Targeted Drilling with Geophysical Characterization

Article by: HGI staff I HydroGEOPHYSICS

Conducting a geophysical survey before, during, or after drilling can offer substantial advantages and reduce uncertainty for a drilling characterization program.?Geophysical characterization?techniques provide a site-wide context to help project managers meet their drilling goals cost-effectively. Such methods help visualize the earth’s subsurface characteristics to target sites by mapping the differences between rock types and physical discontinuities.

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Using geophysical methods to determine targets for drilling provides significant investment returns by identifying the optimum areas for borehole placement.

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Before drilling, geophysics can help predefine the area to pinpoint the best location for hole placement. This may be especially important if targeting small or vertical features such as a fault or dike.??Electrical resistivity?or?seismic methods?are best suited to define the structural geology to depths of at least 100 to 200 meters. On a broader scale, geophysical surveying using the?gravity method?can map the?depth to bedrock?in large basins ensuring water wells can be placed in deep alluvial aquifers. With gravity, we can explore depths exceeding 2 kilometers. When geophysics is appropriately applied, significant savings in cost and time are realized, ensuring optimum borehole or well placement.

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Geophysics can also help extend the information from existing boreholes. Downhole drilling data can provide detailed information about a localized region, but confidence in data projected beyond the confines of the well falls off rapidly away from the borehole. For example, does the borehole exemplify the formation, or is it an anomaly in the formation? Drilling more holes can answer the question but is an expensive proposition fraught with uncertainty as project managers attempt to understand where to drill and what the drilling will tell them. Tying down-hole drilling data to geophysical information such as seismic, electrical resistivity,?induced polarization, or other methods enable project managers to extrapolate subsurface drilling information over larger areas, providing context towards positive project outcomes.

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Geophysical surveys help managers prioritize borehole placement strengthening subsurface drilling characterization at a fraction of the cost of a more excessive drilling program.

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Project example

A mine was looking to expand its tailings facility based on a previous drilling and geologic assessment. However, when the second round of targeted validation drilling began, weak layers were discovered deep in the subsurface that the original geological assessment did not identify. The newly discovered weak layers had severe ramifications for the stability of the expansion.

Identifying variations in soil stiffness

An?MASW seismic survey?was performed throughout the expansion area to map the weak areas of the subsurface. Weak areas were anticipated to be of low seismic velocity due to a lack of cohesion and compaction. Following the geophysical survey, project managers reconfigured the drilling campaign, targeting the newly discovered low-velocity zones verifying the subsurface areas to be geotechnically weak.

The client remarked:?“It was almost like cheating,”?describing the geophysics’ accuracy and ease data acquisition relative to the drilling results. With these findings, the client concluded that the geophysics program saved them close to a million dollars on drilling and helped them design a stable structure to mitigate geotechnical challenges associated with expanding the tailings facility.

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Whether your project is a?geotechnical investigation,?enhanced recovery for heap leach mining, targeting?groundwater?wells, or?delineating voids, combining drilling and geophysical datasets is an extraordinarily valuable tool. Geophysical surveys help managers prioritize borehole placement strengthening subsurface drilling characterization at a fraction of the cost of a more excessive drilling program.

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