Case Study: Delta Geophysics Conducts a Karst Study for Site Design
Customer:
Potential Purchaser

Size:
2.5-acre Commercial Building Lot

Purpose:
Foundation and Storm Water Design/Retention Basin Placement

The Method:
Resistivity & Seismic Profiling

The Scope:
Bedrock/Sinkhole Characterization
Developer benefits from Delta Geophysics’ ability to adapt quickly to field conditions.

In June 2010 Delta Geophysics was requested to complete a karst study of a lot proposed for commercial development. Prior test borings met shallow refusal or were limited to a depth of 15 feet below ground surface, and did not provide adequate information for evaluating potential karst features. Both outcrops and active sinkholes were found at the Site.

The Approach

Delta Geophysics’ team opted to conduct the survey using resistivity profiling rather than seismic methods.

Seismic refraction was selected for the survey because velocity data that has geotechnical meaning would be a by-product of the bedrock/sinkhole survey; additionally seismic was the most cost-effective method technique for this project. Because of suspected bedrock depth, an air-gun source was selected as the seismic signal. However, even with this robust set-up, initial data was too noisy for proper stacking. It was later determined that numerous boulders in the fill material were scattering the acoustic signal preventing reliable data collection using the seismic refraction techniques. This unusual condition was noted during the first several hours of the survey and the client was contacted immediately to discuss the issue. With less experience field personnel the survey could have been completed and erroneous data provided for the project.

Geophysical surveys can be greatly affected by site-specific conditions which are difficult to detect in the field by inexperience personnel.


THE SOLUTION 

Delta recommended resistivity profiling for completing the project.  The client agreed, Delta did not charge for the initially seismic work as a client courtesy, and the project was completed successfully.

The Resistivity cross sections were processed individually and digitized to produce a bedrock topography plot (D100608-01-03).  Depth ranges are color coded so shallow rock occupies the purple end of the contour spectrum and deepest rock the blue end of the contour spectrum.  The geophysical data shows relatively shallow rock on the west and south sides of the property with the deepest soil on the east side of the Site.  These areas can be discerned as red (shallow) or blue (deeper) on D100608-01-03.

Generally, the data collected on-site was consistent with a typical Karst environment showing undulating bedrock with dissolution features and pinnacle structures.  Interpretation of the data suggests a significant “wormhole” anomaly through the central portion of the Site (high-resistivity

blue area in middle of cross section).  A second prominent feature is the area of deeper soil on the eastern side of the property (blue area on plot D100608-03).

Based on these interpretations it was recommended that storm water discharge associated with

Site development be contained and directed toward the area shown in blue on plot D100608-01-03, and that geotechnical drilling be used to determine if the worm-hole anomaly needed to be considered for foundation design.

Client Benefit
The geophysical survey gave a complete site overview that identified specific areas of concern and provided focus to allow cost-effective deployment of more costly geotechnical investigations.