Large-scale structures logging

Large-scale structures represent through-going faults that extend from inter-ramp to overall pit slope and regional scale. They are weakening features that usually are widely spaced. They may form the boundaries to large-scale structural domains or define a major structural pattern within a particular structural domain.

■ Number of joint sets, Jn. This value represents the number of individual open joint sets intersected by the drill hole. Joint angles to the core axis (Alpha) and joint characteristics can help to determine the number of sets present (Figure 2.17).

■ Typical angle of the individual joint set to the core axis (a). The Alpha angle captures the angle between the joint plane (the maximum dip vector) and the core axis (Figure 2.18).

■ Joint conditions for the individual set (Jc). Joint conditions are expressed by small-scale irregularities (roughness) on the surface of the joint, alteration of the joint wall and the nature of the joint infill.

The nature of the joint infill should be captured using the engineering terminology of the Unified Soils

Classification System (Table 2.7), which enables empirical estimates of the shear strength of the infill. Estimates of the alteration of the joint wall allow comparison of the relative strength of the joint wall against the strength of the intact rock. It should be noted whether the wall is fresh or altered. If it is altered, it should be noted if it is weaker or stronger than the intact rock.

Figure 2.15: Example of core disking during drilling due to locked-in stress. The surfaces are fresh, without staining or infill and are generally perpendicular to the core axis

Source: Photo courtesy J. Jakubec

Figure 2.16: Example of micro-defect density, heavy to the left and moderate to the right

Source: Photos courtesy J. Jakubec

Figure 2.17: Example of two dominant joint sets intersected by the drill core. Although the Alpha angle is the same for both sets, the core pieces exhibit a different orientation

Source: Photo courtesy J. Jakubec

Figure 2.18: The Alpha angle (a) should be estimated for each individual joint set

At a minimum, the following parameters should be captured on the large-scale structural logging sheet:

■ rock type;

■ length of intersection of fault zone along the core axis;

■ quality of material within the fault zone. The quality of the material within the boundaries of the fault struc-ture can be described using the following terms:

→ crushed material (Table 2.6), containing angular, sand-sized fragments of rock in a matrix of silt and clay (Figure 2.20). This material is equivalent to the descriptive geological term ‘gouge’. It should be described using the Unified Soils Classification System (Table 2.7) as an aid to establishing the shear strength of the material;

→ sheared material (Table 2.6), comprised dominantly of angular sand to gravel-sized rock fragments that are smaller than the core diameter, with some silt and clay. Frequently, the angular fragments exhibit slickensided surfaces formed as the fault ruptured the rock mass (Figure 2.21). It should be possible to log this material using the Unified Soils Classifica-tion System (Table 2.7);

→ broken material comprised almost entirely of core fragments smaller than the core diameter with only traces of silt and clay (Figure 2.22);

→ jointed material comprising a zone of higher joint density than the rest of the core (Figure 2.23). In these zones the joint frequency per metre and the condition of the joints (Jc) should be captured on the logging sheets.

Figure 2.19: Examples of small-scale joint roughness geometries.

Stepped slickensided (top left), stepped rough (top right), undulating slickensided (middle left), undulating rough (middle right), planar slickensided (bottom left) and planar rough (bottom right)

Source: Photos courtesy J. Jakubec

Table 2.11: Small-scale roughness criterion, J_r

Note that the chart is not to scale. The length of the individual surfaces illustrated should be approximately 10 cm. JRC 200 mm and JRC 1 m correspond to joint roughness coefficients when the profiles are scaled to lengths of approximately 200 mm and 1 m respectively.

Source: Barton (1987b)

Figure 2.20: Example of crushed material (gouge) formed by a large-scale fault. The core recovery is poor since much of the clay and silt-sized material was washed away during drilling Source: Photo courtesy J. Jakubec

Figure 2.21: Example of a sheared zone. Note that the fragments are smaller than the core diameter. The core recovery is also poor since much of the fine material was washed away during drilling Source: Photo courtesy J. Jakubec

represent only a local variation (undulation) in the geometry rather than general orientation of the structure.

There are often insufficient data points to determine the true orientation of larger structures from individual drill holes.

Digital photography:

Two new digital photographic systems have been developed specifically for geotechnical logging and analysis – StereoCore PhotoLog™ (Orpen 2007) and CoreProfiler™ (Sliwa et al. 2007).

Using StereoCore Photo Log™ the oriented core can be digitally photographed in the tray within a reference frame and the image processed to compensate for perspective and produce a depth-registered virtual 3D model of the core cylinders. The processed model can then be used pick the a and b angles before the system performs the structural analyses with drill hole survey data loaded from either measurement while drilling or independent drill hole survey records. Lithological logs, stereo plots and fracture frequency and RQD histograms can be reported at selected depth intervals for drill hole path depth or true vertical depth. The depth registration process also allows the driller’s stick-up logs to be rigorously checked, enabling the correct identification of core loss and/or gain.

CoreProfiler™ (Figure 2.24) has been developed from CSIRO’s Sirovision™ technology to reconstruct a scaled continuous image of drill core from handheld core tray or core split photographs. The development was funded by the Australian Coal Association Research Program (ACARP Project C15037) and Release 1.0⁶ is freely available to the Australian coal industry.

Imported photographs of core can be digital

photographs or high-quality scans of paper prints, in TIFF or JPEG format. Lens corrections can be applied to each photograph as it is imported, with perspective correction applied by identifying the corners of a rectangle of known dimensions on the image; precise depth controls can be added later. The angle of a joint or bedding to the core axis (a, Figure 2.18) can be estimated directly from the core image and the import/export logging data.

Downhole imaging

Acoustic (ATV) and optical (OTV) televiewers provide continuous and oriented 360° views of the drill hole wall from which the character, relation and orientation of lithologic and structural planar features can be defined (Figures 2.25 and 2.26).

ATVs were first developed by the petroleum industry in the late 1960s, with the optical OTVs following in the 1980s (Williams & Johnson 2004).

ATV imaging systems emit an ultrasonic pulse-echo and record the transit time and amplitude of the acoustic 2.4.9.5 Determining the orientation of structures

There are at least three different means for determining the orientation of any joints or faults that may intersect the drill hole: direct measurement from the oriented core using a goniometer; direct measurement from the oriented core using digital photography and virtual 3D imagery;

and downhole imaging using optical or acoustic televiewers.

Goniometry

The angle of the joint to the core axis (a, Figure 2.18) and the circumference angle (b), which represents the angle from the reference line around the core to the maximum dip vector of the joint, can be measured with a goniometer.

These measurements can then be combined with the bearing and plunge of the drill hole to calculate the actual dip and dip direction of the joint (Savely & Call 1981;

Brennan & Inouye 1988). Individual orientations can be quickly determined using a stereographic net, but if a large number of structures have been measured at a range of depths down the hole it is preferable to use either an Excel spreadsheet or a computer program to convert the goniometer measurements to dip and dip direction by vector mathematics and the drill hole survey data.

Caution must be used when attempting to use the Alpha angle to help orient large-scale structures. Although the Alpha angle can sometimes be used for determining the structure’s orientation, it must be remembered that, because of the larger scale, the measured angle could

Figure 2.23: Example of a highly jointed section of core, relative to the background joint frequency. Note the limonite staining on the joints

Source: Photo courtesy J. Jakubec

Figure 2.22: Example of a broken zone. Angular fragments combine to provide almost 100% recovery

Source: Photo courtesy J. Jakubec

Figure 2.24: Core Profiler™ screen snapshot of the main core image builder interface used to manage the imported photographs and their division into core sticks and sample intervals

Source: After Sliwa et al. (2007)

Figure 2.25: ATV and OTV images Source: Courtesy Wellfield Services Ltda

signal reflected back from the lithological or structural features intersected by the bore hole as photographic-like images. The images can be collected in water or lightly mud-filled intervals of drill holes. Irrespective of the medium, the best images are obtained when there is sufficient drill hole relief or acoustic contrast.

OTV imaging systems use lights to illuminate the drill hole and a reflector housed in a transparent cylindrical window to focus a 360° slice of the drill hole wall in the lens of a charge-coupled device (CCD) camera that measures the intensity of the colour spectrum in red, green and blue. The lithology and structures present in the wall of the drill hole are viewed directly on the OTV images. Optimum viewing conditions are provided by air or clear water-filled drill hole intervals.

In both systems the lithological and structural planar features are classified and fitted interactively with sinusoidal traces and the true orientation calculated using the associated software. The classified features are usually displayed on ‘tadpole’ plots and stereo plots. On the

‘tadpole’ plot the dip is plotted on the x-axis of the plot and the tadpole tail points in the down dip direction. The tadpole plots are depth-dependent; the stereo plots can be reported for selected depth intervals of singular or multiple drill holes (Martel 1999).