Marine CSEM Inversion Based on Joint Constraints of Structure and Petrophysical Property

We present a study of marine controlled-source electromagnetic (CSEM) inversion using joint constraints based on structural and petrophysical properties. Marine CSEM can directly evaluate the oil and gas distribution within a reservoir and improve drilling success rates. However, marine CSEM inversion suffers from significant nonuniqueness. Constrained inversion is an effective method to reduce nonuniqueness and improve resolution. There are two main approaches to constrained inversion: one involves applying constraints based on subsurface structural information, and the other relies on known petrophysical information. Current constrained inversion techniques typically utilize only one of these methods in isolation. In this study, we develop a joint constraint method that combines structural constraints with petrophysical property constraints for marine CSEM inversion. First, we obtain a high-precision velocity structure using seismic full-waveform inversion and introduce the velocity structure constraint into the objective function of marine CSEM inversion using a cross-gradient function. This enables marine CSEM inversion based on seismic velocity structure constraints. In addition, during the iterative inversion process, we apply petrophysical property constraints using the fuzzy C-means (FCMs) clustering method. The joint constraint method integrates the advantages of both types of constraints, effectively improving inversion accuracy. Synthetic model analyses and field data applications demonstrate that marine CSEM inversion, guided by joint constraints from seismic velocity structures and petrophysical properties, not only enhances the imaging accuracy of resistivity structures but also recovers realistic resistivity properties. Furthermore, the FCMs clustering method sharpens the boundaries of inversion results, making them clearer and more interpretable for geological analysis.