Introduction to the magnetotelluric method

Introduction: Over the past three decades, and particularly over the intervals 1985–1995 and 2005–2010, magnetotellurics has undergone a revolution driven by four main factors: (1) the emergence of low-power, low-cost, 24-bit digital electromagnetic sensing and recording technologies, (2) dramatic improvements in the understanding of noise in electromagnetic measurements, with the concomitant evolution of data processing algorithms, (3) substantial advances in the ability to recognize and remove distortion by near-surface structure local to the measurement point that is the bane of practical magnetotellurics, and (4) the development of fast two- and three-dimensional (2D and 3D) modeling and inversion capabilities concurrent with the constantly increasing power of computers. In the 1970s, a typical magnetotelluric survey consisted of a handful of sites whose data were analyzed using ordinary least-squares methods, smoothed in the frequency domain to reduce data scatter, and interpreted using one-dimensional (1D) models “stitched” together into a 2D pseudo-section that may, or may not, be tested through 2D forward modeling. By the 1990s, surveys comprising several tens of sites along a single line were common, data were processed using robust methods, which produced substantially more reliable response estimates that were subsequently analyzed for galvanic distortion, and rapid 2D modeling and inversion were standard. By the 2010s, magnetotelluric surveys consisting of many hundreds of sites, with areal rather than linear coverage, are being carried out, data processing is semi-automatic, usually using bounded influence or multivariate approaches, multi-site distortion removal is being applied routinely, 2D interpretation, often including anisotropy, is routine, while 3D interpretation based on 3D inversion is becoming commonplace. The key purposes of this book are documenting this magnetotelluric revolution and providing an up-to-date, rigorous reference on the field that is much more than a practical guide, and that is useful to the novice, expert practitioner and interested non-magnetotelluric geoscientist alike.