Amplitude and phase measurement in electromagnetic logging

• US 4,547,733 A
• Filed: 12/18/1981
• Issued: 10/15/1985
• Est. Priority Date: 12/31/1980
• Status: Expired due to Fees

• Current Assignee: Schlumberger Technology Corporation (SLB)
• Original Assignee: Schlumberger Technology Corporation (SLB)
• Inventors: Thoraval, Yvon
• Primary Examiner(s): Strecker, Gerard R.
• Assistant Examiner(s): Snow, Walter E.
• Application Number:
  US06/332,119
• Time in Patent Office: 1,397 Days
• Field of Search: 324/338, 324/83 D, 324/339, 324/340, 324/341, 324/225, 324/233, 324/74 D, 328/133, 328/134, 328/114, 328/109, 328/110, 307/514-517
• US Class Current: 324/338
• CPC Class Codes: G01V 3/30 operating with electromagnetic waves

Abstract

An electromagnetic logging apparatus for investigating properties of an earth formation surrounding a borehole comprises an electromagnetic energy transmitter and four receivers supported by a support member, a measurement channel, and a multiplexer for selectively coupling the measurement channel to the receivers. The measurement channel comprises an amplitude processing channel including a highly gain-stabilized amplifier and gain control circuit; a phase processing channel including a highly phase-stabilized amplifier; and a circuit for measuring the amplitude and phase. The gain of the gain-stabilized amplifier is controlled by the gain control circuit in a closed loop feedback arrangement, while the gain of the phase-stabilized amplifier is controlled by the same gain control circuit without the closed loop arrangement. Phase is detected in the phase processing channel by a zero crossing detector and a mode detector, and is measured in the amplitude and phase measurement circuit by a rising edge detector, a falling edge detector, and means for determining a phase value. Amplitude is detected in the amplitude processing channel with an amplitude detector. A related electromagnetic logging method is disclosed.

Claims

1. An electromagnetic logging apparatus for investigating properties of an earth formation surrounding a borehole, comprising:

a support member movable through said borehole;

a transmitter supported by said support member for transmitting electromagnetic energy toward said formation;

a receiver supported on said support member a predetermined distance from said transmitter for receiving electromagnetic energy from said formation to provide a received signal;

means for amplifying with high gain stability said received signal;

means for amplifying with high phase stability said received signal;

means for controlling the gain of said gain stabilized amplifying means, the output thereof further being provided to said phase stabilized amplifying means for controlling the gain thereof;

means coupled to said gain stabilized amplifying means for detecting the amplitude of said received signal;

means coupled to said phase stabilized amplifying means for detecting a zero crossing of said received signal;

means for measuring said amplitude; and

means for determining a value of the phase of said received signal relative to the phase of the electromagnetic energy transmitted by said transmitter comprising;

a rising edge detector coupled to said zero crossing detecting means and responsive to a reference signal indicative of the electromagnetic energy transmitted by said transmitter;

a falling edge detector coupled to said zero crossing detecting means and responsive to said reference signal; and

a counter coupled to said rising edge detector and said falling edge detector for determining an offset-compensated phase value.

2. An apparatus as in claim 1 wherein said phase determining means further comprises:

a mode detector responsive to said received signal and to said reference signal for determining the phase condition of said received signal and said reference signal relative to one another; and

means coupled to said mode detector for modifying the phase of said reference signal in response to said phase condition, to compensate for jitter of said received signal about 0 and π

.

3. An apparatus as in claim 1, wherein said phase value determining means comprises:

means for counting nonoverlapping segments of the respective output signals of said rising edge detector and said falling edge detector at a first preselected counting frequency; and

means for counting overlapping segments of the respective output signals of said rising edge detector and said falling edge detector at a second preselected counting frequency equal to twice said first counting frequency.

4. An electromagnetic logging apparatus for investigating properties of an earth formation surrounding a borehole, comprising:

a support member movable through said borehole;

a transmitter supported by said support member for transmitting electromagnetic energy toward said formation;

a receiver supported on said support member a predetermined distance from said transmitter for receiving electromagnetic energy from said formation to provide a received signal;

means for amplifying with high gain stability said received signal;

means for amplifying with high phase stability said received signal;

means for controlling the gain of said gain stabilized amplifying means, the output thereof further being provided to said phase stabilized amplifying means for controlling the gain thereof;

means coupled to said gain stabilized amplifying means for detecting the amplitude of said received signal;

means coupled to said phase stabilized amplifying means for detecting a zero crossing of said received signal;

means for measuring said amplitude; and

means for determining a value of the phase of said received signal relative to a reference signal indicative of the electromagnetic energy transmitted by said transmitter, comprising;

a mode detector responsive to said received signal and to said reference signal for determining the phase condition of said received signal and said reference signal relative to one another; and

means coupled to said mode detector for modifying the phase of the reference signal supplied to said phase determining means in response to said phase condition, to compensate for jitter of said received signal about 0 and π

.

5. An apparatus for investigating the electromagnetic properties of an earth formation traversed by a borehole, comprising:

an elongated support member adapted for movement in the borehole;

a transmitter for transmitting electromagnetic energy to said formation, said transmitter being mounted on said support member and providing a reference signal;

a receiver for receiving electromagnetic energy from said formation and providing a signal thereof, said receiver being mounted on said support member; and

a measurement channel coupled to said receiver and said transmitter, said measurement channel including an amplitude measurement channel for measuring the amplitude of said received signal and a phase measurement channel for measuring the phase of said received signal relative to said reference signal, said phase measurement channel comprising

a zero crossing detector for detecting the zero crossings of said received signal;

a rising edge detector coupled to said zero crossing detector and responsive to said reference signal for determining the phase difference on rising edges;

a falling edge detector coupled to said zero crossing detector and responsive to said reference signal for determining the phase difference on falling edges;

a counter coupled to said rising edge detector and to said falling edge detector for determining an offsetcompensated phase value;

a mode detector responsive to said received signal and to said reference signal for determining the phase condition of said received signal and said reference signal relative to one another; and

means coupled to said mode detector for modifying the phase of the reference signal supplied to said rising edge detector and to said falling edge detector in response to said phase condition.

6. An apparatus as in claim 5, wherein said measurement channel further comprises:

means for accumulating the phase value of the electromagnetic energy received at said receiver over a plurality of measure periods; and

means for determining the number of measure periods over which the phase value is accumulated.

7. An apparatus as in claim 5, wherein said counter comprises:

means for determining a phase value when a phase shift between the outputs of said rising edge detector and said falling edge detector is less than π

; and

means for determining a phase value when a phase shift between the outputs of said rising edge detector and said falling edge detector is greater than π

.

8. An apparatus as in claim 5, wherein:

said mode detector provides at its output a transfer function of 1 around a phase condition of 0 and a transfer function of 0 around a phase condition of π

, said transfer function changing from 1 to 0 at 2π

/3 as the phase condition increases from 0 to π

, and changing from 0 to 1 at π

/3 as the phase condition decreases from π

to 0; and

said modifying means comprises an exclusive-or gate having an input receiving said reference signal and another input coupled to the output of said mode detector, the output of said exclusive-or gate being coupled to respective inputs of said rising edge detector and said falling edge detector.

9. An apparatus as in claim 5, wherein said counter comprises:

means for counting nonoverlapping segments of the respective output signals of said rising edge detector and said falling egde detector at a first preselected counting frequency; and

means for counting overlapping segments of the respective output signals of said rising edge detector and said falling edge detector at a second preselected counting frequency equal to twice said first counting frequency.

10. An electromagnetic logging apparatus for investigating properties of an earth formation surrounding a borehole, comprising:

a support member movable through the borehole and generally elongated in the direction of movement;

a transmitter supported on said support member for transmitting electromagnetic energy toward the formation;

first, second, third and forth receivers supported on said support member separately from one another and disposed thereon generally along a line parallel to the direction of elongation of said support member, said transmitter being disposed along said line and said first, second, third and fourth receivers being located at respective predetermined distances from said transmitter, for receiving electromagnetic energy from said formation;

a multiplexer coupled to said first, second, third and fourth receivers;

a highly gain-stabilized amplifer coupled to said multiplexer;

a gain control circuit coupled to said gain-stabilized amplifier in a closed loop feedback arrangement;

an amplitude detector coupled to said gain-stabilized amplifier;

a highly phase-stabilized amplifier coupled to said multiplexer and further coupled to the output of said gain control circuit;

a mode detector coupled to said phase-stabilized amplifier and responsive to the phase of the electromagnetic energy transmitted by said transmitter for determining whether the output of said phase-stabilized amplifier is in-phase or out-of-phase with the electromagnetic energy transmitted by said transmitter;

a zero-crossing detector coupled to said phase-stabilized amplifier;

means coupled to said mode detector and responsive to the phase of the electromagnetic energy transmitted by said transmitter for offsetting the phase of the electromagnetic energy transmitted by said transmitter.

a rising edge detector coupled to said offsetting means and said zero crossing detector;

a falling edge detector coupled to said offsetting means and said zero crossing detector;

means coupled to said rising edge detector and said falling edge detector for obtaining a phase value when a phase shift between the outputs of said rising edge detector and said falling edge detector is less than π

;

means coupled to said rising edge detector and said falling edge detector for obtaining a phase value when a phase shift between the outputs of said rising edge detector and said falling edge detector is greater than π

;

means coupled to said less than π

phase value determining means and to said greater than π

phase value determining means for accumulating the phase value obtained thereby over a plurality of measure peroids; and

means for determining the number of measure periods over which the phase value is accumulated.

11. An apparatus as in claim 10 further comprising:

means for providing an amplitude measurement from said amplitude detector; and

means for calculating a phase measurement from said accumulated phase values and said number of measure periods.

12. A method of electromagnetic logging for investigating properties of an earth formation surrounding a borehole, comprising the steps of:

generating electromagnetic energy at a first position in the borehole;

receiving at a second position in the borehole electromagnetic energy having propagated through the formation to provide a received signal having an amplitude and phase relative to the generated electromagnetic energy influenced by the formation;

independently amplifying said received signal with low amplitude distortion and a rapidly controlled gain;

detecting the amplitude of said received signal following said step of amplifying said received signal with low amplitude distortion and a rapidly controlled gain;

determining an amplitude measurement following said amplitude detecting step;

amplifying said received signal with low phase distortion and and a rapidly controlled gain;

detecting a zero crossing of said received signal following said step of amplifying said received signal with a low phase distortion and a rapidly controllable gain;

detecting a phase difference between the rising edges of the detected zero crossing and a reference signal synchronized with the transmitted electromagnetic energy;

detecting a phase difference between the falling edges of the detected zero crossing and said reference signal; and

averaging said rising edge detected phase difference and said falling edge detected phase difference.

13. A method as in claim 12 further comprising the step of offsetting said reference signal by π

when a phase difference between said received signal and said reference signal is near 0.

14. A method of electromagnetic logging for investigating properties of an earth formation surrounding a borehole, comprising the steps of:

generating electromagnetic energy at a first position in the borehole;

receiving at a second position in the borehole electromagnetic energy having propagated through the formation to provide a received signal having an amplitude and phase relative to the generated electromagnetic energy influenced by the formation;

independently amplifying said received signal with low amplitude distortion and a rapidly controlled gain;

detecting the amplitude of said received signal following said step of amplifying said received signal with low amplitude distortion and a rapidly controlled gain;

determining an amplitude measurement following said amplitude detecting step;

amplifying said received signal with low phase distortion and and a rapidly controlled gain;

detecting a zero crossing of said received signal following said step of amplifying said received signal with a low phase distortion and a rapidly controllable gain;

detecting a phase difference between the detected zero crossing of said received signal and a reference signal synchronized with the transmitted electromagnetic energy;

offsetting said reference signal by π

when a phase difference between said received signal and said reference signal is near 0;

accumulating said detected phase difference over a plurality of measure periods; and

averaging said accumulated detected phase differences over the number of said measure periods.