A local property developer in Harrisburg requested that GTS evaluate his five-acre property to determine the depth to bedrock and locate any sinkhole or other structural features that would impact his plans to develop the property in a high-tech business park. The surrounding area has a long history of sinkhole activity that has created headaches for many developers and property owners. GTS recommended a two step geophysical survey using terrain conductivity to rapidly determine the distribution of shallow and deep bedrock and any potentially suspicious karst areas. Then, several continuous electrical resistivity profiles using an AGI SuperSting resistivity array were located to provide detailed subsurface information where indicated by the terrain conductivity data.

GTS has found such two-step geophysical investigations to be very effective in evaluating properties from 2 to greater than 350 acres. Larger properties cannot be surveyed using slow, higher resolution geophysical methods like electrical resistivity except at great cost. We therefore use lower resolution, but very fast, geophysical applications like terrain conductivity coupled with high spatial resolution GPS satellite instruments to allow our field geophysicists to quickly survey a site. This usually enables us to target boreholes or higher resolution geophysical methods over a few locations where some concern is indicated. This may be shallow bedrock, fault zones, sinkholes or soft, low-bearing capacity soils, for example. The result is a vastly higher density of subsurface information that what would be obtained from boreholes or test pits alone.




For this particular property the terrain conductivity information showed that the site had shallow bedrock in the lower topographic areas with three sets of pinnacles crossing the higher topographic areas. Several high conductivity anomalies suggested areas of deep weathering that were probably due to sinkhole development. From this information GTS did four SuperSting electrical resistivity surveys over these anomalies to determine their cause and structure.

GTS found that the lower, nearly level parts of the property were underlain by shallow bedrock while the upper slopes were underlain by large bedrock pinnacles separated by areas of deep weathering and probable sinkholes (though not apparently active). A fault contact was also delineated between the limestone and a sandy siltstone near the top of the hill. Based on nearby properties, which had a great thickness of soil, the developer wanted to level much of the property and offer the excavated soil to other building sites in need of clean fill. However, the geophysics indicated that little overburden soil was available for this purpose. A single confirmatory borehole also found that the bedrock was approximately six feet below the surface. This meant that the site could not be easily used for any purpose that required significant excavation, because of the cost of removing the bedrock. The developer is currently reviewing building alternatives that can be profitably built on this site.



METHODS
Electrical Resistivity
Terrain Conductivity
Very Low Frequency
Spontaneous Potential
Seismic Refraction
Ground Penetrating Radar
Magnetometry
Vibration Monitoring
Soil Resistance Testing


CASE HISTORIES
Archaeology
Bedrock Delineation
Ground Penetrating Radar
Groundwater Availablity
Mining 1
Mining 2
Sinkhole Investigations
Sinkhole Investigations 2
Soil Resistance Testing
Underground Storage Tanks
Vibration Monitoring




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