CA2941995C - System for acquiring and processing electric field signals relating to subsurface geologic structures - Google Patents
System for acquiring and processing electric field signals relating to subsurface geologic structures Download PDFInfo
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- CA2941995C CA2941995C CA2941995A CA2941995A CA2941995C CA 2941995 C CA2941995 C CA 2941995C CA 2941995 A CA2941995 A CA 2941995A CA 2941995 A CA2941995 A CA 2941995A CA 2941995 C CA2941995 C CA 2941995C
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- G—PHYSICS
- 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
- G01V3/08—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices
Abstract
A system for acquiring and processing low-frequency electric field signals, such as magnetotelluric signals, relating to subsurface geologic structures. At least one capacitive line antenna acquires the signals. A feedback circuit increases input impedance for the signals to reduce the frequency at which signal attenuation occurs.
Description
SYSTEM FOR ACQUIRING AND PROCESSING ELECTRIC FIELD SIGNALS
RELATING TO SUBSURFACE GEOLOGIC STRUCTURES
Field of the Invention The present invention relates to systems for acquiring and processing electric field signals, and more specifically to systems for acquiring and processing magnetotelluric signals relating to subsurface geologic structures.
Back2round of the Invention In various situations it may become desirable to characterize the subsurface of the Earth. For example, the mining and oil industries are concerned with subsurface structures such as hydrocarbon reservoirs and ore deposits.
One way to accomplish subsurface characterisation is by a technique referred to as magnetotellurics (MT). MT involves the measurement of naturally occurring oscillations in the Earth's magnetic field and the corresponding electric field fluctuations induced within the conducting Earth, the size of which are determined by local Earth resistivity.
Based on these measurements the subsurface composition can be inferred. A benefit of the MT
technique is that the energy measured is initially derived from an external natural source, such as lightning, which partially penetrates into the Earth. This avoids the use of an artificial transmitter which becomes difficult and costly to implement when the required depth of exploration is large.
The electric field is normally estimated by the difference between point measurements of voltage, usually obtained with porous pot electrodes on the order of 50 to 100 m apart. However, the point measurements of voltage are susceptible to being influenced by near surface anomalies that may be near the location of the porous pot electrodes. These near surface inhomogeneities collect electrical charge on their peripheries, which can amplify (or diminish) the voltage measured at that point. This causes an artificial "shift" (up or down) of the apparent resistivity 2272279v12 curve, which is arguably the largest source of uncertainty in MT inversion.
This shift can be mostly frequency independent, hence the term "static shift".
The most popular technique to remove static shift effects from MT data is to collect in-loop Time-Domain-Electromagnetic (TEM) sounding data at every MT station and thus obtain a resistivity-depth profile from a purely inductive measurement. As long as the TEM data have sufficient bandwidth to overlap with the MT data, the MT apparent resistivity curve can be level shifted so as to tie-in with the TEM defined apparent resistivity. However, this is only appropriate for the MT TE (Transverse Electric) mode apparent resistivity, as it is purely inductive for a 2D structure. Furthermore, this technique may not be valid in areas of extreme topography, as is often the case in geothermal and porphyry copper settings, as the TEM data itself may be severely distorted by topography.
In the general 3D case, both apparent resistivities will be mixed mode, inductive and galvanic to some extent, thus static shift removal can be problematic in these cases.
As mentioned above, there are problems when removing static shift effects from MT data acquired by porous pot electrodes, especially in a 3D environment. What is needed, therefore, is a different system for measuring an electric field that further reduces the static shift effect observed when using porous pot electrodes.
RELATING TO SUBSURFACE GEOLOGIC STRUCTURES
Field of the Invention The present invention relates to systems for acquiring and processing electric field signals, and more specifically to systems for acquiring and processing magnetotelluric signals relating to subsurface geologic structures.
Back2round of the Invention In various situations it may become desirable to characterize the subsurface of the Earth. For example, the mining and oil industries are concerned with subsurface structures such as hydrocarbon reservoirs and ore deposits.
One way to accomplish subsurface characterisation is by a technique referred to as magnetotellurics (MT). MT involves the measurement of naturally occurring oscillations in the Earth's magnetic field and the corresponding electric field fluctuations induced within the conducting Earth, the size of which are determined by local Earth resistivity.
Based on these measurements the subsurface composition can be inferred. A benefit of the MT
technique is that the energy measured is initially derived from an external natural source, such as lightning, which partially penetrates into the Earth. This avoids the use of an artificial transmitter which becomes difficult and costly to implement when the required depth of exploration is large.
The electric field is normally estimated by the difference between point measurements of voltage, usually obtained with porous pot electrodes on the order of 50 to 100 m apart. However, the point measurements of voltage are susceptible to being influenced by near surface anomalies that may be near the location of the porous pot electrodes. These near surface inhomogeneities collect electrical charge on their peripheries, which can amplify (or diminish) the voltage measured at that point. This causes an artificial "shift" (up or down) of the apparent resistivity 2272279v12 curve, which is arguably the largest source of uncertainty in MT inversion.
This shift can be mostly frequency independent, hence the term "static shift".
The most popular technique to remove static shift effects from MT data is to collect in-loop Time-Domain-Electromagnetic (TEM) sounding data at every MT station and thus obtain a resistivity-depth profile from a purely inductive measurement. As long as the TEM data have sufficient bandwidth to overlap with the MT data, the MT apparent resistivity curve can be level shifted so as to tie-in with the TEM defined apparent resistivity. However, this is only appropriate for the MT TE (Transverse Electric) mode apparent resistivity, as it is purely inductive for a 2D structure. Furthermore, this technique may not be valid in areas of extreme topography, as is often the case in geothermal and porphyry copper settings, as the TEM data itself may be severely distorted by topography.
In the general 3D case, both apparent resistivities will be mixed mode, inductive and galvanic to some extent, thus static shift removal can be problematic in these cases.
As mentioned above, there are problems when removing static shift effects from MT data acquired by porous pot electrodes, especially in a 3D environment. What is needed, therefore, is a different system for measuring an electric field that further reduces the static shift effect observed when using porous pot electrodes.
2 2272279v12 Summary of the Invention The present invention seeks to provide a system for acquiring and processing low-frequency electric field signals relating to subsurface geologic structures.
According to a first broad aspect of the present invention, there is provided a system for acquiring and processing electric field signals relating to subsurface geologic structures, the signals selectively in a frequency range of less than 100 kHz, the system comprising:
at least one capacitive line antenna for acquiring the signals; and a feedback circuit for increasing input impedance for the signals and thus reducing the frequency at which signal attenuation occurs.
The signals according to the first broad aspect of the present invention may represent an average potential along the length of the at least one capacitive line antenna.
Preferably, the average potential is used to reduce static shift in measured horizontal electric fields. Where an apparent resistivity curve has been produced using porous pot cells, the average potential may be used to correct the apparent resistivity curve produced using the porous pot cells by reducing the static shift.
Where an apparent resistivity curve and a phase curve have been produced using porous pot cells, the apparent resistivity curve and the phase curve may be manifesting contact resistance distortion, in which case, signals according to the first broad aspect of the present invention may be used to correct for the contact resistance distortion.
In some exemplary embodiments of the first aspect, the frequency range is about 0.1 Hz to about 40 kHz.
The at least one capacitive line antenna may have a capacitance in the range of 1 nF to 4 nF.
Optionally, the at least one capacitive line antenna comprises a plurality of legs.
According to a first broad aspect of the present invention, there is provided a system for acquiring and processing electric field signals relating to subsurface geologic structures, the signals selectively in a frequency range of less than 100 kHz, the system comprising:
at least one capacitive line antenna for acquiring the signals; and a feedback circuit for increasing input impedance for the signals and thus reducing the frequency at which signal attenuation occurs.
The signals according to the first broad aspect of the present invention may represent an average potential along the length of the at least one capacitive line antenna.
Preferably, the average potential is used to reduce static shift in measured horizontal electric fields. Where an apparent resistivity curve has been produced using porous pot cells, the average potential may be used to correct the apparent resistivity curve produced using the porous pot cells by reducing the static shift.
Where an apparent resistivity curve and a phase curve have been produced using porous pot cells, the apparent resistivity curve and the phase curve may be manifesting contact resistance distortion, in which case, signals according to the first broad aspect of the present invention may be used to correct for the contact resistance distortion.
In some exemplary embodiments of the first aspect, the frequency range is about 0.1 Hz to about 40 kHz.
The at least one capacitive line antenna may have a capacitance in the range of 1 nF to 4 nF.
Optionally, the at least one capacitive line antenna comprises a plurality of legs.
3 2272279v12 Preferably, the signals are natural source electric field signals. Even more preferably, the natural source electric field signals are magnetotelluric signals.
Optionally, the signals are controlled source electric field signals.
According to a second broad aspect of the present invention, there is provided a use of a capacitive line antenna for acquiring electric field signals relating to subsurface geologic structures in a geophysical exploration method, the signals selectively in a frequency range of less than 100 kHz, the capacitive line antenna operably connected to a feedback circuit for increasing input impedance for the signals and thus reducing the frequency at which signal attenuation occurs.
The signals according to the second broad aspect of the present invention may represent an average potential along the length of the capacitive line antenna. Preferably, the average potential is used to reduce static shift in measured horizontal electric fields. Where an apparent resistivity curve has been produced using porous pot cells, the average potential may be used to correct the apparent resistivity curve produced using the porous pot cells by reducing the static shift.
Where an apparent resistivity curve and a phase curve have been produced using porous pot cells, the apparent resistivity curve and the phase curve may be manifesting contact resistance distortion, in which case, signals according to the second broad aspect of the present invention may be used to correct for the contact resistance distortion.
In some exemplary embodiments of the second aspect, the frequency range is about 0.1 Hz to about 40 kHz.
The capacitive line antenna may have a capacitance in the range of 1 nF to 4 nF.
Optionally, the capacitive line antenna comprises a plurality of legs.
Optionally, the signals are controlled source electric field signals.
According to a second broad aspect of the present invention, there is provided a use of a capacitive line antenna for acquiring electric field signals relating to subsurface geologic structures in a geophysical exploration method, the signals selectively in a frequency range of less than 100 kHz, the capacitive line antenna operably connected to a feedback circuit for increasing input impedance for the signals and thus reducing the frequency at which signal attenuation occurs.
The signals according to the second broad aspect of the present invention may represent an average potential along the length of the capacitive line antenna. Preferably, the average potential is used to reduce static shift in measured horizontal electric fields. Where an apparent resistivity curve has been produced using porous pot cells, the average potential may be used to correct the apparent resistivity curve produced using the porous pot cells by reducing the static shift.
Where an apparent resistivity curve and a phase curve have been produced using porous pot cells, the apparent resistivity curve and the phase curve may be manifesting contact resistance distortion, in which case, signals according to the second broad aspect of the present invention may be used to correct for the contact resistance distortion.
In some exemplary embodiments of the second aspect, the frequency range is about 0.1 Hz to about 40 kHz.
The capacitive line antenna may have a capacitance in the range of 1 nF to 4 nF.
Optionally, the capacitive line antenna comprises a plurality of legs.
4 2272279v12 Preferably, the signals are natural source electric field signals. Even more preferably, the natural source electric field signals are magnetotelluric signals.
Optionally, the signals are controlled source electric field signals.
According to a third broad aspect of the present invention, there is provided a method for acquiring and processing electric field signals, the signals selectively in a frequency range of less than 100 kHz, the method comprising the steps of:
a. providing at least one capacitive line antenna;
b. operating the at least one capacitive line antenna to acquire the signals;
c. providing a feedback circuit for increasing input impedance for the signals; and d. increasing the input impedance and thus reducing the frequency at which signal attenuation occurs.
The signals according to the third broad aspect of the present invention may represent an average potential along the length of the at least one capacitive line antenna.
Preferably, the average potential is used to reduce static shift in measured horizontal electric fields. Where an apparent resistivity curve has been produced using porous pot cells, the average potential may be used to correct the apparent resistivity curve produced using the porous pot cells by reducing the static shift.
Where an apparent resistivity curve and a phase curve have been produced using porous pot cells, the apparent resistivity curve and the phase curve may be manifesting contact resistance distortion, in which case, signals according to the third broad aspect of the present invention may be used to correct for the contact resistance distortion.
In some exemplary embodiments of the third aspect, the frequency range is about 0.1 Hz to about 40 kHz.
The at least one capacitive line antenna may have a capacitance in the range of 1 nE to 4 nF.
Optionally, the signals are controlled source electric field signals.
According to a third broad aspect of the present invention, there is provided a method for acquiring and processing electric field signals, the signals selectively in a frequency range of less than 100 kHz, the method comprising the steps of:
a. providing at least one capacitive line antenna;
b. operating the at least one capacitive line antenna to acquire the signals;
c. providing a feedback circuit for increasing input impedance for the signals; and d. increasing the input impedance and thus reducing the frequency at which signal attenuation occurs.
The signals according to the third broad aspect of the present invention may represent an average potential along the length of the at least one capacitive line antenna.
Preferably, the average potential is used to reduce static shift in measured horizontal electric fields. Where an apparent resistivity curve has been produced using porous pot cells, the average potential may be used to correct the apparent resistivity curve produced using the porous pot cells by reducing the static shift.
Where an apparent resistivity curve and a phase curve have been produced using porous pot cells, the apparent resistivity curve and the phase curve may be manifesting contact resistance distortion, in which case, signals according to the third broad aspect of the present invention may be used to correct for the contact resistance distortion.
In some exemplary embodiments of the third aspect, the frequency range is about 0.1 Hz to about 40 kHz.
The at least one capacitive line antenna may have a capacitance in the range of 1 nE to 4 nF.
5 2272279v12 Optionally, the at least one capacitive line antenna comprises a plurality of legs.
Preferably, the signals are natural source electric field signals. Even more preferably, the natural source electric field signals are magnetotelluric signals.
Optionally, the signals are controlled source electric field signals.
Brief Description of the Drawin2s In the accompanying drawings, which illustrate exemplary embodiments of the present invention:
Figure 1A is simplified view of a first exemplary embodiment of the present invention;
Figure 1B is simplified view of a another embodiment of the present invention;
Figure 2 is a photo of potential capacitive line antennas of the first exemplary embodiment of the present invention;
Figure 3A is a photo of a pre-amplifier of the first exemplary embodiment of the present invention Figure 3B is a photo of a pre-amplifier of the first exemplary embodiment of the present invention;
Figure 4A is a schematic of a bootstrap circuit;
Figure 4B is a schematic of a feedback circuit that may be employed in the first exemplary embodiment of the present invention;
Preferably, the signals are natural source electric field signals. Even more preferably, the natural source electric field signals are magnetotelluric signals.
Optionally, the signals are controlled source electric field signals.
Brief Description of the Drawin2s In the accompanying drawings, which illustrate exemplary embodiments of the present invention:
Figure 1A is simplified view of a first exemplary embodiment of the present invention;
Figure 1B is simplified view of a another embodiment of the present invention;
Figure 2 is a photo of potential capacitive line antennas of the first exemplary embodiment of the present invention;
Figure 3A is a photo of a pre-amplifier of the first exemplary embodiment of the present invention Figure 3B is a photo of a pre-amplifier of the first exemplary embodiment of the present invention;
Figure 4A is a schematic of a bootstrap circuit;
Figure 4B is a schematic of a feedback circuit that may be employed in the first exemplary embodiment of the present invention;
6 2272279v12 Figure 4C is a schematic of a feedback circuit that may be employed in the first exemplary embodiment of the present invention;
Figure 5A is a chart containing resistivity curves for data acquired by capacitive line antennas and porous pots;
Figure 5B is a chart containing resistivity curves for data acquired by capacitive line antennas and porous pots; and Figure 5C is a chart containing resistivity curves for data acquired by a capacitive line antennas and porous pots.
Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings.
Detailed Description of Exemplary Embodiments Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. The following description of examples of the technology is not intended to be exhaustive or to limit the invention to the precise form of any exemplary embodiment. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
Referring now to the accompanying drawings, embodiments of a system according to the present invention are illustrated. It is to be understood that the illustrated embodiments are exemplary only and other embodiments may properly fall within the scope of the claims.
The present invention is directed to systems for acquiring and processing electric field signals relating to subsurface geologic structures. Specifically, the systems are for acquiring and processing signals selectively in a frequency range of less than 100 kHz.
Preferably, the systems
Figure 5A is a chart containing resistivity curves for data acquired by capacitive line antennas and porous pots;
Figure 5B is a chart containing resistivity curves for data acquired by capacitive line antennas and porous pots; and Figure 5C is a chart containing resistivity curves for data acquired by a capacitive line antennas and porous pots.
Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings.
Detailed Description of Exemplary Embodiments Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. The following description of examples of the technology is not intended to be exhaustive or to limit the invention to the precise form of any exemplary embodiment. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
Referring now to the accompanying drawings, embodiments of a system according to the present invention are illustrated. It is to be understood that the illustrated embodiments are exemplary only and other embodiments may properly fall within the scope of the claims.
The present invention is directed to systems for acquiring and processing electric field signals relating to subsurface geologic structures. Specifically, the systems are for acquiring and processing signals selectively in a frequency range of less than 100 kHz.
Preferably, the systems
7 2272279v12 are for acquiring and processing signals selectively in a frequency range of about 1 Hz to about 40 kHz. The signals referred to throughout the description may be AC signals and they may vary with time.
Preferably, the signals acquired and processed by the present invention are natural source electric field signals. The signals may be magnetotelluric signals.
Turning to Figure 1A, a first exemplary embodiment of the present invention is illustrated. A
system 10 comprising a plurality of capacitive line antennas 12 connected to a pre-amplifier 14 is shown. The capacitive line antennas 12 are for acquiring electric field signals which may relate to subsurface geologic structures. Preferably, the plurality of capacitive line antennas 12 are arranged such that they are spread apart from one another while laying on the surface of the ground.
The capacitive line antennas 12 may comprise a plurality of legs 18. Legs 18 provide the directionality of the measurement since it may be needed to establish a measurement co-ordinate system. Using this preferred setup, a differential measurement of the electric field is obtained which also serves to cancel any common mode noise that may be present in the North/South (and East/West) capacitive line antennas. Alternatively, the capacitive line antennas 12 may only comprise one leg 18 as shown in Figure 1B. This single-ended measurement is more convenient for field operations but provides no common mode noise cancellation and therefore may give lower quality results in some cases.
The capacitive line antennas 12 may permit effective operations where traditional porous pot measurements would be impossible or very difficult, such as, on frozen ground, very rocky ground, very dry ground or other rugged conditions. This is because capacitive line antennas are non-contacting; there is no galvanic or direct electrical contact with the earth required. When the ground is frozen or comprises rocks or very dry material with almost no clay, pounding a conventional stainless steel stake into frozen ground, or into the side of a rock, or into dry sand, may be ineffective as the contact resistance is substantial, and the quality of contact may therefore be extremely poor. On the other hand, the earth potential that is measured with a
Preferably, the signals acquired and processed by the present invention are natural source electric field signals. The signals may be magnetotelluric signals.
Turning to Figure 1A, a first exemplary embodiment of the present invention is illustrated. A
system 10 comprising a plurality of capacitive line antennas 12 connected to a pre-amplifier 14 is shown. The capacitive line antennas 12 are for acquiring electric field signals which may relate to subsurface geologic structures. Preferably, the plurality of capacitive line antennas 12 are arranged such that they are spread apart from one another while laying on the surface of the ground.
The capacitive line antennas 12 may comprise a plurality of legs 18. Legs 18 provide the directionality of the measurement since it may be needed to establish a measurement co-ordinate system. Using this preferred setup, a differential measurement of the electric field is obtained which also serves to cancel any common mode noise that may be present in the North/South (and East/West) capacitive line antennas. Alternatively, the capacitive line antennas 12 may only comprise one leg 18 as shown in Figure 1B. This single-ended measurement is more convenient for field operations but provides no common mode noise cancellation and therefore may give lower quality results in some cases.
The capacitive line antennas 12 may permit effective operations where traditional porous pot measurements would be impossible or very difficult, such as, on frozen ground, very rocky ground, very dry ground or other rugged conditions. This is because capacitive line antennas are non-contacting; there is no galvanic or direct electrical contact with the earth required. When the ground is frozen or comprises rocks or very dry material with almost no clay, pounding a conventional stainless steel stake into frozen ground, or into the side of a rock, or into dry sand, may be ineffective as the contact resistance is substantial, and the quality of contact may therefore be extremely poor. On the other hand, the earth potential that is measured with a
8 2272279v12 capacitive line antenna is coupled capacitively, so no direct electrical contact is required. Thus, the capacitive line antenna may be less influenced by conditions of the ground.
The pre-amplifier 14 is preferably in communication with a data logger 16 that controls and preforms the acquisition process of the signal that comes from the pre-atnplifier. The pre-amplifier 14 may be followed by another amplifier which further filters and amplifies the signal prior to digitization. A person skilled in the art would know of data loggers suitable for application to embodiments of the present invention. A data logger 16 may comprise a 16 bit, 8 channel, 1 MHz throughput, successive approximation analog-to-digital converter, connected to a ruggedized laptop via a USB cable. The laptop may run software that comes with the analog-to-digital converter and thus controls the triggering of events and stores those events on its hard drive.
Preferably, the capacitive line antennas 12 comprise a wire of a gauge causing the antennas 12 to be of a weight that prevents movement of the antennas 12 during conditions such as high winds.
Less movement of the antennas may result in acquiring better quality data at low frequencies as measurements are typically sensitive to antenna motion. An example of such a wire that may be used is the Belden 1857A tri-axial cable, which is shown in Figure 2 as 24. A
higher gauge wire (smaller diameter) such as wire 22 shown in Figure 2 may be acceptable for use during calm to moderately breezy conditions. Furthermore, line antennas 12 comprising a lower gauge wire (larger diameter) may provide higher capacitance than wires of a higher gauge (smaller diameter). Preferably, the capacitive line antennas 12 have a capacitance in the range of 1 nF to 4 nF. A person skilled in the art would know of other suitable wires or cables that could be used with embodiments of the present invention. An appropriate length of the leg 18 of an antenna 12 may range from 20 to 100 m, however, a person skilled in the art would know of other suitable lengths that could be used with embodiments of the present invention 3D features at or near the smallest skin depth can be recognized and modeled with 3D inversion code. Skin depth relates to the depth at which the electromagnetic field decays and is relevant to the depth of investigation for a given frequency of an electromagnetic wave.
As frequency lowers, skin depth (depth of exploration) increases.
The pre-amplifier 14 is preferably in communication with a data logger 16 that controls and preforms the acquisition process of the signal that comes from the pre-atnplifier. The pre-amplifier 14 may be followed by another amplifier which further filters and amplifies the signal prior to digitization. A person skilled in the art would know of data loggers suitable for application to embodiments of the present invention. A data logger 16 may comprise a 16 bit, 8 channel, 1 MHz throughput, successive approximation analog-to-digital converter, connected to a ruggedized laptop via a USB cable. The laptop may run software that comes with the analog-to-digital converter and thus controls the triggering of events and stores those events on its hard drive.
Preferably, the capacitive line antennas 12 comprise a wire of a gauge causing the antennas 12 to be of a weight that prevents movement of the antennas 12 during conditions such as high winds.
Less movement of the antennas may result in acquiring better quality data at low frequencies as measurements are typically sensitive to antenna motion. An example of such a wire that may be used is the Belden 1857A tri-axial cable, which is shown in Figure 2 as 24. A
higher gauge wire (smaller diameter) such as wire 22 shown in Figure 2 may be acceptable for use during calm to moderately breezy conditions. Furthermore, line antennas 12 comprising a lower gauge wire (larger diameter) may provide higher capacitance than wires of a higher gauge (smaller diameter). Preferably, the capacitive line antennas 12 have a capacitance in the range of 1 nF to 4 nF. A person skilled in the art would know of other suitable wires or cables that could be used with embodiments of the present invention. An appropriate length of the leg 18 of an antenna 12 may range from 20 to 100 m, however, a person skilled in the art would know of other suitable lengths that could be used with embodiments of the present invention 3D features at or near the smallest skin depth can be recognized and modeled with 3D inversion code. Skin depth relates to the depth at which the electromagnetic field decays and is relevant to the depth of investigation for a given frequency of an electromagnetic wave.
As frequency lowers, skin depth (depth of exploration) increases.
9 2272279v12 Turning to Figures 3A and 3B, a pre-amplifier 14 according to a first exemplary embodiment of the present invention is shown. Preferably, the pre-amplifier 14 employs a bootstrapping circuit 32 that a person skilled in the art would know of This typically involves a circuit that takes an output and feeds it back to the input, thus altering the input impedance.
Extra damping resistors may be added for stability of the pre-amplifier 14 and to dampen its output.
The pre-amplifier 14 also comprises a battery source 34, a power switch 36 and input means 38 for which the legs 18 of each antenna 12 connect to. Figures 3A and 3B show two input means 38, thus, two legs 18 of an antenna 12 could be connected to the pre-amplifier. The other legs 18 of another antenna would be connected to another pre-amplifier 14. In other embodiments, several legs 18 from several antennas 12 may be connected to a pre-amplifier 14 with several input means.
A conventional bootstrap circuit schematic is shown in Figure 4A. The pre-amplifier 14 may comprise a feedback circuit for increasing input impedance for the signals that are input to the pre-amplifier 14, thus reducing the frequency at which signal attenuation occurs. The feedback circuit increases the input impedance for AC signals so that low frequency AC
signal recovery may occur with little or no attenuation. In other words, the larger the input impedance, the lower the frequency that can be acquired before the AC signals start to become attenuated by what is essentially a voltage divider (formed by the antenna impedance and the pre-amplifier input impedance). Schematics of feedback circuits that may be employed are shown in Figures 4B and 4C.
Preferably, the signals acquired by the capacitive line antennas 12 of the first exemplary embodiment represent an average potential along the length of the capacitive line antennas 12.
Therefore, the effects of small, near-surface inhomogeneities may be averaged over to some extent and their influence lessened, thus potentially reducing static shift.
In other signal-acquiring methods, such as porous pots, contact resistance and finite line capacitance may work together to form a low pass filter. If contact resistance is large enough, an artificial distortion of the high frequency apparent resistivity and phase curves occurs. However, as discussed previously, capacitive line antennas are non-contacting and as a result are typically 2272279v12 free of contact resistance distortion. Therefore, capacitive line antennas can be used to provide a corrected version of the apparent resistivity and phase curves compared to the curves obtained with porous pots alone.
Turning to Figures 5A to 5C, resistivity curves for data acquired by capacitive line antennas and porous pots are illustrated. In Figure 5A, a resistivity curve is illustrated in edited and corrected forms. For example, the correction procedure may comprise the following two steps. The first step is to remove the static shift by observing low frequency level shifts between an apparent resistivity curve collected with porous pot electrodes and an apparent resistivity curve collected at the same station with the capacitive line antenna. After static shift correction, contact resistance correction can occur by simply replacing the high frequency (¨ >
3000 Hz) portion of the (porous pot) static shift corrected apparent resistivity curve with the apparent resistivity curve derived from the capacitive line antenna as the latter is free of contact resistance distortion.
Lastly, the static shift and contact resistance corrected apparent resistivity curve is edited with a graphical editor with knowledge of the fact that the true underlying apparent resistivity curve must be a smoothly (slowly) varying function of frequency, so very sharp or quickly changing portions of the curve, such as may exist in the "dead-band" region (500 Hz to 5000 Hz) may be replaced with a smooth, linear interpolant. This is done mainly for the purpose of plotting (presentation), and interpolated (edited) portions of the curve are naturally down weighted with very large error bars so editing the curve in a noisy portion will have little, if any, effect on the final inversion since the error bars are very large in edited portions of the curve.
The phase is a static-free quantity and therefore requires no static correction; however, it is greatly affected by contact resistance distortion. As can be seen in Figures 5A to 5C, where the curve from the porous pot survey data drops vertically (at the "corner frequency" of the unwanted filter, caused by line capacitance and contact resistance), the phase curve will already have been distorted by 45 degrees. As this unwanted corner frequency can be in the range of
Extra damping resistors may be added for stability of the pre-amplifier 14 and to dampen its output.
The pre-amplifier 14 also comprises a battery source 34, a power switch 36 and input means 38 for which the legs 18 of each antenna 12 connect to. Figures 3A and 3B show two input means 38, thus, two legs 18 of an antenna 12 could be connected to the pre-amplifier. The other legs 18 of another antenna would be connected to another pre-amplifier 14. In other embodiments, several legs 18 from several antennas 12 may be connected to a pre-amplifier 14 with several input means.
A conventional bootstrap circuit schematic is shown in Figure 4A. The pre-amplifier 14 may comprise a feedback circuit for increasing input impedance for the signals that are input to the pre-amplifier 14, thus reducing the frequency at which signal attenuation occurs. The feedback circuit increases the input impedance for AC signals so that low frequency AC
signal recovery may occur with little or no attenuation. In other words, the larger the input impedance, the lower the frequency that can be acquired before the AC signals start to become attenuated by what is essentially a voltage divider (formed by the antenna impedance and the pre-amplifier input impedance). Schematics of feedback circuits that may be employed are shown in Figures 4B and 4C.
Preferably, the signals acquired by the capacitive line antennas 12 of the first exemplary embodiment represent an average potential along the length of the capacitive line antennas 12.
Therefore, the effects of small, near-surface inhomogeneities may be averaged over to some extent and their influence lessened, thus potentially reducing static shift.
In other signal-acquiring methods, such as porous pots, contact resistance and finite line capacitance may work together to form a low pass filter. If contact resistance is large enough, an artificial distortion of the high frequency apparent resistivity and phase curves occurs. However, as discussed previously, capacitive line antennas are non-contacting and as a result are typically 2272279v12 free of contact resistance distortion. Therefore, capacitive line antennas can be used to provide a corrected version of the apparent resistivity and phase curves compared to the curves obtained with porous pots alone.
Turning to Figures 5A to 5C, resistivity curves for data acquired by capacitive line antennas and porous pots are illustrated. In Figure 5A, a resistivity curve is illustrated in edited and corrected forms. For example, the correction procedure may comprise the following two steps. The first step is to remove the static shift by observing low frequency level shifts between an apparent resistivity curve collected with porous pot electrodes and an apparent resistivity curve collected at the same station with the capacitive line antenna. After static shift correction, contact resistance correction can occur by simply replacing the high frequency (¨ >
3000 Hz) portion of the (porous pot) static shift corrected apparent resistivity curve with the apparent resistivity curve derived from the capacitive line antenna as the latter is free of contact resistance distortion.
Lastly, the static shift and contact resistance corrected apparent resistivity curve is edited with a graphical editor with knowledge of the fact that the true underlying apparent resistivity curve must be a smoothly (slowly) varying function of frequency, so very sharp or quickly changing portions of the curve, such as may exist in the "dead-band" region (500 Hz to 5000 Hz) may be replaced with a smooth, linear interpolant. This is done mainly for the purpose of plotting (presentation), and interpolated (edited) portions of the curve are naturally down weighted with very large error bars so editing the curve in a noisy portion will have little, if any, effect on the final inversion since the error bars are very large in edited portions of the curve.
The phase is a static-free quantity and therefore requires no static correction; however, it is greatly affected by contact resistance distortion. As can be seen in Figures 5A to 5C, where the curve from the porous pot survey data drops vertically (at the "corner frequency" of the unwanted filter, caused by line capacitance and contact resistance), the phase curve will already have been distorted by 45 degrees. As this unwanted corner frequency can be in the range of
10's of kHz, the amount of phase distortion can still be quite significant, many degrees, at frequencies as low as 1000 Hz. Therefore, the effect of contact resistance distortion is greater on the phase in terms of the bandwidth of data that is affected. To correct for distortion on the phase, the frequency below which the porous pot phase curve and the capacitive line antenna
11 2272279v12 phase curve nicely overlie is noted, above this frequency the undistorted capacitive line antenna phase curve is used. Alternatively, a frequency dependent correction could be applied to the porous pot phase curve based on the difference between the undistorted capacitive line antenna phase curve and the distorted porous pot phase curve.
As shown in Figure 5A, the resistivity curve obtained by capacitive line antennas reduce static shift and contact resistance distortion, while the resistivity curve obtained by porous pots have areas where static shift and contact resistance distortion is observed. A
larger static shift is shown in Figure 5B. A resistivity curve obtained by porous pots where only contact resistance distortion is observed and not static shift is shown in Figure 5C. Resistivity curves shown in Figures 5A to 5C were generated with EMpulse Geophysics proprietary Adaptive Polarization Stacking algorithm (Goldak, D.K. and Goldak, M.S., 2001, Transient Magnetotellurics with Adaptive Polarization Stacking, presented at the SEG International Exposition and 71'st annual meeting, San Antonio, 1509-1512).
According to a second exemplary embodiment of the present invention there is provided a use of the system according to the first exemplary embodiment of the present invention.
According to a third exemplary embodiment of the present invention there is provided a method analogous to the system according to the first exemplary embodiment of the present invention.
Turning again to Figure 1, the plurality of capacitive line antennas 12 are provided such that they are spread apart from one another while lying on the suiface of the ground.
The antenna legs are pulled out and laid down as close to the ground as possible, being careful to avoid small shrubs and tall grass. Having the antenna legs as close to the ground as possible helps to minimize antenna motion due to wind and also maximizes the antenna capacitance, both of these aspects being beneficial for low frequency data collection (< 20 Hz). The plurality of capacitive line antennas 12 are operated to acquire signals. The acquired signals are input to the pre-amplifier 14 that may comprise a feedback circuit. This typically involves a circuit that takes an output and feeds it back to the input, thus altering the input impedance. The feedback circuit is for increasing the input impedance for AC signals so that low frequency AC signal recovery may occur with little or no attenuation. Signals from the pre-amplifier 14 are then delivered to the
As shown in Figure 5A, the resistivity curve obtained by capacitive line antennas reduce static shift and contact resistance distortion, while the resistivity curve obtained by porous pots have areas where static shift and contact resistance distortion is observed. A
larger static shift is shown in Figure 5B. A resistivity curve obtained by porous pots where only contact resistance distortion is observed and not static shift is shown in Figure 5C. Resistivity curves shown in Figures 5A to 5C were generated with EMpulse Geophysics proprietary Adaptive Polarization Stacking algorithm (Goldak, D.K. and Goldak, M.S., 2001, Transient Magnetotellurics with Adaptive Polarization Stacking, presented at the SEG International Exposition and 71'st annual meeting, San Antonio, 1509-1512).
According to a second exemplary embodiment of the present invention there is provided a use of the system according to the first exemplary embodiment of the present invention.
According to a third exemplary embodiment of the present invention there is provided a method analogous to the system according to the first exemplary embodiment of the present invention.
Turning again to Figure 1, the plurality of capacitive line antennas 12 are provided such that they are spread apart from one another while lying on the suiface of the ground.
The antenna legs are pulled out and laid down as close to the ground as possible, being careful to avoid small shrubs and tall grass. Having the antenna legs as close to the ground as possible helps to minimize antenna motion due to wind and also maximizes the antenna capacitance, both of these aspects being beneficial for low frequency data collection (< 20 Hz). The plurality of capacitive line antennas 12 are operated to acquire signals. The acquired signals are input to the pre-amplifier 14 that may comprise a feedback circuit. This typically involves a circuit that takes an output and feeds it back to the input, thus altering the input impedance. The feedback circuit is for increasing the input impedance for AC signals so that low frequency AC signal recovery may occur with little or no attenuation. Signals from the pre-amplifier 14 are then delivered to the
12 2272279v12 data logger where the signals are processed. The signals acquired may be processed by 3D
inversion code which may enable recognition and modeling of 3D features at or near the smallest skin depth.
Preferably, the components used for the third exemplary embodiment of the present invention are analogous to the components described for the first exemplary embodiment of the present invention.
Unless the context clearly requires otherwise, throughout the description and the claims:
= "comprise", "comprising", and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".
= "connected", "coupled", or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof = "herein", "above", "below", and words of similar import, when used to describe this specification shall refer to this specification as a whole and not to any particular portions of this specification.
= "or", in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.
= the singular forms "a", "an" and "the" also include the meaning of any appropriate plural forms.
Words that indicate directions such as "vertical", "transverse", "horizontal", "upward", "downward", "forward", "backward", "inward", "outward", "vertical", "transverse", "left", "right", "front", "back", "top", "bottom", "below", "above", "under", and the like, used in this description and any accompanying claims (where present) depend on the specific orientation of the apparatus described and illustrated. The subject matter described herein may assume various alternative orientations. Accordingly, these directional terms are not strictly defined and should not be interpreted narrowly.
inversion code which may enable recognition and modeling of 3D features at or near the smallest skin depth.
Preferably, the components used for the third exemplary embodiment of the present invention are analogous to the components described for the first exemplary embodiment of the present invention.
Unless the context clearly requires otherwise, throughout the description and the claims:
= "comprise", "comprising", and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".
= "connected", "coupled", or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof = "herein", "above", "below", and words of similar import, when used to describe this specification shall refer to this specification as a whole and not to any particular portions of this specification.
= "or", in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.
= the singular forms "a", "an" and "the" also include the meaning of any appropriate plural forms.
Words that indicate directions such as "vertical", "transverse", "horizontal", "upward", "downward", "forward", "backward", "inward", "outward", "vertical", "transverse", "left", "right", "front", "back", "top", "bottom", "below", "above", "under", and the like, used in this description and any accompanying claims (where present) depend on the specific orientation of the apparatus described and illustrated. The subject matter described herein may assume various alternative orientations. Accordingly, these directional terms are not strictly defined and should not be interpreted narrowly.
13 2272279v12 Where a component (e.g. a circuit, module, assembly, device, etc.) is referred to herein, unless otherwise indicated, reference to that component (including a reference to a "means") should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention.
Specific examples of methods and apparatus have been described herein for purposes of illustration. These are only examples. The technology provided herein can be applied to contexts other than the exemplary contexts described above. Many alterations, modifications, additions, omissions and permutations are possible within the practice of this invention. This invention includes variations on described embodiments that would be apparent to the skilled person, including variations obtained by: replacing features, elements and/or acts with equivalent features, elements and/or acts; mixing and matching of features, elements and/or acts from different embodiments; combining features, elements and/or acts from embodiments as described herein with features, elements and/or acts of other technology; and/or omitting combining features, elements and/or acts from described embodiments.
The foregoing is considered as illustrative only of the principles of the invention. The scope of the claims should not be limited by the exemplary embodiments set forth in the foregoing, but should be given the broadest interpretation consistent with the specification as a whole.
Specific examples of methods and apparatus have been described herein for purposes of illustration. These are only examples. The technology provided herein can be applied to contexts other than the exemplary contexts described above. Many alterations, modifications, additions, omissions and permutations are possible within the practice of this invention. This invention includes variations on described embodiments that would be apparent to the skilled person, including variations obtained by: replacing features, elements and/or acts with equivalent features, elements and/or acts; mixing and matching of features, elements and/or acts from different embodiments; combining features, elements and/or acts from embodiments as described herein with features, elements and/or acts of other technology; and/or omitting combining features, elements and/or acts from described embodiments.
The foregoing is considered as illustrative only of the principles of the invention. The scope of the claims should not be limited by the exemplary embodiments set forth in the foregoing, but should be given the broadest interpretation consistent with the specification as a whole.
14 2272279v12
Claims (30)
1. A system for acquiring and processing electric field signals relating to subsurface geologic structures, the system comprising:
at least one capacitive line antenna for acquiring the signals selectively in a frequency range of less than 100 kHz;
a feedback circuit for increasing input impedance for the signals and thus reducing a frequency at which signal attenuation occurs; and wherein the signals represent an average potential along the length of the at least one capacitive line antenna.
at least one capacitive line antenna for acquiring the signals selectively in a frequency range of less than 100 kHz;
a feedback circuit for increasing input impedance for the signals and thus reducing a frequency at which signal attenuation occurs; and wherein the signals represent an average potential along the length of the at least one capacitive line antenna.
2. The system of claim 1 wherein using the average potential reduces static shift in measured horizontal electric fields.
3. The system of claim 2 wherein an apparent resistivity curve has been produced using porous pot cells, the average potential used to correct the apparent resistivity curve produced using the porous pot cells by reducing the static shift.
4. The system of claim 1 wherein an apparent resistivity curve and a phase curve have been produced using porous pot cells, the apparent resistivity curve and the phase curve manifesting contact resistance distortion, the signals used to correct for the contact resistance distortion.
5. The system of claim 1 wherein the frequency range is about 0.1 Hz to about 40 kHz.
6. The system of claim 1 wherein the at least one capacitive line antenna has a capacitance in the range of 1 nF to 4 nF.
7. The system of any one of claims 1 to 6 wherein the at least one capacitive line antenna comprises a plurality of legs.
8. The system of any one of claims 1 to 7 wherein the signals are natural source electric field signals.
9. The system of claim 8 wherein the natural source electric field signals arc magnetotelluric signals.
10. The system of any one of claims 1 to 7 wherein the signals are controlled source electric field signals.
11. Use of a capacitive line antenna for acquiring electric field signals relating to subsurface geologic structures in a geophysical exploration method, the signals selectively in a frequency range of less than 100 kHz, the capacitive line antenna operably connected to a feedback circuit for increasing input impedance for the signals and thus reducing a frequency at which signal attenuation occurs, the signals represent an average potential along the length of the capacitive line antenna.
12. The use of claim 11 wherein using the average potential reduces static shift in measured horizontal electric fields.
13. The use of claim 12 wherein an apparent resistivity curve has been produced using porous pot cells, the average potential used to correct the apparent resistivity curve produced using the porous pot cells by reducing the static shift.
14. The use of claim 11 wherein an apparent resistivity curve and a phase curve have been produced using porous pot cells, the apparent resistivity curve and the phase curve manifesting contact resistance distortion, the electric field signals used to correct. for the contact resistance distortion.
15. The use of claim 11 wherein the frequency range is about 0.1 Hz to about 40 kHz.
16. The use of claim 11 wherein the capacitive line antenna has a capacitance in the range of 1 nF to 4 nF.
17. The use of any one of claims 11 to 16 wherein the capacitive line antenna comprises a plurality of legs.
18. The use of any one of claims 11 to 17 wherein the signals are natural source electric field signals.
19. The use of claim 18 wherein the natural source electric field signals are magnetotelluric signals.
20. The use of any one of claims 11 to 17 wherein the signals are controlled source electric field signals.
21. A method for acquiring and processing electric field signals, the signals selectively in a frequency range of less than 100 kHz, the method comprising the steps of:
providing at least one capacitive line antenna;
operating the at least one capacitive line antenna to acquire the signals representing an average potential along the length of the at least one capacitive antenna;
providing a feedback circuit for increasing input impedance for the signals;
and increasing the input impedance and thus reducing a frequency at which signal attenuation occurs.
providing at least one capacitive line antenna;
operating the at least one capacitive line antenna to acquire the signals representing an average potential along the length of the at least one capacitive antenna;
providing a feedback circuit for increasing input impedance for the signals;
and increasing the input impedance and thus reducing a frequency at which signal attenuation occurs.
22. The method of claim 21 wherein using the average potential reduces static shift in measured horizontal electric fields.
23. The method of claim 22 wherein an apparent resistivity curve has been produced using porous pot cells, the average potential used to correct the apparent resistivity curve produced using the porous pot cells by reducing the static shift.
24. The method of claim 21 wherein an apparent resistivity curve and a phase curve have been produced using porous pot cells, the apparent resistivity curve and the phase curve manifesting contact resistance distortion, the signals used to correct for the contact resistance distortion.
25. The method of claim 21 wherein the frequency range is about 0.1 Hz to about 40 kHz.
26. The method of claim 21 wherein the at least one capacitive line antenna has a capacitance in the range of 1 nF to 4 nF.
27. The method of any one of claims 21 to 26 wherein the at least one capacitive line antenna comprises a plurality of legs.
28. The method of any one of claims 21 to 27 wherein the signals are natural source electric field signals.
29. The method of claim 28 wherein the natural source electric field signals are magnetotelluric signals.
30. The method of any one of claims 21 to 27 wherein the signals are controlled source electric field signals.
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| Application Number | Priority Date | Filing Date | Title |
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| CA2941995A CA2941995C (en) | 2016-09-15 | 2016-09-15 | System for acquiring and processing electric field signals relating to subsurface geologic structures |
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| CA2941995A CA2941995C (en) | 2016-09-15 | 2016-09-15 | System for acquiring and processing electric field signals relating to subsurface geologic structures |
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN111965712A (en) * | 2020-10-21 | 2020-11-20 | 国网江西省电力有限公司电力科学研究院 | Method for correcting static effect of controllable source audio magnetotelluric method |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN111965712A (en) * | 2020-10-21 | 2020-11-20 | 国网江西省电力有限公司电力科学研究院 | Method for correcting static effect of controllable source audio magnetotelluric method |
| CN111965712B (en) * | 2020-10-21 | 2021-03-02 | 国网江西省电力有限公司电力科学研究院 | Method for correcting static effect of controllable source audio magnetotelluric method |
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