Seismic
"Seismic" (size-mik) is one of the most important tools available to the Petroleum Geologist. It is a method used to determine the depth and orientation of rock formations without actually having to drill a well.
Although the principles of seismic are extremely
complicated, it can be be explained rather
easily. Look at the diagram to the right.
A "source" of energy is applied at the surface of the ground. This energy may be applied by a "thumper" truck, which uses a large, heavy iron plate to strike the ground. More likely, the source will be dynamite, placed in a shallow hole and exploded.
Sound waves from the source travel down into the ground
and are reflected back by the rock layers back up to the surface. A system
of listening microphones, or "geophones" pick up the reflected sound
waves. The simple picture shows only one receiver. In practice, many
such receivers are used. The more receivers used, the better the
quality of the received data.
In
a similar way, seismic data can be acquired for the sea floor and the rock
layers beneath the sea.
To do this, a ship at sea tows a seismic source
behind it. The seismic source provides very strong waves of sound energy
(supplied by blasts of compressed air from air guns towed behind the ship) that penetrate the sea floor, bounce off the rock layers, and are then picked by
hydrophones (listening devices located on the streamer) towed behind the ship.
The seismic is then processed the same way as land seismic, and the result looks like
the "line" of seismic shown below.
Once
the data has all been "shot", the information is fed into computers
for processing. The result is a "line" of seismic, shown at the
left.
Layers of rock deep in the earth can be clearly seen. On this "2D" line, stratigraphic traps containing possible oil or gas have been shaded green. The line is called "2D", or two-dimensional seismic, because it shows a single cross-section through the earth along a relatively straight line.
In
the 1990's a new method of acquiring seismic became popular. This was
called "3D" seismic. "3D" seismic is shot much the
same way as 2D except that the "shotpoints", or dynamite holes, are much
closer together and are laid out on a grid, instead of in a straight
line. The geophones that receive the reflected sound waves are also laid
out in a grid. A huge amount of data is collected, and this data is
processed by powerful computers. The result is a "3D data set"
that can be manipulated on a computer to display fantastic three-dimensional
images of the rock formations deep underground.
You
might think that 3D seismic would solve all the problems of the petroleum
geologist. It's not quite that simple. First of all, 3D seismic is extremely
expensive. Second, 3D seismic is good at showing structural
traps but not so good at showing stratigraphic
traps.
Finally, 3D seismic cannot help the PG tell if the rocks contain contain oil or
gas. Only by drilling a well can this be established.
Many,
many different types of colorful and exciting displays can be created with 3D
seismic datasets.
Petroleum
Geologists and Geophysicists study the images carefully before they decide to
drill a very expensive well!