Theoretical studies and some microwave measurements of soil penetration capabilities suggested that microwave radiometers may be able to detect subsurface voids associated with karst development beneath relatively thick soil cover. However, from a theoretical aspect, it is difficult to model the effects of surface roughness, moisture content, vegetation coverage, and a nonhomogeneous sky. To prove the potential of microwave surveys in locating and mapping subsurface voids, a mobile laboratory unit was used to obtain in situ data. This unit is equipped with passive microwave radiometers operating at 13.4 GHz (2.22 cm), 37 GHz (8.1 mm), and 94 GHz (3.2 mm), as well as geophysical support equipment.

An area near the town of Cool, El Dorado County, California, was used to obtain data. This, area is known to have well-developed subsurface karst development and has been surveyed by both the California Highway Department and the California Rock and Gravel Company.

The microwave survey showed significant radiometric “cold” anomalies associated with void-space development beneath several tens of feet of soil cover. Detection was positive in almost all cases, evidence of the strong probability that microwave systems may be used to detect and rapidly map karst systems from a remote platform. This will greatly reduce surveying and construction costs in areas where caves and sink holes have developed beneath obscuring soil cover.

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