Abstract:
An underground marker and methodology for locating the same is discussed. The marker has a tuned circuit and an energy storage device. The marker also has a transponder that is coupled to the tuned circuit. When the tuned circuit receives electromagnetic energy at its tuned frequency, the tuned circuit resonates and provides power to the energy storage device, which in turn powers a transponder. The transponder has a memory containing a coded signal. The transponder transmits this coded signal using the tuned circuit. The transponder alters the impedance of the tuned circuit in accordance with the coded pattern. In this manner, an addressable underground marker can be used to locate particular types of buried structures. Nonaddressable markers can be used to locate addressable markers by providing a reference point for searches.

Description:
SPECIFICATION 
     This application is a continuation-in-part of application Ser. No. 60/163,925, filed Nov. 5, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electrical markers that are located underground for the purpose of locating buried structures. 
     BACKGROUND OF THE INVENTION 
     Buried structures include pipelines, cables, etc. Once a structure is buried in the ground, it becomes difficult to locate. Location is useful, for example, to dig up the structure for repair or to avoid the structure when performing nearby excavation. 
     Electrical markers are used to locate buried structures. The markers are located adjacent to a structure and then are buried with that structure. 
     In the prior art, each marker contains one or more tuned LC circuits. Each circuit typically includes a coil of wire. In order to locate a buried marker, an operator moves across the surface of the ground with a transmitter and a receiver. The transmitter sends out an electromagnetic signal tuned to the frequency of the marker. Upon receiving the transmitted signal, the marker resonates and thus produces an electromagnetic response. This response is received by the above ground receiver and converted to a signal that is detectable by the operator (for example, an audio tone). The operator marks the pinpointed location on the ground using paint and then moves on to find the next marker buried along the structure. 
     Thus, with the prior art, the location of the marker and the location of the buried structure can be determined. 
     In the prior art, each utility has a particular frequency of marker. For example, a marker intended for use by a water utility is made to operate at a first frequency, while a marker intended for use by a telephone utility is made to operate at a second frequency. Thus, it is possible to distinguish between a telephone utility buried structure (for example, a cable) and a water utility buried structure (for example, a water pipe) by the frequency of the buried markers. 
     Buried structures can be nonuniform and have substructures that are of interest. For example, a buried telephone cable can have a splice. Locating the splice with prior art markers is difficult. This is because it is difficult to identify a particular one of the many markers that may be buried along the length of the cable. Each marker is virtually indistinguishable from the other marker except by its location. 
     It is desirable to have an underground marker that can be distinguished from all of the other underground markers that lie along the same buried structures. Such a marker would allow the location of particular substructures, such as a splice, a vault hatch, a valve, etc. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an electrical marker for use in locating buried structures, which marker can be specifically identified. 
     It is another object of the present invention to provide an electrical marker for use in locating buried structures, which marker is addressable. 
     The present invention provides an apparatus for use in locating buried structures. The apparatus has a tuned circuit having an inductance and a capacitance. An energy storage device is connected to the tuned circuit. A transponder has a power input that is connected to the energy storage device and a trigger input connected to the tuned circuit. The transponder has a memory for containing and identifying code. The transponder has an output that is connected to the tuned circuit. The transponder operates to transmit by way of the tuned circuit the identifying code when the trigger input is activated. 
     In accordance with one aspect of the present invention, the inductor is a flat circular coil. 
     In accordance with another aspect of the present invention, the transponder comprises a shift register. 
     In accordance with still another aspect of the present invention, the apparatus further comprises a phase shift transmitter coupled to the transponder output and to the tuned circuit. 
     In accordance with still another aspect of the present invention, the apparatus further comprises a waterproof housing surrounding the tuned circuit, the energy storage device and the transponder. 
     In accordance with another aspect of the present invention, the tuned circuit is a first tuned circuit. The apparatus further comprises a second tuned circuit with a second transponder electrically coupled thereto. The first and second tuned circuits are nonplanar with respect to each other. 
     In accordance with still another aspect of the present invention, the apparatus further comprises a third tuned circuit with a third transponder electrically coupled thereto. The first, second and third tuned circuits are nonplanar with respect to each other. 
     The invention provides a method of identifying a buried structure. A tuned circuit having an impedance is provided. The tuned circuit is tuned to a selected frequency. Electromagnetic energy at the selected frequency is received by way of the tuned circuit, wherein the tuned circuit resonates. The resonating in the tuned circuit is interrupted in accordance with a coded pattern. 
     In accordance with one aspect of the method, the step of interrupting the resonating of the tuned circuit in accordance with a coded pattern further comprises the step of changing the impedence of the tuned circuit in accordance with the coded pattern. 
     In accordance with another aspect of the present invention, the method further comprises the step of burying the tuned circuit adjacent to the buried structure before the step of receiving electromagnetic energy. 
     There is also provided the method of identifying a buried structure in the ground. A buried first marker is provided adjacent to the structure. The marker has an identifying code and a transponder. A second marker is buried some distance away from the structure. The ground is subjected to electromagnetic energy at a selected frequency. A second response is obtained from the second marker so as to locate the second marker. The ground is subjected to the electromagnetic energy, searching from the location of the second marker for the first marker. Then, the identifying code in the first response is detected so as to locate the first marker. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view of a buried structure with markers of the present invention adjacent thereto. 
     FIG. 2 is a schematic elevational view of a cross-section of earth, showing the use of the present invention. 
     FIG. 3 is a block diagram of a transmitter and receiver that is used to locate a marker. 
     FIG. 4 is a block diagram of the marker. 
     FIG. 5 is a plan cross-sectional view of the marker. 
     FIG. 6 is an isometric view of the marker, in accordance with a preferred embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The disclosure of my earlier U.S. Pat. No. 5,699,048 is hereby incorporated by reference. 
     In FIG. 1, there is shown a diagram of a buried structure  11 . The buried structure  11  shown could be a cable, a pipeline, etc. The buried structure  11  has substructures  13 ,  15  which are of interest. For example, in a cable, there are enclosures  13 , splices  15 , etc. For a water pipe, there are T-fittings  13 , valves  15 , etc. 
     Markers  17 ,  19 ,  21  are buried with the structure  11 . The markers are located adjacent to the structure. Some of the markers  17  are conventional and may be such as are disclosed in my U.S. Pat. No. 5,699,048. These markers resonate at a selected frequency and provide information on the location of the buried structure. The other markers  19 ,  21  located adjacent to the substructures  13 ,  15  are addressable in that they provide a unique identification code. For example, the marker  19  that is buried next to the substructure  13  will provide a first identification code (for example, binary 10010011) when prompted. The marker  21  that is buried next to the substructure  15  will provide a second identification code (for example, binary 11100010) when prompted. With the unique identification code, an operator can identify a particular marker  19 ,  21  among the many buried markers  17 ,  19 ,  21  and can thus pinpoint the location of the substructure relative to the remainder of the buried structure. 
     For example, a telephone cable splice  15  may be identified in telephone company records as number 8321 located at the intersection of Throckmorton and Fifth Streets (or in the alternative as being located at specific Global Positioning System coordinates or other mapping coordinates). A marker  21  having a unique identification code is located adjacent to the splice  15 . The specific location of the splice can be determined when that marker is found by an operator who utilizes surface equipment. 
     Other markers, whether they provide a unique code or merely a resonant signal common to other markers, can be used to “home” in on the particular marker of interest and thus to the particular substructure of interest. For example, the operator can locate a marker  17 . Once located, the operator can continue along the length of the cable using the other markers  17  along the way until reaching the marker  19  or  21  of interest. Alternatively, the Global Positioning System (GPS) can be used to find the approximate location of the marker (if the GPS coordinates are known), wherein the exact location of the marker can then be determined using the techniques discussed herein. 
     FIG. 2 shows in general the operation of the marker system. The marker  19 ,  21  is first buried adjacent to the structure  11 . After burial, the marker can be located by surface equipment  23 . The surface equipment  23  utilized by an operator contains a transmitter and a receiver (or a transceiver) and is located above the ground or the earth  27 . The transmitter produces a signal  25  at the frequency of interest. This signal penetrates the earth  27  and impinges on the marker  19 . The marker  19  has an antenna that receives the signal. Inside of the marker is a transponder that transmits the unique identification signal  29  back to the surface equipment. On the surface, the surface equipment  23  receives the signal  29  and processes it to extract the identification code that is associated with that marker. Identifying information about the marker  19  is then provided to the operator in visual, audible, or other forms. 
     The identifying code can be unique to a single marker. Alternatively, the identifying code can be unique to a group of markers, such as for those markers that are to be used adjacent to valves. 
     In FIG. 3, there is shown a schematic block diagram of the surface transmitter and receiver  23 . The transmitter portion has a signal generator  31 . The output of the generator is connected to an amplifier  33 , which in turn is connected to an antenna  35 , such as a coil. The signal generator  31  produces a signal (such as a sine wave) at the frequency of interest. If desired, the transmitter can produce plural signals, each at a selected frequency. The receiver portion is connected to an antenna  35 A (or to the antenna  35 ) and has a filter  37  connected thereto. The filter  37  can be a bandpass filter that excludes noise above and below the frequency band of interest. The output of the filter  37  is connected to a decoder or demodulator  39 . The decoder  39 , which has memory, extracts the coded information from the marker signal  29 . The signal may have header information or other information, which is decoded using information in the memory. The output of the decoder is connected to an indicator  41 . The indicator provides an audible or visual indication of the marker&#39;s identity. For example, the coded information can be converted into a base  10  (ten) number, which is then provided to the operator. The number can then be correlated to the particular buried structure that is marked by the marker. One or more bits of the coded information can be used to identify characteristics of the buried structure. For example, a segment of the code can be used to identify the buried structure as a valve or a splice, or as a water utility structure. 
     FIG. 4 illustrates the transponder electronics located inside of a marker  19 ,  21 . FIG. 4 is exemplary, as other types of transponders can be used. For example, a transponder is described in U.S. Pat. No. 5,211,129, the disclosure of which is incorporated herein by reference. 
     The marker has an antenna  43  in the form of a coil. A capacitor  45  is connected in parallel with the coil  43  to make an LC circuit  43 ,  45  that is tuned to the frequency of the generator  31 . Connected in parallel with the tuned LC circuit is an ac/dc regulator and energy storage device  47 . For example, the device  47  can be a relatively large capacitor that can be used to store energy received by the LC circuit. The device  47  has a diode that rectifies and regulates the energy. The output  49  of the regulator and energy storage device  47  is a dc voltage. 
     A rotating type of shift register  51  is provided. The regulator and energy storage device  47  powers the shift register  51 . The shift register  51  has programmable inputs  53 , which allow a string of data bits (D N ) to be programmed therein. The inputs can be programmed, for example, by being open or grounded. The shift register has a clock, or trigger, input  55  that is connected to the LC circuit  43 ,  45 . A voltage limiter  57  can be provided in series with the input  55  if needed. The output  59  of the shift register  51  is connected to a switch  61 . The switch  61  and an impedance  63  (such as a small capacitor) are connected across the LC circuit  43 ,  45 . When the switch  61  is closed, continuity is provided and the impedance is connected across the LC circuit. When the switch  61  is open, continuity is broken and the impedance is no longer connected across the LC circuit. The switch  61  is operable between its open and closed positions by the output  59  of the shift register  51 . 
     The electronics are contained within a waterproof housing  65 . 
     In operation, when the signal  25  (see FIGS. 2 and 4) from the surface transmitter impinges on the marker  19 ,  21 , the LC circuit  43 ,  45  receives the signal and resonates. The regulator and energy storage device  47  captures some of the resonant energy in order to power the shift register  51 . 
     Once the shift register  51  becomes powered, it can operate the switch  61  according to the coded format as represented by its stored data bits. When the received signal from the LC circuit  43 ,  45  is sufficiently high in amplitude, the clock input  55  is triggered and one bit is provided to the output  59 . The output  59  operates the switch depending on the value of the bit. The received signal provided to the clock input goes low because of the ac nature of the signal. When the received signal becomes once again sufficiently high in amplitude, then the clock input  55  is triggered and the next bit in the shift register is provided to the output. The data bits and the shift register are thus provided in a sequential manner to operate the switch  61 . 
     The switch  61  is operated in accordance with the value of the bit on the output  59  of the shift register. For example, if the bit is a “1”, then the switch is closed; if the bit is a “0”, the switch is opened. 
     When the switch  61  is closed, the impedance  63  is connected across the LC circuit  43 ,  45 . The signal  29  (see FIG. 2) produced by the resonating LC circuit shifts in phase due to the added impedance. If the switch  61  is open, the impedance is disconnected from the LC circuit and there is no phase shift or change in the signal  29 . Thus, the code as represented by the bit sequence stored in the shift register is transmitted by the LC circuit as phase shifts. The decoder  39  in the surface equipment  23  detects these phase shifts in the signal  29 . 
     The beginning of the coded information can be represented by a header. The header is a bit sequence that is common to each addressable marker. The shift register  51 , being of the rotary type, can thus transmit the coded information a number of times until the energy provided by the regulator and energy storage device  47  becomes insufficient to operate the shift register  51 . Thus, the coded information need not be transmitted with the header bits as the initial bits. Instead, the surface receiver utilizes the header to identify the beginning of the coded information. Alternatively, the shift register can output its string of bits once and then reset, so that the same bit is always transmitted first. 
     The code is programmed into the shift register during manufacture and before the housing  65  is sealed. The code can be imprinted onto the housing  65  for visual reference. This allows an installer to notate in a database which marker is installed where. 
     The transponder can have separate transmit and receive antennas/coils. Also, various types of encoding and modulation can be used by the transponder. 
     FIG. 5 shows a marker  19 ,  21  in accordance with a preferred embodiment. The coil  43  is relatively large, on the order of 2-4 inches. The coil of course can be smaller or larger, depending on the particular need. The electronics  69 , which includes the capacitor  45  and the remaining components  47 ,  51 , etc. as shown in FIG. 4, are located inside of the coil  43  and are electrically coupled to the coil as required. In order to minimize shock damage, the electronics  69  can be taped or otherwise secured to the coil  43 . In addition, foam can fill the interior space of the coil. The coil  43  and the electronics  69  are put into the waterproof housing  65 . 
     FIG. 6 illustrates another embodiment of the marker  71  (the housing is not shown in FIG.  6 ). The marker has three orthogonal tuned circuits  73 , as discussed in U.S. Pat. No. 5,699,048. This orthogonal arrangement provides an omnidirectional response, which is particularly desired in buried markers. Each tuned circuit has electronics  69  coupled thereto. Thus, each tuned circuit has a transponder. The transponders in a single marker can all be programmed with the same coded information. Alternatively, the transponders in a single marker can have unique codes. Such a marker is useful in determining if the orientation in the marker and the surrounding structure has changed over time. 
     The foregoing disclosure and the showings made in the drawings are merely illustrative of the principles of this invention and are not to be interpreted in a limiting sense.