I have examined the possibility of using a 3D modeling algorithm in studying tunnel detectability using the electromagnetic gradiometer (EMG) response. A detailed comparison of different source-receiver configurations reveals that the EMG response is stronger for a configuration when the transmitter and receiver are orthogonal to each other than the parallel configurations. Orthogonal configurations perform better at a relatively lower frequency than the parallel configuration. The EMG response is enhanced with the broadside offset defined as the distance between transmitter and receiver pair. The response of a tunnel is generally weak; consequently, deviation from the configuration in which both receivers are equidistant from the transmitter masks the response of a tunnel. The impact analysis of tunnel-floor conductivity revealed that a one-order-higher conductive floor does not change the behavior of response compared to a tunnel without a conductive floor. However, the two-order-higher conductive floor changes the shape of the response curves, yet the change in the magnitude is not significant. The presence of a metal conductor substantially enhances the response of an EMG system. The parallel configuration is more suitable for the depth estimation of the tunnel than the orthogonal configuration. The distortion in the EMG response due to the heterogeneity in the case of an orthogonal configuration depends on the broadside offset. For a zero broadside offset, the response is severely distorted by the small-scale inhomogeneity. For a broadside offset such as 20 m, the impact of small-scale inhomogeneity is almost invisible.

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