Abstract

This paper presents analytical modeling results for a triaxial frequency-domain electromagnetic-induction (EMI) sensor over a homogeneous earth containing a long linear conductor. Although the conductor studied is intended to represent an underground wire or pipe, it can represent any subsurface, linear geologic structure that can channel current. Treating the sensor transmitter as a vertical magnetic dipole, the model combines the well-known solution for the magnetic field arising from the interaction with the earth with the solution for the induced magnetic field from the excited subsurface conductor. Expressions for the three components of the magnetic field at an arbitrary point above the earth are presented. Two types of coupled, moving transmitter-receiver configurations (coaxial and coplanar) wereconsidered, and the model is sufficiently flexible to allow for many other sensor variations to be studied. Characteristics of the sensor signals were explored through several parametric modeling studies that demonstrate the sensitivity of the signals to transmitter frequency, earth conductivity, conductor depth, sensor geometry, and crossing angle. Using simple relationships developed from analysis of the sensor signals, key parameters such as conductor depth and orientation can be estimated. The ability of the model to predict and characterize sensor output should prove helpful in distinguishing between geologic features and man-made underground infrastructure. These modeling results also are expected to facilitate frequency-domain EMI data analysis and interpretation, sensor design and operation, and the development of detection and classification algorithms.

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