The largest number of geophysicists find employment in the petroleum industry. Since most of the petroleum deposits are buried deep below the surface, petroleum geophysicists find oil and gas by building a clear picture of what is below the Earth's surface. How do geophysicists build this picture?

A seismic source is used to create shock waves that reflect off the rock layers. The upcoming energy is received by the receivers, and the signal is transmitted through the cable to the recording truck. The signal is digitally recorded and stored on disk.

The Seismic Method

In the petroleum industry, the most common and effective method of obtaining a clear picture of the Earth's subsurface is the seismic method. This technique is based on principles of Earth physics. Shock waves set off at the surface penetrate the earth like sonar and the returning echo can be used to tell what is below.

Petroleum geophysicists usually specialize in the acquisition, processing or interpretation of seismic data. However, most will have a good knowledge of the other areas, as well as geology and some engineering principles, as there is a high degree of collaboration between the different groups.

Petroleum deposits occur in several different shapes. Two types of deposits are shown here

Petroleum geophysicists can be divided into three categories:

• AcquisitionGeophysicists
  take 'pictures' of the subsurface much like X-Rays reveal details of the human body;
• Processing Geophysicists
  develop and sharpen the 'picture';
• Interpretation Geophysicists
  use the sharpened 'picture' to construct the shape of petroleum deposits.

Each of the above three types of geophysicists is described further in the following sections.

Tractor mounted Vibroseis units are used to put shock waves into the ground by pounding large steel plates onto the surface of the earth.