Response of an Electromagnetic Gradiometer to a Subsurface Wire

The response of an electromagnetic gradiometer (EMG) system to a subsurface wire is analyzed in terms of experimental and analytical modeling results. Our objective is to explore characteristics of the response and assess the fidelity of our model. The EMG system consists of a static transmitter and a man-portable sensor, which uses a pair of receivers that yield a gradiometric measurement. Experimental results were collected over a range of wire depths from 3.4 to 8.5 m. A number of different transmitter positions were explored, and the tests studied were conducted at 200 kHz. Modeling results were consistent with the experimental results and supported a number of key findings. Results are presented showing that, in order to maximize the strength of the wire response, the transmitter should be positioned approximately 5 m off the wire axis. Furthermore, in order to avoid unwanted transmitter influence on the response, the EMG should be at least 30 m from the transmitter. Using the experimental and modeling results, we found a linear relationship between the width of the magnitude response peak and the wire depth. Based on our experimental results, the EMG is able to yield a discernible target response at a depth of at least 7 m. Lastly, an example of how the model can be used to optimize survey planning is presented. This paper illustrates how an EMG can be used to locate underground wires with applications ranging from underground utility mapping to the detection of shallow subsurface tunnels.