Probing the Earth by Projectile

To aid their endless search for water, oil and minerals beneath the surface of the earth, prospectors and scientists have used everything from divining rods to sophisticated seismic devices. But more often than not, they have had to fall back on costly and time-consuming drilling to probe the earth's secrets. Now, New Mexico's Sandia Corp. has developed new tools for preliminary subsurface exploration that may do in minutes or hours what now takes days and even months to accomplish. The new devices: high-speed, instrumented projectiles dropped from aircraft or propelled by rockets.

The projectile probes, developed by Sandia engineers in the course of nuclear-weapons research for the Atomic Energy Commission, operate on a simple principle: the deceleration of a projectile as it penetrates the earth is determined by the material through which it passes. A projectile penetrating loose to medium-dense sand, for example, will be slowed down more quickly than one passing through soft clay.

Antenna Tail. To determine the behavior of objects penetrating the earth at high speed--a science that Sandia has named "terradynamics"--engineers have used projectiles weighing from 5 Ibs. to 6,000 Ibs. that strike the earth vertically at speeds of from 41 m.p.h. to 1,870 m.p.h., depending on the drop altitude and method of release. Some are merely shoved out of airplanes or hovering helicopters; others are dive-bombed or rocketed to boost their velocities. The best penetrators, Sandia has found, are pencil-shaped missiles of heavy metal that are at least 8 to 10 times longer than their diameters. Some have plunged more than 200 ft. into the earth.

Before the projectile hits the ground, a small parachute tucked into its hollow tail is released, pulling out a long wire antenna. As the projectile pierces the earth, a small, insulated accelerometer responds to the sudden impact and subsequent slowing by producing a voltage that varies with the rate of deceleration. The voltage is amplified and transmitted through the antenna, which, unfurled, is long enough to remain extended above the surface.

Anchor Planting. Picked up by a receiver above ground, the data are plotted on deceleration v. time and deceleration v. depth curves that are characteristic of the substance and structure of the soil that has been penetrated. Sandia engineers are already able to tell when the projectiles have passed through materials Ifke sand, silt, clay, water, mud and certain kinds of rock.

Sandia's earth-probe projectiles have been proposed to investigate the suitability of remote sites along Colombia's Atrato River for the location of dams. Eventually, in addition to a role in mineral exploration, the projectiles may be used to find water, to place deep-sea anchors, and to bury radioactive fuels re-entering the atmosphere after the flights of nuclear rockets. Shot from unmanned spacecraft orbiting distant planets, one Sandia scientist proposes, the projectile probes could even help determine if there are water tables beneath the surface of Mars and Venus.

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