Lunar and Planetary Institute

Apollo 14 Mission

Science Experiments - Active Seismic

Astronaut using thumper portion of Active Seismic Experiment. The geophone line is visible in the foreground. Part of the Active Seismic Experiment's geophone line, extending from the flag to the bottom right edge of the image.

Two experiments, the Active Seismic Experiment on Apollo 14 and 16 and the Lunar Seismic Profiling Experiment on Apollo 17, were performed to determine the detailed structure of the upper kilometer of the lunar crust. Both experiments involved detonation of a series of small explosives. The seismic waves or ground motions caused by these explosions were measured by a network of geophones. On Apollo 14 and 16, up to 19 explosions were detonated by an astronaut using a device called a "thumper" along a 90-meter-long geophone line. On Apollo 16, three mortar shells were also used to lob explosive charges to distances of up to 900 meters from the ALSEP. On Apollo 17, eight explosive charges were positioned during the three EVAs at distances of up to 3.5 kilometers from the LM. These charges had masses of 57 grams to 2.7 kilograms. Both the Apollo 16 mortar shells and the Apollo 17 explosives were detonated by radio control after the astronauts left the lunar surface.

These experiments showed that the seismic velocity (P wave) is between 0.1 and 0.3 kilometers per second in the upper few hundred meters of the crust at all three landing sites. These velocities are much lower than observed for intact rock on Earth, but are consistent with a highly fractured or brecciated material produced by the prolonged meteoritic bombardment of the Moon. At the Apollo 17 landing site, the surface basalt layer was determined to have a thickness of 1.4 kilometers, slightly higher than the 1-kilometer thickness determined from the Traverse Gravimeter Experiment.