PURPOSE: To early find the occurrence of a new fracture by filling a conducting material into a spouting geothermal well in advance, lowering an electric well logging sonde in the geothermal well, and measuring the electric specific resistance of a geothermal fluid.

CONSTITUTION: After the pit mouth valve of a production well 1 in a geothermal power plant in operation is closed, a conducting material such as a KBr aqueous solution is filled through a nozzle provided at a pit mouth. An electric well logging sonde 2 having current electrodes A1 and A2 and voltage electrodes V1 and V2 is lowered from the upper section of the pit mouth, and the specific resistance of the preset stratum is measured.