Seismic Survey

The most accurate and widely used means of finding good drilling locations is the seismic survey. Seismic surveying involves sending sound waves down into the ground and recording the echoes that bounce back off the various sedimentary layers.The sound or shock waves are generated by; setting off small explosive charges just below the surface; hitting the ground with a heavy weight; or shaking the ground using large vibrator trucks. The echoes returning from the subsurface are detected by sensitive instruments called geophones which are strung out along the ground in a straight line. The geophones are connected by electrical cable to a recording system. The recording system precisely records, to the nearest one thousandth of a second on magnetic tape, the time it takes for the echoes to return to the surface. By knowing the amount of time it takes for a sound wave to reach a certain layer and then bounce back to the surface, as well as the speed of sound through the rock
layers in between, the geophysicist is able to determine the depth to that layer at that location. By determining the depth at a large number of points along the seismic line, the geophysicist is able to create a profile of the underground layers along the line.