Abstract:
A system for efficiently and cost effectively monitoring the status of the interface between two dissimilar media is provided. In a preferred embodiment, the system uses principles applied from the theory of time domain reflectometry (TDR), together with novel circuitry and low cost narrow band telemetry, to provide real time monitoring on a continuous basis, as needed. The circuitry involved permits operation of the system without relying on relative values of signal amplitude while employing a novel feedback function that sets the pulse repetition frequency instantaneously to permit an optimum data collection rate as well as a separate measure of the status based on the system operating parameters. It has particular application to real time monitoring and alerting to the effect of scour events in waterways.

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. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to sensing, monitoring and alerting functions. In particular, it employs novel electronic circuits to detect, monitor and alert to a present condition based on the location of the interface between materials having different coefficients of refraction, such as may occur typically at the interface between water in a stream and sediments thereunder during a scour event. 
     2. Description of the Prior Art 
     Scour is a severe problem that results in millions of dollars of damage to infrastructure and substantial loss of life annually. Scour occurs during times of high tides, hurricanes, rapid river flow, and icing conditions, when sediment, including rocks, gravel, sand, and silt, are transported by currents, undermining bridge and pier foundations, submarine utility cables, and pipelines, and filling in navigational channels. Scour is dynamic; ablation and deposition can occur during the same high-energy hydrodynamic event. The net effect of scour has not been easily predicted, nor readily monitored, in real-time heretofore. 
     Bridge scour monitoring technologies are known. In U.S. Pat. No. 5,784,338, issued Jul. 21, 1998 to Norbert E. Yankielun et al, an instrument called a “time domain reflectomer” (TDR) is directly connected to a parallel transmission line consisting of a pair of robust, specially fabricated non-corroding rods or wires (hereinafter “leads”). The principle of TDR is generally known, described in the technical literature, and applied to numerous measurements and testing applications. The technique was applied to scour detection and monitoring in the aforesaid &#39;338 patent, which is incorporated herein by reference. TDR operates by generating an electromagnetic pulse, or a fast rise time step, and coupling it to a transmission line. The pulse travels down the transmission line at a fixed and calculable velocity, a function of the speed of light and the electrical and physical characteristics of the transmission line. The pulse propagates down the transmission line until the end of the line is reached, and is then reflected back toward the source. The time in seconds that it takes for the pulse to propagate down and back the length of the transmission line is called the “round trip travel time” and is calculated as described in the &#39;338 patent. For a two-wire parallel transmission line, changes in the dielectric media in the immediate surrounding volume cause a change in the roundtrip travel time of a pulse initiated thereon. Further, at any boundary between differing media located along the transmission line (e.g., air/water, water/sediment, etc.) a discontinuity exists that is characterized by a change in the refractive index from one medium to the next. As a pulse imposed on the transmission line encounters these boundaries, a portion of the pulse is reflected back to its source. The remaining portion of the pulse continues on to encounter other boundaries with like results, or the end of the transmission line from which it is reflected, in whole or part, back to its source. Measuring the time of flight of the reflected pulse(s), while knowing the refractive index of the media through which it passes enables one to determine where along the transmission line these boundaries are 
     Freshwater has a relatively high dielectric constant and dry sedimentary materials (e.g.: soil, gravel and stone) have a relatively low dielectric constant. Wet sediment has a dielectric constant that is a mixture of the constants of water and dry soil. The dielectric constant of this mixture will vary, depending upon the local sedimentary material constituency. However, in all cases of bulk dielectric, the bulk index of refraction of the mixture will be less than that of liquid water alone and significantly greater than that of the dry sedimentary materials. Some sediment materials, particularly clay-based sediments, can be extremely “lossy”. This lossy behavior of the soil is exhibited by a severe attenuation of an electromagnetic pulse as it propagates along a transmission line surrounded by such materials. The pulse, when launched from a TDR, dissipates as it travels along the transmission line. Sufficient dissipation reduces the reflected pulse energy below a detectable level. For lossy consolidated soils, such as clay, the electromagnetic signal is attenuated greatly as it propagates along transmission line leads embedded in these soils. Levels of signal attenuation may be as great as tens of dBs/m in clay, yielding undetectable reflected signals in some cases. To protect the transceiver from scour action in a stream, it may be beneficial to bury it in the sediment below the expected level of scour. In this scenario, the soil, typically clay, may absorb all or most of a pulse&#39;s energy, some on transmission, and the rest on reflection. For the case in which the transceiver is located in the water above the sediment, a pulse will be minimally attenuated in the water and will reflect strongly from the boundary with the sediment, the sediment having a significantly different refractive index. This occurs because the amplitude of the reflection correlates directly to the ratio between the refractive indices at the boundary. 
     In either of the above scenarios, once a portion of the transmitted pulse is reflected from the water/sediment boundary, the remainder of the pulse propagates to the end of the transmission line leads whereupon it also is reflected. If the reflection from the water sediment/boundary is difficult to detect, the reflection of its complement that must traverse the entire distance of the transmission line will be even more difficult to detect. Discernment of the occurrence of these two significant events thus complicates the problem of identifying a location at which scour in a streambed is occurring, for example. 
     Thus, needed is a real time scour detection and monitoring system that uses information gleaned from its own operation to set optimal operating parameters for purposes of establishing reflected signals that are able to be differentiated. Further, this system should be both operationally and fiscally efficient, able to broadcast continuous data, if need be, in real time using inexpensive narrow-bandwidth transmitters and data processors. 
     SUMMARY OF THE INVENTION 
     A system for efficiently and cost effectively monitoring the status of the interface between two dissimilar media is provided. The system uses principles applied from the theory of time domain reflectometry (TDR), together with novel circuitry and low cost narrow band telemetry, to provide real time monitoring on a continuous basis, as needed. 
     In a preferred embodiment, a system employing TDR techniques using a pulsed signal generator but having novel circuitry unique to this invention, is emplaced in an environment that permits access to a boundary between one media and a second media of interest. This may be, e.g., a streambed in which the first media is water and the media of interest is the sediment thereunder. Using basic principles of TDR, an electromagnetic pulse is imposed on parallel transmission lines embedded so as to traverse portions of both media, traversal through the interface therebetween being of most importance. The time of travel of this pulse to a first boundary, that is ostensibly the boundary of interest, is used in a feedback line to establish the pulse repetition frequency of operation of the pulse generator of the system via operation of a portion of the circuitry that is unique to this invention. The reflected pulse is also provided to a signal processing circuit that prepares the pulse for transmission on a low cost narrowband telemetry system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a preferred embodiment of the present invention as installed to monitor a boundary between differing media together with relative amplitude and polarity of representative pulses appearing on a transmission line thereof. 
     FIG. 2 is a side view of the embodiment of FIG. 1 inverted in a similar installation. 
     FIG. 3 depicts representative circuitry associated with a preferred embodiment of the present invention as connected to the transmission line leads via an impedance matching transformer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Refer to FIG. 1. A boundary monitoring system  112  of the present invention is shown emplaced such that an electronics package  114 , including a pulse generator circuit  116  employing TDR techniques internal thereto, are located at an uppermost position with respect to the generally parallel transmission line leads  110  to be pulsed via connection with the pulse generator circuit  116 . In a more permanent installation, the electronics package  114  may be powered via an external source, preferably a DC power supply, connected via a cable to the electronics package  114 . Neither the source nor cable is separately shown. 
     As depicted, the boundary  120  of interest includes a first media having a refractive index, n 1 , and a second media having a refractive index, n 2 , where n 1 &gt;n 2 . In operation, a transmitted pulse  130 , that may be of positive polarity as depicted, is imposed on the transmission line leads  110  in the direction indicated by the arrow  122 . At the boundary  120 , a portion  132  of the transmitted pulse  130 , is reflected back towards the source, i.e., the pulse generator circuit  116 . The remainder (not separately shown) of the transmitted pulse  132  continues along the transmission line leads  110  to the termination  118  thereof. Depicted in FIG. 1 is a short circuit termination  118 . At the termination  118 , this remainder portion is reflected back, in whole or in part, as depicted by the reflected pulse  134  of opposite polarity. Note that as the pulse travels further along the transmission line leads  110  and encounters media of a smaller refractive index, because a portion of the transmitted pulse has already been reflected and then enters a media having a lower refractive index, n 2 , the reflected pulse  134  is attenuated significantly as shown by comparing the amplitude of the reflected pulse  134  from the termination  118  with that of the reflected pulse  132  from the boundary  120 . In some cases the reflected pulse  134  from the termination  118  may not be detectable by a standard detector. 
     Refer to FIGS. 1 and 2. The transmission line leads  110 , comprising wires or rods as used in a system for monitoring scour in a streambed or along a shoreline, may be approximately 1-2 meters (3-6 ft) in length. The diameter of the wire or rods  110 , nominally approximately 1.6-3.2 mm ({fraction (1/16)}-⅛), as well as spacing thereof, may be selected to achieve an impedance match with a first media into which the leads  110  of the system  112  are installed. As is seen by comparing FIGS. 1 and 2, this first media may be either that comprising the greater refractive index, n 1 , as in FIG. 1 or that of the lesser refractive index, n 2 , as shown in FIG.  2 . The leads  110  may be terminated in a short circuit  118  as indicated in FIG. 1 or have an open circuited termination  218  as shown in FIG.  2 . An open-circuited transmission line results in a reflected pulse  244  at its termination  218  of the same polarity as is transmitted whereas a short-circuited transmission line, as depicted in FIG. 1, reverses the polarity of the reflected pulse  134  at its termination  118 . 
     In FIG. 2, note that because the transmitted pulse  240  initiated in the direction indicated by the arrow  224  first traverses media having a refractive index, n 2 , then encounters a boundary of media having a refractive index, n 1 , where n 1 &gt;n 2 , the reflected pulse  242  from the boundary  120  is reversed in polarity. This fact is key in designing installations of the present invention. Also note that the relative amplitude of the initiating pulse  240  of FIG. 2 is shown as being greater than that of the initiating pulse  130  of FIG. 1 while that of the reflected pulse  242  from the boundary  120  is less than that of the reflected pulse  132  of FIG.  1 . This pictorially conveys the significant attenuation encountered when a preferred embodiment of the present invention is installed so that the transmission line&#39;s “transmitting end” is installed in media having a relatively low refractive index, n 2 . 
     The amplitude of a reflected pulse  132 ,  134 ,  242 ,  244  reflected from a boundary  120  between media having refractive indices of n 1  and n 2 , respectively, is proportional to a reflection coefficient, ρ, given by:              ρ   =       (       n   1     -     n   2       )       (       n   1     +     n   2       )               (   1   )                                
     such that Eon. (1) describes the reflection coefficient of the configuration of FIG. 1, since the first media encountered by the transmitted pulse  130  is that with the refractive index n 1 . Substituting n 2  for n 1  and vice versa in Eon. (1) yields the reflection coefficient for the configuration of FIG.  2 . Thus, since n 1 &gt;n 2 , ρ is positive for the configuration of FIG.  1  and negative for the configuration of FIG.  2 . The reflected pulse  132  at the boundary  120  for the configuration of FIG. 1 is thus positive polarity while the reflected pulse  242  at the boundary  120  of the configuration of FIG. 2 reverses polarity to the negative. 
     For either configuration represented in FIGS. 1 and 2, the terminal reflected pulses  134 ,  244  are of opposite polarity to their respective “boundary reflected” pulses  132 ,  242 . This phenomenon is useful in designing a simple circuit to make use of this difference in polarity so that even relative amplitude does not have to be determined or employed. It is particularly useful in those cases where pulse amplitude of these reflected pulses  132 ,  134 ,  242 ,  244  may be severely attenuated by passing through, not once, but twice, media having a relatively low refraction coefficient. Thus, the difficulty induced in having to detect low amplitude signals due to significant signal attenuation has been removed if one deals only in ascertaining the polarity of the reflected pulses  132 , 134 , 242 , 244 . 
     Either the configuration of FIG. 1 or FIG. 2 may be used to determine the location of the boundary  120 , such as a water/sediment boundary at a pre-specified location in a streambed, while monitoring and alerting to changes therein in real time. Although either configuration represented by FIGS. 1 and 2 may be suitable for operation with the present invention, the configuration of FIG. 1 is preferred because of the greater relative amplitude levels available in the reflected pulses  132 ,  134 . This inherent capability of the configuration of FIG. 1 also means that the transmit pulse imposed on the transmission line leads  110  may be of lower amplitude than that of the configuration of FIG. 2 to achieve a minimally discernible signal with a low cost detector while requiring less energy to power and a concomitant smaller physical embodiment to achieve its function. 
     Refer to FIG. 3. A pulse generator  320 , capable of being triggered in real time, generates a narrow pulse that may be conditioned in a first conditioning circuit  321  where it may be amplified as needed for a specific application by an amplifier  322 . A pulse thus generated is provided to a circulator  324  or Tee (not separately shown). From the circulator  324 , the pulse is provided to an impedance matching transformer  326 , if needed. This impedance matching transformer  326  may be designed with an impedance ratio that assures that “boundary reflected” pulses  132 ,  242  will have either the same polarity of the transmitted pulses  130 ,  240 , i.e., the configuration of FIG. 1, or the reverse polarity, i.e., the configuration of FIG.  2 . In some applications an impedance matching transformer may not be needed so that the physical configuration of the transmission leads  110  may be set to match the expected impedance of the environment into which it is inserted given that the environment maintains relatively constant impedance. As required for a specific application, the impedance matching transformer  326  permits the impedance of the circuit  321  to approximate that of the media, e.g., water or sediment for an in-stream installation, that will initially surround the “transmitter ends” of the transmission line leads  110 . Once imposed on the transmission line leads  110 , the pulse traverses the length of the leads  110 , reflecting at least in part from any boundary  120  and in whole or part from the termination  118 ,  218  of the leads  110 . Upon reflection, the individual reflected pulses  132 ,  134 ,  242 ,  244  re-enter the impedance matching transformer  326  (if present), are blocked from returning to the pulse generator  320  by the circulator  324  and thus encounter a second conditioning and selection circuit  323  incorporating an amplifier  328 , where they are amplified to a usable level prior to being provided to a half-wave rectifier  330 . It is within the half-wave rectifier  330  that a first novel implementation of the present invention occurs. The half-wave rectifier  330  is configured to pass only the reflected pulses  132 ,  242  from the boundary  120 , thus its polarity is chosen to match whatever configuration in which the system  112  is installed, i.e., only a positive polarity pulse  132  would be processed for the configuration of FIG.  1  and only a negative polarity pulse  242  for the configuration of FIG.  2 . The relative amplitude of these pulses  132 ,  242  is immaterial, it being necessary only for them to be sufficient amplitude for use by the half-wave rectifier  330 . The “boundary reflected” pulse  132 ,  242  is then provided to a first inverting amplifier  332  where its polarity is reversed and then on to a first low pass filter (LPF 1 )  334 . The LPF  334  removes the DC component of the signal and provides a “cleaner” pulse  132 ,  242  for further use. 
     The half-rectified pulse  132 ,  242  is then provided for further processing along two paths  336 ,  340 . The first  336  inputs to an output circuit  327  providing the system  112  output while the second  340  inputs to a feedback circuit  325 . Feedback may be initialized through an optional time delay device  344 , generating a time delay, τ, that may be used to establish a minimum pulse repetition frequency (prf) to cycle the transmitted pulses  130 ,  240 . Once the delay, τ, has been imposed on the conditioned half-rectified pulse  132 ,  242 , it is provided to a diode limiter  346  for further conditioning. The pulse  132 ,  242  is configured to have a steep rise time and a “flat top” suitable for use as a trigger pulse. It is then amplified by a third amplifier  348  for use as a trigger pulse to the pulse generator  320 . Thus, a trigger pulse is initiated at the pulse generator  320  based on an interval of time required for a transmitted pulse  130 ,  240  to travel to the boundary  120  of interest and return as “boundary reflected” pulses  132 ,  242  to the amplifier  348  providing the trigger pulse. This “roundtrip time” (and its inverse, the prf of the transmitted pulses  130 ,  240 ) will vary with the position of the boundary  120 , thus the instantaneous prf of the transmitted signal  130 ,  240  provides information that may be translated to a distance value suitable for use in real time monitoring, such as for determining the level of scour in a streambed. Thus, by monitoring the operating parameters of the pulse generator  320 , one may glean useful, precise, time-critical information on occurrences being observed with the sensor system  112 . Further, since it is performed in real time, it provides other useful information, such as the instantaneous rate of scour, so that it may be used for predicting events, taking preventive action, and issuing timely warnings. 
     The output of the system is provided via an output circuit  327  by amplifying, via a fourth amplifier  350 , the conditioned half-rectified pulse  132 ,  242 . The amplified conditioned half-rectified pulse is then provided to a mixer  352  where it is mixed with a signal from a local oscillator (LO)  358 . The operating frequency of the LO  358  is chosen to correlate to that of the prf of the system  112 , generating a frequency difference within the mixer  352  suitable for creating a signal of narrow bandwidth for transmission via a cable (not separately shown) or a low cost telemetry system (not separately shown). This signal is then provided to a second low pass filter (LPF 2 )  354  to remove any high frequency elements and amplified by a fifth amplifier  356  prior to being output for transmission to a remote location for its ultimate use. 
     A cable-based system  112  based on TDR principles may be used for long term or permanent monitoring scenarios in which an umbilical, low-loss coaxial cable (not separately shown) is easily installed in a permanent configuration. This configuration implies a physically short distance, typically a few hundred feet, between the leads  110  and the pulse generator  320 . An implementation using batteries (not separately shown) and a wireless communications device (not separately shown), or submerged acoustic telemetry link (not separately shown), may be used in temporary installations in which the sensor system  112  is retrieved periodically for replacement of batteries and refurbishing as needed. 
     Refer to FIGS. 1 and 2. In a practical installation, a preferred embodiment of the present invention, e.g., a scour sensor system  112 , is buried in river bottom sediments having a refraction coefficient, n 2 , and anchored (not separately shown) at a point below the maximum expected depth of scour. For low-loss sediments, the sensor system  112  may be installed with the electronics package  114  buried deeply in the sediment as shown in FIG. 2, thus providing some protection from scouring action for the electronics package  114 . In those cases where the sediment is consolidated soil, such as clay, the attenuation of the pulsed signal may be severe. For this scenario, the configuration of FIG. 1 is preferred, although some risk of damage to the electronics package  114  from scour events is unavoidable. 
     A preferred embodiment of the present invention may be installed in a streambed or at other water/sediment interfaces by “air jetting” or “hydro-jetting.” In soft sediment, it may be installed by “pile driving” it in or hydraulically forcing it into the sediment. Once emplaced, the top of the sensor system  112  is “surveyed in” relative to a local survey benchmark (not separately shown) to permit ready identification of the geographic location being monitored. 
     After emplacement of the system  112  in a streambed or other waterway, an initial reference level is established for the response of the sensor system  112  to an imposed (transmitted) pulse  130 ,  240 , thus establishing an initial location for the water/sediment boundary  120 . This is done by determining the travel time for a boundary reflected pulse  132 ,  242  to return as well as the roundtrip time for a transmitted pulse  130 ,  240  imposed on the leads  110  to traverse to the termination  118 ,  218  and return to the source  116 . This roundtrip time may be used to assist in calibrating the sensor system  112  and the travel time of the boundary reflected pulse  132 ,  242  is used for initializing the feedback  325  and output  327  circuitry. This information is stored in a suitable storage device such as a computer (not separately shown). 
     Responses received during subsequent operation of the system  112  are acquired, processed, and compared with the stored reference data. A computer algorithm, operated in real time, may be used to compare the reference values with real time data and trigger an alarm when a pre-specified threshold has been exceeded. In one embodiment, output of the sensor system  112  may be multiplexed with signals from other sensor systems  112  that may be used in an array (not separately shown) to monitor the foundation of a structure or sediment field of interest. 
     It is to be understood that the present invention is by no means limited to the particular constructions herein disclosed and/or shown in the drawings, but also comprises any modification or equivalent within the scope of the claims.