Geoscience Reference
In-Depth Information
Trace
Graph showing the amplitude of a seismic signal against time, conventionally plotted with time
increasing vertically downwards. The signal can be of many different types, e.g. as-recorded,
stacked, or migrated. Originally the amplitude of a trace would be shown by a conventional
wiggly line, but increasingly colour is used to convey amplitude information, as explained in
Transgression
Landward migration of the shoreline, owing to relative rise in sea-level.
Turbidites
A turbidity current is a suspension of sediment in a turbulent water flow; such currents are able to
move coarse-grained sediment far out to sea and into deep water. Turbidites are the deposits of these
turbidity currents. They are widespread deep-water deposits, with individual beds up to several metres
in thickness, and ranging from coarse- to fine-grained sediment.
TWT
The Two-Way Time to a seismic reflector is the time taken for a seismic signal to travel from the
surface to the reflector and back to the surface again. This is the usual vertical scale for seismic section
display.
Unconformity
An unconformity is a surface across which there is a gap in sediment deposition; this may be a result
of erosion or of non-deposition. Where the time-gap is substantial, the properties of the sediments are
often quite different on either side of it, giving rise to a prominent seismic reflection. Sometimes the
unconformity surface cuts across bedding planes of the sediment below it; such angular unconformities
are often easily recognised on seismic displays.
Well synthetic
To make it easier to tie seismic to well data, it is useful to make a well synthetic. From the wireline log
data in the well, acoustic impedance is calculated as a function of depth by multiplying together the
recorded velocity and density logs. From this a reflectivity log is calculated, and an expected seismic
response calculated by converting it into a function of TWT using checkshot information if available,
and then convolving it with a seismic wavelet. See
section 3.1
for details.
Zero-offset
A zero-offset trace is one recorded with source and receiver at the same location. This implies a simple
seismic geometry in which an outgoing ray strikes a reflector at right angles and is reflected back
along exactly the same path as it has come.
Zero-phase
There are an infinite number of seismic wavelets that share the same amplitude spectrum. One of
these is the zero-phase wavelet, which is symmetrical about zero time. In practice, the wavelet will
have a strong central loop and a number of smaller sidelobes
(fig. A2.2)
.
This is not a wavelet
that could be generated by any real source, because it would need to begin before the source was
triggered at time zero. However, the seismic traces generated by the real source wavelet can be
manipulated by processing into the form they would have if the wavelet were zero-phase. This is
useful because it is much easier to understand a zero-phase seismic section. Every reflecting interface
produces a signal with its maximum value centred on the interface. This means that a picked horizon
representing an interface follows a maximum loop excursion (positive or negative depending on
the impedance change), which makes autotracking simple. Where reflections are closely spaced,
