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Hi everybody,
As some of you have probably seen on the mag forum, we are now out of the design and prototyping stages of our Mark II project (i.e. a proton magnetometer) and in production phase.
I plan my next challenging project to be the feasibility study followed eventually by the design and build of a "Multi-frequency EM Induction Sensor".
I already have accumulated a pile of useful data on that subject and I have defined a number of possible design tracks.
My simple question is: Is there anybody out there who is willing to make this trip with me?
In that case, I am ready to uncover what I have already learned about the subject and what is the current status of my project.
Thanks,
Willy
For those who are not yet documented about this, this is short functional description:
What can an EMI sensor do?
A magnetometer is used in a geophysical survey to measure magnetic susceptibility variations in earth. It is a passive sensor because it uses the ambient earth magnetic field as the source of excitation.
An electromagnetic (EM) sensor operating below the radio frequency (RF), or in an EM induction (EMI) mode, is commonly used to measure electrical conductivity variations in earth. (An EM sensor operating above the RF, commonly called ground-probing radar or GPR, can measure variations in dielectric permitivity, which we do not consider here.)
For this reason, an EMI sensor is often called a conductivity meter. It is an active sensor because it carries its own source of excitation. It has been common to consider that a magnetometer is the principal sensor for measuring magnetic susceptibility, and that an EMI sensor is the principal sensor for measuring electricalconductivity.
An EMI sensor responds to both electrical conductivity and magnetic susceptibility. In fact, an EMI sensor operating at sufficiently low frequencies acts more as a magnetometer than as a conductivity meter. At the so-called “resistive limit” where the conductivity-frequency product is small, an EMI sensor responds only to magnetic susceptibility and ignores electrical conductivity. It would be a serious misnomer in this case to call an EMI sensor a conductivity meter. A correct designation would be an active magnetometer or, as we propose here, an electro-magnetometer.
A properly designed electro-magnetometer can serve two simultaneous functions: that of a magnetometer and of a conductivitymeter.
EMI surveys are used to:
· Locate buried tanks and pipes
· Locate pits and trenches containing metallic and/or nonmetallic debris
· Delineate landfill boundaries
· Delineate oil production sumps and mud pits
· Map conductive soil and groundwater contamination
· Characterize subsurface hydrogeology
· Map buried channel deposits
· Map geologic structure
· Conduct groundwater exploration
· Locate conductive fault and fracture zones
See GEM-2 Data examples for a few typical multi-depth, apparent conductivity and apparent susceptibility profiling results.
Old EMI Technology
· The EMI instruments were originally made of two separate and quite distant coils due to the influence of the powerful transmitter magnetic field on the receiving coil. This made those instruments difficult to deploy and operate; two operators were necessary. The modern instruments use a bucking coil to cancel the transmitted field allowing assembling the coils in a more compact packaging that can be carried by a single operator.
· Geometrical sounding versus frequency sounding.
By technical necessity, most old frequency-domain (FD) EMI sensors have employed the principle of “geometrical sounding” where the coil separation is the only variable since all coils operate at a factory-set frequency. Because these coils are tuned to a particular frequency by their inductance and external capacitance, one cannot change the operating frequency without replacing all coils or associated electronics.
A tuned coil derives its signal strength from its Q (called the figure of merit—the sharper the resonance, the higher the Q), a voltage amplification factor at the tuned frequency.
Therefore, the only means left to the user of such a system is changing the coil separation, which often requires multiple operators tending separate coils connected by cables to a measuring console. Furthermore, the system must maintain a considerable coil separation to avoid RX saturation from the primary TX field. It is obvious that such a sensor cannot be made into a small, handheld package.
In contrast, depth sounding by changing frequency (or “frequency sounding”) measures the earth response at multiple frequencies at a fixed TX-RX geometry. In frequency sounding, there is no exclusive relationship between the coil separation and the depth of exploration.
· Multi-depth Profiling
Measurements are taken at successive intervals along a profile with multiple or stepping frequencies. Data are presented as profiles or contour maps and interpreted qualitatively. The depth of the profile essentially depends on the operational frequency used to plot it (lower frequencies show deeper ground layers, higher frequencies show shallower ground layers).
Objectives
Advantages of a broadband, multi-frequency EMI sensor are obvious. The idea of using multiple frequencies stems from the so-called “skin-depth”, also known as the depth of exploration, which is inversely proportional to frequency: a low-frequency signal travels far through a conductive earth and, thus, "sees" deep structures, while a high-frequency signal can travel only a short distance and thus, "sees" only shallow structures. Therefore, scanning through a frequency window is equivalent to depth sounding. However, it is also possible to use the I and Q values coming from one particular frequency (i.e. one depth) to produce the 2D apparent Susceptibility and/or apparent Conductivity profile of the survey area at that depth.
As some of you have probably seen on the mag forum, we are now out of the design and prototyping stages of our Mark II project (i.e. a proton magnetometer) and in production phase.
I plan my next challenging project to be the feasibility study followed eventually by the design and build of a "Multi-frequency EM Induction Sensor".
I already have accumulated a pile of useful data on that subject and I have defined a number of possible design tracks.
My simple question is: Is there anybody out there who is willing to make this trip with me?
In that case, I am ready to uncover what I have already learned about the subject and what is the current status of my project.
Thanks,
Willy
For those who are not yet documented about this, this is short functional description:
What can an EMI sensor do?
A magnetometer is used in a geophysical survey to measure magnetic susceptibility variations in earth. It is a passive sensor because it uses the ambient earth magnetic field as the source of excitation.
An electromagnetic (EM) sensor operating below the radio frequency (RF), or in an EM induction (EMI) mode, is commonly used to measure electrical conductivity variations in earth. (An EM sensor operating above the RF, commonly called ground-probing radar or GPR, can measure variations in dielectric permitivity, which we do not consider here.)
For this reason, an EMI sensor is often called a conductivity meter. It is an active sensor because it carries its own source of excitation. It has been common to consider that a magnetometer is the principal sensor for measuring magnetic susceptibility, and that an EMI sensor is the principal sensor for measuring electricalconductivity.
An EMI sensor responds to both electrical conductivity and magnetic susceptibility. In fact, an EMI sensor operating at sufficiently low frequencies acts more as a magnetometer than as a conductivity meter. At the so-called “resistive limit” where the conductivity-frequency product is small, an EMI sensor responds only to magnetic susceptibility and ignores electrical conductivity. It would be a serious misnomer in this case to call an EMI sensor a conductivity meter. A correct designation would be an active magnetometer or, as we propose here, an electro-magnetometer.
A properly designed electro-magnetometer can serve two simultaneous functions: that of a magnetometer and of a conductivitymeter.
EMI surveys are used to:
· Locate buried tanks and pipes
· Locate pits and trenches containing metallic and/or nonmetallic debris
· Delineate landfill boundaries
· Delineate oil production sumps and mud pits
· Map conductive soil and groundwater contamination
· Characterize subsurface hydrogeology
· Map buried channel deposits
· Map geologic structure
· Conduct groundwater exploration
· Locate conductive fault and fracture zones
See GEM-2 Data examples for a few typical multi-depth, apparent conductivity and apparent susceptibility profiling results.
Old EMI Technology
· The EMI instruments were originally made of two separate and quite distant coils due to the influence of the powerful transmitter magnetic field on the receiving coil. This made those instruments difficult to deploy and operate; two operators were necessary. The modern instruments use a bucking coil to cancel the transmitted field allowing assembling the coils in a more compact packaging that can be carried by a single operator.
· Geometrical sounding versus frequency sounding.
By technical necessity, most old frequency-domain (FD) EMI sensors have employed the principle of “geometrical sounding” where the coil separation is the only variable since all coils operate at a factory-set frequency. Because these coils are tuned to a particular frequency by their inductance and external capacitance, one cannot change the operating frequency without replacing all coils or associated electronics.
A tuned coil derives its signal strength from its Q (called the figure of merit—the sharper the resonance, the higher the Q), a voltage amplification factor at the tuned frequency.
Therefore, the only means left to the user of such a system is changing the coil separation, which often requires multiple operators tending separate coils connected by cables to a measuring console. Furthermore, the system must maintain a considerable coil separation to avoid RX saturation from the primary TX field. It is obvious that such a sensor cannot be made into a small, handheld package.
In contrast, depth sounding by changing frequency (or “frequency sounding”) measures the earth response at multiple frequencies at a fixed TX-RX geometry. In frequency sounding, there is no exclusive relationship between the coil separation and the depth of exploration.
· Multi-depth Profiling
Measurements are taken at successive intervals along a profile with multiple or stepping frequencies. Data are presented as profiles or contour maps and interpreted qualitatively. The depth of the profile essentially depends on the operational frequency used to plot it (lower frequencies show deeper ground layers, higher frequencies show shallower ground layers).
Objectives
Advantages of a broadband, multi-frequency EMI sensor are obvious. The idea of using multiple frequencies stems from the so-called “skin-depth”, also known as the depth of exploration, which is inversely proportional to frequency: a low-frequency signal travels far through a conductive earth and, thus, "sees" deep structures, while a high-frequency signal can travel only a short distance and thus, "sees" only shallow structures. Therefore, scanning through a frequency window is equivalent to depth sounding. However, it is also possible to use the I and Q values coming from one particular frequency (i.e. one depth) to produce the 2D apparent Susceptibility and/or apparent Conductivity profile of the survey area at that depth.
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