Introduction:

The Tensense Optical Borehole Televiewer (OTV) is a cutting-edge subsurface exploration tool that employs advanced optical imaging technology to capture high-resolution, color images of the borehole wall. This device provides geologists with a detailed, 360-degree, oriented view of geological features, enabling precise mapping and analysis of rock formations, lithologic boundaries, fractures, bedding planes, foliation, and other critical geological attributes.

Functionality:

Tensense Optical Televiewers offer magnetically oriented, high-resolution color images of the borehole wall, which are instrumental in mapping rock types and the orientations of planar features such as fractures, joints, lithologic contacts, and bedding that intersect the borehole. The OTV utilizes a digital high-resolution camera directed towards a conical mirror, capturing 360-degree images of the borehole wall. This technology allows for continuous imaging in both air- and water-filled boreholes.

Principle:

The advent of charged coupled device (CCD) cameras and complementary metal-oxide semi-conductor (CMOS) sensors has revolutionized optical imaging technology. These cameras digitize the intensity of the color spectrum in red, green, and blue. The camera lens is directed towards a conical mirror, and measurements are collected with azimuthal increments ranging from 180 to over 1800. Depth measurements are collected at high vertical resolution, producing high-resolution images. A high precision 3D compass, incorporating a 3-axis magnetometer and 3-axis accelerometer, is used to determine the azimuthal direction and inclination, allowing for the orientation of the borehole deviation and magnetic declination to be applied to the interpreted features.

Value:

Digital data from the Tensense OTV can be processed and interpreted manually or using automated structural algorithms with post-processing software. Advanced 2- and 3-D imaging software packages permit corrections for magnetic declination, borehole deviation, borehole diameter, and decentralization. Structural interpretations can be represented in various formats, including tadpole plots, rose diagrams, and stereographic projections.

Applications:

Optical imaging with the Tensense OTV is valuable for both qualitative and quantitative interpretations of lithology, structure, bedding, breakout and stress analysis, thin-bed detection, referencing or correcting core, casing inspection, and determination of vuggy porosity. These investigations are crucial for water-resources, contaminant, and geologic mapping investigations. The OTV has been widely used in open-hole fractured rock applications, allowing for direct mapping of rock types and weathering from the images. Analysis of fracture orientations, breakout zones, and pop-out structures can determine the in-situ stress fields.

Limitations:

A critical limitation of OTV logging in fluid-filled boreholes is the requirement for clear fluid and adequate lighting to illuminate the borehole wall. Without variable illumination and/or shutter speed, the lighting conditions may not permit the detection of features of interest. Additionally, if the tool is not centered, the images can have shadows and will be distorted. Borehole images can also get “disoriented” in the presence of magnetic minerals and steel casing; however, this artifact can sometimes be removed in post-processing.

Conclusion:

The Tensense Optical Borehole Televiewer represents a significant advancement in geological exploration, providing a comprehensive and detailed insight into subsurface structures. Its applications span across various industries, making it an indispensable tool for accurate geological analysis and interpretation.