Borehole acoustic and optical televiewers provide a means to visualize, orient and measure borehole features in-situ. Televiewer logging can augment and in some instances replace coring in areas with low-core recovery (low RQD) or missing core. The acoustic televiewer produces the borehole wall image by transmitting ultrasound pulses with a focused beam, rotating, sonar transducer and recording the travel time and amplitude of multi-echo reflections from the borehole wall. A 360 degree image is produced with accurate hole deviation and tool position provided by a three-axis magnetometer and three accelerometers. Televiewers are not adversely effected by inclined holes, with sensors limited only in near-horizontal boreholes.
The optical televiewer provides a very high resolution, continuous, oriented, true colour wall image in both air or water filled boreholes. The borehole is illuminated by a circular array of white LEDs mounted on the head of a charge-coupled device (CCD) camera. A projection of the virtual core can be rotated and viewed at any orientation. Televiewers record highly accurate, detailed, oriented caliper and structural information in the logging process. The optical televiewer provides images in natural rock colours representing actual core and requires clear fluid, or an empty, clean borehole. The acoustic televiewer is more versatile in application and works in open or plastic-lined boreholes, and in clear or low visibility, ‘dirty’ borehole fluids.
Deep bedrock environments can be surveyed with a high temperature, high pressure acoustic televiewer. This development extends televiewer applications to deeper, more hostile environments for geothermal exploration, deep-seated mining, oil and gas exploration, and geotechnical evaluations. When a dipping plane intersects the borehole, the intersection forms an ellipse. Unrolling the ellipse on a two-dimensional plot produces a sine wave, the radial position of which indicates the dip direction and the amplitude indicates the dip angle. The interpretation procedure involves fitting sine waves to the structure evident in the logs. The aperture size of the larger structural features can also be measured. Structural bedding and fractures are illustrated in the acoustic televiewer borehole image. Interpreted modelling of the structural features is shown by the tadpole symbols overlain on the reflected amplitude images. The dip angle is indicated by the horizontal position of the tadpole. The strike direction, dip direction, and dip angle results of the structural interpretations are shown plotted on a polar diagram. This plot displays strike direction and dip angle as a single point in the projection plane. The distance from the centre of the projection plane represents the dip angle and the azimuth position on the circle represents the strike or dip direction.