A buoy for use in seismic surveying at marine locations comprises an elongated housing with a flotation member positioned near the normal waterline of the device. The lower portion of the housing or battery section is of greater diameter than the upper section, and a tethering ring for connection to an anchor is located at the upper end of the battery section so that the cross sectional area below the tethering ring is substantially equal to the cross sectional area of the housing above the tethering ring. A seismic detection apparatus, conveniently a hydrophone, is used to detect seismic signals and preferably rests on the water bottom. A seismic amplifier within the housing amplifies the geophone signals. The gain of the amplifier is controlled by a circuit that varies the gain in accordance with a weighted function of the amplitude of the output signals of the amplifier over an interval of at least two minutes. The output signal of the amplifier also is used to amplitude modulate a signal generator of constant frequency or constant repetition rate, and the amplitude modulated signal is used to frequency modulate a radio transmitter.