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| Project Branson Airport, Branson, Mo. |
| Geotechnical engineer Geotechnology, Inc., St. Louis |
| Product application Electromagnetic, resistivity, and seismic refraction surveys provide non-invasive exploration of subsurface conditions beneath a new airport. |
In March 2007, during construction of the new Branson Airport in Branson, Mo., a sinkhole was discovered at the north end of the runway. The airport construction team immediately contacted St. Louis-based Geotechnology, Inc., to perform non-invasive geophysical exploration to identify the location of suspected karstic voids or solution features within the footprint of the runway area. The runway is approximately 150-feet wide and 7,140-feet long.
The geology of the area, which is located in the heart of the Ozark Mountains, consists predominantly of limestone overlain by residual cherty clay soils. Karst features are prevalent throughout the region in the form of pinnacled bedrock surfaces and caves.
Geophysical exploration was performed in two expedited phases. Geophysical methods included electromagnetic (EM) dipole-dipole resistivity and seismic refraction surveying, which are based on transmitting and recording EM, electrical, and seismic energy. Geotechnology’s geophysicists analyzed the measurements collected and correlated them to subsurface features, including voids. A team of nine worked day and night shifts to complete the fast-track assignment and provide the client with information while keeping the project on schedule.
The airport is slated to begin operations in May 2009.
Phase 1
Electromagnetic survey—Frequency-domain EM surveying is a geophysical method used to measure ground conductivity. Ground conductivity is a physical parameter of soil and rock that is directly related to water content and mineralogy of the measured media. In general, media with higher water content will exhibit higher conductivities. Typical conductivity values corresponding to clays will be high, ranging between approximately 20 and 80 millimhos per meter (mmhos/m). Limestone bedrock will typically exhibit conductivities less than 20 mmhos/m. Karst features may be detected within limestone bedrock because clay-filled solution features often exhibit high-conductivity anomalies and voids may exhibit low-conductivity anomalies.
EM surveying is performed by generating a continuous primary electromagnetic field in the subsurface using a transmitting coil. The primary field causes a secondary electromagnetic field in conductive zones in the subsurface. The secondary magnetic field is measured using a receiving coil. The strength of the secondary electromagnetic field is proportional to ground conductivity. For this survey, Geotechnology used a Geonics EM34 terrain conductivity meter calibrated to measure ground conductivity directly. An intercoil spacing of approximately 32 feet for this project limited the effective depth of investigation for the EM34 to approximately 50 feet.
The electromagnetic survey was conducted March 11-14, 2008. Electromagnetic data were collected with a vehicle mounted Geonics EM34 coil assembly along parallel lines at approximately 25-foot or less spacing within the survey area. The survey area was approximately 180 feet wide and 7,500 feet long over the proposed runway. Additional surveying was performed over some adjacent aprons and in the vicinity of the suspected location of the previously encountered sinkhole, which was located immediately northeast of the north end of the runway.
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| Geophysical Technician Kristy Cross is setting up one of the electrical resistivity lines along the 7,200-foot-long runway. |
The dipole-dipole resistivity method was the configuration used on this project. The greater the distance between the current dipole and the voltage dipole, the greater the depth of the measured electrical field. By placing the electrodes at successive locations along a survey line, profiles of subsurface resistivity are determined for the region beneath the survey area. Raw resistivity data are modeled to obtain "true" resistivity and the interpreted depths of subsurface features. Resistivity data are interpreted by comparing the resulting "true" resistivity with typical resistivity values for various geological materials.
Typically, clay will exhibit a low resistivity, ranging between 1 and 100 ohm-meters (ohm-m). Limestone bedrock at this site exhibited resistivity values ranging between 100 and 105 ohm-m. Karst features may be detected within limestone bedrock because clay-filled solution features often exhibit low-resistivity anomalies and voids may exhibit high-resistivity anomalies.
Geotechnology conducted the resistivity survey March 12-21, 2008. Inclement weather precluded surveying during three work shifts and abbreviated three more work shifts. For the resistivity survey, the runway was segmented into seven sections and we collected six resistivity lines in each section. The first and last lines were spaced 15 feet from the edge of the runway. Interior survey lines were spaced 30 feet apart. The dipole-dipole resistivity surveys were conducted using a SuperSting-R8 earth resistivity meter, manufactured by Advanced Geosciences, Inc., with 112-electrodes at 10-foot electrode intervals. Each line included approximately 1,110 linear feet of electrodes and cables. The lines were arranged end-to-end with approximately 50 feet of overlap to allow meshing of the data at the ends of each survey line. We collected a total of approximately 44,820 feet of resistivity data.
The surface material comprised crushed rock or bedrock, both of which were extremely resistive. Field personnel poured salt water on the electrodes that displayed high resistance at the surface to provide better conductivity with the near surface materials, resulting in better current generation and voltage measurements.
Apparent resistivity recorded during the survey are not true resistivity of subsurface materials and do not directly indicate the depth or structure of subsurface features. The depth and true resistivity of subsurface features are estimated by modeling the recorded data. Geotechnology modeled the apparent resistivity data recorded during the surveys using AGI’s resistivity inversion program Earth Imager 2D. This inversion program automatically determines a 2-D resistivity model using the Gauss-Newton least-squares method.
Phase 1 conclusions—Data from the first phase led us to believe the potential exists that near-surface karstic features were located within the runway area. Geophysical surveys identified the presence of 21 subsurface, low-resistivity or high-conductivity anomalies that did not appear to be the result of fill placement activities at the site. These features were consistent with other karst features positively identified through subsurface exploration at similar sites. These anomalies typically represent clay- or water-filled voids within the bedrock formations. At this point, Geotechnology recommended and was given authorization to perform seismic refraction surveys at select locations within the runway footprint. In addition, we were authorized to perform EM and electrical resistivity surveys over adjacent terminal, apron, and taxiway areas.
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| Kevin Roberts, geophysical technician, headed up the night crew and is shown recording electrical resistivity data along the runway. |
The purpose of this phase of the project was to use a non-invasive seismic method to further evaluate potential karst features within the runway and to identify the location of any additional voids or solution features within the cut areas of the terminal, apron, and taxiway. Electromagnetic and resistivity methods were used in the terminal, apron, and taxiway surveys, while seismic refraction was used in the runway survey.
The seismic refraction method involves generating seismic waves at the ground surface using an impact source. The seismic waves travel from the source through the subsurface along a variety of paths, including refracting along interfaces between soil and rock layers having different seismic velocities. The seismic waves return to the ground surface where they are recorded at various distances from the source using geophones and a seismograph. Seismic velocity calculations are made by analyzing the differences in elapsed time from the source to each geophone. The resulting profile is a representation of velocities of the soil and bedrock directly beneath the survey line.
We conducted seismic refraction surveys at the site on April 2, 4, and 5, 2008, recording data along nine survey lines using a 24-channel Seistronix, RAS 24 seismograph for the survey. Twenty-four geophones were placed at 10-foot intervals along each survey line. Sixteen to 18 "shots" were acquired along each line, which included end shots, off-end shots, and multiple mid-spread shots. A propelled weight-drop source was used to generate the seismic energy by striking a metal plate on the ground surface. Generally, 10 to 15 successive impacts were made on the metal plate at each shot point. The seismograph was used to "stack" the recorded data from each impact. Stacking the recordings enhanced source signals and reduced the effects of spurious noise in each recording. The line locations were surveyed using a Trimble ProXRS differential GPS unit.
Phase 2 conlusions—Surveys appeared to confirm the presence of possible locations of karstic features within the runway area and also appeared to indicate several potentially karstic features within the terminal, apron, and taxiway areas. We recommended that Branson Airport, L.L.C. commission additional investigations of the anomalous features by performing confirmatory borings.
Anomalies in the EM, electrical resistivity, and seismic geophysical data were interpreted and areas of possible karst were selected and ranked according to a variety of characteristics and perceived severity. Most of the identified areas were drilled by a local firm and discontinuities in the rock were found, primarily in the form of clay-filled seams/zones. The design team evaluated the information from both the geophysical surveys and the drilling and determined that the hazards were not a significant risk and the design did not need to be altered. Construction of the new airport continued on schedule with the confidence that the performance of the runway wouldn’t be affected.
This article was contributed by Geotechnology, Inc., St. Louis, which offers consulting services in applied earth and environmental sciences, geotechnical engineering, materials testing, geophysics, drilling, and construction observation.
