Method and apparatus for measuring the depth of a tool in a borehole

A method for determining the depth of a tool suspended from a cable inside a borehole penetrating an earth formation comprising the steps of:

• deriving at a surface location a measurement of cable depth of the tool from a measurement of the length of the cable from which the tool is suspended;
• deriving a measurement of the acceleration of the tool along the borehole axis;
• estimating the depth of the tool from the cable depth and acceleration measurements with a Kalman filter,
• sensing a motionless tool;
• and
• modifying the tool depth estimating step at least during the interval the tool is sensed to be motionless so as to maintain the tool depth estimate constant while the tool is motionless.

A method for determining the depth of a tool suspended from a cable inside a borehole penetrating an earth formation comprising the steps of:

• generating a signal representative of the cable depth of the tool;
• generating a signal representative of the acceleration of the tool along the borehole;
• estimating the depth of the tool from the acceleration and cable depth signals with a Kalman filter;
• sensing when the tool becomes stuck in the borehole;
• and
• maintaining the tool depth estimate constant while the tool is stuck.

An apparatus for determining the depth of a tool suspended from a cable inside a borehole penetrating an earth formation comprising:

• means located at the surface for generating a cable depth signal indicative of the length of cable from which the tool is suspended;
• means for measuring the acceleration of the tool along the borehole axis and producing a signal representative thereof;
• means responsive to the cable depth and acceleration signals for generating an estimate of the depth of the tool;
• means for detecting a motionless tool and producing a signal indicative thereof;
• and
• means for modifying said estimate generating means with the latter signal to maintain the tool depth estimate constant while the tool is motionless.

An apparatus for determining the depth of a tool suspended from a cable inside a borehole penetrating an earth formation comprising:

• means for generating a cable depth signal indicative of the length of the cable from which the tool is suspended;
• means for measuring the acceleration of the tool along the borehole axis and producing a signal indicative thereof;
• Kalman filter means responsive to the cable depth and acceleration signals for generating an estimate of the depth of the tool;
• means for detecting a motionless tool and producing a signal indicative thereof;
• and
• means for applying the latter signal to the Kalman filter means to maintain its tool depth estimate constant while the tool is motionless.