Abstract:
A device for detecting an underground power line includes, for example, a sensor unit that has a magnetic field sensor element that is configured for receiving a received signal depending on the features of the power line and the underground, a control and evaluation unit configured for controlling the sensor unit and for evaluating the received signal, and a display unit for displaying the received signal evaluated by the control and evaluation unit. The sensor unit includes, for example, at least one additional magnetic field sensor element configured to receive a received signal depending on the features of the power line and the underground, wherein the magnetic field sensor elements can be independently controlled by the control and evaluation unit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to German Patent Application DE 10 2011 079 261.9, filed Jul. 15, 2011, which is hereby incorporated by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to devices and methods for detecting an underground power line. 
         [0003]    Typical objects, which are detected underground, include, for example, water pipes, rebar, electric and power lines, accumulations of moisture, and hollow spaces. The term “object” within the scope of this patent application includes, for example, arbitrary solid, liquid, and gaseous objects embedded underground. A need exists for power lines to be detected with high reliability by a detection device, due to the risk of accidents when severing a power line. A power line is defined as an electric line with a current flowing, which during operation generates a magnetic field that can be used for detecting the power line. Although telephone and antenna cables represent electric lines, they only carry very low currents and need not be included in the definition of the term “power line” as used within the scope of this application. 
       BACKGROUND OF THE INVENTION 
       [0004]    Prior art devices for detecting an underground power line include a sensor unit with a magnetic field sensor element, which is embodied as a coil or another magnetic field sensor (e.g., semiconductor magnetic field sensor, fluxgate sensor, magneto-impedance sensor), a control and evaluation unit, and a display unit. The display unit is an LED display unit or a signal strength display unit. 
         [0005]    A disadvantage of prior art detection devices includes that the spatial arrangement of the underground power line is not illustrated realistically for the user. The spatial arrangement of the underground power line is deduced by the user by a repeated scanning of the power line and marked on the underground. 
       SUMMARY OF THE INVENTION 
       [0006]    An objective of the present invention includes further developing a device and a method for detecting an underground power line of the type mentioned at the outset such that the progression of the underground power line is visualized realistically for the user. 
         [0007]    In one embodiment, a device for detecting a power line in the underground includes a sensor unit, a control and processing unit and a display unit. The sensor unit includes a magnetic field sensor element that is configured to receive a received signal based on the characteristics of the power line and the underground. The control and processing unit is configured to control the sensor unit and to process the received signal. A display unit is configured to display the received signal processed by the control and processing unit. The sensor unit includes at least one additional magnetic field sensor unit configured to receive the received signal dependent on the characteristics of the power line and the underground, in which the magnetic field sensor elements are configured to be controlled independently by the control and processing unit. 
         [0008]    In another embodiment, the device for detecting an underground power line is characterized according to the invention such that the sensor unit includes at least one additional magnetic field sensor element, which is configured to receive a received signal dependent on the features of the underground power line, with the magnetic field sensor elements being independently controlled by the control and evaluation unit. An arrangement of several magnetic field sensor elements is useful in that the spatial arrangement of the underground power line can be determined and displayed on the display unit. The magnetic field sensor elements record a magnetic field or a magnetic field gradient as the received signals. 
         [0009]    Suitable magnetic field sensor elements include, for example, coils, echo sensors, magneto-impedance sensors, magneto-inductive sensors, fluxgate sensors, giant magneto resonance sensors, colossal magneto resistance sensors, and anisotropic magneto resistance sensors, as well as all other sensors suitable for detecting magnetic fields. 
         [0010]    In one embodiment, the sensor unit includes first and second magnetic field sensor elements. The first magnetic field sensor elements detects a magnetic field or a magnetic field gradient in a first direction and the second magnetic field sensor elements detects a magnetic field or a magnetic field gradient in a second direction, which is different from the first direction. The second direction can be aligned perpendicular with respect to the first direction. 
         [0011]    In another embodiment, the first and second magnetic field sensor elements are arranged alternatingly along a horizontal direction. By the alternating arrangement along a horizontal direction, the number of magnetic field sensor elements used to determine the spatial progression of an underground power line can be reduced. An average amount is calculated from the measurements of adjacent magnetic field sensor elements. A number N of first and second magnetic field sensor elements yields N−1 measurements. This type of alternating arrangement is primarily suitable for expensive magnetic field sensor elements. 2N−2 magnetic field sensor elements are configured for magnetic field sensor elements measuring a magnetic field in the first and second direction at the same position. 
         [0012]    In yet another embodiment, the sensor unit shows third magnetic field sensor elements, which detect a magnetic field or a magnetic field gradient in a third direction different from the first and second directions. The third direction is aligned in a direction perpendicular with respect to the first and second directions. Due to the fact that the magnetic field or the magnetic field gradient of the power line is detected in a third direction the reliability of the measurement and the precision of the spatial allocation of the underground power line is improved, primarily in case of inclined, curvy, and/or twisted power lines and in multi-phase power lines. 
         [0013]    In another embodiment, the magnetic field sensor elements detect a magnetic field or a magnetic field gradient in a first direction and in a second direction, which is different from the first direction. The magnetic field sensor elements detect a magnetic field or a magnetic field gradient in a third direction, which is different from the first and the second directions. Small, cost-effective magnetic field sensors (e.g., echo elements) are suitable as magnetic field sensor elements detecting the magnetic field or the magnetic field gradient of the power line in two and/or three directions. 
         [0014]    In one embodiment, the magnetic field sensor elements each include two magnetic field sensors, which are arranged parallel and at a distance in reference to each other. Due to the parallel arrangement of two magnetic field sensors a difference can be calculated between the measurements of the magnetic field sensors. Due to the formation of the difference, homogenous magnetic unidirectional fields interfering with the measurements are eliminated, and the reliability and the precision of the measurement are improved. 
         [0015]    In another embodiment, a modulation unit is provided, which can be connected, via a communication connection, to the control and evaluation unit and which, upon an order by the control and evaluation unit, modulates a power signal of the power line. By the modulation of the power signal with a known pattern, for example, the received signals of the power line can be better identified by the magnetic field sensor elements. The modulation unit is configured, for example, such that it is plugged into an outlet provided in the underground and coupled to a phase of the power line. The control and evaluation unit includes an evaluation module for demodulating the received signals. 
         [0016]    In one embodiment, another sensor unit is provided to detect an underground object. The additional sensor unit to be embodied as an inductive sensor unit, capacitive sensor unit, radar sensor unit, magnetic field sensor unit, or another sensor unit suitable to detect underground objects. Depending on the field of application of the detection device all known sensor units may be combined with each other. 
         [0017]    Power lines are detected with a high degree of reliability by a detection unit due to the risk of accidents when a power line is severed. When using several sensor units with different sensor features, the quality and reliability of the measurement can be increased. The additional sensor unit includes several sensor elements differing in at least one sensor feature from the magnetic field sensor elements. The term “sensor features” summarizes any and all features of sensor units, such as type of sensor, size, position, alignment. The combination of a sensor unit for detecting arbitrary underground objects with a sensor unit for detecting an underground power line is useful in that power lines are detected by both sensor units and the reliability of the measurement and the precision can be increased for the spatial allocation of the power line. Using the sensor unit for detecting arbitrary objects allows primarily the determination of the spatial alignment of underground objects and by using the sensor unit for detecting a power line it can be ensured that power lines are securely detected. 
         [0018]    In one embodiment, a method for detecting a power line in an underground is provided. The method may include, for example, one or more of the following: detecting a received signal by a magnetic field sensor of a sensor unit; evaluating the received signal by a control and evaluation unit; displaying the evaluated received signals on a display unit; and detecting at least one additional received signal by another magnetic field sensor element of the sensor unit. 
         [0019]    In another embodiment, the method for detecting an underground power line can include an additional step of detecting at least one further received signal by another magnetic field sensor element of the sensor unit. Due to the fact that several received signals are detected by the magnetic field sensor elements a horizontal illustration can be determined representing the progression of the underground power line. 
         [0020]    In one embodiment, a first magnetic field gradient is detected in a first direction and a second magnetic field gradient in a second direction. The first and the second direction can be arranged perpendicularly with respect to each other. In one embodiment, as the sensor unit is moved in the travel direction over the underground, the first magnetic field gradient is detected in a horizontal direction perpendicular with respect to the travel direction, and the second magnetic field gradient is detected in the travel direction, and in a depth direction perpendicular to the horizontal direction. The progression of a power line can be determined in the underground from the magnetic field gradients in the horizontal direction and the depth direction. 
         [0021]    In another embodiment, the first and second magnetic field sensor elements are arranged alternatingly in the horizontal direction and from the first magnetic field gradient and the second magnetic field gradient an average amount can each be calculated from adjacent first and second magnetic field sensor elements. By the alternating arrangement of the magnetic field sensor elements along the horizontal direction the number of the magnetic field sensor elements is reduced which is used to determine the spatial progression of an underground power line. 
         [0022]    Based on the average amounts of the adjacent first and second magnetic field sensor elements, a horizontal illustration is calculated by the control and evaluation unit, the horizontal illustration is transmitted by the control and evaluation unit to a display unit, and displayed on the display unit. From the horizontal illustration the user is provided with a spatial impression of where the power line extends in the underground. 
         [0023]    In one embodiment, additional received signals are received by the sensor elements of another sensor unit. By the use of different sensor types or the use of a sensor type with different sensor features different objects or objects at different depths in the underground are reliably detected. 
         [0024]    In another embodiment, using the control and evaluation unit, joint depth cross sections and from the joint depth cross sections a joint top view are calculated from the received signals of the sensor unit and the received signals of the additional sensor unit. Joint depth cross sections and a joint top view are useful such that all objects are displayed in one illustration. Additionally, the reliability during the detection of a type of object is increased when the type of object has been detected by different manners. 
         [0025]    In yet another embodiment, the control and evaluation unit calculates from the received signal of the sensor unit and the received signal of the additional sensor unit separate depth cross sections and from the separate depth cross section separate top views. Separate depth cross sections and a separate top view are useful in allowing the adjusting of the display and calculation parameters for the depth cross sections and the top view to the depth range and the objects to be detected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  shows the application of a detection device according to the invention in an interior space, including a concrete floor with an embedded iron grating and a masonry rear wall made from brick with horizontally and vertically extending electric lines. 
           [0027]      FIG. 2A  shows a top view of a first embodiment of a manually guided detection device according to the invention. The top view of the detection device faces away from the underground to be detected. 
           [0028]      FIG. 2B  shows a bottom view, facing the underground to be detected, of the detection device shown in  FIG. 2A . A measuring device is arranged inside with a first sensor unit and a second sensor unit. 
           [0029]      FIG. 3A  shows a power sensor unit from  FIG. 2B . 
           [0030]      FIG. 3B  shows a first magnetic field sensor element of the power sensor unit in  FIG. 3A . 
           [0031]      FIG. 3C  shows a second magnetic field sensor element of the power sensor unit of  FIG. 3A . 
           [0032]      FIG. 4  shows another embodiment of a magnetic field sensor element of the power sensor unit from  FIG. 2B . 
           [0033]      FIG. 5  shows a display of a measurement of the detection device of  FIGS. 2A-B , which is moved along a travel direction over the underground to be detected. The display of the measurement includes a top view and a depth cross section view. 
           [0034]      FIG. 6  shows another embodiment of a power sensor unit with a first sensor unit and a second sensor unit aligned in two horizontal directions perpendicular with respect to each other. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]      FIG. 1  shows the application of a device  1  according to the invention for detecting a power line in an interior space  2 . The detection device  1  is embodied as a hand-held or guided detection device. A hand-held detection device is held over the underground to be detected without any driving device and a guided detection device is guided along a linear track or an arbitrary track over an underground to be detected. Detection devices are called hand-held and/or hand-guided when a user manually guides or holds it over the underground to be detected. 
         [0036]    The interior space  2  includes a floor  3 , a right and left lateral wall  4 ,  5 , a rear wall  6 , and a ceiling  7 . The bottom  3  includes a concrete slab with an embedded iron grating  8 . The rear wall  6  is made from cinder blocks and/or bricks  9 . A horizontally arranged power line  11  extends in the rear wall  7  and three vertically arranged power lines  12 . 1 ,  12 . 2 ,  12 . 3 , branch off the horizontally arranged power line  11 . 
         [0037]      FIG. 2A  shows a first embodiment of a hand-held detection device  21 , including a housing  22 , a handle  23 , a motion unit  24  with four wheels  25 , a display unit  26 , and an operating unit  27 . 
         [0038]    The user guides the detection device  21  with the help of the handle  23  and the motion device  24  in a travel direction  28  over the underground to be detected, for example, embodied as the floor  3  or the rear wall  6 . The handle  23  is arranged on a top  29  of the detection device  21  facing away from the underground  3 ,  6  during the measurement and connected with the housing  22 . The display unit  26  includes a display  30 , on which the measurements of the detection device  21  are displayed as a measurement. 
         [0039]    The operating unit  27  serves to start the measurement and to adjust the detection device  21 . The operating unit  27  includes a first and second operating unit  31 A,  31 B, which are arranged on the top  29  in addition to the display  30 . The first operating unit  31 A includes an on/off switch  32  to activate and deactivate the detection device  21 , a toggle switch  33  by which a marker line or a marker cross is positioned in an illustration on the display  30  and can be displaced, as well as two additional operating buttons  34 ,  35 . The second operating unit  31 B includes five functional controls  36 A- 36 E for activating different functions of a functions menu, shown on the display  30 . The operating unit  27  additionally includes two start/stop buttons  37 A,  37 B serving to start and stop a measurement and arranged at the handle  23 . 
         [0040]    The detection field of the detection device  21  is limited and does not coincide with the full length of the housing  22 . The limit of the detection field is displayed at the right housing edge of the housing  22  via an upper and lower right marker  38 A,  39 A and the left housing edge via an upper and lower left marker  38 B,  39 B. Using the markers the operator can place the detection device  21  on the underground to be detected. The center of the detection field is displayed at the upper and lower housing edge via an upper and lower marker  40 A,  40 B. 
         [0041]      FIG. 2B  shows the detection device  21  in a view of the bottom  42 , facing an underground during measuring. A measuring device  43  is located inside the housing  22  at the bottom  42 . The measuring device  43  includes a first sensor unit  44 , a second sensor unit  45 , a control and evaluation unit  46 , a power source  47 , and a coordinates detection unit  48 . 
         [0042]    The control and evaluation unit  46  serves to control the first and second sensor unit  44 ,  45  to evaluate the received signals provided by the sensor units  44 ,  45  and to display the measurements in the form of a measurement image on the display unit  26 . The control and evaluation unit  46  is connected via real-time communication connections to the first and second sensor unit  44 ,  45  and via another real-time communication connection to the display unit  26 . The term “real-time communication connection” also includes, in addition to communication connections without any time lags, communication connections, in which the time lag between the detection of the received signal by the sensor unit and the display of the measurements on the display unit  26  is so short that the measurements are displayed on the display unit  26  essentially at the present position of the sensor unit. The power source  47  is connected to the first sensor unit  44 , the second sensor unit  45 , the control and evaluation unit  46 , and the display unit  26 , and provides the units  44 ,  45 ,  46 ,  26  with the electric energy used for the measuring operation. 
         [0043]    The detection device  21  is moved during the measuring process in a travel direction  28  with the travel speed over the underground to be detected. The coordinate detection unit  48  detects the coordinates in a travel direction  28 . In a guided detection device by which an arbitrary motion can be performed the coordinates are detected by the coordinates detection device in a level parallel with respect to the bottom of the housing  22 . 
         [0044]    The first sensor unit  44  is embodied as a magnetic field sensor unit for detecting a power line and is also called a power sensor unit. The second sensor unit  45  is embodied as a sensor unit for detecting an arbitrary object in the underground and includes a first sensor element  49 . 1 , a second sensor element  49 . 2 , and a third sensor element  49 . 3 . The sensor elements  49 . 1 - 49 . 3  are embodied as inductive sensors, capacitive sensors, radar sensors, magnetic field sensors, or as other sensors suitable for detecting underground objects and arranged nested in two rows. The power sensor unit  44  is arranged between the first row of the sensor elements  49 . 1 ,  49 . 2  and the housing  22  of the detection device  21 . 
         [0045]    In the embodiment of  FIG. 2B  the sensor elements  49 . 1 - 49 . 3  are embodied as radar sensor elements. The radar sensor elements  49 . 1 - 49 . 3  are controlled during the measuring operation via the control and evaluation unit  46  such that in the transmission mode at any given time only one radar sensor element transmits a transmission signal and in the receiving mode all radar sensor elements  49 . 1 - 49 . 3  simultaneously receive a received signal. In another first partial measuring step the first radar sensor element  49 . 1  transmits a first transmission signal and the three radar sensor elements  49 . 1 - 49 . 3  respectively receive a received signal. In a second partial measuring step the second radar sensor element  49 . 2  transmits a second transmission signal and the three radar sensor elements  49 . 1 - 49 . 3  each receive a received signal. In a third partial measuring step the third radar sensor element  49 . 3  transmits a third transmission signal and the three radar sensor elements  49 . 1 - 49 . 3  each receive a received signal. 
         [0046]    The nine received signals of a measuring step include three mono-static and six bi-static received signals, with mono-static referring to a mode in which the sensor element transmits and simultaneously receives, and bi-static to a mode in which a sensor element transmits and another sensor element receives. The nine received signals are allocated in the XY-level to three mono-static and three bi-static area sections. Each sensor element  49 . 1 - 49 . 3  is allocated to a mono-static area section, at which the allocated mono-static received signal is illustrated. The bi-static received signals of the first and second sensor elements  49 . 1 ,  49 . 2  are averaged and the averaged signal is allocated to a first bi-static area section, which is arranged between the first and second mono-static area section. Similarly, the bi-static received signals of the first and third sensor elements  49 . 1 ,  49 . 3  and/or the second and third sensor elements  49 . 2 ,  49 . 3  are averaged, and the averaged signals are allocated to a second and third bi-static area section, with the second bi-static area section being arranged between the first and third mono-static area section and the third bi-static area section between the second and third mono-static area section. In addition to forming averages, for example a median, a maximum value, or a weighed total can be calculated from the received bi-static signals. The term “averaged signal” is understood as a signal which is calculated by a suitable mathematical function from the received bi- static signals. 
         [0047]    The six area sections which are moved with the travel speed along the travel direction  28  form five receiving channels in the first horizontal direction. During the travel motion the received signals are detected and from the detected received signals a portion of the depth cross section is already calculated. This part of the depth cross section is transmitted from the control and evaluation unit  46  via the real-time communication connection to the display unit  26 . The depth cross section is regularly updated during the travel motion. The receiving channels form the lanes showing the received signals and are regularly updated. 
         [0048]    In order to increase the reliability when detecting a power line and ensure that the received signal is actually generated by a power line present in the underground the power sensor unit  44  includes a modulation unit  50  for modulating a power signal. The modulation unit  50  can be connected via a communication connection  51  to the control and evaluation unit  46  and is for example embodied such that it is plugged into an outlet, present in the underground, and is coupled to a phase of the power line. The control and evaluation unit  46  transmits a control command via a communication connection  51  to the modulation unit  50 , modulating the power signal with a predetermined pattern. In order to evaluate the received signals the control and evaluation unit  46  includes a respective evaluation module for demodulating the received signal. 
         [0049]    The measuring device  43  shown in  FIG. 2B  includes two sensor units  44 ,  45  differing from each other in at least one sensor feature. The detection device  21  can also be operated with only a single power sensor unit  44 . By using several sensor units with different sensor features the quality and reliability of the measurement can be increased. 
         [0050]      FIG. 3A  shows the power sensor unit  44  of the detection device  21  in an enlarged illustration. The power sensor unit  44  includes four first magnetic field sensor elements  61 . 1 ,  61 . 2 ,  61 . 3 ,  61 . 4  and three second magnetic field sensor elements  62 . 1 ,  62 . 2 ,  62 . 3 , fastened alternating on a circuit board  63 . 
         [0051]    The circuit board  63  serves as a fastening element for the mechanic fastening and for an electric connection for the first and second magnetic field sensor elements  61 . 1 - 61 . 4 ,  62 . 1 - 62 . 3 . A connection element  64  is provided on the circuit board  63 , by which the circuit board  63  can be connected to a control and evaluation unit  46 . The first and second magnetic field sensor elements  61 . 1 - 61 . 4 ,  62 . 1 - 62 . 3  are aligned in two horizontal directions  65 ,  66  perpendicular in reference to each other. The depth direction  67  is defined as the direction into the underground perpendicular in reference to the horizontal directions  65 ,  66 . 
         [0052]      FIG. 3B  shows the first magnetic field sensor element  61  of the power sensor unit  44  in detail. The first magnetic field sensor element  61  includes a circuit board section  68 , a first pair of magnetic field sensors  69 A,  69 B, and an amplifier  70 . The magnetic field sensors  69 A,  69 B are shaped as coils in the embodiment of  FIG. 3A  and aligned along the second horizontal direction  66 . The magnetic field sensors  69 A,  69 B are aligned parallel and distanced with respect to each other in the depth direction  67  and measure a magnetic alternating field B x,A , B x,B  (e.g., 50/60 Hz) in a first horizontal direction  65 . 
         [0053]      FIG. 3C  shows the second magnetic field sensor element  62  of the power sensor unit  44  in detail. The second magnetic field sensor element  62  includes a circuit board section  71 , a second pair of magnetic field sensors  72 A,  72 B, and an amplifier  73 . The magnetic field sensors  72 A,  72 B are shaped as coils in the embodiment of  FIG. 3A  and aligned along the depth direction  67 . The magnetic field sensors  72 A,  72 B are in parallel with respect to each other, arranged distanced in a second horizontal direction  66 , and measure a magnetic alternating field B z,A , B z,B  (e.g., 50/60 Hz) in the depth direction  67 . 
         [0054]    In order to eliminate a homogenous magnetic unidirectional field (homogenous alternating field) during the detection, a difference value ΔB x =B x,A −B x,B  is calculated between the magnetic field sensors  69 A,  69 B of the first pair and a difference value ΔB z =B z,A −B z,B  between the magnetic field sensors  72 A,  72 B of the second pair. From the difference values ΔB x , ΔB z  of the adjacent first and second pairs of magnetic field sensors  61 . 1 - 61 . 4 ,  62 . 1 - 62 . 3  an average ΔB xz =sqrt[(ΔB x ) 2 +(ΔB z ) 2 ] is calculated. The power sensor unit  44  shown in  FIG. 3B  with four first magnetic field sensor elements  61 . 1 - 61 . 4  and three second magnetic field sensor elements  62 . 1 - 62 . 3  yields six measurements ΔB xz,1 −ΔB xz,6 , which are allocated to six different X-coordinates along the first horizontal direction  65 . The control and evaluation unit  46  calculates from the measurements ΔB xz,1 −ΔB xz,6  the progression of the power line in the underground and transmits a horizontal illustration (XY-illustration) of the underground with the power line to the display unit  26 . 
         [0055]    From the detection device  21  of  FIG. 2B  with the first and second sensor elements  44 ,  45  the first and second magnetic field sensor elements  61 . 1 - 61 . 4 ,  62 . 1 - 62 . 3  are arranged alternating along the first horizontal direction  65  and detect a measuring zone in the first horizontal direction  65  which is equivalent to the detection field of the second sensor unit  45 . The measurements of the first and second sensor unit  44 ,  45  may be shown as separate measurements or in a combined measurement. 
         [0056]      FIG. 4  shows an alternative embodiment of the magnetic field sensor element  81 , which replaces the first and second magnetic field sensor elements  61 . 1 - 61 . 4 ,  62 . 1 - 62 . 3  in the power sensor unit  44  of  FIG. 3A . In this case the power sensor unit  44  includes seven identically designed magnetic field sensor elements  81  arranged side-by-side on the circuit board  63 . The magnetic field sensor element  81  includes a circuit board section  82 , a first pair of magnetic field sensors  83 A,  83 B, a second pair of magnetic field sensors  84 A,  84 B, as well as a first amplifier  85  for the first sensor pair  83 A,  83 B, and a second amplifier  86  for the second sensor pair  84 A,  84 B. 
         [0057]    The magnetic field sensors  83 A,  83 B,  84 A,  84 B are embodied as coils. The first pair of coils  83 A,  83 B is arranged in parallel with respect to each other, along the second horizontal direction  66 , and the coils  83 A,  83 B are arranged distanced from each other in the depth direction  67 . The second pair of coils  84 A,  84 B is arranged in parallel with respect to each other, aligned along the depth direction  67 , and the coils  84 A,  84 B are arranged distanced from each other in the second horizontal direction  66 . The first pair of coils  83 A,  83 B measures a first difference in the first horizontal direction  65  and the second pair of coils  84 A,  84 B measures a second difference in the depth direction  67 . 
         [0058]      FIG. 5  shows the display  30  of the display unit  26  with a measurement of the detection device  21 , which is moved in a linear motion along the travel direction  28  over the underground. The width of the measurement in the X-direction is limited to the width of the detection field. The width of the detection field is displayed to the user via the upper and lower markers  38 A,  38 B,  39 A,  39 B on the housing  22  of the detection device  21 . 
         [0059]    The display  30  is divided during the display of the measurement in a first operating mode into three primary fields: at the left edge of the display  30  a menu of functions is shown in a first primary field  90 , which includes up to five functions  91 A- 91 E. Each function  91 A- 91 E is activated by the functional button  36 A- 36 E, located at the left, of the second operating unit  31 B. A second primary field  92  is arranged in the central area of the display  30  and serves to display the measurement. The second primary field  92  is divided into three partial fields, which are arranged underneath each other. In an upper partial field  93  a top view is shown, in a central partial field  94  a depth cross section, and in a bottom partial field  95  an allocated measuring scale. At the right edge of the display  30 , in a third primary field  96 , various data are displayed for the user. The third primary field  96  is divided into an upper status area  97  and a lower information area  98 . The status area  97  includes, among other things, information concerning the charge status of the power supply  48  or a memory card, with the information being displayed in the form of pictograms in the status area  97 . In the information area  98  updated coordinates of the measurement are shown. 
         [0060]    A depth cross section is a two-dimensional display of the measurements in a level extending perpendicular in reference to the XY-level; the depth direction is displayed on the vertical axis of the depth cross section and a horizontal direction in the XY-level on the horizontal axis. In a linear travel motion the horizontal direction is particularly equivalent to the travel direction; in a hand-held detection device or the motion of a hand-held detection device along an arbitrary path, the horizontal direction is particularly equivalent to a travel direction determined by the detection device, for example, a housing edge. In the depth cross section, raw data, e.g., the received signals embodied as hyperboles, or received processed signals are shown. The received signals are processed using image processing and sample detection methods in order to gain information regarding objects in the underground. In depth cross sections using received processed signals the objects are shown geometrically as objects; the shape and size of the objects is displayed by different colors. 
         [0061]    A top view represents a two-dimensional illustration of the measurements in the XY-level, calculated from the depth cross sections as averages, medians, maximums, weighed totals, or other suitable mathematical functions regarding the depth range between a first and a second depth. The depth range is determined via the first and second depth or by a layer depth and a layer thickness. The depth range, by which the top view is averaged, is embodied adjustable via the toggle switch  33  of the first operating unit  31 A. In the top view only those objects are shown located within the adjusted depth range. All other objects located outside the set depth range are not shown in the top view. 
         [0062]    The average partial range  94  shows a first depth cross section  99 . 1 , in which the objects were identified in the underground by detection patterns; a power line is discernible in the cross section. The depth cross section is stretched from the depth direction Z as a vertical axis and the travel direction  28  as a horizontal axis. In addition to the first depth cross section  99 . 1  additional depth cross sections  99 . 2 - 99 . 5  are stored. The transition between the depth cross sections  99 . 1 - 99 . 5  remains unprocessed or is interpolated using interpolation methods known per se. The operator can switch back and forth via the toggle switch  33  between the depth cross sections  99 . 1 - 99 . 5 . 
         [0063]    The upper partial field  93  shows a top view  100 , which was calculated from the depth cross sections  99 . 1 - 99 . 5  over a depth range between a first depth z and a second depth z+Δz. The power line has been detected in the received signals via pattern detection methods and displayed as a power line in the top view  100 . The operator can select from several color schemes to display in color the depth cross sections  99 . 1 - 99 . 5  and the top view  100 . The color schemes serve for a differentiated display and to adjust to the ambient brightness; they have no other function. 
         [0064]    In the second primary field  92  of the display  30  several vertical and horizontal marking lines are arranged, partially displaceable via the toggle switch  33 .  FIG. 5  shows a continuous, vertical marking line  101 , two dotted, vertical marking lines  102 A,  102 B, as well as a continuous and a dot-dash, horizontal marking line  103 ,  104 . The continuous, vertical marking line  101  characterizes the center of the detection field and is equivalent to the position of the markings  40 A,  40 B at the upper and lower edge of the housing  22 . The dotted, vertical marking line  102 A shows the right hosing edge and the dotted, vertical marking line  102 B the left housing edge of the housing  22  of the detection device  21 . The continuous horizontal marking line  103  defines the layer depth z and the dot-dash horizontal marking line  104  the layer thickness Δz of the depth range. The top view  100  shown in  FIG. 5  is averaged over the depth range from 20 mm to 80 mm, the layer depth z amounts to 20 mm, and the layer thickness Δz amounts to 60 mm. The center of the detection field is located at the X-coordinate 0.96 m. 
         [0065]    The measurement shown in  FIG. 5  with the depth cross section  99 . 1  and the top view  100  is a joint measurement of the first and second sensor unit  44 ,  45 . Using the power sensor unit  44  described in  FIG. 3A  the spatial arrangement of a power line in the underground can be determined; the power sensor unit  44  is not suitable for determining the depth, though, at which the power line is embedded in the underground. When the measurement of the power sensor unit  44  is shown as a separate measurement, the control and evaluation unit  46  calculates an XY-illustration (an XY-cross section) and transmits this XY-illustration to the display unit  26 . 
         [0066]    By the combination of the power sensor unit  44  with the second sensor unit  45  the measurements of both sensor units  44 ,  45  can be displayed as joint measurements with depth cross sections and a top view. 
         [0067]    In a schematic illustration  FIG. 6  shows another embodiment of a power sensor unit  111 , which is suitable, among other things, for the use in a hand-held detection device or a detection device guided along an arbitrary track. The power sensor unit  111  includes a first power sensor unit  112  and a second power sensor unit  113 , which are aligned in two horizontal directions  114 ,  115  perpendicular in reference to each other. The depth direction  116  is defined as the vertical direction into the underground perpendicular in reference to the horizontal directions  94 ,  95 . 
         [0068]    The first power sensor unit  112  includes six magnetic field sensor elements  117 , arranged on a first holding element  118 . The magnetic field sensor elements  117  include a magnetic field gradient in the first horizontal direction  114  and in the depth direction  116 . The second power sensor unit  113  includes three magnetic field sensor elements  119 , mounted on a second fastening element  120 . The magnetic field sensor elements  119  include a magnetic field gradient in the second horizontal direction  115  and in the depth direction  116 . The power sensor units  112 ,  113  include another magnetic field sensor element  121 , which detects a magnetic field gradient in the first horizontal direction  114 , the second horizontal direction  115 , and the depth direction  116 . The magnetic field sensor element  121  is fastened on the first and/or the second fastening element  118 ,  120 . In order to eliminate any homogenous magnetic unidirectional fields the magnetic field sensor elements  117 ,  119 ,  121  are embodied as gradient sensor elements. 
         [0069]    While particular elements, embodiments, and applications of the present invention have been shown and described, it is understood that the invention is not limited thereto because modifications may be made by those skilled in the art, particularly in light of the foregoing teaching. It is therefore contemplated by the appended claims to cover such modifications and incorporate those features which come within the spirit and scope of the invention.