Abstract: A gas membrane sampling device, particularly for geological investigations, comprises a membrane element including a liquid-tight, gas-permeable membrane, a filler material contained in and stabilizing the membrane element, and a connection cable being adapted for conducting gas out of the membrane element through a bore hole to a surface of the earth's. A gas sensor device for geological investigations, comprises the gas membrane sampling device and an analyzing device connected with the connection cable of the gas membrane sampling device. Furthermore, a method for the investigation of a geological formation is described.