Inversion of marine magnetotelluric and controlled source electromagnetic data with site gap and low signal-to-noise ratio

Abstract

SUMMARY Marine magnetotelluric (MT) and controlled source electromagnetic (CSEM) methods have been routinely applied to survey the crustal and upper mantle electrical resistivity structures beneath the sea floor. In practice, there are inevitably site gaps and contamination by noises in the collected data because of lost ocean bottom electromagnetic receivers, unusable data, and difficulties in deploying instruments near deep trenches. So far, it remains unclear to what degree those factors will lower the resolution and the credibility of marine MT and CSEM inversion models. In this paper, we investigate the individual and combined effects of site gaps and data noises on the inversion models through synthetic analyses based on a simple block resistivity model and a realistic resistivity structure derived from the Mariana Trench. The results suggest that data with a sufficiently high signal-to-noise ratio can reasonably recover the subseafloor structures in the area of data gap. The transverse electric mode and tipper data from the MT method are much more sensitive to the structure near the site gap. The joint inversion of MT and CSEM data would improve the model's resolution at the site gap area. The inversion of data with a relatively low signal-to-noise ratio, for example, 10 per cent, can recover the structures with few artefacts if there is no site gap. But if the site gap and noisy data are combined, even a joint inversion cannot correctly recover the burial depths and geometries of the anomalous bodies beneath the site gap where vertical strips are likely present. To improve the model's resolution and suppress inversion artefacts, we propose constraining part of the model with as much a prior information as possible. Specifically, for a survey in the subduction zone, we could reduce the penalties on the model's smoothness at the upper and low interfaces of the resistive subduction slab, or even fix the resistivity of the resistive slab with the help of other information, if any. The inversion models shown in this paper provide valuable references for the site design before marine MT and CSEM surveys as well as for interpreting real data inversion models that may be subject to the same biases introduced by the site gap and noise.