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and vibrates the weight. This vibrating weight, like an explosion, also
creates sound waves that enter the layers of rock. Searchers often use
several such trucks. Because explosions in water can kill marine life,
offshore explorationists use specific sound generators.
Regardless of how oil seekers make the low-frequency sound,
it penetrates the many layers of rock. Where one layer meets another,
a boundary exists. Each boundary reflects some of the sound back to
the surface. The rest continues downward. On the surface, special
devices, termed "geophones", pick up the reflected sounds. The
sounds carry information about the many layers. Cables from the
geophones or hydrophones transmit the information to sophisticated
recording devices in a truck or on a boat.
Explorationists take the recordings to a special laboratory
where personnel use powerful computers to analyze and process the
recordings. The computers display and print out the seismic signals as
two- or three-dimensional views. Some seismic readouts show a sort
of cross section of the earth. Others display a top view of buried rock
layers. This type of display, in effect, removes thousands of feet of
rock lying above a given layer to reveal the layer from above. Seismic
displays indicate to knowledgeable personnel where oil and gas may
exist. Unquestionably, seismic exploration is valuable; indeed, modem
seismic technology pinpoints buried oil and gas reservoirs with great
accuracy. Because of this accuracy, operating companies can be
reasonably sure that when they drill a well, the reservoir it taps will
produce oil or gas.
TYPES OF WELLS

The industry generally classifies wells as “exploration wells”,
“confirmation wells”, and “development wells”. They also speak of
drilling “infilling” wells and “step-out” wells. An exploration, or
wildcat, well is one a company drills to determine whether oil or gas
exists in a subsurface rock formation. A wildcat well thus probes the
earth where no known hydrocarbons exist.

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