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Mapping the depth to basement from gravity anomalies is an essential geophysical survey in sedimentary basins. We propose an iterative method to simultaneously estimate the basement relief, exponential density-depth model, and constant offset. The iterative process is achieved by applying the approximate invariance, which directly relates the basement depth to density parameters and constant offset. The method can be employed provided that there are a few known depth constraints. By testing the sediment and basement models, we find that the observation plane height and random noise of gravity anomalies have almost no impact on the stability of the method. However, the method is slightly affected by the number of constraints and is sensitive to the uncertainty of constraint depth. Finally, we successfully use the new method to map the depth to basement in the Pannonian basin using several depth constraints from drilling data and obtain the almost same density-depth model as that assumed based on drilling density. The method can be extended to recover the discontinuous basement relief and estimate other density-depth models, such as polynomial density function, once appropriate adjustments are made to the method.

This study regards the assessment of surface gravity surveying for CO2 plume monitoring in the deep Johansen saline aquifer, a potential offshore site for CO2 geologic storage. We used the available benchmark model and geological information to simulate the injection and postinjection phases. We calculated the gravity response at the surface from the estimated models of reservoir density and saturation at different time intervals and for different injection rates. The forward calculation is achieved by assuming a tetrahedral mesh discretization such as to ensure an accurate and detailed reconstruction of the complex reservoir. The results show a gravity anomaly extending radially around the well position and reaching a peak of about $-15\,\,\mu $ Gal for an injection rate of 60 kg/s. During the postinjection period, the gravity maps clearly show an increase in brine saturation around the injection well and the migration of the CO2 plume toward shallower portions of the reservoir. We also show that time-lapse gravity data can be used to successfully estimate the CO2 stored mass by means of depth from extreme point (DEXP) multiscale analysis, even when the anomaly is incompletely defined due to a not proper areal coverage of the survey. The DEXP method has proven to be very stable with respect to noise and to be an efficient technique for simultaneously determining the CO2 plume depth, its geometrical features, and stored mass.