by Kym L Morton, CEO and Consulting Hydrogeologist at KLM Consulting Services Pty Ltd
Valuable groundwater information becomes available as soon as drilling starts, particularly during early mineral exploration campaigns. Often groundwater information is not collected because its value does not become evident until the exploration sites become under scrutiny for mine development.
Drilling and logging of exploration coreholes most often only emphasize the characterization of an orebody, even neglecting to properly log the overburden (future waste-stripping). But capturing basic groundwater information when logging exploration coreholes will significantly reduce the future cost of initial hydrogeological studies and can be valuable at all feasibility and operational stages.
All holes drilled encounter water ranging from damp to undrillable. Drillers record a lot of data on water strikes, water levels and drilling fluid loss circulation records which give important information on zone(s) of high permeability. Useful information that can be collected by the site geologist include depth to water intersections, drilling fluid circulation losses, basic water chemistry and rest water levels. Each drill hole may be used to determine the depth of the water table whether aquifers are confined or not. Thus, some holes can be equipped for use as long-term water level monitoring, water chemistry information and a long-term record of water level movement prior to mining or preserved for use at a later stage. Such holes can reduce the cost of the design of water supply, dewatering and environmental studies.
Often a planned mine area has a very high density of drill holes used for exploration, but if these holes have not been used to measure water levels, additional new holes will have to be drilled at pre-feasibility stage creating additional cost. Figure 1 shows an example of drill hole density for a project in plan.
Best practice is to seal drill holes so that they cannot become conduits for flow into mine workings, but where possible the drill holes must be sealed using cemented-in vibrating wire piezometers (VWP) to provide a 3D monitoring network of the groundwater prior to and during mining. This valuable information can be used in dewatering design, water supply, water balance reporting, environmental impact studies, closure design and post-closure planning.
It is very important to log the depth of weathering as this can be a significant aquifer or water susceptible zone when the mine is designed. Geotechnical engineers are also interested in the depth and types of weathering, which can be recorded on the standard drilling and geological logs.
During core drilling, the driller monitors a minimum of three gauges on a drill rig: water pressure, feed pressure and torque pressure. They are integrated onto the control panel. When the water table is encountered the driller notices that water pressure will start to increase. Also, when lowering the overshot to lift the core tube, there will be slack in the line – also the sign of a water table in front of the core bit. The website drilling.fordia.com/essential-guide-to-drilling-parameters describes all the parameters monitored by a driller and their use in recording the water encountered in the formation.
During drilling by any method, the depth of the first water strike should be recorded by the driller on daily drill logs and the drill sheets. Most drill sheets are primarily focused on details for invoicing the client, however, it can be specified in the drilling contract that all water strikes, water losses and water levels must also be recorded and shared with the site geologist. Water levels should be reported in meters above mean sea level.
Water strikes and the subsequent measurement of water levels can indicate the presence of confined aquifers. When encountered in confined aquifers, the resultant water level is the phreatic or pressure surface for the area, thus the measurements may enable understanding of the groundwater regime and possible impact on the proposed mine.
Air percussion and rotary drilling can be used to measure the yield of a water strike in liters per second (l/sec) or meters cubed per hour (cubic m/hr). Core drilling does not enable the measurement of yield, however, if a permeable zone (possibly an aquifer or water-bearing fracture) is intercepted, the driller will note circulation losses and need to top up the corehole with water and drilling fluids. The more water and fluid used indicates greater permeability and provides information on the groundwater potential for pumping yields. Heinz, in his book Diamond drilling handbook (1989) describes all the types of formations that can be noted from circulation losses including:
- Sands and gravels
- Cavernous or vugular limestone and dolomite
- Naturally fractured/fissured rocks
- Induced fractures in solid rocks
A recommended daily drill log is available below this article.
During core drilling, the level of drilling fluid in the core hole represents the hydraulic pressure being maintained by the driller to keep the hole open, the bit lubricated and to optimize the efficiency of the drilling. If the drilling is stopped for any reason (e.g., a drillers’ break weekend or at the end of the hole), the fluid level will equilibrate to the actual rest groundwater level. Hole depth, water level and date/time should be recorded. As the drill hole deepens, different aquifers with different groundwater pressures may be encountered. The level of fluid in the corehole should therefore be measured after any cessation of drilling over 24 hours. It is possible that artesian conditions, where the fluid flows out of the corehole, may be encountered and these too should be noted.
During drilling, the pH and the electrical conductivity of the water encountered may change indicating interception of different aquifers. Some drilling companies, such as Geomechanics (Pty) Ltd (‘Geomechanics’), offer automated logs of drilling information. Figure 3 is an example of a log created by Geomechanics.
When plotted with the geological log the information can highlight more permeable zones.
Down the hole geophysics
On drill hole completion, exploration coreholes are often surveyed using down-the-hole (DTH) geophysical probes to record information on the ore body. The DTH logging probes can also provide information on the aquifers encountered in the corehole (Bouw and Morton, 1987). At the very least, the probes will record when water is entered and thus provide a record of the water level in the corehole on a specific date. Gamma-gamma and resistivity probes reveal porosity and permeability. Temperature and electrical conductivity probes indicate zones of water flow or different aquifers. Groundwater geophysical logging can be piggybacked on the mineral exploration DTH logging for very little additional cost
Drill hole closure
Following drilling, the hole is often contaminated with mud, grease, and oil. This needs to be cleaned out to unclog the side walls and enable the hole to be used for monitoring either as a temporary (it will collapse if not kept open) open hole or as a permanent water level monitoring hole. A hole can be kept open using plastic conduit or more robust casing. The conduit/casing requires slotting, a cap at the bottom and protection at surface. Figure 4 shows a recommended design:
When water strikes or multiple aquifers have been encountered during drilling then multi-stage open piezometers can be installed. Figure 5 shows a multi open hole piezometer constructed in one hole or using several holes.
If open pit mine or underground mine dewatering is anticipated, then sealed vibrating wire piezometers (VWP) are recommended as these measure groundwater pressure for known depths. A network of VWPs will give the distribution of pressure and plot flowlines in 3D providing valuable information for the interception of mine water and accurate dewatering design. Figure 6 shows the construction of a VWP type point piezometer and data logger sealed into the backfilled core hole using a tremie pipe.
Once exploration is complete, or at least its first phase, the project is evaluated at either pre-feasibility (PF) or feasibility levels to obtain funding for either further exploration to improve confidence or to design and open the mine. The initial monitoring network adds confidence to the studies needed for environmental permitting, water supply and mine dewatering designs. For very little expenditure, the monthly plotting of groundwater level fluctuations significantly increases the understanding of the groundwater regime and provide essential data for use in modelling and calibration of the water flow for the planned mine.
As more disciplines become involved drill holes can provide multiple uses. An example of a drilling sign-off log is available below. The sign-off log is to ensure a new drillhole supports Geology, Geotechnical studies, Mine planning, Engineering and Environmental stakeholders. The form was developed at the De Beers Group for use prior to the drilling of all holes. The document shows the layout and length of the planned hole, its construction and planned use. The design is signed off by all before the hole is drilled.
As the mine project advances and new exploration holes are added, the monitoring network can be expanded, and data collection frequency can be increased. The more hydrological seasons that are covered by monitoring water levels improves confidence in numerical modelling, the initial water control, water supply and dewatering designs.
It is important that the groundwater information is filed in a dedicated data base for use later in the project development. AcQuire and Quick-Log are two of the geological logging programs that can store water
level and water strike information. Other water data packages include water chemistry information.
The data needs to be easily accessible and attached to the dates measured for use in conceptual and numerical modelling.
Exploration drilling programs can be used, at very little extra cost, to collect information on groundwater. Drill holes can be used to create an initial water level monitoring network. The longer the monitoring record, the greater the confidence in the hydrological studies required for permission to mine.
Groundwater data can be obtained at all stages of drilling and provides valuable information for project water supply, mine dewatering design, water balance reporting, mine water management and environmental impact studies.
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Read Issue 19 here:
• The assistance of Geomechanics Drilling, Grant Rijsmus (the Geogroup) is gratefully acknowledged.
• Dr Steve Westhead of AIMC and Dr Matthew Field of Mining Plus are thanked for their comments.
Bouw, P C and MORTON K L, (1987). ‘The importance of geophysical logging of boreholes’. Borehole water Journal Vol 6. South Africa
Heinz WF 1989 Diamond drilling handbook Johannesburg, South Africa ISBN 0-620-13785-1