Page 51 - Hydrocarbon Exploration and Production Second Edition

38 Exploration Methods and Techniques

travel time of the incident wave is the same as the travel time of the reflected wave.
Point a at depth z is recorded at position au at the surface and associated with depth
zu; both the position and the depth are correct: a ¼ au and z ¼ zu.
In the case of steeply dipping reflectors the travel time of the incident wave is
different to the travel time of the reflected wave. In the picture the travel time of the
reflected wave is much smaller than the travel time of the incident wave. This leads
to point a being recorded updip of its true position with a shift in surface position
(a 6¼ au) and a shift in depth (z 6¼ zu); the same occurs at point b and so on. The true
dip (y true ) of the reflector is imaged incorrectly and the apparent dip (y app )is
shallower.
Migration is the process of repositioning reflected signals to show an event
(geological boundary or other structure) at its true position in the subsurface and at
its correct depth. There are two main types of migration: pre-stack and post-stack
migration. The first involves migrating the seismic data prior to the stacking
sequence, the second after stacking has occurred.
If the geological layers are almost flat and the seismic velocities are uniform, a
simple post-stack time migration will give a good result. If the seismic velocities
vary only a little or the dips are small then a pre-stack time migration will give a
good solution. In areas of complex geological structures, for example sub-salt or
sub-basalt, neither technique will image the events below the salt or basalt correctly
and pre-stack depth migration (PSDM) will need to be applied.
PSDM requires the processor to draw up a model of the seismic velocities of the
subsurface, this in itself can be quite challenging. The input model allows the
reflectors to be restored to their true position in the subsurface and corrects
apparent dips to true dips.
Although PSDM is an important tool in the imaging of complex structures it is an
expensive and time-consuming process. PSDM is often only applied when other
methods have failed to give a working solution. With advances in computer
technology and processing capability, however, PSDM is likely to be become
economic and more readily applied.

3.2.2.5.6. Multiples. An additional step that is often required is the removal of
‘multiples’. Multiples are signals that have been reflected at more than one interface
and they are common in a layer cake scenario. The seabed multiple is a common
feature on many offshore seismic sections. The sea surface is a strong reflector and
waves travelling upwards can bounce off it before being reflected a second time at
the seabed. Multiples can be very difficult to remove and can severely impede
seismic interpretation if they mask true reflectors.

3.2.2.5.7. Seismic output. A 2D seismic survey consists of a network of lines,
usually arranged in an orthogonal grid at regular spacing, for example 500 m. The
processed result is a series of seismic sections in time or depth (Figure 3.22) that tie
at the nodes or intersections of the lines. A single 2D line typically contains several
hundred traces.
A 3D seismic survey is acquired in a series of parallel swathes each containing a
large number of inlines (sail lines) and crosslines (perpendicular to the sail lines)