PATENT DOCUMENT

Abstract:
A method and apparatus are provided for detecting and transmitting geophysical data from a plurality of electrodes inserted into the soil utilizing a set of identical dynamically reconfigurable voltage control units located on each electrode and connected together by a communications and power cable. A test sequence is provided in each voltage control unit. Each voltage control unit records data measurements for transmission to a central data collector. Each voltage control unit incorporates and determines its positional relationship to other voltage control units by logging when the unit is attached to the electrode. Each voltage control unit I equipped with a magnetic switch for detecting when they are in contact with the electrode.

Full Description:
This application is a Continuation-In-Part claiming priority benefit from U.S. patent application Ser. No. 11/472,901 which was filed on Jun. 22, 2006, now U.S. Pat. No. 7,386,402, issued on Jun. 10, 2008 and from U.S. patent application Ser. No. 11/982,484 which was filed on Nov. 1, 2007, now U.S. Pat. No. 7,788,049, issued on Aug. 31, 2010. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This preferred embodiment relates to a method for producing a subsurface electrical resistivity model. More specifically, the preferred embodiment relates to a method for resistivity modeling for locating anomalies, such as groundwater, leaks, tree roots, and other vegetation, particularly beneath and around existing composite concrete structures and roadbeds. Raw resistivity data is collected by a reconfigurable network of sensors that distributes current and measures voltage. A regression correlation is performed on the raw resistivity data to create a resistivity model, which is converted to graphical form, then analyzed to detect and locate subsurface soil anomalies. 
     BACKGROUND OF THE INVENTION 
     Modern concrete structures are usually composites which utilize embedded materials such as steel and aggregate stone for strength and durability. Some examples of concrete structures include roadways and slab-on-grade foundations. The most common modern roadway paving materials are concrete and asphalt while slab-on-grade foundations are primarily concrete. Typical concrete exhibits extraordinary resistance to compressive forces but lower resistance to tensile forces and vibration. Concrete composites include steel bars, steel wire mesh or fiber materials added to vary the characteristics of the concrete and to increase its resistance to tensile forces and vibration. 
     The use of reinforced concrete composite was prevalent during construction of the interstate highway system in the United States. An updated form of steel reinforced concrete is still used today for urban traffic applications. Bridge decks and bridge footings also extensively employ reinforced concrete. 
     Roadway structures use other forms of surfacing that incorporate composite materials and steel reinforcement. Asphalt pavements are applied to a compacted gravel base generally at least as thick as the asphalt layer, but some asphalt pavements are built directly on the native subgrade. In areas with very soft or expansive subgrades such as clay, thick gravel bases or stabilization of the subgrade with cement or lime can be required. Subgrade stabilization can include use of steel mesh or reinforced concrete applied to the roadbed under asphalt pavement. 
     Slab-on-grade foundations are shallow foundations that are often constructed of concrete using reinforcing methods. Slab-on-grade foundations are typically prone to cracking due to defication when the subgrade becomes unstable. To reduce the effects of subgrade instability, steel, wire mesh, fiber composites and tension cables are employed in the concrete. Post-tensioning is a method of strengthening concrete using high-strength steel strands or cables, typically referred to as tendons. The tendons rest on anchors in the concrete. Tension is applied to the cables used to place the concrete in compression. 
     Two cases arise where it is often necessary to determine fluid leaks, subsurface fluid flow and subsurface anomalies such as land faults and buried inclusions. 
     In the first case, preparation of a roadbed or construction site for building requires that an analysis of subsurface anomalies and subsurface fluid flow in order to adequately plan for and construct either a road or a concrete foundation. 
     In this case, detecting subsurface anomalies is a difficult task because limited time and resources often prevents a sufficient sample size to be adequately confident that the site located is free from anomalies or subsurface fluid flow. 
     In the second case, a reinforced concrete or asphalt structure is already in place. Testing is necessary to determine subsurface leaks, subsurface anomalies or subsurface fluid flow after construction. 
     Detecting and locating fluid leaks and other subsurface anomalies beneath composite concrete and asphalt structures is often a difficult task because the concrete or asphalt prevents access to the soil underneath. Removal of sections of the concrete to inspect the subjacent soil is often required. Methods of the prior art often require destruction of large sections of the composite structures to locate leaks and other subsurface anomalies because the exact location of the leak or anomaly is unknown. 
     Methods currently exist for detecting and locating leaks from landfills, hazardous waste dumps, impoundments, and other outdoor fluid containment areas by measuring changes in the conductivity and/or resistivity of the adjacent soil. Daily et al. &#39;406 discloses a “mise-a-la-masse” technique and an electrical resistance tomography technique. 
     The mise-a-la-masse technique involves placement of several electrodes, one inside and several outside the facility. An electronic potential is applied to different pairs of electrodes, but always includes the electrodes in the fluid containment facility. Voltage differences are then measured between various combinations of electrodes. The leak location is determined from the coordinates of a current source profile that best fits the measured potentials within the constraints of the known or assumed resistivity distribution. However, because the potentially leaking fluid must be driven to a potential, mise-a-la-masse methods can monitor for leaks in continuous fluid systems only, such as ponds, lined fluid containment areas, and tanks. 
     Electrical resistivity tomography (ERT) involves placing electrodes around the periphery of, beneath, or, in the case of subsurface containment vessels, above the facility. A known current is applied to alternating pairs of electrodes, and then the electrical potential is measured across other alternating pairs of electrodes. The measurements allow calibration of electrical resistivity (or conductance) over a plurality of points in the soil. Differences in resistivity correlate directly with migration of leaking fluid. However, Daily does not disclose a method or apparatus that allows the electrodes to be placed directly under the leak source, after construction of a structure or paved roadway. 
     Henderson &#39;202 and &#39;045 both disclose directly monitoring the soil subjacent to a fluid containment area by burying electrodes directly beneath the containment. Both Henderson patents disclose a plurality of four-plate electrode systems. A voltage and a known current are applied across the outer pair of plates. The resulting potential difference is measured across the inner pair. Henderson &#39;045 also discloses a system of individual electrodes that, by varying the spacing between the electrodes impressing a current into the ground and the spacing of the potential measurement electrodes, can indirectly measure the resistivity at a calculated depth. However, Henderson &#39;045 does not disclose a method of directly monitoring the subgrade beneath a structure without permanently burying the electrodes or a method to place electrodes beneath an existing structure. 
     Woods et al. &#39;244 discloses a leak detection system for radioactive waste storage tanks. The system comprises a metal tank, an AC generator connected between the tank and a reference electrode, and a plurality of reference electrodes. When the generator is energized, it creates an electric field in the ground between the tank and the reference electrode. A voltmeter measures the potential difference between the sensing electrodes and the tank. A significant change in the potential at one or more of the sensing electrodes indicates that the tank has developed a leak. Woods et al. has a number of disadvantages. First, it requires an electrically conductive fluid container. Second, it requires that the electrodes be permanently buried in the soil surrounding the tank. Finally, it requires the use of an AC generator, which is less convenient than a DC power source. 
     Bryant &#39;625 discloses a method and apparatus for creating an electrical resistivity map of the volume beneath a slab foundation by placing electrodes through a foundation, and applying a current through them. Bryant &#39;625 further discloses a method for converting the measured potential to a resistivity value, assigning the resistivity value to a spatial coordinate, and storing a plurality of values in computer files. The apparatus includes an array of electrodes that are used to impress a known current in the soil and measure the resulting electrical potential of electrodes. Typically, a pair of electrodes is used to impress a constant current, and another pair is used to measure a voltage potential. 
     The array of electrodes is interconnected by electrical conducting cables that connect to the various electrodes at predetermined intervals. The interconnecting cables transmit electrical current that passes through certain electrodes to create the electrical field within the underlying and subjacent soil, and return electrical signals from other measuring electrodes that detect the electrical field within the soil. However, Bryant does not disclose the ability to switch current between nodes or to conduct an orderly permutation of voltage measurements between nodes. 
     None of the prior art is entirely satisfactory to locate fluid leaks beneath composite and reinforced concrete structures or to analyze them in near real time. For instance, it is not practical to electrify the potentially leaking fluid and because there may exist multiple sources of fluid, mise-a-la-masse is not a practical option. Nor is it practical to embed permanently a series of electrodes beneath an existing massive concrete structure or roadway to monitor soil resistivity. Further, because some of the ERT methods use multiple-plate electrodes where a large hole is bored to insert the electrodes into the subjacent soil making the method impractical and destructive. In addition, placing the electrodes around the periphery of a roadbed or foundation is less accurate compared to placing the electrodes directly beneath or adjacent the potential leak source. 
     The current state of the art is unsatisfactory because it does not provide a method to remotely change injection current locations or to conduct an orderly progression through a permutation of voltages and currents between nodes. Moreover, the state of the art does not provide for dynamically addressable sensors whose location and address can be changed on the fly. 
     Furthermore, the present state of the art requires that the electrodes be placed in a linear, regularly spaced grid pattern that does not provide needed flexibility in the physical layout of arrays of electrodes in multiple, non-linear arrangements. The present state of the art presumes the locations of the electrodes to one another. It may be necessary to arrange the electrodes in a non-linear grid if the physical layout of the concrete or underlying area to be measured prevents the layout of electrodes in a typical, linear grid arrangement. It may be necessary to locate certain electrodes in a non-linear pattern to accommodate obstructions in the existing structures foundation or to conform the layout of the grid to a particular stretch or curvature of a paved roadway or bridge, or other geographic anomaly. Furthermore, the prior art does not provide for a means to easily adjust an array of electrodes to avoid encapsulated steel structures such as rebar. Where a need arises to arrange the electrodes in a non-linear grid, it may be necessary to identify the spatial relationship of the electrodes. Likewise, it may be necessary to adjust the location of certain electrodes to accommodate obstructions and thus, it may be necessary to identify the new location. 
     Furthermore, the present state of the art requires that electrodes be placed according to a measured or surveyed pattern at the physical location. The requirement of physically measuring and placing electrodes is hampered by structures which include buildings, medians, or other obstructions which makes placing the electrodes accurately difficult. The inaccurate placement of electrodes leads to errors in the mathematical calculations required to analyze the locations of the anomalies and therefore reduces the overall efficiency and accuracy of the system. 
     Moreover, the state of the art does not provide for interchangeability of sensors. This limitation requires extensive time in installation and replacement of defective sensors. The uniqueness of sensors required by the prior art creates a need for specific sensors uniquely identified by their order in a specific grid. 
     Further, a need exists for accurate measurement of voltage offsets to accurately calibrate voltage readings among sensors. 
     SUMMARY OF THE INVENTION 
     Soil, rocks, and vegetative matter can conduct electricity to varying degrees. The resistance, or resistivity, of these materials to an electrical current will vary depending upon density, particle composition, moisture content, and the chemical composition of fluid in the spaces between the particles. A fluid leak from, for example, pipes in a slab or a waste conduit or water main under a paved roadway into the subjacent soil will affect the electrical resistivity (resistance offered by a material to the flow of electrical current, multiplied by the cross-sectional area of current flow and per length of current path). Fluid generally decreases the resistivity of the subgrade. Measuring resistivity at varying depths and at varying locations, both beneath and adjacent to a slab foundation or paved roadway, and comparing these resistivities to one another, allows one the location of soil anomalies based on resistivity variations. These anomalies can include wet soil, vegetative matter and buried metallic and non-metallic objects. Research has shown that the location of resistivity anomalies corresponds well to the location of soil anomalies. 
     Resistivity is generally not measured directly; however, resistivity can be computed if the intensity of a current injected into the ground, and the resulting potential difference established between measurement electrodes are measured. These quantities depend on the geometry of the electric field, the nature of the soil and interstitial fluid, and the method used to measure the injected current and the resulting potential difference. 
     The preferred embodiment contemplates converting the measured potential to a resistivity value, assigning the resistivity value to a spatial coordinate, and storing these values in a computer file. A computer program then performs a least squares data inversion analysis on the resistivity and location values, creating an electrical resistivity model that minimizes the error of the field data. Next, another computer program performs a spatial data analysis, or geostatistical analysis, using kriging or other methods. 
     The preferred embodiment provides for a method and apparatus for impressing current into the ground and gathering resistivity data from a set of voltage control units that operate independently of one another but in synchronization with one another to carry out a sequence of geophysical resistivity tests. The preferred embodiment also provides for synchronous transmittal of test results from a test taken by said set of voltage control units to a data processor for creating 2-D and 3-D graphical representations of areas of equipotential resistivity, the test results being transmitted after every test to the data processor. 
     The preferred embodiment provides for an apparatus and method for controlling the amplitude of injected current with a power distribution unit, the current being ramped up and down during the sequence of geophysical test in order to preserve power source battery charge and to ensure like current and voltage conditions for each measurement. The preferred embodiment of the power distribution unit also causes voltage control unit operating power to be drawn from stored capacitive energy alone during a measurement for creating minimal electrical noise in the system. A novel design for a solid state relay switch is taught whereby the injected current is connected through at least two electrodes in such a way as to avoid the need to draw system power during the measurement and whereby the measurement noise is reduced over prior art methods. 
     The preferred embodiment provides for a method of deployment of voltage control units wherein each voltage control unit incorporates a self-test sequence during deployment so that faulty units may be identified early in the deployment process. The deployment process is a manual process in the preferred embodiment, wherein electrodes are first surveyed and placed, the distances and positions of electrodes with respect to each other being recorded manually, after which the set of voltage control units are connected one to another in sequence from the power distribution unit and proceeding downstream, the subsequent registration of a voltage control unit causing a self-test on that voltage control unit to begin operation. The self-test sequence on a given voltage control unit includes the steps of creating a random address for the voltage control unit under self-test and for verifying the validity of the random address by communications with an upstream voltage control unit. 
     In another aspect of the invention, the control processor can identify and transmit instructions to manage current distribution, data and voltage measurements data at each sensor. 
     In the event of damage to a sensor or damage to an interconnecting cable, a replacement sensor or cable may be added to the array of sensors without excessive downtime and expensive field repairs because all of the sensors are identical. The replacement sensor can unambiguously identify itself with respect to other sensors to the control processor for use by the mapping software. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the preferred embodiment, and for further details and advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1   a  is a schematic diagram of the injection of current into the ground through existing pavement and the monitoring of a resulting voltage. 
         FIG. 1   b  is a schematic diagram of an electrode array. 
         FIGS. 2   a, b  and  c  show a roadbed with various electrode array patterns 
         FIGS. 3   a - 3   f  are schematic diagrams of a series of permutations of the locations of an ammeter and a current source and a volt meter as employed by one preferred embodiment. 
         FIG. 4  is a schematic diagram of one preferred embodiment. 
         FIG. 5  is a schematic diagram of a power distribution unit for controlling the current supplied to electrodes and for controlling the operating power for other system components. 
         FIG. 6  is a schematic diagram of a voltage control unit for gathering and transmitting measurement data from an electrode. 
         FIG. 7  is a resistivity test sequence power and timing diagram. 
         FIG. 8  is a diagram of showing the structure of a test data packet. 
         FIG. 9   a  is an isometric view of a voltage control unit of the preferred embodiment. 
         FIG. 9   b  is an exploded isometric view of a voltage control unit of the preferred embodiment. 
         FIG. 10  is an exploded isometric view of a first embodiment of a power distribution unit. 
         FIG. 11  is an exploded isometric view of a second embodiment of a power distribution unit. 
         FIG. 12  is a flowchart of the power distribution unit self-test method of one preferred embodiment. 
         FIG. 13  is a flowchart of the voltage control unit self-test method of a preferred embodiment. 
         FIG. 14  is a flowchart of the voltage control unit register method in a preferred embodiment. 
         FIG. 15   a  is a sequence diagram of the first embodiment test method of a f the preferred embodiment. 
         FIG. 15   b  is a sequence diagram of the second embodiment test method of one preferred embodiment. 
         FIG. 16  is a sequence diagram of a run method of one preferred embodiment. 
         FIG. 17  is block diagram of the preferred mode of operation of the of the preferred embodiment. 
         FIG. 18  is the analysis method of the preferred embodiment. 
         FIG. 19  is a flowchart of a preferred safety check and pre-setup operation. 
         FIG. 20  is an example of a first resistivity model. 
         FIG. 21  is an example of a second resistivity model. 
         FIGS. 22   a  and  22   b  are circuit diagrams of a solid state relay switch in the preferred, embodiment of the voltage control unit. 
         FIG. 23  is a flowchart of a preferred system setup operation. 
         FIG. 24  is a flowchart of a preferred system start operation. 
         FIG. 25  is a schematic diagram of a voltage control unit configured for differential voltage measurement between adjacent pairs of stakes. 
         FIG. 26  is a schematic diagram of an example set of voltage control units configured to make a differential voltage measurement. 
         FIG. 27  is a schematic diagram of a voltage control unit programmably configured for differential voltage measurement between any pair of stakes. 
         FIG. 28  is a schematic diagram of an example set of voltage control units configured to make a programmable differential voltage measurement between a pair of non adjacent stakes. 
         FIG. 29  is a flow chart diagram of a differential voltage measurement method. 
     
    
    
     DETAILED DESCRIPTION 
     A schematic diagram of the theory used to gather data as used in the preferred embodiment is shown in  FIG. 1   a . A series of electrodes  101  are inserted, typically in a line, into ground  10 .  FIG. 1  shows the preferred embodiment as applied to an asphalt paved roadway. Asphalt layer  12  is applied on top of mid-level layer  14 . Adhering to typical road construction principles, mid-level layer  14  may be a layer of concrete, steel reinforced concrete, or a compacted layer of gravel. In some situations, asphalt layer  12  may not be present. Electrodes  101  are inserted through both asphalt layer  12  and mid-level layer  14  and into ground  10  in order to determine resistivity and to map and detect subsurface soil anomalies under the roadway. The locations of electrodes  101  are typically in a regular pattern with a known interstitial distance between each electrode. In most situations, the known interstitial distance ranges from 6 to 10 feet and an equal interstitial distance is used between each linear array. In some situations, it may be necessary to locate certain electrodes in a non-linear pattern to accommodate obstructions. For example, it may be necessary to avoid a latticework of steel rebar. One electrode, ground electrode  100 , may not be in the regular pattern of electrodes  101 , ground electrode  100  supplying a known earth ground reference point from which an electric potential difference V is measured. 
     Several arrays of electrodes are shown in  FIG. 1   b . Road section  202  is shown with a linear grid of arrays. Each “X” indicates the location of electrodes  101 . The known interstitial distance between electrodes  204  is equal to the known interstitial distance between arrays  206 .  FIG. 1   b  also shows a schematic representation of latticework of steel rebar  205  encapsulated in the concrete surface of the roadbed. It should be noted that the typical regular array of electrodes interferes with the latticework of steel rebar at electrode locations  207 ,  209 ,  211 ,  213 ,  215  and  217 , known as “interference points”. 
     In practice, an array of electrodes such as shown in  FIG. 1   b , is laid out in conjunction with a roadbed and its surrounding surface. Borings are made through the pavement and roadbed for placement of the electrodes. During the process of boring, interference points are determined by actual exposure of rebar or other electromagnetic means such as electromagnetic preferred embodiment or use of magnetometers. Once interference points are located, the electrode positions are changed to avoid the steel latticework. In practice, the electrode location is moved by at least one-one hundredth of the interstitial distance between adjacent electrodes. In a second technique, the electrodes which create interference points are eliminated from the array altogether. 
     Referring to  FIGS. 2   a - 2   c , grid spacing alternatives are discussed. In large projects where many thousand of electrodes must be positioned, the spacing of the electrodes can be manipulated to achieve greater geographic coverage at the expense of accuracy. Three-dimensional data is produced by a two-dimensional rectangular grid pattern of electrodes. The ratio of “length” of the rectangular pattern to its “width” is known as the “aspect ratio” of the grid. For example, the aspect ratio of the rectangular pattern of  FIG. 2   a  is “low” while the aspect ratio of the rectangular pattern of  FIG. 2   b  is relatively “high”. The precision of the measurement obtained is increased as the grid spacing is decreased; however, the time and expense required to deploy the electrodes at a decreased spacing can be considerable. 
     In certain situations, the grid spacing of electrodes must be compressed to adequately investigate subsurface anomalies. For example, when a roadbed includes a bridge or when a roadbed includes a known anomaly such as a submerged river, a compressed grid pattern across the longitudinal axis of the roadbed is recommended.  FIG. 2   a  shows a grid  1798  where the interstitial distance between electrodes  1802  is comparatively less than the interstitial distance  1804  between electrodes. The ratio of interstitial distance  1802  to  1804  is the aspect ratio. An aspect ratio of between about 1/3 to about 1/10 can be used successfully to compress data in a desired direction to achieve increased precision. 
       FIG. 2   a  also depicts the use of a low aspect ratio grid pattern array across a roadbed to be useful in at least two situations. First, the low aspect ratio is useful observe a generalized anomaly  1805  that encounters a roadway  1801  having a generally longitudinal axis  1803 . First, the axis of the anomaly  1799  is generally perpendicular to longitudinal axis  1803 . The grid  1798  is positioned with respect to the axis of the array so that a greater distance along the anomaly is covered. Second, grid  1798  is used when subgrade points outside the roadbed are of interest with respect to the anomaly. In this example, the interstitial distance between electrodes  1802  may range between about 3 to about 30 feet. 
       FIG. 2   b  shows a grid pattern  1812  on a roadway  1807  having a longitudinal axis  1800  and an anomaly  1809  where the interstitial distance between electrodes  1806  is greater than the interstitial distance between arrays  1808 . This grid pattern is of high aspect ratio. A high aspect ratio grid pattern may be used when the subsurface beneath roadway  1807  is the only interest. More roadbed can be mapped along longitudinal axis  1800  using a high aspect ratio arrangement. The high aspect ratio is about 3 to about 10. The interstitial distance between electrodes  1806  may be about 3 to about 30 feet. This type of grid pattern is also used when the terrain adjacent to the roadbed is difficult to map such as in the case of steep grades (a mountainside) or a manmade retaining wall (e.g., adjacent a bridge deck). 
       FIG. 2   c  shows a grid pattern using intersecting arrays on a roadway  1811  having longitudinal axis  1812 . The use of two single-dimensional grids requires fewer electrodes and provides a faster setup time than a two-dimensional array covering a comparable area. For example, in situations where rapid deployment is required, or alternatively where it is expected that few anomalies will be located, two-dimensional grids are preferable. The axis of array  1810  is positioned adjacent to the roadbed and produces a two-dimensional representation of a plane adjacent the roadbed. The axis of array  1814  is generally perpendicular to the roadway and must be positioned by the use of holes through the roadway. Arrays  1820  and  1821  show alternate positions for two-dimensional arrays. 
     Referring again to  FIG. 1 , the first pair of electrodes  101  is connected to a DC or low frequency AC current source  110 . This pair of electrodes is referred to as current electrodes  102 . Current source  110  impresses a current into the ground through current electrodes  102 . In series with current electrodes  102  and current source  110  is an ammeter  112 . Ammeter  112  measures the current injected into the ground by current source  110 . A second pair of electrodes, voltage electrodes  103 , are connected to a volt meter  120  in series between them. Volt meter  120  measures the potential difference across voltage electrodes  103  created by the current impressed into the ground by current source  110  through current electrodes  102 . One of the voltage electrodes  103  is the ground electrode  100 , the other electrode being one of the electrodes  101  in the array. 
     In order to create a complete resistivity map, the voltage must be obtained at every possible permutation of electrodes in the array. Moreover, in order to obtain a complete resistivity map, the current source must be also moved to every possible permutation of electrodes. 
     For example,  FIGS. 3   a - 3   f  show, schematically, one method envisioned by the preferred embodiment for obtaining a resistivity map by various permutations of the position of current source  110  and volt meter  120 . In  FIG. 3   a , ammeter  112  and current source  110  are connected to the first and second electrodes  101 . Volt meter  120  is connected to ground electrode  100  and to the third electrodes in position  122   a . Readings are then taken as will be fully described later. While the volt meter remains connected to the ground electrode  100 , it is also connected to the fourth, fifth, sixth and seventh electrodes in order ( 122   b ,  122   c ,  122   d  and  122   e , respectively) and readings are taken at each connection. 
       FIG. 3   b  shows the ammeter and current source being moved to a second position between the second and third electrodes. The volt meter is then connected between the ground electrode  100  and first electrode  101  in position  123   a . It is then connected to the fourth, fifth, sixth and seventh electrodes in the array shown at positions  123   b ,  123   c ,  123   d  and  123   e , respectively. Readings are taken at each connection of the volt meter. 
       FIG. 3   c  shows the ammeter and current source being moved to a third position between the third and fourth electrodes. The volt meter is connected between the ground electrode  100  and first electrode  101  in position  124   a . It is then connected to the second, fifth, sixth and seventh electrodes in the array shown at positions  124   b ,  124   c ,  124   d  and  124   e , respectively. Readings are taken at each connection of the volt meter. 
       FIG. 3   d  shows the ammeter and current source being moved to a fourth position between the fourth and fifth electrodes. The volt meter is connected between the ground electrode  100  and first electrode  101  in position  125   a . It is then connected to the second, third, sixth and seventh electrodes in the array shown at positions  125   b ,  125   c ,  125   d  and  125   e , respectively. Readings are taken at each connection of the volt meter. 
       FIG. 3   e  shows the ammeter and current source being moved to a fifth position between the fifth and sixth electrodes. The volt meter is connected between the ground electrode  100  and first electrode  101  in position  126   a . It is then connected to the second, third, fourth and seventh electrodes in the array shown at positions  126   b ,  126   c ,  126   d  and  126   e , respectively. Readings are taken at each connection of the volt meter. 
       FIG. 3   f  shows the ammeter and current source being moved to a sixth position between the sixth and seventh electrodes. The volt meter is connected between the ground electrode  100  and first electrode  101  in position  127   a . It is then connected to the second, third, fourth and fifth electrodes in the array shown at positions  127   b ,  127   c ,  127   d  and  127   e , respectively. Readings are taken at each connection of the volt meter. 
     Other permutations of connections for injecting current and taking voltage readings between an array of electrodes are envisioned by the preferred embodiment. Those skilled in the art will recognize that the current can be injected at many different locations in a given array, not only those shown in  FIGS. 3   a ,  3   b ,  3   c ,  3   d ,  3   e  and  3   f . All possible permutations are also envisioned as embodiments of this preferred embodiment. 
     Continuing to  FIG. 4 , a schematic diagram is shown of the novel system of the preferred embodiment which allows for an array of nodes to dynamically reconfigure itself to impress current and take voltage readings in an array of electrodes. System  400  includes a power distribution unit (PDU)  401 , and a set of voltage control units (VCU)  402  comprised of N VCUs arranged in a serial chain such that VCU( 1 ) is connected to PDU  401 , VCU( 2 ) is connected to VCU( 1 ), VCU( 3 ) is connected to VCU( 2 ) and so on until VCU(N) is connected to the end of the serial chain, N being equivalent to the number of VCUs in the set of VCUs  402 . The VCUs in the set of VCUs  402  are connected to one another by a set of eight-conductor flexible cables  405 . Each VCU in the set of VCUs  402  has an electrode  101  to which it may supply a current at high voltage, return a current from another high voltage source, or measure an electric potential. VCU( 1 ) is referenced as the “head electrode” or “pole position” and VCU(N) is referenced as the “tail VCU” herein. 
     System  400  includes a data collection PC  404  with a data connection  403  between PDU  401  and data collection PC  404 . Data collection PC  404  is typically a Pentium class laptop personal computer running an operating system such as Windows. Data collection PC  404  is powered via the PDU  401 . Since system  400  is typically deployed in a remote area, it is helpful to have a power source for data collection PC  404 . Power connection  399  between data collection PC  404  and PDU  401  provides for DC power for data collection PC  404 . 
     A bi-directional communications link is provided between PDU  401  and the set of VCUs  402 . In the preferred embodiment, the bidirectional communications links utilize two conductors in the set of eight-conductor flexible cables  405 . With the data connection  403 , the bidirectional communications link establishes a communications channel between each VCU in the set of VCUs  402 , PDU  401  and the data collection PC  404 . 
     PDU  401  provides a 12V DC battery  406  which serves as the current source for all other functions of the system  400 . Alternate embodiments include multiple DC Batteries or AC-to-DC converters. In the preferred embodiment the 12V DC battery  406  is 12V deep cycle marine battery capable of storing 160 amp-hours of cranking time at approximately 20 amps. 
     In the preferred embodiment, a VCU is an intelligent node which has the capability to run tests independently of other VCUs in the system  400 . The data collection PC  404  is used to select a particular test configuration, to assimilate the data as it is sent from the VCUs as they run tests, and to process the assimilated data. 
     Referring to  FIG. 5 , a diagram of the construction of a PDU  401  is shown. PDU  401  is a power distribution unit containing power distribution electronics  420  for carrying out the logic functions for PDU  401 . PDU  401  has three power converters/supplies, a first DC-DC converter  422 , a second DC-DC converter  424  and a high voltage (HV) power supply  426 ; the output voltage lines of second DC-DC converter  424  and HV power supply  426  being connected into power distribution electronics  420  for further control. 
     PDU  401  has number of power supply related connections, namely DC power connector  407  which connects 12V DC battery  406  to the power converters/supplies, main on/off switch  410  which connects 12V DC power between DC power connector  407  and the power converters/supplies, HV on/off switch  411  which connects 12V DC power between the HV power supply and the main on/off switch  410 , HV reference port  417  for connecting ground electrode  100  to system  400 , and computer power port  416  which provides a port for connecting power between first DC-DC converter  422  and data collection PC  404 . Note that ground electrode  100  may be placed at a location physically remote from PDU  401  and connected by a single conductor cable to HV reference port  417 . 
     Continuing with input/output and control connections, PDU  401  has test start switch  412  which connects to power distribution electronics  420  for initiating a sequence of tests similar to the sequence of tests illustrated in  FIGS. 3   a - 3   f ; a USB port  414  also connected to power distribution electronics  420  for connecting PDU  401  to a USB port on data collection PC  404 ; a RS-232 port  413  also connected to power distribution electronics  420  for alternatively connecting PDU  401  to an RS-232 port on data collection PC  404 ; LED  409  connected to power distribution electronics  420  for visually signaling a system  400  state of operation; speaker  408  connected to power distribution electronics  420  for audibly signaling a system  400  state of operation; RS-485 communications link  438  for communicating downstream to set of VCUs  402  further comprised of a set of physical wires, “down+” wire  436  and “down−” wire  437  wherein RS-485 communications link  438  is connected and controlled by power distribution electronics  420 ; and control line  439  connected to power distribution electronics  420  used for controlling test functions. 
     PDU  401  functions to control a set of output power lines, HV DC power  431 , HV RET  432 , HV REF  433 , LV DC power  434  and LV DC return  435  for powering the set of VCUs and for providing HV current for tests. The set of power lines, control line  439 , “down+” wire  436  and “down−” wire  437  are connected to set of VCUs  402  by an output connector  440  which is further connected to one of 8-conductor flexible cables  405 . Table 1 is a list of signals being carried on the eight conductor flexible cable  405  in the preferred embodiment. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Flexible cable specifications 
               
             
          
           
               
                   
                   
                 MAX DC 
                   
                   
                 Strand- 
               
               
                 Wire 
                 Signal 
                 Voltage 
                 Current 
                 Gauge 
                 ing 
               
               
                   
               
             
          
           
               
                 1 
                 HV DC Power 431 
                 800 
                 2 
                 A 
                 22 
                 19 × 34 
               
               
                 2 
                 HV RET 432 
                 800 
                 2 
                 A 
                 22 
                 19 × 34 
               
             
          
           
               
                 3 
                 HV REF 433 
                 0 
                 0 
                 22 
                 19 × 34 
               
             
          
           
               
                 4 
                 LV DC power 434 
                 48 
                 2 
                 A 
                 22 
                 19 × 34 
               
               
                 5 
                 LV DC return 
                 48 
                 2 
                 A 
                 22 
                 19 × 34 
               
               
                   
                 435 (Gnd) 
               
               
                 6 
                 COM+ 
                 3.3 
                 100 
                 mA 
                 22 
                 19 × 34 
               
               
                   
                 (up/down+) 
               
               
                 7 
                 COM− 
                 3.3 
                 100 
                 mA 
                 22 
                 19 × 34 
               
               
                   
                 (up/down−) 
               
               
                 8 
                 Control line 439 
                 3.3 
                 100 
                 mA 
                 22 
                 19 × 34 
               
               
                   
               
             
          
         
       
     
     Power distribution electronics  420  contains a microprocessor  445 , an ammeter  446 , a digital to analog converter, DAC  447 , and a voltmeter  448 . Microprocessor  445  executes a pre-defined set of instructions to control the functions of HV power supply  426 , second DC-DC converter  424 , ammeter  446 , voltmeter  448 , test start switch  412 , LED  409 , speaker  408  and to aid in communications between VCUs  402  and data collection PC  404 . In particular for communications, power distribution electronics  420  generates “down+” wire  436  and “down−” wire  437 , “down+” wire  436  and “down−” wire  437  forming half-duplex differential RS-485 communications link  438  according to the TIA-485-A standard. Microprocessor  445  also translates signals to and from RS-485 communications link  438  to and from USB port  414  so that the set of VCUs  402  may communicate to and from data collection PC  404  using RS-485 communications link  438 . Power distribution electronics  420  also translates RS-485 communications link  438  to and from RS-232 port  413  which optionally provides a channel between RS-485 communications link  438  and data collection PC  404 . 
     Microprocessor  445  connects to second DC-DC converter  424  so that LV DC power  434  is controlled independently of main on/off switch  410 . Microprocessor  445  also connects to DAC  447  so that control of HV power supply  426  is accomplished whereby any set voltage between zero and the maximum output of HV power supply  426  is output to HV DC power  431 . 
     Ammeter  446  is connected between HV power supply  426  and HV RET  432  so as to measure the current being delivered to system  400  current electrodes  102 . Ammeter  446  is connected to and read by microprocessor  445  so that current measurements may be sent to data collection PC  404  in relation to a test. 
     Voltmeter  448  is connected to HV DC power  431  and to microprocessor  445 . Microprocessor  445  measures the output voltage of HV power supply  426 , for proportional control of HV DC power  431  and for test purposes. Microprocessor  445  measures voltage of LV DC power  434  in order to check the charge condition of 12V DC battery  406 . HV reference port  417 , HV REF  433 , and HV power supply  426  ground are connected together at line  418  to a reference ground point defined by the position of ground electrode  100  connected to HV reference port  417 . Additionally, line  418  is connected to 12V DC source negative input at DC power connector  407 , to local ground for power distribution electronics  420  and to the ground outputs of DC-DC converters  422  and  424 . 
     In the preferred embodiment the HV power supply is a 1C24-P250 from UltraVolt, Inc. with a maximum output voltage of 800V and a current limit of 2 A. 
       FIG. 6  is a block diagram of a VCU  450  in the set of VCUs  402 . The VCUs in the set of VCUs  402  are identical to VCU  450 . VCU  450  is connected to electrode  451  for impressing or returning a high voltage current or for measuring an electric potential. VCU  450  has upstream connector  441  for connecting power and RS-485 communications signals to upstream VCUs and/or PDU  401  via eight-conductor flexible cable  405 . For the purpose of description, “upstream connections” are shown in  FIG. 4  as from right to left toward PDU  401 . “Downstream connections” are shown in  FIG. 4  from left to right away from PDU  401 . VCU  450  has downstream connector  442  for connecting power and RS-485 communications signals to downstream VCUs via eight-conductor flexible cable  405 . The six power signals from the upstream connector  441  are passed through to the downstream connector  442 , the passed power signals being HV DC power  431 , HV RET  432 , HV REF  433 , LV DC power  434 , LV DC return  435 , and control line  439 . 
     Output connector  440 , upstream connector  441 , downstream connector  442  are Amphenol PT Series type connectors from Amphenol Corporation with service rating I, solder contact size 20 and shell size 12. A cable connecting receptacle, part number PT01J-12-8P003 from Amphenol, is used to terminate the eight-conductor flexible cables  405  and to plug into the connectors  440 ,  441  and  442 . Of course, other connector systems may be employed. 
     VCU  450  has a microcontroller  460 , having internal ROM and RAM  465  for storage in addition to flash memory  463 . RAM  465  is volatile while flash memory  463  is non-volatile. VCU  450  performs controlling and processing functions which are stored as predefined programs in ROM. An analog-to-digital converter, ADC  461 , is attached to and read by microcontroller  460 . ADC  461  has inputs attached to electrode  101  and to HV REF  433 . Microcontroller  460  is connected to a set of LEDs  456  which are powered on and off by microcontroller  460  as required, the set of LEDs having at least a red LED and a green LED. Microcontroller  460  is further connected to a set of RS-485 transceivers  467  via TX line  464  and RX line  462 . VCU  450  has register button  453  which is connected to microcontroller  460  and used for registering VCU  450  in a system start-up process. 
     ADC  461  is integrated into microcontroller  460  in the preferred embodiment wherein microcontroller  460  is preferably model MSP430 from Texas Instruments. ADC  461  essentially measures the potential difference between electrode  101  and HV REF  433 . 
     VCU  450  has a RS-485 transceiver  467  which is further comprised of two RS-485 transceiver ICs arranged in a back to back configuration suitable for regenerating pass-through RS-485 communication signals and for generating communication signals to and from the microcontroller  460 . Set of RS-485 transceivers  467  drive the “up+” wire and “up−” wire of RS-485 communications link  454  connected to upstream connector  441  in addition to the “down+” wire and “down−” wire  449  of RS-485 communications link  454  connected to downstream connector  442 . RS-485 communications link  454  to downstream connector  442 . In the case of VCU( 1 ), upstream connector  441  is connected by 8-conductor flexible cable  405  to output connector  440  of PDU  401 . In all other cases upstream connector  441  of VCU(i) is connected by a 8-conductor flexible cable  405  to the downstream connector  442  of VCU(i−1) for i=2, . . . N. In all cases, the “up+” wire of VCU(i) connects to the “down+” wire of VCU(i−1) and similar for the “up−” and “down−” wires. Maxim model MAX3535E is a suitable RS-485 transceiver IC in the preferred embodiment. 
     LV DC power  434  is connected to the common input of DC regulator  468 , the output of DC regulator  468  being connected to and providing regulated DC power  469  to microcontroller  460 , RAM  465 , flash memory  463 , LEDs  456  and RS-485 transceiver  467  of VCU  450 . Capacitor  455  is connected between ground at LV DC return  435  (by line  459 ) and regulated DC power  469 . Capacitor  455  is charged while LV DC power  434  is on and applied. During test operations LV DC power  434  is purposely turned off to reduce system noise. VCU  450  operating by drawing power from energy stored in charged capacitor  455 . 
     Solid state relays, first SSR  457  and second SSR  458 , contained within VCU  450  are used to connect or disconnect high voltage signals to and from electrode  451 . SSR  457  is connected to HV DC power  431  on one side and to electrode  451  on the other side, essentially opening and closing a current path between HV DC power  431  and electrode  451 . Input control to SSR  457  is connected to microcontroller  460  which independently signals SSR  457  to open or close. SSR  458  is connected to HV RET  432  on one side and to electrode  451  on the other side, essentially opening and closing a current path between HV RET  432  and electrode  451 . Input control to SSR  458  is connected to microcontroller  460  which independently signals SSR  458  to open or close. 
     In  FIGS. 22   a  and  22   b , circuit diagrams of the preferred embodiment of the solid state relays SSR  457  and SSR  458  are shown. SSR  457  controls the delivery of positive high voltage from HV DC power  431  to VCU electrode  451  which is connected to the vpin output  510 . SSR  457  circuit comprises isolation transformer  502 , charge pump  505 , transistor  506 , transistor  503 , transistor pair  507 , transient surge protector  508  and transient surge protector  509 ; the transient surge protectors providing protection in the case of lightning. Isolation transformer  502  is powered from DC regulator  468  at Vdd pin  500  and grounded through GND pin of isolation transformer  502  in conjunction with transistor  503 . Transistor  503  is connected to signal line  501   c . Signals  501   a ,  501   b  and  501   c  from microcontroller  460  drive SSR  457  into various states: signal  501   a  is pulsed on/off at about 1 kHz rate when energized and drives SSR  457  into a conducting state wherein current from HV DC power  431  is delivered to vpin output  510  and VCU electrode  451 ; an “on” signal at signal  501   b  cancels the previous conducting state, disconnecting HV DC power  431  from vpin output  510  and VCU electrode  451 ; signal  501   c  is used to disconnect SSR  457  from power when SSR  457  is not being used thereby saving VCU system power. 
     Isolation transformer  502  isolates the signals  501   a ,  501   b  and  501   c  from SSR  457  and generates a regulated output voltage at  504   a , the output voltage being 5V in the preferred embodiment. Charge pump  505  is connected to signal  501   a  through isolation transformer  502 . Charge pump  505  is further connected to the regulated output voltage  504   a . Charge pump  505  is a Dickson type charge pump comprised of capacitors C 1 , C 3 , C 8 , and C 9  and diodes D 1  and D 4 , the charge pump output voltage  504   b  being approximately 12V when the signal  501   a  is energized and the signal  501   b  is off. Resistors R 3  and R 6  are current limiters. Resistor R 18  and capacitor C 7  are connected so as to smooth output voltage  504   b  transient. When charge pump  505  is operating, the transistor pair  507  gates acquire a charge at 12V. At 12V, transistor pair  507  conducts causing HV DC power  431  to connect to vpin output  510 . When charge pump  505  is subsequently turned off by turning signal  501   a  off, the charge on transistor pair  507  floats and remains so that they continue to conduct, the transistors being of low leakage MOSFET type. When signal  501   b  is turned on transistor  506  is made to conduct and discharges transistor pair  507  so that they no longer conduct. HV DC power  431  is then isolated from vpin output  510 . Resistor R 17  and capacitor C 10  are connected to the input of transistor  506  to reduce transients on the input signal generated from signal  501   b.    
     SSR  458  operates the same way as SSR  457  having input signals  501   d ,  501   e  and  501   f  similar to respective input signals  501   a ,  501   b  and  501   c , but driven separately by microcontroller  460 . SSR  458  comprises transistor  513 , transformer  512 , charge pump  515 , transistor  516  transistor pair  517  tied to HV RET  432  and transient surge protector  518 . Charge pump  515  is connected to isolated signal  501   d  through isolation transformer  512 . Charge pump  515  is further connected to the regulated output voltage  514   a . Charge pump  515  is comprised of capacitors C 2 , C 4 , C 5 , and C 6  and diodes D 2  and D 3 , the charge pump output voltage  514   b  being approximately 12V when the signal  501   d  is energized and the signal  501   f  is off. Resistors R 2  and R 8  are current limiters and the resistor R 1  and capacitor C 7  is connected so as to smooth the voltage  504   b  transient. When charge pump  515  is operating, the transistor pair  517  gates acquires a charge at 12V and the transistor pair  517  conducts further causing HV RET  432  to connect to vpin output  510 . When charge pump  515  is subsequently turned off by turning signal  501   d  off, the charge on transistor pair  517  becomes floating and remains on the transistor pair  517  so that the transistors continue to conduct, the transistors being of low leakage MOSFET type. When signal  501   e  is turned on, transistor  516  is made to conduct and discharges transistor pair  517  so that they no longer conduct and HV RET  432  is then isolated from vpin output  510 . Resistor R 5  and capacitor C 11  are connected to the input of transistor  516  to reduce transients on the input signal generated from signal  501   e.    
     In the preferred embodiment isolation transformer is part number ADUM5240ARZ from Analog Devices; transistors  503 ,  506 ,  513  and  516  are type 2N7002; transistor pair  507  is IXTA3N120 from IXYS corporation; transient surge protectors  508 ,  509  and  518  are of type GTCA28-102k from Tyco Electronics; diodes D 1 , D 2 , D 3  and D 4  are Shottky-Barrier type transistors BAT54S from Fairchild Semiconductor. 
     SSR  457  and SSR  458  have the feature of latching into a conducting state for the duration of a test run without the need for further power consumption by working off of a floating charge. In the preferred embodiment, measurements may be made faster and with much less noise than in the prior art as each of the VCUs draws on locally stored capacitative power during a test run. The design of SSR  457  and SSR  458  enables such noise free operation. 
     Referring to  FIG. 6 , microcontroller  460  contains VCU software programs for executing test sequence instructions and for communicating with other system  400  components. VCU software programs are optionally recorded in ROM or may be stored in flash memory  463 . One function of the VCU software programs is to operate on a test sequence table stored in RAM  465 . The test sequence executes instructions that carry out a series of measurements including setting internal VCU connections to electrode  451 . Test sequence tables are discussed further below. 
     Since a 12 VDC battery is used as a current source for the tests accomplished with system  400 , it becomes a finite resource. The preferred embodiment contemplates conservation of the finite resource in the tradeoff between the number of tests that can be completed in a given time and the power consumption per test.  FIG. 7  is a graph  600  of power usage during a typical test cycle. Trace  602  is a trace of the HV DC power consumed by the electrodes in test. Trace  603  is a trace of the LV DC power consumed by the set of VCUs  402  during the same period. Trace  605  is a trace of the HV DC power consumed by PDU  401  including the data collection PC  404 . Setup time  611  defines test setup time as the time it takes for the system to ramp up the high voltage on a given electrode from zero to a preset voltage limit. Sample time  612  is the sample time in which a VCU makes an electric potential measurement. Data transfer time  613  is the data transmit time which is the time required to transmit the electric potential measurement data from the VCU to the data collection PC  404 . HV DC power  431  is powered down during a measurement. Test time for one test cycle is the sum of the setup time  611 , sample time  612  and data transfer time  613  is provided by the following equation:
 
Timecycle= T setup+ T sample+ T data.
 
As an example,
 
Timecycle typlar =2 sec+1 sec+0.035 sec=3.035 sec
 
     The number of test cycles determined according to the equation: 
             Testcycles   =         [     N   ×     (     N   +   1     )       ]     2     ×   polarity           
where,
         N is the number of VCUs in the set of VCUs  402 , and   Polarity=the number of times that the polarity is switched (e.g., in the preferred embodiment, polarity=2 (positive and negative)).       

     The required battery ratings for different values of N are given in TABLE 2. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Battery rating vs. number of tests. 
               
             
          
           
               
                   
                   
                 Number of test 
                   
                 Test 
               
               
                 Battery AH 
                 Number of 
                 sequences on 
                 Total number 
                 sequence 
               
               
                 rating 
                 VCUs 
                 one charge 
                 of tests 
                 duration 
               
               
                   
               
             
          
           
               
                 145 
                 100 
                 1 
                 10100 
                 8.5 
               
               
                 100 
                 80 
                 1.1 
                 6480 
                 5.5 
               
               
                 70 
                 50 
                 2.1 
                 2550 
                 2.1 
               
               
                 20 
                 30 
                 1.8 
                 930 
                 0.8 
               
               
                 10 
                 16 
                 3.1 
                 272 
                 0.2 
               
               
                 5 
                 8 
                 5.9 
                 72 
                 0.1 
               
               
                   
               
             
          
         
       
     
     In the data communications between set of VCUs  402  and data collection PC  404 , a data packet is transmitted at the end of a test cycle during data transfer time  613 .  FIG. 8  is a diagram of the data packet format  620  used in the preferred embodiment. A data packet is comprised of first test data byte  622 , second test data byte  624 , first miscellaneous data byte  626  and second miscellaneous data byte  628 . The test data is transmitted as a 16-bit value contained in first test data byte  622  and second test data byte  624 . The miscellaneous data bytes are designated for miscellaneous information, 2 bits of which are reserved for VCU version information. More than one 16-bit test data value may be taken during a test cycle when the environment is noisy, enough samples to reduce external measurement noise levels to an acceptable level. In most cases, disconnecting the LV DC power  434  from the VCU reduces measurement noise sufficiently. 
       FIGS. 9   a  and  9   b  show exploded isometric drawings of VCU  800  typical of the VCUs in set of VCUs  402 . The mechanical components of VCU  800  comprise an injection molded enclosure  801  into which a stainless steel plate  820  is molded so that a steel surface exists on the outside of enclosure  801  for contacting electrode  451 . Enclosure  801  has a cover  803  which secures to enclosure  801  with a set of screws inserted into a first set of molded holes  812  and compresses a rubber gasket  804  that prevents dust and moisture from entering enclosure  801 . A circuit board  802  carrying electronic components for accomplishing the function of VCU  800  is attached to enclosure  801  by screws inserted into a second set of molded holes  811 . Upstream connector  441  and downstream connector  442  are attached to enclosure  801  with screws inserted into a third set of holes  813 : a connector gasket  805  placed between downstream connector  442  and enclosure  801 ; a connector gasket  806  placed between upstream connector  441  and enclosure  801 . A permanent magnet  810  with a slot  814  is molded into a hanging tab  815  which is a molded feature to enclosure  801 . Permanent magnet  810  aids in maintaining a solid electrical and mechanical connection between the stainless steel plate  820  of VCU  800  and the electrode  451  as shown in  FIG. 9   a.    
       FIG. 10  shows an exploded isometric drawing of PDU  401 . PDU  401  has a first enclosure  816  comprised of PDU enclosure base  817  connected by a hinge to PDU enclosure lid  821 . PDU  401  contains PDU circuit board  822  which further comprises the power distribution electronics  420 , main on/off switch  410 , HV on/off switch  411 , test start switch  412 , RS-232 port  413 , USB port  414  and output connector  440 . First DC-DC converter  422 , second DC-DC converter  424  and HV power supply  426  are securely fastened beneath circuit board  822  to the PDU enclosure base  817 . Circuit board  822  is fastened to first enclosure  816  just underneath PDU enclosure lid  821  so that a front panel interface is accessed upon opening PDU enclosure lid  821 , the front panel interface allowing ease of access to main on/off switch  410 , test start switch  412 , RS-232 port  413 , USB port  414 , DC power connector  407 , and computer power port  416 . The 12V DC battery  406  is attached externally to the DC power connector  407  in the preferred embodiment. First enclosure  816  with enclosure base  817  and enclosure lid  821  is realized by Pelican model 1400 case purchased from Pelican Products Inc. in the preferred embodiment. 
     In an alternate embodiment of the preferred embodiment, the DC battery is stored internal to PDU  401 .  FIG. 11  shows an exploded isometric drawing of PDU  401  for use with an integrated rechargeable DC battery. PDU  401  has second enclosure  830  comprised of second PDU enclosure base  840  connected by a hinge to second PDU enclosure lid  841 . Second enclosure  830  contains PDU circuit board  842  which further comprises the power distribution electronics  420 , main on/off switch  410 , HV on/off switch  411 , test start switch  412 , RS-232 port  413 , USB port  414  and output connector  440 . 12V DC battery  406 , first DC-DC converter  422 , second DC-DC converter  424  and HV power supply  426  are securely fastened beneath circuit board  842  to the second PDU enclosure base  840 . Circuit board  842  is fastened to second enclosure  830  just underneath second PDU enclosure lid  841  so that a front panel interface is accessed upon opening second PDU enclosure lid  841 , the front panel interface allowing ease of access to HV on/off switch  411 , test start switch  412 , RS-232 port  413 , USB port  414 , DC power connector  407 , and computer power port  416 . 12V DC battery  406  is attached securely to second PDU enclosure base  840  and has an external recharging port  845  on the outside of second enclosure  830  for charging 12V DC battery  406 . Second enclosure  830  with second PDU enclosure base  840  and second PDU enclosure lid  841  is realized by Pelican model 143 Top Loader Case purchased from Pelican Products Inc. in the preferred embodiment. 
     Turning to the embedded software on the PDU and VCUs, PDU  401  has a self-test software program in memory that initiates and runs when DC power is applied. PDU self-test  960 , diagrammed in  FIG. 12 , initiates at system power up event  961  wherein a 12V DC battery is connected to PDU  401  and PDU self-test  960  runs until PDU  401  no longer has DC power applied. The 12V DC battery voltage is checked in step  962 . If the 12V DC battery voltage is low, then a signal is sent from PDU  401  to data collection PC  404  in step  963 . A warning signal is displayed using PDU LEDs and displayed in data collection PC  404  in step  964 . PDU  401  and system  400  is then powered down automatically in step  965 . If the battery voltage is acceptable in step  962 , then the PDU waits a predetermined time interval in step  966 . After the predetermined time interval, the check battery voltage step  962  is repeated. The predetermined time interval is two minutes in the preferred embodiment. The acceptable battery voltage is consistent with a state of charge sufficient to support approximately five minutes of discharge at the current discharge rate: for example, V must be greater than 11.0V for a discharge rate of C/10 corresponding to about 5 minutes of remaining discharge time for a 160 A-h (amp hour) battery. 
     PDU  401  monitors test start switch  412  in step  967 . If test start switch  412  is pressed then a signal is sent from PDU  401  to data collection PC  404  in step  968 . PDU  401  monitors USB port  414  for a signal from data collection PC  404  in step  969 . A “stop” signal received from data collection PC  404  causes PDU  401  to stop a test run in step  809 . The test run is stopped by turning off all high voltage and low voltage power to all downstream VCUs. A “run” signal received from data collection PC  404  simulates test start switch  412  being pressed: PDU  401  operates to run a test according to test method  1000  and run method  909  described in relation to  FIGS. 15   a ,  15   b  and  16  below. 
     A set of concurrent and continuously running self-tests are implemented as a software program and caused to operate on each VCU of the set of VCUs  402  when LV power is applied to the set of VCUs  402 .  FIG. 13  shows a flow chart for VCU self-test  970 . 
     At power on event  971 , VCU self-test  970  begins by verifying a given VCU&#39;s firmware integrity in step  972 . If the firmware is verified to be in good working order, then VCU self-test  970  proceeds to verify storage memory integrity of the given VCU in step  974 . If both the firmware and the storage memory are good then the VCU self-test  970  continues to step  978  wherein a green flashing LED  978  is displayed, after which a software loop is entered to continuously check the given VCU&#39;s register button at step  979 . If the register button is pressed then the given VCU is registered in step  980 . 
     If firmware integrity is bad at step  972 , then a corresponding audible tone is generated on speaker  408  in step  975 . An appropriate error code is displayed using LED  409  in step  976 . Also, if storage memory integrity is bad then a corresponding audible tone is generated on speaker  408  in step  975  and an appropriate error code is displayed on LED  409  in display error step  976 . 
       FIG. 14  is a flowchart of VCU registration step  980  which is implemented on each VCU as a software program initiated by pressing the register button on a given VCU in event  981 . A random address ID  911  is generated in step  982  wherein an address is generated from a software controlled random number generator in the given VCU microcontroller. The given VCU then broadcasts the generated address ID  911  to all VCUs in step  983 . The given VCU monitors its RS485 communication RX port in step  984  taking actions based on steps  985 ,  986  and  987 . The RS485 communications RX port is checked in step  985  for a received character in response to the broadcasted address ID  911 . If a character is received then step  986  is performed with the received character  910 . If a character is not received then step  985  is repeated. If after a predefined elapsed time, a character still has not been received, a timeout occurs and step  988  is performed. If the received character  910  is a NAK character as checked in step  986 , then the address ID  911  is invalid and step  982  is repeated, generating a new address ID. If the received character  910  is not a NAK character then step  987  checks received character  910  to signify a valid position character n, for example, corresponding to a number in the range [1,255]. If n is not valid, then step  983  is repeated. If n is valid, then step  998  is performed. 
     In step  998 , wherein position character n is valid, the given VCU position number is set equal to n and stored in the given VCU memory. The given VCU is now registered as VCU(n). A green LED is lit continuously in step  999  to visibly indicate a valid registration. In step  997 , the given VCU executes software code to wait for a test sequence to be received by the RS485 communications RX port. 
     In step  988 , following the occurrence of a timeout in step  985 , the given VCU automatically sets its position number equal to 1 and is now registered as VCU( 1 ). Furthermore, in step  989 , VCU( 1 ) initializes the total number of VCUs connected, nVCUs, to a value of 1. A green LED is lit continuously in step  990  to indicate a valid registration. Then VCU( 1 ) executes two software loops: first software loop step  997  listens for a test sequence to be received by the RS485 communications RX port; and second software loop in step  992  listens on RS485 communications RX port for broadcasted address IDs from requesting VCUs as their register buttons are activated. VCU( 1 ) functions as a VCU registrar according to the steps that follow. 
     When an address ID is received in step  992 , the received address ID  912  is checked for validity in step  993 . Checking for validity is done by comparing address ID  912  with list of address IDs  959 : if a matching address ID exists then address ID  912  is invalid. If a matching address ID does not exist, then address ID  912  is valid and is stored in address list  959  with its position number, the position number being equal to nVCUs after the nVCUs is incremented in step  994 . In step  995  the position number is broadcast on VCU( 1 )&#39;s RS485 communications TX port indicating a position number n=nVCUs to the VCU that requested address ID  912 . In step  996 , a NAK is broadcast on VCU( 1 )&#39;s RS485 communications TX port, indicating an invalid address ID to the requesting VCU. 
     The handling of test sequence configurations by the set of VCUs  402  is explained as follows. In the preferred embodiment, identical test sequence tables are preloaded into each RAM  465  and executed as a set of instructions on each local microcontroller  460 . In an alternate embodiment, the test sequence tables are stored in data collection PC  404  and a selected test sequence table downloaded to each VCU as a step in the test setup process. 
     Test sequence tables are comprised of rows and columns, the kth row defining a test with test number k, and ith column defining a test sequence for VCU(i). The value of the (k, i) entry in the table identifies one of three possible test configurations for VCU(i) while running test k: a value of “+1” identifies a HV current source configuration; a value of “−1” identifies a HV current sink configuration; and a value of “0” (zero) identifies a HV measurement configuration. In the HV current source configuration, HV DC power  431  is connected to the local electrode  451 . In the HV current sink configuration, HV RET  432  is connected to the local electrode  451 . In the HV measurement configuration, ADC  461  is connected to the local electrode  451 . Table 3 shows an example test sequence table. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Example of a TEST SEQUENCE TABLE 
               
             
          
           
               
                   
                 VCU(1) 
                 VCU(2) 
                 VCU(3) 
                 VCU(4) 
                 VCU(5) 
                 VCU(6) 
                 VCU(7) 
                 VCU(8) 
                 VCU(9) 
                 VCU(10) 
               
               
                   
                   
               
             
          
           
               
                 Test1 
                 0 
                 0 
                 0 
                 0 
                 +1 
                 −1 
                 0 
                 0 
                 0 
                 0 
               
               
                 Test2 
                 0 
                 0 
                 0 
                 +1 
                 0 
                 0 
                 −1 
                 0 
                 0 
                 0 
               
               
                 Test3 
                 0 
                 0 
                 +1 
                 0 
                 0 
                 0 
                 0 
                 −1 
                 0 
                 0 
               
               
                 Test4 
                 0 
                 +1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 −1 
                 0 
               
               
                 Test5 
                 +1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 −1 
               
               
                 Test6 
                 −1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 +1 
               
               
                   
               
             
          
         
       
     
     According to Table 3, for example, “Test1” in row 1 has VCU( 5 ) electrode connected to HV DC power  431 , VCU( 6 ) electrode connected to HV RET  432  and all other VCUs taking electric potential measurements of their electrodes; “Test2” in row 2 has VCU( 4 ) electrode connected to HV DC power  431 , VCU( 7 ) electrode connected to HV RET  432  and all other VCUs taking electric potential measurements on their electrodes; “Test6” in row 6 wherein “Test6” has VCU( 10 ) electrode connected to HV DC power  431 , VCU( 1 ) electrode connected to HV RET  432  and all other VCUs taking electric potential measurements on their electrodes. 
     Further analyzing the test sequences of Table 3: VCU( 1 ) executes a test sequence of [0,0,0,0,+1,−1] configurations; VCU( 2 ) executes a test sequence of [0,0,0,+1,0,0] configurations; and so forth. VCU( 1 ) will configure itself to take an electric potential measurement in the first four tests (k=1, 2, 3 and 4) then configure itself to connect HV power  431  to its electrode becoming the HV current source in the fifth test (k=5) and then configure itself to connect HV ret  421  to its electrode becoming the HV current sink in the sixth test (k=6). The other VCUs configure their test sequences according to the columns of Table 3. 
     The example given in Table 3 is intended to be a non limiting example. A vast number of test sequence tables may be constructed in the present invention, each sequence table constructed according to the number of VCUs, the geometry of the VCUs, and the test configurations required by a user&#39;s analysis methods. 
     Test method  1000  is explained according to the sequence diagrams of  FIGS. 15   a  and  15   b . The sequence diagrams show interactions between system entities in sequential order with time. Software programs to support the interactions and their sequential order in test method  1000  are implemented on data collection PC  404 , PDU  401 , and set of VCUs  402 . The system entities of data collection PC  404 , PDU  401 , and set of VCUs  402  indicated by VCU( 1 )  1003   a , VCU( 2 )  1004   a  and VCU(m)  1005   a  have respective timelines  1001   b ,  1002   b ,  1003   b ,  1004   b  and  1005   b  associated with them. Time progresses from top to bottom along the timelines. As time progresses, blocks on a given timeline appear indicating that a software program executes steps to perform a function by the system entity associated to the timeline. Messages are shown as horizontal lines with arrows, a solid line indicating a request and a dashed line indicating data sent. 
     Prior to test method  1000 , set of VCUs  402  have executed VCU self test  970  and register method so that step  997  of listening for a test sequence is functioning. Also, for PDU  401 , the PDU self test  960  has been executed so that the battery is being checked, the test button is being checked and the communications from data collection PC  404  is being monitored. The invocation of the methods  960 ,  970 ,  980  are not shown in  FIGS. 15   a  and  15   b  because said methods are independent of the test method  1000 . Also, a set of test sequence tables have been pre-stored in the flash memory of all VCUs in set of VCUs  402 . 
     Beginning from the left of the diagram in  FIG. 15   a , the data collection PC  404  when stimulated by a run command event  1006  will begin to operate a test setup program  1007  which is a menu driven program by which data collection PC  404  displays a set of options allowing an operator to choose to do any of the following: enter VCU coordinates in step  1011 , enter a test filename in step  1012 , or select a test sequence in step  1013 . Run command event  1006  is typically invoked by clicking on a program icon in the Windows operating system to have the operating system begin execution of the associated test setup program, although the program may be started in any number of ways known in the art. The menu driven program is a Windows program which may be written in Visual Basic or a similar Windows programming environment also known in the art. When a menu item is invoked by selection a corresponding subroutine is executed on the PC. 
     Upon invoking step  1011  to enter VCU coordinates, message  1008  is sent from data collection PC  404  to VCU( 1 )  1003   a  to obtain the number of VCUs in the system. VCU( 1 )  1003   a  operates subroutine get VCUs  1009  which returns the number of VCUs, nVCUs  1010 . Step  1011  continues by gathering coordinate data for each VCU in the system. In the preferred embodiment, the coordinate data is gathered by entering coordinates into a spreadsheet that is opened when step  1011  is invoked. In an alternate embodiment, step  1011  may cause a coordinate data file to be downloaded from a handheld computer device. 
     Upon invoking step  1012  to enter a test filename, data collection PC  404  presents a dialog box in which a user may browse the computer file system for a folder and then the user may do one of: selecting an existing filename or typing a new filename. In an alternate embodiment, step  1012  may automatically generate a filename in a default folder in the computer file system of PC  404 . 
     Upon invoking step  1013  to select a test sequence table, data collection PC  404  presents a menu box in which a user may select a test sequence table from a menu of test sequence tables. Each test sequence table has an associated test sequence ID  1018 . Test sequence ID  1018  for the selected test sequence table is stored by step  1013  for later use by test method  1000 . 
     Once the steps  1011 ,  1012  and  1013  have been executed in test setup program  1007 , background process  1014  begins to wait for a test button event. A test button event occurs when test start switch  412  is pressed on PDU  401  indicated by event  1015 . On event  1015 , PDU  401  executes subroutine  1016  that returns signal  1017  indicating that test start switch  412  has been pressed. Upon receiving signal  1017 , test setup program  1007  causes data collection PC  404  to broadcast test sequence ID  1018  to the set of VCUs  402  via broadcast message  1020 . 
     At time  1021   a  VCU( 1 ) receives broadcast message  1020  containing test sequence ID  1018 . VCU( 1 ) then executes subroutine  1021   b  to retrieve the test sequence table associated to test sequence ID  1018  from its flash memory. In subroutine  1021   c , VCU( 1 ) loads column  1  from the associated sequence table as test sequence  1021   d  for execution. After subroutine  1021   c , VCU( 1 ) waits for HV DC power  431  to change from 0V to 48V in subroutine  1021   e.    
     Similarly, at time  1022   a  VCU( 2 ) receives broadcast message  1020  containing test sequence ID  1018 . VCU( 2 ) then executes subroutine  1022   b  to retrieve the sequence table associated to test sequence ID  1018  from flash memory. In subroutine  1022   c , VCU( 2 ) loads column  2  from the associated sequence table as test sequence  1022   d  for execution. After subroutine  1022   c , VCU( 2 ) waits for HV DC power  431  to change from 0V to 48V in subroutine  1022   e.    
     Accordingly, at time  1025   a  VCU(m) receives broadcast message  1020  containing test sequence ID  1018 . VCU(m) then executes subroutine  1025   b  to retrieve the sequence table associated to test sequence ID  1018  from flash memory. In subroutine  1025   c , VCU(m) loads the mth column from the associated sequence table as the test sequence  1025   d  for execution. After subroutine  1025   c , VCU(m) waits for HV DC power  431  to change from 0V to 48V in subroutine  1025   e.    
     All other VCUs in the set of VCUs  402  operate programs similar to those given for VCU(m). After a predefined elapsed time from broadcast  1020 , run method  909  begins. 
       FIG. 15   b  shows an alternative embodiment of the present invention wherein test sequence tables are not pre-stored in flash memory on the set of VCUs. Instead, data collection PC  404  broadcasts a complete test sequence table which is downloaded by each VCU in the set of VCUs. The alternative embodiment of  FIG. 15   b  offers advantages in flexibility, since new test sequences may be constructed in the field to better fit the testing needs. For example, a pre-stored set of tables may not optimally fit the geometry or time requirements for a given test situation. 
     Beginning from the left of the diagram in  FIG. 15   b , the data collection PC  404  when stimulated by a run command event  1036  will begin to operate a test setup program  1037  which is a menu driven program by which data collection PC  404  displays as a set of options allowing an operator to choose to do any of the following: enter VCU coordinates in step  1041 , enter a test filename in step  1042 , or select a test sequence in step  1043 . Run command event  1036  and menu driven program are similar to those described in  FIG. 15   a.    
     Upon invoking step  1041  to enter VCU coordinates, message  1038  is sent from data collection PC  404  to VCU( 1 )  1003   a  to obtain the number of VCUs in the system. VCU( 1 )  1003   a  operates subroutine get VCUs  1039  which returns the number of VCUs, nVCUs  1040 . Step  1041  continues by gathering coordinate data for each VCU in the system. In the preferred embodiment, the coordinate data is gathered by entering coordinates into a spreadsheet that is opened when step  1041  is invoked. In an alternate embodiment, step  1041  may cause a coordinate data file to be downloaded from a handheld computer device. 
     Upon invoking step  1042  to enter a test filename, data collection PC  404  presents a dialog box in which a user may browse the computer file system for a folder and then the user may do one of: selecting an existing filename or typing a new filename. In an alternate embodiment, step  1042  may automatically generate a filename in a default folder in the computer file system of data collection PC  404 . 
     Upon invoking step  1043  to select a test sequence table, data collection PC  404  presents a menu box in which a user may select a test sequence table from a menu of test sequence tables. The selected test sequence table  1048  is stored by step  1043  for later use by test method  1000 . 
     Once the steps  1041 ,  1042  and  1043  have been executed in test setup program  1037 , background process  1044  begins to wait for a test button event. A test button event occurs when test start switch  412  is pressed on PDU  401  indicated by event  1045 . On event  1045 , PDU  401  executes subroutine  1046  that returns signal  1047  indicating that test start switch  412  has been pressed. Upon receiving signal  1047 , test setup program  1037  causes data collection PC  404  to broadcast test sequence table  1048  to the set of VCUs  402  via broadcast message  1050 . 
     At time  1051   a  VCU( 1 ) receives broadcast message  1050  containing the test sequence table  1048 . VCU( 1 ) then executes subroutine  1051   b  to download test sequence table  1048  into VCU( 1 ) random access memory. In subroutine  1051   c , VCU( 1 ) loads column  1  from test sequence table  1048  as test sequence  1051   d  for execution. After subroutine  1051   c , VCU( 1 ) waits for HV DC power  431  to change from 0V to 48V in subroutine  1051   e.    
     Similarly, at time  1052   a  VCU( 2 ) receives broadcast message  1050  containing test sequence table  1048 . VCU( 2 ) then executes subroutine  1052   b  to download test sequence table  1048  into VCU( 2 ) random access memory. In subroutine  1052   c , VCU( 2 ) loads column  2  from test sequence table  1048  as test sequence  1052   d  for execution. After subroutine  1052   c , VCU( 2 ) waits for HV DC power  431  to change from 0V to 48V in subroutine  1052   e.    
     Accordingly, at time  1055   a  VCU(m) receives broadcast message  1050  containing test sequence table  1048 . VCU(m) then executes subroutine  1055   b  to download test sequence table  1048  into VCU(m) random access memory. In subroutine  1055   c , VCU(m) loads the mth column from test sequence table  1048  as test sequence  1055   d  for execution. After subroutine  1055   c , VCU(m) waits for HV DC power  431  to change from 0V to 48V in subroutine  1055   e.    
     All other VCUs in the set of VCUs  402  operate programs similar to those given for VCU(m) in  FIG. 15   b  for the alternate embodiment. After a predefined elapsed time from broadcast message  1050 , run method  909  begins. 
       FIG. 16  is a sequence diagram of run method  909 . The steps described in run method  909  are implemented as software programs on the system entities of data collection PC  404 , PDU  401  and set of VCUs  402 . In particular, the program steps taken by VCU( 1 ) in the embodiments of the present invention are preprogrammed into all VCU units, so that any VCU unit that happens to be positioned as VCU( 1 ) (or any other position VCU(n)) may operate accordingly. 
     In the description of run method  909 , it should be understood that stored test sequences such as test sequences  1021   d ,  1022   d ,  1025   d ,  1051   d ,  1052   d  and  1055   d  exist in all of the set of VCUs  402  and are used in run method  909 . 
     Run method  909  starts with the step  1060  in which PDU  401  applies 48V to HV DC power  431  to signal  1061  the initiation of a test run. In other embodiments, a voltage other than 48V (such as a range, for example, of about 0.1 volts to about 50 volts) may be used in step  1060 . Upon receiving signal  1061 , the VCUs each turn on a red flashing LED in step  1062 . The red flashing LED provides an alert that high voltage is being applied to one of the electrodes and a test is underway. VCU( 1 ) in step  1063 , sends VCU address list  959  to data collection PC  404  in order of VCU( 1 ), VCU( 2 ), . . . VCU(n). VCU( 1 ) then requests that PDU  401  turn HV DC power  431  to off (zero V) in step  1064  followed by the step  1065  of initializing T, the number of tests in the test sequence, and k, the current test number. In step  1065 , T is set equal to the number of configurations in its stored test sequence  1021   d  and  k  is set equal to 1 (one). 
     Run method  909  continues in loop  1070  (designated by a box structure with the label “loop” in the upper lefthand corner) of  FIG. 16 . All sequential operations inside of loop  1070  are repeated while condition  1067  is true, condition  1067  being that k is less than or equal to T. In step  1068 , the current test number k is sent to the set of VCUs  402  by VCU( 1 ). VCU( 1 ) then instructs each VCU to load the kth test configuration from its stored test sequence. VCU( 1 ) then invokes start_test( ) on PDU  401  in signal  1069  whereupon PDU  401  raises control line  439  in step  1072 . PDU  401  sets its LED flashing red in step  1074 . 
     Raising the control line in step  1072  is a signal to the set of VCUs to connect their electrodes. The VCU electrodes are connected at step  1075  according to their loaded configurations from previous step  1068 . The two VCUs, one with HV DC power  431  connected, the other with HV RET  432  connected leave their red LEDs flashing while all other VCUs turn off their red LEDs and turn on their green LEDs to flashing. PDU  401  then ramps the HV DC power  431  to its maximum voltage in ramp HV step  1076 , the maximum voltage being the voltage limit of HV power supply  426  or the voltage at which the current limit of HV power supply  426  is reached, whichever is reached first. In the preferred embodiment, the high voltage is between about 250 volts and about 2,000 volts. PDU  401  then sets the control line low in step  1078  and disconnects DC-DC converter  424  from LV DC power  434  in step  1080 , so that local VCU power reverts to stored capacitive power in each VCU. After an approximately 100 ms time delay, the VCUs in measurement configuration operate their respective ADCs  461  to measure and record the electric potential on their corresponding electrodes  451  in VCU measurement step  1084 . During the VCU measurement step  1084 , PDU  401  measures HV current, I, and HV voltage, V, of HV DC power  431  and HV RET  432  circuit in step  1085 . Referring back to step  1084 , an electric potential measurement is also performed in step  1084  by the two VCUs with HV DC power  431  connected and with HV RET  432  connected, the latter measurement being done to correct for any voltage drops between either of the two VCUs and the PDU where the HV current is sourced and sunk. 
     Run method  909  continues at step  1088  when PDU  401  reconnects LV DC power  434 . In step  1090 , PDU  401  disconnects HV DC power  431 . In step  1091 , the two VCUs connected to HV DC power  431  and HV RET  432  disconnect their electrodes after sensing HV power is off. Also, the two VCUs HV put their green LEDs to flashing after turning off their red LEDs. In step  1092 , PDU  401  turns off its red LED and turns on a flashing green LED. 
     In data transfer  1093 , PDU  401  sends HV current, I, and HV voltage, V, to data collection PC  404 . In step  1094 , data collection PC  404  accepts the I, V data sent in step  1093  and accepts a set of data transfers  1095  from the set of VCUs wherein VCU( 1 ), VCU( 2 ), VCU(m) report their electric potential measurements obtained in previous step  1084 . In the preferred embodiment, step  1095  is accomplished using a protocol with predefined sequential time slots for data transfer over the RS-485 communications link wherein VCU( 1 ) reports data in time slot  1 , VCU( 2 ) reports data in time slot  2 , . . . , VCU(m) reports data in time slot m up to m=nVCUs. Data collection PC  404  then collects and stores the reported data into stored test data  1012  in step  1096 . 
     In step  1098 , run method  909  increments test number k; loop  1070  is repeated if k less than or equal to T according to condition  1067 . If k is greater than T, then all tests in the test sequence are complete and run method  909  causes VCU( 1 ) to broadcast a shutdown command  1099  to all VCUs and PDU  401 . The set of VCUs respond to shutdown command  1099  by turning off their LEDs in step  2000 . VCU( 1 ) then sends test_complete( ) command  2001  to data collection PC  404  and to PDU  401 . PDU  401  then responds to test_complete( ) command  2001  by turning off LV DC power  434  to set of VCUs  402  at step  2002 . Step  2002  completes the run method  909 . 
     A diagram of the preferred method of operation  900  for a preferred embodiment of system  400  is shown in  FIG. 17 . Method of operation  900  requires participation of a field operator  902  interacting with and performing a number of operations, in order: safety operation  904 , system setup operation  905 , system start operation  906 , self test operation  907 , test method  1000  and analysis  1700 . 
       FIG. 19  shows a flow chart of safety operation  904 . In safety operation  904 , field operator  902  is responsible for transport and setup of system  400 . Safety operation  904  begins with step  914  of transporting system  400 , eight-conductor flexible cables  405 , set of VCUs  402 , 12V DC battery  406 , data collection PC  404 , and electrodes  451  to a field site for setup. A safety inspection is then performed in step  915  in which system  400  is inspected for damages, the inspection including all power cords and eight-conductor flexible cables  405  to ensure that the insulation on said cords and cables is not damaged. If any damage conditions exist, the damaged part is replaced in step  916 . While seemingly a detail, this step provides a critical benefit of the preferred embodiment. In this step, any defective PDU may be replaced by another PDU and any defective VCU may be replaced by another VCU because the system components are interchangeable. This advantage allows transportation of less overall equipment than prior art systems with duplicated components and cabling. 
     If no damaged parts exist or damaged parts are replaced in step  916 , safety operation  904  continues with step  917 . In step  917 , the test site survey area is marked with warning signs. Safety operation  904  includes step  918  of checking for thunderstorms in the vicinity of the test site. If a thunderstorm or rainy condition is impending then the test operation  900  is aborted in step  919  and if no thunderstorms or rainy conditions exist, test operation  900  proceeds with system setup operation  905 . 
     System setup operation  905 , is described in further detail with reference to  FIG. 23 . At step  921 , a tape measure is placed along the survey line, and marks are placed at predetermined spacings along the survey line forming marked locations  932 . Then electrodes  101 , which are typically metal stakes, are driven into the ground at marked locations  932  in step  922 . While staking in step  922 , field operator  902  manually records the positional coordinates of each electrode stake location in step  931 . The recorded positions  931  are kept for use later in the test process. 
     Recorded positions  931  of each electrode stake may be written on paper or alternatively stored into a personal digital assistant, PDA, or other suitable handheld computing device. In another embodiment of step  922 , a GPS enabled handheld computing device may be held in collocation with the marked locations  932  to store recorded positions  931  wherein GPS coordinates of the electrode stake are entered automatically into the handheld computing device. 
     In step  923 , the VCUs in the set of VCUs  402  are laid out in order from upstream to downstream near the marked locations  932 . Eight-conductor flexible cables  405  are connected from each VCU downstream connector  442  to each VCU upstream connector  441 . After step  923  is complete, second cabling step  924  is performed wherein the eight-conductor flexible cables  405  are attached between PDU  401  output connector  440  and VCU( 1 ) upstream connector  441 . After step  924  is complete, step  925  connects 12V DC battery  406  to PDU  401  via DC power connector  407  and then in step  926 , data collection PC  404  is connected to PDU  401  wherein data collection PC  404  power is connected to computer power port  416  and data collection PC  404  USB port is connected to USB port  414  for data communications. In step  927 , data collection PC  404  is turned “on”. Optionally, in step  926 , data collection PC  404  is connected to RS-232 port  413  for data connection to PDU  401 . 
     The main on/off switch  410  is turned “on” in step  928  which results in low voltage power being applied to the set of VCUs  402 . A flashing green LED on a given VCU at step  930  indicates that the given VCU is correctly cabled and powered, having run self-test  970 . System setup operation  905  concludes with a system integrity check in step  929 . In step  929 , field operator  902  checks VCU LEDs to be flashing green. If there is a misconnected or damaged item the affected LED will not be lit and flashing. The misconnection or damaged item is replaced in step  934  and the new item is reconnected in step  935 . Step  929  is repeated until the set of VCUs  402  are all checked and working properly. System  400  is now in a state in which the set of VCUs are cabled together and are positioned next to their respective stakes, the set of VCUs having power and communications connections to the PDU, the PDU operatively connected to a DC battery and to a PC, and the VCUs having LV power applied to them. The system is then started by proceeding with system start operation in step  906 . 
     Step  906  is further described in reference to  FIG. 24 .  FIG. 24  begins at step  940  where VCU( 1 ) is attached to its electrode  451 . It is significant to test operation  900  that field operator  902  begin system start operation  906  with VCU( 1 ) and work downstream away from PDU  401  to attach subsequent VCUs to their respective electrodes in downstream order. Register button  453  on VCU( 1 ) is pressed in step  941  causing VCU( 1 ) to run VCU register method  980 . As a result of register method  980 , VCU( 1 ) recognizes that it is the first VCU in the set of VCUs to be connected in step  942  having assigned position  943  to m=1. The number of VCUs in the string is also set to 1 at step  944 . 
     System start operation  906  continues as all of the set of VCUs  402  to their electrodes  451  in downstream order. The next downstream VCU, VCU( 2 ) is attached to its electrode in step  950  after which VCU( 2 )&#39;s register button  453  is pressed in step  951 . Step  951  causes VCU( 2 ) to run VCU register method  980 . In step  952 , the result of register method  980  being that VCU( 1 ) increments the number of VCUs in the system, nVCUs, ( 945 ) stores an address ID for VCU( 2 ) and gives VCU( 2 ) its position number m=2 ( 946 ). At step  948 , the system waits a predetermined period of time to determine if other VCUs exist and require registration. If so, the system loops to step  950 . Steps  950 ,  951  and  952  are repeated for VCU( 3 ), VCU( 4 ), VCU(m) including all of the set of VCUs  402 . 
     After the set of VCUs  402  are connected and registered, test is initiated by executing test method  1000 . Test method  1000  is used repeatedly to complete a set of test runs which results in a set of test data  1012 . The test data is analyzed according to analysis method  1700 . 
     To analyze the set of test data  1012  gathered by system  400 , the steps at  FIG. 18  are followed. Referring to  FIG. 18 , a flow chart is shown depicting the steps involved in producing the subgrade resistivity map. In step  1701 , data collection PC  404  calculates electrical resistivity from the measured voltages and currents in test data  1012  and stores them into a file resistivity data  1702  according to the following equation: 
     
       
         
           
             
               ρ 
               
                 n 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 m 
               
             
             = 
             
               K 
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                   V 
                   
                     n 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     m 
                   
                 
                 I 
               
             
           
         
       
     
     Where, 
     K is an electrode geometric constant, 
     Vnm is the electric potential difference between two electrodes, electrode at position n and electrode at position m, 
     I is the injected current which corresponds to the measured HV current, and 
     □nm is the current density between the electrodes at positions n and m. 
     The data collection PC  404  also calculates, based on the known locations of the current electrodes  102  and voltage electrodes  103 , virtual resistivity locations corresponding to the calculated resistivity values. In step  1703 , the data collection PC  404 , using SWIFTCNV software from SAGA Geophysics, or an equivalent, sorts the virtual resistivity locations and resistivity data from resistivity data  1702  into a number of discrete sets  1712 , each consisting of a spatial set of coordinates and a resistivity values at those coordinates. 
     Next, in step  1704 , the data collection PC  404  performs a least squares data inversion analysis on the discrete sets  1712  to create a first electrical resistivity model  1705 . To perform this least squares data inversion, software such as RES2DINV by SAGA Geophysics may be used; however, a number of programs are available that can perform the least squares inversion and produce a two- or three-dimensional graphical output. First electrical resistivity model  1705  minimizes the error of the field data.  FIG. 20  is an example of a two-dimensional graphical output of the least squares data inversion analysis of step  1705 .  FIG. 20  shows ill-defined voids and subsurface features such as a sandbar. 
     At step  1706 , a spatial data analysis is performed using geostatistical methods known in the art, such as kriging. Step  1706  produces a second electrical resistivity model (step  1707 ) that minimizes the error of the spatial variability of the first electrical resistivity model. SURFER is a typical commercially-available geostatistical analysis program; however, any geostatistical analysis program may be used. Again, the output is typically a two- or three-dimensional graphical representation of location and resistivity.  FIG. 21  is an example of a two-dimensional graphical output of the kriging analysis.  FIG. 21  shows with much greater clarity voids, saturated soil, ground water and sandbar deposits. 
     In another embodiment of the preferred embodiment, an alternative computer other than the data collection PC  404  is used to analyze the data, the alternative computer typically being off-site and having much greater computing capacity than the data collector. Test data files may be transferred by to the alternative computer by portable storage means such as a diskette or a compact disc or the test data files may be transferred over a computer network such as the internet. 
     The embodiments of the system apparatus described thus far have relied upon measuring the voltage of the electrode pin, electrode  451  of  FIG. 6 , of each VCU referenced to the ground electrode which is tied to reference port  417  of  FIG. 5 . The voltage measured by an analog-to-digital converter (ADC) in this way may be a quite large voltage approaching the full HV DC voltage. An improved embodiment of the present invention allows for the simultaneous measurement of differential voltages between VCUs while high voltage current is being injected into the ground. The measured voltage per ADC is effectively reduced, thereby proportionally reducing absolute ADC quantization noise and any multiplicative inter-electrode noise. Expected measurement times and number of averaging cycles are reduced due to the lower noise levels. 
       FIG. 25  is a drawing of a voltage control unit, VCU  750 , capable of measuring the voltage difference between its own electrode  751  and the electrode of the adjacent connected downstream VCU. The internal components and external connectors of VCU  750  are similar to the internal components and external connectors of VCU  450  as described previously in relation to  FIG. 6  except for those items explicitly numbered in  FIG. 25 . VCU  750  includes at least a controller  752 , two solid state switches  757  and  758 , a switch  759 , and electrode  751  staked into earth and an analog-to-digital converter ADC  754 . Controller  752  is programmed to at least measure voltage data using ADC  754 , control solid state switches  757  and  758 , and control switch  759 . Controller  752  is further programmed to communicate voltage measurements to upstream components and interpret instructions received from upstream components. VCU  750  has control line  739 , low voltage LV DC power line  735 , low voltage DC return line  734 , HV DC power line  730  and HV DC return line  731 , all of which are ultimately connected upstream to the PDU via an eight-conductor flexible cable (not shown). 
     Solid state switch  757  is programmed by controller  752  to one of (a) connect HV DC+ line  730  to electrode  751 , or (b) isolate HV DC+ line  730  from electrode  751 . If solid state switch  757  connects HV DC+ line to electrode  751 , then VCU  750  is injecting high voltage current into the earth as a part of a test procedure. 
     Solid state switch  758  is programmed by controller  752  to one of (a) connect HV DC− line  730  to electrode  751 , or (b) isolate HV DC− line  730  from electrode  751 . If solid state switch  757  connects HV DC− line to electrode  751 , then VCU  750  is returning high voltage current from the earth as a part of a test procedure. 
     VCU  750  includes HV pin (−) line  733  which is connected to a downstream VCU electrode and further connected to ADC  754  for voltage measurement. HV pin (+) and HV pin (−) lines are connected to upstream and downstream VCUs, respectively, via the eight-conductor flexible cable. ADC  754  is capable of measuring the differential voltage between HV pin (+)  732  and HV pin (−)  733 . 
       FIG. 26  indicates how a set of VCUs  740  are interconnected to effect differential voltage measurements between their electrodes. VCU-A has ADC-A for voltage measurement and electrode PIN-A inserted into the earth at its physical location. Similarly, VCU-B has ADC-B for voltage measurement and electrode PIN-B inserted into the earth at VCU-B&#39;s physical location and VCU-C has ADC-C for voltage measurement and electrode PIN-C inserted into the earth at VCU-C&#39;s physical location. VCU-A, VCU-B and VCU-C are similar to VCU  750  of  FIG. 25 . The VCUs are interconnected such that HV pin (−) of VCU-A is connected to HV pin (+) of VCU-B via HV pin line  741  and HV pin (−) of VCU-B is connected to HV pin (+) of VCU-C via HV pin line  742 . 
     VCU-A effects a measurement of the voltage difference VAB between PIN-A and PIN-B. VCU-B effects a measurement of the voltage difference VBC between PIN-B and PIN-C. The voltage difference VAC between VCU-A and VCU-C may be obtained by adding: VAC=VAB+VBC. The set of VCUs  740  may be generalized to include a number N of VCUs connected together in series to form a system for which a number of combinations, larger than N, of differential voltages may be measured between the electrodes of the VCUs. 
     Further improvements over the embodiments of  FIGS. 25 and 26  may be conceived. For example,  FIG. 27  is a drawing of a voltage control unit, VCU  770 , capable of measuring the voltage difference between its own electrode  771  and the electrode of any downstream VCU. The internal components and external connectors of VCU  770  are similar to the internal components and external connectors of VCU  450  as described previously in relation to  FIG. 6  except for those items explicitly numbered in  FIG. 27 . VCU  770  includes at least a controller  772 , two solid state switches  777  and  778 , a switch  779 , and electrode  771  staked into earth and an analog-to-digital converter ADC  774 . Controller  772  is programmed to at least measure voltage data using ADC  774 , control solid state switches  777  and  778 , and control switch  779 . Controller  772  is further programmed to communicate voltage measurements to upstream components and interpret instructions received from upstream components. VCU  770  has control line  789 , low voltage LV DC power line  785 , low voltage DC return line  784 , HV DC power line  780  and HV DC return line  781 , all of which are ultimately connected upstream to the PDU via an eight-conductor flexible cable (not shown). 
     Solid state switch  777  is programmed by controller  772  to one of (a) connect HV DC+ line  780  to electrode  771 , or (b) isolate HV DC+ line  780  from electrode  771 . If solid state switch  777  connects HV DC+ line to electrode  771 , then VCU  770  is injecting high voltage current into the earth as a part of a test procedure. 
     Solid state switch  778  is programmed by controller  772  to one of (a) connect HV DC− line  780  to electrode  771 , or (b) isolate HV DC− line  780  from electrode  771 . If solid state switch  777  connects HV DC− line to electrode  771 , then VCU  770  is returning high voltage current from the earth as a part of a test procedure. 
     VCU  770  has a HV pin (+) line  782  which is connected to switch  779  and further connected to ADC  774  for voltage measurement. VCU  770  also has a HV pin (−) line  783  which is connected to switch  779  and further connected to ADC  774  for voltage measurement. HV pin (+) and HV pin (−) lines are connected to upstream and downstream VCUs, respectively, via the eight-conductor flexible cable. ADC  774  is capable of measuring the differential voltage between HV pin (+) line  782  and HV pin (−) line  783 . 
     Switch  779  is programmed by controller  772  to connect one of (a) HV pin (+) line  782  to electrode  771 , or (b) HV pin (+) line  782  to HV pin (−) line  783 . 
       FIG. 28  indicates how a set of VCUs  760  are interconnected to effect differential voltage measurements between their electrodes. VCU-D has ADC-D for voltage measurement, switch SW-D for configuration and electrode PIN-D inserted into the earth at its physical location. Similarly, VCU-E has ADC-E for voltage measurement, switch SW-E for configuration and electrode PIN-E inserted into the earth at VCU-E&#39;s physical location; VCU-F has ADC-F for voltage measurement, switch SW-F for configuration and electrode PIN-F inserted into the earth at VCU-F&#39;s physical location. VCU-D, VCU-E and VCU-F are similar to VCU  770  of  FIG. 27 . The VCUs are interconnected such that HV pin (−) of VCU-D is connected to HV pin (+) of VCU-E via HV pin line  761  and HV pin (−) of VCU-E is connected to HV pin (+) of VCU-F via HV pin line  762 . HV pin line  763  is connected to an upstream component while HV pin line  764  is connected to a downstream component. 
     In the example of  FIG. 28 , switch SW-D is configured to connect PIN-D to HV pin line  763 , and similarly, switch SW-F is configured to connect PIN-F to HV pin line  762 . Switch SW-E is configured to connect HV pin line  761  to HV pin line  762 . 
     VCU-D effects a measurement of the voltage difference VDF between PIN-D and PIN-F, bypassing the electrode PIN-E. The set of VCUs  760  may be generalized to include a number N of VCUs connected together in series to form a system for which a number of combinations, larger than N, of differential voltages may be measured between the electrodes of the VCUs by configuring their HV pin line switches. 
     One benefit of the present invention compared to prior art techniques is the parallel collection of electrode electric potential measurements so that a set of test data may be accumulated rapidly. Analysis methods may be conceived that could take advantage of such a rapid data collection. 
     A method consistent with differential electrode potential measurements and the parallel collection of the measurements is shown in method  700  of  FIG. 29 . Method  700  utilizes a communications system between a data collection computer, a PDU connected to the data collection computer, a master VCU and the set of VCUs, both connected to the PDU and all components communicatively connected as in the apparatus of  FIG. 1  and  FIG. 25 . Alternatively, the apparatus of  FIG. 27  may be used with method  700 . 
     To begin the method, step  705  sets up one test configuration from a set of predefined configurations  704 . A current injection pair of electrodes and a set of differential voltage measurements between a set of electrode measurement pairs are configured using the communications system wherein the master VCU sends out the test configuration to the set of VCUs. Once step  705  is complete, the background voltage is measured in steps  706 , repeating a differential background voltage measurement for each electrode pair in the set of electrode measurement pairs wherein there is zero injection current being delivered to the earth. 
     Then in step  710  the injection current is applied by the PDU between a first electrode and a second electrode in the current injection pair of electrodes, the first electrode having a positive high voltage DC electric potential with respect to the second electrode. Once the high voltage current is stable, the PDU measures the injected current. The set of differential electrode voltages are then measured simultaneously in steps  711 , for each electrode pair in the set of electrode measurement pairs. 
     Moving to step  715 , the injection current is applied by the PDU between the current injection pair of electrodes such that the second electrode has a positive high voltage DC electric potential with respect to the first electrode. Step  715  thus reverses the electrical polarity of the measurement performed in step  710 . Once the high voltage current is stable, the PDU again measures the injected current. The set of differential electrode voltages are measured simultaneously in steps  716 , for each electrode pair in the set of electrode measurement pairs. 
     The measurements in steps  710 ,  711 ,  715  and  716  may be repeated via step  720  for higher accuracy using a predefined set of rules  718 . An example of the predefined set of rules  718  is simply to repeat the steps  710  through  716  two times for each polarity. Other examples of rules may incorporate the calculation of the signal-to-noise ratio for the set of measurements, using for example the measured background voltage, and iterate the steps  710  through  716  until a predefined signal-to-noise threshold is surpassed. 
     Once the repetitions of step  720  have completed, the VCUs report the set of differential voltage measurements to the data collection computer in step  722 . 
     Also in step  724 , the PDU sends its measured injection current data to the data collection computer. 
     In step  728 , the master VCU determines if all test configurations for the set of configurations  704  have been completed. If the configurations have all been tested, then master VCU signals the other components to stop further action. If the configurations have not all been tested, then method  700  is repeated in step  727 . 
     While this preferred embodiment has been described in reference to a preferred embodiment along with other illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the preferred embodiment, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.