## 2 Method  
3D marine controlled-source electromagnetic (MCSEM) surveys collect data using a setup in which a dipole source is towed while a receiver mesh is spread on the seafloor. The data can be categorized as inline and broadside, each with three electric and three magnetic field components at various frequencies. The inline data are recorded when the source line is active and is towed along the receiver line, while the surrounding receivers record the lateral signal, known as broadside data. This coordinated effort helps map the lateral variation and anisotropy effects. The resulting setup (Figure 1) allows multiple source-receiver pairs to contribute to each CMP gather, which are then combined to formulate the mathematical problem and solve for a model that accounts for the lateral connection between the CMP columns. Figure 1: Acquisition configurations. Triangles represent receivers. The transmitters are assumed to be over the appropriate receiver positions in each source-receiver pair. The CMP inversion allows for building resistivity columns in points not associated with a receiver position, as illustrated in Figure 1b. Therefore, a more detailed discretization of the area can be achieved, when sufficient coverage is provided by the data, in comparison with more traditional common-source or common-receiver schemes. For a true layered earth, both approaches are equivalent, but when there are lateral resistivity variations, these are better mapped by organizing the data in CMP gathers, which receive information from multiple directions around the CMP. In our approach, a maximum offset in a CMP gather is determined based on the cutoff level of the amplitude of the electric field. All source-receiver pairs that share the same CMP, and have offsets within this limit, define and contribute to that particular CMP. Therefore, the number of transmitter-receiver pairs defining a CMP varies in the area: CMPs close to the borders of the survey have the least number of data points, and those at the center of the survey area have contributions from the greatest number of data points. This organization of the data for inversion is an efficient way to make use of most of the information that is available in a 3D survey, going beyond what is the common practice in published works performing 1D laterally constrained inversions, in which only the true in-line and broadside data are inverted [8; 9; 10]. For example, in the case of the Pelotas Basin survey presented in Section 4, the CMP with the biggest number of data points has contributions from 1573 transmitter-receiver pairs, and only 207 data points contribute to the one with the lowest number. Although it is not common practice, it is possible to consider the case in which a common-receiver approach would use all data from transmitters around the receiver position, using the same maximum offset as in the CMP inversion. In this case, the data in the CMP inversion would be more closely localized around the CMP, because it is gathered in an area with a smaller radius. This closer concentration of the information around the central point benefits the inversion with lateral constraints because it tends to better separate the influences of localized resistive targets in the area, and also because over a given 3D target, the anomaly generated by the stacking of the CMP data is stronger than that generated by common-receiver data around the same point, as shown by the analysis of [5] for a model of a hydrocarbon reservoir. To reduce the running time, only two electric field components are used in the data volume in this investigation, which provide sufficient information to generate quality inversion results, as the results show.