MAGNETIC DETECTION AND MAGNETOMETER SYSTEM THEREFOR

• US 3,731,752 A
• Filed: 06/25/1971
• Issued: 05/08/1973
• Est. Priority Date: 06/25/1971
• Status: Expired due to Term

• Current Assignee: Charles A. Schad
• Original Assignee: Charles A. Schad
• Inventors: Schad, Charles A.
• Primary Examiner(s): Purser, Ernest R.
• Application Number:
  US05/156,821
• Time in Patent Office: 683 Days
• Field of Search: 175/40,45,57,61 324/43
• US Class Current: 175/45
• CPC Class Codes: E21B 47/022 of the borehole, e.g. using geomagnetism
• CPC Class Codes: E21B 7/04 Directional drilling
• CPC Class Codes: G01R 33/04 using the flux-gate principle

Abstract

A highly sensitive magnetometer, consisting of at least one pair of generally rectangular core elements having square hysteresis loops, for use in a guidance system for off-vertical drilling, is disclosed. The primary windings of each of the cores are driven by an oscillator, with the secondary windings of each core being differentially connected through a null circuit, which serves to balance the effects of background fields, and a detector to a suitable indicator. The primary windings of the cores are operated in resonance and the secondary windings are arranged to provide a high null output which eliminates the dead band caused by the threshold value of the diodes in the output detector circuit. This high null output may be accomplished by unbalancing the core windings or by feeding a portion of the primary winding drive current, or a signal synchronous therewith, to the corresponding secondary winding, thereby permitting measurement of the small magnetic fields which previously fell within the dead band of the device.

Claims

1. A method of guidance wherein a highly sensitive magnetometer utilizing first and second sensor elements permits detection of a magnetic target field of the order of 0.01 gammas, comprising the steps of locating an electromagnet at a target position, locating said megnetometer at a device to be guided in movement with respect to said target, driving the primary winding of said sensor elements in ferroresonance;

feeding to the secondary winding of each sensor element a direct current selectively variable as to polarity and magnitude to establish in each sensor element a null condition;

feeding to each secondary winding an alternating current component to produce a predetermined threshold output signal at said null condition, generating and applying to the winding of said electromagnet a current having an identifiable repetitive waveform capable of driving said electromagnet into transient-free saturation, thereby generating a magnetic target field varying in a predetermined identifiable manner, measuring at said magnetometer the direction and strength of tHe magnetic field caused at said magnetometer by said target electromagnet, and guiding in accordance with said measurement said device to be guided.

2. A guidance system comprising:

a target, said target comprising a highly sensitive magnetometer having first and second sensor elements driven in resonance by a regulated alternating current, null balance means for establishing in said first and second sensor elements a null condition, first and second diode detector means for converting the outputs of said first and second sensor elements, respectively, to direct currents representative of the strength of a sensed magnetic field, and unbalancing means for introducing a portion of said regulated alternating current into the said outputs of said first and second sensor elements for overcoming the thresholds of said first and second detector means;

a device to be guided in motion with respect to said target, said device to be guided carrying an electromagnet for producing an identifiable magnetic field which may be sensed by said target magnetometer, said device further including means for sensing the attitude of said device and electric motor means operable to vary said attitude;

means for directing said device in response to measurements derived from said target of the distance and direction of said device, said means for directing said device comprising means for generating and applying to said electromagnet a waveform of predetermined shape, whereby said identifiable magnetic field is generated, means for measuring the attitude of said device, and means for energizing said electric motor means; and

stepping switch means in said device to be guided and in said means for directing said device for providing selective measurement of said attitude, energization of said motor to vary said attitude and application of said waveform to said electromagnet.

3. The guidance system of claim 2, wherein said means for generating a waveform comprises a source of direct current voltage and a pair of motor-driven potentiometers connected across said source, variation of said potentiometers by said motor serving to produce a waveform which does not produce transients in said electromagnet.

4. The guidance system of claim 2, further including mode switch means having monitoring and control positions and cooperating with said stepping switch means for selectively connecting control signals to said device to be guided and for connecting said device to said means for directing, whereby said device may be operated, or the attitude of said device monitored, respectively.

5. The guidance system of claim 2, wherein said device to be guided comprises a drill located in a first well and adapted to drill an off-vertical bore and said target comprises a magnetometer located in a second well remote from said first well.

6. The guidance system of claim 5, wherein said magnetometer sensor elements, a source of regulated alternating current, detector means, and unbalancing means are locatable downhole in said second well, said null balance means including a polarity and amplitude variable direct current supply located uphole out of said second well, said magnetometer further including amplifier and indicator means and selector switch means located uphole, said selector switch serving to activate one or the other of said sensor elements and to connect the output of the corresponding detector to said amplifier and indicator means, whereby the magnetic field sensed by each sensor element can be individually measured and indicated, thereby to provide increased sensitivity and accuracy to the measurement.

7. The guidance system of claim 2, further including stepper advance means for advancing said stepping switch means in said device to be guided, and position identification means connected to monitor means for indicating the position of said last named stepping switch means.

8. The guidance system of claim 7, further including position indicator means for said stepPing switch means in said device to be guided.