Final answer:
The first method for determining sea floor depth was using a lead line marked with fathoms. As technology advanced, sonar using sound wave reflections became the standard for precise depth measurements and understanding undersea features.
Step-by-step explanation:
The first method used to determine the depth of the sea floor was by using a lead line weighted with lead and marked with fathom units. Historical sailors would lower this rope with knots at every six feet intervals until it hit the seafloor. The number of knots submersed indicated the depth. This method evolved over time as technology advanced, leading to techniques like sonar (Sound Navigation And Ranging), which uses the reflection of sound waves to measure ocean depths more accurately.
Today, sonar is a common method for depth measurement, where a sound wave is transmitted from a ship and the time it takes for the echo to return from the seabed is used to calculate the distance. Seismic surveys have become more sophisticated; they also involve sending sound waves into the subsurface and recording how they are reflected back. These reflections provide data that can identify subsurface features and materials, including strata implicated in tectonic activity, as done by the International Ocean Discovery Program off the New Zealand coast.