This patent deals with methods to extract and analyze seismic signals due to resonance phenomena in an enclosed oil, gas, or water reservoir, which are passively excited by seismic background noise, locating thereby the presence of the reservoir by doing qualitative and quantitative estimates via forward modeling. Measurements are first used in a qualitative analysis in eight steps. The influence of gas bubbles is important. A quantitative method in form of a numerical simulation using one of several specific physical concepts is used for further analysis. By using successive forward modeling, together with investigation and feedback and in conjunction with the Monte Carlo method, more details are gained. It is also possible to determine the fluid type. The uniqueness of the methods is first that it is purely passive; second it is directly sensitive to the oil or gas, because the resonance effect is only present for a fluid.