Borehole acoustic logging system having synchronization

• US 4,969,128 A
• Filed: 02/06/1990
• Issued: 11/06/1990
• Est. Priority Date: 02/06/1990
• Status: Expired due to Fees

• Current Assignee: Mobil Oil Corporation (Exxon Mobil Corporation)
• Original Assignee: Mobil Oil Corporation (Exxon Mobil Corporation)
• Inventors: Dennis, Charles L., Lenhardt, Will K.
• Primary Examiner(s): Lobo, Ian J.
• Application Number:
  US07/475,547
• Time in Patent Office: 273 Days
• Field of Search: 367/35, 367/25, 367/69, 367/911, 181/105, 324/161, 324/166
• US Class Current: 367/25
• CPC Class Codes: G01V 1/46 Data acquisition
• CPC Class Codes: Y10S 367/911 Particular well-logging apparatus

Abstract

An elongated borehole logging tool employs an acoustic energy transducer for directing acoustic energy pulses toward a borehole wall at a repetition rate synchronized with rotational speed of the tool within the borehole.

Claims

1. A system for the acoustic logging of subsurface formations surrounding a borehole, comprising:

(a) a transducer for directing pulses of acoustic energy toward the wall of said borehole and for receiving direct reflections of said acoustic energy from said borehole wall,

(b) means for rotating said transducer about a rotational axis parallel to the axis of the borehole,

(c) a wheel rotated in synchronism with said transducer and having a plurality of teeth equally spaced about its outer periphery,

(d) a sensor spaced in juxtaposition with said wheel for detecting the rate of movement of said teeth past said sensor and for producing an electrical signal having a plurality of pulses with a repetition rate representative of the rate of movement of said teeth past said sensor, and

(e) means responsive to said electrical signal for controlling said transducer to direct said pulses of acoustic energy toward said borehole wall at a repetition rate synchronized with the rotational speed of said transducer.

2. The system of claim 1 wherein said control means converts said electrical signal to a transducer excitation signal having a pulse rate synchronized to the repetition rate of the pulses comprising said electrical signal.

3. The system of claim 2 wherein said control means comprises:

(a) an oscillator for generating said transducer excitation signal having a pulse rate that is a multiple N of the repetition rate of the pulses of said electrical signal,

(b) means for dividing said transducer excitation signal by N, and

(c) a detector for comparing the phase of said transducer excitation signal as divided by N with the phase of said electrical signal and for providing a phase error signal to said oscillator for synchronizing said excitation signal to said electrical signal so that the rate at which said transducer directs pulses of acoustic energy toward said borehole wall is synchronized with the rate of rotation of said transducer.

4. A system for the acoustic logging subsurface formations surrounding a borehole, comprising:

(a) a transducer for directing pulses of acoustic energy toward the wall of said borehole and for receiving direct reflections of said acoustic energy from said borehole wall,

(b) a motor for rotating said transducer about a rotational axis parallel to the axis of the borehole with a plurality of speed variations during each revolution,

(c) a wheel rotated in synchronism with said motor and having a plurality of teeth equally spaced about its periphery,

(d) a sensor spaced in juxtaposition with said wheel for detecting the rate of movement of said teeth past said sensor and for producing an electrical signal having a plurality of pulses with a repetition rate representative of the rate of movement of said teeth past said sensor,

(e) an oscillator for producing a transducer excitation signal having a pulse rate that is a multiple N of the pulse rate of said electrical signal,

(f) a divide-by-N counter that divides the pulse rate of said transducer excitation signal by said multiple N, and

(g) a phase detector for comparing said electrical signal with the output of said counter to provide a phase error signal for controlling said oscillator so that the pulse rate of said transducer excitation signal is phase locked to the pulse rate of said electrical signal wherein the rate at which said transducer directs acoustic energy pulses toward the borehole wall is in synchronism with the speed variations of said motor during each revolution so that said acoustic energy pulses are directed from said transducer at equally spaced azimuthal positions about the rotational axis of said transducer.

5. A system for the acoustic logging of subsurface formations surrounding a borehole, comprising:

(a) a transducer for directing pulses of acoustic energy toward the wall of said borehole and for receiving direct reflections of said acoustic energy from said borehole wall,

(b) a motor for rotating said transducer about a rotational axis parallel to the axis of the borehole at a rate of R revolutions per second with a plurality of acceleration and deceleration cycles per revolution,

(c) a wheel having T teeth equally spaced about its outer periphery and rotated in synchronism with said motor,

(d) a sensor spaced in juxtaposition to said wheel for detecting the rate of movement of said teeth past said sensor and for producing a R×

T hertz electrical signal representative of the rate of movement of said teeth past said sensor each second,

(e) a voltage controlled oscillator for producing a P/N hertz transducer excitation signal,

(f) a counter that divides said transducer excitation signal by N to provide a P/N hertz counter signal, and

(g) a phase detector for comparing the phase of said R×

T hertz electrical signal with the phase of said P/N hertz counter signal to provide a phase error signal to control the voltage of said voltage controlled oscillator to cause said P hertz transducer excitation signal to be phase locked to said R×

T hertz electrical signal so that said transducer excitation signal causes said transducer to direct P/R acoustic energy pulses toward the borehole wall at equally spaced azimuthal positions about said rotational axis for each revolution of said transducer.

6. The system of claim 5 wherein:

(a) R represents 6 revolutions per second,

(b) T represents 64 teeth,

(c) R×

T represents a 384 hertz electrical signal,

(d) P represents a 1536 hertz transducer excitation signal,

(e) N represents a division of said transducer excitation signal by 4,

(f) P/N represents a 384 hertz counter signal, and

(g) P/R represents 256 acoustic energy pulses per revolution of said transducer.