Gravity data inversion of the Pyrenees range using Taguchi sensitivity analysis and ADMM bound constraints based on seismic data

SUMMARY Gravity inversion methods are able to recover density distributions in the Earth but they require strong constraints based on various prior information. In the present study, our aim is to invert gravity anomalies using existing geological models and density information in orogenic areas like the extensively studied Pyrenees region. Geophysical exploration for resource assessment using gravity and seismic data already accessible in this area has been performed to identify potential fluid/gas reservoirs of economic significance. For a given pre-existing model defined by different geological units and summarizing our knowledge of the area, we aim at retrieving the density distribution within each unit. For this, we use an Alternate Direction Multiplier Method to perform gravity data inversion by constrained by Interval Bound Constraints (IBC-ADMM) defined as bounding distribution intervals of possible density values. To estimate these bounding intervals, we first use a prior density model geologically compatible and obtained using geological information, gravity modelling, seismological data and seismic models. In a second step, we apply a Taguchi statistical analysis on representative density variations inside each unit of the prior density model to estimate their impact on data residuals, and reduce the parameter space prior to inversion. We perform the gravity data inversion constrained by these Taguchi-derived density intervals and we estimate model uncertainties for these density intervals. We apply the technique to the entire Pyrenees range at 2 km resolution. The sensitivity analysis shows that the strongest variations on gravity data are obtained essentially in the Axial Zone, in the middle and lower crust, and in the Upper mantle. Inverted densities are compared to the prior model. They are increased in the central part of the Pyrenees Axial Zone while, in the deeper structures (from middle crust down to upper mantle), they are decreased in the Southern (Spanish) Iberian upper mantle and increased in the Northern (French) Eurasian Upper mantle. A possible shortening of the Iberian lower crust–upper mantle system can be assumed below the western part of the Axial Zone.