Inversion of Marine Multifrequency Electromagnetic Profiling Data: A New Approach to Resolve Surficial Sediment Stratification

baasch, b. ; oberle, h. m¨uller, f. k. j. ; dobeneck, t. von;

Electromagnetic induction (EMI) methods are widely use donland to mapelectric conductivity and/or magnetic susceptibility distributions of surficial sediments. In contrast, the application of these methods in marine environments is relatively novel. Based on the recently developed electromagnetic benthic profiler MARUM-NERIDIS III we investigate the potential of concentric-loop EMI methods to recover conductivity and susceptibility of layered marine sediments. Sensitivity analyses based on a data and model normalized Jacobian matrix were performed to compare the influence of conductivity and susceptibility to in-phase and quadrature components at different frequencies. Both parameters substantially affect the EM response. However, the influence of susceptibility decreases more with depth and offers lower depth resolution than that of conductivity. A 1-D inversion algorithm to reconstruct vertical conductivity distributions was developed from existing non-linear inversion methods using apparent conductivity and apparent susceptibility recovered from simultaneous half-space inversion as apriori information. This algorithm was tested on synthetic and real marine EM data from a commercial multi frequency concentric loop EMI system (GEM-3). The results indicate that our inversion algorithmyi elds meaningful results down to approximately 3mdep thunder typicals hallow marine conditions. The comparison of inversion results recovered with1-Dand 2-D constraints showed that combining lateral with vertical constraints substantially improves the resolution of the inversion outputs. Field data from the NW Iberian shelf was calibrated according to a processing flow specifically designed for underwater conditions and analysed. Inversion outputs are in good agreement with ground-truthing stratigraphic investigations and deliver relevant clues on past and present sediment dynamics.