Abstract:
In a method of identifying a passive electromagnetic signal emitted from an underground utility, a portable above-ground antenna is disposed at a location at which the passive electromagnetic signal is expected. Measurement signals are received from the antenna corresponding to electromagnetic signals received by the antenna. The measurement signals are measured at a plurality of different frequency bands within a measurement frequency range expected to include a frequency of the passive electromagnetic signal, and at least one of the plurality of frequency bands is selected based on a predetermined criteria applied to the measurements.

Description:
This is a continuation of U.S. application Ser. No. 10/871,491, filed Jun. 18, 2004, now U.S. Pat. No. 7,088,105, which is a continuation of U.S. application Ser. No. 10/263,154 filed Oct. 2, 2002, now abandoned, the entire disclosure of which is incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   Underground utility lines may emit passive electromagnetic signals for various reasons. An electric utility, for example, carries its own electric signal at 50-60 Hz. Other utility lines emit electromagnetic fields within a frequency range of 13 to 22 kHz responsively to ambient fields that induce signals on the utility. Gas providers typically place 100-120 Hz signals on gas lines to provide cathodic protection against corrosion. CATV lines typically carry signals at about 31 kHz. As used herein, any electromagnetic field emitted by a utility line, whether generated by signals carried directly by the utility or by signals induced on the utility by signals ambient to the utility, where the signal is not applied by a cable location system for the purpose of locating the cable, is considered a “passive” signal. 
   It is known to provide an above-ground portable locator having an antenna tuned to receive signals within a desired frequency range for locating the underground utility. Typically, the operator has a general idea of the underground utility&#39;s location. If so, the operator carries the locator so that the antenna is parallel to the ground and perpendicular to the expected line of the utility and walks generally perpendicular to the expected line. The operator views a signal intensity display on the locator and finds the position at which a peak signal appears. At that point, the above-ground position may be marked and a depth reading taken. Where the operator does not know the utility&#39;s general location, he may find the utility by walking in a grid pattern while carrying the locator so that the antenna is parallel to the ground. The operator first walks in parallel lines over the area. If he does not find a peak value, he may be carrying the antenna parallel to the utility. Since underground lines emit a generally cylindrical magnetic field, the magnetic flux lines are perpendicular to the coil antenna&#39;s axis in this orientation and do not induce a significant signal in the antenna. Thus, the operator then turns 90° and walks in parallel paths over the same area. In such a pattern, the antenna is more likely to be perpendicular to the utility and therefore more likely to detect a peak signal. Of course, the operator may not be exactly parallel or exactly perpendicular to the utility in either direction. By using the grid pattern, however, the operator should be able to cross over the utility with the antenna at such an angle that a peak can de detected. At such a point, the operator may rotate the locator about a vertical axis until the antenna is perpendicular to the utility and a maximum signal is measured. Thus, the locator&#39;s orientation notifies the operator of the utility&#39;s direction. An example of a system capable of locating existing underground utility lines carrying active or passive signals is described in U.S. Pat. No. 6,102,136, the entire disclosure of which is incorporated by reference herein. 
     FIG. 1  schematically illustrates components of a portable above-ground locator  10  used to detect passive signals within a frequency range of 13 to 22 kHz resulting from ambient radio frequency signals. A coil antenna  12  receives electromagnetic signals emitted by the utility line, generates a signal responsively thereto and outputs the signal to an amplifier  14  and a pair of bandpass filters  16  and  18 . Filter  16  has a bandpass range of 13 to 17 kHz, while filter  18  passes a range of 18 to 22 kHz. Two bandpass filters are used, as opposed to a single filter of larger bandwidth, to improve the signal-to-noise ratio. A multiplexer  20  selects the output of either of the two filters, depending on instructions received from a CPU  22  controlled by the operator through a keyboard at the locator display. Multiplexer  20  directs the signal from the selected filter to an analog-to-digital converter  24 , and the resulting digital signal passes to CPU  22 . The locator includes a display  26  that indicates the strength of the signal detected by antenna  12  so that the operator may locate a signal peak and, therefore, a point above the underground utility. 
   The locator may also be used to locate signals in other frequency ranges. Generally, the locator includes low pass and bandpass filters, which are configured to pass the respectively desired frequency ranges, in parallel with filters  16  and  18 . The CPU selects the filter for a desired frequency range through a multiplexer. 
   In one known locator, three antennas are used to determine location and depth of underground utilities. A locator housing (not shown) includes three parallel horizontally-aligned antennas (not shown). The bottom antenna (hereafter “C 1 ”) is disposed in the housing so that it is parallel to the ground when the operator carries the locator during operation. The second antenna (hereafter “C 2 ”) is directly above the first antenna and, being parallel to antenna C 1 , is also parallel to the ground. The third parallel antenna (hereafter “C 3 ”) is directly above antenna C 2  in the housing a distance equal to the distance between antennas C 1  and C 2 . The distance between C 1  and C 3  is referred to herein as “d.” The three antennas are connected to amplifier  14  through a multiplexer (not shown) controlled by the CPU. 
   In operation, the CPU repeatedly samples the signals on the three antennas and relies on the difference in signal strength between C 1  and C 3  (C 1 −C 3 =E1) and between C 2  and C 3  (C 2 −C 3 =E2) to locate an underground utility and determine its depth. In locating an underground utility, the CPU monitors and displays the value of E1, and the operator finds a point above the utility by finding the peak value of E1 in a manner as described above. Once finding such a position, the operator activates a button on the locator to determine the utility&#39;s depth, which the CPU calculates according to the function: depth=(d)(E2)/(E1−(2(E2))). 
   SUMMARY OF THE INVENTION 
   The present invention recognizes and addresses the foregoing considerations, and others, of prior art constructions and methods. 
   Accordingly, it is an object of the present invention to provide an improved system and method for detecting passive signals emitted from underground utilities. 
   One embodiment of a system for identifying a passive electromagnetic signal emitted from an underground utility includes a portable housing, an antenna disposed within the housing and a control circuit. A frequency bandpass circuit is disposed operatively between the antenna and the control circuit so that the bandpass circuit receives measurement signals from the antenna corresponding to electromagnetic signals received by the antenna and passes to the control circuit a portion of the received measurement signals within a frequency pass band. The control circuit sequentially sets the bandpass filter circuit to a plurality of different frequency pass bands within a measurement frequency range expected to include a frequency of the passive electromagnetic signal. The control circuit receives the measurement signal portion corresponding to each frequency pass band set by the control circuit and selects at least one of the frequency pass bands based on a predetermined criteria applied to the corresponding measurement signal portions. 
   In an embodiment of a method for identifying a passive electromagnetic signal emitted from an underground utility, a portable above-ground antenna is disposed at a location at which the passive electromagnetic signal is expected. Measurement signals are received from the antenna corresponding to electromagnetic signals received by the antenna. The measurement signals are measured at a plurality of different frequency bands within a measurement frequency range expected to include a frequency of the passive electromagnetic signal, and at least one of the plurality of frequency bands is selected based on a predetermined criteria applied to the measurements. 
   The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended drawings, in which: 
       FIG. 1  is a schematic illustration of a prior art above-ground portable utility locator; 
       FIG. 2A  is a schematic illustration of an above-ground portable locator in accordance with an embodiment of the present invention; 
       FIG. 2B  is a diagrammatic illustration of a part of the schematic illustration of  FIG. 2A ; 
       FIG. 2C  is a diagrammatic illustration of a part of the schematic illustration of  FIG. 2A ; 
       FIG. 2D  is a diagrammatic illustration of a part of the schematic illustration of  FIG. 2A ; 
       FIG. 2E  is a diagrammatic illustration of a part of the schematic illustration of  FIG. 2A ; 
       FIG. 2F  is a diagrammatic illustration of a part of the schematic illustration of  FIG. 2A ; 
       FIG. 3  is a perspective view of an operator and an above-ground locator in accordance with an embodiment of the present invention; 
       FIG. 4  is a schematic illustration of a locator display in accordance with an embodiment of the present invention; 
       FIG. 5  is a schematic illustration of a locator display in accordance with an embodiment of the present invention; 
       FIG. 6  is a schematic illustration of a locator display in accordance with an embodiment of the present invention; 
       FIG. 7  is a flow chart illustrating operation of an above-ground locator in accordance with an embodiment of the present invention; 
       FIG. 8  is a pass band diagram of the filter stages shown in  FIG. 2E ; and 
       FIG. 9  is an exemplary pass diagram for a filter in accordance with an embodiment of the present invention. 
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     FIG. 2A  schematically illustrates an above-ground locator configuration for detecting radio frequency passive signals within a range of 9.6 kHz to 33.2 kHz. A locator  28  includes three coil antennas  30  selected by a CPU  32  through a multiplexer  34 . Although it should be understood that any suitable antenna may be used, each antenna  30  comprises a coil extending about a ferrite core. A first antenna  30  is disposed within the lower end of the housing of a locator  54  ( FIG. 3 ) so that as the operator holds the locator in the vertical orientation shown in  FIG. 3 , first antenna  30  is parallel to the ground. Second and third antennas  30  are disposed in the housing above and parallel to the first antenna in the manner described above. A magnetic field emitted by an underground utility induces measurement signals in antennas  30  that are amplified by an amplifier  36 .  FIG. 2B  provides a circuit illustration of an antenna  30  and amplifier  36 . Only one antenna  30  is illustrated in  FIG. 2B , and multiplexer  34  is omitted. 
   As noted above, the arrangement illustrated in  FIG. 2A  is configured to locate passive signals within a range of 9.6 kHz to 33.2 kHz. Thus, amplifier  36  outputs to a 9.5 kHz high pass filter  38 . CPU  32  controls a variable low pass filter  40 , which follows filter  38 , to frequency thresholds of 12 kHz, 18 kHz, 25 kHz and 33 kHz. As described in more detail below, the CPU scans signals received by the antennas in frequency steps over the 9.6-33.2 kHz range. As the CPU scans through this range, it steps filter  40  through the four threshold levels to eliminate or reduce false signals caused by higher-order harmonics within the scanning filters.  FIG. 2C  provides a circuit illustration of filters  38  and  40 . Filter  40  includes a multiplexer  100  controlled by the CPU to select a combination of input resistors  102  and output resistors  104  to establish the desired frequency threshold. 
   Referring to  FIGS. 2A and 2D , low pass filter  40  outputs to a gain control  42  that includes a multiplexer  106 , a series of resistors  108  and an op amp/resistor pair  110 . As should be understood in this art, each resistor  108  defines a different resistance, for example ranging from approximately 470 ohms to 1.5 Mohms. As described in more detail below, CPU  32  controls multiplexer  106  through control lines  112  to select a desired input resistance  108  to op amp/resistor pair  110 , thereby creating a desired amplification. 
   Referring to  FIGS. 2A and 2E , the now-amplified signals are output from the automatic gain control to a mixer circuit  44  and a bandpass filter  46 . Together, mixer  44  and bandpass filter  46  comprise a selectable bandpass filter. Mixer  44  mixes a clock signal from CPU  32  with the measurement signal from the gain control so that the signal output from the mixer has a frequency equal to the difference between the clock signal frequency and the measurement signal frequency. Bandpass filter  46  includes a pair of filter stages  113  and  114  that pass a frequency range of approximately 1.1 kHz to 1.5 kHz, as indicated in  FIG. 8 . As described below, the CPU controls mixer  44  to cycle the overall variable bandpass filter formed by mixer  44  and filter  46  through frequency bands within a predetermined measurement frequency range in order to find a passive signal emitted from an underground utility of interest, despite the presence of competing background signals. Referring also to  FIG. 2F , bandpass filter  46  outputs the signal to a detection circuit  47 . Detection Circuit  47  AC-couples the signal prior to input to an analog-to-digital converter  48 , which passes a digitized signal to CPU  32 . The CPU communicates with a memory  50 , which may be wholly or partly maintained within the CPU, to store and retrieve signal values. A display  52  assists the user in finding the desired signal. 
   The CPU steps the clock frequency through predetermined levels (from 10.8 kHz to 34.4 kHz) so that the overall variable bandpass filter formed by mixer  44  and filter  40  consecutively steps through the measurement frequency range (9.6 kHz-33.2 kHz in the present example) at 0.4 kHz increments. For example, assume the operator stands above an underground utility that emits a passive signal at 9.6 kHz, disposes antennas  30  so that the antennas are perpendicular to the utility line, and activates the CPU to measure the strength of the signals received by the antennas. The CPU initially sets the frequency threshold of low pass filter  40  to 12 kHz. Due to high pass filter  88  and low pass filter  40 , therefore, the mixer receives a measurement signal ranging in frequency from 9.5 kHz to 12 kHz. Since the utility is emitting a passive signal at 9.6 kHz, the measurement signal contains the peak signal. 
   The CPU also initially sets the clock signal to 10.8 kHz. As noted above, the signal output by the Mixer has a frequency equal to the difference between the clock signal frequency and the measurement signal frequency. Thus, when mixer  44  mixes the measurement signal with the clock signal, the signal output from the mixer has a frequency range of −1.2 kHz (i.e. a 1.2 kHz signal with opposite phase) to 1.3 kHz. Since the peak signal is at 9.6 kHz, the peak in the mixed signal is located approximately at 1.2 kHz, which is within the 1.1-1.5 kHz pass band of bandpass filter  46 . That is, the 1.1-1.5 kHz pass band of filter  46  corresponds to a pass band of 9.3-9.7 kHz (limited by 9.5 kHz filter  88 ) in terms of the measurement signal frequencies, and this includes the peak signal at 9.6 kHz. Being the first measurement, the CPU stores and displays the signal strength and the approximate center frequency of the pass band in which the signal was detected, i.e. the clock frequency step (10.8 kHz) minus 1.2 kHz, or 9.6 kHz. 
   CPU  32  maintains this frequency band for about 0.5 seconds and then increases the clock frequency by 0.4 kHz, to 11.2 kHz. The variable low pass filter remains at 12 kHz. The mixer again receives a measurement signal having a range of 9.5 kHz to 12 kHz. Again, this signal includes the strong portion at 9.6 kHz. The mixer, however, changes the measurement signal&#39;s frequency to a range from −0.8 to 1.7 kHz. The signal peak is at 1.6 kHz (i.e. 11.2 kHz-9.6 kHz) and is, therefore, slightly beyond the filter&#39;s peak pass band of 1.1-1.5 kHz. In other words, the overall filter&#39;s peak pass band now ranges from 9.7 kHz to 10.1 kHz, which excludes the signal peak. The CPU compares the new signal strength with the previously stored signal strength. Since the new measurement is weaker, the CPU does not store the new signal strength or its measurement frequency. 
   The CPU increases the clock frequency by 0.4 kHz, to 11.6 kHz. At this step, the bandpass filter passes the measurement signal corresponding to the frequency range 10.1-10.5 kHz. Thus, the peak signal is again outside the pass band. The signal strength measured by the CPU is less than the stored value, and the CPU therefore maintains the previously stored signal strength and measurement signal frequency values in memory  50 . 
   The CPU then consecutively increases the clock frequency, intermittently increasing the threshold level of low pass filter  40 , as described in more detail below, to accommodate the increase in frequency range, and measures the signal strength at each step. Since each step moves the filter pass band farther from the 9.6 kHz peak value, the CPU maintains the signal values stored at the 10.8 kHz clock frequency step. Upon completely scanning the measurement frequency range, the CPU causes display  52  to display the approximate center frequency (9.6 kHz) of the measurement signal frequency range (9.3 kHz to 9.7 kHz) at which the peak signal was found. 
     FIG. 9  illustrates the pass bands defined by the overall filter comprised of mixer  44  and bandpass filter  46  ( FIG. 2A ) when the clock frequency is 10.8 kHz. As discussed above, a pass range  107  has a peak between 9.3 kHz and 9.7 kHz. As should be understood in this art, however, the filter also defines a pass band  109  having a peak between 11.9 kHz and 12.3 kHz. Accordingly, when the clock frequency is 10.8 kHz, the filter passes peak signals to detector  47  both in the 9.3-9.7 kHz range and the 11.9-12.0 kHz range. Thus, the CPU detects a peak signal and stores the received signal strength in association with the center frequency (9.6 kHz) of pass range  107 , whether the peak occurs in range  107  or range  109 . Low pass filter  40  may, however, limit the existence of signals within range  109 . 
   Because the system correctly records the peak signal when the peak later appears in pass band  107 , the apparent uncertainty regarding the center frequency is resolved as the clock frequency steps up through the operative range. Assume, for example, that the utility emits a signal at 11.9 kHz. The signal peak passes through low pass filter  40 , which has a pass band at this step of 12 kHz, and the CPU stores the detected signal&#39;s strength in association with the 9.6 kHz pass frequency, i.e. in association with incorrect pass band. But when the clock frequency steps to 13.2 kHz, pass band  107  has a peak range from 11.7 kHz to 12.1 kHz, and the detector therefore again detects the signal emitted by the utility. Since the signal strength is equal to or greater than the stored signal strength, the CPU stores the newly-detected signal strength in association with the correct frequency, i.e. 12.0 kHz. 
   The overall filter also defines a harmonic set of pass bands  111  centered at 21.6 kHz when the filter clock frequency is at 10.8 kHz. Low pass filter  40 , however, the threshold of which is set to 12 kHz, removes any signals received by the antennas that would otherwise be detected in this pass range. Thus, the system does not improperly respond to this or other higher order harmonics. 
   As the CPU steps the filter up through the operative frequency range, it intermittently changes the threshold level of filter  40 . In the presently-described embodiment, the CPU sets the low pass filter threshold with respect to the clock frequency as described in the following table: 
                               TABLE 1                       Clock Frequency   LP Filter Threshold Frequency                           &lt;12 kHz   12 kHz           &gt;=12 kHz and &lt;18 kHz   18 kHz           &gt;=18 kHz and &lt;25 kHz   25 kHz           &gt;=25 kHz   33 kHz                        
It should be noted that at the clock frequency&#39;s last step, i.e. 34.4 kHz, the pass range of pass band  107  is 32.9-33.3 kHz. At this point, filter  40  is set to 33 kHz. Thus, the detector sees only signals in the early portion of pass band  107  (32.9-33.0 kHz) and does not see signals within pass band  109 .
 
   As noted above, CPU  32  also controls automatic gain control circuit  42  through control lines  112 . More specifically, and referring to  FIGS. 2A and 2D , CPU  32  controls multiplexer  106  at the beginning of the frequency scan procedure to select a resistor  108  providing the highest gain. That is, at the beginning of the scan, gain is at a maximum. As should be understood in this art, however, it is possible that the A/D converter may “clip” signals above or below the component&#39;s operative range. Accordingly, the CPU compares the intensity levels of incoming signals to predetermined maximum and, optionally, minimum thresholds within the A/D converter&#39;s operative range. The particular threshold settings within the operative range may vary, for example, with the particular component used in the system. If signal intensity reaches the maximum threshold, the CPU switches multiplexer  106  to select another resistor option so that gain is reduced by a factor of ten. If the signal reaches the minimum threshold, the CPU does not increase the gain. As illustrated below, automatic control is unnecessary for low signals in the described embodiment. The CPU monitors intensity levels in this manner throughout scanning of the measurement frequency range. 
     FIG. 7  illustrates operation of the exemplary scan procedure described above. When the operator initiates the procedure at  116 , the CPU sets the initial stored signal voltage level to zero, sets the initial clock frequency at 10.8 kHz and sets the initial center measurement frequency to 9.6 kHz. As indicated in the flow chart, the CPU does not directly store the 9.6 kHz frequency. Instead, a MAX_FREQ variable, which corresponds to the frequency at which the high signal intensity is detected, is initially set to 10.8 kHz. Since the locator displays are offset by −1.2 kHz, however, 10.8 kHz setting corresponds to 9.6 kHz, and the CPU is considered to have “stored” the 9.6 kHz level. At  118 , the CPU determines whether the clock frequency is greater than 34.4 kHz, i.e. the maximum clock frequency covering the scan&#39;s last operative measurement signal frequency range of 9.6 kHz to 33.2 kHz. If not, the routine has not yet reached the end of the scan, and the CPU displays the measurement frequency (i.e. the clock frequency less 1.2 kHz) on locator display  52  ( FIG. 2A ) at  117  and sets low pass filter  40  ( FIG. 2A ) at  119  to its threshold as defined by Table 1 above. 
   At  120 , the CPU measures the signal voltages on each of the three receiver coils. As described above, the three locator coils (C 1 , C 2  and C 3 ) are parallel and vertically aligned in the locator housing. The CPU adds the signal voltages measured on the bottom and top coils (i.e. C1+C3) and considers this value to be the measured signal voltage. 
   At  122 , the CPU compares the measured signal voltage to the maximum voltage threshold corresponding to the analog-to-digital converter overflow. If the signal is below the threshold maximum, the CPU compares the measured signal voltage to the previously stored measured signal voltage (MAX_VOLT) at  124 . If the measured signal voltage is greater than or equal to MAX_VOLT, the CPU stores at  126  the measured signal voltage as MAX_VOLT and stores the center frequency of the measurement signal frequency range at which the measured signal voltage was found. As described above, the CPU stores the measurement frequency through setting MAX_FREQ to the clock frequency at which the signal was detected. The CPU increments the clock frequency at  128  and returns to  118  for the next measurement. If the measure signal voltage is less than MAX_VOLT at  124 , the CPU increments the clock frequency at  128  without changing MAX_VOLT. 
   If the measured signal voltage overflows the voltage threshold at  122 , the CPU automatically reduces the gain by a factor of ten (as discussed above) at  130 . Since this occurrence necessarily means the measured signal voltage is greater than the previously stored voltage level, there is no need for a comparison, and the CPU stores the measured signal voltage and corresponding center frequency at  126 . 
   When the clock frequency incremented at  128  exceeds 34.4 kHz at  118 , the last-stored center frequency is considered the frequency at which the peak signal was found, and the CPU therefore stores this final frequency at  132  (again, through storage of the clock frequency, which is understood to be 1.2 kHz offset from the measurement frequency), displays the final frequency at  133  and ends the frequency scan routine. 
   As discussed above, the pass range of pass band  107  ( FIG. 9 ) is 32.9-33.3 kHz when the clock frequency is 34.4 kHz. Since low pass filter  40  is set to 33 kHz at that time, however, pass band  107  only detects signals from 32.9 kHz to 33.0 kHz. In an alternate embodiment, step  118  checks whether the clock frequency is greater than or equal to 34.4 kHz. Thus, the maximum clock frequency is 34.0 kHz, not 34.4 kHz. The last operative pass band is 32.5 kHz to 32.9 kHz, and the last center frequency is 32.8 kHz rather than 33.2 kHz. 
   It should be understood that the schematic and circuit illustrations in  FIGS. 2A through 2F , and the procedural illustration in  FIG. 7 , are provided for purposes of example only. Thus, for example, it should be understood that various circuit configurations, frequency step dimensions and measurement frequency ranges may be used. Furthermore, as noted above, the present locator may be used to locate underground utilities emitting signals at active frequencies and at frequencies other than within the presently-described radio frequency band. For example, the locator may also be used to locate passive signals at 50-60 Hz for power lines, 110-120 Hz for gas lines, and 512 Hz for fault location. Accordingly, and referring to  FIG. 2A , locator  28  includes three additional band pass filter circuits (not shown), each defining a pass band encompassing a respective one of these three desired frequency levels. The three filter circuits are disposed in parallel with each other and with the series filters  38  and  40 . When the operator selects a desired frequency level as described below, the CPU automatically selects the corresponding one of the three parallel filter circuits through a multiplexer controlled by CPU  32 . 
   The locator may be used to locate utilities emitting signals resulting from signals actively placed on the utility by the operator through direct connection to the utility or through a probe passed through the utility, as described in U.S. Pat. No. 6,102,136. In one presently preferred embodiment, these signals are generated at frequencies of 512 Hz, 9.5 kHz and 38 kHz. As noted above, a parallel band pass filter is provided for 512 Hz signals. When the operator selects the 9.5 kHz and 38 kHz frequencies, however, the multiplexer selects the path of filters  38  and  40 . When detecting 38 kHz signals, filter  40  is set to a level above 38 kHz. 
   Similarly, the locator includes four additional bandpass filter circuits (not shown) in parallel with bandpass filter  46  to accommodate location of signals at the 50-60 Hz, 100-120 Hz, 512 Hz and 9.5 kHz/38 kHz ranges. Although a separate band pass filter is provided for the 9.5 kHz and 38 kHz signals, it should be understood that band pass filter  46  may instead be used for these frequencies. When the operator selects a desired frequency level, the CPU automatically selects the corresponding one of the four parallel bandpass filter circuits through a multiplexer controlled by the CPU. 
   When the locator is not executing an automatic frequency scan, the system operator may manually control the gain through an input interface provided as part of display  52 . CPU  32  drives an indicator on display  52  that describes the magnitude of incoming signals as a percentage relationship to benchmark gain setting. Upon activation of a “Gain” button on display  52 , the CPU determines whether the signal level from the A/D converter is at or above the converter&#39;s maximum threshold value. If so, gain is reduced by a factor of ten as described above. The resulting high (1) and low (0) levels of signals from the converter become the new benchmark levels, and the indicator at display  52  is re-set so that signals at these levels are displayed as a “70%” power level. The percentage power level displayed for any subsequent signal is determined relative to the 70% benchmark signal. Thus, a reading of 100% may indicate that the incoming signal might be too large, and the operator may therefore choose to again activate the Gain button. Additionally, the system provides an indicator on the display whenever the signal level from the converter is at or above the converter&#39;s maximum value that prompts the operator to activate the Gain button. 
   If, when the operator activates the Gain button, the signal level from the A/D converter is below the converter&#39;s maximum value, the high and low signal levels from the converter become the new 70% benchmark levels, but the system does not change the gain. 
   As described above, and referring to  FIG. 3 , a system according to the present invention may be used as part of an above-ground locator  54  carried by an operator  56 . The bottom-most antenna  30  ( FIG. 2 ) is disposed at the bottom of locator  54  so that it is proximate the ground, and the second and third antennas are above and parallel to the first antenna. Display  52  is provided on an upper surface of locator  54  so that it is easily viewed by the operator during the process of locating an underground utility  58 . Upon finding the utility, the operator may record the utility&#39;s location to avoid later collisions with the utility by an underground boring tool. 
   Referring to  FIGS. 2 and 4 , display  52  may be presented in any suitable fashion to the operator, for example by way of an LED display. In the example provided in  FIG. 4 , a touch pad area is provided having a series of six buttons that can be activated by the operator to place the locator in a desired mode of operation and to execute certain functions within one or more given modes. A display screen area  62  may change from mode-to-mode to display information relating to the selected mode. The various operating modes are described below. 
   A “Power” button  80  activates the locator. The locator may be provided with a speaker to permit the system to issue audible alarms and to backlight the display screen. Accordingly, when the operator activates a “Mode” button  78 , display  52  provides a screen (not shown) at  62  that provides sound and backlight options. From this screen, multiple activations of a “Gain” button  72  cycles the device through various sound level options, while a “Depth” button  74  cycles the device through backlight options. 
   Activation of Mode button  78  a second time allows the operator to put the locator in a “probe” mode, in which the locator is configured to receive signals emitted from a probe moved through the underground utility. In the present embodiment, the probe may emit signals at 38 kHz, although it should be understood that the particular frequency used may vary. The operator may put the receiver in or out of probe mode by activating a “Frequency” button  76 . Activating the probe mode selects an appropriate band pass filter (not shown) in parallel with filter  46 , as described above. The CPU selects filters  38  and  40 , with low pass filter  40  being set to an appropriate level. Activation of the “Mode” button a third time returns the locator to location functions and the receiver is set to operate at 38 kHz. 
   The operator may also turn probe mode off and then re-activate the Mode button. At this point, or following power-up without going into probe mode, “Frequency” button  76  allows the operator to set the receiver for non-probe reception of signals at five predetermined frequency levels. Three frequency levels are used with signals directly or indirectly placed on the underground utility by an above-ground transmitter. In direct modes, the transmitter places a location signal on the utility through a cable that attaches to the utility or to a conduit encasing the utility. In the presently described embodiment, the signal is either a combination of 9.5 kHz and 38 kHz or a combination of 512 Hz and 9.5 kHz. Generally, 9.5 kHz is preferred for locating cables in areas congested with existing utilities and for power cables and tracer wires, while 38 kHz signal is preferred for CATV lines and metal pipes and cables. Thus, in such conditions, the transmitter applies the 9.5/38 kHz signal in direct connection or external connection mode, and the operator sets the locator to receive at 9.5 kHz or 38 kHz, as appropriate. The 9.5 kHz/512 Hz signal may be used for cable location but is preferred to detect faults. In the induction mode, the transmitter is placed on the ground above the utility and emits an electromagnetic field that induces a signal on the utility at 38 kHz. Additionally, the locator may be set to locate signals at 50-60 Hz (power frequencies) and 110-120 Hz (gas line signals). 
   Each of these five frequency positions (9.5 kHz, 38 kHz, 50-60 Hz, 110-120 Hz, and 512 Hz) is stored by the system CPU and memory, and the Frequency button allows the operator to cycle through the frequency options to select a desired range. Upon activating Frequency button  76 , and referring to  FIG. 5 , screen  62  displays at  66  the last frequency band used in this mode. If, for example, the operator had last used the locator to find natural gas lines, indicator  66  presents “120” to indicate that the locator&#39;s last setting was 110-120 Hz. If the operator wishes to again search for a gas line, he activates Frequency button  76  a second time, thereby causing CPU  32  ( FIG. 2A ) to select the appropriate band pass filter in parallel with filters  38  and  40  ( FIG. 2A ) and the appropriate band pass filter in parallel with band pass filter  46  ( FIG. 2A ). If, however, the operator wishes to move the locator from the 120 Hz mode to 38 kHz, he activates button  76  two additional times to move CPU  42  through the 512 Hz setting to the 38 kHz position. Generally, regardless of the initial frequency setting, the operator activates button  76  to move through the frequency settings (9.5 kHz, 38 kHz, 50-60 Hz, 110-120 Hz, 512 Hz) to a desired frequency range. 
   Generally, the operator sets the receiver to one of these frequency settings in order to locate and determine the depth of a utility line that is emitting signals at the known frequency. Then, the operator searches for and locates the utility as described above. 
   Note that the operator may set the locator to 38 kHz reception either in probe mode or in cable location mode. While the receiver&#39;s filter settings are the same in the two modes for this frequency, the depth calculations are different. 
   Referring again to  FIG. 4 , an RF Search button  64  sets the locator to a search mode for passive frequencies. As described above, the present embodiment searches for passive frequencies within an operative range of 9.6 kHz to 33.2 kHz, although larger, smaller and/or different ranges could be used as desired. Again, and referring also to  FIG. 2A , system memory  50  retains the last frequency level at which the system was set in this mode. Thus, upon activation of the RF Search button, CPU  42  sets the mixer to locate that frequency, which is displayed as a blinking number at  66 . Remaining battery life is indicated at  67 . At this point, the CPU measures signals received at the indicated frequency and provides the intensity level of those signals at a bar graph  69  and numerically at  70 . The bar graph is a percent of the “maximum” magnitude level as described above. 
   If the frequency level of passive signals from the desired utility is known, or if the operator desires to manually search for the frequency level, the operator may manually change the CPU&#39;s frequency setting though “up” button  72  and “down” button  74 . Each button activation moves the frequency displayed at  66  up or down 0.4 kHz, depending on which button is pushed. If the operator is moving the receiver to a known frequency level, the operator again activates RF Search button  64  upon reaching that level, thereby causing CPU  42  to set the bandpass filter to the desired frequency band. Frequency indicator  66  stops blinking, and the operator proceeds with utility measurements. If the operator does not know, or is not confident of, a utility&#39;s passive frequency level, the operator may step through several frequency levels using this manual method until finding a peak signal. 
   Alternatively, the operator may execute an automatic frequency scan as discussed above. After initially activating the RF Search button, so that frequency indicator  66  blinks, the operator starts an automatic scan by activating either of up or down buttons  72  and  74 . If the operator presses up button  72 , CPU  42  starts at the bottom of the operative frequency range (9.6 kHz in the present example) and samples upward in 0.4 kHz steps to the top of the range (33.2 kHz in the present example). If the down arrow is pressed, the CPU starts at the top of the frequency range and moves down. When searching down through the operative range, the measurement frequency is considered to be 1.2 kHz above the clock frequency, and the CPU displays the measurement frequency (clock frequency plus 1.2 kHz), at steps  117  and  133  ( FIG. 7 ). That is, and referring also to  FIG. 9 , pass band  109  is the measurement band, as opposed to pass band  107 , when scanning downward. 
   While a 0.4 kHz frequency step is provided as an example, it should be understood that different step sizes may be used as desired. The frequency step size may depend, for example, upon the performance level of the CPU, for example a Hitachi HD64F3644H microprocessor operating at a 7.37280 MHz base clock. As should be understood in this art, increasing clock frequency and/or CPU efficiency may permit smaller step size, for example 0.3 kHz or 0.1 kHz. 
   As the CPU scans the frequency range in the automatic scan mode, frequency indicator  66  continues to blink. Bar graph  68  and numerical indicator  70  show the signal intensity currently stored as the strongest signal received by the system, and frequency indicator  66  blinks at the corresponding frequency. Thus, these indications change as the system finds stronger signals. Because smaller peak levels may indicate the presence of other underground cables in the area, the operator may note the frequencies at which such smaller peaks occur for later attempts to locate the other cables. 
   When the system finds the peak level, CPU  42  has stored the frequency level in memory  50 . Indicator  66  stops blinking, thereby notifying the operator that the automatic scan is complete. Bar graph  68  and numerical indicator  70  show the signal intensity at the selected frequency, and the operator proceeds with utility measurements. 
   If an operator presses button  72  before actuation of another mode button, button  72  acts as a “Gain” button by which the operator sets the signal gain to 70% as described above. 
   If an operator presses button  74  before actuation of another mode button, button  74  acts as a “Depth” button by which the operator may use the locator to find the depth of the underground utility. The locator&#39;s three antennas ( FIG. 3 ) are used to determine location and depth of underground utilities. The bottom antenna (C 1 ) is disposed in the housing so that it is parallel to the ground when the operator carries the locator during operation. The second antenna (C 2 ) is directly above the first antenna and, being parallel to antenna C 1 , is also parallel to the ground. The third parallel antenna (C 3 ) is directly above antenna C 2  in the housing a distance equal to the distance between antennas C 1  and C 2 . The distance between C 1  and C 3  is referred to herein as “d.” Referring also to  FIG. 2A , the antennas are connected to downstream amplifiers through multiplexer  34 . 
   During the “probe” mode described above, in which the locator is used to locate an underground utility through location of electromagnetic signals emitted from a probe in the utility, the CPU repeatedly samples the signals on the three antennas. The CPU relies on the difference between, and the sum of, the signal strengths on C 1  and C 3  (C1+C3=E1, C1−C3=E2) to locate an underground utility and determine its depth. In locating an underground utility, the CPU monitors and displays the value of E1, and the operator finds a point above the utility by finding the peak value of E1 in a manner as described above. Alternatively, the locator could display E2 in locating the utility. Once finding such a position, the operator activates button  74  to determine the utility&#39;s depth, which the CPU calculates according to the function: depth=(3/2)(d)(E1−E2)/(E2). Referring to  FIG. 6 , activation of button  74  in this manner provides a screen  62  that displays a depth measurement at  86 , the frequency level at  66  and the remaining battery life at  88 . 
   If the locator is set (through activation of “Frequency” button  76  as discussed above with respect to  FIG. 5 ) to locate power lines, gas lines, faults or lines upon which the transmitter has actively placed a signal, E2 is equal to C1−C3, and E3 is equal to C2−C3. In locating an underground utility, the CPU monitors and displays the value of E2, and the operator finds a point above the utility by finding the peak value of E2. Once finding such a position, the operator activates button  74  to determine the utility&#39;s depth, which the CPU calculates according to the following function: depth=(d)(E3)/(E2−(2(E3))). 
   If the locator is set to locate RF signals through the “RF Search” button, the CPU repeatedly samples the signals on the three antennas. E1 is again equal to C1+C3, and E2 is equal to C1−C3. In locating an underground utility, the CPU monitors and displays the value of E1, and the operator finds a point above the utility by finding the peak value of E1. Once finding such a position, the operator activates button  74  to determine the utility&#39;s depth, which the CPU calculates according to the following function: depth=(d/2)(E1−E2)/E2=(d)(C3)/(E2). 
   When locating an underground utility when the locator is set through the “Frequency” button, the operator sets the locator to the desired frequency and walks with the locator in the cross-wise or grid patterns discussed above or other patterns as desired. When locating an underground utility when the locator is set through the “RF Frequency” button, the operator may first place the locator near an above-ground exposure of the utility, or near an above-ground junction box or post to which the utility is connected, and execute either of the manual or automatic frequency searches discussed above to locate the peak frequency at which the utility emits passive RF signals. After activating the “RF Search” button a second time to set the locator to the peak frequency, the operator proceeds to locate the utility in the cross-wise or grid patterns discussed above or other pattern as desired. 
   While one or more preferred embodiments of the invention have been described above, it should be understood that any and all equivalent realizations of the present invention are included within the scope and spirit thereof. The embodiments depicted are presented by way of example only and are not intended as limitations upon the present invention. Thus, it should be understood by those of ordinary skill in this art that the present invention is not limited to these embodiments since modifications can be made. Therefore, it is contemplated that any and all such embodiments are included in the present invention as may fall within the literal or equivalent scope of the appended claims.