Abstract:
A method of land surveying that electronically registers together multi-layer underground and surface images of a surface volume with buried utilities and other infrastructures. Such method further comprises assembling and presenting the combination to a device in the field that visually guides crews in their safe digging of the ground nearby. The orienting, scaling, and registering of a first image layer is to a standardized orientation and scaling on a map of a photograph of a land surface from a zenith point in space above. Then the orienting, scaling, and registering of a second image layer is made to the standardized orientation and scaling on the map. This layer is a result of a ground penetrating radar investigation of buried objects point-by-point in an immediate search area of a corresponding ground surface. Underground buried objects and utilities are thereby located to make safe digging nearby.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to electronically finding, locating, and mapping buried urban utilities electronically from the immediate surface above, and more particularly to presenting a composite on a mobile device to enable safe digging. 
     2. Description of the Problems to be Solved 
     Finding stray, broken, lost, forgotten, misplaced, prohibited, or unlawful segments of pipes, utilities, and wires buried in the soil can be important for any number of construction, maintenance, security, defensive, economic, and operational reasons. Old maps, diagrams, notes, memories, and visual spotting are unreliable and fatiguing. Conventional electronic methods have not been performing as well as needed. 
     Electro-magnetic fields can both be induced into and sensed from wires and pipes from above on the ground surface of an area. Re-radiated magnetic fields can provide telltales of the locations and orientations of metallic and other electrically conductive objects, especially long metal pipes and wires. 
     Long-wavelength, gradiometric ground-penetrating radar (gGPR) methods can do better at mapping than conventional ground-penetrating radars (GPR) which use short-wavelength radar techniques like those developed during WW-II by the MIT Radiation Laboratory. The construction industry, in particular, has suffered a high rate of failures in the use of modern commercial versions of such short-wavelength GPR. A recent example of such a technology failure was demonstrated at Los Angeles airport during a reconstruction project when ground drilling ruptured a high-pressure gas line and the resulting leaking flammable gas exploded. A second misadventure hit an underground utility power supply to Terminal-I knocking out all power, and a third misadventure cut a large sewage line and flooded the nearby TSA headquarters with black water. 
     Gradiometric ground penetrating radar (gGPR) instrumentation and signal processing functionality takes the spatial or time domain derivative of phase-coherent illuminating and scattered/reflection radar signals. In the near field, Taylor&#39;s theorem derivatives of these signals determine distance to scattering/reflecting objects buried below a spatial cluttering geologic reflecting (SCGR) interface. The derivative of far field illuminating and reflecting interface plane wavefront is zero, making for high gain and high signal-to-noise-ratio detection. 
     Signal gradients can be very pronounced and change rapidly as electromagnetic gradiometer detection equipment is moved over areas with buried pipes and wires. These conductive objects will naturally re-radiate secondary electro-magnetic emissions. In contrast, far-field primary emissions from distant sources are characterized by near-zero difference signal gradients in the local area. Near-fields are dominated by dipole-type electric or magnetic fields. The sensing equipment has only to distinguish between the electric primary and magnetic secondary signal types to recognize nearby targets. 
     At least two basic kinds of gradiometer instruments have been used to measure magnetic field gradients. An axial gradiometer type places two magnetometers head-to-head on the same axis and wires them in series so they automatically cancel the common mode signals. Such measure the difference in the magnetic flux at a point. A planar gradiometer type spaces two magnetometers in parallel apart from one another to subtract one from the other at the receiver. The difference is the differential magnetic flux between the two magnetometers. For example, refer to our U.S. Pat. No. 7,675,289, issued Mar. 9, 2010. The axial and planar types will respond differently to spatial signals. The magnetic field component directions relative to the device&#39;s spatial orientation can be measured with vector magnetometers. 
     A single magnetometer can be used to collect readings point-by-point over a survey area. Each reading at each point is stored in a table and then the apparent magnetic gradients can be computed and displayed on a map. A three-axis magnetometer in a single pod can provide x,y,z magnetic axes measurements for even more complex calculations and information extraction. 
     Gradiometer instrumentation can sometimes be challenging to accommodate because of its size. Even full-size ground, air, and water vehicles can be pressed to provide enough room. The size, weight, and power (SWAP) requirements of new kinds of onboard sensors and devices can sometimes be too much for first generation systems. This is especially true of small remote-controlled vehicles, robots, and drones that are not fully integrated. In contrast, stand-alone sensors developed as individual mix-and-match add-ons waste too much in resources, and the combinations are difficult to house in tight accommodations. 
     Stolar Research Corporation (Rio Rancho, N. Mex.) has developed a broad line of vertical magnetic gradiometers (VMG) that can be used singularly, in pairs, and in arrays to sweep over roadways and open fields. Stolar VMG&#39;s can be carried by personnel on foot, vehicles, airplanes, kites, robots, and even as EMG sondes on drones. These products all descended from Stolar&#39;s earlier work with EM gradiometers on measuring coal deposits in underground mines. 
     New advances here in the remote sensing sciences combine multi-purpose, collocated sensors that also share system resources. 
     The American Public Works Association (APWA) has been promulgating its Uniform Color Code for Marking Underground Facilities Underground facilities. They want everyone to use colors of spray paint to mark on the ground surface the locations of buried utilities in accordance with a following designated color code. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 White 
                 Pre-marking of the outer limits of the  
               
               
                   
                   
                 proposed excavation or marking the  
               
               
                   
                   
                 centerline and width of proposed lineal  
               
               
                   
                   
                 installations of buried facilities.  
               
               
                   
                 Pink 
                 Temporary Survey Markings.  
               
               
                   
                 Red  
                 Electric power lines, cables or conduit,  
               
               
                   
                   
                 and lighting cables.  
               
               
                   
                 Yellow 
                 Gas, oil, steam, petroleum, or other  
               
               
                   
                   
                 hazardous liquid or gaseous materials.  
               
               
                   
                 Orange 
                 Communications, cable TV, alarm or signal  
               
               
                   
                   
                 lines, cables or conduits.  
               
               
                   
                 Blue 
                 Potable Water, irrigation, and slurry  
               
               
                   
                   
                 lines.  
               
               
                   
                 Purple 
                 Slurry and reclaimed.  
               
               
                   
                 Green 
                 Sewers, drainage facilities or other drain  
               
               
                   
                   
                 lines. 
               
               
                   
                   
               
             
          
         
       
     
     The APWA guidelines for uniform temporary marking of underground facilities provides for universal use and understanding of the temporary marking of subsurface facilities to prevent accidents and damage or service interruption by contractors, excavators, utility companies, municipalities or any others working on or near underground facilities. Their One-Call damage prevention system is often required by Law to be contacted prior to excavation. White marks are painted or chalked to show the location, route or boundary of proposed excavation. Surface marks on roadways do not exceed 1.5″ by 18″ (40 mm by 450 mm). The facility color and facility owner identity may be added to white flags or stakes. Color-coded surface marks are used to indicate the location or route of active and out-of-service buried lines. Color-coded vertical markers (stakes or flags) are used to increase visibility and supplement surface marks. Marks and markers indicate the name, initials or logo of the company that owns or operates the line, and width of the facility if it is greater than 2″ (50 mm). Marks placed by other than line owner/operator or their agents are used to identify the designating firm. Multiple lines in a joint trench are marked in tandem. If the surface over the buried line is to be removed, supplementary offset markings are used. Offset markings are on a uniform alignment and meant clearly indicate the actual facility is a specific distance away. 
     Any excavation within the tolerance zone must be done with non-powered hand tools and other non-invasive methods until the marked facility is exposed. The width of the tolerance zone may be specified by Law. If not, a tolerance zone including the width of the facility plus 18″ (450 mm) measured horizontally from each side of the facility is recommended. The American Public Works Association encourages public agencies, utilities, contractors, other associations, manufacturers and all others involved in excavation to adopt the APWA Uniform Color Code, using ANSI standard 2535.1 Safety Colors for temporary marking and facility identification. 
     However, marking all the buried utilities in a whole city or even an airport is not very practical using the APWA methods. The conventional utility locating tools commonly used are only useful in small areas when lots of time is available. 
     SUMMARY OF THE INVENTION 
     Briefly, a method of land surveying electronically registers together multi-layer underground and surface images of a surface volume with buried utilities and other infrastructures. Such method further comprises assembling and presenting the combination to a device in the field that visually guides crews in their safe digging of the ground nearby. The orienting, scaling, and registering of a first image layer is to a standardized orientation and scaling on a map of a photograph of a land surface from a zenith point in space above. Then the orienting, scaling, and registering of a second image layer is made to the standardized orientation and scaling on the map. This layer is a result of an ground penetrating radar investigation of buried objects point-by-point in an immediate search area of a corresponding ground surface. Underground buried objects and utilities are thereby located to make safe digging nearby. 
     These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures. 
    
    
     
       IN THE DRAWINGS 
         FIG. 1  is a perspective view diagram representing how each vertical column of earth is scanned with various GPS, camera, GPR, and EMG sensors to each produce a “tile” in a stack, and how many such stacks are assembled to electronically represent the buried infrastructure in a search area; 
         FIG. 2  is a schematic diagram of a multi-sensor data collection vehicle that is moved over the surface of a search area shown here in cross-section, these produce the tile scans of  FIG. 1  representing the buried infrastructure in a search area; 
         FIG. 3  is a perspective diagram of how a whole search area of multi-sensor data collection is trickled point-by-point into data base and image processing is used to update and refresh a user display screen of maps with overlays of the buried infrastructure in a search area; 
         FIG. 4A  is a perspective diagram of a resonant microwave patch antenna showing how a relationship of a feedpoint to a groundpost; 
         FIG. 4B  is a cross sectional diagram of the resonant microwave patch antenna of  FIG. 4A  and shows the relationships between the feedpoint, groundpost, fringing E-field, and a propagating E-field; 
         FIG. 5A  is a schematic diagram of a horizontal magnetic dipole antenna and a graph of its gradiometer response to a buried object scattering an electromagnetic radio signal; 
         FIG. 5B  is a schematic diagram of a vertical magnetic dipole antenna and a graph of its gradiometer response to a buried object scattering an electromagnetic radio signal; 
         FIG. 6  is a plan view diagram of how four antennas can be multiplexed into pairs that yield a variety of gradient orientations and improve data collection of the EMG; 
         FIG. 7  is a schematic block diagram of a software defined radio transceiver for operation with the ground penetrating radar and RMPA; 
         FIG. 8  is a frequency plan diagram for the software defined radio transceiver of  FIG. 7 ; 
         FIG. 9  is a perspective view diagram of how six VMG antennas can be flown in two arrays on outriggers over both shoulders of a road looking for buried pipes, wires, and other infrastructure; 
         FIG. 10  is a schematic block diagram of a tri-axial VMG antenna and software defined radio transceiver for GPR use as in  FIGS. 1-9 ; 
         FIGS. 11A-11C  are side, cross-section, and top view diagrams showing one way to construct shielded, magnetic field antennas; 
         FIG. 11D  is a perspective, exploded assembly view diagram showing how three antennas in a tri-axial arrangement can be orthogonally disposed in a weatherproof housing; 
         FIG. 12  is a diagram representing the signals obtained from VMG measurements over a survey area, and especially the noise floor that tends to increase when scanning over areas contaminated by bacteria that make the soils and waters conductive to electricity; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A method of mapping underground utility infrastructures and other buried objects begins by investigating buried objects point-by-point in an immediate search area of a ground surface radio illuminated by a radio broadcast transmitter in the 500-kHz to 1,000-kHz band transmitting from a location outside the immediate search area. Then, sensing together short-wavelength reflections of 100-MHz to 2,000-MHz and long-wavelength scattering of the electro-magnetic radio energy of the radio broadcast transmitter from buried objects underneath in layered soils with a surface-based measurement of buried-object signals using at least a phase-coherent elimination of ground surface reflection noise of at least sixty decibels in digital signal processing with a field programmable gate array (FPGA). Then interpreting signals and displaying visual representations and characteristic descriptions of underground utility infrastructures and the buried objects sensed together, located, and clarified, with a mapping apparatus capable of displaying aeronautical charts, satellite images, elevation maps, and geographically referenced overlays that can be displayed and printed over any map background. 
     Buried-utility-detection embodiments of the present invention leverage the electromagnetic (EM) radiation from conductive objects in the soils that results from the radio energy they absorb from local longwave AM-radio broadcasts in the 500-kHz to 1-MHz radio spectrum. The horizontal electric fields from such broadcasts will induce secondary currents in conductive soils and any underground conductors if the electric fields get captured and scattered. Long-wavelength scattered electro-magnetic fields will return reflections in the form of spherically and/or cylindrically spreading waves through and up to the surface where their gradients over short distances are detectable by a gradiometer. Our U.S. Pat. No. 7,675,289, issued Mar. 9, 2010, provides important details of construction, operation, and use of electromagnetic gradiometers (EMG&#39;s), but the EMG&#39;s we now use in embodiments of the present invention are improvements over our 2010 state-of-the-art. 
     Buried-utility-detection embodiments of the present invention also leverage special earth penetrating radars in the 100-MHz to 2,000-MHz range with special phase-coherent elimination of ground surface reflection noise of at least sixty decibels in digital signal processing with a field programmable gate array (FPGA). For background, see United States Patent Application, Published Application US 2014/0125509 on May 8, 2014, filed Jan. 10, 2014 as Ser. No. 14/152,823, and titled RADAR FOR REJECTING AND LOOKING PAST SURFACE REFLECTIONS, is incorporated herein by reference. 
     Herein lies a first advantage of our electromagnetic gradiometers in this application, the radio energy used for detection of underground utility infrastructures travels only once through the overburden. Active radar from the surface, by its nature, forces the radio energy used for detection to travel twice through the overburden, e.g., down through the surface to a buried object, and then back up through the surface. Making radar only useful in imaging shallow depths. Conventional active radar also suffers from a “surface glare” of radio energy because the ground surface is very reflective to radar. Even more so if wet with puddles and dew. 
     The cylinder centerline, e.g., of a wire or pipe, will typically be nearby, down in the ground only a few feet. So the gradiometric spatial differences in magnetic tilt will be measurable between even modest displacements of antenna locations of six feet or less. The illuminating and scattered EM field components are always phase-coherent with respect to each other, and therefore synchronous radio detection methods are usefully employed. 
     Differences in the EM fields measured at different antenna locations are related to the spatial gradient, differential, difference or derivative. 
     A vertical magnetic gradiometer (VMG) can usually be carried over a search area surface pretty easily by a motor vehicle. The VMG has a spot of sensitivity which is oriented to stay nadir on the ground to the VMG itself. A Faraday shield surrounds each VMG antenna to screen out near-field electric dipole signals. A ferrite rod core and winding inside can then act as an antenna sensitive to only near-field magnetic dipole signals. Such magnetic dipole signals will radiate from nearby conductors if the conductors are in an electric radio field. The VMG on its vehicle calls on all the points in the search area to collect and measure the available magnetic signal gradients and reversals point-by-point. The points nadir to the VMG which coincide with buried conductors will express characteristic signatures in the signal gradients and phase reversals measurable in the VMG antenna. Even changes in the receiver&#39;s background noise will indicate important features in the soils. 
       FIG. 1  represents a point-by-point vertical column of earth  102  topped by a ground surface  104  with x-axis and y-axis dimensional portions of a survey area  106 . This vertical column of earth  102  is abstracted in a z-axis (depth) and rendered in batch data processing later as a voxel in a false-color visual display apparatus of the present invention. Such is continually updated and improved with new bits of information as they are gathered from the field. For example the way Google Maps is a patchwork of many inputs from many sources over random time periods. 
     The “false-colors” used would be more familiar to utility workers if the APWA Marking System were employed for the display screens. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 APWA Marking System 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 White 
                 Pre-marking of the outer limits of the  
               
               
                   
                   
                 proposed excavation or marking the  
               
               
                   
                   
                 centerline and width of proposed lineal  
               
               
                   
                   
                 installations of buried facilities.  
               
               
                   
                 Pink 
                 Temporary Survey Markings.  
               
               
                   
                 Red 
                 Electric power lines, cables or conduit,  
               
               
                   
                   
                 and lighting cables.  
               
               
                   
                 Yellow 
                 Gas, oil, steam, petroleum, or other  
               
               
                   
                   
                 hazardous liquid or gaseous materials.  
               
               
                   
                 Orange 
                 Communications, cable TV, alarm or signal  
               
               
                   
                   
                 lines, cables or conduits.  
               
               
                   
                 Blue 
                 Potable Water, irrigation, and slurry  
               
               
                   
                   
                 lines.  
               
               
                   
                 Purple 
                 Slurry and reclaimed.  
               
               
                   
                 Green 
                 Sewers, drainage facilities or other drain  
               
               
                   
                   
                 lines. 
               
               
                   
                   
               
             
          
         
       
     
     A “voxel” in computer-based modeling or graphic simulation is an element of an array of elements of volume that create a imaginary three-dimensional space, especially each of an array of discrete elements into which a representation of a three-dimensional object is divided. The purpose is finding, locating, and mapping buried urban utilities  108  electronically from the immediate surface above, and more particularly to incrementally improving blending of multiple sensors and painting composite images. 
     Essentially, only one electronic voxel of a vertical column of earth  102  can be collected at a time as stacked tiles  111 - 114 , and so many snapshots are needed to be collected before a survey area abstract  106  can be fully painted or rendered into an informational display. Here, stacked tiles  111 - 114  comprise GPS location fixes  111 , a photograph  112 , a GPR measurement  113 , and an EMG measurement  114 , all mutually registered with one another. Together these are electronically assembled by image processors into a composite tile  116 . Such has neighboring composite tiles  117 - 119 . 
     Embodiments of the present invention measure the search area in several dimensions, one is to measure the differences in magnitude at changing locations of an EMG antenna to observe spatial derivatives. Subsequent antenna locations yield more spatial derivatives. These derivatives are subtracted to yield a local gradient and to suppress the far stronger planar wavefronts of the illuminating AM-Radio Broadcasts. Conventional gradient-subtraction algorithms require many antenna receivers and phase coherency between them to suppress the illuminating EM-waves. 
     Other measurement dimensions include shortwave radar, photography, and navigation fixes. One recent development of ours places neutron generators above ground on the surface over where plastic PVC pipes are suspected to lay. The piping is filled with N 2  nitrogen gas if possible. When a neutron from the neutron generator strikes a nitrogen molecule, it will create an isotope with a very brief half-life. The chlorine molecules in the PVC pipe may also be struck with such neutrons. When these nitrogen and/or chlorine isotopes decay they will emit gamma rays with energy signatures that can be detected and recognized by gamma ray detection equipment very common to the explosives detectors now in widespread use at airports and security checkpoints. 
     Toxic plume contamination can be detected by increases in the background noise (EM fields) that appears when scanning over abnormal zones. Earlier test observations later confirmed such readings to actually be the result of toxic plume contamination. Toxic plume contaminations are conductive and electrically contrast with surrounding soils because anaerobic bacteria metabolic processes produce hydrogen sulfide, methane and enzymes. Non-uniform scattered wavefronts result and are detectable with gradiometric processing. 
                                           OPERATIONAL SUMMARIES                                        EMG (Orion) operates with very low frequency                200 kHz to 1 MHz nominal                Excellent penetration depth, less inherent resolution                Resolution recovered in our EMG with signal processing                GPR operates at medium frequencies                100 MHz to 2 GHz nominal                100 to 20,000 shorter wavelength than EMG (Orion)                Good resolution, poor to extremely poor penetration depth                Inherent physics of higher frequency signal propagation in                lossy materials                Two-way propagation: out and back; EMG is one-way as                background environment provides source signal                        
Materials can be electronically characterized by their conductivity, permittivity, and permeability. Such characteristics can easily be probed with radio energy.
 
     GPR fares even worse in even moderately mineralized or wet soil. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                   
                 EMG (Orion) IS BEST USED WHEN 
               
               
                   
                   
               
               
                   
                   
                 detection depth is pre-eminent requirement;  
               
               
                   
                   
                 need to “see past” smaller objects, e.g., rebar, small metal  
               
               
                   
                   
                 debris, etc. 
               
               
                   
                   
               
               
                   
                   
                 GPR IS BEST USED WHEN 
               
               
                   
                   
               
               
                   
                   
                 spatial resolution is pre-eminent requirement 
               
               
                   
                   
               
               
                   
                   
                 EMG (Orion) and GPR ARE BEST USED TOGETHER 
               
               
                   
                   
               
               
                   
                   
                 For maximum detection depth, probability of detection, and  
               
               
                   
                   
                 to avoid clutter (EMG), and best 3-axis resolution (GPR) 
               
               
                   
                   
               
               
                   
                   
                 A RECENT PROTOTYPE USED IN TESTING HAD: 
               
               
                   
                   
               
               
                   
                   
                 An EMG (Orion) system with three antennas  
               
               
                   
                   
                 Combinations of the antennas fashioned three separate  
               
               
                   
                   
                 gradients;  
               
               
                   
                   
                 The individual gradients have unique characteristics  
               
               
                   
                   
                 Each antenna was capable of receiving two frequencies  
               
               
                   
                   
                 simultaneously;  
               
               
                   
                   
                 Total combination of antennas and frequencies means there are  
               
               
                   
                   
                 a total of six gradients available from a single pass of data  
               
               
                   
                   
                 collection (G1-G6) 
               
               
                   
                   
               
             
          
         
       
     
     The gradients realized from the ORION EMG prototype were:
         G 1 : Widest antenna separation for deeper target resolution at frequency F 1 ;   G 2 : Widest antenna separation for deeper target resolution at frequency F 2 ;   G 3 : Medium antenna separation for mid-depth target resolution at frequency F 1 ;   G 4 : Medium antenna separation for mid-depth target resolution at Frequency F 2 ;   G 5 : Narrow antenna separation for shallow target resolution at frequency F 1 ;   G 6 : Narrow antenna separation for shallow target resolution at frequency F 2 
 
The ORION EMG prototype was operated at two frequencies simultaneously. The lower frequencies reached deeper depths of penetration but required long linear targets. The higher frequencies had only shallow depth penetration, but could “see” short length targets.
       

     The Detection Depths and gradients were: 
     Shallow depths: 0-8 ft. in general; 
     Mid-deep ranges: 8-15 ft. in general; 
     Deep ranges are: 15 ft. plus. 
     Typically, for targets of adequate length:
         1. G 1  and G 2  could detect deep and shallow targets but could not distinguish between closely spaced targets;   2. G 3  and G 4  could detect targets up to about 15 ft. (depending on soil type and frequency) with stronger gradients that provided a good overall picture;   3. G 5  and G 6  had trouble resolving targets longer than ten feet, but were the gradients of choice for detecting closely spaced targets.       

     The ORION EMG prototype was primarily capable of detecting non-linear targets, and secondarily capable of detecting zones of soil contamination. The ORION EMG does not detect gradients in soil contamination, but rather, it detects a rise in the noise floor over a contaminated zone where the contamination had raised the local soil conductivity. 
     As such, a detectable rise in RF background “noise” will be interpreted as soil contamination. Such soil contamination was not limited to hydrocarbon or other chemical contaminates. Any increase in water moisture properties in situ can also increase background RF “noise” over these “wet” zones. Background RF “noise” increases are seen across all gradients, but they are more pronounced in the mid to deep looking gradients. 
     The image processing algorithms for the ORION EMG prototype parsed data into one of three categories. The imaging software did not operate on the phase of the gradients, only their magnitudes. An “MAG” imaging process operated on the magnitude of the gradients. It took the highest and lowest amplitude signals from a set of data and did a linear interpolation across the set, assigning colors for visual display. A “SmallMag” imaging process took a set of data, removed the highest amplitude data, and then interpolated the remaining data to span full-scale. The process removed large peaks in the data and highly emphasized small/weak signals. A “CMBD” imaging process combined the other two processes. It removed the large peaks, amplified the weak signals, then re-inserted the large peaks. This process worked good for contaminated soil analysis. 
     Regardless of which imaging form was selected, the rendered data sets were then color coded relative to the processed magnitude of individual gradient set being worked with. The color scale ranges used ran from deep blue to intense red. The deeper blue color represented the smaller magnitude gradients, meaning nothing was detected. Intense red represented a maximum in gradient amplitudes, indicating a target was detected. Color blends in between were proportional the value of the gradient magnitude at that position, depending on the strength of the signal from a nearby target. The form of imaging used in the ORION EMG prototype was not designed to predict target depths. 
       FIG. 2  represents a data collection method  200  in which a data collection vehicle  202  is rolled over a ground surface  204  of a survey area. The data collection vehicle  202  is outfitted with several sensors, e.g., a GPS navigation receiver  206 , a downward pointing digital camera  208 , a ground-penetrating radar (GPR)  210 , and an electro-magnetic gradiometer (EMG)  212 . A very full description of a suitable GPR for use here will be found in our recently allowed United States Patent Application published as US 2014/0125509, on May 8, 2014, and titled RADAR FOR REJECTING AND LOOKING PAST SURFACE REFLECTIONS. Such Application is a parent to this continuation-in-part, and is incorporated herein in full by reference. A very full description of a suitable EMG for use here will also be found in our United States Patent Applications: (1) published as US 2014/0139224, on May 22, 2014, and titled STRAY WIRE LOCATION SENSOR; and, (2) published as US 2014/0368196, on Dec. 18, 2014, and titled PRACTICAL ELECTROMAGNETIC GRADIOMETER. Such Applications too are parents to this continuation-in-part, and are each incorporated herein in full by reference. 
     Sensors  206 ,  208 ,  210 , and  212  are simultaneously operated point-by-point in a search area to obtain their respective data outputs for each point on surface  204  that data collection vehicle  202  visits in a survey. The method objective is to locate by sensing, and map by combination of their outputs, the depths and locations of pipes, conduits, wires, structures, and other buried objects  214  beneath surface  204 . A local AM-Radio Broadcast Station  220  outside the search area is depended upon to radiate a strong longwave radio carrier  222  in the 500-kHz to 1-MHz band. Such local AM-Radio Broadcast Station  220  being nearby is serendipitous. Buried objects  214  will capture some of this radio energy and re-radiate electromagnetic signals  224  and  226 . These are respectively sensed as magnetic gradients by EMG  212 . 
     GPR  210  operates by sending radio signals  230  in the middle frequencies of 100-MHz to 2,000-MHz into the surface  204 . A very strong radio reflection  232  always occurs from the surface. But some of the energy  230  that does penetrate will be returned from buried objects  214 . The strong surface reflections  232  are suppressed in signal processing in accordance with our disclosures in Published United States Patent Application US 2014/0125509, May 8, 2014, titled RADAR FOR REJECTING AND LOOKING PAST SURFACE REFLECTIONS, and our other issued patents. 
       FIG. 3  represents how a large search area  300 , like a city airport or the whole city itself, can be visually rendered as a map with overlays on a display screen bit by bit, point-by-point, using embodiments of the present invention. Such large search area  300  is, in one regard, a mosaic of tiles, such as composite tiles  116 - 119  ( FIG. 1 ). There will be tiles representing locations that-have-been-visited  302  and those next-to-be-measured  304 . Stacked tile information  306  includes GPS position fixes and photographic surface location fixes and is communicated by a link  308  and collected by a database  310 . An image processing algorithm  312  renders the whole visually to a display screen  314 . For example, FALCONVIEW™ and SKYVIEW™ by the Georgia Tech Research Institute would be useful. Even partially rendered, display screen  314  will provide important information to construction and utility workers about the buried infrastructures underlying area  300 . 
     FalconView is a mapping system created by the Georgia Tech Research Institute. It was initially developed for the Windows family of operating systems. It displays various types of maps and geographically referenced overlays. Many types of maps are supported, but the primary ones of interest to most users are aeronautical charts, satellite images and elevation maps. FalconView also supports a large number of overlay types that can be displayed over any map background. A current overlay set is directed toward military mission planning users and is focused on aviators and aviation support personnel. FalconView is an integral part of the Portable Flight Planning Software (PFPS) and includes FalconView, Combat Flight Planning Software (CFPS), Combat Weapon Delivery Software (CWDS), Combat Air Drop Planning Software (CAPS) and several other software packages built by various software contractors. Current work includes the development of FalconView as part of XPlan, the Department of Defense&#39;s most recent mission planning system. The Joint Mission Planning System is also being added to FalconView as a plugin. The program has an active user community, and the Georgia Tech Research Institute plans events supporting the program. Wikipedia 
       FIGS. 4A and 4B  represent a resonant microwave patch antenna (resonant microwave patch antenna)  400  useful in connection with GPR  210  ( FIG. 2 ). Such resonant microwave patch antenna is detailed further in our United States Patent Application Publication US 2014/0306839, published Oct. 16, 2014, and titled ELECTROMAGNETIC DETECTION AND IMAGING TRANSCEIVER (EDIT) AND ROADWAY TRAFFIC DETECTION SYSTEM, Ser. No. 13/862,379, filed Apr. 13, 2013, and incorporated herein in full by reference. 
       FIG. 4A  represents a way to construct resonant microwave patch antenna  400  using common FR4 printed circuit board material. A copper-foil backplane  402  and radiating patch  404  are separated by an epoxy substrate  406 . A feedpoint  408  is drilled through the backplane  402  and substrate  406  so a 50-ohm coaxial cable can be attached to the radiating patch  404 . A groundpost  412  is constructed by drilling and plating a copper via. The relationship of the feedpoint  408  to the groundpost  412  creates a forward radiating edge  414  and an aft radiating edge  416 . The resonant microwave patch antenna has a characteristic input impedance (Z in ) and resonant frequency (F R ) that are a function of the dielectric constant of substrate  406 , objects in the radiated field, the separation distance of backplane  402  and patch  404 , the distance between feedpoint  408  to the groundpost  412 , and the plan dimension of patch  404 . Herein, these all add up to a resonant frequency in the range of 100-MHz to 2-GHz, and a Z in  of about 50-ohms when the radiation field is substantially comprised of air. Varactors  420  or other types of trimming capacitors can be added around the edges of resonant microwave patch antenna  400  to fine-tune its resonant frequency. 
     It is important to good operation in this use here that the antenna be narrow band. Conventional antennas used in the GPR&#39;s we reference herein typically employ wideband antennas. 
     In the illustrated configuration, the resonant microwave patch antenna is fed a constant frequency and the varactors are tuned to keep it at resonance despite changes in the media environment surrounding the resonant microwave patch antenna. The “auto-correction” voltages sent to the varactors to keep the balance will therefore respond proportionally to changes in the media environment. The resonance is verified by observing minimas in the Z in . Interpretations of the placement and relative movements of buried objects can, in one embodiment, be made by tracking the correction voltages sent to the varactors  420  necessary to minimize Z in . 
     Scattering parameters (s-parameters) describe the scattering and reflection of traveling waves when a network is inserted into a transmission line. Here, the transmission line includes the soils and buried objects. S-parameters are normally used to characterize high frequency networks, and are measured as a function of frequency. So frequency is implied and complex gain and phase assumed. The incident waves are designated by the letter a n , where n is the port number of the network. For each port, the incident (applied) and reflected waves are measured. The reflected wave is designed by b n , where n is the port number. When the incident wave travels through a network, its gain and phase are changed by the scattering parameter. For example, when a wave a 1  travels through a network, the output value of the network is simply the value of the wave multiplied by the relevant S-parameter. S-parameters can be considered as the gain of the network, and the subscripts denote the port numbers. The ratio of the output of port-2 to the incident wave on port-1 is designated S 21 . Likewise, for reflected waves, the signal comes in and out of the same port, hence the S-parameter for the input reflection is designated S 11 . 
     For a two-port network with matched loads: 
     S 11  is the reflection coefficient of the input; 
     S 22  is the reflection coefficient of the output; 
     S 21  is the forward transmission gain; and 
     S 12  is the reverse transmission gain from the output to the input. 
     S-parameters can be converted to impedance by taking the ratio of (1+S 11 ) to (1−S 11 ) and multiplying the result by the characteristic impedance, e.g., 50-ohms or 75-ohms. A Smith chart can be used to convert between impedance and S-parameters. 
     The frequency and impedance, or reflection coefficient (S 11 ), of resonant microwave patch antenna  400  are measured to provide sensor information and interpretive reports. resonant microwave patch antenna  400  is electronically tuned by a sensor controller either adjusting oscillator frequency and/or varactors to find the resonant frequency of the resonant microwave patch antenna each time a measurement is taken. The S 11  (reflection coefficient) parameter is measured in terms of magnitude. The sensor controller seeks to minimize the magnitude of S 11 , meaning resonant microwave patch antenna  400  is near its resonant point and 50-ohms. 
     During an automatic steady state calibration, an iterative process is used in which a sensor controller seeks a minimum in S 11  by adjusting the applied frequency through an oscillator. Once a frequency minimum for S 11  is found, sensor controller adjusts a bias voltage on varactors connected to the edges of resonant microwave patch antenna  400 . The voltage variable capacitances of varactors are used to fine tune resonant microwave patch antenna  400  into resonance, and this action helps drive the impedance as close to 50-ohms as possible. Sensor controller simply measures the S 11  magnitude minimum. Once voltage adjustments to varactors find a minimum in S 11  magnitude, the process is repeated with very fine adjustment steps in an automatic frequency control to find an even better minimum. The voltages to varactors are once again finely adjusted to optimize the minimum. 
     After calibration, an independent shift away from such minimum in S 11  magnitude means a buried object is affecting the balance. The reflection coefficient (S 11 ) will change away from the original “calibrated” resonance value. Typically a buried object passed overhead within the field will cause a peak maximum in the measured data. The rate of change of the measured signal in the area is directly related to the speed of the vehicle carrying resonant microwave patch antenna  400 . 
     S 11  has both magnitude and phase, a real and imaginary part. Changes in magnitude indicate a disturbance in the EM-field of resonant microwave patch antenna  400 , and changes in the phase provide the directionality of travel  110 - 113 . resonant microwave patch antenna  400  is a linearly polarized antenna, the fields on one edge of resonant microwave patch antenna  400  are 180-degrees out of phase from the field on the other edge. With a proper alignment of resonant microwave patch antenna  400  in situ, buried objects passing in front of resonant microwave patch antenna  400  from left to right, will produce a phase signature that is 180-degrees out of phase from other objects moving right to left. The phase at resonance can be corrected to provide a constant 180-degree shift. 
       FIG. 4B  schematically represents resonant microwave patch antenna  400  taken through a normal plane that longitudinally bisects both the ground post  412  and feedpoint  408 . Varactor  420  is typical of many that can be connected to be voltage-controlled by electronics controller  422  to enable fine tuning of the resonant frequency of resonant microwave patch antenna  400  to help with calibration and measurement sensing. The electronics controller  422  is able to measure parameter S 11  at the feedpoint  408  and thereafter issue interpretive reports. 
     At resonance, the electromagnetic fields radiate away from resonant microwave patch antenna, as shown in  FIG. 4B . A linearly polarized electric field fringes from the edges of the metalized, copper foil parts of resonant microwave patch antenna  400 . Such type of polarization is an important operational element, this polarization enables a directional indication. As applied here, the antenna radiation pattern has a very broad 3-dB beam width of ±30 degrees from the perpendicular to the plane of patch  404 . This pattern is important in the present applications because the wide antenna pattern allows a large area to be electronically swept. 
       FIGS. 5A and 5B  represent how a horizontal-type magnetic dipole antenna  501  and a vertical-type magnetic dipole antenna  502  respectively respond with an EMG receiver and processor to a buried object  503  that is re-radiating radio energy from a local AM-Broadcast carrier. A waveform  511  in  FIG. 5A  results from the EMG receiver as horizontal-type magnetic dipole antenna  501  is moved left and right laterally overhead to buried object  503 . A waveform  512  in  FIG. 5B  results from its EMG receiver when vertical-type magnetic dipole antenna  502  is moved left and right laterally overhead relative to buried object  503 . The lateral distance that either antenna  501  or  502  must be moved to change between a peak and a null in EMG receiver waveforms  511  and  512  is approximate to the depth the object  503  is buried below the surface. It is therefore important to have very fine position fixes on antennas  501  and  502  at the instants they provide gradient waveforms  511  and  512 . The position fixes provided by ordinary GPS receivers is usually not good enough without being assisted by photographic registrations of the ground surface at those same instants. 
       FIG. 6  represents a data collection method  600  in which four electromagnetic dipole antennas  601 - 604  and an antenna multiplexer  606  are mounted on a data collection vehicle  608 . The data collection vehicle  608  is rolled over the surface of a search area looking for buried infrastructure like pipe  610 . The antenna multiplexer  606  allows the four electromagnetic dipole antennas  601 - 604  to be selected in gradient-view pairs and coupled to an electromagnetic gradiometer receiver and processor in at least ten different ways. E.g., 
                                 (1) gradient between A-B;        (2) gradient between B-A;        (3) gradient between A-C;        (4) gradient between C-A;        (5) gradient between A-D;        (6) gradient between D-A;        (7) gradient between B-C;        (8) gradient between C-B;        (9) gradient between B-D; and        (10) gradient between D-B.                    
Taking all ten gradient pair measurements will yield information about the pipe  610  and its relative lay under the surface.
 
       FIG. 7  represents a GPR implementation in a software defined transceiver radar (SDTR)  700  that includes a digital signal processor  701 , an analog radio frequency stage  702 , a heterodyne frequency synthesizer  703 , an RF adder  704 , a local oscillator adder  705 , a directional coupler  708 , and an resonant microwave patch antenna  710 . These all launch RF transmissions through a protective polycarbonate radome window  711  into a ground surface  712 , into soils  714 , and reach a buried object  716 . Return reflections are collected by a port  718  and beat down by a mixer  720  into an intermediate frequency  722 . This is filtered by a bandpass  724  for processing by DSP  701 . 
     In a prototype implementation of a software defined transceiver radar, the analog printed circuit board included a quadrature up converter, RF power amplifier, coupler (for a monostatic radar), phase locked loop (PLL), or several quad DOSs (Analog Device AD9959) and a down converter. 
     Software-defined transceiver radar (SDTR)  700 , included digital and analog printed circuit boards (PCBs) for  701 ,  702 , and  703 . The digital PCB  701  produces four synthesized digital frequencies ω 1 , ω 2 , ω 3 , and ω 4 , respectively described by equations 7, 8, 7′, and 8′, in Table-V. Analog PCB  702  uses these to produce the radio frequency (RF) signals described by equations 13 and 14, and analog PCB  703  produces heterodyne signals described by equations 13′ and 14′, of Table-V (in our U.S. Pat. No. 7,656,342, issued Feb. 2, 2010, and titled, DOUBLE-SIDEBAND SUPPRESSED-CARRIER RADAR TO NULL NEAR-FIELD REFLECTIONS FROM A FIRST INTERFACE BETWEEN MEDIA LAYERS). Adders  704  and  705  sum these to produce a transmitter signal described by equation 15 and a synchronous mixer signal described by equation 15′ of Table-V. A directional coupler  708  sends the transmitter signal through for launching into a radar media by an antenna  710 . A first interface  712 , a coal seam  714 , and a second interface  716  are typical in the radar media. Any return reflections  718  extracted by directional coupler  708  are described by equation 16 of Table-V (in US and are detected by a mixer  720 . The mixer output  722  is described by equation 17 of Table-V. A bandpass filter  724  removes the carrier and one of the sidebands for an output signal  726 , and is described by equation 18 of Table-V. The digital PCB  701  then interprets signal  726  to estimate the character of first interface  712 , coal seam  714 , and second interface  716 . 
       FIG. 8  diagrams how the various equations of Table-V can be interrelated, and suggests how the circuitry of SDTR  700  can be configured to do the required signal processing. The inputs w 1  and w 2  are in the range of 750-1000 kHz and are summed with θ 1  using phase splitters to produce upper and lower sidebands with a completely suppressed carrier at the output of an adder  804 . Such is the equivalent of adder  704  in  FIG. 7 . This is amplified by an amplifier (G) before being applied to a directional coupler  808  and antenna  810 . Such are equivalent to directional coupler  708  and antenna  710  in  FIG. 7 . 
     The mixer  720  must accommodate a reflection of +0-dB from the first-interface  712  reflected EM wave that is up to 80-dB greater than the second interface  716  reflected EM wave. This requires a radar receiver dynamic range greater than 80 dB (10,000). The mixer  720  performs sinusoidal waveform multiplication. The band pass filter  724  rejects all mixer output frequencies except the intermediate frequency (IF). The directional coupler  708  recovers the reflected wave. Phase-coherent detection is achieved by mixing the DDS with the reflected point signal and bandpass filtering the mixer output signal. An important feature of the phase-coherent detection scheme is that the in phase (I) and quadrature (Q) terms are simultaneously measured in the digital electronics  701 . Simultaneous measurement improves noise immunity. 
     Surface reflection suppression is processed as illustrated in  FIG. 8 , an algorithm  800 . Such is described in great detail in our U.S. Pat. No. 7,656,342, issued Feb. 2, 2010, and titled, DOUBLE-SIDEBAND SUPPRESSED-CARRIER RADAR TO NULL NEAR-FIELD REFLECTIONS FROM A FIRST INTERFACE BETWEEN MEDIA LAYERS. And such is incorporated herein by reference. 
       FIG. 8  diagrams how the various equations of Table-V in the Reference can be interrelated, and suggests how the circuitry of SDTR  700  can be configured to do the required signal processing. The inputs w 1  and w 2  are in the range of 750-1000 kHz and are summed with θ 1  using phase splitters to produce upper and lower sidebands with a completely suppressed carrier at the output of an adder  804 . Such is the equivalent of adder  704  in  FIG. 7 . This is amplified by an amplifier (G) before being applied to a directional coupler  808  and antenna  810 . Such are equivalent to directional coupler  708  and antenna  710  in  FIG. 7 . 
       FIG. 9  represents a ground vehicle application  900  in which six VMG&#39;s  902 - 907  are carried in two single-file arrays, one array on each side of a vehicle. These two VMG arrays are hung out on flip-down outriggers so that their ground nadirs  909 - 914  will run along the shoulders of a roadway  916 . Pipes and wires  920  are illustrated here as an example, and for purposes of discussion here is shown to come from off one side of roadway  916  and to terminate about in the middle. Such wires can easily be 100-300 feet in length. The end point of such wires will uniquely radiate a spherically spreading pattern, while the rest of the wire will radiate a cylindrically spreading pattern. Precise end-point determinations will look for this transition. 
     It should be obvious that VMG&#39;s  902 - 904  on the “wrong side” will not pass over pipes, wires  920  and therefore have little chance of detecting it. However, VMG&#39;s  905 - 907  on the “right” side will pass over, one at a time beginning with VMG  905  and quickly ending with VMG  907 . So, if VMG&#39;s  902 - 907  are connected to a suitable signal processor while the vehicle carrying them is in forward motion, detection “blips” will occur in a string in VMG&#39;s  905 - 907 , and none at the same time for VMG&#39;s  902 - 904 . Wires that end under roadway  916  are of particular interest, and so they must be positively identified if stumbled upon. 
     It will usually be only the electric field component of the primary electromagnetic wave that can reach far enough to illuminate and induce significant current flow in surface and subsurface conductors. Such current flows cause cylindrically spreading secondary waves to propagate out over the conductors&#39; length and are readily observable on and above the surface. These secondary waves will decay with the one-half power of distance from the radiating conductor. Electromagnetic surveys of the observables using magnetic dipole antennas will reveal the orientation, depths, and locations of infrastructures and networks otherwise hidden or not apparent. 
     The characteristics of the things radiating the secondary waves can be inferred by measuring the magnitudes, orientations, and phases of the secondary waves as they are received at various points on and above the ground surface. A difference in measurement between two identical receiving antennas, or two different measures of a moving antenna, can be plotted as a gradient. Embodiments of the present invention take measurements from the same antennas at different points in a survey area. 
     The primary wave, the electric field that travels directly from the transmitter to the receiver, is rejected by the way the receiving magnetic antennas are constructed. The magnetic gradients can then be measured without significant intrusion. 
     Gradient measurement devices are able to resolve much finer electronic details than can total field measurement devices. As the antennas and receiver pass over radiating conductive targets during a typical gradiometer survey of an area, many dips and peaks in the secondary wave magnitudes, and phase shifts and reversals will be observed. A total field measurement for the same area can appear as a homogeneous blur without any details. Small changes in the gradient fields will not look important in total field measurements. Gradient measurement receivers can be configured to naturally reject interfering, unwanted signals. 
     More recent advancements by Stolar Research in gradiometer technology have shown that single vertical magnetic dipole antennas configured as vector magnometers can provide useful, detailed information about the presence and nature of wires, pipes, and other surface and subsurface conductors. 
     There are at least three practical ways to move a gradiometer over the ground surface: (a) handheld, (b) on a ground vehicle, or (c) on a low-flying airframe. The ground vehicle has a better chance of being able to tolerate larger power demands, heavier weights, and larger sizes, but all applications will benefit if these issues are minimized. 
     But, ground vehicles, especially small ones can jostle and bounce the receiving antennas so much that the gradiometer measurements are degraded by changing noise floors. Airframes can require minimum flying heights that demand too much separation distance, and the secondary waves are as a consequence made too faint to be detectable. 
       FIG. 10  represents a VMG-type sensor cube and receiver embodiment of the present invention, and is referred to herein by the general reference numeral  1000 . Three vector magnetometer type magnetic dipole antennas  1001 - 1003  are orthogonally arranged into X, Y, and Z orientations relative to a nadir spot on the ground surface of a search area. Each antenna is equipped with a low noise amplifier (LNA)  1004 - 1006 . Capped cylindrical all-copper Faraday shields  1008 - 1010  on each antenna  1001 - 1003  are used to screen out primary wave electric fields from outside and still readily pass through the near-field secondary magnetic waves. 
     A software-defined radio (SDR)  1012  uses programming to define its radio modes. The three antennas are individually connected in a tri-axial arrangement to a multiplexed analog-to-digital converter (ADC)  1014 . ADC  1014  samples and digitizes the analog electromagnetic signals for signal processing by a field programmable logic array (FPLA)  1016  according to algorithms and programming provided in a program memory  1018 . See, Roger H. Hosking, “Putting FPGAs to Work in Software Radio Systems”, (c) 2005-2013, Pentak, Inc. Upper Saddle River, New Jersey 07458 http://www.pentek.com. FPLA  1016  outputs digital in-phase (I) and quadrature (Q) samples. 
     Any additional input channels available in ADC  1012  could be useful, e.g., in monitoring voltage, current, and temperature measurements to report and understand the system health. A single board computer (SBC)  1020  takes in the I-Q data and at the same time matches it with navigation data provided by a GPS receiver or inertial management unit (IMU)  1022 . A temperature sensor  1024  is included to measure SBC operating temperatures. A FIFO register  1026  may need to be included to sync to other data links downloaded from the vehicle. 
     SDR  1014  provides a number of in-phase (I) and quadrature (Q) measurements of the magnetic signals respectively received by antennas  1001 - 1003 . The number of I-Q channels provided is user selectable. In remote controlled vehicle applications, the I-Q channel data is synchronized with other systems and command data using first-in, first-out (FIFO)  1026  data registers. In some applications as many as 20,000 samples of raw ADC data needed to be buffered, e.g., to observe the noise signatures better, or to help throttle for downlink streaming. Vehicle ignition noise from the platform can be synchronized and blanked out to control the noise floor, e.g., by as much as 15-dB. 
     Software-defined radio (SDR) modules for unmanned aerial vehicle (UAV), radar, and communications applications can be implemented with the 4-channel, 200 MHz ADC XMC module and the Xilinx Virtex®-7 FPGA family. IP can be developed for it using the Xilinx ISE® Design Suite and the Pentek GateFlow FPGA Design Kit. 
     In general, the Y-antenna  302  is used to receive magnetic signals-of-interest from a small spot on the ground immediately below. The Z-antenna  301  and X-antenna  303  are used to locate and identify nearby transmitters-of-opportunity that may already be radio illuminating metallic subsurface and surface objects. 
       FIGS. 11A-11C  illustrate one way to construct each of antennas  1001 - 1003 .  FIG. 11D  shows how three antennas in a tr-axial arrangement can be orthogonally disposed in a weatherproof housing  1120 . A vector magnetometer  1100  comprises a ferrite rod  1102  with a helical-wound coil  1104  about 3″ long. The ferrite rod concentrates the magnetic fields received (on axis) with the same orientation. A hollow phenolic cylinder  1106  is coaxially placed around the ferrite rod and is filled with an insulating and stabilizing foam fill  1108 , e.g., a two-part urethane foam. 
     A pattern of longitudinally arranged copper foil strips  1110  on the outside of the phenolic cylinder  1106  are connected together at each end by copper grounding rings  1111  and  1112 . The result of this is to create a Faraday shield  1114  that allows only magnetic fields to penetrate to the inside to be sensed up by the helical-wound coil  1104 . The best spacing, widths, and lengths of copper foil strips to use depend on the operating frequencies involved and can be empirically derived. Such copper foil strips can be etched from copper cladding laminated on the outside surface of the hollow phenolic cylinder  1106 , as is done with common printed circuit boards. 
       FIG. 12  represents a tri-axial sensor cube  1200  that packs, mounts, and weatherproofs three of the vector magnetometer devices, e.g., antennas  1100  as illustrated in  FIGS. 11A-11C . A plastic weatherproof top  1202  screws onto and seals with a plastic housing  1204 . Inside, three magnetic dipole antennas  1206 - 1208  are arranged orthogonal to one another. Each has a ferrite rod core  1210 - 1212 , and a low noise amplifier (LNA)  1214 - 1216 . These are wired to a bulkhead connector  1220 . The insides of the three magnetic dipole antennas  1206 - 1208  are filled with insulating foam, like urethane, to provide support and resistance to strong vehicle vibrations. 
     In some applications, only the vertically oriented magnetic dipole antenna  1206 - 1208  is used to receive and measure magnetic gradients. The others are used like reference sampling antennas. 
     Referring again to  FIG. 10 , a hardware description language is used in electronic design automation to describe field-programmable gate arrays, integrated circuits, and other digital and mixed-signal systems. E.g., very-high-speed integrated circuit (VHSIC) Hardware Description Language (VHDL). VHDL can also be used as a general purpose parallel programming language. An algorithm was found to be needed in the VHDL programming  1018  used by FPGA  1016  to monitor data from ADC  1012  and to automatically blank noisy data from the signal processing. Such involved an M-of-N criteria to test if the noisy data rose above the rest of the data by a threshold amount, for example 20-dB. The data that met the criteria was blanked out. A delay/storage was therefore included in the original signal processing chain and additional state machines were used to check for the M-of-N criteria. 
     The various I-Q channels from FPGA  1016  can be processed locally or remotely after downloading by a general purpose microcomputer with a gradiometer software application program to detect and annunciate the locations, orientations, and depths of conductors in or on the ground in a survey area. Navigation receivers and maps are combined with the sensor cube  1200  to make the results more useful and easy to understand. For example, the instantaneous GPS navigation locations at which each I-Q data sample was obtained are registered and logged with the data downloaded for processing. FalconView, by Georgia Tech Professional Education, is a PC-based mapping application that can be advantageously included to provide meaningful and informative user displays. See, www.falconview.org. 
     The interpretation of gradiometer data has been described by Larry G. Stolarczyk, et al., in “Detection of Underground Tunnels with a Synchronized Electromagnetic Wave Gradiometer”, AFRL-VS-HA-TR-2005-1066, c. 2005, Proceedings of SPIE Vol. 5778; and also, U.S. Pat. No. 7,336,079, “Aerial electronic detection of surface and underground threats”, issued Feb. 26, 2008. 
     Operating from a ground vehicle or airframe usually involves fighting off electrical interference caused by internal combustion ignition systems and generator brushes. The electrical noise coming from an engine contributes to the overall noise floor seen by a receiver. Noise floors can rise high enough to inundate any signals of interest. 
       FIG. 12  represents how algorithms  1018  ( FIG. 10 ) can be employed to overcome strong interference and produce more reliable detection results. A raw receiver input signal  1200  includes regular, periodic spikes caused by an engine ignition system. These can obscure or bury a signal-of-interest  1202  that is also a spike, but that is produced as a VMG passes over a buried conductive object. Field personnel call this a “hit”. A vehicle ignition interference blanking gate  1204  is therefore added. This results in a partially cleaned up signal  1206 . 
     A signal noise floor  1208  is represented by the ragged tops of signal  1206  in  FIG. 12 . Such noise floor  1208  can rise and fall with vehicle motion and terrain changes. We have observed that the noise floor rises when passing over areas of contaminated soils in which bacteria have been active and left ionized elements conductive to electricity. We also observed this same phenomenon inside tunnels on their wet floors. The very high humidity in mines promotes both aerobic and anaerobic bacteria blooms. Pools of water inside mines are unusually conductive and will reflect and scatter radio energy. 
     Changes in the noise floor make using a fixed squelch levels impractical because the best squelch level to use changes minute-by-minute. The best noise floor level to use is therefore automatic, being computed constantly from only the most recent samples. This is graphically represented by a moving window gate  1210  that is 3-5 seconds wide which allows noise floor samples to be read. A detection threshold  1212  is automatically raised and lowered according to a most recent noise floor calculation from inside moving window gate  1210 . If any hit  1202  exceeds the detection threshold  1212 , that hit becomes validated as a real target detection. The real-time position of the VMG at the time of the validated hits are logged and used to paint a map and/or sound an operator alarm. 
     The ill effects of electromagnetic interference (EMI) proved difficult to control in prototypes that were built. Multiple shields were needed in the interconnecting cables as were shielded connectors. The receiver electronics required an eight-layer PCB board design with full power planes and generous decoupling. The receiver and a general purpose single board computer (SBC) were isolated into separate compartments. Differential line drivers and signal lines were needed from the antenna. Low-pass and bandpass filters on the low noise amplifier and receiver boards. Diesel motors have an advantage over gasoline engines in that ignition systems are not needed and the interference they cause are not present and need not be contended with. The Faraday shields  1008 - 1010  ( FIG. 10 ) further reduce EMI. 
     Single point VMG measurements are rather meaningless in isolation. The conductor to be detected needs to move, or the VMG needs to move relative to the conductor, and several measurements with location positions are collected and compared. 
     The double-sideband gradiometric ground penetrating radar resonant microwave patch antenna is driven with two different phase-coherent reflected double-sideband waveform signals from the cluttering geology air-to-soil interface (e.g., cluttering geologic noise) caused by variations in moisture, type of buried object rock and fragmentation of the trona oil shale buried object rocks. The early arrival time cluttering geologic noise from the air-to-soil interface has an average magnitude of −6.6 dBm. The detection problem now becomes evident. The magnitude of the late arrival time (late arrival time) reflected double-sideband signal (S) from the floor trona oil shale buried object interface is a factor of 5.5 times less than the early arrival time double-sideband signal reflected cluttering geologic noise from the air-to-soil interface interface. 
     There is a significant difference in round trip travel time (t) between the early arrival time cluttering geologic noise and the late arrival time double-sideband signal (S) reflected from the trona oil shale buried object interface. The EM wave velocity (v) in the trona layer is C (e.g., 3×108 m/s) divided by the square root of the relative dialectic constant of trona) and slows down to 1.09×10 8 m/s. When the cutting edges are 609 mm (1 foot) from the trona oil shale buried object interface, the round trip travel time (trona oil shale buried object) is 5.6 nanoseconds. The ERT double-sideband signal round trip travel time (tATB) is less than 0.05 nanosecond. 
     The double-sideband gradiometric ground penetrating radar transmission and receiving paths are not totally isolated from each other, and this makes detection more difficult. When a single resonant microwave patch antenna sensor is used in a double-sideband gradiometric ground penetrating radar design, an integrated directional coupler (DC) is needed. The directional coupler has a directivity specification that seldom exceeds −30 dB. The magnitude of early arrival time coupler transmit path leakage double-sideband signals is −30 dBm in the receiver channel. 
     Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the “true” spirit and scope of the invention.