Geoscience Reference
In-Depth Information
measurements taken on a limited time-window around a reflector or interval of interest, and very often
to measurements made on single traces, though some attributes are measured over a small cluster of
traces (e.g. the local dip or curvature of a reflector).
Autotracker
Software to follow a reflector automatically through a seismic trace volume, starting from a limited
amount of manually picked data ( seed points/lines ).
AVO
Amplitude Versus Offset: the variation of reflector amplitude with source-receiver separation (see
chapter 5 ). Sometimes denoted AVA (Amplitude Versus (Incidence) Angle).
Avulsion
Overbank sedimentation from an active river channel causes the growth of a ridge above the sur-
rounding floodplain, within which the channel is contained. When a channel bank is breached in a
flood, the river can find a new path on a lower part of the floodplain, leading to the abandonment of
the old channel. This process is called avulsion, and occurs in delta distributary channels as well as
meandering rivers.
Bin
The final processing output of a 3-D survey will be a set of traces on a rectangular grid. It is not
feasible to acquire data on a perfectly regular grid. Therefore, a uniform grid of rectangular bins is
superposed on the acquisition map. Each acquisition trace is assigned to the bin located at the point
midway between its source and receiver positions. The traces falling within a bin will be stacked after
correction for normal moveout, giving the uniform output grid of traces. Some refinements of this
basic idea are discussed in chapter 2 .
Bright spot
The classic hydrocarbon indicator is the bright spot, when pay (usually gas-bearing) sands are very
soft and brine sands are similar in impedance to the overburden. The top reservoir event will then
be a strong soft loop in the pay zone, and a weak event elsewhere. Such an amplitude anomaly will
conform to structure, unless there is a strong lateral change in lithology across the area of hydrocarbon
fill.
Bulk modulus
If a volume V of a medium is acted on by a pressure P , the volume will be decreased by
V . The
dilatation
is defined as
=
V
/
V , and the bulk modulus k is defined as
k
=−
P
/
where the minus sign is inserted to make k positive.
Checkshot survey
In order to tie well information to seismic data, it is important to know the relationship between depth
and TWT at the well. This can be established by measuring the travel-time from a source on the
surface to a receiver at a known depth in the well. This information is usually collected at a number
of receiver depths. In the case of a deviated well, either the source can be located vertically above
the receiver (and thus at some distance from the wellhead), or corrections can be applied for the slant
travel path.
CDP gather
Synonym for CMP gather ( see CMP gather ) .
 
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