EP-95 ON THE MOON WITH APOLLO 16

FIGURE 31.-Principle of the active seismic experiment.

FIGURE 31.-Principle of the active seismic experiment. The geometry of the experiment is shown in the lower figure. At the source, which resembles the explosion of a shot gun or a grenade, elastic waves are sent in all directions. Some of these waves (marked A) travel in the direction of the three geophones, G1, G2, and G3. The relative times of the source and the arrivals; at the three geophones are shown in the upper figure In the actual case, the signals continue ringing longer than is shown in this figure; the signals have been abbreviated for clarity. If there is a layer of rock beneath the lunar soil, and the depth to it is not too great, then some of the elastic energy is returned to the surface. Such waves arrive at the geophones later than the direct waves.

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