DMT has released the new generation of its Slim Borehole Scanner (SBS) for imaging extra slim drill holes with a minimum diameter of 25 mm. One of its primary tasks is the logging of rock- or roof-bolt boreholes.

There has been keen interest from South African mining companies with regard to this technology.

Rock- or roof bolting (RB) is a common technology to stabilise underground mine workings, tunnels or cliffs. The physical principal is a simple transfer of load from the exposed, and hence less stable rock face to the intact internal rock mass. Since the first use of RB in the 19th century, many types of rock bolts have been designed with variable lengths, materials and methods to ensure contact to the rock. Depending on the deformation of the rocks and the shape of the mine workings or tunnels, a pattern with suitable rock bolts is designed. This implies that changing ground conditions may force to change the design pattern. More stable conditions will enable the mine operator to lower the density and the type of rock bolt, while less stable conditions may initiate a shift to a denser pattern and/or different rock bolt.

RB adds significant costs to underground operations but is crucial for the safety of workers and production. Because of costs, rock bolts are typically slim, i.e. less than 25 mm in diameter. This diameter has in the past forced geotechnical staff to inspect ground conditions in RB boreholes using an endoscope, a camera-like device originally designed for medical investigations or, in many cases, not at all. The endoscopic method resulted in a verbal description by geotechnical staff of observations made in the pictures. Despite efforts to standardise this method a large source of uncertainty is the subjectivity or personal impression of the description. Hence, repetition and transparency are compromised, and companies are often reluctant to shift to more cost-saving RB patterns.

In response to these constraints, DMT developed the RB borehole scanner, called the Slim Borehole Scanner (SBS), some years back together with the German hard coal mining industry. Because of this cooperation the tool developed is rated intrinsically safe e.g. for use in coal mines (I M1 Ex ia I Ma) or explosive environments. Figure 1 shows the complete tool kit.

Latest improvements made are:

• Depth control using a SlimDepthRecorder – easy to operate, 1/10 mm resolution, weight 150 g and low power requirements (See Figure 2);
• Integrated stabilizers;
• SBS (ex-proof version) is equipped with batteries and not with rechargeable accumulators.and
• Exchangeable data storage chip. 

Basic design and configuration of the tool was untouched in order to maintain the intrinsically safe rating and retain the ability to enter RB drill holes. A key characteristic is the extra-slim diameter of 23 mm with total length of 120 cm. It is a stand-alone solution with its own power supply and data storage unit. 

Its scanning device is combined with a ring of LED, which will illuminate bore hole walls up to diameter of 45 mm. The imaging assembly with camera and LED-ring is shown in Figure 4. Self-containment implies that settings made prior to logging cannot be changed and quality of the data can be only be verified after down-loading.

The optical assembly will generate a 360° optical image, which will be stored on the data storage chip for down-loading after completion of the measurement. The high-resolution image is generated by stacking slice by slice and thus producing an image of the complete unwrapped borehole (Figure 5). Measurements are taken by pushing the tool with carbon-fibre rods by hand into the RB drill hole or any other short extra-slim drill hole. Its light weight (1,5kg) also allows logging of upward holes in the roof. Hence, it is ideal for scanning of roof bolt boreholes.

The stored image can be down-loaded into a designated software and is thus, available for qualitative and quantitative analyses, statistical evaluation and record stored for further work or reference, if required. Because of the possibility to orientate the image to actual conditions real orientations of bedding, jointing, discontinuities and other geological features (faulting, veins, etc.) can be quantitatively determined and interpreted. 3D-positions of dividing planes and their density in the roof or sidewall of an underground working can be defined.

Due to the high frequency of RB drilling the database builds up rapidly and the geotechnical and rock mechanic database is improved significantly.

In summary, the tool allows to identify geological and tectonic structures, orientated in space, to be statistically analysed. This data can be used for a geological-geotechnical model during construction or maintenance of underground workings and provide objective information, which form a solid foundation for decisions to change the design of the RB pattern. This adaption of the RB pattern is exemplary, shown in 7.  Finally, using a similar methodology, RB of cliffs and rock faces can also be planned very cost-efficiently but technically appropriate thus reducing one of the most common geo-hazards - falling rocks.