Oil prospectors have a method called ‘Seismic shooting’ in which they use small artificial Earthquakes. In the area they have chosen to explore, the prospectors bury a charge of dynamite.

They plant detectors called geophones at various distances, and run wires from them to a recording instrument in a truck.

The surface rocks of the area are Sedimentary. This means they are made of ancient sediments-sand and mud which came from the crumbling of earlier rocks.

Where sedimentary rocks exist, they lie in layers on top of crystalline rocks like granite. When the prospectors fire their ‘shot’, compression waves through the rock layers, hit the boundaries between them, and echo back.

The returning waves reach the geophones, where they set off electric signals. These go to the instrument in the truck, and are recorded as a seismogram.

Later, the seismogram is carefully studied. What will its zigzag lines reveal? The prospectors look for signs of a structure they call an oil trap.

Somewhere in the area, they believe, the layers of rock make an upward fold. One of the layers is sandstone, which may be soaked with oil and water.

It is sandwiched between other layers through which the oil and water do not leak. The water in the sandstone forces the lighter oil to the top of the fold, and there it lies, trapped.

Such a structure can be located because waves from the shot hit the top of the fold and bounce back.

What Wave speeds show:

Waves travel at different speeds in different rocks. When rock that is very elastic is compressed in a shock wave, it jumps back fast, so the wave goes fast.

In a less elastic rock the reaction is slower, and so is the wave. Scientists have learned test shots just how fast compression waves travel in granite, sandstone, basalt, and other rocks.

They can therefore tell from wave speeds the nature of rocks beyond their reach. When seismic shots are made at sea, the waves travel under the ocean floor at a higher speed than waves passing through granite and other rocks of the continents.

They travel at the speed of waves in basalt. Is the rock under the oceans basalt? A few samples have drilled from the ocean floor, and they have indeed turned out to be basalt.

Clues to Structure:

In 1909 the Yugoslav seismologist, Andrija Mohorovicic, made an important discovery.

Seismic waves speed up gradually as they go deeper, but at certain levels they make sudden changes in speed.

Thy also change direction. A level at which these changes happen is called a discontinuity. It seems to mark a boundary between different kinds of material.

The first important one lies about 20 miles down under the continents, and as little as three miles down under the ocean floor.

It is known as the Mohorovicic discontinuity, after it’s discovered. Scientists have nicknamed it the ‘Moho’.

A seismic wave going down towards the Moho speeds up gradually. But at the Moho it suddenly makes a jump in speed of about 20 miles a minute.

Depth of Earth