Well casing potential measurement tool with compensated DC errors

A well logging apparatus for measuring DC voltage differences occurring between first and second points on a well casing, said apparatus comprising:

• means for detecting DC voltages at said first and second points on said well casing;
• first connecting means, having first and second inputs coupled to said detecting means to receive said DC voltages, having a control input, and having first and second outputs, for selectively supplying said DC voltages at said first and second connecting means outputs at a variable polarity;
• a differential amplifier having first and second inputs DC coupled to said first connecting means first and second outputs, respectively, and having an output which produces a first signal;
• an inverter having an input DC coupled to said differential amplifier output and having an output which produces a second signal;
• second connecting means, having a first input DC coupled to said differential amplifier output, a second input DC coupled to said inverter output, a control input, and an output, for selectively supplying one of said first signal and said second signal at said second connecting means output;
• a filter having an input DC coupled to said second connecting means output and having an output for supplying an amplified DC voltage signal;
• and
• clock means, having an output coupled to said first and second connecting means control inputs, for causing said first connecting means to alternate said variable polarity of said DC voltages at said outputs of said first connecting means in synchronization with alternate supply of said first and second signals at the output of said second connecting means, said clock means generating a clock signal having a duty cycle established to compensate for errors in said inverter and said filter.

An amplifier for low level DC signals comprising:

• first switching means, having a control input and first and second outputs, for supplying a differential DC voltage at said first switching means outputs so that said differential DC voltage has a variable polarity;
• a differential amplifier having first and second inputs DC coupled to said first switching means first and second outputs, respectively, and having an output;
• an inverter having an input DC coupled to said differential amplifier output and having an output;
• second switching means, having a first input DC coupled to said differential amplifier output, a second input DC coupled to said inverter output, a control input, and an output, for selectively connecting one of said differential amplifier output and said inverter output to said second switching means output;
• a filter having an input DC coupled to said second switching means output and having an output for supplying an amplified DC signal;
• and
• clock means, having an output coupled to said first and second switching means control inputs, for causing said first switching means to alternate the polarity of said differential DC voltage in synchronization with alternate connection of said differential amplifier and inverter outputs to said second switching means output, said clock means generating a clock signal having a duty cycle established to compensate for errors in said inverter and said filter.

A method for amplifying a differential DC voltage comprising the steps of:

• alternating polarity of said differential DC voltage to produce a switched low level DC signal;
• amplifying said switched low level DC signal to produce an amplified DC signal;
• inverting said amplified DC signal to produce an inverted DC signal;
• alternately filtering said amplified DC signal and said inverted DC signal to produce a filtered output signal;
• synchronizing said alternating polarity and alternately filtering steps;
• and
• dynamically controlling said synchronizing step to compensate for offset errors in said amplifying, inverting, and alternately filtering steps.