Abstract : The subject of this thesis is related with the DEMETER project, and much particularly to the processing of its data. DEMETER is the first scientist micro-satellite which was launched by CNES in June 2004 with a payload under the responsibility of LPCE. DEMETER measures the electric field and the magnetic field in the ionosphere in various frequency bands, as well as the characteristic plasma parameters at the satellite altitude. The main objective of DEMETER is to study the ionospheric perturbations related with the seismic activity. Before the launching of DEMETER, the variations of Total Electronic Content (TEC) measured by DORIS with the others satellites evolving in the ionosphere (Spot, Topex-Poseidon, and Jason) have been studied to search the perturbations due to the seismic activity. After, the study of the general characteristics of the electrostatic turbulence at mi-latitude and low latitude was done with the DEMETER data. At the same time, we search if these characteristics of the electrostatic turbulence are changed when the satellite is over a seismic zone. In this study we have shown that: the TEC parameter and the electrostatic turbulence could vary as function of seismic activity. The perturbations of these two ionospheric parameters before the earthquakes have been well proven. But these results were obtained with a statistical method and we can't still confirm that they can be used as precursors.