Page 45 - Hydrocarbon

32 Exploration Methods and Techniques

(a) x 1 (b) x 1
x 2 x 2
x 3 x 3
S 1 S 2 S 3 R 3 R 2 R 1 S 1 S 2 S 3 R 3 R 2 R 1
depth depth
z z
a a
CMP CMP
The anomalous rock (say a volcanic
a = reflection point common sill) affects the seismic wave for shot-
midpoint CMP receiver pair S -R . At increasing
3
3
S = source offsets the seismic waves can pass
through the rock unit without adverse
R = receiver
effects of the volcanic sill.
x = offset/shot-receiver separation
2
Figure 3.14 Source^receiver geometry for multiple o¡sets.
Seismic sources generate acoustic waves by the sudden release of energy. There are
various types of sources and they differ in

the amount of energy released: this determines the specific depth of penetration
of the wave
the frequencies generated: this determines the specific ‘vertical resolution’, or
ability to identify closely spaced reflectors as two separate events.

There is usually a trade-off between the two depending on the objectives of the
survey. Studies of deep crustal structures require low frequency signals capable of
penetrating over 10 km into the earth, whereas a shallow geological survey requires
a very high frequency signal which is allowed to die out after only a few hundred
metres.
Typical sources for land surveys are truck-mounted vibrating sources or small
dynamite charge sources detonated in a shallow hole. The most common marine
sources are pneumatic sources such as air guns and water guns that expel air or water
into the surrounding water column to create an acoustic pulse. There are also
electrical devices such as sparkers, boomers and pingers that convert electrical
energy into acoustic energy. Typically the latter produce less energy and have a
higher frequency signal than pneumatic sources.
Seismic detectors are devices that register a mechanical input (seismic pulse) and
transform it into an electrical output which is amplified before being recorded to
tape. On land the receivers are called geophones and they are arranged in a spread on
the ground or in shallow boreholes. At sea the receivers are called hydrophones, often
clustered in arrays, and they are either towed in the water behind the boat or laid
out on the sea floor in the case of OBC (Figure 3.15).
The acquisition geometry, or the configuration of source(s) and receivers depends
on the objectives of the survey, characteristics of the subsurface geology and
logistics. Seismic surveys can be acquired along straight lines, zig-zag lines, in a