Abstract:
Man-portable locator systems for locating buried or otherwise inaccessible pipes, conduits, cables, wires and inserted transmitters using detector arrays and stochastic signal processing and similar techniques to analyze and display multiple target objects at differing frequencies in a layered user interface (UI) are disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority to co-pending U.S. Utility patent application Ser. No. 13/108,916, entitled ADAPTIVE MULTICHANNEL LOCATOR SYSTEM FOR MULTIPLE PROXIMITY DETECTION, filed May 16, 2011, which is a continuation of and claims priority to U.S. Utility patent application Ser. No. 12/785,826, now U.S. Pat. No. 7,948,236, entitled ADAPTIVE MULTICHANNEL LOCATOR SYSTEM FOR MULTIPLE PROXIMITY DETECTION, filed May 24, 2010, which is a continuation of and claims priority to U.S. Utility patent application Ser. No. 11/854,694, now U.S. Pat. No. 7,741,848, entitled ADAPTIVE MULTICHANNEL LOCATOR SYSTEM FOR MULTIPLE PROXIMITY DETECTION, filed Sep. 30, 2007 which claims priority under 35 U.S.C. $119(e) to U.S. Provisional Patent Application Ser. No. 60/826,064, filed Sep. 18, 2006, entitled MULTICHANNEL LOCATOR WITH MULTIPLE PROXIMITY DETECTION. This application claims priority to each of the above-described applications. The content of each of the above-described applications is incorporated by reference herein in its entirety for all purposes. 
     
    
     FIELD 
       [0002]    This disclosure relates generally to electronic systems and methods for locating buried or otherwise inaccessible pipes and other conduits, cables, conductors and self-contained transmitters. More specifically, but not exclusively, the disclosure relates to portable locators for operation in a multiple signal environment. 
       BACKGROUND 
       [0003]    There are many situations where is it desirable to locate buried utilities such as pipes and cables. For example, before starting any new construction involving excavation, it is important to locate existing underground utilities such as underground power-lines, gas lines, phone lines, fiber optic cable conduits, CATV cables, sprinkler control wiring, water pipes, sewer pipes, etc., collectively and individually referred to hereinafter as “utilities” or “objects.” As used herein the term “buried” refers not only to objects below the surface of the ground, but in addition, to objects located inside walls, between floors in multi-story buildings or cast into concrete slabs, etc. If a backhoe or other excavation equipment hits a high voltage line or a gas line, serious injury and property damage may result. Severing water mains and sewer lines leads to messy cleanups. The destruction of power and data lines may seriously disrupt the comfort and convenience of residents and cost businesses huge financial losses. 
         [0004]    Buried objects may be located, for example, by sensing an alternating current (AC) electromagnetic signal emitted by the same. Some cables such as power-lines are already energized and emit their own long cylindrical electromagnetic field. Location of other conductive lines may be facilitated by energizing the line sought with an outside electrical source having a frequency typically in the region of approximately 50 Hz to 500 kHz. Location of buried long conductors is often referred to in the art as “line tracing,” a term that is so used herein. 
         [0005]    A “sonde” (also referenced in the art as a “transmitter,” “beacon” or “duct probe”, for example) is a term used herein to denominate a signal transmitter apparatus that typically includes a coil of wire wrapped around a ferromagnetic core. The coil is energized with a standard electrical source at a desired frequency, typically in the frequency region of approximately 50 Hz to 500 kHz. The sonde may be attached to a push cable or line or it may be self-contained so that it may be flushed through a pipe with water. A sonde typically generates a dipole electromagnetic field, which is more complex than the long cylindrical pattern produced by an energized line. However, a sonde may be localized to a single point. A typical low frequency sonde does not strongly couple to other objects and thereby avoids the production of complex interfering field patterns that may occur during the tracing. The term “buried objects” is used herein in a general sense and includes, for example, sondes and buried locatable markers such as marker balls. 
         [0006]    When locating buried objects before excavation, it is also very desirable to determine the depth of the buried objects. This may be done by measuring the difference in field strength at two locations. Although various methods of determining depth of buried conductors are well-established in the art, it is also well-known that existing methods may produce variable and therefore unreliable results leading to potentially dangerous errors in depth estimation when operating in the presence of complex or distorted electromagnetic fields. 
         [0007]    Portable locators that heretofore have been developed offer limited functionality insufficient for quickly and accurately locating buried utilities. Accordingly, there is still a clearly-felt need in the art for an improved compact man-portable locator system with user interface (UI) features permitting the locator operator to quickly and accurately manage the simultaneous detection and localization of a plurality of buried and/or inaccessible targets. Accordingly, there is a need in the art to address the above-described as well as other problems. 
       SUMMARY 
       [0008]    This disclosure relates generally to electronic systems and methods for locating buried or otherwise inaccessible pipes and other conduits, cables, conductors and self-contained transmitters. More specifically, but not exclusively, the disclosure relates to portable locators for operation in a multiple signal environment. 
         [0009]    In one aspect, this disclosure relates to a system for simultaneously searching and analyzing multiple frequency bands, and sorting the resultant target detections according to their proximity to the locator instrument for presentation to the operator by means of a user interface (UI) system, which may include a multi-layered display of real-time or near-real-time target analysis information. 
         [0010]    The UI system may operate to simultaneously filter and process outputs from multiple detection channels in a manner that improves locator operator effectiveness. Embodiments may also be adapted to seeks objects of interest by dynamically forming and revising signal filters to focus locator assets on the particular objects found within detection range. 
         [0011]    Various additional aspects, features, and functionality are further described below in conjunction with the appended Drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present disclosure may be more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, wherein: 
           [0013]      FIG. 1A  is a general view of an exemplary multiple electromagnetic field source environment suitable for use of the locator system embodiments as described herein; 
           [0014]      FIG. 1B  is a breakaway view of a lower three-dimensional (3D) sensor array embodiment suitable for use in the sensor assembly of the locator system of  FIG. 1A ; 
           [0015]      FIG. 1C  is a block diagram illustrating an embodiment of the sensor and sensor conditioning assemblies suitable for use with the locator system of  FIG. 1A ; 
           [0016]      FIG. 1D  is a block diagram illustrating an embodiment of the processor and user interface (UI) circuit assemblies suitable for use with the locator system of  FIG. 1A ; 
           [0017]      FIG. 1E  is a block diagram illustrating an embodiment of an assembly for integrating multiple sensor inputs from the upper and lower omnidirectional sensors for transfer to a digital signal processing block suitable for use with the locator system of  FIG. 1A ; 
           [0018]      FIG. 1F  is a flowchart illustrating an exemplary embodiment of a signal processing method suitable for use with the locator system of  FIG. 1A , including steps of accumulating samples formed into vector sums, processing the accumulated signals responsive to signal amplitude as a function of the number n of frequency bins, and filtering the accumulated signals with respect to the frequency bins; 
           [0019]      FIG. 1G  is a flowchart illustrating an exemplary process suitable for use with the method of  FIG. 1F  when applied to data from one channel, including the steps of sampling signal data, processing the samples by means of Fourier transforms, and adaptively filtering the transform results; 
           [0020]      FIG. 2A  is a flowchart illustrating an exemplary embodiment of a method of this invention whereby signal values (S.sub.t) are sampled, processed into frequency bins and processed bin by bin based on proximity; 
           [0021]      FIG. 2B  is a schematic diagram illustrating an exemplary graphical user interface (GUI) display embodiment showing the results of the filtering step of  FIG. 2A ; 
           [0022]      FIG. 2C  is a flowchart illustrating two exemplary filtering methods using preconfigured or adaptive notch filtering selected according to the proximity of the source for each frequency detected; 
           [0023]      FIG. 3A  is a flowchart illustrating another exemplary embodiment of a method of this invention whereby signal values (S.sub.t) on multiple channels are filtered and processed to identify vector groupings selected according to magnitude and orientation values; 
           [0024]      FIG. 3B  is a flowchart illustrating an exemplary method for differentiating distortion values by comparing the vector alignments in the vector groupings determined according to the method of  FIG. 3A ; 
           [0025]      FIG. 4  is a schematic diagram illustrating an exemplary GUI display embodiment for displaying the processing results of the method of  FIG. 3B ; 
           [0026]      FIG. 5A  is a schematic diagram illustrating an exemplary GUI layered display having a wider, closer trace line superimposed over other deeper detected traces; 
           [0027]      FIG. 5B  is a schematic diagram illustrating an exemplary GUI layered display of a sonde detection in the foreground superimposed over a simultaneously-detected 33 kHz trace in the background; 
           [0028]      FIG. 6  is a schematic diagram illustrating an exemplary GUI display of two tracing lines each having curved elements representing the current flow direction detected by the locator; 
           [0029]      FIG. 7A  is a schematic diagram illustrating an exemplary GUI display of a gradient line disposed parallel to a tracing line with the direction of offset displayed as a dynamically-updated indicator arrow; 
           [0030]      FIG. 7B  a schematic diagram illustrating an exemplary GUI display employing symbols to communicate alignment information to a locator operator; 
           [0031]      FIG. 7C  a schematic diagram illustrating an exemplary GUI display of balanced gradient coil signals (equal signal strength) represented as a gradient line centered in the tracing line. 
           [0032]      FIG. 7D  is a schematic diagram illustrating a locator disposition suitable for producing the display of  FIG. 7C ; 
           [0033]      FIG. 7E  is a perspective view of several exemplary locator system embodiments, each showing the physical gradient coil sensor dispositions suitable for performing a particular method embodiment of this invention; 
           [0034]      FIG. 8A  a schematic diagram illustrating an exemplary GUI display demonstrating the use of a variable-time bandpass filter using a filter half-width of ½ Hz; 
           [0035]      FIG. 8B  is a schematic diagram illustrating an exemplary GUI display demonstrating the use of a variable-time bandpass filter using a filter half-width of 2 Hz;  FIG. 8C  a schematic diagram illustrating an exemplary GUI filter display demonstrating the use of a filter half-width of 8 Hz, the filter having adjusted responsive to the circumstances of the locating task; 
           [0036]      FIG. 9A  is a schematic diagram illustrating an exemplary embodiment of the method of this invention for operating several filters with time-multiplexing; 
           [0037]      FIG. 9B  is a block diagram illustrating a process for summing the outputs of filters tuned to the 5.sup.th, 9.sup.th and N.sup.th harmonics of 60 Hz; 
           [0038]      FIG. 10  is a block diagram illustrating a process for using a notch-filter to remove a particular band from a search band; 
           [0039]      FIG. 11A  is schematic diagram illustrating an exemplary GUI layered display of target location information at multiple frequencies; 
           [0040]      FIG. 11B  is a schematic diagram illustrating the tracing at 33 kHz of a push cable coupled to a 512 Hz sonde at its far end and an exemplary GUI layered display with auto-switched images illustrative of an exemplary automatic sonde detection procedure of this invention; 
           [0041]      FIGS. 12A ,  12 B and  12 C are schematic diagrams illustrating the evolution of an exemplary GUI display using a change in trace line width to represent locator movement toward a target 33 kHz conductor; 
           [0042]      FIGS. 13A ,  13 B and  13 C are schematic diagrams each illustrating an exemplary GUI display using a centering pair of arrows, a tracing line and a gradient guidance line in combinations representing a multi-dimensional view of a locating situation in real time; and 
           [0043]      FIGS. 14A ,  14 B and  14 C are graphs illustrating the effects of shifting the analog to digital converter (ADC) clocking rate to optimize noise characteristics and smooth filter bandwidth. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0044]    This application is related by common inventorship and subject matter to the commonly-assigned patent application Ser. No. 10/268,641 entitled “Omnidirectional Sonde and Line Locator” filed on Oct. 15, 2002 by Mark Olsson et al. and published on Apr. 15, 2004 as U.S. Patent Application No. 2004/0070399A1, and to the commonly-assigned patent application Ser. No. 10/308,752 entitled “Single and Multi-Trace Omnidirectional Sonde and Line Locators and Transmitter Used Therewith” filed on Dec. 3, 2002 by Mark Olsson et al. and published on Apr. 15, 2004 as U.S. Patent Application No. 2004/0070535A1, which are both entirely incorporated herein by this reference. This application is also related by common inventorship and subject matter to the commonly-assigned patent application Ser. No. 10/956,328 entitled “MultiSensor Mapping Omnidirectional Sonde and Line Locators” filed on Oct. 1, 2004, patent application Ser. No. 11/106,894 entitled “Locator with Apparent Depth Indication” filed on Apr. 15, 2005, patent application Ser. No. 11/184,456 entitled “A Compact Self-Tuned Electrical Resonator for Buried Object Locator Applications” filed on Jul. 19, 2005, and patent application Ser. No. 11/248,539 entitled “A Reconfigurable Portable Locator Employing Multiple Sensor Arrays Having Flexible Nested Orthogonal Antennas” filed on Oct. 12, 2005, which are all entirely incorporated herein by this reference. This application is also related by common inventorship and subject matter to the Provisional Patent Application No. 60/806,708, filed on Jul. 6, 2006, and entitled, “Mesh Networked Wireless Buried Pipe and Cable Locating System”, as well as to U.S. Pat. No. 7,136,765, issued on Nov. 14, 2006, entitled “Buried Object Locating and Tracing Method and System Employing Principal Components Analysis for Blind Signal Detection” (Maier, et al.), both of which are entirely incorporated herein by this reference. 
         [0045]    Embodiments of the invention described herein provide an unexpectedly advantageous cable, pipe and sonde location method and apparatus by using a streamed concatenation of a plurality of detector data channels; for example, by using vector-summation and Fast Fourier Transform (FFT) or similar techniques. Such data streams are made useable for the first time by introducing an advantageous user interface (UI) for presenting to the operator (also denominated “user” herein) information representing multiple objects, frequencies and changes in the subterranean landscape simultaneously using graphical, numeric, and acoustic representations. 
         [0046]    By using advantageous combinations of correlations, proximity calculations, vector calculations and adaptive filtering, this locator system facilitates the limiting of operator attention to signals of interest selected automatically from among a larger set of signal detections at a particular location. For example, the signals of interest may be selected according to the estimated proximity of the sources of detected signals. The locator system preferably includes some means for converting antennas and coil analog signal input to digital data, some means for storing the digital data and for performing calculations therewith, and some means for displaying results from such calculations to an operator, including graphical images, for example. The simultaneous analysis and display of source detections at multiple frequencies, and the adaptive filtering described herein below, operate to enhance operator utility in complex signal environments. 
         [0047]    In general, the exemplary embodiments described herein are intended to be useful examples of the system of this invention and are not intended to be limited to any particular embodiment of the method of this invention, nor to any particular number or orientation of antenna arrays, for example. One versed in the art may readily appreciate that the system and method of this invention may be readily adapted for use with many other useful antenna configurations and/or calculation methods, as is evident from the following discussion. 
         [0048]      FIG. 1A  is a general view of an operator  102  and a locator system embodiment  104  working in a exemplary multiple electromagnetic field source environment. Operator  102  holds locator  104 , which is equipped with upper and lower omnidirectional antenna nodes  106  and  108  affixed to a central shaft  110 . Locator  104  is also equipped with right and left gradient-coil antennas  112  and  113  (antenna  113  is hidden), which, in this example, are affixed above lower omnidirectional antenna node  108  and extend to the operator&#39;s right and left from central shaft  110 . An underground utility line  114  is disposed at a depth Z.sub. 1  below locator  104  and a second buried utility line  116  is disposed at a depth Z.sub. 2  below locator  104 . An overhead electromagnetic (EM) signal source  118  is shown embodied as overhead power lines, which are disposed at a distance Z.sub. 3  above the bottom of lower antenna node  108 . As may be readily appreciated, each of utility lines  114 ,  116 , and  118  is separated from lower antenna node  108  by a different proximity value, Z.sub. 1 .noteq.Z.sub. 2 .noteq.Z.sub. 3 . As discussed in more detail below, it is an important feature of the locator system of this invention that these differing signal proximities and frequency spectra are employed to spatially separate and identify the discrete signal sources. 
         [0049]    The user (also denominated “operator” herein) of locator system  104  cannot change the underlying conditions of a difficult location environment, but the user can improve the location results from locator  104  by, for example, changing the frequency, grounding conditions, and transmitter location or by isolating the target line from a common ground, for example, by making a better ground connection, avoiding signal splits, or taking steps to reduce local magnetic field (B-field) distortion. 
         [0050]      FIG. 1B  is a breakaway view of the three-dimensional omnidirectional sensor array  109  contained within lower antenna node  108  ( FIG. 1A ), and is exemplary of the internal structure of the sensor arrays within other nodes. Array  109  includes three orthogonally-aligned antenna windings  120 ,  122 , and  124  fixed within a rigid casing  126 . Windings  120 ,  122 , and  124  are thereby disposed to define the three orthogonal sensor axes  128 - i ,  128 - j , and  128 - k  having a fixed orientation with respect to central shaft  110 . 
         [0051]      FIG. 1C  is a block diagram illustrating an 8-sensor signal-conditioning assembly embodiment  128  suitable for use in locator system  104 . A sensor assembly  130  includes eight identical EM field sensor coils (like winding  120  in  FIG. 1B ) organized physically into three orthogonal sensor coils (C 11 , C 12 , C 13 ) constituting a Top Ball (upper) array  133  (like upper three-dimensional (3D) sensor array  106 ), three orthogonal sensor coils (C 21 , C 22 , C 23 ) constituting a Bottom Ball (lower) array  137  (like lower 3D sensor array  108 ), and two sensor coils (C 3 , C 4 ) constituting the horizontal gradient sensor assemblies  132  and  135  (like antennas  112  and  113  in  FIG. 1A ). Each coil (C 11 , C 12 , C 13 , C 21 , C 22 , C 23 , C 3 , and C 4 ) is independently coupled to one of eight identical channels in the analog signal conditioning and digitizing assembly  128 , each of which may be appreciated with reference to the following description of the fourth conditioning channel coupled to EM field sensor  132  (C 3 ). Coil C 3  is coupled by way of the appropriate frequency-response conditioning and signal attenuating elements to a preamplifier  134 , which produces a low-impedance differential analog time-varying signal S 4 ( t )  136 . Signal S 4 ( t )  136  is routed directly to the switch  138  and also to the mixer  140  where it is mixed with a local oscillator (LO) signal  142  from a numerically-controlled oscillator (NCO)  144  governed by clock-control signal  156  to produce the usual sum and difference frequencies, which may be lowpass filtered in the usual manner to remove the sum frequencies from the difference frequencies at the input of the isolation amplifier  146 , for example. Thus, amplifier  146  produces an intermediate frequency (IF) signal  148  representing time-varying signal S 4 ( t )  136  shifted up or down in frequency by an amount corresponding to LO signal  142 . 
         [0052]    Switch  138  may be set to present either time-varying signal S 4 ( t )  136  or IF signal  148  or both to the 24-bit Analog-to-Digital Converter (ADC) assembly  150 , which produces a digital data signal representing a sample of the selected analog time-varying signal (either signal  136  or signal  148 ) in the usual manner. Signal S 4 ( t )  136  may be preferred when the signal frequency of interest is within a range of values that may be sampled by the ADC; and signal  148  may be preferred when the signal frequency of interest is higher than the range of values that may be sampled by the ADC, for example. 
         [0053]    Responsive to the external control signals  152 , ADC assembly  150  thereby produces K=8 streams of digital signal samples representing the K=8 time-varying signals {S.sub.k(t)} from sensor assembly  130 . These signals are transmitted via, for example, a Texas Instruments Multi-Channel Buffered Serial Port (McBSP)™  154 . ADC assembly  150  provides a new signal sample for each of K=8 sensor signals for every t-second interval, which is herein denominated the sampling interval. For example, the inventors have demonstrated the usefulness of a 73,245 Hz sampling rate, which imposes a sampling interval T=13.65 microseconds. These data may optionally be stored, transmitted, or displayed to the operator by way of some aspect of the User Interface (UI). Any known storage, transmission, or display means may be used. One or more filters may then optionally be matched to any desirable range of available frequency as indicated by those frequency bins with the highest signal energy values, to maximize the transmittance of the signal of a given detected utility. These filters might be chosen from a pre-calculated set of possible filters, or determined analytically and subsequently formed by software. By way of example, one filter might be chosen (or adaptively formed) to maximize the transmittance of certain lower frequency power-line signal components, and another filter might be chosen (or adaptively formed) to maximize the transmittance of certain higher frequency signal components. This adaptive process may operate continuously in the background or may be initiated by a user command, for example. It is apparent to one skilled in the art that alternative embodiments may be described according to the system of this invention wherein the function of any MCBSP is performed by some other useful high-speed serial link, serial port interface (SPI), low-voltage differential signaling (LVDS) element, or the like. 
         [0054]      FIG. 1D  is a block diagram illustrating a processor circuit assembly embodiment  160 , including a user interface (UI) circuit assembly embodiment of this invention. The processor assembly  160  accepts digital signal samples  154  from ADC assembly  150  ( FIG. 1C ) at a digital signal processor (DSP)  162 , which includes internal memory  164  for storing and executing the accumulator and evaluator software program elements  166  required to produce digital data representing buried object emission field vectors on the data bus  168  in any useful manner described in the above-cited commonly-assigned patent applications fully incorporated herein by reference. For example, software program elements may be provided in DSP  162  to evaluate a B-field vector magnitude for each of the K=8 channels of digital data  154  arriving from analog signal conditioning and digitizing assembly  128 . Indications of the 3D field vector BU(x, y, z) at the upper array node  106  ( FIG. 1A ) and indications of the 3D field vector BL(x, y, z) at the lower array node  108  may, under control of DSP  162 , then be presented to the user by means of a UI assembly that comprises the liquid crystal display (LCD)  174 , the audio interface  185 , the keypad  182  and various associated memory chips and data buses in the example shown, for example. Additionally, indications of the independently measured horizontal magnetic gradient equal to the difference between the horizontal B-field component B 1 ( x ) at the centroid of coil (C 3 )  132  (like left gradient-coil sensor  113  in  FIG. 1A ) and the horizontal B-field component B 2 ( x ) at the centroid of coil (C 4 )  135  (like the right gradient-coil sensor  112  in  FIG. 1A ) may also be presented to the user by means of the UI assembly under control of DSP  162 , for example. Moreover, these B-field vector indications may be limited to certain frequency bands and may be updated with the passage of time to reflect changes in any useful manner described in the above-cited fully-incorporated patent applications, for example. 
         [0055]    DSP  162  operates under the control of a microcontroller  170  and also produces external control signals  172  for controlling ADC assembly  150  and the clock control signals transmitted by means of the Multi-Channel Buffered Serial Port  156  for controlling NCO  144  ( FIG. 1C ). The Graphical UI (GUI) LCD  174  is disposed to accept and display images and data representing buried object emission field vectors from data bus  168  under the control of various specifications transferred on the address and control bus  176 . Data bus  168  and control bus  176  are also coupled to a flash memory  178  and a synchronous dynamic random-access memory (SDRAM)  180 , which all operate under the control of DSP  162  and serve to store data for program control and display purposes, for example. The keypad matrix or other user input device  182  is coupled to microcontroller  170  by, for example, a standard matrix scan bus  184 , whereby a user may insert commands to processor assembly  160 . An Audio user interface (AUI)  186  operates to transfer various audio signals to a user from the serial bus  188  under the control of DSP  162 . Processor assembly  160  may provide a new set of field vectors for every accumulation interval, which is herein defined as a plurality N.sub.T of the t-second sampling intervals T.sub. 1  . . . n, thereby providing continuing indications as a function of time. This plurality N.sub.T of the t-second sampling intervals is indexed by the integer i=1, N, where N may vary from one accumulation interval to the next and where sequential accumulation intervals may be either disjoint or overlapping, for example. The t-second sampling interval may also vary. The inventors have demonstrated the usefulness of a T=64 sample buffer interval, for example. An external data interface module  190  is also provided to allow data communication between processor assembly  160  and external devices such as a personal computer or external storage devices such as external removable memory media or a universal serial bus (USB) drive (not shown), for example. 
         [0056]      FIG. 1E  is a schematic showing the several inputs from multiple sensors as they relate to the DSP  162 . The three orthogonal Top Ball signals T.sub.i, T.sub.j, and T.sub.k from top (upper) signal array  133  ( FIG. 1C ) are routed to the DSP  162 , as are the three orthogonal Bottom Ball signals B.sub.i, B.sub.j, and B.sub.k from lower signal array  137  ( FIG. 1C ). 
         [0057]    Turning now to the flow chart shown in  FIG. 1F , according to one aspect, the method of this invention combines (as a vector sum) three or more channels of digital data from a detector array (for measuring the total field) into a single digital data stream representative of the total signal magnitude measured by the detector array. A transform process or power spectrum estimation technique performed on a vector or block of these data produces signal energy data as a function of frequency allocated to some number of predetermined frequency bins. In  FIG. 1F , it is shown in Step  201  that sensor data is combined to form vector sums, in this example for lower-antenna array values i, j, and k. In Step  202 , n samples of the processed values are collected in a data block. In Step  203 , the data block is processed to show the collected signal-strength values in terms of n frequency bins, which may be sent for display on the UI at step  207 . From this arrangement, the system selects filters from a pre-formed set or adaptively forms filters in Step  204 , based on fB(n). The filtered data may be combined with direct data from the sensor array as shown in Step  205 . Finally in step  206 , the resultant data is sent for display on the UI. 
         [0058]    According to another aspect, a method of this invention combines (as a vector sum) three or more channels of digital data from a detector array (for measuring the total field) into a single digital data stream representative of the total signal magnitude measured by the detector array. A Fast Fourier Transform (FFT) or similar technique is performed on a vector or block of these data. The result represents signal energy as a function of frequency allocated to some number of predetermined frequency bins. By way of example, a sample rate of 73,245 samples per second (corresponding to a Nyquist frequency of 36,622.5 Hz) might be allocated into a 2048 element channel data vector to yield a frequency bin size of 35.76 Hz. These frequency bin data may optionally be stored, transmitted, or displayed to the operator by way of some aspect of the UI. Any known storage, transmission, or display means may be used. One or more filters may then optionally be matched to any desirable range of available frequency as indicated by those frequency bins with the highest signal energy values, to maximize the transmittance of the signal of a given detected utility. These filters might be chosen from a pre-calculated set of possible filters, or determined analytically and subsequently formed by software. By way of example, one filter might be chosen (or adaptively formed) to maximize the transmittance of certain lower frequency power-line signal components, and another filter might be chosen (or adaptively formed) to maximize the transmittance of certain higher frequency signal components. This adaptive process may operate continuously in the background or may be initiated by a user command, for example. It should be appreciated that the number n of samples in a buffer does not necessarily define the number of frequency bins into which the same samples are processed, although the two merely happen to be identical in  FIG. 1F . 
         [0059]      FIG. 1G  illustrates exemplary channel signal data (S(t)) graphically as a waveform mapped against time (t) in Step  211 . These channel signal data (S(t)) are presented over one path to an FFT processor at Step  212  in the example shown, and from there to a selected or adaptively formed filtering process at Step  213  for the peaks identified as A, B, C and D in Step  212 , and from there on a second path directly to the filtering process from the original data channel in Step  214 . The results of this comparison may be represented by a GUI display in Step  205 , which shows, for example, an image representing separate frequencies sorted by proximity. 
         [0060]      FIG. 2A  is a flowchart illustrating an exemplary approach to computing frequencies relative to proximity. In an exemplary embodiment of the method of this invention, individual channel data from two or more spaced apart detector arrays capable of measuring the total field are passed to a transformation or power spectrum estimation process performed on a block of preferably synchronous data from each detector array channel. In another exemplary embodiment of the method of this invention, three or more channel data from two or more spaced apart detector arrays capable of measuring the total field are vector summed into multiple data streams representative of the total signal magnitude as measured by each detector array. A transform or power spectrum estimation procedure performed on a block of preferably synchronous data from each detector array produces data for relative signal energy as a function of frequency. Calculations identify frequency bins associated with signals that originate closer to one of the several detector arrays. It should be appreciated that other embodiments of this method may incorporate other useful methods for proximity calculation and the specific method(s) discussed herein are not intended to limit the scope of the claimed invention. 
         [0061]    In  FIG. 2A , sensor data S(t) are formed at Step  221  into vector sums for the top (upper) and bottom (lower) antenna arrays, S.sup.T(t), S.sup.B(t). In Step  222 , n samples of this process are accumulated. The block of data thus formed is processed in Step  223  into frequency bins for the Top and Bottom signal data, respectively, fB.sup.T(n), fB.sup.B(n). In Step  224 , the frequency bins are processed to yield proximity values P(n) for each of the n sources. Step  224  introduces the constant K, which depends on the spacing between the upper and lower antenna arrays in this example. Locator  108  may be reconfigures to accommodate different array numbers and/or spacing. The calculation performed in this example is provided in Eqn  1 : 
         [0000]      .times..function..function..function..times.  ##EQU00001##
 
         [0000]    After Step  224 , in Step  225 , one or more filters is selected or adaptively formed responsive to the computed value of P(n) from Eqn. 1. These filter(s) are applied to the unprocessed sensor data {S(t)} at Step  226  and the resultant filtered data are sent for conversion to a GUI image for display at Step  227 . 
         [0062]      FIG. 2B  provides an exemplary GUI image  230  portraying a representation of data produced from the process described in  FIG. 2A . In  FIG. 2B , the closest signal is a 33 kHz trace, shown as the top layer  231 . Beneath layer  231 , two other signals are shown, a broadband &gt;10 kHz trace  232  and below it, a 512 Hz detection  233 . These signal representations are displayed as layers ordered according to their calculated proximity, with the closest disposed uppermost. 
         [0063]      FIG. 2C  illustrates three exemplary embodiments  241 ,  242  and  243  of the method of Step  225  ( FIG. 2A ) for selecting or adaptively forming filters responsive to the proximity values {P(n)}. In Example  241  (preconfigured filtering for proximity), the frequency bin associated with the largest proximity value P(max) is selected in Step  251  and a preconfigured narrow-band filter centered on this P(max) frequency at Step  252 . This preconfigured narrowband filter is applied to the signal data stream and the output is processed to generate an image for display at the GUI at Step  253 . 
         [0064]    In  FIG. 2C , Example  242  (adaptive broadband filtering for proximity) begins at Step  261  with the computation of the proximity values P(n) in each frequency bin to identify the n adjacent bins having the highest average proximity value P, where n is chosen in some useful manner. At Step  262 , a broadband correlation filter is generated so that it is centered on the average frequency value of the n identified bins. The broadband filter is applied and the output is processed for display at the GUI at Step  263 . 
         [0065]    In  FIG. 2C , Example  243  (adaptive filtering to enhance target signal SNR) begins at Step  271  where adaptive filtering and/or notch filtering is applied to enhance the signal-tonoise ratio (SNR) of the target signal and a frequency bin with the largest proximity value is then selected, or alternatively, a frequency bin containing values for a target frequency is selected. In Step  272 , frequency-bin values are evaluated to identify any significant “jammers” in the locating environment. In Step  273 , the notch filtering necessary to reduce the signal levels of any significant jammers with respect to the target signal frequency bin from Step  271  is adaptively generated. Finally, at Step  274 , the new notch filter(s) are applied to the signal data stream and the necessary images are generated and presented to the GUI display. Any such “jammers” might be, for example, local EM noise generators or, for example, a known active transmitter frequency (sonde) used simultaneously in the target location effort. 
         [0066]    The exemplary methods described above in connection with  FIGS. 2A and 2C  are useful embodiments. A Fast Fourier Transform (FFT) or similar procedure is performed on a block of preferably synchronous data from each detector array channel. The result is data representing signal energy as a function of frequency allocated to some number of predetermined frequency bins for each detector array channel. A corresponding signal vector and associated vector magnitude may then be computed for each frequency bin associated with each spaced apart detector array. A proximity calculation is then performed for each bin and from these calculations, filters may be adaptively formed and/or preconfigured filters may be selected and applied to the stream of sensor data (S.sub.t). In these examples, the three sensors in the Top and Bottom arrays or nodes of the locator are combined to create vector magnitude values for each array. 
         [0067]    According to one aspect of the system of this invention, three channel data from a three-or-more-channel full field detector array are passed to a power spectrum estimator and/or transformation technique to yield signal energy as a function of frequency by channel allocated to n frequency bins. Vector magnitude is then computed for each frequency bin and filters are chosen or developed adaptively to optimize the transfer of signal frequencies unique to each vector grouping. As an example of a power spectrum estimator and/or transformation technique, a FFT is performed on data from each channel of each detector array. The result, for each channel, is signal energy as a function of frequency, allocated to some number of predetermined frequency bins (as in  FIG. 1F ). A signal vector and associated vector magnitude are then computed for each frequency bin. By any useful means known in the art, vector groupings are identified from vector orientation and magnitude values corresponding to signal energy from different sources. Separate filters may then be chosen to optimize transfer of the frequencies unique to each vector grouping. 
         [0068]      FIG. 3A  is a flowchart illustrating an exemplary embodiment of the method of this invention in which a number of samples for each of a number of channels are first processed to determine field vectors and signal proximities. The flowchart illustrates the principle of bin sorting by proximity when applied to each data channel separately rather than to the combined vector sums. Step  311  accumulates n samples for each channel and Step  312  sorts the samples into frequency bins segregated by data channel. Step  313  forms a series of frequency bin vectors, one per data channel and Step  314  organizes these vectors into frequency-bin vector magnitudes, which may also be transferred to the analysis process illustrated in  FIG. 3B  if no filtering is desired. 
         [0069]    Continuing with  FIG. 3A , Step  315  analyzes the frequency bin magnitude values to identify vector groups according to the orientation and magnitude of the several vectors by any useful means known in the art and the vector group data are processed for GUI display at Step  316 . Alternatively, the process may branch to Step  317  for selection or adaptive generation of filters responsive to vector orientation or magnitude values. 
         [0070]      FIG. 3B  is a flowchart illustrating an exemplary method for differentiating distortion values by comparing the vector alignments found in the vector groupings determined at Steps  314  and/or  317  ( FIG. 3A ). In  FIG. 3B , Step  321  passes the vector magnitude and orientation values to Step  322  for a calculation to determine a proximity value associable with each frequency-bin sensor array vector. Step  322  is accomplished using Eqn. 2: 
         [0000]      .times..function..function..function..times.  ##EQU00002##
 
         [0000]    In Eqn. 2, the constant K is proportional to the fixed distance between the upper and lower antenna nodes  106 ,  108  ( FIG. 1 ). From these Eqn. 2 proximity values, Step  323  may (optionally) calculate distortion according to Eqn. 3 to identify those frequency bins with less signal distortion, which is related to the variable .THETA. from Eqn. 3: 
         [0000]      .THETA..function..times..times..times..times..times..fwdarw..fwdarw..time-s.  ##EQU00003##
 
         [0000]    In Eqn. 3, f is some generally monotonic function so that a .THETA. value close to zero indicates less distortion between Top and Bottom sensor arrays. The use in the divisor of the scalar product of two vectors assumes an orthonormal vector space. 
         [0071]    This distortion measure allows Step  324  to identify signals of interest having larger proximity values (Eqn. 2) and lower distortion values (Eqn. 3), which are then processed for display at the GUI in Step  325 . Alternatively, Step  326  may select or adaptively generate filters for producing results filtered responsive to the proximity, distortion, magnitude and/or orientation values of the several vectors. 
         [0072]    Note that to save computation time it is possible to calculate only Top (upper) array magnitude values rather than the complete vectors, to provide proximity results only, omitting calculation for distortion. 
         [0073]    Signals of interest may be filtered based on distortion values alone when these are available from Eqn. 3 calculations. One useful method for such a calculation is to determine the difference angle .theta..sub.n between each corresponding T vector (Top Array) and corresponding B vector (Bottom Array), frequency bin by frequency bin. With this method, the corresponding vectors are most closely aligned and the .theta..sub.n values are small when less field distortion is found between the two or more spaced apart 3D sensor arrays. 
         [0074]      FIG. 4  illustrates an exemplary GUI display image  330  for portraying the results of the processes of  FIGS. 3A-B .  FIG. 4  may be appreciated with reference to the above discussion of  FIG. 2A . Other exemplary GUI display images and methods are now discussed in connection with  FIGS. 5A ,  5 B, and  6 . 
         [0075]    According to another aspect of the system of this invention, information from the locator instrument processors is displayed in independent layers so that the graphic display image responds to filtered signals to indicate the presence and relative location of either one or more lines or a sonde or both.  FIG. 5A  is a display screen image  340  portraying a method of displaying detection information from a locator in a layered fashion. In  FIG. 5A , a frequency detection with a near proximity is portrayed as a wide trace  700  in the top image layer of the display area  706 . A second trace  702  represents a signal with a more distant proximity value in an intermediate image layer, and the trace  704  represents a third signal having a yet more distant proximity value in a lower image layer. In operation, these trace representations  700 ,  702  and  704  might represent sonde signals or line-trace detections. The layer of each detection represented changes according to changes in the relative proximity during the locate effort. 
         [0076]      FIG. 5B  is a display image  350  demonstrating that the display can provide the locator operator with real-time (or near real-time) information on two or more targets simultaneously, including line conductors (utilities) or dipole transmitters (sondes), and that such information may include the relative proximities of all such targets. Image  350  shows two signals portrayed simultaneously in the display area  708  of locator  108  ( FIG. 1A ) where the line  710  in the upper image layer represents the detection of a proximate sonde and the trace  712  in the lower image layer represents the detection of a more distant 33 kHz line. 
         [0077]    According to another aspect of the system of this invention, the current flow direction in the detected conductor is computed and graphically displayed to the operator using icons such as, for example, a series of curved segments associated with the gradient line.  FIG. 6  shows two exemplary images  350  and  360  using curved segments to represent the direction of current flow in a detected conductor. In image  360 , the current direction in conductor  1100  is toward the bottom of the screen, and in image  370 , the current direction in conductor  1102  is in the direction opposite to that in conductor  1100 . Alternatively, the curved segments showing direction of current may be displayed as moving in the current direction at a rate that corresponds to the calculated current magnitude in the detected conductor. Alternatively, current direction may be shown by such movement alone without curvature in the segments. 
         [0078]    Gradient Calculation and Display 
         [0079]    According to another aspect of the system of this invention, separate gradient coils are disposed on the left and right side of the locator for detecting the magnetic field gradient between them. In a preferred embodiment, the signals from these gradient coils and the lower antenna array signals together permit the computation of proximity and depth values from a buried utility detection. This combination of gradient coil and lower antenna array signals may be processed to yield depth and proximity values with useful accuracy when the axis between the two gradient coils is generally disposed perpendicular to the buried utility line with the line generally centered and the gradient coil pair. 
         [0080]    According to another aspect of the present invention, separate gradient coils in a locator are used to detect the gradient values of the detected magnetic field on the left and right side of the locator. In a preferred embodiment, the gradient coils of the locator are used in conjunction with the lower antenna array signals to provide a basis for the computation of proximity and depth of a detected buried utility. Using these values, depth and proximity may be calculated with useful accuracy when the gradient coils are approximately centered over the line and perpendicular to the axis of the target utility. 
         [0081]    The inputs provided are the signal strength as measured on the three axes of the bottom antenna node, and the signal strength measured by the two gradient coils. A component is calculated of S.sub.B in the direction of the gradient axis. An estimate is made of field strength at a virtual receiver location at the gradient axis. From these values, a depth value is calculated as shown below: 
         [0000]      .times..times.  ##EQU00004##
 
         [0000]    Where: B.sup.B.sub.i,j,k=signal values from lower antenna G.sub.R, G.sub.L=Signal strength values from right and left gradient coils
 
d=Distance between bottom antenna and a virtual receiver location at the gradient axis
 
         [0082]    (Note: For dipole fields the final square root would be a 6.sup.th root) 
         [0083]      FIG. 7A  is a schematic diagram illustrating an exemplary GUI display image  380  showing a gradient line  1200  disposed parallel to a trace line  1202  with the direction of offset displayed as a dynamically-updated indicator arrow  1204 . Gradient line  1200  is determined by a comparison of the field detection (G.sub.R) of the left-side gradient coil sensor  113  ( FIG. 1A ) with that (G.sub.L) of the right-side gradient coil sensor  112  ( FIG. 1A ). If the two signals strengths are equal (G.sub.R=G.sub.L), the gradient line  1200  is displayed as concentric with trace line  1202  to indicate that locator  108  ( FIG. 1A ) is disposed directly above the center of the detected field. In image  380 , gradient line  1200  is displayed with an offset of distance Z from trace line  1202  to indicate that the left-side gradient coil  113  is sensing a higher signal strength than the right-side gradient coil  112 . In this exemplary embodiment, the value of Z is calculated from Eqn. 6: 
         [0000]      .times..times..times..function..times..times..angle..times..times.  ##EQU00005##
 
         [0000]    Where: Rm=the map radius in pixels in image  380 ; d=calculated depth; Polar.angle..sub.Bottom=Polar Angle computed from lower sensor array  108 ; and G.sub.R, G.sub.L=Signal strength values from left and right gradient coils ( 112 ,  113 ). 
         [0084]    The gradient line is preferably displayed whenever any one or more of the following three criteria is satisfied: (a) The field measured by the two offset gradient coils, located in proximity to a full field vector sensing array comprising the Top and Bottom nodes in  FIG. 1A  ( 106 ,  108 ), is approximately balanced within some predetermined range; (b) The azimuthal angle of the field measured by full field vector sensor is aligned with the axis of the gradient coil pair ( 110 ,  112 ) to within some predetermined range of angles; or (c) The measured depth or proximity is positive and the magnetic field source is determined to be in the ground below the receiving unit. 
         [0085]    In image  380 , the angle of gradient line  1200  is set to the azimuthal B-field orientation at the centroid of the Bottom Array, so .theta.=Azimuthal.angle.Bottom. Gradient line  1200  is therefore parallel to but displaced from trace line  1202 , depending on the signal balance between the two gradient sensor signals G.sub.R, and G.sub.L. 
         [0086]      FIG. 7B  provides a display image embodiment  390  that uses symbols to present alignment information to the locator operator (user). In one aspect of the system of this invention, misalignment between the portable locator and a target conductor may be indicated to the operator through the display of, for example, curved arrows indicating the rotational correction needed to align the locator. In image  390 , curved arrows  1204  and  1206  are displayed to direct the locator operator to rotate the locator in the direction required to align the gradient axis (defined by the coil pair) with the target conductor. Clockwise curved arrows  1204 ,  1206  direct the operator to rotate the locator  108  ( FIG. 1A ) clockwise to align with the trace line  1208 , and counterclockwise arrows (not shown) would direct the operator to rotate the locator  108  counterclockwise to do so. 
         [0087]      FIG. 7C  shows an exemplary display image  400  for displaying a condition in which the signals from the left gradient coil  113  ( FIG. 1A ) and the right gradient coil  112  ( FIG. 1A ) are balanced. In this exemplary embodiment, a gradient balance indication image provides a pair of displacement arrows  1214  disposed generally perpendicular to the azimuthal projection  1212  into the plane of the display of the local magnetic field vector determined by a 3-D full-field vector sensing array. This gradient balance indication  1214  is displayed only when one or more field measurement criteria are satisfied. The gradient balance indication of this invention may be a line displayed on a graphical user interface, and may be displayed at the center of the mapping area of a graphical user interface when the two gradient coil signals are generally balanced and the measured field is generally equal in each coil. In  FIG. 7C , the gradient line  1210  is centered along the trace line  1212 , and the displacement arrows  1214  point inward toward the gradient line  1210 . The display image  400  indicates that the locator  104  ( FIG. 1A ) is disposed directly over the center of the detected trace line field.  FIG. 7D  illustrates a disposition of a locator  1216  with gradient coil antennas  1218 ,  1220  embodied as side-wheels, relative to the normal field  1240  under the conditions indicated in image  400  ( FIG. 7C ). 
         [0088]      FIG. 7E  shows a perspective view of three exemplary locator embodiments  1216 ,  1222 , and  1228 . Each locator embodiment  1216 ,  1222 , and  1228  employs a useful physical gradient coil sensor arrangement suitable for use with the method of this invention. In locator  1216  (also denominated the SR-20 model), the gradient coils  1218  and  1220  are disposed at an offset above the lower antenna node  1217 . In locator  1222  (also denominated the SR-60 model), the gradient coils  1226  and  1224  are disposed similarly to those in locator  1216 . In locator  1228  (also denominated the “self-standing” locator), three omnidirectional 3D antenna nodes  1234 ,  1236 , and  1238  are disposed to replace the usual single lower antenna node  1217  seen in locator  1216 . Operating together, the three lower nodes  1234 ,  1236 , and  1238  provide three sets of three-dimensional B-field data, which may be processed to define the azimuthal magnetic gradient so that the separate pair of single-axis gradient coils are unnecessary for this purpose. With locator  1228 , Eqns. 4-6 above are revised to accommodate the additional available 3-axis field data, from which the azimuthal gradient components may be quickly derived with reference to one or more of the above-cited commonly assigned patent applications incorporated herein by reference. 
         [0089]    Variable-Time Bandpass Filter 
         [0090]    In one aspect of system of this invention, an adjustable variable-time bandpass filter is coupled to a signal quality determining means to facilitate adjustment of the filter timeconstant responsive to a signal quality measure. In one embodiment, such adjustment is made automatically. 
         [0091]      FIG. 8A  is an exemplary locator display image for indicating when a variable-time bandpass filter is applied with the filter half-width set to ½ Hz. In narrow-band filtering applications, the filter half-width define a passband frequency window outside of which the signal is rejected. Thus, in  FIG. 8A , any signal frequency component more than ½ Hz above or below the nominal seeking frequency is rejected, thereby eliminating nearby jamming signal frequencies in noisy environments, for example. 
         [0092]    In broadband filtering applications, the variable-time bandpass filter operates to enhance the signal-to-noise ratio (SNR) by changing the sampling size per unit of time (and the sampling rate). When the filter half-width is reduced, as in  FIG. 8A , more signal samples of smaller size are collected, requiring a longer period to process a block of data and thereby providing a higher SNR. Conversely, when the filter in broadband situations is set to a higher filter half-width value, such as 8 Hz, the SNR is lower, but the locator response time changing conditions is proportionately faster. 
         [0093]      FIG. 8B  is the exemplary locator display image of  FIG. 8A  revised to indicate that the variable-time bandpass filter is applied with the filter half-width set to 2 Hz. In narrowband filtering applications, this filter half-width reduces the SNR over that available for the ½ Hz filter, but increases the SNR over that available from the 8 Hz filter, the display image for which is shown in  FIG. 8C . Conversely, the response time for the filter shown in  FIG. 8B  is reduced by the filter setting shown in  FIG. 8C , and increased (slower) by the filter setting shown in  FIG. 8A . In narrowband applications, the higher 8 Hz setting in  FIG. 8C  permits processing of signals having frequencies up to 8 Hz above or below the nominal line search frequency, which may be useful when signal distortion is present, for example, or when other environmental factors increase the importance of margin frequencies. 
         [0094]    In another aspect of the system of this invention, input from an on-board 3-axis compass and an on-board 3-axis accelerometer may be used to provide additional data useful for describing relative locator motion. When the locator is moving, the system of this invention may automatically select the time-variable settings needed for faster refresh rates to accommodate the increased rate of change to the locate situation. If locator motion declines or halts, the system may responsively adjust the time-variable bandpass values to improve SNR by lowering system response time. 
         [0095]    In another aspect of the system of this invention, a first harmonic of a detected signal is filtered by a first bandpass filter and a second harmonic of the same signal is filtered by a second bandpass filter to produce a composite signal. This composite signal, produced by a combination of these filters, is then used to create a GUI display image adapted to indicate the presence of a hidden utility or sonde. 
         [0096]    Alternatively, two or more of such bandpass filters may be applied serially to a signal.  FIG. 9A  illustrates the signal spectrum  1900  with the 5.sup.th, 9.sup.th and n.sup.th harmonics of 60 Hz marked. Applying the first filter  1902  in a series limits the output to the 300 Hz signal frequencies as shown. Applying the second filter  1904  in the series limits the output to the 540 Hz signal frequencies as shown. Applying the n.sup.th filter  1906  in the series limits the output to the x Hz signal frequencies as shown. The combined results of two or more of such bandpass filters may then be processed to produce a GUI display image. 
         [0097]      FIG. 9B  is a flow chart of an exemplary embodiment of this process. In  FIG. 9B , signal S(t) is passed simultaneously through a filter  1908  for 300 Hz, a filter  1910  for 540 Hz and a filter  1912  for x Hz. The outputs from these filters are then presented to a summing block  1914  for producing a composite output that is then presented to the user interface  1916  for generation of the required GUI display image (not shown). 
         [0098]    In another aspect of the system of this invention, a display image for indicating the presence of one or more hidden sondes or utilities is produced by employing a combination of a first narrowband notch filter for attenuating one or more predetermined frequencies and a second broadband filter having a passband overlapping the same predetermined frequencies. 
         [0099]    In another aspect of the system of this invention, a display image for indicating the presence of one or more hidden sondes or utilities is produced by employing a combination of a first narrowband notch filter for attenuating one or more predetermined frequencies and a second cross-correlation process according to the method shown  FIG. 10 . A broadband filter  2000  for frequencies greater than 10 kHz is first applied to signal S(t). The output is presented to the notch filter  2002  to remove a predetermined frequency (33 kHz in this example). This filter  2002  may, for example, be used to mask an active-trace signal frequency while passively detecting other search environment frequencies that may otherwise be masked by the stronger active-trace signal. 
         [0100]    In  FIG. 10 , the notch filter  2002  removes the 33 kHz signal frequencies from the &gt;10K signal band so that the &gt;10 kHz signals may be separately detected passively, for example, during the simultaneous pursuit of a 33 kHz active trace signal on a separate channel. This prevents leakage of the actively pursued 33 kHz trace signal from leaking into the passive &gt;10 kHz search data processing. This feature advantageously permits the simultaneous search for sources in a passive search band and a predetermined active search trace frequency, which facilitates alerting the locator operator to unknown conductors in the vicinity of his active search, for example. It may be readily appreciated that this aspect of using such filtering to facilitate the simultaneous search for multiple target frequencies is not limited by the exemplary embodiments described herein. 
         [0101]    In the exemplary multiple electromagnetic field source environment of  FIG. 1A , for example, an active frequency such as 33 kHz may be placed on a first conductor  114  while a more deeply buried conductor  116  is re-radiating some passive energy in the &gt;10 kHz range. The filtering method described above facilitates the clear simultaneous detection of both signals and the discrimination between their source locations by preventing active 33 kHz signal components from masking the weaker passive signals from conductor  114  in the &gt;10 kHz band. Alternatively, a single filter with a passband for passing all frequencies &gt;10 kHz and a notch-band for attenuating the 33 kHz signal may be provided to the same effect, or the individual filters may be applied serially in either order, for example. A cross-correlation process, such as is described in the above-cited U.S. Pat. No. 7,136,765 and incorporated herein in its entirety by reference, may then be applied to the filtered output. Referring again to  FIG. 10 , the correlation process  2004  is applied to extract eigenvalues and eigenvectors from the correlation matrix generated from the filtered signals and the resultant field vector of the signal emission is processed in UI  2006  to produce the required GUI display image. 
         [0102]    In another aspect of the system of this invention, a first bandpass filter is configured to filter some predetermined sonde frequency and a second bandpass filter is configured to filter a predetermined line-tracing frequency, thereby providing a signal useful for indicating at the UI the presence and general location of a line, a sonde, or both. In yet another aspect of the system of this invention, one bandpass filter is configured to select a predetermined sonde frequency while a second filter is configured for broadband detection of line trace frequencies. Signal information from both filters is then used to create a display image for indicating the presence of a hidden sonde at the predetermined sonde frequency and any line signals emanating within the second broadband bandwidth. 
         [0103]    In another aspect of the system of this invention, the locator mode is automatically switched responsive to the kind of target detected. For example, the locator system may automatically switch to sonde mode if a 512 Hz frequency is detected while tracing at some line frequency (e.g., 33 kHz) and while simultaneously seeking passive signals in a broad band region such as &gt;10 kHz. The locator may permit the operator to “lock” to a frequency mode as an override of the automatic mode-change feature, thereby retaining control to focus on a particular frequency. Alternatively, the locator may display information related to several bands or frequencies with, for example, a line-trace frequency or sonde frequency locked to appear “uppermost” in a layered display image. In another aspect of this embodiment, sound signals may be used to signal an automatic mode-change to the operator, or to signal to the operator when new frequencies are detected during the search, for example. In another aspect of this embodiment, four channels of information from four different frequencies may be displayed simultaneously, such as a sonde frequency of 512 Hz, a passive AC power frequency, a 33 kHz line-trace frequency, and detections in a passive &gt;10 kHz band, for example. 
         [0104]      FIGS. 11A and 11B  illustrate an exemplary embodiment suitable for use with these aspects.  FIG. 11A  shows an embodiment of the layered display method of this invention suitable for use when simultaneously tracing several frequencies. A selected primary frequency (such as a 512 Hz sonde detection) image layer  2202  is moved up in the display stack above the displays of other frequencies. Using the above-described signal filtering method, the locator system display may automatically switch to the sonde mode and display image layer  2202  automatically pushed to the top of the display image stack responsive to the detection (above a useradjustable threshold) of any 512 Hz signal, for example. Alternatively, when a 33 kHz signal is detected in the absence of a 512 Hz sonde signal, then the 33 kHz display image  2200  is of primary interest and its display image  2200  may be pushed to the top of the display image stack. The locator system may also be scanning simultaneously for passive signals in the &lt;4 kHz power band, for example, applying a notch filter to eliminate all 512 Hz signals from the &lt;4 kHz power band display image layer  2206 , and may also be scanning for passive signals in the &gt;10 kHz band, for example, applying a notch filter to remove any 33 kHz frequencies from the &gt;10 kHz band display image layer  2204 . In such configurations, an inserted cable energized at 33 kHz and having a sonde and camera at the end may be traced, while also continuously scanning for the 512 Hz sonde itself as it moves into detection range, while also searching for other frequencies in the two &lt;4 kHz and &gt;10 kHz passive-search bands, for example. The system may also provide means for an operator selectable “lock” on a particular frequency of interest to hold the corresponding display image at the top layer of the GUI stack to facilitate concentration on a particular target by the locator operator while also updating other target frequencies at lower display layers without regard to their proximity, for example. The frequencies filtered out in a particular display may be indicated to the operator by, for example, displaying their numerical values prefixed by a minus (“−”) sign to remind the operator that they have been filtered from the display image. Display image stack layers may be ordered by, for example, proximity, signal strength, by cyclic or manual rotation, or any other useful criterion. 
         [0105]      FIG. 11B  is a schematic diagram illustrating the tracing at 33 kHz of a sewer snake  2210  (energized using 33 kHz or other traceable frequency) coupled to a 512 Hz sonde  2208  (powered by wires within sewer snake  2210 ) at its far end, and an exemplary GUI layered display with auto-switched images  2214  and  2216  illustrative of an exemplary automatic sonde detection procedure mentioned above. Such a scenario may be encountered, for example, when seeking the location of a blockage in a domestic drain line  2218 . In the scenario of  FIG. 11B , the locator system  2212  includes a first bandpass filter (not shown) configured to a predetermined sonde frequency (e.g., 512 Hz), a second bandpass filter (not shown) configured to a predetermined line frequency (e.g., 33 kHz), and a third broadband filter (not shown) configured for known line-tracing frequencies. Locator system  2212  also includes GUI processing means for displaying images indicating the detection of one or more of (a) a known-frequency target sonde, (b) a known-frequency line such as might be built into the cable connected to the sonde, and (c) other hidden lines which may be encountered. The display image  2214  is illustrated as displaying a trace line  2215  at 33 kHz and the display image  2216  indicates the detection of sonde  2208 , whereby the system advances image  2216  to the top layer of the layered GUI image stack. 
         [0106]    In another aspect of the preferred system embodiment of this invention, the GUI tracing line display image is produced to vary in width inversely (or as a generally monotonic function of the inverse) to the depth computed for a detected conductor or buried object. Alternatively, the GUI tracing line display image is produced to vary in width inversely (or as a generally monotonic function of the inverse) to the proximity computed for the detected object or conductor. Thus, for example, a buried cable detection produces a display image having a wider tracing line when closer to the locator and a narrower tracing line when further away (using the alternative proximity proportionality). Of course, any other useful image characteristics may alternatively be used to indicate relative calculated depth or proximity, such as color density, graphic patterns, and the like. 
         [0107]      FIGS. 12A ,  12 B and  12 C are schematic diagrams illustrating the evolution of an exemplary GUI display image using a change in trace line width to represent locator movement toward a conductor target radiating at 33 kHz. In  FIG. 12A , the trace line  410  is displayed as relatively narrow to indicate a depth of 6 feet (1.8 m). In  FIG. 12B , the calculated depth has dropped to 4 feet (1.2 m) and trace line  410  is displayed as wider line. In  FIG. 12C , the locator has moved to within 2 feet (0.6 m) of the target conductor and trace line  410  is displayed as a yet wider line. This UI display image method provides a rapid and intuitive visual cue to the operator of relative locator movement with respect to the detected target. Other useful techniques may also be used with the system of this invention, such as variable-density cross hatching or color-coding, for example. In another aspect of a preferred embodiment described above, the gradient coils of the locator are used in conjunction with the lower antenna array signals to provide a basis for the computation of proximity and depth of a detected buried utility. 
         [0108]    In another aspect of the system of this invention, signals from the two gradient coil antennas  1218  and  1220  ( FIG. 7D ) located on either side of the locator shaft are used to compute alignment information relative to the detected conductor. Detection information from these gradient coils alone is sufficient to determine the lateral disposition of a guidance line on a GUI display image, while the primary trace line disposition on the display image requires the full vector information supplied by the upper and lower 3D omnidirectional antennas (e.g.,  1217  in  FIG. 7D ). 
         [0109]      FIGS. 13A ,  13 B and  13 C are schematic diagrams each illustrating an exemplary GUI display image using a centering pair of arrows, a tracing line and a gradient guidance line in combinations representing a multi-dimensional view of a locating situation in real time. In FIG.  13 A, a locator display image  1300  is shown presenting a central circular area  1302  in which the multiple frequency bands sensed by the locator are simultaneously displayed graphically as lines  1306 ,  1310 , and  1312 . Using computational methods discussed above, the primary trace line  1306  is disposed to represent a bearing calculated for a signal emission detected in the frequency band containing the signal having the nearest calculated proximity; the 4 k-15 kHz band in this example. A guidance arrow  1308  is displayed to indicate the direction of lateral locator movement required to better align the locator with the source of primary tracing line  1306 , and a guidance line  1304  indicates the target bearing by its displayed angle. Guidance line  1304  also indicates by its relative length the degree of alignment with the source of primary tracing line  1306 , growing longer to indicate the approach of locator alignment to that of the source of primary tracing line  1306 . Primary tracing line  1306  also indicates the computed level of signal distortion by adding a visual “fuzzing” or defocusing effect to graphic line  1306 , which quickly and intuitively communicates a qualitative sense of signal conditions in the local area and thereby a qualitative sense of target detection reliability. Thus, in a region with little or no signal distortion, primary tracing line  1306  is presented as a clean straight line. With increasing signal distortion, primary tracing line  1306  is presented with correspondingly increased “fuzziness.” The two secondary trace lines  1310  and  1312  represent signal detections in two other bands (e.g., &lt;4 kHz and &gt;15 kHz). 
         [0110]    In  FIG. 13B , primary tracing line  1306  represents a detection in the &gt;15 kHz band and the secondary tracing lines  1312  and  1313  represent detections in other bands such as 4-15 kHz and &lt;4 kHz, for example. The two guidance arrows  1308  and  1314  are centered on the guidance line  1318  in  FIG. 13B  to indicate that the locator is disposed directly over the primary conductor detected in the &gt;15 kHz band. The display image  1322  shows that significant signal distortion is present by the degree of “fuzziness” in primary tracing line  1306  and by the lateral displacement between primary tracing line  1306  and guidance line  1308 . 
         [0111]    In  FIG. 13C , a display image  1324  shows an undistorted primary tracing line  1316  well-aligned with the guidance line  1320  bearing. Guidance arrows  1308  and  1314  indicate that the locator is disposed directly over the primary detected conductor. Primary trace line  1316  indicates a relatively undistorted detection in the &gt;15 kHz band. A secondary trace line  1312  indicates a simultaneous detection in another band (e.g., &lt;4 kHz or 4-15 kHz). Guidance line  1320  is displayed at or near maximum length to indicate that it is closely aligned with conductor represented by primary trace line  1316 . There is little or no lateral displacement between guidance line  1320  and primary tracing line  1316  and guidance arrows  1308  and  1314  indicate centering over the primary conductor emitting in the &gt;15 kHz band. 
         [0112]    In another aspect of the system of this invention, a process for smoothing filter bandwidth variations and for simultaneously improving the inverse-frequency (1/f) noise characteristics of the analog-digital conversion process, is embodied as a method including the step of varying the clocking rate of the analog to digital converter (ADC). The clock-rate varying step may be performed stepwise or continuously. In one embodiment of this process, the clock rate is adjusted to optimize the calculated proximity of a given signal. The inventors have discovered that optimizing the ADC clock rate in this manner provides a more stable detection signal for passive locating. In particular, for lower frequencies, smoothly and adaptively reducing the ADC clocking rate should yield gradually slower response times but with gradually narrower filter widths, and hence improved SNR, which is of particular value in a noisy environment. 
         [0113]    In  FIGS. 14A ,  14 B and  14 C, this ADC clock rate adjustment method is illustrated graphically. In  FIG. 14A , the filter frequency responses are normalized to a fraction of the Nyquist rate (determined by the ADC sampling clock rate).  FIGS. 14B and 14C  show the filter frequency responses for two different Nyquist rates.  FIG. 14B  shows a filter frequency response with a bandwidth of 10 kHz for a Nyquist value of 40 kHz, and  FIG. 14C  shows a filter frequency response with a bandwidth of 5 kHz for a Nyquist value of 20 kHz. 
         [0114]    Clearly, other embodiments and modifications of this invention may occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims and their equivalents, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.