Question

The following passage is adapted from an article on mid-twentieth-century oceanography and plate tectonics:

In the summer of 1947, oceanographer Maurice Ewing led an expedition aboard the research vessel Atlantis to map the Mid-Atlantic Ridge. At the time, prevailing geological theory held that the ocean floor consisted of a thick layer of continental granite covered by miles of accumulated sediment. Ewing utilized seismic refraction measurements, sending acoustic shockwaves from small explosive detonations down to the sea floor and recording the echo timings with hydrophones.

The results shattered long-held assumptions. Ewing discovered that the oceanic crust was composed entirely of dense basalt, not granite. Furthermore, the sediment layer measured less than 1,000 feet thick, a fraction of the 12,000-foot thickness predicted by conventional models of an ancient, static ocean floor. This thin sediment layer indicated that the Atlantic floor was geologically young, providing crucial empirical evidence that eventually supported the theory of seafloor spreading.

According to the passage, what specific equipment did Ewing use aboard the Atlantis to record the echoes of the acoustic shockwaves?