Robertson Geo is a global market leader in slim-hole logging instrumentation with in-house design and manufacturing facilities and offers a proven logging service worldwide.
Borehole televiewers provide a continuous, orientated, high-resolution unwrapped image of the borehole wall, offering many advantages to geologists and geotechnical engineers. The data provides information about geology, structure, fractures, fault 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 fluid filled boreholes the acoustic televiewer logs the borehole wall in terms of hardness, measuring the amplitude and travel time of a high-frequency reflected sonic pulse at very high resolution. Hard rocks reflect high-amplitude signals and soft rocks and fractures reflect low ones. The result is a map of the borehole wall with an individual resolution 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 via a conical mirror. Resolution is very high, with pixel sizes down to well below 1mm at HQ borehole diameter.
BATTLE PROVEN TECHNOLOGY
Robertson Geo has been providing borehole televiewer equipment and logging services throughout the world in some of the harshest environments. Within the mining industry Robertson Geo have provided televiewer services in the search for gold, zinc, lead, coal and aggregates. Televiewers are also routinely deployed on major infrastructure projects worldwide, both onshore and offshore. However, the ease of deployment and cost effectiveness of the technology means affordability for all scales of project.
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. 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 whereby the drill string is withdrawn in stages to provide some open borehole while providing protection for problematic zones.
The data is monitored in real time as the logging proceeds and upon completion of the log the image can be viewed in detail if immediate results are required. The fast turnaround of images in the field is now a common requirement for geologists and geotechnical personnel who need to make rapid decisions based on the results.
All televiewer data is fully oriented, usually with respect to magnetic north. The tilt and azimuth of the borehole are always recorded to allow correction to true vertical depth and to provide automatic correction for the delineation of features. Geologically relevant features on the log are identified and a pure sinusoid is fitted over which defines the azimuth and dip. After picking, the features are displayed as a tadpole plot. Feature dips can also be displayed from different view angles and the data can be summarised into various histogram plots and Stereonet and Rose diagrams over selected depth ranges and grouped by dip angles or azimuth.
Towards the future, televiewer images providing a wealth of unambiguous information will continue to be a principle requirement for mineral and geotechnical investigation.
Graham Comber, Logging Services Manager, Robertson Geo