The schematic figure shows seismic energy being generated by
four vibroseis trucks.
Some experiments use explosives as the energy source, while other
experiments utilize earthquakes. The waves propagate through
the earth and reflect or refract when reaching a transition between
rocks with different physical properties. The waves are reflected
or refracted (bent) back to the earth's surface where the energy
is detected by geophones (marked by yellow flags) and recorded
by a computer. A typical LITHOPROBE geophone array includes 240
geophone groups that extend 8 km ahead of the vibrators and 4
km behind. When vibrating every 100 m, this type of seismic reflection
crew covers approximately 20 km per day. Measuring the time it
takes the energy to follow the various paths allows the transitions
to be mapped. Enough energy is generated by 4 vibrators to successfully
map crustal structures to the base of the crust (approximately
30 to 50 km beneath continents).