Big-box stores, warehouses, housing developments, and many other facilities seek out large undeveloped land tracts to build upon. However, if limestone bedrock lies underneath, the risk of sinkholes increases as they develop the land. Detecting sinkhole-prone areas over such vast expanses can be daunting. In such cases, RETTEW uses a phased approach to detect sinkholes. First, we conduct a reconnaissance-level electromagnetic or resistivity survey to find areas that might be taking on water preferentially (a sinkhole driver). Once we have identified those areas, we can conduct higher-resolution surveys in smaller places, identified as safer areas, to provide more detailed sinkhole mapping.
Where, Oh Where, Are the Sinkholes?
Before developing an approximately 100-acre farm property, one of our clients first needed to know if any areas could be sinkhole-prone (Figure 1). To tackle the problem cost-effectively, RETTEW completed a phased-approach karst survey.
First, we conducted a site-wide electromagnetic (EM) survey using an ATV for dense, rapid data collection to detect areas of higher conductivity, which can result from either increased moisture (an essential ingredient for sinkhole development) or buried metals (Figure 2 – the areas of dark red/white). We were able to see what portion of the conductive areas might be due to metal (Figure 3 – no dark red/blue, so minimal metallic influence) and concentrate our next phase on the area of concern in the northern portion of the site (Figure 2 – the area of dark red/white).
Across this identified anomalous zone, we completed electrical resistivity profiling. We developed multiple cross sections showing where fracture zones could focus on sinkhole activity (Figure 4 – areas of dark blue).
Finally, we conducted a few seismic refraction profiles to map rock depths in key areas (Figure 5) where the EM survey indicated shallow rock (Figure 2 – low-conductivity areas of blue). Correlating seismic depths to low-conductivity EM values helped calibrate what rock depths they could expect over much larger site areas, which enabled the client to proceed confidently with the project!
Meet Matt Bruckner
Environmental & Geotechnology Regional Director
Meet Matt Bruckner, our Environmental & Geotechnology Regional Director, whose passion for collaboration and learning from diverse disciplines drives his success in delivering comprehensive solutions for clients. Go “beneath the surface” and get to know Matt!