Two single-channel seismic profiles were recorded in 1981 during a cruise of R/V Jean Charcot along the axis of the East Pacific Rise between lat 11°50′ N and 12°54′ N. The seismic source was a 1.3−1 (80 in3) water gun. The ship's track was kept on the rise axis by using the real-time mapping capabilities of the SeaBeam echo sounder. The seismic data were recorded in analog form and no signal processing was performed. The seismic sections revealed the existence of segments of 7- to 30-km-long coherent arrivals located at about 0.6 s two-way traveltime below the sea-floor reflection. The same time difference observed for the water-borne multiple of these arrivals is a strong indication that these segments of reflection are not side echoes but vertical-incidence reflections originating from the subbottom. The correlation between the seismic data and some geologic observations in the area near lat 12°–13°N indicates that the observed reflections are probably associated with the roof of discontinuous magma chambers situated at an average depth of about 1.5 km below the sea floor.

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