The Making of Oil
Detection 

How does remote sensing from the surface find oil? 

The search for oil begins by looking for a reservoir -- a layer of rock that can hold oil -- and an impermeable seal that stops the upward movement of oil and traps it in the reservoir. Until a well is drilled, it is difficult to confirm the presence of oil -- only the presence of a structure that might contain it.

There are several kinds of remote sensing. Each works by extending the eyes of the geoscientist into the earth.

Surveys of anomalies in gravity, like this view of California, can reveal rock bodies that may trap oil.

Similar maps of magnetic anomalies, like this one of Alaska, taken from an airplane, can also reveal oil traps.

Likewise, shallow structures can be discerned with ground-penetrating radar.

But perhaps the most powerful tool is the seismic survey, which uses echoes to map rock layers as far down as four miles. These surveys paint the big picture, but miss the details. Still, these techniques generally find structures that may contain oil. They usually don't detect oil directly.

How do you detect oil from the earth's surface?
How does direct sensing in wells find oil?