Magnetotellurics: What You Need To Know

Investigation and development of ground water resources are a continuing and increasingly important issue for local and national governments. As the depths to fresh water resources increase, geophysical techniques that are most sensitive to depth ranges from 10 m to 800 m, such as controlledsource audio magnetotellurics (CSAMT), e.g., Geometric’s STRATAGEM® system, have become increasingly useful. Magnetotellurics (MT) and Audio-Magnetotellurics (AMT) have been successfully utilized to delineate fresh and saline water, lithology, and subsurface geologic structures that impact ground water flow. However, it has become apparent that while more and more practitioners are using MT/CSAMT, sometimes successfully and sometimes not, their training and knowledge of the basic magnetotelluric technique has not kept pace with their use. The purpose of this talk is to present a primer on how to set up a field survey (TM vs. TE modes) and what to look for when recording AMT data. Discussion focuses on what constitutes good signals, bad signals, effects from lightning, and cultural noise effects (usually power lines and actively pumping water wells and pipe lines), and how to mitigate these effects. MT and AMT are examples of geophysical techniques that can be used to help map subsurface geology and effectively support hydrogeologic investigations. However, magnetotelluric surveying, as with most geophysical methods, works best when it is integrated into a omprehensive geophysical investigation.