Compact underwater electromagnetic measurement system

• US 20080246485A1
• Filed: 12/11/2007
• Published: 10/09/2008
• Est. Priority Date: 12/11/2006
• Status: Active Grant

• Current Assignee: QUASAR Federal Systems, Inc. (Quasar Group LLC)
• Original Assignee: QUASAR Federal Systems, Inc. (Quasar Group LLC)
• Inventors: Hibbs, Andrew D., Nielsen, Thomas Kurt
• Granted Patent: US 10,082,546 B2
• Time in Patent Office: Days
• US Class Current: 324/332
• CPC Class Codes: G01R 29/0871 Complete apparatus or systems; circuits, e.g. receivers or amplifiers G01R29/0878, G01R29/0892 take precedence; dosimeters, warning devices G01R29/0857
• CPC Class Codes: G01R 33/04 using the flux-gate principle
• CPC Class Codes: G01R 33/10 Plotting field distribution ; Measuring field distribution
• CPC Class Codes: G01R 33/12 Measuring magnetic properties of articles or specimens of solids or fluids involving magnetic resonance G01R33/20
• CPC Class Codes: G01V 3/02 operating with propagation of electric current
• CPC Class Codes: G01V 3/08 operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices with electromagnetic waves G01V3/12
• CPC Class Codes: G01V 3/083 Controlled source electromagnetic [CSEM] surveying
• CPC Class Codes: H03F 2200/261 Amplifier which being suitable for instrumentation applications
• CPC Class Codes: H03F 2203/45138 Two or more differential amplifiers in IC-block form are combined, e.g. measuring amplifiers
• CPC Class Codes: Y02A 90/30 Assessment of water resources

Abstract

An underwater EM measurement system, which is substantially smaller, much simpler to use, and more robust than prior systems, is formed as a sensor package integrated into a single pressure vessel includes two magnetic sensors including induction coils disposed substantially horizontally so as to measure fields in orthogonal directions. The package also includes two electric field sensors including electric potential antennas adapted to couple to a water potential via a capacitive electrode having a conducting material and an electrically insulative layer formed of an insulating material. The capacitive electrode has a capacitance to the medium of greater than 1 mF. Preferably, the insulating material is a metal oxide.

Claims

1. A system for measuring magnetic and electric fields in an electrically conducting medium, the system comprising:

a group of components constituting an integrated sensor package including;

two magnetic sensors, each magnetic sensor having a magnetic field induction coil for producing a magnetic field signal and a magnetic field signal amplifier connected to the induction coil for producing an amplified magnetic field signal based on the magnetic field signal, each induction coil being disposed substantially horizontally so as to measure fields in orthogonal directions;

two electric field sensors, each electric field sensor having an electric field signal amplifier connected to at least two electric potential antennas for producing an amplified electric field signal, each electric field sensor being disposed substantially horizontally so as to measure fields in orthogonal directions, the electric potential antennas being adapted to couple to the electric potential via a respective capacitive electrode having a conducting core and an electrically insulative layer covering the core, said insulative layer preventing transfer of a resistive current from the core to the medium;

a data acquisition system for receiving the amplified electric field signals and the amplified magnetic field signals and storing data representative thereof; and

a power supply for providing electrical power to the data logging processor, the electric sensors and the magnetic sensors.

2. The system according to claim 1, further comprising:

magnetic source coils and conductors which create calibration magnetic and electric fields that are measured by the field induction coils and electric field antennas, respectively.

3. The system according to claim 1, further comprising:

a magnetic field induction coil for measuring a component of the magnetic field in a direction orthogonal to the two magnetic sensors.

4. The system according to claim 1, further comprising:

a fifth electric potential antenna disposed above the electric potential antennas of the two electric field sensors so the electric field in any direction can be determined.

5. The system according to claim 1, wherein the induction coils, the magnetic field signal amplifiers, the electric potential antennas, the electric field signal amplifiers, and the data acquisition system are integrated into a common pressure vessel.

6. The system according to claim 1, wherein the capacitive electrode has a capacitance to the medium of greater than 1 mF.

7. The system according to claim 1, wherein the insulative layer is a metal oxide.

8. The system according to claim 1, wherein the electric field signal amplifier includes a preamplifier integrated with a respective said electric potential antenna.

9. The system according to claim 1, wherein the electric field signal amplifier includes a preamplifier and an isolation stage prevents DC coupling of an output of the preamplifier from an eventual downstream data recording stage, whereby a DC potential difference between each capacitive electrode and the medium is minimized in order to minimize electrochemical reactions at the capacitive electrodes.

10. The system according to claim 5, wherein at least one of the electric potential antennas spans the pressure vessel.

11. The system according to claim 1, further comprising:

an anchor for deploying the system to a seafloor and a flotation device for retrieving the system from the seafloor and the system is adapted to determine the resistivity of the seafloor to quantify hydrocarbon deposits.

13. The system according to claim 1, further comprising:

an antenna connected to the data acquisition system that receives commands and transmits data wirelessly to a receiver.

14. The system according to claim 5, further comprising:

an orientation module including a magnetic compass is located inside the pressure vessel.

12. The system according to claim 12, wherein the flotation device serves as the structure which connects all system elements.

15. A system for measuring very low electrical potentials in a medium comprising:

an electrode having a low electrical impedance to the medium;

a connecting element made of a conducting material attached to the electrode; and

an amplifier including a low noise preamplifier attached to the connecting element, said amplifier containing an input bias circuit that minimizes the current drawn from the electrode through the amplifier inputs in order to minimize electrochemical reactions at the electrode.

16. The system according to claim 15, wherein the electrode further includes a conducting core and an electrically insulative layer covering the core, said insulative layer preventing transfer of a resistive current from the core to the medium, said connecting element being attached to the core of the electrode.

17. The system according to claim 16, wherein the electrode has a capacitance to the medium of greater than 1 mF.

18. The system according to claim 16, wherein the insulative layer is a metal oxide.

19. A method for measuring an electric field in seawater below a surface of the water comprising;

deploying electrodes on a rigid structure that provides a defined electrode separation;

preventing a transfer of a resistive current from the electrodes to the water;

producing an amplified electric field signal by either amplifying a differential signal from the electrodes or amplifying a signal from each electrode first and then taking the difference; and

storing the amplified electric field signal.

20. A method for measuring electric and magnetic fields below a surface of a body of water comprising;

deploying a system including electric field sensors, each connected to at least two electric potential antennas having associated electrodes, and magnetic field sensors, having at least two magnetic field induction coils, arranged in a substantially horizontal plane under the water;

measuring the electric and magnetic fields in orthogonal directions to produce signals representative of the electric and magnetic fields;

preventing a transfer of a resistive current from at least one of the electrodes to the water;

amplifying the signals from the electric potential antennas and magnetic field induction coils to produce amplified electric field signals and amplified magnetic field signals, respectively; and

storing the amplified electric field signals and amplified magnetic field signals.

21. The method according to claim 20, further comprising:

minimizing a DC potential difference between the electrodes and the water in order to minimize electrochemical noise at the electrodes; and

isolating amplified outputs from outside ground potentials in order to minimize electrochemical noise at the electrodes.

22. The method according to claim 20, further comprising:

generating electromagnetic fields near at least one of the electric and magnetic field sensors; and

confirming correct operation of the at least one of the electric and magnetic field sensors by using the generated electromagnetic fields.

23. The method according to claim 20, further comprising:

keeping a single pressure vessel that houses the electric field sensors, the magnetic field sensors and a data acquisition system sealed at all times during shipborne deployment.

24. The method according to claim 20, further comprising:

sending a command by an acoustic signal to start or stop the operation of any component of the sensor package, or setting the system internal clock at a desired time.

25. The method according to claim 20, further comprising:

sending data to the surface via a cable connected to a data acquisition system housed with the electric and magnetic field sensors.

26. The method according to claim 20, further comprising:

determining the resistivity of the seafloor to quantify hydrocarbon deposits.