Sensitivity analysis on gravity change data in order to observe mantle convection

With the GRACE-FO mission well underway and the plans of the MAGIC/NGGM mission in the future, there will be several decades of satellite gravity change data available. Both periodic and secular mass changes are studied with this data, mostly surface mass changes like hydrology, ice melt, Glacial Isostatic Adjustment, and large Earthquakes. With the increasing time period of the gravity data set, smaller variation in the signal can be detected, especially linear secular changes. One of the processes that would result in small secular gravity rate is the mass change due to mantle convection.

We perform various sensitivity analysis studies to understand the added benefit of detecting mantle convection with satellite gravity change observations. A fast stoke solver (FLAPS) is developed that is based on an axisymmetric half annulus geometry. A density and viscosity structure can inserted as well as test anomalies to understand the effect of the mantle flow on the gravity change observations. The model evolves over 50 years after which the difference between the initial and final state is computed. This will also give the rate of change information. Realistic Earth models (PREM) as well as synthetic models are tested to better understand the sensitivity of the gravity change data.

The gravity change observations are sensitive to the absolute viscosity state of the mantle. This is contrary to dynamic topography and geoid data, which do not have this sensitivity and studies using these data always have an ambiguity wrt. viscosity state. Moreover, it seems that the gravity change data is more sensitive to the lower mantle of the Earth. This sensitivity can be very helpful in further exploration of Core-Mantle Boundary structures. The modelled magnitude of the gravity change linked to global mantle convection seems to be larger then the formal error estimates of the GRACE and GRACE-FO instrumentation. A longer acquisition period will reduce the secular errors in the ocean, atmosphere and tidal correction models, such that eventually mantle convection can be studied directly by satellite gravimetry.