Why borehole televiewers have become the tool of choice for geotechnical and mineral investigation. By Robertson Geo services manager Graham Comber.
Borehole Televiewers provide a continuous, orientated, high-resolution unwrapped image of a borehole wall, offering many advantages to geologists and geotechnical engineers. The data provides information about geology, structure, fractures and stress orientation, and acts as a template for orientating cores and providing depth control where core recovery is incomplete.
How Televiewers work
Working in a fluid filled boreholes th eacoustic televiewer logs the borehole wall in terms of hardness, measuring the amplitude and travel time of a high frequency relfected sonic pulse at very high resolution. Hard rocks reflect high-amplitude signals and soft rocks and fractures relfect low ones. The individual measurements of reflected amplitude are made continuously by a rotating transducer or a rotating sonic mirror aligned with a stationary transducer. The result is a map of the borehole wall with an individua lresolution of about 2mm in ideal conditions. Fractures and bedding planes appear as sinusoidal lines where the deepest point on the line is the direction of dip.
If data is required from dry boreholes, or if the borehole fluid is clear, the optical televiewer should be employed. It measures the colour and shade of reflected light. The borehole wall is lit by a ring of diodes on the tool and reflections are directed to a light-sensitive sensor via a conical mirror. Resolution is very high, with pixel sizes down to well below 1mm at HQ borehole diameter.
Robertson Geo High Resolution Optical Televiewer (Hi-OPTV) Probe
Battle proven technology
Robertson Geo has been providing borehole televiewer equipment and logging services throughout the world in some of the harshest environments.Off shore, operating from jack-up platforms or from drill ships, marine resistant technology has allowed televiewers to be included in the armoury of tools available to the geotechnical engineers. Televiewers have also been deployed extensively in mineral exploration and on major infrastructure projects worldwide. However, the ease of deployment and cost effectiveness of the technology means affordability for even the smallest of projects.
Borehole conditions play a big part in image quality with rotary drilled boreholes always providing the best images (low rugosity). Televiewers can produce good images in boreholes from 60mm to 300mm with the optimum range being from 75mm to 150mm. It is important that the borehole be prepared to offer the best conditions for the selected methodology. Flushing and settling time is often vital for the optical televiewer to allow particulates in the fluid time to settle. The acoustic televiewer is wholly dependent on a fluid filled borehole but is quite tolerant to mud filled boreholes. Where borehole stability is an issue the logging is often split into separate runs. A shooting plan can be made where the drill string is withdrawn in stages to provide some open borehole while providing protection for problematic zones. Using this staged logging method necessitates the televiewer being able to pass through the drill string and drill bit into the open borehole while retaining sufficient centralisation. Drill bit types with a relatively large internal diameter compared to borehole diameter allow the centralisation to be optimised.
In addition to the image data beingi nspected in real time as the logging proceeds; the image can be viewed in detail on completion of the log if immediate results are required. The fast turnaround of images in the field is now a common requirement for geotechnical personnel and geologists who need to make quick decisions on depths for further tests or for installations.
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