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In recent years, miniaturization of electronic components has led to development of "seeing-eye" TV cameras capable of entering small-diameter boreholes. Commercially available cameras now can operate in 3-inch holes to depths exceeding 5,000 feet. Some cameras look downhole; others view sideward by means of rotating mirrors. Built-in compass systems permit accurate surveys of hole orientation and attitudes of planar elements intersecting the borehole.
TV borehole cameras have many geological and engineering applications. In dry uncased holes, uses include determination of lithologic character and stratigraphic contacts, measurement of bedding and fracture orientation, identification of textural features (porosities), and establishment of causes of drilling problems including recognition of caved zones and intervals of circulation loss. A TV camera survey supplements or replaces coring. If hole walls are clean, inspection of the entire interval provides direct information normally lost where core recovery is poor. In holes containing buckled or perforated casing, lost tools or unknown obstructions, a TV traverse may guide the driller to an effective, time-and cost-saving remedy.
The Lawrence Radiation Laboratory has built two cameras for use in pre- and post-shot studies associated with underground nuclear explosions. These 2½-inch diameter cameras can reach 3,000-foot depths and will operate in vertical or inclined holes up to diameters limited only by illumination capability. Both cameras have remote-control focussing. One camera, having a fixed side-viewing mirror, is equipped with a specially designed compass. The other camera looks down the axis of the hole. Field experience with these cameras is described and photos of the monitor image illustrate the results obtainable during in-hole operation. TV pictures showing the Gnome explosion cavity are compared with actual photographs taken inside this void.
The current status of TV borehole camera technology and possible future developments in both instrumentation and applications are reviewed.
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