The ocean floor is often considered the last frontier on earth, as it is a domain that remains greatly unexplored. Bathymetry, also known as sea-floor topography, involves measuring and mapping the depths of the underwater world. Today much of the ocean floor still remains unmapped because collecting bathymetry data in waters of great depth is a time consuming and complex endeavor.
Two hundred years ago most people assumed that the ocean floor was similar to the beaches and coastlines. During the nineteenth century, attempts to produce maps of the seafloor involved lowering weighted lines from a boat and waiting for the tension of the line to change. When the handline hit the ocean floor, the depth of the water was determined by measuring the amount of slack. Each of these measurements was called a sounding, and thousands of soundings had to be done just to get a rough measurement of a small portion of the ocean floor. Besides estimating the depth, these surveys helped in identifying large shipping hazards, especially near the shoreline. A naval officer published the first evidence of underwater mountains in a bathymetric chart in 1855.
During World War I, scientists developed the technology for measuring sound waves in the ocean. Anti-Submarine Detection Investigation Committee (ASDICs) was the original name for these underwater sound projectors, but by World War II the term sonar was adopted in the United States and many other nations. Sonar, which stands for Sound, Navigation, and Ranging, was first used to detect submarines and icebergs. By calculating the amount of time it took for a sound signal to reflect back to its original source, sonar could measure the depth of the ocean as well as the depth of any objects found within it. The first sonar devices were passive systems that could only receive sound waves. By the 1930s, single-beam sonar was being used to transmit sound waves in a vertical line from a ship to the seafloor. The sound waves were recorded as they returned from the surface to the ship. However, this type of sonar was more useful in detecting submerged objects than mapping the seafloor. Throughout World War II, technology improved, and active sonar systems that both received and produced sound waves were being used. It was the invention of the acoustic transducer and the acoustic projector that made way for this modem sonar. The newer systems made it possible to identify certain material, such as rock or mud. Since mud absorbed a good portion of a sound signal, it provided a much weaker echo than rocks, which reflected much of the sound wave.
The multi-beam sonar, which could be attached to a ship’s hull, was developed in the 1960s. With this type of sonar, multiple beams could be adjusted to a number of different positions, and a larger area of the ocean could be surveyed. Maps created with the aid of multi-beam sonar helped to explain the formation of ridges and trenches, including the Ring of Fire and the Mid-Ocean Ridge. The Ring of Fire is a zone that circles the Pacific Ocean and is famous for its seismic activity. This area, which extends from the coast of New Zealand to the coast of North and South America, also accounts for more than 75 percent of the world’s active and dormant volcanoes. The Mid-Ocean Ridge is a section of undersea mountains that extends over 12,000 feet high and 1,200 miles wide. These mountains, which zigzag around the continents, are generally considered the most outstanding topographical features on earth.
The invention of the side-scan sonar was another modem breakthrough for the field of bathymetry. This type of sonar is towed on cables, making it possible to send and receive sound waves over a broad section of the seafloor at much lower angles than the multi-beam sonar. The benefit of the side-scan sonar system is that it can detect very specific features over a large area. The most modem form of bathymetry, which is also the least accurate, is done with data collected by satellite altimetry. This method began to be used in the 1970s. This type of mapping relies on radar altimeters that receive echoes from the sea surface. These signals measure the distance between the satellite and the ocean floor. Unfortunately, due to water vapor1 and ionization, electromagnetic waves are often decelerated as they move through the atmosphere; therefore, the satellite receives inaccurate measurements. The benefit of using satellites to map the ocean is that it can take pictures of the entire globe, including areas that have not yet been measured by sonar. At this time, satellite altimetry is mainly used to locate areas where detailed sonar measurements need to be conducted.
Due to a constant flux of plate activity, the topography of the seafloor is ever-changing. Scientists expect bathymetry to become one of the most important sciences as humans search for new energy sources and seek alternate routes for telecommunication. Preserving the ocean’s biosphere for the future will also rely on an accurate mapping of the seafloor.
Questions 28-33
Complete the table below. Write NO MORE THAN THREE WORDS.
MAPPING THE OCEAN FLOOR
| Method | First Used | Used For | How it Works |
|---|---|---|---|
| Weighted line | (28)……………….. | determining (29)……………….. | drop a line until it hits the bottom |
| (30)……………….. | 1930s | detecting objects underwater | send (31)………………to ocean floor |
| Multi-beam solar | (32)…………… | mapping larger areas of the different directions | send multiple sound waves in |
| Satellite altimetry | 1970s | taking pictures of (33)…………………… | send signals from satellite |
Questions 34-37
Match each description below with the ocean region that it describes. In boxes 34-37 on your Answer Sheet, write
A if it describes the Ring of Fire
B if it describes the Mid-Ocean Ridge
34 It is known for the earthquakes that occur there.
35 It is over one thousand miles wide.
36 It is a mountain range.
37 It contains the majority of the earth’s volcanoes.
Questions 38-40
The list below gives some possible reasons for mapping the ocean floor.
Which THREE of these reasons are mentioned in the reading passage?
Write the appropriate Roman numerals i-vi in boxes 38-40 on your Answer Sheet.
i Predicting earthquakes
ii Finding new fuel resources
iii Protecting ocean life
iv Understanding weather patterns
v Improving communications systems
vi Improving the fishing industry