Borehole induction coil transmitter

• US 6,489,772 B1
• Filed: 01/23/1995
• Issued: 12/03/2002
• Est. Priority Date: 01/23/1995
• Status: Expired due to Fees

• Current Assignee: Lawrence Livermore National Security LLC (Government of the United States of America)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Wilt, Michael J., Holladay, Gale
• Primary Examiner(s): Snow, Walter E.
• Application Number:
  US08/376,725
• Time in Patent Office: 2,871 Days
• Field of Search: 324/323, 324/338, 324/339, 324/340, 324/341, 324/345, 324/346, 324/334, 324/333, 324/342, 324/343, 324/355, 324/356
• US Class Current: 324/339
• CPC Class Codes: G01V 3/28 using induction coils

Abstract

A borehole induction coil transmitter which is a part of a cross-borehole electromagnetic field system that is used for underground imaging applications. The transmitter consists of four major parts: 1) a wound ferrite or mu-metal core, 2) an array of tuning capacitors, 3) a current driver circuit board, and 4) a flux monitor. The core is wound with several hundred turns of wire and connected in series with the capacitor array, to produce a tuned coil. This tuned coil uses internal circuitry to generate sinusoidal signals that are transmitted through the earth to a receiver coil in another borehole. The transmitter can operate at frequencies from 1-200 kHz and supplies sufficient power to permit the field system to operate in boreholes separated by up to 400 meters.

Claims

1. A borehole induction coil transmitter comprising:

an enclosed casing adapted to be positioned in a borehole;

a core positioned in and enclosed by said casing and composed of material selected from the group consisting of ferrite and mu-metal;

said core having a coil of electrically conductive material wrapped there around;

said coil being composed of a wrapping of a wire composed of said electrically conductive material, such that the wire wrapping is selected from the group of even spaced wraps along a length thereof, and even spaced wraps along a portion of the length with more closely spaced wraps near the ends thereof;

an array of capacitors positioned in and enclosed by said casing and connected in series with said coil of electrically conductive material to tune the coil to a resonant frequency;

a flux monitor positioned in and enclosed by said casing including a wire winding around a central section of said coil;

a current driver positioned in and enclosed by said casing for driving the transmitter at the resonant frequency; and

means connected to said enclosed casing and adapted to retain said enclosed casing in an associated borehole.

2. The transmitter of claim 1, wherein said casing is constructed of fiberglass.

3. The transmitter of claim 1, wherein said electrically conductive material is selected from the group of copper and aluminum.

4. The transmitter of claim 1, wherein said core is composed of ferrite, and said coil is composed of copper wire.

5. The transmitter of claim 1, wherein the wire wrapping contains about 100-500 turns.

6. The transmitter of claim 1, wherein said core is composed of mu-metal.

16. The transmitter of claim 1, wherein said core is ferrite and said transmitter is operated as a self-excited oscillator, with direct current power supplied thereto.

7. A transmitter for use in a cross-hole electromagnetic imaging system comprising:

an enclosed casing adapted to be positioned in a borehole;

a core located in said enclosed casing;

a coil of electrically conductive wire wrapped around the core;

a plurality of capacitors located in said enclosed casing and connected in series with said coil of wire;

means located in said enclosed casing for driving the transmitter; and

means located in said enclosed casing for monitoring the time derivative of flux of the transmitter;

said flux monitoring means including a separate turn of wire around said coil for measuring the magnetic flux emitted by the transmitter.

8. The transmitter of claim 7, wherein said core is composed of material selected from the group consisting of ferrite and mu-metal.

9. The transmitter of claim 7, wherein said electrically conductive wire is selected from the group consisting of copper and aluminum.

10. The transmitter of claim 7, wherein said flux monitoring wire wrapped around said coil is constructed of material selected from copper and aluminum.

11. The transmitter of claim 7, wherein said driving means comprises a current driver circuit operatively connected to said coil.

12. The transmitter of claim 7, wherein said plurality of capacitors are tuning capacitors to tune the coil to a resonant frequency, and wherein said driving means drives the transmitter at that resonant frequency.

13. The transmitter of claim 7, wherein said core is composed of ferrite, and said coil is composed of copper wire.

14. The transmitter of claim 7, wherein said core is composed of mu-metal, and said coil is composed of copper wire.

15. The transmitter of claim 7, wherein said coil is composed of about 25 to about 500 wraps of wire.

17. The transmitter of claim 7, wherein said core is ferrite and said transmitter is operated as a passive antenna, with an amplified signal supplied thereto.