Graphing sound waves at the bottom of the ocean

[MyNewPony]

Homework Statement

Ships measure the distance to the ocean bottom with sonar. A pulse of sound waves is aimed at the ocean bottom, then sensitive microphones listen for the echo. The graph shows the delay time as a function of the ship's position as it crosses 60 km of ocean.

Draw a graph of the ocean bottom. Let the ocean surface define y = 0 and ocean bottom have negative values of y. This way your graph will be a picture of the ocean bottom. The speed of sound in ocean water varies slightly with temperature, but you can use 1500 m/s as an average value.

http://session.masteringphysics.com/problemAsset/1071083/10/20.P46.jpg

Homework Equations

v = d/t

The Attempt at a Solution

I'm not how to interpret the graph given. Do the time values represent how long it takes for the waves to get to the bottom? Or do they represent how long it takes for the waves to reach the bottom and echo back?

 

[LowlyPion]

Or do they represent how long it takes for the waves to reach the bottom and echo back?

Doesn't it say:

A pulse of sound waves is aimed at the ocean bottom, then sensitive microphones listen for the echo.