Geoscience Reference
In-Depth Information
Alluvial fan
On land, rivers are the main means of transporting sediment. Alluvial fans are localised areas
of high sediment accumulation, formed at a place where laterally confined flows are able to ex-
pand, for example on leaving a gorge to flow into a broad valley floor. The expansion of the flow
leads to a velocity decrease and a reduction in ability to transport sediment, which is therefore
deposited.
Amplitude spectrum
The methods of Fourier analysis allow us to envisage a seismic trace as the sum of a series of traces
that are pure sine waves. If we perform this analysis and then plot the amplitude of the sine waves
against frequency, we have the amplitude spectrum of the original signal. Seismic processing often
aims to produce a reasonably flat amplitude spectrum in the frequency range from, say, 8 to 50 Hz,
though what is possible depends on the seismic source, the propagation path length and the degree to
which the rocks absorb high frequencies.
Angle stack
It is often possible to infer subsurface lithology or fluid fill from the way that reflection amplitude
varies with incidence angle on a reflector (AVO). One way to investigate this effect is to make sub-
stacks of the data, including only data from a particular angle range. Thus, one might compare a stack
containing incidence angles 0-15
◦
with a 20-35
◦
stack. These datasets are produced by stacking the
appropriate range of offsets, which will vary with TWT; the required offset range for a particular set
of angles at a given TWT can be calculated if the seismic velocity is known as a function of depth,
either from well information or seismic velocity analyses.
Anisotropy
In general, anisotropy means that a physical property varies with the direction in which it is measured.
In our case, the property of interest is usually seismic velocity. In most cases, this exhibits transverse
isotropy; there is a symmetry axis, and velocity is the same in all directions in the plane perpendicular
to this axis, though different from the value parallel to the axis. Fine layering (e.g. in a sand-shale
sequence) of materials with different velocities will produce rocks with a vertical symmetry axis. Ver-
tical cracks, all oriented in the same plane, would produce a horizontal symmetry axis, perpendicular
to the cracks.
Attributes
The first measurements made on seismic traces were of travel-time to a reflector, with a view to making
a structural map of it. As data quality improved, it was realised that the amplitude of a reflection
could carry useful information; changes in amplitude along the reflector might relate to lithology or
reservoir quality, for example. There are many different ways in which we might measure amplitude.
The maximum excursion of a loop is the most obvious, and would, for example, be appropriate if
we were looking at an isolated reflector at the top of a massive sand overlain by a massive shale. On
another occasion, we might be looking at an interbedded sand-shale sequence, where there would be
a whole series of reflections, each one may be laterally discontinuous, between the top and the base
of the sequence. If we wanted to characterise the variability of the sequence, it would be useful to
measure the average amplitude (without regard to sign, so average absolute value or rms average)
over the interval between the top and base of the package. Or again, we might also get information
from the shape of a seismic loop, for example where we are trying to follow thin beds near the limit
of seismic resolution; the most obvious measure might be the width of the loop, i.e. its duration in
TWT between zero-crossings, but many other shape measures are possible. 'Attribute' is in use as
a loose generic term to cover all these types of measurement on a seismic trace. It usually refers to
