Full 3D mapping of shallow structures on land, water and snow using a small towed transient electromagnetic system (tTEM) - new advances and possibilities

We present a multi-purpose towed- transient electromagnetic system (tTEM), capable of imaging the top 100 m of the subsurface. The system can be operated on land, water and snow. The tTEM system can map the subsurface with high resolution both vertically (shallow resolution is about 2-3 m) and horizontally (down to 10 x 10 m). It has a one-turn 4 by 2 m² transmitter loop mounted on a frame on sledges and towed by an all-terrain vehicle (ATV). The receiver coil has an effective area of 20m² and placed in a 9 m offset configuration. The system operates at low and high transmitter moments (LM, HM) to achieve both shallow and deep information. The LM transmits 9.0 Amp with a turn-off time of 2.8 us and a first usable gate at ~4 us (times from beginning of the ramp) while HM transmits 30 Amp. The repetition frequencies for the two moments are approx. 2000 Hz and 800 Hz. A raw sounding is obtained every 0.8 sec corresponding to 3 - 4 m spacing between soundings with a production speed of 15 - 25 km/h. Data are processed and inverted using Aarhus Workbench. In Denmark and other part of Europe, the tTEM system has been used for mapping geology, raw materials, aquifer vulnerability, shallow soil with respect to construction, nitrate-retention potential, and pathways contributing to flow and transport of contaminants. The other two version of the tTEM system used on water and snow, are called FloaTEM and SnowTEM, respectively. FloaTEM surveys were conducted in several parts of United States including the Mississippi delta region for mapping and characterizing the alluvial aquifer system below the river beds. The results show that the FloaTEM system produces detailed subsurface resistivity images in quick time compared to other geophysical methods. We conducted over 300 km of survey along several rivers within a week in the Mississippi delta region. The SnowTEM system uses a 4 turn, 4 m x 4 m transmitter coil and a specially designed low noise receiver coil. It has been put on skies and is towed behind a snow scooter or a snow robot. The SnowTEM has been successfully used to locate the missing part of an airplane engine in Greenland. In the presentation, we discuss the system design and demonstrate the imaging capability of the system with examples.