GTS Technologies -- Geophysical

GTS was asked to help identify the sources of groundwater inflow into a limestone quarry that had been excavated below the level of a nearby stream. The quarry operators were opening a deeper lift that intersected with open fractures and voids probably due to karst weathering. These deep openings led back to the nearby stream resulting in a blowout causing a huge quantity of groundwater and stream water to enter the quarry. Available pumps were just keeping up with the inflow, but any worsening of the situation would flood the quarry.

Previous work had identified several preferred flow zones from the stream to the quarry. A grout curtain was in the process of being installed between the quarry and stream, which had decreased the flow, but not substantially. GTS was asked to perform a spontaneous potential (SP) geophysical survey to help identify the locations where the grout curtain required additional work to prevent this inflow. A spontaneous potential survey is a simple, old, and excellent tool for locating flowing water in fractures, joints and open voids, and because of the depth of the inflow (>200 feet), other geophysical methods would be stretched to or beyond their effective limits.

GTS ran the survey as a profile of SP measurements between the grout curtain and the stream. These measurements detect very small changes in natural earth potentials flowing through the shallow subsurface of the earth. Flowing water concentrates these potentials creating slightly higher measurements that can be measured with a very sensitive millivolt meter using to widely-spaced electrodes.

The result of the SP survey found that the grout curtain was continuing to leak in a particularly rich area of fracture and void development. This indicated that additional drilling and grouting using specialized grout mixes would be required to seal the remaining problem areas. The SP measurements provided the precise locations of these problem areas for setting up the drill rig and grout operation. The quarry was allowed to flood to stop the flow of groundwater into the pit and to prevent the grout mix from being washed away before it could harden. After the grout current was improved, the pumps easily removed the water from the quarry.

Additional SP surveys are anticipated to monitor the grout curtain and to locate any other areas that may be developing a blow out.

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