Limitations Of Small Em Sensors In Resistive Terrain

Handheld electromagnetic (EM) sensors, for example the GEM-2, have been increasingly used in a variety of geotechnical applications. The sensors provide users with great flexibility in<br>selecting operating frequencies, sensor height, and coil configuration to optimize the measurement parameters for specific targets and mapping applications. However, a good understanding of sensor limitations is needed in planning surveys for specific environments, especially when the ground is resistive. Having resistive ground and small separation between transmitter and receiver, combined with a limited frequency span, can place the instrument in the low induction number range. Within this low induction number range the sensor response is mostly dominated by the frequency used (i.e., is linearly dependent on frequency) and not by the ground resistivity. This allows for detecting conductors, but low induction numbers are not suitable for depth imaging. Theoretical diagrams may be used to establish system limits and to choose an optimal set of frequencies for specific ground resistivity. To illustrate various conditions encountered while using small EM systems in resistive terrain we will use data from two GEM-2 surveys: one conducted in an area with high environmental noise on Sacramento, California, levees, and another in a karst environment in Texas where the environmental noise was low.