Rock mass characterization using geophysical methods
Rock mass characterization is a key component of any mining operation in regards to optimizing the mine design as well as guaranteeing safety in the mine. Although geophysics has for a long time played a crucial role in mineral exploration, its use in rock mass characterization is fairly recent and continues to be developed. The push to maximize safety and minimize operating costs drive the use of geophysics in hard rock mines and increasingly geophysical techniques are supplementing or replacing more traditional geological and engineering practices. In contrast, geophysics has long been used in coal mines and oil fields to achieve these same goals. This disparity can be attributed to several technical and logistical difficulties in utilizing techniques and equipment traditionally developed for soft rock mines and oil fields as well as the established ‘culture’ in hard rock mines. Several advances on both these fronts have now been made and the use of geophysics for rock mass characterization in hard rock mines continues to expand.
One of the principal threats to mine performance is the uncertainty related to rock quality. It is not unusual in mining to find very significant sums of money being spent on the basis of very little information and incomplete data sets. This can often cause delays or shortfalls in production which can lead to significant loss of capital due to lost ore or bad ground. The use of geophysics can serve to rapidly expand data sets and provide new information in a cost effective way. Impacts may be direct, such as cost reduction compared to traditional methods, or more indirect such as an improved mine design or timely detection of hazards.
Although new techniques such as laser imaging and microseismic stress inversion are emerging, geophysics carried out inside the mine typically relies on acquisition of data in boreholes. As such, much instrumentation has been developed, often inherited from their usage in soft rock mining, to measure a wide range of geophysical properties such as density, electrical conductivity, magnetic susceptibility, sonic velocity, natural or activated radioactivity, chargeability, and porosity. These techniques can be applied from individual holes or between holes. A summary of the usage of these techniques is presented.
Applications of geophysical borehole logging parameters and their applications. Indicates primary and secondary uses of techniques as well as whether or not fluid should be present and if casings can be used.
|Hole conditions||Dip/strike||Hole diameter/depth||Clay and shale content||Hole to hole correlation||Lithology determination||Porosity/Moisture||Bulk density||Rock fractures||Rock strength||Water flow|
Abbreviations: p, primary; s, secondary; c, casing can be used; f, fluid required in hole; h, hole trajectory only.
Some difficulties in borehole logging in metalliferous mine as identified by  are listed:
- Small hole diameters and inclined holes can make borehole logging technically challenging
- Mineral deposits of often hosted in geologically complex areas where ore boundaries may not coincide with lithological boundaries
- Dewatering programs can compromise some techniques
- Geologists and mine engineers often have limited knowledge and/or experience in borehole geophysics
Presence of a Casing
Some methods require uncased holes while others allow for a casing to be used. The type of casing may matter. The technique of borehole radar, for example, can be used with a PVC casing but not a steel one.
Presence of Fluid
Some techniques such as electric techniques, sonic techniques, and any technique which has as a goal fluid characterisation, require fluid such as ground water to be present in the borehole.
Of particular interest when applying the acoustic televiewer (ATV) technique is centralisation of the probe inside the borehole. When the instrument is not properly centered ATV data will feature characteristic banding as well as traces which will not be sinusoidal and will not match optical televiewer data.
Also of particular note for ATV data is deviation of the borehole from a straight line. Since ATV data must be properly centered in the borehole and since the instrument slides parallel to the borehole, strike and dip data need to be corrected for deviation.
Borehole logging data may need to be calibrated or corrected for not only borehole deviation but also other factors such as temperature, borehole diameter, and fluid resistivity.
Borehole Logging for Rock Mass Characterization
When seeking information about fracture or bedding orientation, the dipmeter tool can be used. This tool works via analysis of high resolution resistivity data recorded simultaneously from several electrodes located around the circumference of the probe. Cross-correlating the resistivity traces defines the dip and azimuth of any feature cutting the hole. Recently however, more sophisticated techniques can be used such as acoustic televiewers, which work by transmitting a pulse in the ultrasound range from a rotating sensor and measuring the travel time and amplitude of the reflected signal, and borehole cameras which allow for very detailed structural analysis.
Borehole Geophysical Imaging A brief overview of some of the more widely used techniques is presented here.
Single Hole Techniques
Borehole electromagnetic (EM) techniques are used to detect electrical conductors such as ore or other conductive features of interest. This technique works by driving an alternating current through the ground, inducing secondary currents in subsurface conductors. The secondary field generated can be detected by receiver coils located in boreholes.
Borehole magnetics is a technique achieved by lowering a three-component magnetometer downhole where it will record the magnetic profile of perturbations in the earth magnetic fields due to both off-hole and intersected magnetic bodies. This can be used to detect ore as well as any magnetic features of interest such as magnetic dykes which can cause issues for ground support.
Ground Penetrating Radar
Ground penetrating radar (GPR), which can be considered an electromagnetic analogue to seismics, can be used to acquire similar resolution data to seismic methods but over smaller range. GPR is typically easier to deploy into a borehole and does not require water to be in the hole. In mining, borehole GPR can be used to map geological contacts, define structure, and to detect cavities making it a very useful technique for mine planning.
Tomographic (Hole to Hole) Techniques
Tomographic imaging relies on the propagation of a signal between two boreholes from a transmitter in the first hole to a receiver in the second. The receiving hole will typically have several receivers at a series of depths while the transmitter will be fixed. This method makes use of some important physical assumptions such as the one that the signal travels in a straight line between the holes. In mining, resolution of such surveys can be limited by factors such as access, background noise, and geology. These techniques are used in mining to delineate geological boundaries and structures as well as to determine in situ rock mass characteristics such as stress. Changing stress conditions in specific locations can be monitored using repeat tomographic surveys.
Seismic tomographic methods can be used to map contacts or structures to a resolution of a few meters over fairly large ranges (hundreds of meters. Most modern hard rock mines now boast an array of both uniaxial and triaxial borehole mounted microseismic sensors which are used to monitor mine-wide seismicity and changing conditions using a variety of parameters, either inferred or measured directly. When an adequate 3D array of triaxial sensors is present, more advanced techniques can be used to determine fault-plane solutions and conduct stress inversions to determine the nature of the stress field in a mine. This information can then be used to supplement numerical modeling and can aid mine planning and ground support implementation.
The borehole radar technique, sometimes called radio, imaging method produces data on intermediate resolution (in the tens of meters range) between more conventional EM methods and GPR. The method relies on the attenuation of the radio signal by conductive zones between holes.
Example geophysics probes from Terraplus and their properties. Different manufacturers will offer different models and types.
Full Waveform Sonic
|Length||111.0 cm||38 cm||159 cm||158 cm||52 cm||119 cm||218 cm|
|Diameter||39 mm||39 mm||39 mm||39 mm||39 mm||39 mm||39 mm|
|Weight||4.3 kg||1.4 kg||7.3 kg||2.7 kg||-||-||-|
|Pressure rating||13,790 kPa||13,790 kPa||13,790 kPa||10,500 kPa||13,790 kPa||20,685 kPa||20,685 kPa|
|Operating temp||0 to 70°C||-20 to 80°C||-20 to 80°C||-20 to 50°C||-20 to 80°C||0 to 70°C||-20 to 70°C|
|Sensor type||stainless steel electrodes with digital measuring circuitry||7-electrode mirrored Wenner array (FR),
fast response semiconductor (T)
|Microprocessor-controlled potentiometer||Magnetic-induced electric field coil @ 25 & 50 cm||7-electrode mirrored Wenner array (FR),
fast response semiconductor (T)
|Microprocessor corrected 3-axis magnetometer/ 3-axis accelerometer||Monopole-dipole configurable wide-band transducer @ 2-30 kHz|
Gamma/SP/Single Point Resistance
Neutron Thermal Neutron
Heat-Pulse Flow Meter
4-pi Omni-Directional Density
Spinner (Impeller Flow Meter)
|Length||79.5 cm||169 cm||116.8 cm||122 cm||142 cm||246 cm||122 cm|
|Diameter||41 mm||39 mm||39 mm||39 mm||39 mm (nominal)||40 mm||39 mm (nominal)|
|Weight||-||5.0 kg||5.0 kg||5.5 kg||7.4 kg||7.4 kg||9 kg|
|Pressure rating||13,790 kPa||20,685 kPa||20,685 kPa||20,685 kPa||20,685 kPa||16,000 kPa||13,790 kPa|
|Operating temp||10 to 70°C||-10 to 70°C||-10 to 70°C||0 to 70°C||0 to 70°C||0 to 70°C||0 to 70°C|
|Sensor type||Nal(tl) 22.2 mm dia. x 76.2 mm long||2 pairs G-M tubes + Nal scintillation detector||Helium -3,4 Atm||Thermisters, 2 cm above/below heat grid||CS 137 source/scintillation detector||1.4 MHz, 3x3mm (beamwidth), user-controlled transducer head; 3-axis magnetometer/ accelerometer||4-pulse per revolution fiber-optic impeller|
3D Laser Imaging
Determination of joint orientation from 3D data can be achieved automatically using a 3D pole density contouring method by using a 3D polygonal model. Such a model meshes the original point cloud data (made up of thousands of (X, Y, Z) points) to a 3D surface made of small triangular elements. Each triangular mesh element is then considered to be a plane where strike and dip can be computed. In order for this method to work the strike and dip of the camera head must be known. The poles of each mesh element are then plotted on a stereonet where joint sets can be identified easily. This method is much more suitable to the mining environment than manual measurement since significantly more joints can be mapped this way. This method also eliminated sampling bias and has the potential to be more accurate than manual compass measurements since surface roughness which is on the compass scale can significantly contribute to measurement spread.
Surface roughnessThe same data can also be used to automatically extract information about surface roughness. In order to do this a rectangular subset, or bin, is selected. The bin size is selected in such a way that it is not too big (otherwise features would be smoothed out), and not too small (error and noise would dominate the roughness measurement). A 2D surface profile is then extracted from the bin and a straight line is generated on the surface between the two local maxima to simulate a straightedge being apposed to the rock surface. The normal distance from the line to the surface is then computed such that the maximum asperity amplitude can be determined. The Joint Roughness Coefficient (JRC) is then automatically read from Barton’s empirical graphs. The result can then be plotted in color.
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 Fallon, G. N., Fullagar, P. K., & Sheard, S. N. (1997). Application of geophysics in metalliferous mines. Australian Journal of Earth Sciences, 44(4), 391-409.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Fullagar, P. K., & Fallon, G. N. (1997). Geophysics in metalliferous mines for ore body delineation and rock mass characterization. Proceedings of Exploration (Vol. 97, pp. 573-584).
- ↑ 3.0 3.1 3.2 Johnson, C. D., Borehole Geophysical Methods. USGS lecture slides. Retrieved online 10/12/2013. Available at: http://www.engr.uconn.edu/~lanbo/G228378Lect0511BH.pdf
- ↑ Milkereit, B. (2012). Borehole Geophysics for Engineers and Geoscientists. Introductory lecture notes for MIN540S, University of Toronto. Retrieved online 10/12/2013. Available at: http://www.physics.utoronto.ca/~exploration/courses/min540s/intro.pdf
- ↑ 5.0 5.1 5.2 5.3 United States Environmental Protection Agency. (1993). Use of Airborne, Surface, and Borehole Geophysical Techniques at Contaminated Sites - A Reference Guide. Available at: http://www.epa.gov/region9/qa/pdfs/geophysics-guide.pdf
- ↑ 6.0 6.1 6.2 Terraplus geophysical equipment supplier. http://terraplus.ca/index.aspx
- ↑ Ku, C. Y., Hsu, S. M., Chiou, L. B., & Lin, G. F. (2009). An empirical model for estimating hydraulic conductivity of highly disturbed clastic sedimentary rocks in Taiwan. Engineering Geology, 109(3), 213-223.
- ↑ Hayles Geoscience Surveys Ltd. Borehole Surveys. Retrieved online 19/12/2013. Available at: http://www.haylesgeoscience.ca/Borehole%20Surveys.html
- ↑ 9.0 9.1 9.2 Mah, J., Samson, C., & McKinnon, S. D. (2011). 3D laser imaging for joint orientation analysis. International Journal of Rock Mechanics and Mining Sciences, 48(6), 932-941.
- ↑ Barton N. (1981) Shear strength investigations for surface mining. Proceedings of the 3rd international conference on stability in surface mining, Vancouver. p.171–92.
- ↑ 11.0 11.1 11.2 Mah, J., Samson, C., McKinnon, S. D., & Thibodeau, D. (2013). 3D laser imaging for surface roughness analysis. International Journal of Rock Mechanics and Mining Sciences, 58, 111-117.