Geoscience Reference
In-Depth Information
CMP gather
The Common Midpoint gather is a collection of traces having different source-receiver offsets but the
same midpoint location between source and receiver. For horizontal reflectors, the traces of the CMP
gather have the same reflection point and can therefore be stacked together after NMO correction. In
the presence of reflector dip, DMO correction will be needed before stack.
Crevasse splay
Where a river is confined between levees, from time to time during floods the water level will reach
the top of the levee and spill over. At a place where this happens, a shallow crevasse is formed on the
crest of the levee. From the crevasse, lobes of silty or sandy sediment spread onto the floodplain as
crevasse splays. Similar features are formed in the distributary channels of deltas.
Crossline
See
Inline
.
Datum
On land, there may be appreciable topography across a survey area. Furthermore, if explosive charges
are used as the source then they may be buried at varying depths across the survey. Also, there will
usually be lateral changes in the thickness of the near-surface low-velocity layer. To remove all these
complications, a reference datum is selected and corrections are made to travel-times so that they
represent what would be recorded if shots and receivers were placed on the datum surface, it being
assumed that there is no further low-velocity layer below this surface. For onshore surveys, the datum
surface may be flat or it may be a more or less smoothed version of the topography. Offshore, mean
sea-level provides a convenient datum.
Decimation
Reduction of the number of traces in a dataset by systematic removal of, for example, every other
trace. It is not restricted, as the name suggests it might be, to the removal of every tenth trace.
Deconvolution
The seismic trace can be thought of as the result of convolving the earth reflectivity with a wavelet.
Ideally, we would like to have a seismic source which gave out a single sharp spike signal, and record
areflected signal which was simply a series of spikes. Unfortunately, any real source will emit a signal
of finite length. Furthermore, the source signal will be modified as it passes through the earth, because
of absorption, scattering, and other causes. The result is that our recorded signal will be the sum of
the reflections of a wavelet from the series of subsurface reflectors
(fig. A2.1
)
. Mathematically, this
process is represented by the convolution of the wavelet with the earth reflectivity. Deconvolution is
a signal processing step that attempts to undo this convolution, to leave us with the earth reflectivity
series, thus improving the resolution of the seismic data.
Delta
A depositional system formed where a river supplies sediment to a coast, forming a shoreline protu-
berance. The input of mud and sand from the river is reworked by marine processes (wave and tide
action). The components of a delta system (from onshore to offshore) are: (1) the delta plain, largely
sub-aerial and consisting of distributary channels separated by inter-distributary areas of marsh and
swamp; (2) the delta front, where fluvial and marine processes interact and form features such as sand
bars at the distributary mouths; (3) the prodelta, a zone of quiet deposition, typically of mud and fine
silt, from suspension. As time goes by, if sediment continues to be supplied, the delta builds out-
ward from the coast. In vertical succession, the prodelta muds will then be overlain by progressively
shallower-water deposits, generally with higher silt and sand content.
