Application of the Helicopter Frequency Domain Electromagnetic Method for Levee Characterization
Levee characterization requires many miles of ground to be surveyed. Remote sensing methods, particularly those capable of installation on an aircraft, are capable of quickly surveying large areas. The helicopter frequency domain electromagnetic technique (HEM) involves towing an electromagnetic transmitter and receiver that measure signals proportional to the electrical conductivity of the ground. Information about the electrical conductivity of the ground can be used to make inferences about the distribution of soils or rocks in the subsurface. HEM data can be interpreted by correlating the apparent conductivity to soil or rock type, as well as for looking for lateral and depth extent of anomalous zones. HEM data provide depth information by performing measurements at different frequencies. Here, we provide a brief review of the frequency domain method, highlighting the transformation of HEM data to apparent resistivity, which is critical for interpretation. We then discuss two case histories using an HEM system for levee characterization and hazard detection. With clay being the primary building material for the levees, the minimum layer thickness that can be accurately resolved with an upper frequency of 140 kHz is about 1 m. The HEM data are particularly useful at detecting anomalous zones for follow up investigation. These zones were caused by underground channels (which provide pathways for water flow) and in one case by significant cracking of a levee. The examples provided here highlight the utility of HEM surveying for levee characterization.
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