Abstract:
A method and apparatus is disclosed for measuring electrical properties of objects by the transmission of electromagnetic waves through a non-magnetizable material. The disclosure also describes a method of measuring changes in electromagnetic signals as the amplitude and frequency of the electromagnetic waves is varied to determine the thickness of an object.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation-in-part application of the application of Bijan Amini, U.S. Ser. No. 60/170,172, filed Dec. 10, 1999, entitled “Apparatus And Method For The Measurement Of Electrical Properties Of Materials Through Non-Magnetizable Materials.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to utilizing controlled transmissions of electromagnetic (EM) energy through or across non-magnetizable materials that have previously been barriers to penetration to determine the thickness and EM characteristics of materials. 
     BACKGROUND OF THE INVENTION 
     It has long been possible to measure metallic thickness variations by electromagnetics. Prior methods have typically excited the metal by eddy currents or D.C. fields. After excitation, the known methods looked for variations in amplitude of the signal corresponding to variations in metallic thickness. 
     Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized by means of the combinations and steps particularly pointed out in the appended claims. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing objects, features, and advantages and in accordance with the purpose of the invention as embodied and broadly described herein, a method for the measurement of electrical properties of materials through non-magnetizable materials is provided. The method is also used to calculate the thickness of a material with unknown permeability and conductivity using transparencies. The method comprising the steps of creating a first set of electromagnetic waves having specific constant amplitude of a known frequency, the first set of electromagnetic waves for engaging a system, impinging the first set of electromagnetic waves on the system under investigation, nulling the system, receiving a nulled signal, creating a change in the system, and receiving a modified signal associated with the change from the nulled signal such that the modified signal contains sufficient information to determine the change in the system. 
     The method further comprising the steps of testing empirically to approximate the conductivity, testing empirically to approximate the permeability, creating a second set of electromagnetic waves adjacent to the system to be measured, the second set of electromagnetic waves being of a relatively low frequency and of lower frequency that the first set of electromagnetic waves, and impinging the second set of electromagnetic waves on the system for saturating a portion of the material in the system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings which are incorporated in and constitute a part of the specification, illustrate a preferred embodiment of the invention and together with the general description of the invention given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention. 
     FIGS. 1A,  1 B,  1 C and  1 D illustrate geometric nulling as practiced by the present invention with respect to spacial equivalence, equivalence of windings, current equivalence and magnetic moment equivalence, respectively. 
     FIG. 2 illustrates the flux circuit core of one embodiment of the magnetic transparency generator used to generate the transparency current required in practicing the present invention. 
     FIG. 3 is a block diagram of one embodiment of a magnetic transparency generator of the present invention. 
     FIG. 4 illustrates a graph of current versus time with respect to the present invention. 
     FIG. 5 illustrates the relationship between signal frequency and penetration depth for a cross-section of a material with a conductivity, a permeability and several imposed frequencies, f x , for the present invention. 
     FIG. 6 illustrates one embodiment of a magnetic transparency generator used to saturate and generate the transparency current required in practicing the present invention. 
     FIG. 7 illustrates one embodiment of a magnetic transparency generator  600  of the present invention. 
     FIG. 8 illustrates the relationship between the flux field β and the change in H (ΔH) in ampere-turns/meters. 
     FIG. 9 illustrates the relationship between the receiver amplitude A Rx  and H in ampere-turns/meters. 
     FIG. 10A illustrates an embodiment of a magnetic transparency generator used to generate a transparency with respect to a material for practicing the present invention. 
     FIG. 10B illustrates another embodiment of a magnetic transparency generator used to generate a transparency with respect to a material for practicing the present invention. 
     FIG. 11 illustrates a bistatic transmitter/receiver configuration  1100  used in practicing the present invention. 
     FIGS. 12A,  12 B and  12 C illustrate examples of tristatic transmitter/receiver configurations as practiced with the present invention. 
     FIGS. 13A,  13 B,  13 C,  13 D and  13 E are illustrations and graph of an axially symmetric configuration of transmitters and receivers such that the receiver coils are centered along the same axis. 
     FIG. 14A illustrates an embedded transmitter/receiver configuration as practiced by the present invention. 
     FIG. 14B is another embedded transmitter/receiver configuration as practiced by the present invention. 
     FIG. 15A is a tristatic configuration of the present invention. 
     FIG. 15B is another tristatic configuration of the present invention. 
     FIG. 16 is another configuration of nulling combining the axially symmetric relationships of FIGS. 13A,  13 B,  13 C and  13 D with the static configurations illustrated in FIGS. 15A and 15B. 
     FIG. 17 is an embedded, rotating receiver/transmitter configuration as practiced by the present invention. 
     FIG. 18 is a combination transmitter/receiver configuration using the same antenna Tx for practicing the present invention. 
     FIG. 19A is an illustration of an application of the present invention utilizing a loop transmitter with a tank for detecting the level within the tank. 
     FIG. 19B is an illustration of an application of the present invention utilizing a loop transmitter with a pipe for detecting the level associated with the pipe. 
     FIG. 20 is a graph of the output of the embedded transmitter/receiver configuration as illustrated in FIGS.  19 A and  19 B. 
    
    
     The above general description and the following detailed description are merely illustrative of the generic invention, and additional modes, advantages, and particulars of this invention will be readily suggested to those skilled in the art without departing from the spirit and scope of the invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention as described in the accompanying drawings. 
     FIGS. 1A 1 B,  1 C and  1 D illustrate various configurations for geometric nulling as practiced by the present invention. Particularly, FIGS. 1A 1 B,  1 C and  1 D illustrate geometric nulling as practiced by the present invention with respect to spacial equivalence, equivalence of windings, current equivalence and magnetic moment equivalence, respectively. In practicing the present invention, several magnetic relationships or phenomena are of assistance. The magnetomotive force F can be expressed in ampere-turns, NI, where N is the number of turns linked with the magnetic circuit and I is the current. The magnetic field intensity H is equal to the amperes per meter I/L, which can be expressed as the flux density divided by the permeability β/μ. The flux Φ can be expressed as: 
     
       
         Φ= BA=k   1   NI ( L /Aμ)  
       
     
     where 
     k1 is a constant, 
     NI is the ampere-turns, 
     L is the length of the magnetic path, 
     A is the area, and 
     μis the permeability. 
     Therefore, 
     
       
         
           NI=k 
           2 
           BL  
         
       
     
     where 
     k 2  is a constant, 
     B is the flux density, and 
     L is the length of the magnetic path. 
     The direction of the magnetizing force of a current is at right angles to its direction of flow of the current. For a cylinderical conductor carrying a current, the magnetic lines about the cylinderical conductor are in circular planes concentric with the conductor and normal to the conductor. The curved lines used to represent a magnetic field are drawn such that the number of lines is related to the strength of the magnetic field at a given point and the tangent of any curve at a particular point is along the direction of magnetic force at that point. 
     The direction of the current I and the resulting magnetic field H are related to each other by the so-called “corkscrew rule.” The corkscrew rule compares the forward travel of the corkscrew with the current I, and the direction in which the corkscrew is rotated with the resulting magnetic field H. Another tool for visualizing the effects associated with the present invention is to use the “right hand rule.” The right hand rule requires that the conductor is grasped in the right hand with the thumb pointing in the direction the current is flowing. With the conductor in the right hand, the fingers will point in the direction of the lines of flux. 
     FIG. 1A illustrates geometric nulling using spacial equivalence. Nulling using spacial equivalence provides that the receiver Rx and the transmitters Tx 1 , Tx 2  are equidistant. Thus, the receiver Rx is disposed intermediate between, and equal distance from, the first transmitter Tx 1  and the second transmitter Tx 2 . The distance R between the transmitters Tx 1 , Tx 2  and the receiver Rx is equal. The coils associated with the transmitters Tx 1 , Tx 2  are wound in different or opposite directions. Winding the transmitters Tx 1 , Tx 2  in opposite directions provides a “bucking” relationship for the transmitters Tx 1 , Tx 2 . The coil associated with the first transmitter Tx 1  is wound in a clockwise direction w 1 . The coil associated with the second transmitter Tx 2  is wound in a counter clockwise direction w 2 . The coil associated with the receiver Rx can be wound in either direction. When the transmitters Tx 1 , Tx 2  are bucked, i.e., wound in opposite directions, the field lines generated are typically described using the right thumb rule. Thus, bucking provides a canceling of the respective field lines associated with the bucked transmitters. A quantity of current I is applied to the transmitters Tx 1 , Tx 2 . The receiver Rx is illustrated having a specified radius r. In one embodiment, the transmitters Tx 1 , Tx 2  have a coil with a specified number of turns N. It is appreciated by those skilled in the art that the number of turns N T  for any transmitter Tx 1 , Tx 2  does not have to be equal to the number of turns NR for the receiver Rx. 
     FIG. 1B illustrates geometric nulling using equivalence of windings as practiced by the present invention. Nulling using winding equivalence, or the number of turns N, provides equivalence with respect to the number of turns N associated with the coils of the relevant transducer; be it transmitter Tx or receiver Rx. The coils associated with the transmitters Tx 1 , Tx 2  are wound in different or opposite directions. Winding the transmitters Tx 1 , Tx 2  in opposite directions provides a “bucking” relationship for the transmitters Tx 1 , Tx 2 . The coil associated with the first transmitter Tx 1  is wound in a clockwise direction w 1 . The coil associated with the second transmitter Tx 2  is wound in a counter clockwise direction w 2 . The coil associated with the receiver Rx can be wound in either direction. When the transmitters Tx 1 , Tx 2  are bucked, i.e., wound in opposite directions, the field lines generated are typically described using the right thumb rule. Thus, bucking provides a canceling of the respective field lines associated with the bucked transmitters. A quantity of current I is applied to the transmitters Tx 1 , Tx 2 . The receiver Rx is illustrated having a specified radius r. 
     FIG. 1B illustrates a receiver Rx disposed between a first transmitter Tx 1  and a second transmitter Tx 2 . The distance the receiver Rx is between two transmitters Tx 1 , Tx 2  differs. Particularly, distance between the receiver Rx and the transmitter Tx 2  is the distance R. The distance between the receiver Rx and the first transmitter Tx 1  is the distance kR. The radius r of the coils associated with the receiver Rx is noted on the drawing having a specified radius r. It is know to those skilled in the art that the flux field decreases or drops off as a function of one over R 3 , where R is the distance between the two transmitters. More particularly, given R is greater than or equal to r, then the power varies as the cube of the coefficient k of the distance R. Thus, to achieve geometric nulling, and thus equivalence with respect to the number of turns N, the relationship between the number of turns N for the first transmitter Tx 1  and the number of turns associated with the second transmitter Tx 2  is the number of turns divided by the cube of the coefficient k N/k 3 . Therefore, when the first transmitter Tx 1  has the number of turns N and is a distance kR from the receiver Rx, then the second transmitter Tx 2 , being a distance R from the receiver Rx, requires that the number of turns N be divided by k 3  where the distance between the receiver Rx and the transmitter Tx 1  is kR. 
     FIG. 1C is illustrates a configuration used with the present invention for geometrically nulling with respect to current equivalence. Particularly, a receiver Rx is disposed between a first transmitter Tx 1  and a second transmitter Tx 2  such that the receiver is a distance R from the second transmitter Tx 2 . Also, the first transmitter Tx 1  is a distance kR from the receiver Rx where k is a constant and r is a constant. To maintain current equivalence with respect to geometric nulling, the value of the current I on the first transmitter Tx 1  has the relationship of the current varying as the cube of the distance R 3 . Thus, the second transmitter Tx 2  requires a current of I divided by k 3  or I/k 3 . As with geometric nulling for the number of turns N, geometric nulling for current equivalence in the illustrated configuration requires that R is greater than or equal to the radius of the coil r where r is the radius of the coil windings for the receiver Rx. The coils associated with the transmitters Tx 1 , Tx 2  are wound in different or opposite directions. Winding the transmitters Tx 1 , Tx 2  in opposite directions provides a “bucking” relationship for the transmitters Tx 1 , Tx 2 . The coil associated with the first transmitter Tx 1  is wound in a clockwise direction w 1 . The coil associated with the second transmitter Tx 2  is wound in a counter clockwise direction W 2 . The coil associated with the receiver Rx can be wound in either direction. When the transmitters Tx 1 , Tx 2  are bucked, i.e., wound in opposite directions, the field lines generated are typically described using the right thumb rule. Thus, bucking provides a canceling of the respective field lines associated with the bucked transmitters. A quantity of current I is applied to the transmitters Tx 1 , Tx 2 . The receiver Rx is illustrated having a specified radius r. 
     FIG. 1D illustrates a configuration for the present invention which achieves geometric nulling with magnetic moment equivalence. To achieve magnetic moment equivalence, a receiver Rx is disposed between a first transmitter Tx 1  and a second transmitter Tx 2  such that the receiver Rx and the second transmitter Tx 2  are disposed at a distance R apart. The receiver Rx and the first transmitter Tx 1  are disposed at another distance kR apart, where k is a constant and R is the distance between the receiver Rx and the second transmitter Tx 2 . Magnetic moment equivalence provides for using transmitters having differing coil radius r. The radius r 1  for the coil of the first transmitter Tx 1  is proportionately larger than the radius r 2  for the coil for the second transmitter Tx 2 . Thus, the coil for the first transmitter Tx 1  is proportionately larger than the coil for the second transmitter Tx 2 , and the first transmitter Tx 1  is disposed by a factor k further away from the receiver Rx than the second transmitter Tx 2 . The coils associated with the transmitters Tx 1 , Tx 2  are wound in different or opposite directions. Winding the transmitters Tx 1 , Tx 2  in opposite directions provides a “bucking” relationship for the transmitters Tx 1 , Tx 2 . The coil associated with the first transmitter Tx 1  is wound in a clockwise direction w 1 . The coil associated with the second transmitter Tx 2  is wound in a counter clockwise direction W 2 . The coil associated with the receiver Rx can be wound in either direction. When the transmitters Tx 1 , Tx 2  are bucked, i.e., wound in opposite directions, the field lines generated are typically described using the right thumb rule. Thus, bucking provides a canceling of the respective field lines associated with the bucked transmitters. A quantity of current I is applied to the transmitters Tx 1 , Tx 2 . The receiver Rx is illustrated having a specified radius r, and the transmitters Tx 1 , Tx 2  are illustrated having differing radius r 1 , r 2 . 
     FIGS. 1A,  1 B,  1 C and  1 D all illustrate varying configurations used with the present invention for geometric nulling. It should be appreciated that all of the transmitters Tx and the receivers Rx can be reversed geometrically. Thus, where a receiver Rx is disposed between two transmitters Tx 1 , Tx 2 , geometric reversal would provide for a single transmitter Tx being disposed between a first receiver Rx 1  and second receiver Rx 2 . Further, there are different items that can be varied to achieve geometric nulling in practicing the present invention. As discussed, the number of turns N can be changed, current equivalence can be maintained, spacial equivalence can be maintained and magnetic moment equivalence can be maintained. Also, the wire size can be varied with respect to the gauge or diameter. The core material can be changed, the thickness of the insulation can be changed and the type of insulation can be changed. 
     FIG. 2 illustrates one embodiment of a magnetic transparency generator  500  used to generate the transparency current required in practicing the present invention. The magnetic transparency generator  500  provides for containing flux lines capable of completely saturating the intended barrier material  100  volume region. Also, FIG. 2 illustrates one embodiment of the flux circuit core  501  for use with the present invention. The flux circuit core  501  comprises a top flange  504 , a bottom flange  505  and a core  552 . The core  552  is located between the top flange  504  and bottom flange  505 . The tank wall comprises the barrier material  100 . The magnetic transparency generator  500  incorporates the flux circuit core  501  for providing a transparent volume region that is illustrated having a width W  920 , a height H  930  and a thickness L  960 . The barrier volume region may be termed the target material. It is appreciated that the transmitter coils  300  and the receiver coils  580  are in positions of geometric nulling with respect to the magnetic transparency generator  500  illustrated. 
     FIG. 3 is a block diagram of one embodiment of a magnetic transparency generator  500  of the present invention. The magnetic transparency generator  500  comprises a large coil  551 , a small coil  300 , and a receiver coil  580 . The large coil  551  generates the transparency current. The small coil  300  generates the transmitter signal. The receiver coil  580  accepts the returning transmitter signal. The large coil  551  for generating the transparency current is engaged with a pulser  566 , one or more capacitors  561  and a power source  560 . The small coil  300  of the transmitter and the receiver coil  580  are engaged with a switch  562 , a frequency generator  563 , a low noise amplifier (LNA)  564 , an electrical nulling circuit  581  for digital signal processing and an output means  582 . For the present invention, the components associated with the large coil  551  for generating the transparency current are not used, but are illustrated for completeness. 
     FIG. 4 illustrates a graph of current versus time with respect to the present invention. FIG. 4 illustrates three significant features in practicing the present invention: the current level required for saturation  420 , the higher frequency sensing signal  411  and the lower frequency transparency signal  401 . The higher frequency sensing signal  411  may be imposed on the lower frequency transparency signal  401 . FIG. 4 illustrates as spikes  411  the higher frequency oscillating electromagnetic wave of the sensing signal  411  disposed along a lower frequency oscillating transparency current  401 . In one embodiment of the present invention, the sensing signal  411  may be transmitted only during the duration of each cycle of the oscillating transparency current  401  that is above the level  420  required for saturation. Among other advantages, the latter embodiment minimizes energy consumption. In the latter embodiment, it is possible to have multiple sensing signal transmissions  411  during each phase that the transparency current  401  is above the saturation level  420 . However, for the present invention, saturation is not required because of the application to non-magnitizable materials. The discussion of saturation is provided for completeness. Illustrated schematicly as an apparatus in FIG.  3  and conceptually in FIG. 4, the higher frequency sensing signal  411  may be generated by a transmitter, comprised of a smaller coil  300  of conductive material, powered by alternating current and at a controlled frequency. 
     In FIG. 4, the high frequency sensing signal  411  is illustrated being pulsed at less than 0.5 millisecond rates. If the lower frequency transparency current  401  generated by a larger coil  551  (See FIG.  6 ), is pulsed or activated “on” for 10 milliseconds  430 , there is sufficient time for twenty sensing signals (e.g., with a wavelength of only 0.5 millisecond) to go out to a near object and take 10 wavelengths of measurements during the “on” pulse of the transparency current. During this 10 millisecond pulse, the transparency current will exceed the saturation energy level  420 . The higher frequency signal  411  from the transmitter coil  300  is being pulsed at a 0.5 millisecond rate so that 20 sensing signals will be available during a 10 millisecond pulse of the transparency signal  401 . 
     FIG. 5 illustrates the relationship between signal frequency and penetration depth for a cross-section of a material with a conductivity, a permeability and several imposed frequencies, f x , for the present invention. For a wave of constant amplitude and varying frequency f x , and a material with the same permeability and conductivity, it is known by skin depth theory that a lower frequency penetrates deeper than a higher frequency. Therefore, one can find an optimum frequency range that can characterize the material conductivity. For constant length L and varying frequencies f x , the penetration depth δ is:        δ   =       (     1   e     )        L           and         δ   =     1       σ                   μ   r          μ   o        f                 where                         δ   =     penetration depth,                 f   =     frequency,                   μ   r     =     relative permeability, and                   μ   o     =     absolute permeability.                                    
     In FIG. 5, the relationship of frequencies is: 
     
       
         f 6 &gt;f 5 &gt;f 4 &gt;f 3 &gt;f 2 &gt;f 1 .  
       
     
     FIG. 6 illustrates one embodiment of a magnetic transparency generator  500  used to saturate and generate the transparency current required in practicing the present invention. The magnetic transparency generator  500  provides for containing flux lines to completely saturate the intended barrier material  100  volume region. Also, FIG. 4 illustrates one embodiment of the flux circuit core  501  for use with the present invention. The flux circuit core  501  comprises a top flange  504 , a bottom flange  505  and a core  552 . The core  552 , upon which the coils of the electromagnet are wrapped, is located between the top flange  504  and bottom flange  505 . The tank wall comprises the barrier material  100 . The complete magnetic transparency generator  500  incorporates the flux circuit core  501  for providing a transparent volume region that is illustrated having a width W  920 , a height H  930  and a thickness L  960 . The barrier volume region may be termed the target material. It is appreciated that the transmitter coils  300 , the receiver coils  580  and the transparency coil  551  are in positions of geometric nulling with respect to the magnetic transparency generator  500  illustrated. 
     FIG. 6A illustrates one embodiment of the magnetic circuit  502  for use with the present invention. The magnetic circuit  502  comprises a saturation coil  551 , a transmitter coil  300 , a receiver coil  580  and a barrier material  100 . A magnetic transparency generator  500  comprises the saturation coil  551 , the transmitter coil  300  and the receiver coil  580 . The magnetic transparency generator  500  is disposed from the barrier material  100  by a gap G. The barrier material  100  has a width L. The magnetic circuit  502  operates by energizing the saturation coil  551  for saturating the barrier material  100 , transmitting a sensing signal from the transmitter coil  300 , and receiving a response via the receiving coil  580 . The relative penetration is caused by the change in the saturation current. Thus, as the saturation current increases from i 1 , to i 2 , to i 3 , to i 4  then the penetration depth increases from δ 1 , to δ 2 , to δ 3 , to δ 4 , respectively. FIG. 6A illustrates the incremental increase in penetration by the field lines F 1 , F 2 , F 3  and F 4 . Also, consideration of the cross-sectional area of each component of the magnetic circuit  502  is required to assure that no component goes into total saturation for a specific power requirement necessary to drive the EM wave across the air gap G. 
     FIGS. 6B,  6 C and  6 D illustrate the relationship between the transmitter current amplitude, the saturation current amplitude, and the receiver current amplitude with respect to the magnetic circuit  502  illustrated in FIG.  6 A. FIG. 6B illustrates that the transmitter current amplitude may be constant over time. FIG. 6C shows that the saturation current amplitude is increased as a step function over time. With the transmitter current amplitude held constant over time and the saturation current amplitude increased as a step function over time, the receiver current amplitude will increase as a step function congruent with the saturation current amplitude up to and until the barrier material is in a state of total saturation (FIG.  6 D). When the barrier material is in a state of total saturation, as illustrated in FIG. 6D, the receiver current amplitude is at a maximum and cannot increase because maximum penetration has been achieved. 
     FIG. 7 illustrates one embodiment of a magnetic transparency generator  600  of the present invention. The transmitter  606  and the receiver  608  are geometricly nulled. The displacement distance between the transmitter  606  and the receiver  608  is essentially zero. The displacement distance is essentially zero because of the close configuration of the transmitter  606  and the receiver  608 . The intensity of the frequencies received will show the material thickness. For example, if all the higher frequencies are attenuated, the material is thick. If all the high frequencies are detected with little attenuation of the low frequencies, the material is thin. For a given power, the displacement distance between the transmitter  606  and the receiver  608  determines the resolution of the thickness measurement. The resolution effects the size of the defect measurable. 
     FIG. 8 illustrates the relationship between the flux field β and the change in H (ΔH) in ampere-turns/meters. The permeability μ is plotted. For the relationship between the flux field β and ΔH, the function defining the permeability μ remains the same. Although the function defining the permeability μ remains the same, the value of ΔH for thinner materials moves up the curve faster. Thus, incremental changes in H create a faster advancement up the permeability curve toward saturation. For example, a given H L1  corresponds to the value of β L1  and a corresponding H L2  corresponds to the value of β L2 . Thus, the value for L2 moves faster up the permeability μ curve than the value for L1. 
     FIG. 9 illustrates the relationship between the receiver amplitude A Rx  and H in ampere-turns/meters. As in FIG. 8, the slope of the curve in FIG. 9 is related to the permeability μ. However, the receiver amplitude A Rx  reaches a different maximum value depending on the thickness of the material. For thinner materials, the receiver amplitude A Rx  reaches its maximum value at a lower amplitude A Rx . For thicker materials, the receiver amplitude A Rx  reaches its maximum value at a higher amplitude A Rx . FIG. 9 illustrates a thinner material having a maximum at A R1 , a thicker material having a maximum at A R3 , and an intermediate thickness material having a maximum at A R2 . 
     FIG. 10A illustrates an embodiment of a magnetic transparency generator  200  used to generate a transparency with respect to a material  100  for practicing the present invention. A transmitter coil  210 A is disposed at the remote end of the outside diameter of an inner cylindrical portion  204  of the magnetic transparency generator  200 . A saturation coil  220  is disposed at the inner end of the outside diameter of the inner cylindrical portion  204  of the magnetic transparency generator  200 . A receiver coil  230  is disposed within the inside diameter of the inner cylindrical portion  204  of the magnetic transparency generator  200 . The receiver coil  230  can be located at different positions using a shaft  232  which telescopes within the inside diameter of the inner cylindrical portion  204  of the magnetic transparency generator  200 . The telescoping shaft  232  can also rotate using a set-screw adjustment  206  and a set-screw housing  208 . Also, wiring  234  can be channelled through the shaft  232 . 
     FIG. 10B illustrates another embodiment of a magnetic transparency generator  200  used to generate a transparency with respect to a material  100  for practicing the present invention. A transmitter coil  210 B is disposed at the remote end of the of the outside diameter of an outer cylindrical portion  202  of the magnetic transparency generator  200 . A saturation coil  220  is disposed along the outside diameter of an inner cylindrical portion  204  of the magnetic transparency generator  200 . A receiver coil  230  is disposed within the inside diameter of the inner cylindrical portion  204  of the magnetic transparency generator  200 . The receiver coil  230  can be located at different positions using a shaft  232  which telescopes within the inside diameter of the inner cylindrical portion  204 . The telescoping shaft  232  can also rotate using a set-screw adjustment  206  and a set-screw housing  208 . Also, wiring  234  can be channelled through the shaft  232 . 
     FIG. 11 illustrates a bistatic transmitter/receiver configuration  1100  used in practicing the present invention. The bistatic transmitter/receiver configuration  1100  maybe used to penetrate from the inside through to the outside or from the outside through to the inside of a container  1102 . The container  1102  can be comprised of flat walled sides or cylindrical shaped object, e.g., a pipe or the like made of any non-ferromagnetic material. Non-ferromagnetic materials are, for example, stainless steel, aluminum, bronze, copper, fiberglass or other non-magnetizable material. FIG. 11 illustrates a single transmitter Tx and a single receiver Rx disposed along the container  1102 . An alternate position for the single receiver Rx is also illustrated in FIG.  11 . The alignment of the receiver Rx for use with the bistatic transmitter/receiver configuration  1100  of FIG. 11 is axially radial. 
     FIGS. 12A,  12 B and  12 C illustrate examples of tristatic transmitter/receiver configurations  1200 A,  1200 B,  1200 C, respectively, as practiced with the present invention. FIG. 12A illustrates a single transmitter Tx and two receivers Rx. FIG. 12B illustrates a transmitter/receiver configuration  1200 B using two transmitters Tx and one receiver Rx. FIG. 12C is a bistatic embodiment  1200 C using a transmitter Tx and a receiver Rx. The type of container  1202  is made of non-ferromagnetic material. The receivers Rx and transmitters Tx are wrapped around the pipe or container  1202 . The receivers Rx and transmitters Tx can be disposed inside the container  1202  to penetrate through to the outside. Alternately, the transmitters Tx and receivers Rx can be disposed outside of the container  1202  to penetrate to the inside. 
     FIGS. 13A,  13 B,  13 C and  13 D are illustrations of an axially symmetric configuration of transmitters Tx and receivers Rx such that the receiver coils are centered along the same axis A. FIG. 13A illustrates an axially symmetric configuration  1300 A of two transmitters Tx 1 , Tx 2  and a receiver Rx disposed there between. The transmitter/receiver configuration  1300 A is in operative association with a pipe, container or wall  1302 . The pipe, container or wall  1302  is made from non-ferromagnetic material. The transmitters Tx 1 , Tx 2  are “bucked.” The transmitters Tx 1 , Tx 2  being bucked requires that the coils associated with the transmitters Tx 1 , Tx 2  are wound in opposite directions. As illustrated in FIG. 13A, the coils for Tx 1  are wound in a clockwise direction w 1 . The coil associated with the second transmitter Tx 2  is wound in a counter clockwise direction w 2 . The coil for the receiver Rx can be wound in either direction. When the transmitters Tx 1 , Tx 2  are bucked, i.e. wound in opposite directions, the field lines generated are described by the right thumb rule. Thus, bucking provides a canceling of the respective field lines associated with the bucked transmitters Tx 1 , Tx 2 . In the transmitter/receiver configuration  1300 A illustrated in FIG. 13A, the receiver Rx must be between the two transmitters Tx 1 , Tx 2 . 
     FIG. 13B is another embodiment of a transmitter/receiver configuration  1300 B which is axially symmetric. A first receiver Rx 1  is disposed adjacent a second receiver Rx 2 . A transmitter Tx is disposed remote from the two receivers Rx 1 , Rx 2  such that the transmitter Tx is not between the receivers Rx 1 , Rx 2 . The receivers Rx 1 , Rx 2  are bucked, i.e. wound in opposite directions. The bucked receivers provide that field lines generated are canceling. The transmitter/receiver configuration  1300 B in FIG. 13B, having two receivers Rx 1 , Rx 2,  is functional with the transmitter Tx placed anywhere along the central axis of the transmitter/receiver configuration  1300 B. Further, as with FIG. 13A, the embodiment of the transmitter/receiver configuration  1300 B illustrated in FIG. 13B provides that the transmitter/receiver configuration  1300 B maybe placed inside the wall to penetrate to the outside. Alternately, the transmitter/receiver configuration  1300 B can be placed outside of the wall to penetrate to the inside. The receiver Rx 1  is illustrated with its coil wound in a clockwise direction w 1 . The receiver Rx 2  is illustrated with coil wound in a counter clockwise orientation w 2 . 
     FIG. 13C is another embodiment of the present invention illustrating a transmitter/receiver configuration  1300 C that is axially symmetric. A pair of receivers Rx 1 , Rx 2  are disposed adjacent to pipe, container or wall  1302  oriented so that the receivers Rx 1 , Rx 2  are axially configured along a central axis A. A transmitter Tx is disposed remote from the receivers Rx 1 , Rx 2  which are remote from the pipe  1302 . The receivers Rx 1 , Rx 2  are bucked. The first receiver Rx 1  is wound in a clockwise direction w 1 , and the second receiver Rx 2  is wound in a counter clockwise w 2 . The pipe  1302  is made of a non-ferromagnetic material. The transmitter/receiver configuration  1300 C maybe placed on the outside to penetrate to the inside, or alternately placed inside to penetrate to the outside. 
     FIG. 13D is another illustration of a receiver/transmitter configuration  1300 D which is actually symmetric. A first receiver Rx 1  is disposed adjacent a pipe, container or wall  1302 . The pipe container or wall  1302  is made from a non-ferromagnetic material. A second receiver Rx 2  is disposed remote from the first receiver Rx 1  in the wall  1302 . Between the first receiver Rx 1  and the second receiver Rx 2 , a transmitter Tx is disposed. The receivers Rx 1 , Rx 2  are also bucked. FIG. 13D illustrates the first receiver Rx 1  being wound in a clockwise direction w 1 , and the second receiver Rx 2  being wound in a counter clockwise orientation w 2 . As with the other axially symmetric configurations, the present receiver/transmitter configuration  1300 D maybe placed on the outside of the wall  1302  to penetrate to the inside, or alternately, maybe placed on the inside of the wall  1302  to penetrate to the outside. 
     FIG. 13E is a graph of amplitude versus time illustrating the signals  1311 ,  1312  associated with the bucked transmitters Tx and receivers Rx illustrated in FIGS. 13A,  13 B,  13 C and  13 D. The left cross-hatched signal  1311  is the mirror image of the right cross-hatched signal  1312  such that the bucking relationship cancels portions of the signal. 
     FIG. 14A illustrates an embedded transmitter/receiver configuration  1400 A as practiced by the present invention. The embedded transmitter/receiver configuration  1400 A is operatively associated with a pipe, wall or container  1402 . The pipe  1402  is made from a non-ferromagnetic material. A transmitter Tx is disposed in a plane that is parallel to, or alternately, tangent to, the plane of the surface of the pipe  1402 . A receiver Rx is disposed in such a way as the receiver Rx is embedded within and has a 90 degree relationship with the transmitter Tx. Thus, the receiver Rx can be moveably located to multiple positions D within the transmitter Tx. More particularly, the interior receiver Rx can be moveably located within the transmitter Tx to provide an enhanced null for the best transmitter/receiver configuration  1400 A for the desired application. The transmitter Tx and the receiver Rx are nulled with respect to each other. The enhanced null characteristics of the transmitter/receiver configuration  1400 A illustrated in FIG. 14A provides for a way to avoid the induced characteristics of the evaluated system, which evaluated system may be a pipe, a wall, a pipe with water in it, a wall with a substance on it, an oil well casing, etc. For example, an embodiment of the present invention as illustrated in FIG. 14A can be placed next to a tank with water in the tank. The transmitter/receiver configuration  1400 A is nulled with the water in the tank. If salt is added to the water, the null will be disrupted and the salinity can be determined. Another example is to null the embedded transmitter/receiver configuration  1400 A illustrated in FIG. 1A away from the tank. Thereafter, moving the embedded configuration to the tank will get a change in signal with respect to the tank and what ever is in the tank. The directionality depends on the particular application being evaluated, for example, inside the pipe, outside the pipe, remote from the pipe, etc. 
     FIG. 14B is another embedded transmitter/receiver configuration  1400 B as practiced by the present invention. The embedded transmitter/receiver configuration  1400 B in FIG. 14B provides for the configuration to be operatively associated with the pipe, container or wall  1402  made from a non-ferromagnetic material. The transmitter Tx is disposed in the plane of the pipe  1402 . The receiver Rx is disposed in or embedded within the transmitter Tx so is to have an orthogonal relationship with the transmitter Tx and the pipe  1402 . A receiver Rx is disposed in such a way as the receiver Rx is embedded within and has a 90 degree relationship with the transmitter Tx. Thus, the receiver Rx can be moveably located to multiple positions D within the transmitter Tx. More particularly, the interior receiver Rx can be moveably located within the transmitter Tx to provide an enhanced null for the best transmitter/receiver configuration  1400 B for the desired application. The transmitter Tx and the receiver Rx are nulled with respect to each other. The enhanced null characteristics of the transmitter/receiver configuration  1400 A illustrated in FIG. 14A provides for a way to avoid the induced characteristics of the evaluated system, which evaluated system may be a pipe, a wall, a pipe with water in it, a wall with a substance on it, an oil well casing, etc. The transmitter/receiver embedded configuration maybe placed on the outside of the pipe to look in or alternately, placed on the inside of the pipe to look out. 
     FIG. 15A is a tristatic configuration  1500 A of the present invention. The tristatic configuration  1500 A provides for the transmitters Tx 1 , Tx 2  and the receiver Rx to be on the same plane, but having different central axis. The tristatic configuration  1500 A is adapted with a pipe container or wall  1502  made from non-ferromagnetic material. The transmitters Tx 1 , Tx 2  are bucked. The transmitters Tx 1 , Tx 2  being bucked requires that the coils associated with the transmitters Tx 1 , Tx 2  are wound in opposite directions. As illustrated in FIG. 15A, the coil for Tx 1  is wound in a clockwise direction w 1 . The coil associated with the second transmitter Tx 2  is wound in a counter clockwise direction w 2 . The coil for the receiver Rx can be wound in either direction. The tristatic configuration  1500 A maybe placed outside the pipe  1500 A to penetrate inside the pipe  1500 A or alternately, maybe placed inside the pipe to penetrate to the outside. 
     FIG. 15B is another tristatic configuration  1500 B of nulling with respect to the present invention. The tristatic configuration  1500 B is operatively associated with the pipe container or wall  1502  made from non-ferromagnetic material. Two receivers Rx 1 , Rx 2  are disposed adjacent each other along the plane of the pipe  1502 . A transmitter Tx is disposed along the same plane as the receivers Rx 1 , Rx 2  but having different central axis. The receivers Rx 1 , Rx 2  are bucked. The receivers Rx 1 , Rx 2  being bucked requires that the coils associated with the receivers Rx 1 , Rx 2  are wound in opposite directions. As illustrated in FIG. 15B, the coil for receiver Rx 1  is wound in a clockwise direction w 1 . The coil associated with the second receiver Rx 2  is wound in a counter clockwise direction w 2 . The coil for the transmitter Tx can be wound in either direction. The tristatic configuration  1500 B of FIG. 15B can be placed on the outside of the pipe  1502  for looking into the pipe, or alternately can be placed inside the pipe for looking outwardly. 
     FIG. 16 is yet another configuration  1600  of nulling combining the axially symmetric relationships of FIGS. 13A,  13 B,  13 C and  13 D with the static configurations illustrated in FIGS. 15A and 15B. A pair of receivers Rx 1 , Rx 2  are oriented so as to be axially symmetric along an axis A. A transmitter Tx is disposed remote from the axial relationship of the receivers Rx 1 , Rx 2 . Similarly, the receivers Rx 1 , Rx 2  can be bucked. The receivers Rx 1 , Rx 2  being bucked requires that the coils associated with the receivers Rx 1 , Rx 2  are wound in opposite directions. As illustrated in FIG. 16, the coil for receiver Rx 1  is wound in a clockwise direction w 1 . The coil associated with the second receiver Rx 2  is wound in a counter clockwise direction w 2 . The coil for the transmitter Tx can be wound in either direction. The mixed configuration  1600  can be placed on the outside of the pipe  1602  to penetrate to the inside or alternately, placed on the inside of the pipe  1602  to penetrate to the outside. 
     FIG. 17 is an embedded, rotating receiver/transmitter configuration  1700  as practiced by the present invention. The embedded rotating receiver/transmitter configuration  1700  is operatively associated with a pipe container or wall  1702  made from non-ferromagnetic material. The transmitter Tx is oriented orthogonal to the pipe, container or wall  1702 . The embedded receiver Rx is disposed such that it can be rotated within the transmitter Tx. The rotating of the embedded receiver Rx provides for enhanced nulling of the embedded configuration. The embedded configuration may be placed on the inside of the pipe  1702  for examining outer regions, or alternately may be placed outside the pipe  1702  for examining the inside of the pipe. The characteristics discussed above for the other nulled configurations applies to the embedded, rotating receiver/transmitter configuration  1700  as well. 
     FIG. 18 is a combination transmitter/receiver configuration  1800  using the same antenna Tx for practicing the present invention. Signal detection is by the very accurate measurement of voltage and current absorbed in the target material, for example, a substance flowing through a pipe  1802 . The combined transmittal/receiver configuration  1800  illustrated in FIG. 18 has a single transmitter/receiver coil. The single coil is transmitted, turned off and used as a receiver for making the accurate measurement of voltage in current absorbed into the target material. 
     FIG. 19A is an illustration of an application of the present invention utilizing a loop transmitter Tx with a tank  1902  for detecting the level  1906 A within the tank  1902  or the resistivity. The tank  1902  is illustrated having embedded transmitter/receiver configuration similar to that illustrated in FIG.  14 B. The transmitter Tx has a diameter L. The receiver Rx can be moved within the transmitter Tx throughout the distance L. The tank  1902  is provided having a distance of k times that of the movable area for the receiver Rx within the transmitter Tx. Thus, the tank  1902  has depth of kL. The transmitter/receiver configuration can be nulled with water in the tank. As the water starts to fill the tank as the target material, the null signal is offset until such time as the water engages the location of the receiver Rx which would be the maximum offset of the null. As the fluid continues to fill the tank, the null signal would decrease to a lower value. 
     Similarly, FIG. 19B is an illustration of an application of the present invention utilizing a loop transmitter Tx with a pipe  1910  for detecting the level  1912  associated with the pipe  1910 . The same measurement can be made within a stainless steel casing having an embedded transmitter/receiver configuration. The transmitter/receiver configuration provides that a receiver Rx is moveably embedded within a transmitter Tx so that nulling can be very accurate. 
     FIG. 20 is a graph of the output of the embedded transmitter/receiver configuration as illustrated in FIGS. 19A and 19B. As the fluid begins entering the tank, the volts detected increases  2010 . The volts detected will continue to increase until such point as the fluid is located congruent with the receiver Rx, which will be the maximum amplitude  2010 . As the fluid continues to fill the tank, nulling offset will decrease and approach the nulled value  2010 . The phase of the signal  2020  is disposed over the amplitude  2010  such that the phase increases to a maximum point and as the fluid engages the receiver Rx, the phase  2020  decreases and passes through the zero axis as the fluid rises above the receiver Rx. The phase has a corresponding configuration in the negative as it had in the positive of the graph. 
     Additional advantages and modification will readily occur to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus, and the illustrative examples shown and described herein. Accordingly, the departures may be made from the details without departing from the spirit or scope of the disclosed general inventive concept.