary. The TDR instrument  300  may be “time gated” to display only those reflections from the environment surrounding the probe  301 .  
         [0056]     Refer to  FIG. 4 B  depicting another embodiment  420  of the present invention. This embodiment  420  is the same as that shown in  FIG. 4 A  except for a short length of coaxial cable  102  inserted between the second circulator  403  and the impedance matching transformer  302 . The short length of the coaxial cable  102  introduces little attenuation and dispersion to any signal impressed thereon while this embodiment of the present invention facilitates locating the probe  301 , e.g., a probe  301  that may be hidden in several feet of sediment on a river bottom.  
         [0057]     Refer to  FIG. 5 , depicting a third embodiment  500  of the present invention. Instead of using the TDR instrument  300  of  FIGS. 4 A , B to generate and display pulses  101 , a short broadband pulse  101  (or fast rise time step) is produced by a signal generator  501 . That pulse  101  is applied to the circuit of the embodiment  500  and propagates through the remainder of the circuit, much as in the system version shown in  FIGS. 4 A , B. This embodiment  500  displays the reflected pulses on an oscilloscope  502  instead of the display of a TDR instrument  300 .  FIG. 5  shows the original pulse  101  and reflected pulses (not shown separately) being displayed on two different trace channels, V 1  and V 2 , respectively. Alternatively, with some additional electronics, the original pulse  101  and reflected pulses may be displayed on a single channel (not shown separately) of the oscilloscope  502 .  
         [0058]      FIG. 6  illustrates another an embodiment  600  of the present invention that multiplexes reflected signals (not shown separately) from several sensor probes  301 . A DC power source  401  provides the “copper path” for the electronics needed to operate the sensor probes  301 . Similar to the embodiment of  FIG. 5 , instead of using a conventional TDR instrument  300 , a signal generator  501  capable of producing short (narrow pulse width) broadband pulses  101  and an oscilloscope  502  are employed. A single channel optical-electrical converter/transceiver pair  407 ,  409  provides the pulse  101  simultaneously to all connected sensor probes  301  via a first multiplexer  601 . COTS multi-channel fiber optic/electrical converter/transceiver module pairs  602 ,  603  employ wavelength division multiplexing. The wavelength-multiplexed converter/transceiver pair  602 ,  603  carries simultaneous responses (reflections) from all probes  301  to the second multiplexer  604  connected to the input of the oscilloscope  502 . Reflections from each probe  301  are simultaneously, but individually, propagated through a single path using optical wavelength multiplexing over the fiber optic portion and an electronic multiplexing switch (not shown separately) once the reflected electrical signal is converted first to an optical signal in converter/transceiver  602  and then back to electrical from optical in the converter/transceiver  603 . The electronic multiplexer  604  is connected to the multiple outputs of the multi-channel fiber optic/electrical transceiver  603 . Responses from individual probes  301  may be displayed on the oscilloscope  502  via selection of the appropriate channel of the multiplexer  604 . Alternatively, as shown in  FIG. 5 , a second oscilloscope channel, V 2 , may display the originally transmitted pulse  101  for “time-of-flight” comparison.  
         [0059]     As shown in  FIGS. 4 A , B, with the addition of appropriate electronics (not shown separately), a TDR instrument  300  may be substituted for the pulse generator  501  and oscilloscope  502  of this embodiment  600 .  
         [0060]     Refer to  FIG. 7  illustrating another embodiment  700  of the present invention using lengths of coaxial cable  102  (in a manner similar to  FIG. 4 B ) inserted between the lower circulators  403  and impedance transformers  302  of multiple probes  301 , thus facilitating a localized distribution of probes  301  a short distance (&lt;300 m) from the interconnected fiber optic cables  413 ,  414 .  
         [0061]     Refer to  FIG. 8  illustrating another embodiment  800  of the present invention using a pair  602 ,  603 ,  802 ,  803  of fiber optic-to-electrical (or electrical-to-fiber optic) converter/transceiver pairs. Here, a first electronic multiplexer  801  is used to selectively and sequentially distribute a short broadband pulse  101  to a multi-channel wavelength division multiplexed fiber optic/electronic converter/transceiver pair  802 ,  803  for selected and sequential distribution to a series of sensor probes  301 . The reflected signals (not shown separately) from each probe  301  are selectively and sequentially transmitted through a return path consisting of another wavelength division multiplexed fiber optic transmission pair  602 ,  603  in a fashion similar to the embodiment  700  of  FIG. 7 , and distributed to an oscilloscope  502  as required for storage and display, e.g., in a sequential and selective manner that has been pre-specified. The electronic multiplexers  604 ,  801  are synchronized as indicated by connection path  804 .  
         [0062]     These examples illustrate using two individual optical fibers  413 ,  414 , one  413  for pulse transmission and a second  414  for reception of the pulse reflection. With appropriate arrangement of electronic components, fiber optic components, and configuration of a wavelength-multiplexing scheme, all signals can be simultaneously passed (in both directions) over a single optical fiber.  
         [0063]     In embodiments of the present invention, COTS fiber optic-to-electrical (or electrical-to-fiber optic) converter/transceivers  407 ,  409 ,  406 ,  408  are employed, such as the family of fiber optic links manufactured by MITEQ® CORP. As an example, MITEQ® manufactures a series of fiber optic-to-electrical and electrical-to-fiber optic converter/transceiver pairs  602 ,  603  intended for RF-to-optic link and optic link-to-RF applications, e.g., a 3-GHz LBL fiber optic link, a 6-GHz SCM fiber optic link, and an 11-GHz MDD fiber optic link. These links each comprise a miniature matched fiber optic-to-electrical and electrical-to-fiber optic converter/transceiver pair  602 ,  603  capable of supporting transmission RF-to-fiber optic and fiber optic-to-RF communications at multi-GHz bandwidths. Since a typical FM-CW signal (step or pulse) used in reflectometry is a short broadband RF signal, it is readily communicated using these components.  
       EXAMPLE  
       [0064]     In a practical application, the “land-based” components (such as a pulse generator  501  or TDR instrument  300 ; oscilloscope  502 ; amplifiers  410 ,  411 ; circulator  402 ; multiplexers  604 ,  801 ; certain converter/transceivers  408 ,  409 ,  603 ,  802 , and the like) are connected by armored fiber optic cables (not shown separately) and a copper conductor pair (not shown separately) to supply ground and a DC voltage to electronic components that comprise the submerged part of the system (such as the probes  301 , the impedance matching transformers  302 ; amplifiers  405 ,  406 ; circulators  403 ; multiplexer  601 ; certain converter/transceivers  406 ,  407 ,  602 ,  803 , and the like. Depending on how deep the probes are installed below the water surface, some electronics may be installed on-land but remotely from the display. These include everything but the impedance matching transformers  302  and the probes  301  themselves, especially if the coaxial cable  102  is inserted between the impedance matching transformers  302  and the circulators  403 . The below-the-water TDR sensor probe electronics may be installed as taught in the patents incorporated herein by reference.  
         [0065]     There are several advantages to the implementation of a fiber optic-based range extender for a metallic TDR scour detection and monitoring system. The distance from a sensor to instrumentation can be extended from less than 300 meters to several (or perhaps several tens of) kilometers. The dispersive effects, i.e., frequency “broadening,” on broadband pulse are nearly eliminated. Attenuation effects on a short (narrow) pulse are nearly eliminated. Multiple sensors may be monitored using one system and an electronic multiplexer.  
         [0066]     Thus, implementation of embodiments of the present invention addresses the following challenge. Select embodiments of the present invention permit installation of a probe array on large structures with all broadband signal paths routed to a single “environmentally benign” remote location for most of the instrumentation. For typical systems employing short broadband RF pulses, select embodiments of the present invention extend the maximum “hard-wired” range between installed sensors and the remote instrumentation from 300 m to several kilometers, if not tens of kilometers. In select embodiments of the present invention, expensive instrumentation may be shared by multiple sensor probes.  
         [0067]     Numerous industrial, commercial, and military instrumentation and measurement systems may employ embodiments of the present invention. Applications include measurement and monitoring of change in materials and material depth, bridge scour, navigation channel sedimentation, dredging spoils stability, and infrastructure, as well as geophysics and engineering investigations.  
         [0068]     Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.  
         [0069]     The abstract of the disclosure is provided to comply with the rules requiring an abstract that will allow a searcher to quickly ascertain the subject matter of the technical disclosure of any patent issued from this disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 CFR § 1.72(b). Any advantages and benefits described may not apply to all embodiments of the invention.