There is a strong business case for organisations to replace traditional methods of gathering geotechnical data from boreholes with televiewer surveys.
Televiewer surveys are becoming routinely used to capture structural geotechnical data for use in pit slope and underground design, as a replacement for manual core orientation techniques. Typically, televiewer surveys consist of a continuous log of orientated, unwrapped images of drillhole walls. Two complementary methods are available: an ‘acoustic televiewer’ (ATV) which uses an acoustic signal from a rotating sonar transducer and an ‘optical televiewer’ (OTV) which uses a high resolution digital color camera equipped with a light source.
Televiewer surveys have altered the way geotechnical data is captured, according to Chris Mears, a consultant in geotechnical engineering at SRK Consulting UK. Mears explains to African Mining Brief that, in comparison with manual core orientation techniques, televiewer surveys are increasingly being preferred due to the convenience they offer in capturing structural geotechnical data. The great benefit of using televiewer survey methods is that two methods – acoustical televiewer and optical televiewer – complement each other. He demonstrates: “Optical televiewers do not work well when the borehole is filled with water (the image returned is often muddy or blurred). When water is reached in the borehole, the acoustic televiewer enables an image to be developed using acoustics, which is not disrupted by muddy or cloudy water. Often, the procedure taken is to begin in the top (dry) section of the borehole with the optical televeiwer, and then switch to an acoustic televiewer when the water is met.
“Amplitude and travel time of acoustic waves can be interpreted from the acoustic televiewer and can be used to determine the presence, size and sometimes infill of structures. Software built to deal with televiewer data enables the accurate generation of dip and dip direction of structures in the borehole, for example, WellCad.”
In contrast, in traditional drilling programmes, the dip and dip directions of structures are determined by generating an orientation line on the extruded core (for reference). However, this method often encounters problems and is not always reliable. As a result, the structural geotechnical data that can be captured from borehole televiewers is thought to be more reliable, which can be crucial in structurally-controlled geotechnical designs.
What televiewer surveys offer
Cost cutting and increasing efficiency are the operative words in the present mining atmosphere, and televiewer survey methods offer one of the avenues which can help companies accomplish this objective. Compared with manual core orientation techniques, televiewer data surveys offer more benefits above efficiency and cost effectiveness in gathering and managing geotechnical data.
- Time Efficiency
With the traditional method, taking an orientation mark at every drill run, marking the orientation line, assessing its reliability and manually measuring each structure orientation from the orientation line is highly time consuming. On the other hand, when using the televiewer file in the software WellCAD, the logger only needs to digitize the relevant structures on the images, the orientations and depth of each structure is automatically calculated and collated in a table in the software.
- High Quality Data
When using traditional core orientation methods, the person in charge of the logging needs to assess the nature of every structure in the core. There is a risk of incorrectly accounting for artificial breaks induced by drilling or core handling which reduces the quality of the rock mass. Generating structural data from ATV/OTV surveys means that only the natural structures are taken into account, artificial breaks being absent on the televiewer image. This results in a more accurate estimate of the in-situ rock mass quality.
- Reliable Data
Acquiring the structural orientation information from a televiewer survey file removes the possible human error associated with the manual measurement from orientated cores.
- Better Time Management
Traditional logging of structural data from orientated core requires constant geotechnical supervision of the drill crew while doing orientation acquisition and logging at the drill site. Using ATV/OTV surveys the geotechnical logger can be on site for a single visit to log all the core at the end of the drilling program. This has a time advantage and moreover a significant cost advantage.
- Rapid Orientation Conversion
Televiewer orientation data are rapidly converted in WellCAD from their apparent orientation i.e. relatively to the core axis to true orientation using a high definition borehole deviation survey file recorded during the televiewer survey acquisition.
- Better characterisation of fracture zones
The orientation and true thickness of fracture zones are very important for the geotechnical characterisation as these can play a vital role in the overall pit slope stability. It is very challenging to accurately assess the true thickness and orientations of these zones from the recovered broken core and the orientation line is generally lost when a fractured zone is intersected. Providing that the borehole is stable, a televiewer survey will record the image in such a zone without much difficulty. The true thickness, the dip and dip direction of the fracture zone contacts will easily be assessed from the televiewer file. To guarantee the stability of the borehole, a PVC pipe could, if needed, be inserted in the hole and depending on certain conditions, an acoustic image could be acquired through the PVC pipe.
- Accurate Depth Measurement
Drill core depth can sometimes be quite approximate with the recovered core length not exactly matching the depth given by the drillers. The televiewer survey file can in this case be taken as a reference. The survey is generally undertaken from bottom to the top for a smooth data acquisition, using a high precision meter measurer on the winch, guaranteeing reliable depth measurement.
- Useful for other Disciplines
Acoustic or optical survey files can be used by hydrogeologists to accurately locate water inflow zones and understand which fractures convey water when coupled with a spinner test survey.
- Costs Benefits
Populating a structural database from televiewer surveys is generally more cost effective than using traditional core orientation techniques. Less time is spent orienting the core, measuring the orientation, undertaking QAQC of the measurements etc.
- Benefits to the Design
The use of televiewer survey leads to a more accurate structural database, with more reliable statistical parameters for each joint sets within the rock mass. This ultimately tends to improve the rock mass characterization and increase the confidence given to the design. Using televiewer survey in geotechnical mining studies can lead to the development of steeper inter-ramp angles resulting in reduced waste stripping volumes and potentially significant mining costs savings.
Strong business case
All told, without question, there is a strong business case for organisations to replace traditional methods of gathering geotechnical data from boreholes with televiewer surveys.
Addressing conditions that can affect televiewer surveys
Mears calls attention to the following conditions that can have a bearing on the efficiency of the process:
- Cleanliness of the borehole – optical televiewers will produce poor quality images if the hole is dirty, which reduces the ability to identify key structures.
- Presence of water – acoustic televiewers will only function in fluid-filled boreholes.
- Stability of the borehole – survey tools cannot pass past collapsed zone of boreholes.
- Additional tools are also required, which capture tilt and azimuth of the borehole to enable the dip and dip directions of the structures identified in the televiewer survey to be estimated.
Written by Jimmy Swira