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  | isbn    = 0891816607

  | isbn    = 0891816607

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Borehole imaging devices are high resolution wireline tools that produce maps of physical measurements of the rocks exposed on the borehole wall. They are thus two-dimensional logs, with depth and azimuth as independent variables.

Borehole imaging devices are high resolution wireline tools that produce maps of physical measurements of the rocks exposed on the borehole wall. They are thus two-dimensional logs, with depth and [[azimuth]] as independent variables.

Three main imaging techniques are presently in use:

Three main imaging techniques are presently in use:

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Borehole images are normally displayed in a depth and azimuth reference system, which corresponds to the projection of the cylindrical surface of a drill core onto a plane that is split by tradition along magnetic north. In this projection, an inclined planar feature such as a bedding surface, fracture, or fault shows up as a sinusoid. The minimum, or trough, of a sinusoid points in the direction of the dip azimuth, while the amplitude (divided by the borehole diameter) indicates the dip angle.

Borehole images are normally displayed in a depth and [[azimuth]] reference system, which corresponds to the projection of the cylindrical surface of a drill core onto a plane that is split by tradition along magnetic north. In this projection, an inclined planar feature such as a bedding surface, fracture, or fault shows up as a sinusoid. The minimum, or trough, of a sinusoid points in the direction of the dip azimuth, while the amplitude (divided by the borehole diameter) indicates the dip angle.

Most borehole images are digitized, either downhole or uphole, and are thus amenable to digital computer processing such as image enhancement (e.g., filtering, sharpening, and false coloring), higher level image analysis (e.g., feature recognition and extraction), and three-dimensional projections. Interactive graphic workstations are useful tools for manipulating and interpreting borehole images.

Most borehole images are digitized, either downhole or uphole, and are thus amenable to digital computer processing such as image enhancement (e.g., filtering, sharpening, and false coloring), higher level image analysis (e.g., feature recognition and extraction), and three-dimensional projections. Interactive graphic workstations are useful tools for manipulating and interpreting borehole images.

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Electrical borehole scanning is an extension of the dipmeter technique. In this method, a large number of closely spaced electrodes of 0.2-in. diameter are mounted on a conductive pad and pressed against the borehole wall. The amount of current emitted from each electrode is recorded as a function of azimuth and depth ([[:file:borehole-imaging-devices_fig6.png|Figure 6]]). The tool thus produces a microresistivity map. Currently on the market are versions with two and four imaging pads perpendicular to each other. The resulting image stripes are about 2.5 in. wide and are oriented azimuthally by a downhole magnetometer unit. In a 8.5-in. borehole, the four-pad Formation MicroScanner provides a 45% circumferential coverage. Repeat logging passes can often increase this percentage. By convention, darker gray tones are used for lower resistivities.

Electrical borehole scanning is an extension of the dipmeter technique. In this method, a large number of closely spaced electrodes of 0.2-in. diameter are mounted on a conductive pad and pressed against the borehole wall. The amount of current emitted from each electrode is recorded as a function of [[azimuth]] and depth ([[:file:borehole-imaging-devices_fig6.png|Figure 6]]). The tool thus produces a microresistivity map. Currently on the market are versions with two and four imaging pads perpendicular to each other. The resulting image stripes are about 2.5 in. wide and are oriented azimuthally by a downhole magnetometer unit. In a 8.5-in. borehole, the four-pad Formation MicroScanner provides a 45% circumferential coverage. Repeat logging passes can often increase this percentage. By convention, darker gray tones are used for lower resistivities.

Formation MicroScanner images record changes in rock resistivity caused by variations in [[porosity]] and clay content of a small rock volume in the vicinity of the borehole wall. Increased pad stand-off due to mudcake buildup on the borehole wall may decrease the spatial resolution, while abrupt changes in tool movement may produce a local misalignment or sawtooth effect in the layers ([[:file:borehole-imaging-devices_fig7.png|Figure 7]]). Important bedding types and surfaces can be identified and measured for their dip and azimuth ([[:file:borehole-imaging-devices_fig8.png|Figure 8]]), as are fractures ([[:file:borehole-imaging-devices_fig9.png|Figure 9]]) and stylolites ([[:file:borehole-imaging-devices_fig10.png|Figure 10]]). Faults are often readily recognized on Formation MicroScanner images because of the offset of rock types across the fault plane.

Formation MicroScanner images record changes in rock resistivity caused by variations in [[porosity]] and clay content of a small rock volume in the vicinity of the borehole wall. Increased pad stand-off due to mudcake buildup on the borehole wall may decrease the spatial resolution, while abrupt changes in tool movement may produce a local misalignment or sawtooth effect in the layers ([[:file:borehole-imaging-devices_fig7.png|Figure 7]]). Important bedding types and surfaces can be identified and measured for their dip and azimuth ([[:file:borehole-imaging-devices_fig8.png|Figure 8]]), as are fractures ([[:file:borehole-imaging-devices_fig9.png|Figure 9]]) and stylolites ([[:file:borehole-imaging-devices_fig10.png|Figure 10]]). Faults are often readily recognized on Formation MicroScanner images because of the offset of rock types across the fault plane.