CA2299743A1 - A method and apparatus for the detection of induced polarization and electrical resistivity in rocks and soils free of the effect of the electromagnetic coupling phenomenon - Google Patents
A method and apparatus for the detection of induced polarization and electrical resistivity in rocks and soils free of the effect of the electromagnetic coupling phenomenon Download PDFInfo
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- CA2299743A1 CA2299743A1 CA 2299743 CA2299743A CA2299743A1 CA 2299743 A1 CA2299743 A1 CA 2299743A1 CA 2299743 CA2299743 CA 2299743 CA 2299743 A CA2299743 A CA 2299743A CA 2299743 A1 CA2299743 A1 CA 2299743A1
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- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
- G01V3/02—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with propagation of electric current
Abstract
An improved apparatus and method for conducting Induced Polarization surveys for polarizeable material located in the subsurface. This apparatus and method provides results that are free of the influence of electromagnetic coupling. The apparatus consists of a direct current generator and electrodes that inject a steady direct current into the same volume of earth to which a standard Induced Polarization system is being applied. This direct current perturbs the electrochemical processes that are the cause of the Induced Polarization phenomenon and thereby causes a change of the output of the standard Induced Polarization system that can only be caused by a change in the Induced Polarization effects because a direct current can not cause any change in the electromagnetic effects that are an inherent part of the data provided by any Induced Polarization survey. In any survey which is performed with the invention the recorded dat is the change of the Induced Polarization and Resistivity readings when the biasing current is switched on. This perturbation of the Induced Polarization effect also leads to changes of the measured DC Resistivity of any survey site that contains polarizeable material. This change in the Resistivity permits the presence of polarizeable material to be recognized in Direct Current conditions.
Description
A METHOD AND APPARATUS FOR THE DETECTION OF INDUCED POLARIZATION
AND ELECTRICAL RESISTIVITY IN ROCKS AND SOILS FREE OF THE EFFECT OF
THE ELECTROMAGNETIC COUPLING PHENOMENON.
Field of the Invention The present invention relates to an improved method for the detection of materials within the earth which are capable of temporarily storing electrical charge by means of the electrochemical Induced Polarization (IP) process. The invention improves upon prior art by providing a means for rejecting the unwanted Electromagnetic Coupling effects that are inherently present in any conventional survey of the IP characteristics of the earth. These Electromagnetic effects are inevitably present because of the time varying nature of the current that must be used in studying the IP effect. The invention achieves this rejection by causing a perturbation of the IP
effect and by detecting changes in the measured IP effect that results from that perturbation. This perturbation is entirely confined to the IP effect and has no influence on the Electromagnetic effects. Thus any changes of the output of a system that employs the invention, can only come from changes of the IP effect and not from the Electromagnetic Coupling effects. This perturbation of the IP effect is achieved by adding a steady state (DC) biasing current to the time varying current that is used in any conventional study of the IP effect. The invention requires the application of this biasing current during a conventional Induced Polarization survey. The biasing cun-ent is applied while the time varying current is flowing and is applied to the same volume of ground that is being tested by the IP survey. It is supplied by a generator which is independent of the generator that supplies the conventional time varying current.
However while the biasing generator is independent of the IP generator, the current output of the biasing generator is finely controlled to permit its output to be regulated to maintain a constant ratio with respect to the current output of the IP
generator at each 30 new station as the whole system is moved from station to station throughout a survey.
This regulation of the biasing current can be controlled manually or can be automatically controlled by the output of the IP generator. This biasing current affects the IP phenomenon, because direct current flow through a polarizeable material is achieved by the same electrochemical processes that also generate the IP
effect which is normally only manifest in the time dependent aspects of those processes.
The steady state, direct, biasing current, can not cause any additional electromagnetic induction effects nor influence any existing electromagnetic effects because electromagnetic effects are generated only by time varying currents and the associated magnetic fields that they radiate.
The biasing current is applied to the earth through one or more electrodes that can be either coincident with the electrodes that deliver the standard IP
current in a conventional IP survey, or which can be separate from those electrodes in order that the biasing can be selectively applied. The biasing current will follow paths through the rock that can be identical to the paths followed by the time varying current which generates the IP effect or the biasing current can be applied so as to follow paths which differ from the paths followed by the time varying current. Identical paths will be achieved if the biasing current is applied through the same electrodes that supply the time varying IP current. Different paths will be achieved if the bias cun-ent is applied through a separate set of electrodes.
AND ELECTRICAL RESISTIVITY IN ROCKS AND SOILS FREE OF THE EFFECT OF
THE ELECTROMAGNETIC COUPLING PHENOMENON.
Field of the Invention The present invention relates to an improved method for the detection of materials within the earth which are capable of temporarily storing electrical charge by means of the electrochemical Induced Polarization (IP) process. The invention improves upon prior art by providing a means for rejecting the unwanted Electromagnetic Coupling effects that are inherently present in any conventional survey of the IP characteristics of the earth. These Electromagnetic effects are inevitably present because of the time varying nature of the current that must be used in studying the IP effect. The invention achieves this rejection by causing a perturbation of the IP
effect and by detecting changes in the measured IP effect that results from that perturbation. This perturbation is entirely confined to the IP effect and has no influence on the Electromagnetic effects. Thus any changes of the output of a system that employs the invention, can only come from changes of the IP effect and not from the Electromagnetic Coupling effects. This perturbation of the IP effect is achieved by adding a steady state (DC) biasing current to the time varying current that is used in any conventional study of the IP effect. The invention requires the application of this biasing current during a conventional Induced Polarization survey. The biasing cun-ent is applied while the time varying current is flowing and is applied to the same volume of ground that is being tested by the IP survey. It is supplied by a generator which is independent of the generator that supplies the conventional time varying current.
However while the biasing generator is independent of the IP generator, the current output of the biasing generator is finely controlled to permit its output to be regulated to maintain a constant ratio with respect to the current output of the IP
generator at each 30 new station as the whole system is moved from station to station throughout a survey.
This regulation of the biasing current can be controlled manually or can be automatically controlled by the output of the IP generator. This biasing current affects the IP phenomenon, because direct current flow through a polarizeable material is achieved by the same electrochemical processes that also generate the IP
effect which is normally only manifest in the time dependent aspects of those processes.
The steady state, direct, biasing current, can not cause any additional electromagnetic induction effects nor influence any existing electromagnetic effects because electromagnetic effects are generated only by time varying currents and the associated magnetic fields that they radiate.
The biasing current is applied to the earth through one or more electrodes that can be either coincident with the electrodes that deliver the standard IP
current in a conventional IP survey, or which can be separate from those electrodes in order that the biasing can be selectively applied. The biasing current will follow paths through the rock that can be identical to the paths followed by the time varying current which generates the IP effect or the biasing current can be applied so as to follow paths which differ from the paths followed by the time varying current. Identical paths will be achieved if the biasing current is applied through the same electrodes that supply the time varying IP current. Different paths will be achieved if the bias cun-ent is applied through a separate set of electrodes.
The output data from a survey which is performed with the invention, is the difference in the readout of the Induced Polarization readings when the system is operated first with the bias off and then with it on. This difference output can be derived from any of the standard IP parameters (such as the Four Cole-Cole parameters, or the Phase Delay caused by the IP effect (Pelton et al. 1978);
or the Integral Chargeability). The biasing current also affects the measured Electrical Resistivity of the earth, so that differencing of earth Resistivity measurements is also an inherent feature of the invention. This variation of the measured resistivity of the earth 60 can be understood, because in DC conditions the measured resistance is the sum of effects which are purely ohmic in origin and the additional resistance due to polarization (i.e. due to charge accumulation or the IP effect). When the biasing current changes the IP response of a polarizeable material then it must also change the polarization resistance in that material and therefore change the overall sum of this polarization resistance with the ohmic resistance. It is this combined resistance that is measured as the total resistance of the earth. By convention, in the existing art of electrical exploration by means of Induced Polarization and Resistivity surveys, this total Resistance is presented as Specific Resistance or Resistivity. The conversion from Resistance to Resistivity is achieved by the application of well known geometric factors 70 that are specific to the electrode array that is used in making the measurements.
Application of the invention does not influence these geometric factors. If the invention is applied to measurements of Resistivity alone and no change in that Resistivity is observed with bias on or off, this will indicate that no polarizeable component exists in the material under test. Conversely a change of Resistivity will indicate the presence of polarizeable material without recourse to the use of Conventional IP survey techniques which require more sophisticated equipment than is needed for the measurement of Resistivity alone.
The biasing current wilt not only cause a change in the readings of the IP and 80 Resistivity responses of the material under test but will also cause a steady DC
voltage to be detected by any device that measures the electric field and a steady magnetic field to be detected by a magnetometer on or within the earth as part of an IP
or Resistivity survey. This DC voltage will be indistinguishable from the naturally occurring Spontaneous Polarization (SP) voltages in the earth. Induced Polarization and Resistivity receivers are normally designed to cancel out these natural voltages and will be able to treat the steady state voltages due to the biasing current in the same way. However the natural voltages in the earth seldom exceed 1 volt so that IP
receivers are usually designed to deal with DC voltages no greater than 2 volts. The voltages that biasing will generate, may easily exceed 2 volts so that for the practical 90 application of the invention, it will be necessary that the IP or Resistivity receiver that is used, be capable of dealing with up to 20 volts DC. This is not part of the invention as it will be an extension of already existing capabilities that are currently available in commercially available Induced Polarization and Resistivity receivers.
In conducting a traverse over a survey site it is normal that both the time varying IP current and the DC biasing current that are delivered into the ground vary from station to station depending upon the resistance to ground at the electrodes that are delivering the respective currents. Ideally, the ratio between the IP current and the biasing current should be the same at each location of the whole electrode array and 1 o0 this requires that the generator that delivers the biasing current be regulated to maintain its output as a constant fraction of the IP current. it is a feature of the invention that regulation of the bias current generator output be capable of being derived automatically and directly from the IP current output thereby maintaining a constant nominated ratio between these two currents.
At locations in a rock where both the bias current and the IP current are present, the bias or perturbation current will affect the supply of ions available for the accumulation of polarization charge where change of current flow between different modes such as ion flow and electron flow takes place. This biasing of the supply of ions 110 to sites will cause the Induced Polarization (IP) (i.e. charge accumulation) to be modified. This modification of the IP effect will be detected by changes in the behaviour of the secondary currents that originate from those polarization sites and in the electric and magnetic fields that those currents create. It will also be detected in changes of the polarization resistance of the material under test. An Electromagnetic Coupling effect can not be generated by any steady state magnetic field such as is radiated by this steady state biasing. or perturbation current. Consequently, any changes in the observed output of the system can only originate from changes of the Induced Polarization (charge accumulation) response of materials in the subsurface. If the material through which the time varying IP current is being passed, contains no sites at 120 which charge can be accumulated (e.g. a body of water is incapable of accumulating charge in this manner), then applying the biasing current will cause no change in the recorded responses. In such circumstances, the difference output will be zero.
The output of this difference system can only be none zero if charge accumulation due to Induced Polarization is occurring in the materials through which the time varying current and the perturbation current are flowing.
An infinitely diverse range of possible configurations of the IP and biasing electrodes can be used in conducting a biasing survey. The invention does not relate to any one of this range of possible electrode configurations. The invention constitutes the application of a DC biasing current to the same volume of earth that is being subjected 130 to an Induced Polarization survey with the biasing current being applied at the same time as the time variant current that is used to study the IP effect using any and all combinations of electrodes.
Background of the invention The Induced Polarization phenomenon is a temporary accumulation of charge within a material such as rock or soil, when electrical cun-ent is passed through that material. The charge accumulates at locations where the electrical charge transfer 140 through the rock or soil, undergoes a change of mode, such as the change from ionic to electronic conduction modes at the surface of a metallic mineral grain or in the change from ionic conduction to the migration of ions along the surface of clay particles. At these locations of charge accumulation, the change of mode of current flow is achieved by electrochemical processes as described by Klein, Beigler and Home. (1984). Materials which display strong Induced Polarization effects are the metallic mineral ores such as the sulphides and oxides of metals, the clay minerals or coal or graphite and any buried metal obiect of either man made origin or naturally occurring metals such as native copper or gold or platinum and the metallic meteorites.
The Induced Polarization phenomenon displayed by these materials, is well 150 documented. Some of the earliest work on IP was provided by H.O.Seigel (1959 Geophysics Vo1.24,p.547) Beginning with the work of Seigel, the detection of the Induced Polarization phenomenon in the earth has been employed for approximately 50 years in geophysical exploration surveys aimed at detecting metallic ore minerals. The methods employed in such surveys involve the direct injection of time varying electrical currents into the earth through metal electrodes placed on or in the earth. The charge accumulation (i.e. Induced Polarization) in the earth creates secondary (or polarization) currents within the earth. The detection of the effects of these secondary currents is 160 achieved by means of sensors which monitor either the electric field generated within or at the surface of the earth by these cun-ents or by using sensitive magnetometers to detect the magnetic fields radiated by those currents. The manifestations of the IP
phenomenon that can be detected by such surveys are various and include:
decaying voltages (electric fields) caused by the secondary currents (or polarization currents) that flow as the accumulated charge is dissipated after the primary current has been switched off, or decaying magnetic fields radiated by the decaying polarization currents;
or frequency dependent impedance of the material under test and frequency dependent phase delay in a sinusoidal current passed through the polarizeable material.
The present invention is concerned with overcoming a problem that is common to all currently available Induced Polarization survey systems. This problem arises from the fact that the currents that are injected into the earth in Induced Polarization surveys must vary with time. Because of this time variation, the magnetic fields associated with this current flow, interact by means of electromagnetic induction with any conductive material immersed in that field and cause secondary electrical currents (i.e.
eddy currents} to flow in that material even if that material is not capable of storing charge by means of the electrochemical Induced Polarization process. Thus the effect of this electromagnetic induction, is the generation of secondary electric and magnetic fields in 180 the volume of material under test, which display characteristics which are very similar to the effects that are generated by the Induced Polarization phenomenon.
Consequently, it is inevitable that the effects detected either in the electric field or the magnetic field either on or within the earth, will not only display the influence of the presence of any materials with a charge storing capability (i.e. the IP effect and its associated polarization currents) but will also display the effects of electromagnetic induction and the secondary currents (i.e. eddy currents) that the electromagnetic phenomenon generates within the earth. These electromagnetic effects are termed the Electromagnetic Coupling phenomenon and they display characteristics that are very similar to those of the Induced Polarization effects so that the two effects can not easily 190 be separated. All survey methods so far devised to study the Induced Polarization phenomenon in rocks, produce responses which inherently contain the Electromagnetic Coupling effect.
Existing approaches to the Electromagnetic Coupling problem A number of approaches to dealing with the problem presented by the Electromagnetic Coupling have been developed since the problem was first identified by the work of Sunde (1949), and developed by the work of Wait (1959).
200 The first approach to dealing with electromagnetic coupling was developed by the users of so called time-domain IP systems. In these systems the time variation of the applied current is in the form of a square wave that is generated by switching on the primary current supplied to the earth and holding it steady for a period which is typically as long as 2 seconds and then switching the current off, and leaving it off, for a period which again is typically 2 seconds. In the off period, the IP effects in the ground manifest themselves as decaying secondary currents (polarization currents) which are sustained by the dissipation of the charge stored while the externally supplied current was flowing. As this charge dissipates, the secondary currents can be detected either by means of the electric fields that they generate within the earth or by the magnetic 210 fields that they radiate throughout and above the earth. Regardless of the configuration of electrodes and sensors that is used to supply and record the effects of Induced polarization, it is inevitable that the changing magnetic field radiated by the primary current will induce secondary currents to flow in the earth by means of electromagnetic induction.
The currents that are generated by electromagnetic induction also decay during the period that the primary current is off and they tend to decay more rapidly that the IP
secondary currents. Therefore, by waiting until the electromagnetically generated currents have decayed to almost undetectable values it is possible to record the more 220 slowly decaying IP currents when their effects are much stronger than those of the electromagnetically induced currents. Typically the delay after the primary current is switched off before recording begins, is of the order of 0.5 seconds. Field experience has shown that delays of this magnitude work very well in reducing - but never completely eliminating - the EM coupling effect in the recorded data. This technique is still one of the most effective ways of overcoming this problem but it can fail if the earth at a study site is particularly conductive such as in the South West United States or in Western Australia or in the Paleozoic and Mesozoic rocks of Western Canada because in very conductive earth, such as is found in those areas, the induced eddy currents can decay very slowly over time scales directly comparable to the time scales required 230 for the decay of Induced Polarization currents.
The electromagnetic coupling phenomenon has been shown to consist of two components which are the so called purely inductive component and the grounded or propagation component (Sunde 1949, Wait 1959). Some electrode geometries have been shown to be able to eliminate the purely inductive component of the Coupling phenomenon (lNynn and Zonge 1975, Sunde 1949, Kinghom 1967) but no electrode geometry can completely eliminate the second component.
A second class of IP systems exists which functions in the frequency-domain.
In 24o this type of work, the IP effect is observed as a frequency dependence of the impedance of the earth when observed by means of a system of electrodes placed on or in the earth. These electrodes supply a current that varies as a pure harmonic (or sinsoid; although in practice, the transmitted current often has an alternating square wave form and the sinusoid is filtered out of that square wave by the receiver). The frequency of alternation of this current can be varied in between approximately 0.001 Hz and 104 Hz in but in exploration surveys the frequency range is confined to the range from 0.1 Hz to 100 Hz for practical reasons. In this type of survey the EM
coupling effect becomes progressively dominant as frequency increases, to the extent that at higher frequencies the coupling effect can constitute well over 90% of the overall 250 detected effect (Hohmann 1971 ). The earliest approaches to dealing with this problem in the frequency domain were based upon working at the lowest practical frequencies but this led to difficulties because it increased the noise levels in the signals that were detected.
An approach to dealing with the electromagnetic coupling effect that has been developed for use with frequency-domain IP systems has been one in which the effects of electromagnetic coupling have been calculated and then subtracted from the observed combination of the EM coupling and IP effect in the hope that the result can be left free of the EM coupling effect. The flaw in this concept is that in order to 260 calculate the EM coupling effect for a particular site the user must have a very good knowledge of the conductivity structure of the site before the corrections can be calculated. The necessary level of information about the site is seldom available and if it is available it means that the site has already been intensively investigated so that an additional survey with IP equipment should be redundant. Nevertheless this approach has led to development of methods for dealing with an earth that was assumed to have the following structures: (1 ) A homogenous conducting earth (MiNet 1967). (2) A
mutilayered earth (Hohmann 1971; Wynn and Zonge 1975). The situation quickly degenerates into one in which so much needs to be known about the structure of the site that it becomes pointless to conduct any form of following survey and even if a 270 survey is performed and corrections are made there is no objective way in which it can be determined if the corrections have been completely successful.
A more pragmatic approach was published by Hallof (1974). This depends upon extrapolating the observed phase of the recorded signals over a range of frequencies back to zero frequency. The concept used in this approach is that at zero frequency, the electromagnetic effect will be zero but that the phase effects of IP will be finite.
Curiously this concept ignores the fact that the effect of IP on the phase of the recorded signal will also go to zero at zero frequency. However the method works reasonably well because of the observed fact that in the range of frequencies normally 280 used in IP surveys, recorded phase changes caused solely to the IP effect, are uniform and single valued so that extrapolation of these observations to zero frequency will give a finite none zero value.
The most recent approach to the separation of the electromagnetic and IP
effects was published by Pelton et al. (1978). This approach required the phase of the current that was passed through the ground to be recorded over a very wide frequency band (10-4 to 10'~ Hz). The authors showed that EM coupling effects display a characteristic frequency dependence in this type of data and that by virtue of these characteristics the electromagnetic coupling effect can be recognized and removed.
290 However, their method still involves the removal of a large coupling effect to leave behind a relatively much weaker IP effect so that considerable potential for error exists in this correction.
The present invention will provide a direct readout in the field, of data that will be inherently free of the influence of all of the components of the EM
coupling effect.
There will be no need for any form of correction or for prior knowledge of the conductivity structure of the survey site.
300 Di-a4ram The embodiments of the invention and of its operation will be detailed by means of reference to the following diagram.
Figure 1. Depicts one possible configuration of the standard IP current delivery generator and electrodes with the biasing current generator and electrodes located collinear with the IP electrodes. A particular feature is a connection between the two generators which allows the output of the biasing generator to be regulated to maintain an output level that is in a constant ratio with respect to the IP current regardless of 310 any changes in the IP current level.
Summary of the operational features of the invention 1. The invention consists of an apparatus and procedure that can be employed in conjunction with any conventional Induced Polarization survey system in order to provide an improvement in the operation of that Induced Polarization system.
2. The apparatus comprises a portable electrical generator capable of delivering a steady direct current to two or more metal electrodes placed on or in the earth. The 320 current output level from this biasing generator is finely controlled to permit that output to be set at any desired value either by manual control or by automatic control. The output level of the current supplied by this generator is controlled by the output of the Induced Polarization generator in order to maintain a constant ratio of the output currents from the Induced Polarization generator and the biasing generator.
3. The biasing current can be supplied to the earth through any arbitrary combination of electrodes that cause the biasing current to flow through the same volume of earth that is being tested for Induced Polarization.
330 4. The current delivered by this biasing generator and electrodes is applied to the same volume of earth that is being studied by means of a conventional Induced polarization survey.
or the Integral Chargeability). The biasing current also affects the measured Electrical Resistivity of the earth, so that differencing of earth Resistivity measurements is also an inherent feature of the invention. This variation of the measured resistivity of the earth 60 can be understood, because in DC conditions the measured resistance is the sum of effects which are purely ohmic in origin and the additional resistance due to polarization (i.e. due to charge accumulation or the IP effect). When the biasing current changes the IP response of a polarizeable material then it must also change the polarization resistance in that material and therefore change the overall sum of this polarization resistance with the ohmic resistance. It is this combined resistance that is measured as the total resistance of the earth. By convention, in the existing art of electrical exploration by means of Induced Polarization and Resistivity surveys, this total Resistance is presented as Specific Resistance or Resistivity. The conversion from Resistance to Resistivity is achieved by the application of well known geometric factors 70 that are specific to the electrode array that is used in making the measurements.
Application of the invention does not influence these geometric factors. If the invention is applied to measurements of Resistivity alone and no change in that Resistivity is observed with bias on or off, this will indicate that no polarizeable component exists in the material under test. Conversely a change of Resistivity will indicate the presence of polarizeable material without recourse to the use of Conventional IP survey techniques which require more sophisticated equipment than is needed for the measurement of Resistivity alone.
The biasing current wilt not only cause a change in the readings of the IP and 80 Resistivity responses of the material under test but will also cause a steady DC
voltage to be detected by any device that measures the electric field and a steady magnetic field to be detected by a magnetometer on or within the earth as part of an IP
or Resistivity survey. This DC voltage will be indistinguishable from the naturally occurring Spontaneous Polarization (SP) voltages in the earth. Induced Polarization and Resistivity receivers are normally designed to cancel out these natural voltages and will be able to treat the steady state voltages due to the biasing current in the same way. However the natural voltages in the earth seldom exceed 1 volt so that IP
receivers are usually designed to deal with DC voltages no greater than 2 volts. The voltages that biasing will generate, may easily exceed 2 volts so that for the practical 90 application of the invention, it will be necessary that the IP or Resistivity receiver that is used, be capable of dealing with up to 20 volts DC. This is not part of the invention as it will be an extension of already existing capabilities that are currently available in commercially available Induced Polarization and Resistivity receivers.
In conducting a traverse over a survey site it is normal that both the time varying IP current and the DC biasing current that are delivered into the ground vary from station to station depending upon the resistance to ground at the electrodes that are delivering the respective currents. Ideally, the ratio between the IP current and the biasing current should be the same at each location of the whole electrode array and 1 o0 this requires that the generator that delivers the biasing current be regulated to maintain its output as a constant fraction of the IP current. it is a feature of the invention that regulation of the bias current generator output be capable of being derived automatically and directly from the IP current output thereby maintaining a constant nominated ratio between these two currents.
At locations in a rock where both the bias current and the IP current are present, the bias or perturbation current will affect the supply of ions available for the accumulation of polarization charge where change of current flow between different modes such as ion flow and electron flow takes place. This biasing of the supply of ions 110 to sites will cause the Induced Polarization (IP) (i.e. charge accumulation) to be modified. This modification of the IP effect will be detected by changes in the behaviour of the secondary currents that originate from those polarization sites and in the electric and magnetic fields that those currents create. It will also be detected in changes of the polarization resistance of the material under test. An Electromagnetic Coupling effect can not be generated by any steady state magnetic field such as is radiated by this steady state biasing. or perturbation current. Consequently, any changes in the observed output of the system can only originate from changes of the Induced Polarization (charge accumulation) response of materials in the subsurface. If the material through which the time varying IP current is being passed, contains no sites at 120 which charge can be accumulated (e.g. a body of water is incapable of accumulating charge in this manner), then applying the biasing current will cause no change in the recorded responses. In such circumstances, the difference output will be zero.
The output of this difference system can only be none zero if charge accumulation due to Induced Polarization is occurring in the materials through which the time varying current and the perturbation current are flowing.
An infinitely diverse range of possible configurations of the IP and biasing electrodes can be used in conducting a biasing survey. The invention does not relate to any one of this range of possible electrode configurations. The invention constitutes the application of a DC biasing current to the same volume of earth that is being subjected 130 to an Induced Polarization survey with the biasing current being applied at the same time as the time variant current that is used to study the IP effect using any and all combinations of electrodes.
Background of the invention The Induced Polarization phenomenon is a temporary accumulation of charge within a material such as rock or soil, when electrical cun-ent is passed through that material. The charge accumulates at locations where the electrical charge transfer 140 through the rock or soil, undergoes a change of mode, such as the change from ionic to electronic conduction modes at the surface of a metallic mineral grain or in the change from ionic conduction to the migration of ions along the surface of clay particles. At these locations of charge accumulation, the change of mode of current flow is achieved by electrochemical processes as described by Klein, Beigler and Home. (1984). Materials which display strong Induced Polarization effects are the metallic mineral ores such as the sulphides and oxides of metals, the clay minerals or coal or graphite and any buried metal obiect of either man made origin or naturally occurring metals such as native copper or gold or platinum and the metallic meteorites.
The Induced Polarization phenomenon displayed by these materials, is well 150 documented. Some of the earliest work on IP was provided by H.O.Seigel (1959 Geophysics Vo1.24,p.547) Beginning with the work of Seigel, the detection of the Induced Polarization phenomenon in the earth has been employed for approximately 50 years in geophysical exploration surveys aimed at detecting metallic ore minerals. The methods employed in such surveys involve the direct injection of time varying electrical currents into the earth through metal electrodes placed on or in the earth. The charge accumulation (i.e. Induced Polarization) in the earth creates secondary (or polarization) currents within the earth. The detection of the effects of these secondary currents is 160 achieved by means of sensors which monitor either the electric field generated within or at the surface of the earth by these cun-ents or by using sensitive magnetometers to detect the magnetic fields radiated by those currents. The manifestations of the IP
phenomenon that can be detected by such surveys are various and include:
decaying voltages (electric fields) caused by the secondary currents (or polarization currents) that flow as the accumulated charge is dissipated after the primary current has been switched off, or decaying magnetic fields radiated by the decaying polarization currents;
or frequency dependent impedance of the material under test and frequency dependent phase delay in a sinusoidal current passed through the polarizeable material.
The present invention is concerned with overcoming a problem that is common to all currently available Induced Polarization survey systems. This problem arises from the fact that the currents that are injected into the earth in Induced Polarization surveys must vary with time. Because of this time variation, the magnetic fields associated with this current flow, interact by means of electromagnetic induction with any conductive material immersed in that field and cause secondary electrical currents (i.e.
eddy currents} to flow in that material even if that material is not capable of storing charge by means of the electrochemical Induced Polarization process. Thus the effect of this electromagnetic induction, is the generation of secondary electric and magnetic fields in 180 the volume of material under test, which display characteristics which are very similar to the effects that are generated by the Induced Polarization phenomenon.
Consequently, it is inevitable that the effects detected either in the electric field or the magnetic field either on or within the earth, will not only display the influence of the presence of any materials with a charge storing capability (i.e. the IP effect and its associated polarization currents) but will also display the effects of electromagnetic induction and the secondary currents (i.e. eddy currents) that the electromagnetic phenomenon generates within the earth. These electromagnetic effects are termed the Electromagnetic Coupling phenomenon and they display characteristics that are very similar to those of the Induced Polarization effects so that the two effects can not easily 190 be separated. All survey methods so far devised to study the Induced Polarization phenomenon in rocks, produce responses which inherently contain the Electromagnetic Coupling effect.
Existing approaches to the Electromagnetic Coupling problem A number of approaches to dealing with the problem presented by the Electromagnetic Coupling have been developed since the problem was first identified by the work of Sunde (1949), and developed by the work of Wait (1959).
200 The first approach to dealing with electromagnetic coupling was developed by the users of so called time-domain IP systems. In these systems the time variation of the applied current is in the form of a square wave that is generated by switching on the primary current supplied to the earth and holding it steady for a period which is typically as long as 2 seconds and then switching the current off, and leaving it off, for a period which again is typically 2 seconds. In the off period, the IP effects in the ground manifest themselves as decaying secondary currents (polarization currents) which are sustained by the dissipation of the charge stored while the externally supplied current was flowing. As this charge dissipates, the secondary currents can be detected either by means of the electric fields that they generate within the earth or by the magnetic 210 fields that they radiate throughout and above the earth. Regardless of the configuration of electrodes and sensors that is used to supply and record the effects of Induced polarization, it is inevitable that the changing magnetic field radiated by the primary current will induce secondary currents to flow in the earth by means of electromagnetic induction.
The currents that are generated by electromagnetic induction also decay during the period that the primary current is off and they tend to decay more rapidly that the IP
secondary currents. Therefore, by waiting until the electromagnetically generated currents have decayed to almost undetectable values it is possible to record the more 220 slowly decaying IP currents when their effects are much stronger than those of the electromagnetically induced currents. Typically the delay after the primary current is switched off before recording begins, is of the order of 0.5 seconds. Field experience has shown that delays of this magnitude work very well in reducing - but never completely eliminating - the EM coupling effect in the recorded data. This technique is still one of the most effective ways of overcoming this problem but it can fail if the earth at a study site is particularly conductive such as in the South West United States or in Western Australia or in the Paleozoic and Mesozoic rocks of Western Canada because in very conductive earth, such as is found in those areas, the induced eddy currents can decay very slowly over time scales directly comparable to the time scales required 230 for the decay of Induced Polarization currents.
The electromagnetic coupling phenomenon has been shown to consist of two components which are the so called purely inductive component and the grounded or propagation component (Sunde 1949, Wait 1959). Some electrode geometries have been shown to be able to eliminate the purely inductive component of the Coupling phenomenon (lNynn and Zonge 1975, Sunde 1949, Kinghom 1967) but no electrode geometry can completely eliminate the second component.
A second class of IP systems exists which functions in the frequency-domain.
In 24o this type of work, the IP effect is observed as a frequency dependence of the impedance of the earth when observed by means of a system of electrodes placed on or in the earth. These electrodes supply a current that varies as a pure harmonic (or sinsoid; although in practice, the transmitted current often has an alternating square wave form and the sinusoid is filtered out of that square wave by the receiver). The frequency of alternation of this current can be varied in between approximately 0.001 Hz and 104 Hz in but in exploration surveys the frequency range is confined to the range from 0.1 Hz to 100 Hz for practical reasons. In this type of survey the EM
coupling effect becomes progressively dominant as frequency increases, to the extent that at higher frequencies the coupling effect can constitute well over 90% of the overall 250 detected effect (Hohmann 1971 ). The earliest approaches to dealing with this problem in the frequency domain were based upon working at the lowest practical frequencies but this led to difficulties because it increased the noise levels in the signals that were detected.
An approach to dealing with the electromagnetic coupling effect that has been developed for use with frequency-domain IP systems has been one in which the effects of electromagnetic coupling have been calculated and then subtracted from the observed combination of the EM coupling and IP effect in the hope that the result can be left free of the EM coupling effect. The flaw in this concept is that in order to 260 calculate the EM coupling effect for a particular site the user must have a very good knowledge of the conductivity structure of the site before the corrections can be calculated. The necessary level of information about the site is seldom available and if it is available it means that the site has already been intensively investigated so that an additional survey with IP equipment should be redundant. Nevertheless this approach has led to development of methods for dealing with an earth that was assumed to have the following structures: (1 ) A homogenous conducting earth (MiNet 1967). (2) A
mutilayered earth (Hohmann 1971; Wynn and Zonge 1975). The situation quickly degenerates into one in which so much needs to be known about the structure of the site that it becomes pointless to conduct any form of following survey and even if a 270 survey is performed and corrections are made there is no objective way in which it can be determined if the corrections have been completely successful.
A more pragmatic approach was published by Hallof (1974). This depends upon extrapolating the observed phase of the recorded signals over a range of frequencies back to zero frequency. The concept used in this approach is that at zero frequency, the electromagnetic effect will be zero but that the phase effects of IP will be finite.
Curiously this concept ignores the fact that the effect of IP on the phase of the recorded signal will also go to zero at zero frequency. However the method works reasonably well because of the observed fact that in the range of frequencies normally 280 used in IP surveys, recorded phase changes caused solely to the IP effect, are uniform and single valued so that extrapolation of these observations to zero frequency will give a finite none zero value.
The most recent approach to the separation of the electromagnetic and IP
effects was published by Pelton et al. (1978). This approach required the phase of the current that was passed through the ground to be recorded over a very wide frequency band (10-4 to 10'~ Hz). The authors showed that EM coupling effects display a characteristic frequency dependence in this type of data and that by virtue of these characteristics the electromagnetic coupling effect can be recognized and removed.
290 However, their method still involves the removal of a large coupling effect to leave behind a relatively much weaker IP effect so that considerable potential for error exists in this correction.
The present invention will provide a direct readout in the field, of data that will be inherently free of the influence of all of the components of the EM
coupling effect.
There will be no need for any form of correction or for prior knowledge of the conductivity structure of the survey site.
300 Di-a4ram The embodiments of the invention and of its operation will be detailed by means of reference to the following diagram.
Figure 1. Depicts one possible configuration of the standard IP current delivery generator and electrodes with the biasing current generator and electrodes located collinear with the IP electrodes. A particular feature is a connection between the two generators which allows the output of the biasing generator to be regulated to maintain an output level that is in a constant ratio with respect to the IP current regardless of 310 any changes in the IP current level.
Summary of the operational features of the invention 1. The invention consists of an apparatus and procedure that can be employed in conjunction with any conventional Induced Polarization survey system in order to provide an improvement in the operation of that Induced Polarization system.
2. The apparatus comprises a portable electrical generator capable of delivering a steady direct current to two or more metal electrodes placed on or in the earth. The 320 current output level from this biasing generator is finely controlled to permit that output to be set at any desired value either by manual control or by automatic control. The output level of the current supplied by this generator is controlled by the output of the Induced Polarization generator in order to maintain a constant ratio of the output currents from the Induced Polarization generator and the biasing generator.
3. The biasing current can be supplied to the earth through any arbitrary combination of electrodes that cause the biasing current to flow through the same volume of earth that is being tested for Induced Polarization.
330 4. The current delivered by this biasing generator and electrodes is applied to the same volume of earth that is being studied by means of a conventional Induced polarization survey.
5. The procedure for use of the invention involves initial operation of the conventional Induced Polarization system with the current from the direct current biasing generator switched off. The readings are then repeated with the direct current switched on. They are also repeated with the direct current on again but reversed with respect to the first direction of application of that current.
340 6. The output data provided by the invention is the difference between the induced polarization and resistivity measurements taken with the direct current off and then on.
For the change of readings to be measured accurately it is necessary that the conventional Induced Polarization system be equipped with a receiver that is capable of dealing with DC offset voltages (often referred to as SP voltages) as large as 20 volts.
7. The central concept of the invention is that the application of a direct current to a volume of earth that contains polarizeable material will cause changes in the polarizeability of that material but will not change the electromagnetic effects that are associated with passing any time varying current through the earth.
8. As the direct current can affect only the Induced Polarization characteristics of the earth this means that the difference readings obtained by use of the invention will inherently contain no influence from electromagnetic coupling effects.
9. The electronic details of the direct current generator are not part of the invention because such generators already exist. The invention lies in the application of a direct (or biasing} current to a polarizeable material regardless of the electronic design details of the generator or of the number and distribution of the electrodes that are used to deliver the direct current into the earth.
400 References Hallof, P.G. (1974), The IP phase measurement and inductive coupling, Geophysics, Vo1.39, p.650.
Hohmann,G.W., 1971, Electromagnetic coupling between grounded wires at the surface of a iwo layer earth. Geophysics, Vo1,38, p.854.
Kinghom, G.F., 1967, Electrical methods for deep subsurtace exploration. IEEE
Trans.
Geosci, Electronics., GE-5, p.51.
41o Klein,J.D., BiegIer,T., and Home,M.D., 1984, Mineral intertacial processes in the method of induced polarization. Geophysics, Vo1.49, p.1105.
Millet,F.B., 1967. Electromagnetic coupling of collinear dipoles on a uniform half space.
In Mining Geophysics. Society of Exploration Geophysicists.
Pelton, W.H., Ward,S.H., Hallof, P.G. SiII,W.R., and Nelson,P.H., (1978), Mineral discrimination and removal of inductive coupling with muiltifrequency IP.
Geophysics, Vo1,43, p.588.
420 H.O.Seigel,1959, Mathematical formulation and type curves for induced polarization:
Geophysics, Vo1.24, p.547) Sunde, E. D.,1949, Earth Conduction Effects in Transmission Systems: New York:
Van Nostrand.
Wait,J.R., 1959, The variable-frequency method, Overvoltage Research and Geophysical Applications, Pergamon, Oxford, 1959.
Wynn,J.C. and Zonge,K.L., 1975, EM coupling, its intrinsic value, its removal and the 430 cultural coupling problem. Geophysics, Vo1.40, p.831.
340 6. The output data provided by the invention is the difference between the induced polarization and resistivity measurements taken with the direct current off and then on.
For the change of readings to be measured accurately it is necessary that the conventional Induced Polarization system be equipped with a receiver that is capable of dealing with DC offset voltages (often referred to as SP voltages) as large as 20 volts.
7. The central concept of the invention is that the application of a direct current to a volume of earth that contains polarizeable material will cause changes in the polarizeability of that material but will not change the electromagnetic effects that are associated with passing any time varying current through the earth.
8. As the direct current can affect only the Induced Polarization characteristics of the earth this means that the difference readings obtained by use of the invention will inherently contain no influence from electromagnetic coupling effects.
9. The electronic details of the direct current generator are not part of the invention because such generators already exist. The invention lies in the application of a direct (or biasing} current to a polarizeable material regardless of the electronic design details of the generator or of the number and distribution of the electrodes that are used to deliver the direct current into the earth.
400 References Hallof, P.G. (1974), The IP phase measurement and inductive coupling, Geophysics, Vo1.39, p.650.
Hohmann,G.W., 1971, Electromagnetic coupling between grounded wires at the surface of a iwo layer earth. Geophysics, Vo1,38, p.854.
Kinghom, G.F., 1967, Electrical methods for deep subsurtace exploration. IEEE
Trans.
Geosci, Electronics., GE-5, p.51.
41o Klein,J.D., BiegIer,T., and Home,M.D., 1984, Mineral intertacial processes in the method of induced polarization. Geophysics, Vo1.49, p.1105.
Millet,F.B., 1967. Electromagnetic coupling of collinear dipoles on a uniform half space.
In Mining Geophysics. Society of Exploration Geophysicists.
Pelton, W.H., Ward,S.H., Hallof, P.G. SiII,W.R., and Nelson,P.H., (1978), Mineral discrimination and removal of inductive coupling with muiltifrequency IP.
Geophysics, Vo1,43, p.588.
420 H.O.Seigel,1959, Mathematical formulation and type curves for induced polarization:
Geophysics, Vo1.24, p.547) Sunde, E. D.,1949, Earth Conduction Effects in Transmission Systems: New York:
Van Nostrand.
Wait,J.R., 1959, The variable-frequency method, Overvoltage Research and Geophysical Applications, Pergamon, Oxford, 1959.
Wynn,J.C. and Zonge,K.L., 1975, EM coupling, its intrinsic value, its removal and the 430 cultural coupling problem. Geophysics, Vo1.40, p.831.
Claims
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS.
1. An improved system for the detection of accumulations of electrical charge within the earth comprising:
An electrical generator capable of delivering a steady direct current into the earth through contacts made with the earth at two or more locations on or within the earth, by means of metal electrodes of rod or sheet form;
the generator has the ability to deliver current of either positive or negative polarity;
the direct current delivered into the earth by this generator and electrodes is caused to flow through the same earth volume that is being tested by means of any conventional Induced Polarization exploration system concurrent with the operation of that system;
this direct current is regulated to maintain a constant ratio with the magnitude of the time varying current that is used to perturb the Induced Polarization effect;
this direct current applies a direct current bias to that volume of rock;
the measured output of this improved system is the change of reading of any of the Induced Polarization or Resistivity measurements when the bias is switched on;
this change or difference of reading of the electrical characteristics of the earth as the bias current is switched on comprises the data that is recorded in a survey conducted using the present invention;
the recorded differences in the detected properties of the earth will be generated entirely by changes in the Induced Polarization phenomenon and will contain no influence from the Electromagnetic Coupling phenomenon.
An electrical generator capable of delivering a steady direct current into the earth through contacts made with the earth at two or more locations on or within the earth, by means of metal electrodes of rod or sheet form;
the generator has the ability to deliver current of either positive or negative polarity;
the direct current delivered into the earth by this generator and electrodes is caused to flow through the same earth volume that is being tested by means of any conventional Induced Polarization exploration system concurrent with the operation of that system;
this direct current is regulated to maintain a constant ratio with the magnitude of the time varying current that is used to perturb the Induced Polarization effect;
this direct current applies a direct current bias to that volume of rock;
the measured output of this improved system is the change of reading of any of the Induced Polarization or Resistivity measurements when the bias is switched on;
this change or difference of reading of the electrical characteristics of the earth as the bias current is switched on comprises the data that is recorded in a survey conducted using the present invention;
the recorded differences in the detected properties of the earth will be generated entirely by changes in the Induced Polarization phenomenon and will contain no influence from the Electromagnetic Coupling phenomenon.
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| CA 2299743 CA2299743A1 (en) | 2000-02-22 | 2000-02-22 | A method and apparatus for the detection of induced polarization and electrical resistivity in rocks and soils free of the effect of the electromagnetic coupling phenomenon |
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|---|---|---|---|
| CA 2299743 CA2299743A1 (en) | 2000-02-22 | 2000-02-22 | A method and apparatus for the detection of induced polarization and electrical resistivity in rocks and soils free of the effect of the electromagnetic coupling phenomenon |
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| WO2006101419A1 (en) * | 2005-03-24 | 2006-09-28 | Obshchestvo S Ogranichennoy Otvetstvennostyu 'sibirskaya Geofizicheskaya Nauchno-Proizvodstvennaya Kompaniya' | Electromagnetic sounding method using a transient field spatial derivation on several separations |
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