Rock mass characterization using geophysical methods

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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.[1][2] 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.[1] 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.[2] 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.[2]

Contents

Borehole Logging

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.[1] 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.[1]

  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
Caliper     p           s    
Magnetics c hp                  
Natural gamma c   p p p p          
Gamma-gamma c   s   p p p p p p  
Neutron gamma c         p p        
Self potential f     p p p         s
Spaced resistivity f     p   p          
Focused resistivity f     p p p s       s
Conductivity c         p s        
Magnetic susceptibility c       s p          
Sonic f       p p p   p p  
Temperature c               s   p
Dipmeter f p     p           p
Flowmeter c                    
Nonlinear resistivity f     p   p          
Video     p     p     p    
Televiewer acoustic f         p     p    
Fluid resistivity f                    

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 [1] 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

Implementation

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Centralization of a probe inside a borehole.[3]
Borehole logging instruments for these techniques vary in type, size, and length depending on the manufacturer and the exact goals of the instrument. Although exact implementation depends on the specific instrument being used and manufacturer guidelines should be followed, the following should be considered when implementing borehole techniques:

Borehole Diameter

Mining typically utilizes “slim-hole” borehole instruments. These range in size from approximately 40 to 60 mm diameter.[4] Some techniques may require larger holes than others.[5]

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.[5]

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.[5]

Centralization

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.[3]

Borehole Deviation

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.[3]

Other Considerations

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.[5]

Borehole Logging for Rock Mass Characterization

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Sensitivity of sonic parameters to fracture density.[2]
Rock mass characterization is of critical importance in mine design. Borehole geophysical methods can be used to safely provide the input necessary to determine rock mass characteristics and complete geomechanical models. The most widely used parameters for rock mass characterization are sonic parameters such as seismic velocity and attenuation. This is because these parameters are sensitive to rock stress, rock strength, fracture density, porosity, and the physical properties of any void filling material. An empirical relationship between seismic velocity and uniaxial compressive strength can be established at individual mine sites.[1][2]

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A multi-frequency full waveform sonic probe with a single transmitter and two receivers. This probe (model 2SAA-1000 from Terraplus geophysical equipment supplier) has a diameter of 39 mm and is 2.66 m long.[6]

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Dynamic elastic constants which have been derived from typical sonic and density logs. Shear, bulk, and Young’s moduli as well as Poisson’s ratio are plotted.[2]
When comparing sonic borehole logging to conventional methods, several advantages emerge. A borehole log provides a continuous record of rock characteristics in situ. This information could then be used to aid core sample selection for lab testing saving both time and money. When properly calibrated, sonic velocity data is capable of providing rock strength information in regions of weak rock unsuitable to lab testing due to loss or fragmentation thus overcoming the problem of bias in core sample selection. If logging multiple drill holes, parameters can be mapped in 3D which can significantly improve mine planning. When dealing with full waveform sonic logs, both shear wave (s-wave) and compression wave (p-wave) velocities can be determined. Using this information as well as density, Poisson’s ratio and the three dynamic elastic moduli can be determined. When the strength of rock is high, the dynamic and static elastic moduli are almost equal allowing these parameters to be used in numerical modeling, for example.[1][2]
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.[1]

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Example acoustic televiewer image showing fracture locations and orientations.[7]


Borehole Geophysical Imaging

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Examples of natural gamma and electrical conductivity responses between two adjacent boreholes.[8]
Borehole geophysical imaging techniques are used to delineate or characterize off-hole features up to hundreds of meters away. Although these techniques are most often used for exploration they are also used for structural mapping and geotechnical assessment. In boreholes these methods can be conducted either in a single hole or from hole to hole.[1] A brief overview of some of the more widely used techniques is presented here.

Single Hole Techniques

Borehole EM

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.[1]

Borehole Magnetics

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.[1]

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.[1]

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.[1]

Seismic

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.[1] 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.

Borehole Radar

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.[1]

Example Probes



Example geophysics probes from Terraplus and their properties.[6] Different manufacturers will offer different models and types.

Probe 2PEA-100
Normal Resistivity
2SP A,B-1000
Temperature/Fluid Resistivity
2PCA-1000
Three-Arm Caliper
2PIA-1000
Electromagnetic Conductivity
2PFA-1000
Temperature/Fluid Resistivity
2DVA-1000
Borehole deviation
2SAA-1000
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
Probe 2PGA-1000
Gamma/SP/Single Point Resistance
2GHA-1000
Triple Gamma
2NUA-1000
Neutron Thermal Neutron
HFP-2293
Heat-Pulse Flow Meter
KLP-2780
4-pi Omni-Directional Density
FAC40
Acoustic Televiewer
FLP-2492
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

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Example of an automatically generated stereonet (top) and manual compass measurements (bottom) in the same area.[9]
3D laser imaging, namely in the form of LiDAR (Light Detection And Ranging), has found growing use above ground in recent years. Its use in mining, however, is only recently being developed with the recognition that it can be used for rock mass classification. Although its use in mining is not yet widespread, LiDAR allows for large amounts of data to be taken at a safe distance from potentially hazardous areas in mines very rapidly. The data can be processed quickly resulting in a detailed and high quality 3D model of mine excavations (drifts, stopes, etc…). This model can then be used to quickly and automatically extract rock mass characteristics such as joint orientations and surface roughness, which can then be employed in rock classification systems.[9]

Joint Orientation

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.[9]

Surface roughness

The 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.[10] The result can then be plotted in color.[11]

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Example measurement of maximum asperity amplitude.[11]
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Joint orientation and surface roughness mapped along strike on a rock face. Joint sets are represented by colour and roughness by intensity.[11]

References

  1. 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. 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. 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
  4. 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. 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. 6.0 6.1 6.2 Terraplus geophysical equipment supplier. http://terraplus.ca/index.aspx
  7. 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.
  8. Hayles Geoscience Surveys Ltd. Borehole Surveys. Retrieved online 19/12/2013. Available at: http://www.haylesgeoscience.ca/Borehole%20Surveys.html
  9. 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.
  10. 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. 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.
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