JP2007127565A - Spectrum forcible polarization survey device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct a phase generated by a mechanical characteristic of an isolation amplifier or the like used for detecting a potential, and to allow accurate measurements, by controlling mutually an amplitude and a phase between two direct digital synthesizers (DDSs) to bring the potential in a reference potential point to a minimum, using the two direct digital synthesizers (DDSs), in a spectrum forcible polarization survey device. <P>SOLUTION: In this spectrum forcible polarization survey device, with an impedance measuring object connected between a pair of coaxial cables in the four coaxial cables connected in parallel, and using a 4-terminal pair method for inputting a sine wave signal from one side thereof to measure an impedance, electromagnetic induction and electrostatic induction in the cable are active-controlled by digital-controlling the amplitude mutually and the phase between the DDSs, to minimize the potential in the reference potential point, using the two direct digital synthesizers, and the phase lag in circuit is controlled by utilizing the potential difference between the two direct digital synthesizers. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technique for spectral forced polarization that measures impedance and phase spectrum peculiar to the ground and rock, and more particularly, to a spectral forced polarization exploration apparatus using electromagnetic induction active control.

  Electrical exploration is a physical exploration method that measures the “specific resistance (electricity flow difficulty)” and electrochemical “polarization phenomenon” when an electric current is passed through the ground to grasp the physical properties of the ground.

  Conventionally, it is known that the resistivity method is used for geophysical exploration (see Patent Document 1). The specific resistance method is an electric exploration method in which a direct current (or alternating current) is passed through the ground and the ground properties are grasped from the distribution of the difficulty of the current flow.

  In other words, there is a big difference in the current flow in the ground where groundwater is distributed, the boundary between weathered rock and basement rock, or between the sedimentary layer and basement rock, and the better rock and fault fracture zone. By measuring the difference in current flow, it is possible to estimate in advance the presence of groundwater, the shape of the landslide surface, and the ground conditions on construction surfaces such as tunnels and dams.

  The forced polarization method (IP method: induced polarization method) causes various electrochemical polarization phenomena when a current is passed through the ground, and transient phenomena are observed in the observed waveform of the voltage. This is an exploration method.

  In particular, Spectral Forced Polarization (SIP) measures the impedance and phase spectrum peculiar to the ground and rock by measuring the complex resistivity (impedance and phase) of the ground and rock for each frequency. Technology. The ground and rock mass have characteristic impedance spectra due to polarization caused by conductive mineral particles contained in the interior and membrane polarization generated through a microporous membrane in the rock.

By acquiring these impedance spectra, it is considered possible to grasp the classification and properties of the rocks and ground, and various efforts have been made in the past. However, due to the influence of electromagnetic coupling of the measurement cable used between the measuring instrument and the electrode and the influence of electrostatic induction, the use range is limited to a low frequency of several Hz or less, and its application has been limited.
JP 2001-337175 A

  In the conventional technology, the frequency band that can be used is limited to a low frequency due to the influence of electromagnetic coupling or electrostatic induction of a measurement cable used between the measuring instrument and the electrode. In this frequency domain, the entire spectrum of IP cannot be acquired, and there is a disadvantage that the measurement time becomes long by using a low frequency.

  In order to solve these problems, it is important how to prevent the influence of electromagnetic induction and electrostatic induction of the measurement cable used between the measuring instrument and the electrode. In the present invention, in order to control the influence of electromagnetic induction or electrostatic induction, a means for performing active control of the induction electromagnetic field using a coaxial cable as the current / potential cable and canceling out of it is adopted.

  Conventionally, the spectral compulsory polarization method is a technique that has been used as a four-terminal pair method, whereas the four-terminal method cancels the induced electromagnetic field by performing antiphase feedback as a pair of transmitters. In the present invention, two DDSs (direct digital synthesizers) are used, and the amplitude and phase between the DDSs are mutually controlled so that the potential at the reference potential point is minimized. It is an object of the present invention to enable control including the influence of phase lag in the case.

  At the same time, the phase difference between the DDSs is measured, and the phase difference between the current and the potential is corrected using the phase difference. This correction makes it possible to correct a phase caused by mechanical characteristics of an isolation amplifier or the like used for potential detection, and to enable highly accurate measurement.

  In order to solve the above-mentioned problem, the present invention connects an impedance measurement object between a pair of coaxial cables of four coaxial cables connected in parallel, and inputs a sine wave signal from one of the impedance cables. In the spectral forced polarization probe using the four-terminal-pair method for measuring the signal, the amplitude and phase between the DDSs are digitally controlled to each other so that the potential at the reference potential point is minimized using two direct digital synthesizers. Thus, an active control of electromagnetic induction and electrostatic induction in a cable is possible, and a spectral compulsory polarization exploration device is provided.

  The phase delay in the circuit may be controlled using the phase difference in potential between the two direct digital synthesizers.

According to the spectral forced polarization exploration apparatus according to the present invention having the above-described configuration, the following effects are produced.
(1) It is possible to provide full-color geological section maps and geological columnar charts for logging fields, penetration tests, and tomographic exploration, which had previously been difficult to apply. It becomes possible to grasp.

(2) Since two DDSs (direct digital synthesizers) are used and the amplitude and phase between the DDSs are mutually controlled so that the potential of the reference potential point is minimized, electromagnetic induction and capacitance in the cable and in the circuit It is possible to perform control including the effect of the phase delay.

(3) Also, the phase difference between the DDSs is measured at the same time, and the phase difference between the current and the potential is corrected using the phase difference. This is caused by the mechanical characteristics of the isolation amplifier used for detecting the potential. It enables phase correction and high-precision measurement.

  Embodiments of a spectrally compulsory polarization exploration apparatus according to the present invention will be described below with reference to the drawings based on examples.

(principle)
As shown in FIG. 1, the influence of the IP phenomenon and the electromagnetic coupling becomes significant at a high frequency of 1 Hz or higher, and measurement becomes difficult in a frequency band higher than that. Therefore, the frequency region that can be measured has been limited to a low frequency.

  Conventionally, as a high-frequency impedance measurement method, there is a means called a four-terminal pair method, which has been used as a measurement method for high-frequency impedance at a laboratory level.

  FIG. 2 shows a basic configuration of the spectrally compulsory polarization exploration apparatus according to the present invention, and FIG. 3 shows a configuration of an embodiment of the present invention. In FIG. 2, the principle of the four-terminal pair method will be described. The sinusoidal signal transmitted from the transmitter 1 passes through the coaxial cable 10, is transmitted from the electrode 2 of the coaxial cable 10, passes through the electrode 4 of the coaxial cable 7 from the impedance measurement object 3, and the inner core of the coaxial cable 7. The current control circuit 6 paired with the transmitter 1 is passed through.

  Here, a current whose phase is inverted is supplied from the current control circuit 6 to the outer shield of the coaxial cable 7 so that the potential difference between the P2 electrode 5 and the shield of the coaxial cable 10 becomes 0V.

  A signal that flows through the outer shield of the coaxial cable 7 and has a phase opposite to that of the signal of the electrode 2 passes through the outer ends of the outer shields of the coaxial cables 8, 9, and 10 and finally reaches the transmitter 1. In this process, since the coaxial cables 8 and 9 have high impedance, current does not flow and current flowing through the outer shield flows through the coaxial cables 7 and 10. Since this current has an opposite phase to the current flowing through the cores of the electrodes 2 and 4 of the coaxial cables 10 and 7 and an amplitude that is substantially equal, electromagnetic induction is suppressed.

  The above is the principle of the four-terminal pair method. In FIG. 2, when the wiring or coaxial cable connected here is very long, or when the electrodes C1 and C2 have ground impedance or capacitance, It is difficult to apply the four-terminal pair circuit as it is.

  That is, in the four-terminal pair method, for the coaxial cable 8, the amplitude of the signal entering from the electrode 4 is controlled so that the potential difference between the cable core connected to the P2 electrode 5 and the external shield becomes 0V. The premise is that the phases of the electrodes 4 and 5 are equal.

  However, in reality, the electrodes 4 and 5 have a phase difference due to the ground impedance of C2 and the capacitance of the cable, and it is difficult to control using the signal of the electrode 5. Furthermore, since this phase difference has frequency dependence, it is difficult to correct.

  In the measuring device using the 4-terminal pair method used for impedance measurement of electronic circuits, etc., a specific cable is prepared in advance and tuned accordingly. However, as in physical exploration and logging in the formation, Such a measuring instrument method using the four-terminal pair method cannot be applied to a system that measures an impedance object having a long grounding probe and a different ground impedance every time it is measured.

  Therefore, in the spectral forced polarization exploration device according to the present invention, the current inversion means by feedback of the transmission pairs 1 and 6 is not used, but two independent transmitters are used and the means for optimal control of the phase and amplitude by the computer is applied. To do.

  In the embodiment of the spectral compulsory polarization exploration apparatus 20 according to the present invention, the specific configuration of the present invention will be described in detail with the operation. In FIG. 3, the core wires of the coaxial cables 10, 9, 8, and 7 are connected to the four electrodes C1, P1, P2, and C2, respectively, of the exploration device. Further, the outer shields of the coaxial cables are connected to each other by shield connection lines 24 in the vicinity of the electrodes.

  The CPU 14 can set a frequency to the main DDS 16, and a sine wave having a set frequency is transmitted from the main DDS 16. Thereafter, the output voltage of the sine wave is adjusted by the CPU 14, and the sine wave is transmitted from the transmission electrode C1 to the formation through the core of the current cable connected to the transmission electrode C1. The transmitted signal passes through the C2 electrode, passes through the current cable connected to the C2 electrode, and reaches the sub-transmitter 12.

  On the other hand, a potential signal detected as a potential difference between the potential electrode P2 and the external shield is changed to an effective value through the insulation amplifier 21 and the absolute value rectifier 22, and sent to the CPU 14 through the A / D converter 23. Here, the CPU 14 changes the transmission frequency and phase of the sub DDS 17 so that the potential difference between the P2 core and the external shield becomes 0V.

  The amplitude of the signal transmitted from the sub DDS 17 is further controlled by a command from the CPU 14 and sent to the sub transmitter 12. The signal output from the sub-transmitter 12 passes through the outer shield of the coaxial cable 7 connected to C2, and reaches the main transmitter 11 from the shield connection line 24 of the coaxial cable connected near the electrode through the outer shield of C1. .

  The frequency control and phase control of the sub DDS 17 and the amplitude control by the sine wave D / A are continued until the voltage between the core wire of the coaxial cable 8 connected to P2 and the external shield is adjusted to 0V. It is configured. The potential difference between P1 and the external shield passes through the insulation amplifier 21, is converted to an effective value by the absolute value rectifier 22, is A / D converted, and is recorded in the CPU.

  As for the current, the output from the sub-transmitter 12 is measured by the current sense 15 with a non-inductive resistor, and the current value is converted into an effective value. It is recorded in the CPU 14 through A / D conversion. The impedance is calculated by the CPU 14 from the potential difference and current value of P1 obtained here and displayed on the console 25.

  Further, the signal obtained from P1 and the signal obtained from the current detector 15 ("current sense 15" in the figure) are sent to the phase detection circuit 26, where the phase difference is measured. Similar to the potential measurement, the phase difference is also converted to an effective value by A / D conversion and sent to the CPU 14.

  The CPU 14 displays these impedance and phase difference on the console 25. The CPU 14 can perform both manual setting from the console 25 and automatic control from the personal computer.

  The spectral compulsory polarization exploration device 20 operates with DC input from a 12V battery. As described above, the spectrum forced polarization exploration device 20 is controlled by the CPU 14 and is characterized in that the main DDS 16 and the sub DDS 17 are controlled by digital control.

  The features of the present invention will be further described. In the spectral forced polarization exploration device according to the present invention, the amplitude and the phase are alternately switched using the two transmitters shown in the main transmitter 11 and the sub-transmitter 12, and the potential difference between the core of the shielded cable 8 and the external shield is changed. The amplitude and phase of the sub-transmitter 12 are changed by a command from the CPU 14 so as to be close to 0V. Finally, the process ends when the potential difference between the core of the shielded cable 8 and the external shield is adjusted to be close to 0V.

  Here, the potential difference between the P1 electrode and the P2 electrode is measured by taking the P2 electrode as a reference. The current value monitors the current output from the current detector 15 (“output current sense” in FIG. 3). The phase of the current measured by the current detector 15 is adjusted from the sub-transmitter 12 so that the potential difference between the core wire of the shielded cable 8 connected to the P2 electrode and the external shield becomes 0v. Therefore, the phase difference caused by the capacitance of the current line of the C2 electrode and the ground impedance is canceled by the phase adjustment of the sub-transmitter 12, and can be expressed by an equivalent circuit as shown in FIG.

  Thus, in the present invention, it is possible to accurately measure the phase of the current flowing through the measurement object without considering the influence of the ground impedance of the C2 electrode and the capacitance of the cable. Further, in the present invention, the phase difference associated with the change in the current value is corrected by measuring the phase difference between the two transmitters of the main transmitter 11 and the sub-transmitter 12. That is, since the circuits used for the main transmitter 11 and the sub-transmitter 12 are substantially the same, the characteristic that this phase difference is twice the phase difference of these circuits is used.

  In the spectral compulsory polarization exploration apparatus according to the present invention, control is performed using two DDSs indicated by the main DDS 16 and the sub DDS 17. That is, in response to a command from the CPU 14, a sine wave having a specific frequency is transmitted from the main DDS 16, and the CPU 14 sets the frequency and phase of the sub DDS 17 to the core of the shielded cable 8 connected to the P2 electrode. The potential difference between the line and the outer shield is changed to 0V.

  Here, when the potential difference between the core wire of the shielded cable 8 connected to the P2 electrode and the external shield becomes 0V, the currents of substantially opposite phases are applied to the cable core and the external shield of the respective cables of the C1 electrode and the C2 electrode. Electromagnetic induction does not occur. Further, using the potential difference between the inner core of the shielded cable 9 connected to the P1 electrode and the inner core of the shielded cable 8 connected to the P2 electrode, and the value measured by the current detector 15, the phase and potential difference are calculated. Detect and output impedance and phase difference.

(Experimental example)
The present inventor manufactured a forced polarization exploration device according to the present invention (see the photograph in FIG. 5), connected a 100 m long 6-core coaxial cable and a logging probe, and then introduced the impedance and phase of the non-inductive resistance. Was measured. The measurement results of the device characteristics obtained as a result are shown in FIG.

  According to FIG. 6, it can be seen that the impedance value and the phase accuracy are 1 mrad from 20 Hz to around 2000 Hz, and the measurement is possible in this band without being influenced by the measuring instrument and the cable. The phase offset is caused by the device characteristics of the isolation amplifier used for measuring the potential, but can be corrected by measuring the phase difference between the DDSs. Further, the reason why the phase and impedance errors increase at a low frequency of 20 Hz or less is due to the LPF (low pass filter) used for cutting the DC component.

  Although it is possible to extend the measurement range to the low frequency side by setting the cutoff frequency of the LPF to a lower frequency, on the other hand, the measurement time is significantly deteriorated. Currently, the measurement time per frequency is considered within 30 seconds, and the spectrum measurement at one point and four frequencies is assumed within 2 minutes. If actual logging can be measured in about this time, logging up to a depth resolution of 1 m and a depth of 100 m can be measured in 200 minutes (about 3 hours).

  In addition, measurement at one frequency of 10 frequencies requires a measurement time of about 8 hours, and it is difficult to actually take a measurement time beyond this, so the measurement frequency was set to 20 Hz or higher. One of the reasons that the measurement time of low frequency is significantly deteriorated is because of active control of a four-terminal pair. However, the method of fixing the frequency to a specific frequency or the low frequency with little influence of electromagnetic coupling. It is also possible to widen the measurement area on the low frequency side by restricting the active control of.

  In addition, in order to confirm the effectiveness of the forced polarization exploration apparatus according to the present invention, SIP logging was performed in an actual borehole. The borehole is a borehole having a depth of 60 m. The inner diameter of the borehole is 66 mm, and a strainer of 5% to 7% is cut in a vinyl chloride casing (the strainer is not cut below a depth of 25 m or deeper than 55 m).

  The logging probe is a probe made of vinyl chloride having a diameter of 30 mm, and an electrode made of solder (60% lead) is used as an electrode. The result of connecting a non-inductive resistor to the probe is as shown in FIG. In this drilling hole, logging was performed at three frequencies of 20 Hz, 200 Hz, and 1000 Hz per point for every 1 m, and an 8-frequency spectrum from 20 Hz to 2000 Hz was measured at a specific depth characterized by specific resistance.

  The result is shown in FIG. Each depth shows a characteristic change in spectrum, and a similar spectrum is shown in similar sand and silt layers. Moreover, the phase distribution of each frequency according to depth is shown in FIG. The spectral characteristics according to each stratum are shown in the logging results.

  As described above, the best mode of the spectral compulsory polarization exploration device according to the present invention has been described based on the embodiments. However, the present invention is not limited to such embodiments, and the technical matters described in the scope of the claims can be described. It goes without saying that there are various embodiments within the scope.

  According to the present invention having the above-described configuration, it becomes possible to provide a full-color geological sectional view and a geological columnar map for logging fields, penetration tests, and tomographic exploration, which have been difficult to apply in the past. It can be applied as a device for grasping classification and ground contamination.

It is a figure explaining a prior art. It is a figure explaining the structure of the Example of this invention. It is a figure explaining the structure of the Example of this invention. It is a figure explaining the equivalent circuit of the principal part of this invention. It is a compulsory polarization exploration device used for the experiment of the present invention. It is a graph which shows the experimental result of the SIP characteristic which connected the non-inductive resistance. It is a graph which shows the SIP spectrum acquired from each depth. It is a graph which shows the depth distribution of the SIP spectrum obtained as a result of logging.

Explanation of symbols

1 Transmitter 2 Electrode 3 Impedance Measurement Object 4C2 Electrode 5P2 Electrode 6 Current Control Circuit 7 Coaxial Cable 8, 9, 10 Coaxial Cable
11 Main transmitter
12 Sub-transmitter
14 CPU
15 Current detector
16 Main DDS
17 Deputy DDS
20 Spectral forced polarization probe 21 Insulation amplifier
22 Absolute value rectifier
23 A / D converter
24 Shield connection line
25 Console
26 Phase detection circuit

Claims (2)

A spectrum using a four-terminal pair method in which an impedance measurement object is connected between a pair of coaxial cables of four coaxial cables connected in parallel, and a sine wave signal is input from one of them to measure impedance. In the forced polarization probe,
By using two direct digital synthesizers and mutually digitally controlling the amplitude and phase between DDSs so that the potential at the reference potential point is minimized, active control of electromagnetic induction and electrostatic induction in the cable is possible. Spectral forced polarization exploration device characterized by   A spectrally compulsory polarization exploration device that controls phase delay in a circuit using a phase difference in potential between the two direct digital synthesizers.