Creating 3D images of the subsurface from high resolution towed transient electromagnetic data

The top 30 - 50 m of the subsurface are critical for infrastructure, water supply, artificial infiltration, farming, waste deposit, construction, etc.. Yet, the tools for imaging this zone are limited to boreholes, electrical resistivity imagining (ERT) or other relatively expensive methods with a limited applicability for areas larger than a few hectares. We present a new highly efficient towed transient electromagnetic system, tTEM, which bridges the gap in coverage and resolution between point measurements and airborne electromagnetics. The system yields images in full 3D with a resolution down to 10 x 10 m. The system is towed by and All-Terrain Vehicle (ATV), uses a 2 x 4 m transmitter coil and has a z-component receiver 9 m offset from the transmitter. The first bias free gate is as early as 4 μs from ramp beginning . The turn-off time is 2.6 μs. Data are processed and inverted using methods adopted from airborne electromagnetics. We discuss the system design and present a case study where the system has been used to evaluate the potential contamination risk from a pesticide point source contamination to four water extraction wells.