This abstract reviews the latest methods to optimize marine controlled-source electromagnetic (CSEM) data acquisition to ensure reliable interpretation in the presence of complex bathymetry. We specifically analyze how amplitude and phase of the recorded electric field are affected if the receiver is located at the slope within a seabed trench. The main trends observed on real data offshore Brazil can be reproduced by 3D forward modeling taking into account the precise bathymetry profile. Understanding these trends is very helpful for quality control (QC) and thus increases confidence in the acquired data. Applying error propagation analysis together with known accuracy of receiver positioning we can assess the limiting sensitivity of the CSEM data recorded within a trench with a given steepness.Remarkably, all the data points follow almost the same linear dependence implying that the effect of a trench on CSEM data is mainly determined by a single parameter -the ratio h/w. For example, for h/w = 0.1, whether it is a 600 m wide 60 m deep