Rock mechanics and subsurface imaging at DUSEL, Homestake mine
Contents
1. 1 Overview Monitoring the mine for evidence of local and regional seismic events and long term movement associated with dewatering requires an array of instruments with a combined bandwidth span ning 1x10 1x10 Hz Down hole 3D accelerometer packages platform mounted dual axis bubble type tiltmeters and hydrostatic water level systems have been installed on the 2000 ft level of the mine This instrumentation is connected to a data acquisition system that records da ta and allows adjustments to be made both locally and in real time over internet connections 2 2 3D Accelerometer Package The 3D accelerometer package utilizes three orthogonally mounted Wilcoxon Research 731 207 ultra low frequency seismic accelerometers These high sensitivity piezoelectric accelero meters have a frequency response from 0 2 1 300 Hz and very low electronic noise levels Wilcoxon 2001 These accelerometers are wired with high durability coaxial cable mounted orthogonally and placed inside a 13 cm diameter 80 cm long PVC pipe which is filled with epoxy to protect the accelerometers from damage ensure strong connection with the pod and minimize resonant frequencies The 3D Accelerometer packages are connected to the Magliocco Type Data Acquisition System MTDAS by oil patch quality cable Accelerometer packages were installed in 16 cm diameter holes previously used for drainage or transporting sand slurry for back filling stopes During the operation2. and supported version of the Red Hat Linux operating system compatible with the Diamond analog I O board Kontron motherboard and the Labview Runtime Environment These features allow it to run precom piled Labview programs and enables remote administration networking and customization To capture data from the National Instruments board a basic Labview program was written to read data from the accelerometer channels into a buffer that is monitored for a trigger condition When the trigger condition is met the buffer and subsequent data are recorded The Labview ap plication includes a graphical user interface as well as an initialization file to control the data acquisition parameters and trigger conditions The software used to configure and record the tilt PAPER 4155 4 ROCKENGOS9 Proceedings of the 3rd CANUS Rock Mechanics Symposium Toronto May 2009 Ed M Diederichs and G Grasselli meter readings was created by using the C programming language This software permits the ad justment of sampling interval gain settings and averaging In order to ensure bi directional connectivity through firewalls a virtual private network was employed OpenVPN is a free and open source virtual private network VPN program for creating point to point encrypted tun nels between host computers and is capable of working through proxy servers and firewalls Computers running an OpenVPN client are able to log on to the OpenVPN server in order to join a virt
3. concrete pedestals reinforced by three 1 9 cm diameter rebar rods embedded 30 cm into the intact rock The Fermilab tiltmeters originally designed to monitor the Tevatron particle accelerator at Fermilab determine relative height changes over a distance by measuring water levels in half filled vessels in hydraulic communication via two polyethelene tubes Volk and others 2006 Installed on the 2000 ft level of the mine are two arrays each comprised of two legs Every leg is 120 meters in length and defined by three vessels with a 60 m separation Changes in water level amongst the three vessels can be used to determine the orientation of a vector representative of that leg The two vectors of an array can be used to calculate time dependent orientation changes of a plane representative of the 2000 level rock mass 2 4 Magliocco Type I Data Acquisition System MTDAS The MTDAS was designed to be a quickly deployable flexible and robust unit capable of sur viving the ambient operating conditions within the mine which are characterized by high hu midity and temperatures of up to 35 C The data acquisition system hardware is composed of a PC104 Plus form factor single board computer mated with two analog to digital data acquisition boards and a power supply all housed within a sealed aluminum box The key aspects of the PC 104 Plus standard are the 9 cm x 9 6 cm motherboard footprint and a stackable PCI bus in which extra components can be si
4. of the mine sand holes were drilled from the surface at the slurry pump house to the 1100 level and within the mine be tween levels Sand holes within the mine are typically open therefore installation was accom plished in a four step process packer installation initial grouting package deployment and fi nal grouting PAPER 4155 3 ROCKENGOS9 Proceedings of the 3rd CANUS Rock Mechanics Symposium Toronto May 2009 Ed M Diederichs and G Grasselli 2 3 Tiltmeters Two different types of tiltmeters are presently installed in the Homestake underground includ ing Applied Geomechanics 711 2A Platform Tiltmeters and Fermilab Hydrostatic Water Level Sensors HLS The Applied Geomechanics platform tiltmeters are capable of detecting rota tions as small as 0 5 nano radians d Oreye 1998 with two orthogonal electrolytic level sen sors contained within a rugged hermetically sealed enclosure mounted to a 15 cm by 15 cm anodized aluminum base Applied Geomechanics 2005 Frequencies from DC to 2 5 Hz can be detected with this unit capable of monitoring low frequency components of seismic events earth tides seasonal trends and stress realignment associated with mine dewatering The platform tiltmeter is connected to the MTDAS via a 3 m cable through which power is received and data is sent Currently this type of tiltmeter is installed at the 2000 ft level in the underground See Fig 2 The instruments are mounted on a 30 cm diameter
5. ROCKENGOS9 Proceedings of the 3rd CANUS Rock Mechanics Symposium Toronto May 2009 Ed M Diederichs and G Grasselli Rock mechanics and subsurface imaging at DUSEL Homestake mine J K Van Beek and W M Roggenthen South Dakota School of Mines and Technology Rapid City USA M Magliocco and S D Glaser University of California Berkeley USA ABSTRACT The Deep Underground Science and Engineering Laboratory DUSEL 1s a scien tific facility that will be placed in the deeper reaches of the Homestake mine Lead SD Our team of geologists and engineers is carrying out the first scientific project awarded to the site Towards a Transparent Earth Taking advantage of the thousands of open exploratory and sand line holes we are installing the beginnings of a seismic observatory Each installed station con sists of a 3 D accelerometer package and a nanoradian 2 D tilt meter In order to allow for a variety of monitoring and characterization studies the array instruments are sensitive over the frequency bandwidth of 0 2 1 KHz Three three component accelero meter stations are currently installed in a 140 m deep bore beneath the slurry pump house and in 12 m deep bores on the 2000 foot level 800 and 400 m from the Ross shaft There is a tilt meter installed at each 2000 level station These installations are in 150 mm diameter sand holes From the currently installed tilt sensors we are investigating solid earth deformation as a r
6. asselli the physics experiments Results of data frequency analysis relate primary signal constituents in the semi diurnal range to some values detailed by Wahr 1995 Though longer period tidal con stituents are not currently evident in the data longer monitoring time will likely result in a high er signal to noise ratio in the extremely low frequency range Considerable progress has been made already in transforming the former mine into a labora tory that is conducive to scientific research As progress continues we look forward to further installations in the mine to benefit of future researchers in many fields in addition to those di rectly involved with this project The installation of additional 3D accelerometer pods and tilt meters will greatly increase the ability to interpret rock mass reactions to events of all types Following the completion of the fiber optic network we will be able to make data freely avail able instantaneously to interested parties via the internet and increase the precision and accura cy of the timing at all sites This will enable us to correlate seismic monitoring stations in the mine with local seismic monitoring sites increasing the ability to locate local regional and global events 4 1 Acknowledgments This project would not be possible without the cooperation of the South Dakota Science and Technology Authority Barrick Gold Corporation Fermi National Laboratory and the many subcontractors who ha
7. e sponse to tidal potential The accelerometers are being used to locate and characterize events as sociated with mine dewatering and rehabilitation 1 INTRODUCTION 1 1 Location and History Operating from 1876 to 2001 in the northern Black Hills of South Dakota the Homestake gold mine Fig 1 extracted about 42 000 000 oz of gold from depths reaching 2 4 km In its 125 year history 560 km of drift and several hundred stopes were opened The mine ceased opera tions in 2001 after several years of low gold prices The mine has a long history of close associ ation with particle physics experiments most notably the work of Ray Davis who established a Nobel Prize winning neutrino detection experiment in the mine in 1965 Closure of the mine provided the opportunity to use the mine as a laboratory and has led to the conversion of the mine into a dedicated underground science and engineering laboratory The Sanford lab at Ho mestake currently provides the infrastructure for scientific experiments in the fields of astro physics geology geophysics microbiology and hydrology PAPER 4155 1 ROCKENGOS9 Proceedings of the 3rd CANUS Rock Mechanics Symposium Toronto May 2009 Ed M Diederichs and G Grasselli Sanford Laboratory at Homestake Tertiary aged intrusive igneous rocks Precambrian A if and igneous rocks j Paleozoic sedimentary rocks f Figure 1 Location of the Sanford Lab at Homestake within the Blac
8. k Hills of South Dakota and Wyoming USA 1 2 Geology and Production The Homestake mine is located in the northern Black Hills a Tertiary aged dome uplifted dur ing the Laramide orogeny Redden and others 1990 The primary stratigraphic units found in the mine are from oldest to youngest the Poorman Homestake and Ellison Formations The lower Poorman Formation is hornblende plagioclase schist whereas the upper Poorman is typi fied by well layered sericite carbonate quartz phyllite Conformably overlying the Poorman is the Homestake iron formation dominantly chlorite chert siderite phyllite and grunerite biotite chert schist Caddey and others 1991 The Ellison Formation is interbedded impure quartzite and sericite biotite quartz and has been dated at 1 97Ga Two major deformational periods in the Early Proterozoic formed a steeply dipping sheared set of anticlines and synclines Amphi bolite greenschist facies prograde metamorphism is associated with the first deformational se quence Caddey and others 1991 Ore was removed primarily from nine separate steeply plunging synclinal fold structures known as ore ledges Ore trends within these structures were composed of many sulfide rich elongate cylindrical ore bodies ranging from 2000 to more than 1 000 000 tons Gold is fine grained commonly associated with arsenopyrite and averaged 8 4 g Au t with a Au Ag ratio of 10 1 Although underground mining methods began with shrinkage st
9. mply stacked on the motherboard to add functionality These computers and components feature rugged construction and design intended for embedded and industrial applications The main computer board utilized in the first installation is a Kontron MOPSPM104 which features an Intel Celeron M 600MHz CPU and is outfitted with 1GB of SDRAM and an 8GB flash based hard drive Subsequent installations use the Kontron MOPSIcdLX mother board and AMD Geode LX800 CPU to reduce operating temperatures within the enclosure Tilt meter measurements are collected via a Diamond MM 16 AT 16 bit analog I O PC 104 module manufactured by Diamond Systems Corporation which features autocalibration and 16 analog inputs The accelerometer measurements are digitized by a National Instruments PCI 4462 high performance high accuracy analog signal acquisition board providing 204 8 kS s anti alias filtering and IEPE current excitation capability that is required for the piezoelectric accelerometers To use this board with the PC 104 Plus a PCI adapter board was included as well as a power supply to provide the PCI bus power The MTDAS enclosure is a sealed ex truded aluminum box with internal and external heat sinks and an internal fan to aid in heat dis sipation In addition to excellent heat management the aluminum enclosure is sturdy enough to meet the kick proof requirements of the mine environment The system operates on Fedora Core 7 Linux a community produced
10. n in April 2008 An example of a one minute time window is shown in the upper half of Fig 3 Figure 3 Seismic data from the tri axial seismometer on the 2000 foot level 610 m below sur face Homestake Sanford laboratory recorded on 12 December 2008 at 2 52 pm local 18 52 UTC a Plot showing numerous events of local origin over a 60 sec record b Event with radi al distance of approximately 1200 m y 0 is approximately vertical PAPER 4155 5 ROCKENGOS9 Proceedings of the 3rd CANUS Rock Mechanics Symposium Toronto May 2009 Ed M Diederichs and G Grasselli Although more than twenty five events are obvious in this plot many more events can be seen in the raw data at magnitudes slightly greater than the noise level The bottom portion of Figure 3 shows an expanded scale for one of these events Both the S mode and its associated P mode can be identified in this plot Based upon the time difference the estimated distance to the source is 1200 m A number of events have a similar estimated offset and is the approximate distance to the working shaft 3 2 Tilt Initial evaluation in the frequency content of data recorded from the installed Applied Geome chanics tiltmeters display high signal to noise ratios in the semi diurnal band Fig 4 with de creasing resolution in the lower frequencies due to discontinuous and short recording intervals maximum 48 days continuous data acquisition instrument drift and thermal variatio
11. ns at the recording site Several well defined maxima can be identified which can be correlated after fil tering to remove high frequency noise and low frequency effects due to mechanical drift and possibly the effects of the ongoing dewatering of the underground 4200 T T T T T T X 2 236e 005 Y 1122 1000 4 800 4 X 1158 005 Y 878 7 Magnitude 8 T 1 X 2 193e 005 Y 480 2 n X 2 317e 005 goo Y 378 2 m 200 4 o i i 12 14 16 18 2 22 24 26 28 3 Frequency Hz x10 Figure 4 Principal tidal constituents in the semi diurnal and diurnal bands from windowed FFT of de trended data collected from one axis of the resistive type Applied Geomechanics bubble tiltmeter 4 DISCUSSION This project marks the very beginning of research at the Homestake Sanford lab Three element of a much larger seismic observatory surrounding the mine have been installed The two sta tions on the 2000 ft level are working well and the in house produced data and control system is performing as expected and the accelerometer pods are sensitive to tens of small events per minute Future seismic installations will concentrate on deeper levels of the laboratory and will assist in monitoring the deformation effects of construction of the large cavities associated with PAPER 4155 6 ROCKENGOS9 Proceedings of the 3rd CANUS Rock Mechanics Symposium Toronto May 2009 Ed M Diederichs and G Gr
12. oping timber stoping and drawhole mining they were replaced by mechanized cut and fill and vertical crater retreat methods in the late 1930 s Caddey and others 1991 A schematic representation of the mine cross section is shown in Figure 2 The conversion of the mine to a laboratory will involve the construction of very large cavities in some cases re quiring spans of 20 m at 4850 ft and 7400 ft below the surface In addition extremely large cav ities with spans of 50 m and heights of 50 m are also being contemplated for construction at the PAPER 4155 2 ROCKENGOS9 Proceedings of the 3rd CANUS Rock Mechanics Symposium Toronto May 2009 Ed M Diederichs and G Grasselli 4850 ft level These cavities will provide the location for large arrays of photomultiplier tubes used to detect the interaction of neutrinos with the liquids filling the cavities In addition other physics biology and geosciences experiments will make use of the smaller modules with 20 m spans at the 4850 ft and 7400 ft levels An array of seismic monitoring installations will be em placed to monitor these excavations and the stress response of the surrounding rock mass NE Yates Ross Sw Shaft Shaft pm FN Shaft IL B exheust 2000 f F 4Winze GS r iN ssor ST n Ky Not te Scale Figure 2 Schematic cross section of Homestake Sanford Laboratory showing major levels 2 EQUIPMENT 2
13. ry interpretation of Precambrian events in the Black Hills South Dakota In Lewry J F and Stauffer M R eds The Early Proterozoic Trans Hudson Orogen of North Amer ica Geol Assoc Canada Spec Paper 37 229 251 Rogers H 1990 Geology of Precambrian rocks in the Poorman anticlinorium and Homestake mine Black Hills South Dakota In C J Paterson and A L Lisenbee eds Metallogeny of gold in the Black Hills South Dakota Society of Economic Geologists Guidebook Series v 7 103 111 Volk J T Guerra J A Hansen S U Kiper T E Jostlein H Shilsev V Chupyra A Kon daurov M and Singatulin S 2006 Hydro static water level systems at Fermilab 9 inter national Workshop on Accelerator Alignment September 26 29 Wahr J 1995 Earth tides Global Earth physics AGU Reference Shelf 1 40 45 Wilcoxon Research 2001 Model 731 207 Ultra low frequency seismic accelerometer Users Manual PAPER 4155 7
14. ual network that runs on top of the underlying network connections OpenVPN uses the SSL and TLS security protocols to provide encryption of both the data and control channels and can be authenticated using a shared key system Upon receiving an IP address on the virtual network the system was configured to email the administrator the assigned IP number Remote administration is simple under Linux and several options are available Remote logon through a text based console is easy and secure and can be used for almost any administration task It is also possible to run a graphical application remotely with the interface appearing on the local machine using an X Windows server Finally it is possible to share the entire desktop over the network using standard free software such as Xvnc In order to ensure that the operat ing system and networking continued to run correctly the watchdog timer was used in conjunc tion with the cron scheduling system and simple shell scripts In this way operating system and hardware crashes are resolved with a system reboot The cron system also monitors networking functions with a simple shell script automated to check periodically for network and VPN con nectivity If network connectivity is compromised the script will attempt to restart the network services and will reboot the computer if the problem persists 3 RESULTS 3 1 Seismic The 3D accelerometer packages have recorded several hundred events since installatio
15. ve been accommodating throughout the project G Lillihaug B Pietzeyk T Trancynger Sanford Laboratory and J Volk Fermi National Laboratory were especially helpful Support for this project was provided by NSF grant CMMI 0727726 5 REFERENCES Applied Geomechanics 2005 700 Series platform and surface mount tiltmeters User s Manual Bachman R L and Caddey S W 1990 The Homestake mine Lead South Dakota an over view In C J Paterson and A L Lisenbee eds Metallogeny of gold in the Black Hills South Dakota Society of Economic Geologists Guidebook Series v 7 89 94 Caddey S W Bachman R L Campbell T J Reid R R and Otto R P 1991 The Homes take gold mine an early Proterozoic iron formation hosted gold deposit Lawrence County South Dakota U S Geological Survey Bulletin 1857 J Geology and Resources of Gold in the United States D Oreye N 1998 Qualification test of a dual axis bubble type resistive tiltmeter AGI 700 se ries earth tides recorded and analysed in the underground laboratory of Walferdange Ma rees TerrestresBulletin d Information 129 9953 9961 Meier L F 1990 Structure and ore trend description of the Homestake mine In C J Paterson and A L Lisenbee eds Metallogeny of gold in the Black Hills South Dakota Society of Economic Geologists Guidebook Series v 7 95 102 Redden J A Peterman Z E Zartman R E DeWitt E R 1990 U Th Pb Geochronology and prelimina
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