Geology Reference
In-Depth Information
Fig. 4.60
Sonograph obtained from a dual scan survey of a pipeline route across an area of linear sand waves in the southern North Sea.
The inner edges of the two swathes define the bathymetry beneath the survey vessel. (Scanning range: 100 m).
and the resulting pattern of returned acoustic energy is
known as a
sonograph
. The oblique insonification pro-
duces scale distortion resulting from the varying path
lengths and angles of incidence of returning rays (Fig.
4.59(b)). This distortion can be automatically corrected
prior to display so that the sonograph provides an iso-
metric plan view of sea bed features. A sonograph is
shown in Fig. 4.60.
Although not strictly a seismic surveying tool, side-
scan sonar provides valuable information on, for exam-
ple, the configuration and orientation of sedimentary
bedforms or on the pattern of rock outcrops.This infor-
mation is often very useful in complementing the
subsurface information derived from shallow seismic
reflection surveys. Sidescan sonar is also useful for locat-
ing artefacts on the sea floor such as wrecks, cables or
pipes. As with sub-bottom profiling systems, results in
deep water are much improved by the use of deep-tow
systems.
4.16 Applications of seismic
reflection surveying
The 1980s and 1990s saw major developments in reflec-
tion seismic surveying. Over that period, the general
quality of seismic record sections improved markedly
due to the move to digital data acquisition systems and
the use of increasingly powerful processing techniques.
At the same time, the range of applications of the method
increased considerably. Previously, reflection surveying
was concerned almost exclusively with the search for
hydrocarbons and coal, down to depths of a few kilome-
tres. Now, the method is being used increasingly for
studies of the entire continental crust and the uppermost
mantle to depths of several tens of kilometres. At the
other end of the spectrum of target depths the method
is increasingly applied for high-resolution onshore
mapping of shallow geology to depths of a few tens or
hundreds of metres.
