Testing the limits of AEM bathymetry with a floating TEM system

Abstract

A floating transient electromagnetic (TEM) system (“sea ring”) simulating a low-altitude helicopter airborne electromagnetic (AEM) system was constructed to test the accuracy of the AEM method for measuring water depth and estimating sediment thickness in shallow coastal waters. A square transmitter loop [Formula: see text], plus concentric inner and outer receiver loops, was strung from masts supported by the circular sea-ring base. Data were stacked over periods from 1 to approximately [Formula: see text] and with loop heights ranging between approximately 5.5 and [Formula: see text] above sea level. The towed sea ring provides a stable platform at a known fixed altitude in calm waters. We have undertaken modeling to investigate the effect of vertical and horizontal displacements of the loops, and to compare circular and square loopgeometries, in proximity to the sea surface. With relatively long stacking times, as long as approximately [Formula: see text], the uncertainty in altitude can be reduced to very low levels. The sea ring has been deployed near Port Lincoln, Australia, in an area with known bathymetry, seawater conductivity [Formula: see text], and sediment thickness (from marine seismic). Initial 1D inversion of raw sea-ring data highlighted significant instrument calibration errors. Empirical correction factors were defined to achieve agreement between measured and modeled TEM responses at selected control points. These corrections were then applied to the entire data set. The 1D inversion of corrected sea-ring data predicted seawater depths within approximately [Formula: see text] of known depths down to [Formula: see text]. These results provide an upper limit to the accuracy that can be expected from airborne EM systems for shallow bathymetry using current technology.