Abstract:
A system for detection of linear underground anomalies passing under surface roads comprises an electromagnetic (EM) gradiometer mounted on a vehicle trailer. A transmitter is mounted to the front bumper of a car or vehicle towing the trailer and provides carrier synchronization information to the EM-gradiometer. An opportunistic radio station can be used as an illuminator. The transmitter or ground wave from an opportunistic radio station directs radio waves down into the ground where objects like linear underground anomalies and their equipment will produce reflections and scattered waves. These reflections will have phase angles and magnitudes that can be interpreted for characterizing information about the linear underground anomalies. Each EM-gradiometer measurement is tagged with GPS location information and then stored in a database. Subsequent passes over the same roadways and tracks are compared (change detection) to the earlier stored data. New linear underground anomalies and features become very obvious in these comparisons.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to methods and devices for the detection of underground anomalies, and more particularly to the use of an electromagnetic gradiometer mounted to a vehicle trailer to detect linear underground anomalies crossing under border patrol roadways. 
   2. Description of Related Art 
   In the general case of having to detect linear underground anomalies from the surface, the orientation of the linear underground anomaly, if it exists, will be unknown. And its exact area will also be unknown unless there is some related surface feature or objective the anomalies involve. 
   However, in the case of detecting linear underground anomalies crossing under border patrol roadways, if the roadways are tight against a border, the linear underground anomalies will more or less pass orthogonally underneath. The search area required thus reduces from a two-dimensional field to a one dimensional line, the track of the roadway. 
   Various kinds of conventional technologies have been employed to detect and location underground anomalies, mines, and other structures. Many have used earth penetrating radar techniques. Others look for the secondary emissions from buried objects that occur when they are illuminated by primary radio sources. It is also fairly well understood that some radio frequencies will propagate through the ground better than others, and that will depend on soil conditions. 
   Primary electro-magnetic (EM) waves will interact with underground objects and infrastructures to create scattered EM-waves that are detectable on or above the earth&#39;s surface with a gradiometer. The Stolar, Inc. (Raton, N. Mex.) DeltaEM-gradiometer survey system provides a tool that can generate subsurface geophysical imaging capabilities with greater sensitivity, range (distance), and flexibility over existing instrumentation. In efforts using local radio sources, EM gradiometry has been shown to be a promising technique. The synchronized EM-gradiometer instrumentation is a narrow-band receiver that can discriminate against the spectra noise components and operate in the low ionosphere-earth waveguide noise band, thus maximizing the detection threshold sensitivity of the instrumentation. 
   EM-gradiometers capitalize on their high threshold detection sensitivities to scattered EM-waves in the ELF/VLF bands, 3-3000 Hz and 3-30 kilohertz. Synchronization to the primary wave in the ELF/VLF bands enables very narrow-band detection with threshold detection sensitivity in the picoTesla (pT) range. Theoretical investigations have found that the secondary EM fields are 20-60 dB below that of the primary EM field components. A significant instrument design issue is the detection of the secondary fields in the presence of the much larger primary field components. This has been solved by the careful design of the gradiometer antennas that achieves 70 dB of primary field suppression. 
   Two important advantages in underground anomaly detection have been achieved. First, the magnitude of the scattered secondary wave from them increases as frequency decreases. Thus, waves in the ELF/VLF bands have a significant advantage in detection. Second, the attenuation rate of EM-waves in the ELF/VLF bands through soil/rock is very low, so deeply buried structures can be illuminated and detected. The structures may be empty passageways or may contain electrical conductors serving some utility and ventilation needs. 
   SUMMARY OF THE INVENTION 
   Briefly, a system for detection of linear underground anomalies passing under surface roads comprises an electromagnetic (EM) gradiometer mounted on a largely non-metallic vehicle trailer. A transmitter is mounted to the front bumper of a car or vehicle towing the trailer, and a fiberoptic cable to provide carrier synchronization information is run from the transmitter to the EM-gradiometer. The transmitter directs radio wave down into the ground where objects like linear underground anomalies and their equipment will produce reflections and scattered (secondary) waves. These reflections will have phase angles and magnitudes that can be interpreted for characterizing information about the linear underground anomalies. Each EM-gradiometer measurement is tagged with GPS location information and then stored in a database. Subsequent passes over the same roadways and tracks is compared to the earlier stored data. New linear underground anomalies and features become very obvious in these comparisons. 
   The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a side view diagram of a vehicle and trailer with an EM-gradiometer with VMD and HMD polarized antennas mounted on it, and a cutaway view of the earth with a surface road crossing above over a linear underground anomalies; 
       FIG. 1B  is a side view diagram of a vehicle and a wheels-forward trailer that allows the EM-gradiometer being carried to be advantageously positioned much farther aft of the EM-illuminator on the front bumper for an increased separation distance; 
       FIG. 2  is a functional block diagram of a data collection system with a remote graphics display showing the surface infrastructure and gradiometer response, and which is carried aboard the vehicle and trailer combination of  FIGS. 1A ,  1 B, and  2 ; 
       FIG. 3  is a functional block diagram of a change detection data analysis system carried aboard the vehicle and trailer combination of  FIGS. 1A ,  1 B, and  2 , or that may have its constituent parts distributed in theater command; 
       FIG. 4A  is a graph representing what the phase and magnitude data collected from an EM-gradiometer carried along a road over a linear underground anomalies could look like on a first pass using the equipment of  FIGS. 1A and 1B ; 
       FIG. 4B  is a graph representing what the phase and magnitude data collected later from the same EM-gradiometer carried along the same road could look like on a subsequent pass if a new linear underground anomalies has appeared further down the road track from a first linear underground anomalies detected on a previous pass; and 
       FIG. 5  represents a ground map of points in a matrix in which a road- 1  and road- 2  fork and have some shared points and some not-shared points, the point being that if road- 1  was already surveyed, some points along its track will already be in the measurements database when road- 2  is surveyed later. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A  represents a vehicle and trailer combination embodiment of the present invention, herein referred to by the general reference numeral  100 . The combination  100  includes a car  102  and a utility trailer  104  with an EM-gradiometer  106  mounted to it with vertical magnetic dipole (VMD)  107  and horizontal magnetic dipole (HMD)  108  polarized antennas mounted on it. Trailer  104  comprises little or no metal, so as to reduce interference to radio signals being received. For example, the large structural parts may be make of marine fiberglass I-beams. The combination  100  is driven along a surface road  110  that can be expected to cross orthogonally above over a linear underground anomalies  112 . An EM-illuminator, or transmitter,  114  generates primary waves  116  that penetrate the earth, and it too has VMD and HMD antennas. When the transmitting EM-illuminator is using its VMD antennas, the receiving gradiometer will use its HMD antennas, and vice versa. E.g., if Tx=VMD, then Rx=HMD; otherwise Tx=HMD and Rx=VMD. 
   An electrostatic shield, e.g., Faraday shield, may be used to cover some or all of the magnetic dipole antennas for electrical noise reduction. Such could be constructed as a sheath, bowl, or radome fashioned from sheet aluminum, or from wire mesh. The EMG receiver sensitivity can be very much improved by the use of electrostatic shields on all the magnetic dipole antennas. 
   Typically, carrier frequencies in the range of 2-kilohertz to 1-megahertz are selected for best effect. Linear underground anomalies  112  will reflect and produce scattered (secondary) waves  118  by virtue of its contrasting electrical conductivity and/or dielectric constant with the surrounding geology. Any pipes, wires, or rails inside the linear underground anomalies  112  will further add to the contrast and reflections into scattered (secondary) waves. The phase and magnitude of the scattered (secondary) waves  118  carry important information about the depth and track location of linear underground anomalies  112  back to the earth surface. 
   The scattered (secondary) waves  118  are synchronously detected by EM-gradiometer  106 . Carrier synchronization information is carried back from EM-illuminator  114  to EM-gradiometer  106  by a fiberoptic cable  120 . A military grade GPS receiver  122  capable of producing position fixes and velocity calculations accurate to six inches is mounted above the EM-gradiometer  106 . Each magnitude and phase measurement produced by EM-gradiometer  106  of scattered (secondary) waves  118  is tagged and stored in a database of measurements, see  FIG. 3 . A data logger and user display for the driver could be carried inside vehicle  102 . Continuous logging and display is possible by interpolating discrete points. 
     FIG. 2  represents a data collection system  200  that could be carried aboard the vehicle and trailer combination of  FIGS. 1A and 1B  and used to store EM-gradiometer measurements of magnitude and phase that have been tagged with the precise geographic positions where they were taken. A very accurate GPS navigation receiver  202  provides position solutions better than six inches and is capable of doing that at vehicle speeds of up to sixty miles per hour. For example, GPS navigation receiver  202  is an authorized p-code receiver issued for United States military and government use. An EM-gradiometer  204  continuously receives and synchronously demodulates scattered (secondary) waves it receives from the ground using a transmitter sync signal. The phase and magnitude measurements are stored in a conditions and measurements database  206  that uses GPS solutions to tag the measurements with location and time information. A user display  208 , or a wireless modem  210  transmitting to a laptop computer  212 , provides an operational graphical user interface (GUI) and a map or other display of the surface features and the road being traveled and an interpretation of the measurements being received. 
     FIG. 3  is a functional block diagram of a data analysis system  300  that could be entirely carried aboard the vehicle and trailer combination of  FIGS. 1A and 1B , or distributed in theater command over a network. As in  FIG. 2 , a GPS receiver provides a track of GPS position solutions  302  for tagging the locations of EM-gradiometer measurements  304  obtained on a first pass. The same or another GPS receiver provides another track of GPS position solutions  306  for tagging the locations of subsequent EM-gradiometer measurements  308  obtained on a later pass or passes. Pattern matching techniques are used to synchronize the two tracks along overlapping points. An exceptions digital signal processor  310  issues investigation tags  312  for points where the measurement data should be the same but it is not. One reason it would differ is a new linear underground anomalies has been dug beneath the road between the first and last passes of the vehicle and trailer combination of  FIGS. 1A and 1B . These investigation tags  312  act as flags for immediate expert analysis. Even if only one pass was made over a particular track of points, the detection and recognition of a characteristic M-pattern (HMD gradiometer) or peak pattern (VMD gradiometer) in the measurements, as represented in the detail of  FIG. 3 , could be reason enough to issue an investigation tag  312 . A user display and GUI with interpretation algorithmic code would help make the results easier to understand and more immediately useful. 
     FIG. 4A  is a graph  400  representing what the phase and magnitude data collected from EM-gradiometer  106  carried along road  110  over linear underground anomalies  112  could look like on a first pass using the equipment of  FIGS. 1A and 1B . In particular, a characteristic M-pattern for HMD polarized gradiometer, or peak-pattern for VMD polarized gradiometer  402  will be observed as the vehicle crosses overhead of the linear underground anomalies  112  while moving down the road  110 , and the anomaly causes an imbalance in the signals received by the EM-gradiometer. The HMD polarization has a characteristic M-shaped response. The peak-to-peak separation distance is a function of the target depth. The VMD polarization has a characteristic peak between two minimums. The response&#39;s minimum-to-minimum separation distance(S) belies the approximate target depth, 
   
     
       
         
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     FIG. 4B  is a graph  410  representing what the phase and magnitude data collected later from the same EM-gradiometer  106  carried along the same road could look like on a subsequent pass if a new linear underground anomalies  112  has appeared further down the road track from the first linear underground anomalies  112  detected on a previous pass. Another characteristic M-pattern, or peak pattern  412  will be observed where there was not one before as the vehicle crosses overhead of the new linear underground anomalies. 
     FIG. 5  represents a ground map  500  of points  501 - 564  in a matrix in which a road- 1   566  and road- 2   568  fork and have some shared points  557 - 559  and  552 , and some not-shared points  545 ,  538 ,  539 ,  540 ,  553 ,  554 ,  555 ,  563 , and  564 . The underlying essential idea being that if road- 1   566  was already surveyed, some points along its track will already be in the measurements database  206  when road- 2   568  is surveyed later. 
   A gradiometer antenna array may comprise two ferrite-core left and right magnetic dipole antennas (LMD and RMD) electrically coupled and 180-degrees out of phase. This is called the differential mode of operation. For maximum primary wave cancellation, the HMD polarized antennas are coaxial, antenna rod axes along same axis, and oriented on a base line perpendicular to an intended target&#39;s trend. For VMD polarized gradiometer array, each antenna is parallel to the other. The HMD magnetic dipole antennas may also be connected in the summation mode of operation. In this case, the instrument would not operate as a gradiometer, but as a single magnetic dipole. LMD and RMD antennas may be configured as vertical or horizontal magnetic dipoles. During field tests, a horizontal magnetic dipole configuration was used. The instrumentation used a central electronics enclosure and telescoping antenna assembly enclosed in fiberglass. The antenna assembly tubing had a center section with the synchronization and calibration magnetic dipole antenna (SMD). Measured data was transmitted to a remote lap-top computer via an RF-Modem in the enclosure. The system was operational for six hours on a single lead-acid rechargeable battery. The gradiometer receiver could be carried by an operator using a belt and shoulder strap. 
   For an HMD polarized gradiometer, the source of the primary EM wave is a vertical magnetic dipole (loop antenna) mounted on the lead vehicle in the convoy. The loop antenna generates omni-directional toroidal EM field components. The primary electric field (EP) lies in a horizontal plane. There is a magnetic field (HP) component. When the primary electric field (EP) component illuminates the linear underground anomalies  112 , the induced current flow (I) can be approximately determined from the long wavelength scattering limit of mathematical physics given by; 
                 I   =         2   ⁢   π   ⁢           ⁢     E   p         ωμLog   ⁡     (     κ   ⁢           ⁢   a     )         ⁢           ⁢   amperes             (   1   )               
where κ=β−iα; β is the phase constant and α is the attenuation rate,
 
   ω=2πf and f is the operating frequency in Hertz, 
   a=radius from the linear underground anomalies, and 
   μ=μ r μ o  is the magnetic permeability. 
   Equation (1) shows that the induced current increases as the operating frequency is reduced. The induced current flow produces a cylindrically spreading secondary wave that is observable by vehicle trailer  104 . The secondary magnetic field component is given by 
                   H   s     =       I   2     ⁢       (       i   ⁢           ⁢   κ       2   ⁢   π   ⁢           ⁢   r       )       1   2       ⁢       e       -   i     ⁢           ⁢   κ   ⁢           ⁢   r       ⁡     (     far   ⁢           ⁢   field     )                 (   2   )               
where r=the radial distance in meters from the linear underground anomalies  112  to the vehicle trailer  104 . The secondary EM-wave scattered from an electrical conductor will slow decay with distance from the conductor at radial distances that are large compared with the skin depth. At radial distances that are large compared with the skin depth, the secondary cylindrically spreading EM-waves decay with the half power of distance (r) from the conductor. They are decreased in magnitude by the attenuation factor. In the near field, they decay with first power of (r).
 
   An illuminating transmitter  114  produces a tunable continuous wave (CW) signal of 2-KHz to 1-MHz that can be directed down toward the ground surface by an antenna. For example, such antenna can be wound in a loop and mounted to the front bumper or a front receiver hitch of vehicle trailer  104 . 
   There may be some advantage to slewing or sweeping the CW transmissions through their frequency range to take advantage of the various kinds of reflections that have frequency sensitive signatures, or to image targets hiding behind or underneath something else. A primary EM wave  116  impinges on linear underground anomalies  112  which will re-radiate a secondary EM wave  118 . A synchronous receiver uses a pair of oppositely wound ferrite-core magnetic dipole antennas, e.g., left magnetic dipole (LMD) and a right magnetic dipole (RMD) in a synchronous detection configuration. 
   A reference synchronizing signal from the illuminating transmitter  114  is supplied, for example, over the fiberoptic cable  120 . Phase and amplitude measurements of secondary signal  118  are forwarded to database  206 . A global positioning system (GPS) navigation receiver  122  provides position tags for the measurements. 
   Mounting the illumination transmitter  114  too near the receiver  106  can swamp the RF stages, so a separation distance is needed, e.g., twenty feet. 
   EM-gradiometer ferrite-core magnetic dipole antennas LMD and RMD are electrically coupled and 180-degrees out of phase, e.g., the differential mode of operation. For maximum primary wave  116  cancellation, the antennas are coaxial for the HMD gradiometer, the antenna rod axes run along same axis, and on a base line parallel to the border, and thus orthogonal to the linear underground anomalies run. The magnetic dipole antennas may also be made switchable into the summation mode of operation. In the summation mode, the instrument operates as a single magnetic dipole, and not an EM-gradiometer. The LMD and RMD antennas may also be configured as vertical or horizontal magnetic dipoles. A synchronization and calibration antenna (SMD) may be included for tuning. 
   The system  100  can image subsurface geophysical features with greater sensitivity, range, distance, and flexibility compared to conventional instrumentation. Prior art devices that try to capitalize on preexisting radio sources for target illumination typically suffer from noise and weak signal levels. 
   Good operational results depend on a synchronized EM-gradiometer instrument having a narrow-band receiver that can discriminate against spectra-noise components, and that operates in the low ionosphere, earth waveguide noise band. Such maximizes the detection threshold sensitivity of the instrumentation. EM-gradiometer  106  capitalizes on a high threshold detection sensitivity to scattered EM-waves  118  in the extremely low frequency (ELF) 30-300 Hz, the very low frequency (VLF) 3-30 kHz, and low frequency (LF) 30-300 kHz bands. Synchronization to the primary wave  116  with fiberoptic  120  enables very narrow-band detection with threshold detection sensitivity in the picoTesla (pT) range. Theoretical investigations have found that the secondary EM fields  118  are typically 20-60 dB below that of the directly received primary EM field  116 . A difficult instrument design issue is how to detect the secondary fields in the presence of much larger primary field components. Careful implementation of the gradiometer antennas can result in 70-dB of primary field suppression. 
   The source of the primary EM wave  116  can also be a vertical magnetic dipole, e.g., loop antenna, mounted on a ground vehicle in a convoy, or an opportunistic radio transmitter. Loop antennas generate omni-directional toroidal EM field components. The induced current flow produces a cylindrically spreading secondary wave that is observable by a gradiometer receiver. 
   System  100  could be integrated with conventional Ground Control and Tactical C4 Systems. The output from the gradiometer can be transmitted to the ground base in real time, and still frame images are compressed and transmitted over PSC-5D in conjunction with EM-gradiometer. Live video can be compressed and transmitted in real time over PRC-117F and PSC-5D links. The technology probably should be suitably hardened for use in theatre. 
   A browser-type graphical user interface (GUI) would be useful in trailer combinations  100  and  130  of  FIGS. 1A and 1B . A number of different clickable tabs allow the selection of data, setup, GPS, and graph displays. Here, the phase and amplitude measurements from EMG receiver are plotted according to range distance. A GPS display can be used to show the position of the ground vehicle on a map relative to the suspected linear underground anomalies  112 . 
   The magnitude of the scattered secondary wave  118  detected from an underground conductor (linear underground anomalies) infrastructure increases as the carrier frequency being used decreases. Extremely low frequency (ELF) 30-300 Hz, very low frequency (VLF) 3-30 kHz, and low frequency (LF) 30-300 kHz wavebands have a significant advantage in linear underground anomalies detection. The attenuation rate of EM-waves in the ELF/VLF/HF bands through soil and rock is very low, so deeply buried structures can be readily radio-illuminated and detected. The detected structures may even include empty passageways, or those with electrical conductors serving the utility and ventilation needs. 
   Linear underground anomalies and caves invariably provide electrical pathways. The electrical current flow (I) channel can be a source of the secondary EM field  118 . Primary EM-waves  116  induce current into electrical pathways. In an empty linear underground anomalies case, the higher conductivity layer underlying the linear underground anomalies will channel electrical current. Conductive layers underlying the linear underground anomalies can be created naturally by soluble salts. The result is a cylindrically spreading EM wave that is observable on the surface. In either case, an subsurface induced current results in a surface detectable secondary EM wave  118 . 
   There is a universal similarity amongst all kinds of underground facilities around the world. In part, this stems from the small number of academic institutions that educate and train the world&#39;s mining engineers, geologists, and geophysicists. The same curriculum and textbooks are subscribed to by most all the leading schools. Otherwise disperse members of the world mining community are also drawn together by trade associations and trade shows. 
   Many different kinds of underground structures use reinforced concrete, and the steel reinforcing is very easy to image electronically with the EM-gradiometer. Drug smuggling linear underground anomalies in the Nogales, Ariz., area would collapse if it were not for its aggressive ground control measures. Structures developed into hard rock have similar ground control requirements. Weathering at the adits can make the ground incompetent. Aggressive use of steel/wood supports along with metal screening is required, and reinforced concrete is commonly used in the construction of adits. As the entries are developed, ground control measures intensify with strata depth and with the width of the entry. Mines driven into schists use roof bolts and metal screening to cross through faults. Roof rock falls can be detected seismically with geophones. Such microseismic devices can be integrated into an EM-gradiometer for long-term monitoring. 
   The preferential use of pneumatic drills in mines means that a network of high pressure metal pipes must be installed to supply the compressed air. Such pipe network will reradiate electromagnetic waves as well as power cables and railroad rails. The drills and the blasting with explosives in mines also means seismic and sound detectors can be used to detect activity, especially new construction. 
   The linear underground anomalies boring machines (TBM) used by such operations are specialized equipment that can be tracked by commerce officials. The sale and delivery of TBM&#39;s can signal that a new search could turn up a linear underground anomalies and provide some preliminary information on where to look. 
   No trenching horizontal directional drilling can be used to create pneumatic transfer pipes across the border. Only the 8-hour drilling time period is allowed to detect the ditch witch drill rods. A shallow-buried linear underground anomalies was recently detected by the odd way snow melted overhead on the surface along the center line of the linear underground anomalies. Such linear underground anomalies used wood-support ground-control measures to build the linear underground anomalies. Other nearby linear underground anomalies were driven into schist with drill-and-blast methods. Evidence suggested that rail was used for muck transport. Lighting brackets were seen on the ribs (walls) of the linear underground anomalies, and their electrical conductors were EM-observables on the surface. 
   Mining engineers expect water will most likely be encountered in developing linear underground anomalies entries, and so mined linear underground anomalies are developed upgrade to naturally dewater the workings. The mined linear underground anomalies drainage water therefore must be pumped from the linear underground anomalies. In sulfide-bearing rock mass, the drainage water will be acidic, and discolor the surface soil and retard vegetation. Such water may form an electrical conductor. 
   Bacteria of two types always seem to be associated with mine linear underground anomalies ventilation. When sufficient oxygen is present, the relatively warm and moist underground environment fosters rapid accumulation of aerobic bacteria strains. But in the poorly or not ventilated areas, carbon dioxide (black damp) builds up, and anaerobic bacteria grows rapidly. Septic conditions can also generate hydrogen sulfide and methane. 
   Mine ventilation engineers try to drive fresh air through the mined linear underground anomalies entries and into the working areas with a “primary fan” that is located as near the linear underground anomalies as possible, e.g., in an air door. Overpressure is typically generated by the primary fan in the mined linear underground anomalies to push air out the exhaust vents at the deep end of the linear underground anomalies. Sometimes air ducts are used to carry fresh air to the working area, and the used air exits at the adit. The ventilation system may also be designed around an exhaust fan system. Some ventilation tubing includes electrically conductive spiral wire that can reradiate signals that are observable on the surface with EM-detectors. 
   Large underground mines and other facilities have trouble maintaining adequate air ventilation, so lots of small fans will usually be found to assist the main fans. Three-phase electric utility power is generally required for big ventilation fans because of the large horsepower electric motors they use. The smaller fans are usually connected to single-phase power. 
   The power cables depended on to supply the fans can be expected to radiate secondary EM-waves  118  and also waveguide the primary EM-waves  116  deeper into the lower parts of the underground facility or linear underground anomalies  112 . 
   The electric field vanishes at zero frequency. There is an optimum frequency for inducing maximum current for magnetic dipole sources. Primary EM-waves that propagate in earth-ionosphere waveguide signals are quasi-transverse EM-waves (TEM). 
   A gradiometer antenna is designed to measure the gradient of the cylindrical spreading EM-wave. The reception of secondary EM-waves in the rock mass surrounding the linear underground anomalies or on the surface confirms the existence of nearby electrical conductors. 
   Passageway conductors essentially create an induced current distribution network throughout the linear underground anomalies. The attenuation rate for signals propagating on electrical conductors is typically less than 1.0 dB per kilometer at fifty KHz. The current appears on the electric power and telephone cables entering the complex through any adits. Switches will not disrupt all the induced current flow because the grounding conductors are never switched. However, open switches and any isolation transformers can attenuate the signal. 
   The total field is the sum of the primary and secondary field. Usually the total field changes by only a few percent, but the gradient changes by tens of percent when an EM-gradiometer is passed over a conductor. If quasi-TEM earth-ionosphere waveguide signals are used, EM-waves couple across the air-soil boundary and propagate downward. The attenuation rate and phase shift for a uniform plane wave propagate in natural medium with a typical relative dielectric constant of ten. The propagation constant can be estimated for various types of natural media. 
   The electrical conductivity of most natural media increases with frequency. The lower frequency signal attenuation rate decreases from high frequency values, so deeper targets may be detected using lower frequencies. Ground-penetrating radar (GPR) technologies are inappropriate to find linear underground anomalies, at one-hundred MHz in a 10-1 S/m media, the attenuation rate is just too great. It&#39;s about 39-dB per meter, and such prevents receiving minimum signals at surface. 
   One advantage of an EM-gradiometer is that it can be used on the surface. Radiowave interference from distant sources will be plane waves that can be easily suppressed by the gradiometer antenna. The gradiometer measurements of linear underground anomalies  402  response typically exhibit a high signal-to-noise (SNR) ratio which is favorable for reducing the false alarm rate (FAR). 
   For a sinusoidal signal embedded in white electrical noise, synchronous detection maximizes the threshold detection sensitivity. Typically, a well-designed receiver will exhibit a noise figure near 2-dB. The noise bandwidth (BN) will be the predominating problem in any receiver design. 
   The receiver threshold sensitivity increases as bandwidth is reduced. By synchronizing the receiver to the EM-wave illuminating the target, the receiver bandwidth can be made very small. Alternatively, a wider bandwidth can be used in the design where sampling and averaging can be used to achieve effective bandwidth. However, this type of system would not be able to discriminate the discrete spectrum. 
   An EM-wave magnetic field component threading an area of an induction coil of N-turns produces an electromotive force voltage (EMF). A ferrite rod with an initial permeability of 5,000 and a length/diameter ratio of twelve achieves a relative permeability of one hundred twenty. The induced EMF increases with the first power of N and operating frequency. 
   For a one inch diameter ferrite rod, noise is expected to be 0.02 picoTesla in a 1-Hertz bandwidth. The signal-to-noise ratio is, SNR=50. 
   The primary EM-wave illuminating the ground surface may alternatively be received by a series-tuned sync magnetic dipole antenna (SMD). An EMF signal of typically 32-microvolts per picoTesla will be amplified by a programmable gain controlled amplifier (PGA), e.g., 60 dB of gain. A mixer-filter frequency transposes the signal into the intermediate frequency (IF) signal, and provides an additional gain of 78-dB. The IF signal is filtered and limited to form a square wave. The square wave signal is applied to the phase-locked loop (PLL) phase detector (PD). A phase detector and voltage controlled oscillator (VCO) produce the in-phase (I) and quadrature (Q) sampling gate signals. 
   The EM-gradiometer array signal is typically amplified by programmable gain control amplifier (PGA). The mixer-filter circuit results in the frequency transportation of the gradiometer signal to the intermediate frequency (IF) signal. The IF gradiometer signal can be applied to in-phase (I) and quadrature (Q) sampling gates. 
   The I and Q gate output signals are applied to separate integrators. The output of each integrator is applied to an analog-to-digital converter (ADC). After integration, the rectified signals are processed by a microcomputer. 
   Although particular embodiments of the present invention have been described and illustrated, such is not intended to limit the invention. Modifications and changes will no doubt become apparent to those skilled in the art, and it is intended that the invention only be limited by the scope of the appended claims.