The method proposed in the article implements a synchronous detector for ERT thomography with operating frequency of 4.88 Hz using an almost unchanged recorder module of the small-sized experimental device Siber-32. The detector algorithm is implemented according to the FFT with compensate capacitive influence of the tool internal circuit and external electrodes wires. To increase the information content and add the possibility of visual processing, recording mode include a full wave of current and voltage data is implemented for each measurement of 1024x24 bit words. This method is suitable when it is impossible to obtain small signals at large spacings, or in dipole sequence. The synchronous detector is slower but it can complement the fast way to obtain useful information about the geoelectric section. Due to the low required currents, the use of this method simplifies the equipment, due to softer requirements for the output parameters of the current source and the relay matrix of the generator switch.