The estimation of soil and rock parameters based on field instrumentation data is a common procedure in geomechanics. The use of system identification and optimization techniques allows the performance of this type of analyses in a more rational and objective manner. In this paper a probabilistic formulation for the backanalysis problem is presented. The procedure described involves the evaluation of the measurement covariance matrices, which are derived for some geotechnical instruments used in field instrumentation. The algorithm used to solve the mathematical problem of optimization is also presented, as well as its coupling to a finite element code. The algorithm requires the computation of the sensitivity matrix, which can be evaluated “exactly” in terms of the finite element method. Finally, a synthetic example, based on the excavation of a tunnel, is presented in which the elastic modulus E and the parameter of the material are identified from measured displacements. The effect of the number of measurements and their error structure is also discussed.