Before multibeam technology was invented, surveys of sea mounts were made by simple echo sounders. The sound signal was strong enough, however, that it could penetrate the sea floor, showing some of the structure. In this image from the Deep Sea Drilling Reports, you can see Nashville Seamount and the surrounding sea floor. In this image, horizontal distance is measured in hours that it took the ship to pass over the area, and the vertical measure is the seconds it took the sound to travel back to the ship. At this scale, Nashville seamount is seen rising from the abyssal plain. Actually, the sediment of the abyssal plain has been deposited in the 85 million years since Nashville Seamount was an active volcano.