US 4,736,300 AGranted

Automatic gain control of formation measurements by varying induced current flow

AssigneeWestern Atlas International, Inc.

InventorWade M. Johnson, Jr.

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Abstract

In apparatus for investigating earth formations traversed by a borehole, improved methods and apparatus are provided for performing inductive type formation measurements. A transmitter coil of a well logging tool moving through a borehole emits an alternating field into the adjacent earth formation thus inducing eddy currents therein. A voltage in a receiver coil in the tool is sampled at discrete elevations as the tool traverses the borehole and amplified. This voltage is phase detected with respect to the transmitter coil voltage to insure that the measured receiver voltage corresponds to the currents induced in the formation. An average of these voltage measurements is formed. Variations in the magnitude of the average phase-detected voltage are used to discretely control the amplitude of the alternating current induced into the formation.

Claims19

Dependent claims

A well logging tool adapted to be disposed within a borehole for inductively investigating characteristics of an earth formation, including apparatus for controlling the gain of formation measurements in response to the phase of said formation measurements, comprising:

means for inducing alternating current flows into said formations;
means for generating measurement signals in response to said flows;
means for deriving an average measurement signal functionally related to said measurement signal and at least one other measurement signal derived at a depth within said formation different from the depth at which said measured signal is derived, said means for deriving an average measurements signal being interconnected to said means for generating measurement signals;
means for comparing said average measurement signal to a predetermined range of measurement signals interconnected to said means for deriving said average measurement signal;
means for generating a control signal in response to said comparison interconnected to said means for comparing average measurement signals;
and
means for adjusting the magnitude of said measurement signals by adjusting the magnitude of said induced current flow in response to said control signal, said means for adjusting interconnected to said means for generating a control signal and to said means for generating said induced current flow signals.

4 more elements

A method for inductively investigating characteristics of an earth formation including a well logging tool adapted to be disposed within a borehole including controlling the gain of formation measurements in response to the phase of said formation measurements, comprising:

inducing an alternating current flow into said formation;
generating a measurement signal in response to said induced flow;
deriving an average measurement signal functionally related to said measurement signal and at least one other measurement signal derived at a depth within said formation different from the depth at which said measured signal is derived;
comparing said average measurement signal to a predetermined range of measurement signals;
generating a control signal in response to said comparison;
generating a next measurement signal in response to said flow;
and
adjusting the magnitude of said next measurement signal by adjusting the magnitude of said induced current in response to said control signal.

5 more elements

Dependencies are inferred from the claim text.