Resource: Collecting Data Below the Earth's Surface

Taking a cue from bats and dolphins, humans have developed technologies that use sound to "see" where eyes cannot. Underwater profiling systems are acoustic devices that direct a transmitter, either on or trailing a boat, to emit pulses of sound waves outward to an area of study. Some of these waves bounce off objects they encounter and return to a receiver, which collects the reflected energy and records the time it took the waves to make their round trips. A computer analysis of these data can then determine the depth of the seafloor and the size, shape, and density of underwater objects.

In addition to bouncing off objects, sound waves also penetrate objects. This property makes it possible for geologists to also learn about the composition of the seafloor's underlying layers, or subsurface. Certain physical properties, such as density, influence whether and how sound waves travel through an object. As a sound pulse encounters a boundary between two sedimentary layers, depending on the acoustic properties of the rock, some of the waves may be reflected back and the rest left to continue on to the next boundary. Based on the strength of the returned energy, a computer can create a profile of the subsurface sediments. These data can then be plotted on paper or digitally modeled on screen.

Scientists who study undersea geology to better understand the processes driving the movement of Earth's tectonic plates use these detailed subsurface maps to note unconformities within layers, which may indicate where faulting has occurred in the past and whether it's ongoing. These acoustic mapping techniques are invaluable as an alternative to collecting physical core samples. Although core samples can provide highly detailed information about subsurface sediment structure, because they are taken at intervals, unlike acoustically-produced maps, they provide only a discontinuous record.

A growing body of evidence suggests that generating sound waves at different frequencies may have a detrimental effect on animals living in the marine environment. These impacts may be mitigated by having professional observers on board to control the use of the acoustic sources if, say, mammals are seen in predetermined safety zones, or to prevent their use during nighttime, when darkness precludes such sightings.

To learn more about the geological conditions underlying Nevada's Lake Mead, check out Plate Tectonics: Lake Mead, Nevada.

To learn more about how sound waves travel through water and the equipment used to map underwater surfaces, check out Sound Waves Underwater: The Loch Ness Monster and Sound Waves Underwater: Experiment with Sonar.

To learn more about how marine mammals use sounds to navigate, check out Guess How Whales Hear!.

To learn more about the impact of human activity on marine mammals, check out Sea life is troubled by noise.

• If you want to learn about the earth below a body of water, why would you use sound waves?
• What kinds of questions can the technology featured in the video help to answer?
• Discuss how field science differs from lab science.