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| Remote sensing |
Remote sensing is a technique that gives information about the surface of the Earth and the underwater world without touching the surface. The technique bounces energy off of nonliving or living objects and analyzes the returning signal to collect information.
Remote sensing has many uses in water. Common uses of remote sensing include charting the depth of a lake or ocean bottom. It is vital to the fishing industry and in locating objects at the bottom of the water. Treasure hunters and researchers would find it much harder to detect lost shipwrecks if not for remote sensing.
People interested in finding out where water pollution is occurring can take remote sensing images of water from planes or satellites (orbiting spacecraft) to detect microorganisms such as algae that thrive in polluted water. People who are trying to find deposits of oil and natural gas under the ocean floor also use remote sensing.
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Energy of remote sensing
The first step in remote sensing is to have a source of energy that will be beamed toward the target. The energy comes in the form of light waves of different sizes. Like the waves in an ocean, energy waves can range from waves whose top point (crest) to lowest point (trough) are very tiny to those that are hundreds of feet (meters) long. The distance of one full wave, from crest to crest or trough to trough, is known as the wavelength. The range of waves is known as the electromagnetic spectrum.
At one end of the electromagnetic spectrum lie the tiny waves such as gamma rays and X rays. These waves tend to carry large amounts of energy and can penetrate into solid or liquid material more so than other waves. That is why X rays can pass right through skin to reveal images of the bones and teeth underneath.
At the other end of the spectrum lie waves such as the microwaves that can penetrate a short distance to heat up foods, and radio waves that beam music through a radio speaker. Radio waves are not efficient for remote sensing operations. Microwaves are the longest waves with enough energy to be used for remote sensing.
The regions of the electromagnetic spectrum that is useful for remote sensing contain the waves known as ultraviolet rays (the same rays that give a suntan or sunburn). The term ultraviolet means that the waves are just beyond the portion of the spectrum that contains the waves that are visible, in particular the region of the spectrum that contain violet-colored waves. Indeed, for the visible portion of the electromagnetic spectrum, our eyes are the remote sensors!
Shorter, higher energy wavelengths are preferred for remote sensing because the waves have to move through air or water on their way to the target. Passing through air and water causes some of the waves to be absorbed or deflected (bounced) off the target. (The deflection of different wavelengths of light as they pass through Earth’s atmosphere, the mass of air surrounding Earth, is the reason why the sky appears blue.
Colors with relatively long wavelengths pass straight through the atmosphere. Blue light has a shorter wavelength and the atmosphere scatters it.) A higher energy wave will be better able to blast through any interference to the target, and to bounce back from the target.
The absorption of waves can be useful when trying to figure out the nature of the target. For example, microwaves tend to be absorbed by the gas form of water known as water vapor. The pattern of absorption detected by scientists on their instruments can provide important clues about the amount of water contained in the air above the ground or water.
How remote sensing works
In order to illustrate how remote sensing works, imagine a bathtub full of water. If a bar of soap is dropped into the water, waves will move outward over the surface of the water. As the waves contact the sides of the tub, some the energy will rebound back into the tub. So it is with the energy that is beamed from a satellite, ship or plane.
The returning energy is captured by a detector (also known as a sensor). Instruments and computers that are connected to the sensor can analyze the pattern of the returning waves to help scientists understand the distance and shape of the object on the ground or the ocean floor that deflected the waves.
History of remote sensing
It has been known since the early nineteenth century that sound can move through water. In 1822, scientists measured how fast sound moved underwater in Lake Geneva in Switzerland by suspending a bell from one boat and having someone in another boat listen through a tube lowered down into the water. Their calculations turned out to be very close to those obtained using modern day sensitive electronic technology.
The use of underwater sound became known as sonar, which is a short form for "sound navigation and ranging". Like many technologies, sonar became used in warfare. In World War I (1914-18), British, French, and American forces used sonar to locate submarines and to detect icebergs (massive chunks of ice) that could rip open the hulls of their ships. By World War II (1939-45), sonar had become much more accurate and sophisticated.
During the first half of the twentieth century, scientists realized that sound waves do not move through all ocean water in the same manner. The depth of the water, the amount of salt in the water, and the ocean temperature can all affect wave movement. For example, a technique called acoustic tomography measures the movement of ocean currents (the circulation of ocean waters that produces a steady flow of water in a prevailing direction) by examining the differing properties of the current from the surrounding water.
Sending energy underwater
To chart the depth of a lake or ocean bottom, a transmitter on a boat will beam energy for a short time (a pulse transmission) straight down into the water. A sensor on the boat detects the returning signal. Using a mathematical formula to account for the presence of water, scientists can then determine the one-way distance of the signal. Other uses of vertical (up and down) sonar include detecting other ships and as an aid in navigating.
The energy pulse can also be sent out horizontally through the water, rather than straight down. This is called side scan sonar, and is useful in determining what lies around a ship. Some systems are so sensitive that they can detect an object in the water that is less than 0.4 inches (1 centimeter) in size. Side scan sonar is also useful in investigating underwater archaeological sites.
