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Increases in ground falls over the last few years have prompted the Saskatchewan potash industry into researching new ways to increase mine safety and awareness. Ground conditions in a potash mine are very dynamic and the back stability changes continually due to the pseudo elastic nature of the ore. Also, due to the immense size of the potash mines, any practical ground analysis system needs to be versatile as well as portable. Ground Penetrating Radar (GPR) is the technology having the highest potential for success as a detection system. Early GPR test programs in potash suggested that GPR could produce images of the back conditions, potentially highlighting propagating cracks and clay seams. Follow up testing was slow but further illustrated the technologies positive detection potential. Frequency and operations testing proved that a 200 MHz antenna mounted on an underground vehicle travelling over 10 kph, achieved penetration and interpretation depths of over 10 metres. The proven GPR system technology can now be extended and adapted onto mobile equipment to act as an early warning system for immediately deteriorating back conditions.
CRACK RADAR 10
The Mosaic Potash Colonsay (Colonsay) and Mosaic Potash Esterhazy (Esterhazy) minesites are located approximately 76 and 450 kilometres east of Saskatoon (Saskatchewan), respectively. Production started at the Esterhazy operations in 1962 and Colonsay in 1969. Both operations use continuous mining machines to extract the ore at average extraction ratios ranging from 35% at Colonsay to 50% at Esterhazy. Ground control and stabilization in a potash mine, or in any mine, is crucial to the safety of the mine, but more importantly, critical to the safety of all its employees. Each of the Saskatchewan companies have been involved in their own internal research into ground condition assessment, however, in 2002, the Saskatchewan Potash Producers Association (SPPA) formally embarked on a collaborative initiative to improve the diagnostics involved in ascertaining the stability of the mine opening, and in particular the roof (back) by establishing a Loose Detection Committee. Mine operators have historically relied on their vision or hearing to determine the quality of the ground in the back. Visible cracks can be seen, and audible hollow sounds can be heard when striking the back with a scaling bar. These methods are subjective, and are certainly reactive. A more proactive approach was sought, and a means to determine the quality of the back was required that was simple, accurate, and fast. A literature search of potential geophysical tools was conducted, and the use of a Ground Penetrating Radar (GPR) system emerged as the system with the highest probability of success. Previous work by Annan et al, and each of the Saskatchewan potash companies further substantiated the assessment. Single and Multi-frequency radar tests were conducted with Mosaic Potash (formerly IMC Potash) taking the lead in the Single frequency approach. Mosaic Potash contracted Golder Associates to conduct several tests with numerous single frequency radar systems including Sensors & Software and GSSI. The IDS system from Italy was also tested. All the systems produced better than expected radar results in the potash and salt beds; however, for the purposes of Mosaic Potash, the IDS system proved most versatile as it was easily adaptable for fast acquisition using an underground vehicle. Between the Esterhazy and the Colonsay underground operations are over 1600 km of open travelways, and over 20 active working headings that require monitoring. Additionally, various antenna frequencies were also testing for diagnostic analysis of the Esterhazy Water Inflow area to determine if zones of weakness, loss of potash continuity, or water flow paths could be detected deeper in the back.
Two types of separation that are commonly observed in a potash mine are shown in Figures 2 and 3. Development of the first type (stress arch) will first appear as shear cracks along the far edge of the roof. Visual observations by mine personnel will often detect these cracks early enough to design and implement a ground support program to prevent a potential ground fall. The second type of separation occurs along geological contacts such as clay seams and bedding planes.
Radar experiments in Saskatchewan potash dating back to 1988 showed that radar is an effective method for probing salt/potash environments (Annan et al). However, at that time, a lack of availability of a comprehensive system prohibited its use in this mining environment. However, as shown below (Figure 4), the results of the radar were encouraging.
Not having to have the antenna in contact with the back provided the possibility to complete mine wide surveys in a reasonable amount of time. Further frequency testing indicated that a frequency of around 900 MHz was good for detection of cracks and clay seams (Figure 7). Ultimately, a 950 MHz antenna was developed for and purchased for both the Esterhazy and Colonsay operations.
In the Saskatoon area, the mines are producing ore from the Patience Lake potash member which contains numerous clay seams. From a ground control standpoint, the clay seam approximately 1 metre above the back is the most important. The beam between the back and the clay seam is usually competent if it is at least 0.6 metres thick. Less and the back will tend to loosen and could fall. By mapping this distance, the mine operators can make an assessment on the potential future back conditions. Many cracks also occur along the clay seam. When employing the GPR, distinguishing between the intact clay seam, and a separation or crack in the clay seam is essential in ground assessment. Figure 8 is an example of what the GPR produces from a clay seam and crack. Attenuation of signal was encountered with the first clay seam (1 metre deep), which resulted in poorer imaging of clay seams deeper into the back. However, this first seam is the most important for immediate back condition assessment.
With the success of back condition assessment using the GPR in the production areas, additional frequency testing was undertaken to see if GPR could effectively be used in the Esterhazy water inflow area to assess conditions deeper into the back. Subsequent testing indicated that by using a 200 MHz antenna, features as deep as 10 + metres could successfully be delineated. Figure 9 shows some significant features, lower cracks, and the White Bear marker (10 metre). Figure 10 illustrates potential water filled cracks or zone 7 and 3 metres into the back.
With the amalgamation of a modified underground vehicle (Toyota Landcruiser) and the ground penetrating radar system, the Esterhazy mine developed a mobile radar unit that makes wide scale data acquisition relatively easy and possible for one operator (Figure 11).
The vehicle is fashioned with a radar antenna platform connected to a single stage hydraulic lift that can be controlled by the operator and used to adjust the antenna to address variations in back height (Figure 14). Though presently operated manually, the antenna adjustment process has the capability of becoming automated. On board meters provide a distance traveled as well as a height measurement from the antenna platform to the back. With the turn of a key on the control box, the computer, onboard meters, and hydraulic system are started. The radar vehicle utilizes an isolated two battery system, with each battery handling a portion of the electrical load. One battery powers the truck and hydraulic pump while the other services the needs of the computer, monitor, and meters. Both are simultaneously charged by a single brawny alternator while the vehicle runs. (Figure 14)
For the purpose of mine workings assessment, the 950 MHz, monostatic antenna is being used. The frequency of this antenna was determined to provide the best resolution and ability for crack detection while reaching a penetrating depth of over 3 metres in Esterhazy. A 200 MHz antenna is also available for use and can achieve penetration depths of over 10 metres, albeit, due to the lower frequency, has lower detail or resolution.
Upon activation of the GrasWin software, the user can access menus which allow for the control of various parameters such as radar configuration and display settings. As part the pre-survey routine, an antenna signal test and calculation of an automatic gain factor are performed. Once completed, the operator has the opportunity to enter a custom, user-defined gain curve if so desired. Data collection is continuous and begins with movement of the vehicle once the start of the survey is initiated. Collection pauses with a stop in vehicle motion and ends with the termination of the survey. Acquisition speeds in excess of 10 KPH are possible without any significant loss of data quality. Survey distance is measured by both the computer itself (via wheel mounted pulse transducer) as well as the onboard meter for comparison. The radar section produced is displayed on the monitor located to the right of the operator.
Although loose detection remains as the primary focus for ground penetrating radar at all the Mosaic Potash operations, additional applications for future use are being considered. One such application is the measurement of salt back thickness. Using a low frequency antenna, GPR may be used to determine the thickness of the evaporite sequences that exist above the present mine workings. This data is useful as an additional tool in assessing conditions that might pose concern at a later date or at least a need for monitoring (i.e. an unusually thin salt back).
From a production standpoint, the opportunity to use radar for locating marker beds is a promising possibility. At the Esterhazy mine, a rather competent seam of anhydrite can be consistently found at approximately 6 metres below the mine workings. In a scenario where abnormal geology is encountered, this procedure could be used to help locate the anhydrite layer therefore providing a guide to help navigate miners through the area. 2ff7e9595c
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