Abstract: A test circuit is installed in each channel of a seismic streamer towed behind an exploration vessel in offshore seismic exploration. The test circuit is adapted to generate a signal in the channel that is indicative of a characteristic of the channel. The signal is transmitted to the vessel for recording and observation aboard the vessel through the existing cables in streamers used to transmit seismic signals. A signal from onboard the vessel operates switch means which normally disconnect the test circuit and close the primary circuit of the channel to place the test circuit in the primary circuit of the channel. A test circuit is disclosed which includes a relaxation oscillator adapted to indicate the current leakage resistance of the primary circuit of the channel by changes in the frequency of the oscillator and means for applying a step voltage to the primary circuit and indicating the response of the primary circuit of the channel to the step voltage.