document: BACKGROUND OF THE INVENTION 
       This invention is concerned with locating and tracing concealed elongated conductive objects, such as pipes or cables, and is more particularly concerned with improved locating and tracing of a first object when a second object is adjacent to the first. 
       In the prior art, there are two general techniques of locating buried metallic objects. A passive technique employs a gradiometer or the like as a magnetic locator for detecting the presence of ferrous metal objects, such as iron and steel pipes, iron markers, manhole covers, well casings, etc. An active technique uses a transmitter to induce alternating currents in non-ferrous metal pipes, power cables, or communication cables, for example, and a receiver to sense magnetic fields associated with the currents. 
       The model MAC-51B Magnetic and Cable Locator manufactured by the assignee of the present invention is designed for selective active or passive use. When apparatus of this type is employed to locate and trace a cable (or non-ferrous pipe), for example, a transmitter may be disposed on the ground at a position close to the location (or suspected location) of a portion of the cable so as to induce an alternating current therein that may be traced by moving a receiver back and forth over the ground. When there are no interfering objects close to the cable being traced, this system works admirably, producing a distinct single null in the output signal of the receiver when the receiver is located directly over the cable and is oriented so as to sense a vertical component of a circumferential magnetic field associated with the current in the cable. When, however, another cable (or pipe) is present adjacent to the first cable, e.g., within a few feet of the first cable and extending in the same general direction, the single null output signal characteristic of the receiver becomes distorted, and tracing of the desired cable may become difficult. 
        BRIEF DESCRIPTION OF THE INVENTION 
       The present invention provides a system and method that improves substantially the ease and accuracy of locating and tracing of one concealed object, such as a buried pipe or cable, in the presence of an adjacent object. 
       In one of the broader aspects of the invention, a system for locating at least one of a pair of concealed, elongated, conductive, adjacent objects, comprises, in combination, a transmitter and a receiver, said transmitter having means including a pair of antennae for inducing a pair of distinguishable alternating currents in said objects, respectively, said receiver being movable relative to said transmitter and to said objects, having means for sensing magnetic fields associated with said currents, respectively, and having means for producing an output signal dependent upon the sensing of both of said fields. 
       In another of the broader aspects of the invention, a method of locating at least one of a pair of concealed, elongated, conductive, adjacent objects comprises producing in said objects a pair of distinguishable alternating currents, respectively, moving with respect to said objects a receiver sensitive to a pair of magnetic fields associated with said currents, respectively, and producing an output signal from said receiver dependent upon the sensing by said receiver of both of said fields. 
      
      
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be further described in conjunction with the accompanying drawings which illustrate preferred (best mode) embodiments, and wherein: 
       FIGS. 1 and 2 are diagrammatic views illustrating the use of prior art apparatus in locating and tracing a buried cable; 
       FIG. 3 is a diagrammatic view illustrating an output signal characteristic when a prior art receiver encounters a pair of adjacent cables (or pipes); 
       FIG. 4 is a diagrammatic view illustrating transmitting apparatus in accordance with the invention; 
       FIG. 5 is a diagrammatic view illustrating an optimum position of the transmitting apparatus with respect to a pair of buried pipes or cables; 
       FIG. 6 is a view similar to FIG. 3 and illustrating an improvement in the output signal characteristic due to the invention; 
       FIG. 7 is a view similar to FIG. 5 but illustrating the transmitting apparatus in a non-optimum position; 
       FIG. 8 is a view similar to FIG. 6 and illustrating the output signal characteristic for the disposition of the transmitting apparatus in FIG. 7; 
       FIG. 9 ,is a block diagram of transmitting apparatus employed in the invention; 
       FIG. 10 is a block diagram of receiving apparatus employed in the invention; and 
       FIG. 11 is a diagrammatic view illustrating a modification of transmitting antennae orientation. 
      
      
       DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1 illustrates, diagrammatically, the use of the aforesaid model MAC-51B Cable Locator to locate and trace a buried cable (or pipe) C. A transmitter T with a loop antenna A is placed on the ground over a portion of the cable C (a portion that is known or located experimentally) and generates an electromagnetic field F that is coupled to the cable C and that induces in the cable an alternating current. The current has a circumferential field F&#39; associated therewith that is sensed by a receiver R moved back and forth over the ground by an operator O. Apparatus of this type is well known and need not be described in detail. 
       As shown in FIG. 2, in which the cable C extends perpendicular to the plane of the drawing, when the receiver R is held vertically (so as to sense a vertical component of the field F&#39;) and is moved back and forth across the cable C (three positions of the receiver being illustrated), an output signal characteristic S is produced having a null N directly over the cable and two lobes L and L&#39; at opposite sides of the cable. By sweeping the receiver back and forth across the cable while moving along the general direction of the cable, the position of the cable may be readily traced. 
       When a second cable (or pipe) C&#39; is present adjacent to the first as shown in FIG. 3, the output signal characteristic S may be distorted so that the null N is located between the cables and one of the side lobes has a substantially greater amplitude than the other. The configuration of the output signal characteristic depends, for example, upon the depth of the second cable C&#39; relative to the first cable C, the distance between the cables, and their relative size and conductivity. Thus, when two cables are present, running generally in the same direction, tracing of the desired cable may become difficult. The present invention alleviates this problem to a substantial degree, as will now be described. 
       As shown in FIG. 4, the invention employs a transmitter T&#39; having a pair of antennae A&#39; and A&#34; that are preferably spaced a few feet apart (say 3-5 feet), that are driven by RF signals, and that generate corresponding magnetic fields F1 and F2. Each of the antennae A&#39; and A&#34; may comprise 100 turns of No. 14 wire wound on a 1/2 inch by 8 inch ferrite rod, for example. As described in more detail hereinafter, the signals that drive the antennae are distinguishable, and the fields F1 and F2 induce corresponding distinguishable currents in cables C and C&#39;, respectively. As shown in FIG. 11, the orientation of the antennae may be changed from the horizontal orientation shown in FIG. 4 to enhance the inducement of currents in the respective cables. 
       The transmitting apparatus is optimally positioned relative to the cables as shown in FIG. 5. Sometimes sufficient information as to the location of at least part of the cables is available to permit such positioning initially. At other times, however, such information is not available, and the transmitting apparatus may be initially positioned as shown FIG. 7, i.e., centered over one of the cables, or even completely beside the cables. Usually, sufficient information is available to determine at least the approximate location of a portion of a cable (or pipe) to be located and traced. After initial tracing, using a receiver R of the type referred to earlier, for example, the position of the transmitter may be moved to the position of FIG. 5 to optimize further tracing operations. 
       As described hereinafter in more detail, the system of the invention is capable of producing two distinct output signal nulls N and N&#39; over respective cables C and C&#39;, as shown in FIG. 6. It is thus possible to locate and trace one of the cables (or even both cables) more easily and accurately than with prior art systems and methods. As is apparent in FIG. 6, lobes L and L&#39; are located at opposite sides of the cable C, and although these lobes may have different amplitudes, the null N is readily perceived. 
       When the transmitting apparatus is located as shown in FIG. 7, the output signal characteristic may have the configuration shown in FIG. 8, in which one of the lobes L&#39;, is substantially distorted. By moving the location of the transmitting apparatus in the direction of the distorted lobe L&#39;, it is possible to arrive at the position shown in FIG. 5 and to produce an output signal having the characteristic shown in FIG. 6. The output signal characteristics shown in FIGS. 6 and 8 may be shifted upwardly or downwardly with respect to a base line by adjustment of a receiver deadband control, for example. 
       When the receiver R is employed to trace a cable C in the presence of an adjacent cable C&#39;, the receiver will normally be swept back and forth across both cables to facilitate the desired positioning of the transmitter and to monitor the total output signal characteristic as the receiver is moved in the general direction of the cable(s) to be traced. In accordance with the invention, output signal characteristics of the type shown in FIGS. 6 and 8 are produced only when the receiver senses both fields associated with the currents in the respective cables, which are distinguishable. Among the techniques that may be employed to make the currents distinguishable from one another and to produce an output signal dependent upon the presence of both currents are: (1) currents having different carrier frequencies that may be combined to produce a beat frequency, (2) currents having the same carrier frequency amplitude-modulated by different frequencies that may be combined to produce a beat frequency, and (3) currents that are pulsed at different repetition rates that may be combined to produce a beat frequency. Other techniques may also be employed to distinguish the currents in the respective cables and to produce an output signal dependent upon the presence of both currents. 
       As shown in FIG. 9, in a first embodiment the transmitter T&#39; has carrier generators t and t&#39; that produce sinusoidal carrier currents of 82.300 KHz and 82.682 KHz, for example, which drive antenna A&#39; and A&#34;, respectively. The carrier frequencies when detected in the receiver R, will produce a beat frequency signal of 382 Hz. To produce a pulsating audio output signal which is easier for the operator to distinguish from background noise than a steady tone, each of the carrier frequencies may be pulsed on and off at a 6 Hz rate, for example, by a pulse generator t&#34;. 
       FIG. 10 illustrates a typical receiver employed in the invention (which may be similar to the receiver of the model MAC-51B Magnetic and Cable Locator referred to earlier). The fields associated with the currents in the cables C and C&#39;, for example, are sensed by a sensor coil 10 (which may be wound upon a ferrite core) producing a combined signal that is supplied to an 82.5 KHz amplifier 12. The amplified signal is detected in an 82.5 KHz detector (demodulator) 14. The amplifier 12 amplifies both the 82.300 KHz and the 82.682 KHz carrier components in the combined signal from coil 12, and the detector 14 (a non-linear circuit) detects the envelope of the amplified signal and produces a 382 Hz beat frequency signal (pulsating at 6 Hz) when both components are present. A filter 16 passes the 382 Hz beat frequency signal to a variable gain amplifier 18, and the amplified beat frequency signal is applied to a 382 Hz detector 20. A 6 Hz pulsating signal from detector 20 (a non-linear circuit) is passed by a low pass filter 22 to a voltage controlled oscillator 24, which produces a variable frequency signal that is amplified by an audio amplifier 26 to produce a pulsating output signal that is supplied to a speaker 28. 
       If, instead of using different carrier frequencies to drive the respective antennae A&#39; and A&#34;, the same carrier frequency is used, both currents may be amplitude modulated by the same 382 Hz modulation frequency but pulsed at different and asynchronous pulse rates, such as 20.12 Hz for one antenna and 23.87 Hz for the other. The two signals will blend in the receiver and produce 20.12 Hz or 23.87 Hz pulsations of a 382 Hz signal at the output of detector 20 when a signal from only one cable is present and will produce a beat frequency signal of 3.75 Hz at the output of detector 20 when signals from both cables are present. Thus, if the low pass filter 22 is set to reject frequencies above 4 Hz, for example, an output signal from the speaker 28 will only be produced when currents in both cables are sensed by the receiver. 
       As a further alternative, the same 82.5 KHz carrier (pulsed on and off at 6 Hz, for example) may be employed for both antennae but modulated at 1288 Hz and 906 Hz, respectively, which will produce a pulsating beat frequency signal of 382 Hz at the output of detector 14 when the currents in both cables are sensed. This signal may be processed as in the first embodiment. 
       The invention is especially useful in an environment in which the horizontal separation s between the cables is related to the depth d of the cable to be located and traced in accordance with the relationship s&lt;11/2d. The effect achieved by the invention is enhanced by the fact that the field from the transmitter, and hence the excitation at a cable, decreases by the inverse cube of the distance between an antennae and a cable. For example, if the cables and the antennae were each separated horizontally by 3 feet and the cables were buried 3 feet, then a signal due to a given antenna in a cable under that antenna would be 2.8 times stronger than a signal due to that antenna in a cable 3 feet to one side of the antenna. This phenomenon substantially reduces the inducement of currents from both antennae in the same cable when the transmitter is properly positioned. It also enhances the desired performance of the receiver, which may be optimized by adjustment of a threshold sensitivity control (indicated in FIG. 10). 
       While preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims. For example, the transmitter may be designed so that only one of the antennae may be energized (e.g., by a modulated 82.5 KHz carrier as in the aforesaid Model MAC-51B) for cable locating and tracing when only a single cable is present.
summary: Locating and tracing of a concealed, elongated, conductive object, such as a buried pipe or cable, is enhanced, when a second such object is adjacent to the first, by employing a transmitter having a pair of antennae that induce distinguishable currents in the respective objects. A receiver movable with respect to the transmitter and with respect to the objects produces an output signal dependent upon the sensing of fields associated with both currents. The position of the transmitter relative to the objects is adjusted to optimize the output signal.