Method and apparatus for producing an image log of a wall of a borehole penetrating an earth formation

A method for generating a log of fine conductivity features of a borehole wall with a tool that is suspended from a cable inside the borehole which penetrates an earth formation and which tool has a plurality of survey electrodes arranged in multiple rows in a predetermined overlapping manner to investigate a circumferentially continuous segment of the borehole wall with high spatial resolution comprising the steps of:

• generating high spatial resolution conductivity signals whose amplitudes represent the conductivity of the borehole wall opposite the survey electrodes with the conductivity signals in the aggregate representing conductivity of an effectively continuous vertical and circumferential segment of the borehole wall;
• converting said conductivity signals to high spatial resolution conductivity signals which represent the conductivity of said borehole wall segment as a linear function of borehole depth;
• and
• forming from said latter conductivity signals a visual image of said effectively continuous borehole segment as a linear function of borehole depth and with a grey scale with which fine conductivity features indicative of stratigraphy, vugs, and fractures present in the borehole wall segment are visually enhanced.

A method for generating a log of fine features of a borehole wall with a tool that is suspended from a cable inside the borehole which penetrates an earth formation comprising the steps of:

• generating signals which represent a high spatial resolution measurement of a characteristic of said borehole wall and in the aggregate represent comparable high resolution features of said characteristic over an effectively continuous vertical and circumferential segment of the borehole wall;
• generating signals representative of the cable depth of the tool;
• generating signals representative of the accelerations of the tool;
• determining when the tool is motionless and generating a signal indicative thereof;
• deriving from said cable depth signals, the acceleration signal and the motionless signal, said depth signals with a high spatial resolution that is commensurate with the high spatial resolution of said characteristic signals;
• and
• modifying signals representative of said borehole wall characteristic as a predetermined function of measured deviations of said latter signals from averages thereof as determined over borehole wall regions in the vicinities of the locations to which respective ones of the signals relate so as to equalize said signals for variations attributable to factors such as the borehole environment or unequal processing of signals generated with the tool.

A method for generating a log of fine features of a borehole wall with a tool that is suspended from a cable inside the borehole which penetrates an earth formation comprising the steps of:

• generating signals which represent a high spatial resolution measurement of a characteristic of said borehole wall and in the aggregate represent comparable high resolution features of said characteristic over an effectively continuous vertical and circumferential segment of the borehole wall;
• generating signals representative of the cable depth of the tool;
• generating signals representative of the accelerations of the tool;
• determining when the tool is motionless and generating a signal indicative thereof;
• deriving from said cable depth signals, the acceleration signal and the motionless signal, said depth signals with a high spatial resolution that is commensurate with the high spatial resolution of said characteristic signals;
• modifying signals representative of said borehole wall characteristic as a predetermined function of measured deviations of said latter signals from averages thereof as determined over borehole wall regions in the vicinities of the locations to which respective ones of the signals relate so as to equalize said signals for variations attributable to factors such as the borehole environment or unequal processing of signals generated with the tool;
• determining an average of the signals representative of the characteristic of the borehole wall as a function of depth over a predetermined depth range of the borehole wall segment;
• selecting a reference value representative of a visually desirable grey scale level for the image log;
• forming a grey scale factor with the reference value and said average value;
• modifying signals representative of said characteristic as a function of the grey scale factor to maintain the grey scale of the image log within a visually usable range;
• and
• recording said grey scale factor alongside said image log.

An apparatus for generating a log of fine features of a borehole wall as a function of depth with a signal processor from an investigation of a borehole wall with a tool that is suspended from a cable inside the borehole which penetrates an earth formation comprising:

• means for producing a plurality of high spatial resolution signals respectively representative of a characteristic of the borehole wall and which signals in the aggregate represent high resolution features of said characteristic over an effectively continuous vertical and circumferential segment of the borehole wall;
• means for converting said high spatial resolution signals to signals which represent said characteristic as a linear function of depth;
• and
• means for producing a visual image of said continuous borehole segment with said latter converted signals with the image intensity related to the characteristic, said visual image extending as a linear function of borehole depth with the lateral image positions of the signals being in correspondence with the circumferential locations to which the signals relate so as to form, of the measured characteristic along the borehole wall segment, a visual image that in the aggregate enhances visual recognition of significant fine features of said borehole wall segment.