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October 2019 Bulletin

Near-Surface Geophysical Mapping of Pilot Knob Volcano, Austin, Texas, USA
Mustafa Saribudak¹, and S. Christopher Caran²
¹Environmental Geophysics Associates, Austin, Texas Email: ega@pdq.net
²Texas Development Board, Austin, Texas
This article has been reproduced with permission from the Oil and Gas Journal, with a modification to the title and Figure 1. The original title is ‘Resistivity, magnetic data delineate volcanic tuff in Travis County, Tex’.
Pilot Knob is the eroded core of an extinct volcano 8 miles south of central Austin, TX, near Austin-Bergstrom International Airport and McKinney Falls State Park. This area is 2 miles in diameter and encompasses four small, rounded hills including Pilot Knob proper. A combination of resistivity and magnetic surveys were conducted across the Pilot Knob. Resistivity and magnetic data were collected along two profiles across the Pilot Knob volcano. Profiles 1 and 2 were located in the eastern and western parts of Pilot Knob volcano, respectively. Along profile 1, the resistivity data indicate a series of anomalies, which could be due to a combination of a dike and a volcanic caldera. Magnetic data along the same profile shows significant high and low magnetic anomalies. The magnetic data also display a significant fault anomaly. The sources causing these anomalies are likely solid basalt masses and highly weathered volcanic rocks (palagonized). Profile 2 is located about 1000 feet west of profile 1. Along profile 2, the resistivity data show a sequence of layered volcanic rock; but more significantly, it shows a well-defined low-resistivity unit in the center of the profile. This anomaly could be the mound of Pilot Knob volcano. A series of significant high and low magnetic anomalies is also observed and they correlate well with the locations of resistivity anomalies. In conclusion, the results indicate that a combination of resistivity and magnetic data provide valuable information across volcanic areas in terms of delineating volcanic features such as vent, dike, and mound. And they also help define the geological contacts of tuff and solid basaltic rocks, and faults along profiles.
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