Probe for Optically Monitoring Progress of in-Situ Vitrification of Soil

This invention relates to in-situ vitrification (ISV), and particularly to means for sensing the progress of an ISV process.

In-situ vitrification (ISV) has been proposed as a process for in-place stabilization of a contaminated soil formation. The ISV process is carried out by placing four electrodes in the soil formation at the corners of a square. Tests indicate that electrode separations of at least 5.5 m may be used, allowing treatment of a site having a surface area of over 30 m.sup.2. A conductive mixture of flaked graphite and glass frit is placed in an X pattern between the electrodes, in shallow trenches on the soil surface. Voltages are applied to the electrodes, and the graphite mixture conducts current between the electrodes. The graphite mixture is quickly heated to soil-melting temperatures (1,100 degrees to 1,600 degrees C.). As the surrounding soil melts, it becomes electrically-conductive and the affected soil volume grows downward and outward until the desired soil volume has been completely vitrified and the waste material is thereby encapsulated in the vitreous mass.

It is desirable that the depth to which the ISV process has progressed be monitored, in order to determine that the process is proceeding as expected. It is also desirable to be able to ensure that the soil has been vitrified to the desired depth before the ISV operation is stopped. The soil temperature may reach 2,000.degree. C., and therefore methods for determining depth are restricted to those that do not intrude into the molten zone. At such high temperatures, the soil emits radiant light.

A preferred embodiment of the present invention, in a first aspect, is ISV apparatus comprising at least two electrodes extending within a soil formation and a detector system for sensing the progress along an expected path of an ISV process performed using the electrodes. The detector system comprises a plurality of sensors each having an input port. The input ports are distributed along the expected path of the ISV process between a starting location and an expected ending location. Each sensor generates an electrical signal representative of the temperature in the vicinity of its input port. Signal processor and transmitter means are coupled to the sensors to receive an electrical signal generated by a sensor, generate a signal which is encoded with information which identifies the sensor and whether the ISV process has reached the sensor's input port, and propagate the encoded signal. The signal processor and transmitter means are below ground at a location beyond the expected ending location of the ISV process in the direction from the starting location to the expected ending location. Signal receiver and decoder means are located outside the soil formation for receiving the encoded signal propagated by the signal processor and transmitter means, decoding the encoded signal and providing a human-perceptible indication of the progress of the ISV process.

A preferred embodiment of the present invention in its second aspect is a method of carrying out an ISV process using at least two electrodes extending within a soil formation. The method comprises positioning a plurality of sensors, each having an input port, in the soil formation so that the input ports are distributed along an expected path of the ISV process between a starting location and an expected ending location. Each sensor generates an electrical signal representative of the temperature in the vicinity of its input port. The method also comprises locating signal processor and transmitter means below ground at a location beyond the expected ending location of the ISV process in the direction from the starting location to the expected ending location. The signal processor and transmitter means are coupled to receive the electrical signals generated by the sensors. The signal processor and transmitter means respond to a signal generated by one of the sensors by generating a signal which is encoded with information which identifies the sensor and whether the ISV process has reached the sensor's input port, and propagating the encoded signal to signal receiver and decoder means which are located outside the soil formation. The signal receiver and decoder means decode the encoded signal and provide a human-perceptible indication of the progress of the ISV process.

It is accordingly an object of the invention to provide improved detector means for sensing the progress of an ISV process.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:

FIG. 1 is schematic perspective view of apparatus for carrying out an ISV process, which apparatus includes a transmitter assembly for providing signals representing the depth to which the ISV process has progressed,

FIG. 2 is a partially cutaway side view of the transmitter assembly,

FIG. 3 is a partial sectional view of the transmitter assembly,

FIG. 4 is an enlarged, partly exploded view of a sensor which forms part of the transmitter assembly,

FIG. 5 is a partially schematic block diagram of a signal processor circuit to which the sensor is connected,

FIG. 6 illustrates a series of waveforms to aid in describing the signal provided by the signal processor circuit, and

FIG. 7 is a block diagram showing the path that is followed by signals generated by the transmitter assembly.

The ISV apparatus illustrated in FIG. 1 comprises four electrodes 2a, 2b, 2c and 2d which extend vertically into respective boreholes within a contaminated soil formation and project at their upper ends above the ground surface 6. The electrodes are connected to a high voltage current supply (not shown) for carrying out an ISV process on the soil formation.

An elongate transmitter assembly 10 is located beneath the lower end of the electrode 2a. The transmitter assembly, which is shown in greater detail in FIGS. 2 and 3, comprises two tubes 30 and 32 which are axially aligned and are mechanically coupled by a sleeve 34. A solid disc 36 separates the interior space of the tube 30 from the interior space of the tube 32, and the tubes 30 and 32 are provided with end caps 40 and 42 respectively. The cap 40 is formed with a hole (not shown), whereas the cap 42 is solid. The tubes 30, 32, the sleeve 34, the disc 30 and the end caps 40, 42 are made of dielectric material, such as PCV. The cap 40 is not initially attached to the tube 30. When the transmitter assembly is about to be installed, the electrode 2a is suspended vertically above the ground and the cap 40 is attached to its lower end, e.g. by using a bolt passing through the hole in the cap 40. The tube 30, which is attached to the tube 32, is filled with sand, and is then cemented to the cap 40. The electrode 2a, with the transmitter assembly attached thereto, is lowered into its borehole. The transmitter assembly is thus vertically oriented with the tube 30 disposed between the tube 32 and the electrode 2a. The sand in the tube 30 serves as thermal and electrical insulating material to separate the interior space of the tube 32 from the electrode 2a.

The tube 32 contains an electronics mounting tube 44 which is made of dielectric material. The electronics mounting tube 44 has a sector cut from its wall, and a processor circuit board 50 is fitted in the resulting hole 46. A battery pack 48, comprising several batteries, is accommodated in the tube 44 behind the circuit board 50.

The processor circuit board 50 carries an array of twenty-four photodetectors 52 and a processor circuit 54. The photodetectors are Motorola type MFOD 72 fiber optic detectors, each including a phototransistor 52' (FIG. 5). A transmitter circuit board 56 is mounted over the board 50 by means of stand-offs (not shown) and carries a transmitter circuit 59 which receives the output of the processor circuit 54. The processor circuit and the transmitter circuit are illustrated in FIG. 3 simply as blocks.

The transmitter assembly 10 is designed to be stored in a horizontal orientation and includes a mercury switch 88 (FIG. 7) which is open and isolates the battery pack 48 from the circuits 54 and 59 when the assembly is in its storage orientation. When the assembly is vertical, the mercury switch is closed and the battery pack is connected to the phototransistors 52' and the circuits 54 and 59. The transmitter circuit 59 is coupled to an antenna wire 60. The wire 60 is wrapped about the tube 44, and the tube 44 is supported coaxially inside the tube 32 by spacers 58 of dielectric material.

Twenty-four optical fibers 64 are mechanically and optically coupled to the photodetectors 52 respectively. Preferably, the package of each photodetector has an orifice 110 (FIG. 4), defined by an externally threaded stub 112, for receiving an end of an optical fiber. Each optical fiber comprises a core and a cladding, which are both of transparent material, and a protective jacket. At one end of the fiber, the jacket is removed over a distance of about 1.6 mm and the end of the fiber is inserted through a locking nut 114. The end of the fiber is inserted into the orifice 112 of the photodetector package and the locking nut is screwed onto the threaded stub 112. The twenty-four fibers 64 leave the tube 32 by way of an opening 62 (FIG. 2), which is sealed by a body of RTV (room temperature vulcanizable) silicone sealant material. When the transmitter assembly is installed beneath the electrode 2a, the fibers 64 each extend vertically upwards from the transmitter assembly 10. The fibers are of different respective lengths, so that the upper ends of the fibers are spaced apart along the electrode 2a. At its upper end, each fiber is cut perpendicular to its length so as to provide an input port which collects optical radiation. Optical radiation that is collected by the input port of a fiber is conducted by the core of the fiber and is applied to the associated photodetector.

The collector-emitter path of the phototransistor 52' of each photodetector 52 is connected in series with a resistor 65 (FIG. 5) between two terminals of the battery pack 48. The voltage at the connection point depends on the magnitude of the current conducted by the phototransistor and this in turn depends on the intensity with which the phototransistor is illuminated. The intensity of illumination of the phototransistor depends on the temperature in the soil at the level of the input port. The combination of the optical fiber and its associated phototransistor thus serves as a sensor for detecting the temperature of the soil at the level of the input port.

The connection point of each phototransistor 52' and its associated resistor 65 is connected to the signal processor circuit 54 by way of an inverting buffer. The buffer provides a binary output signal of which the level (logical 1 or logical 0) depends on whether the voltage at its input is below or above a threshold value. The relationship between the level of the binary output signal and the optical power incident on the phototransistor is determined by the value of the resistor 65. The twenty-four buffers that are required are implemented as three type 74 HCT 240 octal buffers 67. A fourth type 74 HCT 240 octal buffer 69 has its input terminals connected to an eight-position switch unit 71. The switch unit 71 is used to encode an identification number. T for the transmitter assembly. In an implementation of the invention, the identification number is encoded using only four switches of the switch unit. A fifth switch is used to invoke a test routine, and the remaining switches are inactive. The output terminals of each octal buffer 67 and 69 are connected to respective lines of the data bus of a microprocessor 68, which operates under control of a program stored in a read-only memory 72. The microprocessor 68 is a type CDP 1805 AC microprocessor, and a listing of the program used to control operation of the microprocessor 68 is contained in Appendix A.

The processor circuit is off when the mercury switch 88 is open. When the switch 88 is closed, the processor circuit has three distinct modes of generation. The power-up mode is entered immediately the switch 88 is closed. In the powerup mode, the microprocessor executes a scan routine. In the scan routine, the microprocessor 68 determines the state of each switch of the switch unit 71 and the state of each phototransistor 52'. This is accomplished by enabling the four buffers successively, using a demultiplexer 70 which operates under control of the microprocessor 68, and polling the eight lines of the data bus sequentially while each buffer is enabled. The microprocessor 68 generates a binary signal Q which is encoded with the information regarding the states of the switches and the states of the phototransistors. The waveform of the typical signal Q has five distinct slots 90, 92, 94, 96 and 98 as shown in FIG. 6. The slot 90 contains a synchronization burst. Each of the slots 92 and 94 contains four bits of data representing the identification number T. The slot 96 contains twenty-four bits representing the states of the phototransistors respectively, and the slot 98 contains a pause. The aggregate duration of the five slots is 3.2 seconds. FIG. 6(a) illustrates the waveform of the signal Q for the case in which the transmitter assembly is in the test mode, no identification number has been encoded and none of the inputs to the buffers 67 exceeds the threshold level, and accordingly shows only the burst and pulses 100 which signify that the switch invoking the test mode is closed. In FIG. 6(b), the pulses 102 in the slots 92 and 94 represent binary 0101, or decimal 5, and the pulse 104 in the slot 96 indicates that the buffer input for the phototransistor coupled to the most distant input port exceeds the threshold level. In FIG. 6(c), the pulses 106 in the slots 92 and 94 represent binary 1010, or decimal 10, and the pulse 108 in the slot 96 indicates that the buffer input for the phototransistor coupled to the nearest input port exceeds the threshold level.

The encoded signal Q is applied to the transmitter circuit 59, and the transmitter circuit uses the encoded signal to modulate a carrier. The modulated carrier is applied to the antenna wire 60 and an electromagnetic signal is radiated thereby. The processor circuit then enters an idle mode unless the buffer input for a phototransistor that is coupled to one of the eight input ports that are farthest from the transmitter assembly exceeds the threshold level, in which case it enters the active mode.

When the processor circuit is in the idle mode, the microprocessor 68 executes the scan routine at intervals of [3.2 sec (2314+3T)]. When a phototransistor that is coupled to one of the eight most distant input ports is conductive, the processor circuit enters the active mode and executes the scan routine at intervals of [3.2 sec (64+3T)]. Once the processor circuit is in the active mode, it remains in the active mode for approximately nine hours after all the phototransistors have become non-conductive. At this time, the processor circuit returns to the idle mode to preserve battery life.

Above the ground surface and outside the area that is expected to be influenced by the ISV process is a receiving antenna 80. The antenna 80 receives the signal that was radiated by the antenna wire 60 and applies the signal to a receiver 82. The receiver 82 demodulates the signal and reconstructs the signal that was applied to the transmitter circuit by the processor circuit 54. A decoding processor 84 decodes the signal provided by the receiver 82, performs error checks on the decoded signal, converts the decoded signal to digital form and transmits the digital signal to a display terminal 86 which displays and prints out the depth status of the melt and an indication of the signal strength of the transmission received. An example of the output data stream that may be displayed is as follows: ##STR1## where "a" indicates that data will follow.

"b" is the identification number of the transmitter assembly.

"c" is the absolute signal strength of the transmission based on a scale from 0 to 9.

"d" designates whether both identification number transmissions were understood by the decoding processor. "G" will appear if both transmissions were received and were consistent. "B" will appear if the transmissions were not consistent or if one or both of the transmissions were not clearly received.

"e" represents the sensor status for the twenty-four sensors.

Each number X representing the status of a sensor is in the range from 0 to 9, depending on the relationship between the voltage level of the signal provided by the receiver 82 during the portion of the time slot 96 that relates to the particular sensor and the absolute signal strength, represented by the portion "c" of the display.

The decoding processor employs a Z80 microprocessor, and a listing of the program used to control operation of the Z80 microprocessor is contained in Appendix B.

It is preferred that each optical fiber have a silica core and a cladding of silica doped with a small amount of fluorine so that it has a smaller refractive index than the core. A suitable fiber is that which is manufactured by Raychem and is sold under the designation VSC-1B-10-17. It has been found that this type of fiber provides a much greater increase in signal strength with increase in soil temperature than other fibers that were tested, such as fibers having a silica core and a polymeric cladding. At moderate temperatures (450.degree. to 550.degree. C.) the polymeric claddings fibers were destroyed and the light conductivity of these fibers dropped essentially to zero. A dramatic increase in the phototransistor current provided by the fiber with the silica plus fluorine cladding took place at about 800.degree. C. Even at temperatures above 900.degree. C., the fiber with the silica plus fluorine cladding provided a phototransistor current output from about 14 to about 100 times the maximum current output provided by phototransistors coupled to the fibers with a polymeric cladding. The fiber with the silica plus fluorine cladding was able to survive exposure to temperatures in excess of 1100.degree. C. The use of a silica-based cladding is considered to be the key to the superior performance at high temperatures of the fiber with the silica plus fluorine cladding. At least part of the phototransistor current provided by the silica plus fluorine fiber may be caused by radiation generated within the fiber itself due to heating of the fiber.

It will be appreciated that the present invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims and equivalents thereof. For example, the electrode 2a acts as a passive repeater and aids in propagating the signal radiated by the antenna wire, but it is not essential to the invention that the transmitter assembly be placed directly beneath one of the electrodes. If the transmitter assembly is not placed beneath an electrode, a metal rod may be placed above the transmitter assembly to assist in data transmission. Multiple transmitter assemblies, encoded with different identification numbers, may be used simultaneously to monitor the progress of the ISV process at different locations, and since the identification numbers are different the decoding processor is able to distinguish among the signals provided by the different transmitter assemblies. In this manner information relating to the progress of the process at the different locations can be displayed and printed out. ##SPC1##