Abstract:
An underground line locating receiver is disclosed which can determine whether the magnetic field it detects is representative of a line to be located or whether distortions in the magnetic field indicate an anomalous condition, and that therefore the reported line location measurement is suspect.

Description:
BACKGROUND 1. Field of the Invention 
     This invention relates to the field of electromagnetic field measurement devices, and in particular to devices for underground line location. 
     2. Discussion of Related Art 
     It is often necessary to locate buried lines, which are employed by numerous utility companies, in order to repair them, replace them or mark them to prevent their damage during excavation nearby. Examples of buried lines include pipelines for water, gas or sewage and cables for telephone, electrical power or cable television. Many of the lines are conductors, such as metallic pipelines or cables. In other applications, it is often useful to locate lines, such as power lines, that are concealed in the walls of buildings. It is well known to locate concealed lines by passing electrical current through them and detecting electromagnetic emissions that then emanate from them thereby. 
     A conducting conduit (a line) may be caused to radiate electromagnetically by being directly connected to an external transmitter or by being inductively coupled to an external transmitter. In some instances, such as with power lines, the line may radiate without an external transmitter. 
     A line locator detects the electromagnetic radiation emanating from the line. Early line locators included a single sensor that detects a maximum signal or a minimum signal, depending on the orientation of the sensor, when the line locator is passed over the line. Later line locators have included two or more sensors to provide information regarding proximity to the line. 
     Some line locators include two detectors oriented to measure magnetic fields in the horizontal direction (i.e., parallel with the surface of the earth) and arranged along a vertical axis. Typically, signals from these two detectors can be utilized to calculate the depth of the line. These line locators, then, detect the magnetic fields from the line to be located and display to an operator information about the location and/or depth of the line. A method of checking for a distorted field with such a locator is to make two measurements of the field while varying the height of the locator and compare the result. The first measurement is made with the locator at ground level. The second measurement is made with the locator at some given distance, for example six inches, above the ground. If the first measurement does not equal the second measurement plus the distance between the two measurements (e.g., six inches), then it is assumed that there is field distortion present and the measurement of depth is assumed to be suspect. Typically, methods of locating a line and determining its depth depend on the assumption that there is a single line of current along the line, that there are no other sources of electromagnetic fields, and that all responses are linear. 
     Problems in the depth measurements can arise when lines, other than the line being detected, interfere with the electromagnetic fields radiated by the target line. For example, other lines may become electrically coupled to the line being detected, either directly or inductively, and re-radiate unwanted electromagnetic fields. There may also be other conditions, which cause other electromagnetic fields, not originating from the line being measured, to be present in the location area such as anomalous soil conditions, metal structures, or ground water. These interfering fields or distortion of the magnetic field from the line being measured cause the line locator to incorrectly calculate the depth of the line. Since there is typically no indication of problems associated with the measurement, the operator may erroneously report the depth of the line and therefore either fail to locate the line or hit the line at too shallow a depth. 
     An incorrect measurement or a measurement on the wrong line can result in injury or damage. If a live power line is dug up by mistake, personnel can be injured and the line and equipment damaged. If a water line is dug into by mistake, the line can be damaged, and water leaking from the line can further cause damage. If a gas line, for example, is damaged during adjacent excavation, injury to persons and damage to property can occur. 
     Therefore, there is a need for a line locator capable of measuring the position and depth of a line and also of providing an indication of the validity of the measurement. The incorporation of such a feature in a locator could prevent harmful and costly damage to buried lines during nearby excavation. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a line locator receiver determines whether a detected electromagnetic field is distorted or not. Distortion can be due to other lines in the ground, power sources, or other anomalies. 
     The electromagnetic field is measured at at-least three different points in space. In some embodiments this is accomplished using a line locator receiver that has three or more detectors at three or more different locations. In some embodiments making depth measurements, the detectors are spaced vertically. However, several detectors at several different positions may be used to make several measurements of an electromagnetic field. 
     Based on the measurements of the magnetic field at the detectors and a model of an expected field, an error term can be calculated from the measurements and compared to a threshold value. If the error term is larger than the threshold value, a warning is communicated to the operator, indicating that an unacceptable distortion of the magnetic field has been detected. 
     In some embodiments, many field measurements are made and mathematically processed to provide detailed information about the detected field. Based on the measurements and/or analysis, an operator can determine if a given depth or position measurement is likely to be accurate. 
     Therefore, by making and processing multiple field measurements, mistakes as to the position, nature, and depth of underground lines can be avoided, thus increasing safety and lowering the risk of damage. 
     These and other embodiments are further discussed below with respect to the following claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a line locator. 
     FIG. 2 is a schematic diagram of an embodiment of a line locator according to the present invention. 
     FIG. 3A is a diagram of a process to detect anomalous fields with a linear estimate according to the present invention. 
     FIG. 3B is a diagram of a process to detect anomalous fields by regression according to the present invention. 
     FIGS. 4A-4C are diagrams of an operator display according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a schematic diagram of a conventional line locator  130  positioned above a line  110 . Transmitter  120  can be coupled to line  110  in order to induce current  121  to flow through line  110 . Current  121  generates electromagnetic field  122 , which is radiated from line  110 . If current  121  is constant, field  122  is a static magnetic field. If current  121  is time varying, so is field  122 . Electromagnetic field  122  penetrates ground  115  and exists above the surface, where it can be detected by coils  131  through  134 . 
     Transmitter  120  is shown in a direct connection mode, i.e. transmitter  120  can be electrically coupled to line  110 . The electrical connection can, for example, be accomplished at a point where line  110  emerges above the surface of ground  115 . In some cases, transmitter  120  can also operate in inductive mode, where current  122  is induced in line  110  by electro-magnetic induction. In some cases, line  110  is already carrying a current, for example, A/C power at 60 Hz. 
     Line locator  130  detects the magnetic field at detectors  131  through  134 . Detectors  131  through  134  in FIG. 1 are shown as coil type detectors, but any detector capable of measuring a magnetic field can be utilized. Detectors  131  through  134  are coupled to detection circuitry  135 . Detection circuitry  135  receives signals from detectors  131  and  134  and provides amplification and filtering for those signals. In some cases, the signals may be digitized in detection circuitry  135 . Processing circuitry  136  receives signals from detector circuitry  135  and performs operations to calculate depth and location of line  110  based on the magnetic fields measured at detectors  131  through  134 . Processing circuitry  136  can be analog circuitry or can be a microprocessor. The results of the calculations can then be displayed to an operator on display  137 . 
     Left right (directional) detection and electronic circuits applicable to line location are further described in U.S. application Ser. No. 09/136767, “Line Locator Having Left/Right Detection,” to Gopal Parakulum and Stevan Polak, herein incorporated by reference in its entirety, and U.S. Pat. No. 6,130,539, “Automatic Gain Control for A line Locator,” to Steven Polak, herein incorporated by reference in its entirety. 
     Referring again to FIG. 1, receiver  130  locates line  110  by detecting a radiated electromagnetic field  122 . Some receivers contain pairs of electromagnetic field sensors (e.g. electric detectors) for determining depth and position of lines to be located. As an example, receiver  130  uses detectors  133  and  134  to determine lateral (i.e. horizontal) position and detectors  131  and  132  to determine depth. Each of detectors  131  through  134  generate signals in response to electromagnetic field  122 . Detection electronics  135  generate signals for processor  136  based on the signals generated by detectors  131  through  134 . Processor  136  compares the signals from detectors  133  and  134  to calculate lateral position of line  110  and processes the signals from detectors  131  and  132  to calculate distance to line  110 . This information is sent to display  137  for the operator. 
     In order to locate line  110 , an operator moves receiver  130  over ground  115  until line locator  130  communicates that the signals detected by detectors  133  and  134  are equal, indicating that line locator  130  is centered over the source of magnetic field  122 , which is also the location of line  130 . 
     To determine depth, receiving detectors  131  and  132  can be used to measure the strength of electromagnetic field  122  at two different distances,  141  and  142 , from line  110 . In some embodiments, the ratio of magnetic field strength in each of detectors  131  and  132  (which are a known distance  143  apart) can be used to calculate the distance to line  110 . 
     The strength of magnetic field  122 , B, as a function of current  122 , i, flowing in (long) line  110  at distance  141 , labeled d, is given in Equation 1 (see for example  Classical Electromagnetic Radiation , Marion and Heald, pg. 433), assuming no distortion of electromagnetic field  122 . 
     
       
           B∝i/d   [1] 
       
     
     Assuming that detectors  131  through  134  have linear responses, or the responses can be linearized in circuit  135 , the output signal from an arbitrary one of detectors  131  through  134  is given by Equation 2. In Equation 2, i is the current  121  induced on line  110 , distance d n  is the vertical distance between the detector and line  110 , response constant k n  is a constant that includes the influence of receiver efficiency, gain, and all other detection parameters, and n indicates an arbitrary one of detectors  131  through  134 . 
     
       
           s   n   =k   n   ·i/d   n   [2] 
       
     
     In line locator  130  shown in FIG. 1, detector  131  is at a distance  141 , or d, from line  110  and detector  132  is at a distance (d+a) from line  110 . Detector  131  can have a response constant k 1  while detector  132  can have a response constant k 2 . Using Equation 2, then, the ratio of signals from detector  131 , s 1 , to the signal from detector  132 , S 2 , is given by Equation (3).                  s   2       s   1       =         k   2       k   1                d   +   a     d     .               [   3   ]                                
     Defining k 12  to be the ratio of k 2  to k 1  (i.e., k 2 /k 1 ) and solving for d yields Equation 4.              d   =       a         k   12          (       s   1     /     s   2       )       -   1       .             [   4   ]                                
     To allow locator  130  to determine an unknown distance d, the ratio k 12  can be determined during a calibration step and fixed. This calibration can typically be accomplished by making measurements on one or more current carrying lines  122  at known distances under known conditions. Because the distance d is known, a is the physical distance between the detectors, and s 1  and s 2  are measured, k 12  for a particular pair of detectors  131  and  132  can be calculated from Equation 5.                  k   12     ≡       k   2       k   1         =         s   2       s   1                d   +   a     d     .               [   5   ]                                
     When distance  141  to line  110  is to be measured, receiver  130  is positioned over line  110 . A measurement consists of recording signals s 1  and s 2  (from detectors  131  and  132  respectively). Equation 4 is then applied to solve for distance  141 , d. 
     In a case where an expected electromagnetic field generated by a single line source was expected, that field is described by Equation 1, and has the form 1/r with distance r being the distance from line  110 . A field not obeying this 1/r relationship would be considered distorted. In order to measure whether an electromagnetic field obeys the 1/r relationship for a single line source, at least three measurements of the electromagnetic field strength can be made for detectors positioned at different distances from line  110 . 
     FIG. 2 shows a schematic diagram of an embodiment of line locator  230  according to the present invention. Line locator  230  shown in FIG. 2 includes at least three detectors  131 ,  132 , and  233 , to determine line depth and the accuracy of the measurement. Detector  233  can be locating in a line with detectors  131  and  132 , at an additional distance  246  from detector  132 . Detection electronics  235  includes a channel for amplifying the current signal from detector  233 . Processor  236  not only calculates lateral position and depth, it includes algorithm  238  for producing an error function based on how well the signals from detectors  131 ,  132 , and  233  fit to an expected relationship (for example Equation 1) that would be produced in response to a single line of current in line  110 . Further, display  137  includes a field distortion indication  239 . Additionally, display  137  may also include a user interface to communicate with processor  236 . Processor  236  may include memory (for example flash memory or other non-volatile memory) for storing software program  238  in addition to data storage memory. 
     In some embodiments of the present invention, receiver  130  includes several additional detectors, such as for example detectors  232 ,  233 , and  234 , to aid in determining the shape of an arbitrary electromagnetic field. While detector  233  is shown above detectors  131  and  132 , additional detectors, such as  232  and  234 , can be used to make measurements at other points in space. Embodiments of line locator  130  can include any number of detectors from which different measurements of the distance between a point in line locator  130  and line  110  can be calculated. 
     FIG. 3A shows a block diagram of an algorithm  300  for determining whether the electromagnetic field in a location area as measured on line locator  130  originates from a single line of current, such as line  110 , or is distorted by other influences. In step  305 , measurements are made at at-least three locations, for example the locations of detectors  131 ,  132 , and  233  in FIG.  2 . In steps  310 ,  315 , and  320 , each possible combination of measurement data is used to calculate three different possible line depths using a model such as that described above in Equations (1) through (5). In some embodiments, more sophisticated modeling can be employed (for example, a model that anticipates distortion of the electromagnetic fields due to soil type, water content, or the presence of other interfering current carrying lines). 
     In the embodiment of line locator  230  shown in FIG. 2, three separate calculations of the distance  141 , d, utilizing combinations of detectors  131 ,  132 , and  233  can be given by                  d   12     =     a         k   12          (       s   1     /     s   2       )       -   1         ,           [   6   ]                   d   23     =       b         k   23          (       s   2     /     s   3       )       -   1       -   a       ,              and           [   7   ]                 d   13     =         (     a   +   b     )           k   13          (       s   1     /     s   3       )       -   1       .             [   8   ]                                
     In Equations (6) through (8), distance  246  (the distance between detectors  132  and  233 ) is b, distance  143  (the distance between detectors  131  and  132 ) is a, s 1  is the signal from detector  131 , s 2  is the signal from detector  132 , s 3  is the signal from detector  233 , k 12  is the ratio of the response constants for detector  132  and detector  131  k 2 /k 1 , k 13  is the ratio of the response constants for detector  233  and detector  131  k 3 /k 1 , and k 23  is the ratio of the response constants for detector  233  and detector  132  k 3 /k 2 . The three calculated distances of d, d 12 , d 23 , and d 13 , from the three combinations of detectors  131 ,  132 , and  233  are then given by Equations 6, 7, and 8. 
     In step  325 , the measurements are compared, and an error term is evaluated. In step  330 , the error term generated from the measurements is compared to a threshold error value, and the result is communicated to the operator via distortion indication  239  of display  137 . 
     In some embodiments of the present invention, for example the above three-measurement example, error term, Err, can be given by Equation 9. Some embodiments use other error analysis techniques (e.g., least squares fitting) to determine if the three measurements fit the profile of a cylindrically symmetric electromagnetic field, of the type that would be generated by a single line current or if a set of measurements fits a more complicated expected field.              Err   =             (       d   12     -     d   13       )     2     +       (       d   12     -     d   23       )     2     +       (       d   13     -     d   23       )     2         .             [   9   ]                                
     The results of the error calculation in step  325  can be compared with a threshold value in step  330 . After analyzing the measurement data, the result can be communicated to the operator in distortion indication  239 , which can be a panel light, display, or any other fashion in display  137  of line locator  230 . In some embodiments, the error result can be displayed on indication  239 . In some embodiments, an indication of whether the error exceeds a threshold can be displayed. 
     In some embodiments, line locator  230  can include more than three detectors, or the detectors may not be positioned in a straight line. Several additional measurements using detectors at different points in space over line  110  can be utilized. It is also possible to have one detector perform multiple functions. For example, if the lateral position detectors  133  and  134  are not at the same elevation as the depth measurement detectors (e.g., detectors  131  and  132 ), one or more lateral position detectors could provide additional field measurements, provided by detector  233  in the above example. 
     FIG. 3B shows a diagram of a process where a curve-fitting method (e.g., linear regression as described in  Advanced Engineering Mathematics , Kreyszig, pp 818-20, herein incorporated by reference in its entirety) produces a set of coefficients defining the electromagnetic field and an error function which can be used to determine an error value for comparison with a threshold. 
     In step  350 , measurements of electromagnetic field strength are made at multiple different points in space, for example with detectors  131 ,  132 ,  233  and  234  of FIG.  2 . Measurements are made at more than two detectors in order to measure discrepancies between the measured magnetic fields and those expected by the model magnetic field. In step  355 , a model for an electromagnetic field is selected. Referring to FIG. 2, this selection can be done through an operator interface in display  137  or through an external input interface  240  to processor  136 . The model may be as described by Equation 3, or a more complicated model (e.g., multiple lines of current, ground water, soil conditions). In some embodiments, a model is programmed into processor  236  of FIG. 2 through a user interface with display  137  or external interface  240 . 
     In step  360 , the set of measurements is mathematically fit to values calculated using the model of step  355 . The distance calculated from the model of step  355  is displayed on display  239  in step  361 . An error term, resulting from the fit, is calculated in step  365 , and input to steps  330 . In step  330 , the error calculated in step  365  is compared with a threshold value and the result displayed on display  239 . 
     The method shown in FIG. 3B can also be generalized to the application of any data-fitting process to determine whether an electromagnetic field is of any shape, not just cylindrically symmetric. In some embodiments, step  361  includes communicating parameters of a fit, the most likely cause of a distorted field, and other analysis from the electromagnetic field measurements to an operator. The models calculating parameters and other results can be executed on processor  236  and may be stored in memory  238 . 
     FIG. 4A shows a diagram of an embodiment of operator display  137 . In some embodiments, display  137  uses signal light  402 , an alarm, or error message  405  (shown in graphics display  404 ) to communicate that the error term is larger than a threshold. 
     FIG. 4B shows a close up view of graphics display  404 . In some embodiments processor  136  calculates coefficients of a polynomial or other mathematical fit of measurement data. Error term  405  and/or coefficients  406  can be accessed through graphics display  404 . 
     FIG. 4C shows a close up view of graphics display  404 . In some embodiments, processor  136  can calculate a source distribution for the measured magnetic field. Graphics display  404  can be used to view picture  408  of calculated sources  410  and  411  of the electromagnetic field. 
     In some embodiments of the present invention receiver  130  measures complex and dynamic electromagnetic fields. In some embodiments, receiver  130  includes more than three detectors and takes many measurements over time and at many places in space so that the number of measurements is larger than the number of degrees of freedom in the model to which the measured field is to be compared. 
     Some embodiments make use of detecting technology that does not use detectors to make field measurements, such as magnetometers, antennas, and electro-optical devices. The invention also may be applied in embodiments detecting energies in optical, acoustical, or other types of fields, with detectors appropriate to those types of energy. The embodiments described above are exemplary only and are not intended to be limiting. One skilled in the art may recognize various possible modifications that are intended to be within the spirit and scope of this disclosure. As such, the invention is limited only by the following claims.