Geoscience Reference
In-Depth Information
required and the fact that lateral variations cannot be included in the reflection
zone.
4.4 Reflection seismology
4.4.1 The reflection method
Although earthquake seismology and refraction seismology enable scientists to
determine gross Earth structures and crustal and upper-mantle structures, reflec-
tion seismology is the method used to determine fine details of the shallow
structures, usually over small areas. The resolution obtainable with reflection
seismology makes it the main method used by oil-exploration companies to map
subsurface sedimentary structures. The method has also increasingly been used
to obtain new information on the fine structures within the crust and at the crust-
mantle boundary.
For land profiles, explosives can be used as a source. Other sources include the
gas exploder
,inwhich a gas mixture is exploded in a chamber that has a movable
bottom plate resting on the ground, and the
vibrator
,inwhich a steel plate pressed
against the ground is vibrated at increasing frequency (in the range 5-60 Hz)
for several seconds (up to 30 s for deep crustal reflection profiling). Vibrators
require an additional step in the data processing to extract the reflections from
the recordings: the cross-correlation of the recordings with the source signal.
Of the many marine sources, the two most frequently used for deep reflection
profiling are the
air gun
,inwhich a bubble of very-high-pressure air is released
into the water, and the
explosive cord
. Many air guns are usually used in an array
towed behind the shooting ship.
Deconvolution
is the process which removes the effects of the source and
receiver from the recorded seismograms and allows direct comparison of data
recorded with different sources and/or receivers. For the details of the methods of
obtaining and correcting seismic-reflection profiles, the reader is again referred
to the textbooks on exploration geophysics (e.g., Telford
et al
. 1990; Dobrin and
Savit 1988;Yilmaz 2001; Claerbout 1985).
The basic assumption of seismic reflection is that there is a stack of horizon-
tal layers in the crust and mantle, each with a distinct seismic P-wave velocity.
Dipping layers, faults and so forth can be included in the method (see Section
4.4.4). P-waves from a surface energy source, which are almost normally inci-
dent on the interfaces between these layers, are reflected and can be recorded
by
geophones
(vertical-component seismometers) close to the source. Because
the rays are close to normal incidence, effectively no S-waves are generated
(Fig. 4.39). The P-waves reflected at almost normal incidence are very much
smaller in amplitude than the wide-angle reflections near to, and beyond, the criti-
cal distance. This fact means that normal-incidence reflections are less easy to rec-
ognize than wide-angle reflections and more likely to be obscured by background
