  This work focuses on the numerical assessment of the accuracy of an
adjoint-based gradient in the perspective of variational data assimilation and
parameter identification in glaciology. Using noisy synthetic data, we quantify
the ability to identify the friction coefficient for such methods with a
non-linear friction law. The exact adjoint problem is solved, based on second
order numerical schemes, and a comparison with the so called "self-adjoint"
approximation, neglecting the viscosity dependency to the velocity (leading to
an incorrect gradient), common in glaciology, is carried out. For data with a
noise of $1\%$, a lower bound of identifiable wavelengths of $10$ ice
thicknesses in the friction coefficient is established, when using the exact
adjoint method, while the "self-adjoint" method is limited, even for lower
noise, to a minimum of $20$ ice thicknesses wavelengths. The second order exact
gradient method therefore provides robustness and reliability for the parameter
identification process. In other respect, the derivation of the adjoint model
using algorithmic differentiation leads to formulate a generalization of the
"self-adjoint" approximation towards an incomplete adjoint method, adjustable
in precision and computational burden.
