Seafloor magnetotelluric system and method for oil exploration

• US 5,770,945 A
• Filed: 06/26/1996
• Issued: 06/23/1998
• Est. Priority Date: 06/26/1996
• Status: Expired due to Term

• Current Assignee: Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Constable, Steven C.
• Primary Examiner(s): Snow, Walter E.
• Application Number:
  US08/670,939
• Time in Patent Office: 727 Days
• Field of Search: 324/350, 324/347-349, 324/323, 324/244, 324/260, 324/329, 324/326, 324/66, 324/67
• US Class Current: 324/350
• CPC Class Codes: G01V 3/082 operating with fields produced by spontaneous potentials, e.g. electrochemical or produced by telluric currents G01V3/26 takes precedence
• CPC Class Codes: G01V 3/15 specially adapted for use during transport, e.g. by a person, vehicle or boat

Abstract

The magnetotelluric system for seafloor petroleum exploration comprises a first waterproof pressure case containing a processor, AC-coupled magnetic field post-amplifiers and electric field amplifiers (the "logger unit"), a second waterproof pressure case containing an acoustic navigation/release system, four silver-silver chloride (Ag-AgCl) electrodes mounted on booms and at least two magnetic induction coil sensors. These elements are mounted together on a plastic and aluminum frame along with flotation devices and an anchor for deployment to the seafloor. The acoustic navigation/release system serves to locate the system by responding to acoustic pings generated by a ship-board unit and receives a release command which initiates detachment from the anchor so that the buoyant package floats to the surface for recovery. The electrodes used to detect the electric field are configured as grounded dipole antennas. Booms by which the electrodes are mounted onto frame are positioned in an "X" configuration to create two orthogonal dipoles, which are used to measure the complete vector electric field. The magnetic field sensors are multi-turn Mu-metal core coils which detect within the frequency range typically used for land-based MT surveys. The magnetic field coils are encased in waterproof pressure cases and are connected to the logger package by high pressure waterproof cables. The logger unit includes the amplifiers for amplifying the signals received from the various sensors, which signals are then provided to the processor which controls timing, logging, storing and power switching operations. Temporary and mass storage is provided within and/or peripheral to the processor.

Claims

1. A unit for incorporation into a system having a plurality of units for magnetotelluric mapping of the seafloor for petroleum exploration, the unit comprising:

a corrosion-resistant frame structure;

a pair of grounded dipole antennas mounted orthogonally on said frame structure, each dipole antenna comprising at least one boom having a pre-determined total length and a non-polarizable electrode suitable for use in sea water disposed at each end of said at least one boom for detecting an electrical current in seawater;

first conductor means for conducting an electric field signal representative of an electric field detected by each said dipole antenna;

an AC-coupled low input impedance, high gain electric field signal amplifier having a first low-frequency roll-off and connected to said first conductor means, for receiving said electric field signal from each said dipole antenna and generating amplified electric field signals;

at least two magnetic field induction sensors, each induction sensor having a frequency flat band region which includes 1 Hz and a second low-frequency roll-off substantially matched to the first low-frequency roll-off, and being enclosed within a first corrosion- and pressure-resistant case, said at least two induction sensors being disposed horizontally, orthogonal relative to each other;

second conductor means for conducting a power supply voltage to said at least two induction sensors and a magnetic field signal representative of a magnetic field detected by each said induction sensor;

a magnetic field signal amplifier connected to said second conductor means for receiving said magnetic field signal from each said induction sensor and generating amplified magnetic field signals;

a data logging processor for receiving said amplified electric field signals and said amplified magnetic field signals and storing data representative thereof, the data logger being enclosed within a second corrosion- and pressure- resistant case and mounted within said frame structure;

a clocking device for synchronizing operation of said data logging processor with other data logging processors on other units of said plurality of units;

a power supply for providing electrical power to said data logging processor, said at least two induction sensors, said clocking device and said electric field and magnetic field amplifiers;

means for deploying said unit to the seafloor; and

means for retrieving said unit from the seafloor.

2. A unit as in claim 1 wherein said retrieving means comprises a navigation system disposed within a third corrosion- and pressure-resistant casing and mounted on said frame structure, said navigation system including an acoustic transponder for receiving an acoustic signal and for providing range and location information for said unit.

3. A unit as in claim 2 wherein said acoustic signal includes a command to release said unit, and said retrieving means further comprises:

a release device responsive to said acoustic signal, wherein said acoustic transponder receives said command to release said unit and activates said release device, separating the frame structure from the deploying means; and

a plurality of buoyant floats mounted on said frame structure for providing positive buoyancy to said unit so that it floats to the surface of the seawater.

4. A unit as in claim 3 wherein said release device comprises comprising a voltage supply connected to a burn wire tying said frame structure to the deploying means.

5. A unit as in claim 1 wherein said magnetic field amplifier is AC coupled.

6. A unit as in claim 1 wherein said non-polarizable electrode is silver-silver chloride.

7. A unit as in claim 1 wherein said at least two induction field sensors are attached to said frame structure.

8. A unit as in claim 1 wherein said at least two induction field sensors are attached to said frame work by a cable so that they fall away from said frame structure to drop separately onto the seafloor.

9. A unit as in claim 1 wherein said deploying means comprises an anchor.

10. A unit as in claim 1 wherein said data logging processor includes a removable mass storage device for storing data collected by said data logging processor and for transferring said data for external processing.

11. A unit as in claim 1 wherein said frame structure is formed from aluminum and plastic, with plastic being disposed at least at mounting points of said corrosion- and pressure-resistant cases on said frame structure.

12. A unit as in claim 1 wherein said electric field amplifier has a -3 dB point of 0.1 Hz.

13. A unit as in claim 1 wherein said electric field amplifier has a first stage comprising a power FET chopper with a chopping frequency of 2 kHz.

14. A unit as in claim 1 wherein each said induction sensor has a -3 dB point of 0.2 Hz.

15. A system for magnetotelluric mapping of the seafloor for petroleum exploration, the system comprising:

a seagoing vessel for deploying, controlling and retrieving a plurality of data collection units, said seagoing vessel including an acoustic transmitter for emitting an acoustic signal to the plurality of data collection units and a data analysis system for analyzing data collected by the plurality of data collection units;

each data collection unit of the plurality of data units comprising;

a corrosion-resistant frame structure;

a pair of grounded dipole antennas mounted orthogonally on said frame structure, each dipole antenna comprising at least one boom having a pre-determined total length and a silver-silver chloride electrode disposed at each end of said at least one boom for detecting an electrical current in seawater;

first conductor means for conducting an electric field signal representative of an electric field detected by each said dipole antenna;

an AC-coupled low input impedance, high gain electric field signal amplifier having a first low-frequency roll-off and connected to said first conductor means for receiving said electric field signal and generating an amplified electric field signal;

two magnetic field induction sensors, each induction sensor having a frequency flat band region which includes 1 Hz and being enclosed within a first corrosion- and pressure-resistant case, said two induction sensors being disposed horizontally, orthogonal relative to each other;

second conductor means for conducting a power supply voltage to said two induction sensors and a magnetic field signal representative of a magnetic field detected by each said induction sensor;

a magnetic field signal amplifier connected to said second conductor means for receiving said magnetic field signal from each said induction sensor and generating amplified magnetic field signals;

a data logging processor for receiving said amplified electric field signals and said amplified magnetic field signals and storing data representative thereof, the data logging processor being enclosed within a second corrosion- and pressure- resistant case and mounted within said frame structure;

a clocking device for synchronizing operation of said data logging processor with other data logging processors on other data collection units of the plurality of data collection units;

a power supply for providing electrical power to said data logging processor, said at least two induction sensors, said clocking device and to said electric field and magnetic field amplifiers;

means attached to said frame structure for deploying said data collection unit to the seafloor; and

a navigation/release system disposed within a third corrosion- and pressure-resistant casing and mounted on said frame structure, said navigation/release system comprising;

an acoustic transponder for receiving said acoustic signal and for providing range and location information for said unit; and

a release device responsive to said acoustic signal, wherein said acoustic transponder receives a command from said acoustic transmitter to release said unit and said release device separates the deploying means from said frame structure;

a plurality of buoyant floats mounted on said frame structure for providing positive buoyancy to said data collection unit so that it floats to the surface of the seawater when said deploying means is released.

16. A system as in claim 15 wherein said data logging processor includes a removable mass storage device for storing data collected by said data logging processor and for transferring said data to said data analysis system.

17. A system as in claim 15 wherein said magnetic field signal amplifier is AC coupled.

18. A system as in claim 15 wherein said two magnetic field sensors are disposed on the frame structure.

19. A system as in claim 15 wherein said two magnetic field sensors are attached to the frame structure by a cable so that they fall away from said frame structure to drop separately onto the seafloor.

20. A system as in claim 15 wherein said electric field signal amplifier has a -3 dB point of 0.1 Hz.

21. A system as in claim 15 wherein each said induction sensor has a -3 dB point of 0.2 Hz.

22. A system as in claim 15 further comprising a land-based local reference magnetic induction sensor and a reference data logging processor for collecting and providing reference data to said data analysis system.

23. A system as in claim 15 further comprising a remote seafloor-based reference magnetic induction sensor and a reference data logging processor disposed up to several hundred kilometers from said plurality of data collection units for collecting and providing reference data to said data analysis system.

24. A system for magnetotelluric mapping of the seafloor for petroleum exploration, the system comprising:

a seagoing vessel for deploying, controlling and retrieving a plurality of data collection units, said seagoing vessel including an acoustic transmitter for emitting an acoustic signal to the plurality of data collection units and a data analysis system for analyzing data collected by the plurality of data collection units;

each data collection unit of the plurality of data units comprising at least;

a corrosion-resistant frame structure;

a pair of grounded dipole antennas mounted orthogonally on said frame structure, each dipole antenna comprising al least one boom having a pre-determined total length and a silver-silver chloride electrode disposed at each end of said at least one boom for detecting an electrical current in seawater;

first conductor means for conducting an electric field signal representative of an electric field detected by each said dipole antenna;

an AC-coupled low input impedance, high gain electric field signal amplifier having a first low-frequency roll-off and connected to said first conductor means for receiving said electric field signal and generating an amplified electric field signal;

a data logging processor for receiving said amplified electric field signals and storing data representative thereof, the data logging processor being enclosed within a first corrosion- and pressure- resistant case and mounted within said frame structure;

a clocking device for synchronizing operation of said data logging processor with other data logging processors on other data collection units of the plurality of data collection units;

a power supply for providing electrical power to said data logging processor, said clocking device and to said electric field signal amplifier;

means attached to said frame structure for deploying said data collection unit to the seafloor; and

means for releasing said data collection unit from the seafloor; and

at least two magnetic field induction sensors, each induction sensor having a frequency flat band region which includes 1 Hz and having a second low-frequency roll-off substantially matched to said first low-frequency roll-off, said two induction sensors being disposed horizontally, orthogonal relative to each other;

a magnetic field signal amplifier for receiving said magnetic field signal from each said induction sensor and generating amplified magnetic field signals representative of magnetic field data;

wherein said data analysis system further analyzes said data collected by the plurality of data collection units in combination with said magnetic field data.

25. A system as in claim 24 wherein each said induction sensor is enclosed within a second corrosion- and pressure-resistant case and connected to said data logging processor by a second conductor means.

26. A system as in claim 25 wherein each said induction sensor is mounted on said frame structure.

27. A system as in claim 24 wherein said at least two induction sensors are deployed at a remote location up to several hundred kilometers from said plurality of data collection units.

28. A system as in claim 24 wherein said remote location is on the seafloor.

29. A system as in claim 24 wherein said remote location is on land.

30. A method for petroleum exploration using magnetotelluric mapping of the seafloor, the method comprising:

deploying from a seagoing vessel a plurality of data collection units by sinking each of the plurality of data collection units to a predetermined location on the seafloor, each data collection unit comprising;

a corrosion-resistant frame structure;

a pair of grounded dipole antennas mounted orthogonally on said frame structure, each dipole antenna comprising al least one boom having a pre-determined total length and a silver-silver chloride electrode disposed at each end of said at least one boom for detecting an electrical current in seawater, and first conductor means for conducting an electrical field signal representative of an electric field detected by said dipole antenna to an AC-coupled low input impedance, high gain electric field signal amplifier having a first low-frequency roll-off;

a data logging processor for receiving the electrical signals generated by said pair of dipole antennas through said first conductor means and storing data representative of the electrical signals, said data logger being enclosed within a second corrosion- and pressure resistant case and mounted within said frame structure; and

a power supply for providing electrical power to said data logging processor, said clocking device and to the amplifiers for said dipole antennas;

synchronizing operation of said data logging processor with other data logging processors on other data collection units of the plurality of data collection units and collecting data;

emitting an acoustic signal from an acoustic transmitter on said seagoing vessel to an acoustic transponder on each of the plurality of data collection units and receiving a return said acoustic signal from each said data collection unit for providing range and location information;

activating a release device in each said data collection unit, causing said data collection unit to float to the surface;

recovering each data collection unit;

transferring the data collected by the plurality of data collection units to a data analysis system for analysis of the magnetotelluric parameters on the seafloor; and

combining the data from the plurality of data collection units in the data analysis system with magnetic field data obtained from at least two magnetic field induction sensors, each induction sensor having a frequency flat band region which includes 1 Hz and having a second low-frequency roll-off substantially matched to the first low-frequency roll-off, said at least two induction sensors being disposed to detect magnetic fields in a horizontal plane.

31. The method of claim 30 wherein the step of deploying the plurality of data collection units comprises positioning the data collection units at a predetermined spacing from each other.

32. The method of claim 30 wherein the step of combining the data comprises attaching two magnetic field induction coils to at least one of the data collection units and inputting the magnetic field data into the data logging processor.

33. The method of claim 30 wherein the step of combining the data comprises obtaining the magnetic field data from at least two remote magnetic field induction sensors positioned on the seafloor at a distance of up to several hundred kilometer from the data collection units.

34. The method of claim 30 wherein the step of combining the data comprises obtaining the magnetic field data from at least two magnetic field induction sensors positioned on land.