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Manufacturer of geophysical resistivity and IP imaging systems
2121 Geoscience Dr., Austin, TX 78726, USA
phone: +1(512)335-3338, fax: +1(512)258-9958
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AGI STING/Swift case history 3
The Sting Cave
Summary:
A cave had been detected by Gasch & Associates Inc. at the
4T ranch north of Austin, Texas. In order to test the cave
detection capability of the Sting memory earth resistivity meter
and the newly developed Swift automatic multi-electrode system, a
dipole-dipole resistivity profile was measured over the known
cave.
After collecting the data with the Sting/Swift system a PC was
used to invert and interpret the results. Based on the 2D section
a location for drilling was selected and a new cave was
discovered.
The Sting/Swift system proved at this site for the first time
that it was capable of detecting caves and voids. Since then the
method has been further developed and is now used in many places.
Description
The survey line was set out perpendicular to the
"strike" of the known cave and 20 ft (6 m) from the
man-made cave entrance. The electrode spacing, dipole size, was
15 ft (4.6 m) and using 28 electrodes the total distance covered
was 405 ft (123 m).
The terrain at the site is flat with a thin layer, 0-1 ft (0-0.3
m), of top soil over lime stone. There is no sign, except the
drilled man-made entrance hole at the surface, indicating that a
cave exists just 6 ft (1.8 m) below the surface.
The resistivity data was acquired in the following way:
The survey was made completely automatic using the Sting/Swift
dipole-dipole system. Dipole, dipole size 15 ft, measurement was
taken out to a dipole separation of n=8 (i.e. 8x15=120 ft).
Each station was measured at least two times and the standard
deviation for the station was automatically calculated. The
maximum deviation encountered among the total measured 175
stations was 0.4 %. The current used varied between 100 and 200
mA for the complete survey.
A two man crew set out the field equipment in about half an
hour and used about the same amount of time to pick it up. The
actual data acquisition took about 40 minutes, therefore the
complete survey took less than two hours.
Data was downloaded from the Sting to an MS DOS type computer
using the STINGDMP download software. Apparent resistivity data
has been automatically inversed to resistivity by the RES2DINV
and then contoured by the Surfer for Windows software. The result
of this processing is shown below in a color resistivity
cross-section image.
The known cave between 56-70 m is evident as a high
resistivity area (dark color) slightly to the right of the center
of the profile. There was however also a clear anomaly centered
at 27 m indicating that another, so far unknown, cave may exist
at this location.
Drilling a 2" hole at the 27 m marker only indicated a small
void at about 12 m depth. The drill rig was moved towards south
and another hole was drilled at the 25 m marker. This time a 2.5
m drop was encountered at 8 m depth.
Using a larger drill rig this pilot hole was reamed to 24 inch
diameter. Just enough so that a rope ladder could be lowered and
cave surveyors be let in.
The new cave, properly named the "Sting Cave", was surveyed, mapped and named by Mike Warton and Associates of Austin, Texas. No biological life could be detected in this new truly interstitial cave.
Cross-sectional map of the Sting Cave (large image).
Last modified: 2006-09-07 18:28:39 EDT
